Tumor microenvironment and signaling pathways in melanoma brain metastasis

Introduction

Mechanisms of metastatic spread in melanoma and other tumors are still poorly understood. Tumor cells migrate through regions of cellular confinement due to dense tissue structure, increased collagen bundling in the tumor microenvironment, and traversing the vascular wall in the tumor microenvironment (1). Melanoma is initiated by transformation of melanocytes by the BRAF V600E mutation (2), NRAS mutation or others, causing hyper activation of the mitogen-activated protein kinase (MAPK) pathway, driving proliferation and migration (3,4). Disease progression occurs from primary melanomas with radial and vertical phases (stages I–II), lymph node invasion (stage III), and ultimately metastasis (IV) (5): the 9-year distant metastasis-free survival in patients with resected stage III or IV melanoma was 54% with nivolumab and 48% with ipilimumab (6). Brain metastases are common in melanoma and other tumors such as lung cancer, in which the brain microenvironment plays a critical function.

The brain metastatic microenvironment is composed of multiple immune and stromal cell populations that contribute to tumor progression through distinct yet interconnected mechanisms. Among the myeloid compartment, neutrophils and microglia appear to play complementary roles. Neutrophils, which are recruited from the circulation, contribute to immune suppression and metastatic niche formation through inflammatory mediators and immune checkpoint-related mechanisms. In contrast, microglia are the resident immune cells of the central nervous system and regulate neuroinflammation, innate immune responses, and interactions between tumor cells and the brain parenchyma. Although their respective contributions to melanoma brain metastasis are incompletely understood, both cell populations may engage common signaling pathways, including inflammatory and TBK1-related networks. Understanding how these myeloid populations cooperate within the brain metastatic ecosystem may provide new therapeutic opportunities.

Studies in other tumor types have provided important insights into biological mechanisms that may also contribute to brain metastasis. For example, in non-small cell lung cancer, insulin-like growth factor 2 mRNA-binding protein 3 (IGB2BP3) promotes brain colonization by stabilizing fatty acid synthase (FASN) mRNA, thereby enhancing lipid metabolic reprogramming and metastatic fitness (7). In addition, lung cancer cells can remodel the brain microenvironment by activating the signal and transducer activator 3 (STAT3) signaling pathway in microglia, increasing the expression of sterol-CoA desaturate (SCD1) and lipid metabolism inflammatory response pathway in the brain tumor microenvironment, which supports metastatic growth (8). Another pathway of potential relevance involves the WW domain-containing oxidoreductase (WWOX) tumor suppressor. WWOX gene expression is often lost or downregulated in several primary tumors, including cutaneous squamous cell carcinoma (9). Experimental studies have shown that WWOX suppresses epithelial-to-mesenchymal transition in several types of cancer cell lines, and WWOX loss is a hallmark of aggressive cancers. In hepatocellular carcinoma, WWOX limits metastasis by simultaneously suppressing STAT3 and Wnt signaling (10). Although these observations have not been fully validated in melanoma brain metastasis, they illustrate how metabolic reprogramming, microenvironmental interactions, and inflammatory signaling can cooperate to promote metastatic progression. A better understanding of these mechanisms in melanoma may facilitate the identification of novel therapeutic vulnerabilities and improve the development of immunotherapy- and targeted therapy-based strategies for patients with brain metastases.

Gaining insights into the mechanisms of melanoma biology could offer the opportunity for a more comprehensive overview of the disease and guide further immunotherapy and targeted therapy management, with particular attention to brain metastases.

Literature search strategy

Relevant literature was identified through searches of PubMed and Google Scholar up to May 2026. Search terms included “melanoma brain metastasis”, “brain metastasis”, “microglia”, “tumor microenvironment”, “TBK1”, “EZH2”, “Src”, “PD-1”, “CTLA-4”, “immunotherapy”, “fatty acid oxidation”, “CPT1A”, and “TCTN1”, alone or in combination. Original research articles, translational studies, and clinically relevant reports were prioritized. Additional publications were identified through manual review of reference lists from selected articles. Articles were selected according to their relevance to the biological mechanisms and therapeutic implications discussed in this review.

Melanoma brain metastasis

In melanoma, metastasis to the brain occurs at a high incidence, and in autopsies, brain metastases are found in 70% of cases (11). It is stated that in 15–20% of melanoma patients, brain metastases are the first site of recurrence, coinciding with metastases at a second site in 41% of cases or a third site in 20%. Risk factors include male gender; the head or neck as the primary site of the melanoma; the presence of nodal or visceral metastases; high serum lactate dehydrogenase (LDH) levels; high Breslow thickness (>3 mm); and ulceration (12). Melanoma brain metastases are predominantly deposited in the frontal lobe, are less frequent in the cerebellum, and uncommon in the hippocampus. It was reported that patients with one to three brain metastases before targeted therapies and immunotherapy had a median overall survival of 6 months, and patients with more than three brain metastases had a median survival of 3.52 months, whereas leptomeningeal metastasis had a median overall survival of less than 2 months (13). With stereotactic radiosurgery, local control has improved.

The first trial to treat melanoma brain metastases with an anti-programmed death 1 (PD-1) antibody (pembrolizumab) showed durable responses in 22% (four of 18 patients) with melanoma brain metastases measuring between 5 and 20 mm, who were asymptomatic and did not require corticosteroids to control symptoms. There were no detectable BRAF mutations in any patient who had a response, and one patient with response had an NRAS mutation (Q61K) (11). Since then, European Society for Medical Oncology (ESMO) guidelines recommend the use of ipilimumab plus nivolumab or BRAF and MEK inhibitors in BRAF V600-mutated symptomatic melanoma (14). Other consensus guidelines of melanoma state that in stage IV melanoma with a BRAF V600 mutation, first-line therapy with BRAF and MEK inhibitors can be offered as an alternative to immunotherapy, only in very select cases (15). Although no phase III trials have been conducted comparing targeted therapy versus immunotherapy, the DREAMseq trial randomized patients to receive dabrafenib plus trametinib upon progression, or the reverse sequence. Two hundred sixty-five patients with BRAF mutant metastatic melanoma were included, and the 2-year overall survival rate was significantly higher in the group receiving first-line immunotherapy with nivolumab plus ipilimumab (72%) than in the group receiving dabrafenib plus trametinib (52%) (16). Notwithstanding, the phase 2 SECOMBIT trial randomized BRAF-mutant patients in three arms. In arm A, patients received encorafenib plus binimetinib as first-line followed by ipilimumab plus nivolumab as the second line; in arm B patients received first-line ipilimumab plus nivolumab and encorafenib plus binimetinib as second-line treatment; whilst in arm C, patients received induction with encorafenib plus binimetinib for 8 weeks followed by ipilimumab plus nivolumab, and, at disease progression, encorafenib plus binimetinib. Two hundred and nine patients were included (17). At a median follow-up of 56 months, the 60-month brain metastasis-free survival rates were 56% for arm A, 80% for arm B, and 85% for arm C (18). Other studies also pointed out the major overall survival benefit of upfront immune checkpoint inhibitors over targeted therapy for melanoma brain metastases (12,19).

Why is the survival effect of immunotherapy dampened in BRAF-mutant melanoma patients receiving first-line targeted therapy with BRAF plus MEK inhibitors? This question has not been investigated and remains to be clarified. It is tempting to surmise that, akin to KRAS inhibitors such as sotorasib, which activate inositol-requiring enzyme 1 alpha (IRE1α), leading to proteostasis reprogramming and facilitating resistance to KRAS inhibitors (20), this observation, coupled with the fact that IRE1α silences taxane-induced double-stranded RNA (dsRNA), causes an immunologically cold tumor. IRE1α inhibitor sensitizes programmed death-ligand 1 (PD-L1)-negative TP53-mutant cold triple negative breast cancers to chemo-immunotherapy (21). It is plausible that either BRAF or MEK inhibitors can equally induce up regulation of IRE1α and silence the activation of dsRNA, which is an important effector pathway for immunotherapy response (21). Putatively, in the future, assessment of IRE1α activation following BRAF and MEK inhibition could confirm the validity of this hypothesis, and further development of IRE1α inhibitors could be positioned in the therapy of melanoma. Despite multiple reviews on the biology of melanoma brain metastasis, the underlying biological insights remains unknown (22-25).

Biological and clinical questions

What are the genetic traits of melanoma brain metastases?

Should the effect of immunotherapy on melanoma brain metastases be improved?

What is learned from immunotherapy-induced central nervous system immune-related adverse events and microglial activation?

What is the crosstalk between microglia and brain metastases in neuroinflammation?

Is it possible to intercept melanoma brain metastasis?

Is TANK-binding kinase 1 (TBK1) a theranostic biomarker of melanoma brain metastases, neuroinflammation, and response to immunotherapy?

Enhancer of zeste homolog 2 (EZH2) phosphorylation at tyrosine 696 drives brain metastases

Immunosuppressive neutrophils are associated with brain metastasis, and in patients with brain metastases and glioblastoma, a high neutrophil-to-lymphocyte ratio in peripheral blood predicts poor prognosis (26). It was discovered that EZH2 is highly expressed in patients with brain metastases (27). In brain metastatic cancer cells, EZH2 is phosphorylated by the oncogenic tyrosine kinase Src at tyrosine (Y)-696 (EZH2 Y696), and EZH2 Y696 induces granulocyte colony-stimulating factor (G-CSF), which recruits programmed death-ligand 1-positive (PD-L1+) immunosuppressive neutrophils into the brain and permits brain metastasis development. Zhang et al. (27) demonstrated that blocking EZH2 phosphorylation with a Src inhibitor hampers recruitment of neutrophils by inhibiting G-CSF, and that targeting immunosuppressive neutrophils with immunotherapy dampens brain metastatic outgrowth in melanoma and other tumor mouse models. RNA sequencing of 24 pairs of primary tumors (breast, lung cancer, and melanoma) and their matched brain metastases showed that EZH2 mRNA was significantly higher in brain metastases than in the corresponding primary tumors (27). Of interest is the fact that EZH2 mRNA is highly expressed in triple-negative and HER2+ breast cancers, which are prone to developing brain metastases. Importantly, higher EZH2 protein expression was detected in all brain metastatic lesions induced by intracarotid artery injection of B16BL6 mouse melanoma cells. Moreover, Western blotting confirmed that EZH2 protein expression was higher in brain metastatic tissued (B16Br) isolated from mice injected with B16BL6 melanoma cells than in their corresponding primary tumors (B16P) (27). Src is a non-receptor tyrosine kinase (RTK) that is dysregulated in cancer. Src intersects with multiple signaling pathways, including EZH2 and Yes-associated protein 1 (YAP1) (Figure 1) (28). EZH2 binds to Src in the nucleus and is phosphorylated by Src at the Y696 site (27). Zhang et al. identified that Src is highly activated in brain metastatic cells, leading to overexpression of EZH2. Inhibition of Src (saracatinib) downregulated H2 Y696 phosphorylation and targeted G-CSF, thereby reverting neutrophil-induced immunosuppression and potentially intercepting or abating brain metastases. In summary, treatment with anti-G-CSF antibodies or immunotherapy (anti-PD-1) and anti-cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) antibodies combined with saracatinib (Src inhibitors) can relieve immune suppression to inhibit brain metastasis outgrowth (27) (Figure 1).

Figure 1 Mechanism of brain metastasis described in breast cancer, lung cancer, and melanoma. Src phosphorylates EZH2, which upregulates inflammatory cytokines, including G-CSF, which recruits immunosuppressive neutrophils into the brain to drive metastases proliferation. See text for further information. Figure created with Biorender. EZH2, enhancer of zeste homolog 2; G-CSF, granulocyte colony-stimulating factor.

Early attempts to unveil genetic reasons for clinical response to CTLA-4 inhibitors in melanoma were reported (29) and soon nivolumab plus ipilimumab was found to yield significant objective response rate and progression-free survival among patients with advanced melanoma (30). In the CheckMate 067 trial, melanoma-specific survival among patients with BRAF mutations at 10 years was 56% with nivolumab plus ipilimumab versus 42% in patients receiving only nivolumab and 27% in those that received only ipilimumab (30). Melanoma-specific survival among patients without BRAF mutations was a 10-year estimate of 50% for those treated with nivolumab plus ipilimumab, 45% for those treated with nivolumab only, and 22% for those treated with ipilimumab only (30). The central nervous system was the site of first progression in 15 patients (5%) in the nivolumab-plus ipilimumab group, 20 (6%) in the nivolumab group, and 28 (9%) in the ipilimumab group (30). These results suggest that the efficacy of the combination of nivolumab plus ipilimumab is approaching a ceiling; however, the potential for complementing this therapy with Src inhibitors and immunosuppressive neutrophil profiling warrants consideration.

Microglia

In brain metastases, microglia and macrophages are referred to as tumor-associated macrophages (TAMs) and form the most abundant non-tumor cell component, accounting for 30% of the tumor mass (31). Microglia are resident immune cells in the central nervous system and physiologically detect and eliminate damage that could disrupt normal brain function (32). Dense clustering of activated microglia supports brain metastatic lesions, favoring tumor cell colonization and a suppressive microenvironment. Recently, one study indicated that lung cancer cells activate STAT3 signaling in microglia, resulting in enhanced expression of the sterol-CoA desaturase 1 (SCD1) lipid metabolism-inflammatory response pathway in the brain tumor microenvironment (8). Patients treated with immunotherapy targeting PD-1/PD-L1 and CTLA-4 could develop central nervous immune-related adverse events with neurological symptoms, estimated in 1% to 12% of all patients. Neuroinflammation caused by immunotherapy has an unpredictable onset and includes encephalopathy, hypophysitis, meningitis, encephalopathy associated with fever, headache, tremors, altered mental state, cognitive impairment, seizures and many others. It has been reported that microglia mediate central nervous system immune-related adverse effects in mice. Spleen tyrosine kinase (Syk) is activated in microglia upon anti-PD-1 treatment. Syk inhibition (entospletinib) reduced microglial activation and improved neurocognitive function without impairing anti-melanoma efficacy (33). Anti-PD-1 cancer immunotherapy induces central nervous system changes (Figure 2). Microglia activation was observed in tumor-free and melanoma-bearing mice after anti-PD-L1 treatment, including morphological changes, major histocompatibility complex class II (MHC-II) and colony-stimulating factor 1 receptor (CSF-1R) upregulation, and activation of Syk. It was also noted that anti-PD-1 treatment causes activation of microglia in patients by examining postmortem tissue from patients receiving anti-PD-1 treatment for different tumors. An increase in IBA-1+ cells in the gray matter of the frontal lobes was observed in patients treated with anti-PD-1 compared with control patients. Imaging mass cytometry with a panel of 37 markers showed microglial activation in patients, confirming findings from preclinical models (33). Entospletinib is a Syk inhibitor that is used in clinical trials for acute myeloid leukemia and chronic lymphocytic leukemia. Syk inhibition interfered with PD-1, PD-L1, and MHC-II expression (33) (Figure 2). In epilepsy, loss of the Mic19 subunit of the mitochondrial contact site and cristae organizing system (MICOS) in neurons triggers activation of the ZBP1-RIPK3-mixed lineage kinase domain-like protein (MLKL) axis, leading to phosphorylated MLKL-mediated mitochondrial DNA release. The released mitochondrial DNA is subsequently taken up by microglia and triggers cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING)-dependent inflammatory signaling through downstream activation of TBK1/IRF3. Pharmacological inhibition of STING with H-151 blocked seizure generation (34) (Figure 2).

Figure 2 Microglia are a main component of the brain tumor microenvironment. Brain tumor microglia activate Syk, which induces neuroinflammation. Entospletinib (Syk inhibitor) reduces microglial activation, improving neurocognitive function. Low Mic 19 alters mitochondria integrity, releasing dsRNA, and activating antiviral response: cytosolic RIG1, MDA5 and downstream components such as TBK1. BTK is activated in the tumor-associated macrophages and microglia interacts with WASp, altering actin and jeopardizing phagocytosis. EG-011 activates WASp. See details in text. Figure created with Biorender. BTK, bruton tyrosine kinase; dsRNA, double-stranded RNA; MDA5, melanoma differentiation-associated protein 5; RIG1, retinoic acid-inducible gene I; Syk, spleen tyrosine kinase; TBK1, tank-binding kinase 1; WASp, Wiskott-Aldrich syndrome protein.

Mitochondria

Mitochondrial cristae and crista junctions are vital for mitochondrial structure and function. Dysregulation of the MICOS complex is composed of eight subunits: Mic60, Mic27, Mic25, Mic26, Mic19, Mic14, Mic13, and Mic10. Dysregulation of MICOS, as observed in epilepsy through loss of Mic19 (34) triggers cGAS-STING pathway-induced neuroinflammation. Mitochondria are signaling hubs of innate immunity and inflammation, protecting against viruses as host organelles prepared to sense foreign nucleic acids. This sensing triggers the mitochondrial antiviral signaling protein (MAVS) and transduces an inflammatory response with high nuclear factor-kB (NF-κB) gene expression, upregulating type I and III interferons, and releasing pleiotropic cytokines/chemokines. Mislocalized or aberrantly processed endogenous nucleic acids, such as dsRNA, are also detected and trigger inflammation, a process referred to as “viral mimicry”. Recently, reduced levels of the mitochondrial scaffold Mic60 (inner mitochondrial membrane protein) have been identified in patients with pancreatic ductal adenocarcinoma, increasing inflammation, NF-κB activation, and production of TNF-α (35). Mic60 levels are undetectable in many human tumors, leading to structurally defective and dysfunctional mitochondria. Low Mic60 mRNA levels in The Cancer Genome Atlas (TCGA) pancreatic cancer dataset, as identified by gene set enrichment analysis (GSEA), exhibit a gene signature of acute inflammation, angiogenesis, and cytokine release. A reduction in Mic60 levels lowers overall mitochondrial fitness and disrupts the organelle outer membrane (36). Mic60-knockdown pancreatic ductal adenocarcinoma (PDAC) cells contain abundant cytosolic dsRNA, as shown by fluorescence microscopy using the J2 antibody to detect dsRNA (35). In addition to PDAC, mitochondrial RNA has been observed in several types of tumors according to TCGA datasets, especially non-small cell lung cancer. It is plausible that melanoma could also display low expression of Mic60. Milcarek et al. (35) observed that Mic60 silencing in PANC-1 cells (pancreatic cancer cell line) upregulates the expression of the endosome-localized toll-like receptor 3 (TLR3) and the cytosolic retinoic acid-inducible gene I (RIG-I), two key dsRNA sensors of antiviral signaling (37). Another cytosolic dsRNA sensor, melanoma differentiation-associated protein 5 (MDA5), was also upregulated by Mic60 loss, although this effect was less pronounced (35), whilst MAVS levels were comparable between control and Mic60-depleted pancreatic cancer cells. Furthermore, TNFR-associated factor-6 (TRAF6), also required for antiviral signaling, was regulated after Mic60 loss. Of interest, siRNA silencing of TLR3 or TRAF6, independently, efficiently suppressed NF-κB promoter activity induced by Mic60 depletion. Similar findings were obtained with a small-molecule inhibitor of the TRAF6-CD40 complex (TRAF6i, compound 6877002), which also suppressed NF-κB promoter activity in Mic60-knockdown PANC-1 cells (35). Intriguingly, Mic60 knockdown in pancreatic cancer cells activated multiple RTK and growth factor receptor networks in a phospho-protein array, including FGFR1, ErbB2, and PDGF-R, as well as regulators of angiogenesis (VEGFR, Tie-2, Eph) and tumor cell motility (DDR1, TrkC). It is plausible that, as in pancreatic cancer, in melanoma and other malignancies, chronic inflammation and sustained interferon signaling lead to immunosuppression. This paradoxical paradigm was first highlighted in B16 melanoma models by Benci et al. (38), where prolonged interferon signaling in tumor cells increased resistance to immune checkpoint inhibitors such as anti-PD-1 and to combinations such as radiation plus anti-CTLA-4, and blocking tumor interferon signaling was required to render tumors responsive. In the same line, in epidermal growth factor receptor (EGFR)-mutant lung cancer, EGFR inhibition triggers type I interferon upregulation via the RIG-I-TBK1-IRF pathway, and inhibition of interferon signaling (anifrolumab) enhances EGFR tyrosine kinase inhibitor sensitivity in EGFR-mutant lung cancer, suggesting that the combination of EGFR tyrosine kinase inhibitors plus interferon-neutralizing antibodies (anifrolumab) could be considered (39). Interferon-γ signaling from activated cells upregulates carnitine palmitoyl transferase 1A (CPT1A) in cancer cells, activating fatty acid oxidation and conferring resistance to immune effector cells by promoting pro-survival signaling with upregulation of Bcl-2 and Bcl-XL and downregulation of Bax. Inhibition of CPT1A with etomoxir increases pro-apoptotic signaling in B16 melanoma cells and in breast cancer models (MDA-MB-231), where cells stimulated with interferon-γ showed increased CPT1A expression by Western blot analysis (40). Valosin-containing protein (VCP) upregulates CPT1A in colorectal cancer (41), and it has recently been shown that the combined use of a VCP inhibitor (CB5083) with an anti-PD-L1 antibody is dramatically improved with deletion of the retromer complex VPS35 subunit, which prevents PD-L1 recycling (42).

Interestingly, Gong et al. reported that combined inhibition of EGFR and type I interferon with anifrolumab was also able to overcome primary resistance in EGFR wild-type non-small cell lung cancer, including KRAS-mutant tumors, as demonstrated in KRAS LSL-G12D transgenic mice. Gong et al. (39) observed that increased levels of IFNA1 mRNA or IFNB1 mRNA confer resistance to EGFR tyrosine kinase inhibitors in patients with EGFR-mutant non-small cell lung cancer and are associated with worse prognosis in these patients. An analysis of TCGA also reveals a correlation between high type I interferon DNA copy number and worse prognosis in EGFR-mutant non-small cell lung cancer (39). Also, Gong et al. (43) showed that inhibition of EGFR in lung cancer rapidly increases TNF secretion and TNF mRNA levels mediated by miR-21, leading to NF-κB hyperactive signaling (Figure 2). Worthy of mention is that treatment with TNF blockers (etanercept or infliximab) has previously been suggested to prevent immune-related adverse effects in combination with CTLA-4 plus PD-1 antibodies for treatment of melanoma, renal cell cancer, and lung cancer (44).

Microglial TBK1 and brain metastases

We learned that the recognition of dsRNA or double-stranded DNA damage in the cytosol activates respective sensors and engages the MAVS and STING, respectively, leading to the downstream activation of the common effector TBK1, which phosphorylates p65 or interferon regulatory protein 3 (IRF3), resulting in the production of interferons and interferon-stimulated genes, thus promoting anti-tumor immunity (37,45). Recently, it has been described that breast cancer brain metastases have abundant infiltration of tumor-associated microglia (TAMG), which promotes the seeding and growth of metastatic breast cancer cells. In tumor samples from patients with breast cancer brain metastases and experimental models, TBK1 signaling was shown to be activated in TAMG, and TBK1 inhibition in TAMG reduced epithelial–mesenchymal transition, migration, invasion, and proliferation of breast cancer cells (46). It is tempting to speculate that the crucial function of TBK1 in regulating microglia biology could be extrapolated to melanoma brain metastasis. Microglia activation is already described in human melanoma brain metastasis (47). In breast cancer brain metastasis models, TBK1 is activated in TAMG, in which it upregulates GM-CSF, promoting metastasis. Whether a similar mechanism operates in melanoma brain metastasis remains to be determined. TBK1 inhibitors (BTX705, Amlex or CMPD-1) reduce metastasis and extend survival (46) (Figure 2). In MDA-MB-231-Br cells, conditioned medium–induced upregulation of CSF2 (encoding GM-CSF) was abolished by treatment with TBK1 inhibitors. The Khan et al. study (46) is surprising, revealing a new facet of the pro-metastatic effect of TBK1 activation in TAMG. Of practical interest is the fact that the TBK1 inhibitor Amlex is an FDA-approved anti-inflammatory and anti-allergic drug used to treat several inflammatory disorders and recurrent aphthous ulcers. It is enticing to surmise, as investigators, that the combination of TBK1 inhibitors with immunotherapy could be of benefit to patients with breast cancer brain metastases (46), pending confirmation of the blood-brain-barrier penetrance of such TBK1 inhibitors and, furthermore, whether, as can be presumed, TBK1–microglia biology is also relevant in melanoma brain metastases. Previously, it was shown that targeting TBK1 enhanced response to PD-1 blockade in multiple experimental models. Tumor cells lacking TBK1 underwent necroptosis via receptor-interacting protein kinase (RIPK) as well as caspase dependent cell death (48).

When reviewing TBK1 functions in cancer biology, multiple mechanisms of TBK1 hyperactivation beyond the MAVS and STING pathways have been described; for example, mutant KRAS lung cancer cells activate TBK1 via RAL GTPases (49,50). The suppression of RALB resulted in significant selective lethality in KRAS-dependent cell lines, and the suppression of TBK1 in A549 or NCI-H2009 (mutant KRAS) cells inhibited tumor formation (49). Therefore, it is not surprising that targeting TBK1 inhibits resistance to MEK inhibitors in mutant NRAS melanoma (51,52). See further information on TBK1 in the reviews of Miranda et al. (53,54) (Figure 2). TBK1 was shown to regulate breast cancer metabolism through aldo-keto reductase 10 (AKRB10) and thymidine phosphorylase (TYMP) genes. Of interest, the TBK1 inhibitor (BX795) effect on cell viability was increased in combination with AKR1B10 inhibitors (diclofenac or flufenamic) and with the TYMP inhibitor (tipiracil) (55).

Tectonic family member 1 (TCTN1) and CPT1A in melanoma

Fatty acid beta-oxidation (FAO) is a multistep process that breaks down long-chain fatty acids to acetyl-CoA, which is oxidized through the Krebs cycle and the electron transport chain to produce ATP (56), and FAO is activated in cancer cells in association with c-Myc, c-Src, and other oncoproteins (56). CPT1A is the rate-limiting enzyme of FAO, catalyzing the transfer of the long-chain acyl group of acyl-CoA esters to carnitine and, hence, shuttling long-chain fatty acids into the mitochondrial matrix through the carnitine transporter for β-oxidation (57). It was shown that killer T cells induce CPT1A expression and FAO activity in cancer cells, driven by interferon gamma (IFN-γ) in an AMP-activated protein kinase (AMPK)-dependent manner. Abrogating the FAO pathway using CPT1A-targeted genetic or pharmacological approaches (etomoxir) renders cancer cells more sensitive to cellular immune cytotoxicity (40). CPT1A was upregulated in murine B16 melanoma cells following coculture with Pmel cells (40). The striking finding is that T cell-derived IFN-γ is a major factor in inducing CPT1A (40). In melanoma, TCTN1, which also increases FAO and ATP production, is overexpressed and correlates with CPT1A (58). It was discovered that TCTN1 promotes melanoma metastasis by increasing FAO. Mechanistically, TCTN1-mediated FAO activates the p38/MAPK signaling pathway in melanoma cells. TCTN1 acts as a scaffolding protein to promote the binding of HADHA and HADHB, subunits of the mitochondrial trifunctional complex activating several genes including CPT1A1. The prostaglandin F receptor agonist fluprostenol can block HADHA/HADHB, thereby inhibiting TCTN1-induced melanoma evasion and metastasis (Figure 3) (58). TCTN1 is upregulated in metastasis. Statistical analysis of 445 patients [from The Cancer Genome Atlas-Skin Cutaneous Melanoma (TCGA-SKCM) cohort] showed that TCTN1 mRNA was strikingly upregulated in cutaneous melanoma compared with normal tissues. TCTN1 in melanoma tissue is also detected using tissue microarray. Patients with high TCTN1 expression have significantly shorter progression-free survival (58). TCTN1 expression was also detected in five melanoma cell lines examined. TCTN1 over-expression regulates CPT1A and FAO-related genes such as CD36 and peroxisome proliferator-activated receptor alpha (PPARα) in melanoma. The analysis by Western blot revealed that TCTN1-induced FAO promotes an epithelial mesenchymal transition phenotype (vimentin, CD44 and loss of E-cadherin) that was reversed by treatment with etomoxir (CPT1A inhibitor). TCTN1 promotes the FAO pathway by increasing HADHA/HADHB binding (58). Of interest, it was previously reported that stimulation with interferon-γ upregulated CPT1A and HADHA in human breast cancer MDA-MB-231 cells (40). Two HADHA/HADHB-binding inhibitors were tested, BRX-220 and fluprostenol. Fluprostenol is an organofluoride compound and a synthetic analog of prostaglandin F2α that is used to treat glaucoma and hypertension and as a contraceptive agent. Fluprostenol reduced cell migration and invasion of A375 melanoma cells and abolished the effect of TCTN1 on these cancer properties (58). Enticingly, overexpression of TCTN1 and CPT1A predicts poor prognosis in melanoma patients. A positive correlation between TCTN1 and CPT1A expression was found in melanoma samples. Overall survival in the TCTN1 and CPT1A high-expression group was much lower than in the TCTN1 and CPT1A low-expression group. The combination of TCTN1 and CPT1A could be a potential biomarker and therapeutic approach (Figure 3, Table 1). TCTN1 promotes HADHA/HADHB complex assembly, and fluprostenol targeting HADHA/HADHB and/or etomoxir targeting CPT1A warrants further investigation (Figure 3, Table 1) (58).

Figure 3 Depiction of TCTN1, a mitochondrial scaffolding protein that forms a complex with the subunits HADHA and HADHB. The trifunctional complex activates CPT1A in melanoma. Fluprostenol blocks the mitochondrial tricomplex, inhibiting TCTN1 and melanoma metastases. Etomoxir is an inhibitor of CPT1A, the rate limiting enzyme of FAO. Overproduction of interferon-γ can induce CPT1A. See details in text. Figure created with Biorender. CPT1A, carnitine palmitoyl transferase 1A; FAO, fatty acid oxidation; TCTN1, tectonic family member 1.

Table 1

Valuable biomarker Melanoma tissue/brain Immunosuppressive neutrophils Microglia Mitochondria TAM Off-target effect immunotherapy Treatment Reference EZH2 mRNA Brain metastases, B16BL6, mouse melanoma Arg+/PD-L1+ – – – – Anti-PD-1 Ab + anti-CTLA Ab + saracatinib + anti-CSF Ab (27) Syk Melanoma – Syk↑ – – Anti-PD-1 Anti-PD-1 Ab + entospletinib (33) Loss of Mic60 PDAC – – Mic60↓ – RIG-I↑/TLR3↓, TBK1↑ TRAF6 inhibitor (35) TBK1↑ TNBC, NRAS mutant melanoma, KRAS mutant lung cancer – TBK1↑ – – GM-CSF Amlex, BX745 (46,49,52) EGFR inhibition Lung cancer – – – – RIG-I↑, TBK1↓, IFNα/β↓ BX745, anifrolumab (39) TCTN1/CPT1A Melanoma – – – – CPT1A↑ Fluprostenol +/− etomoxir (58)

Wiskott-Aldrich syndrome protein (WASp) and brain metastasis

Brain metastases also exhibit high infiltration of TAMs with shared tyrosine kinase dependencies between tumor cells and TAMs. Bruton’s tyrosine kinase (BTK) and bone marrow X-linked nonreceptor tyrosine kinase (BMX), molecular targets of ibrutinib, are highly expressed in glioma stem cells, small cell lung carcinomas, B cell lymphomas, and melanoma cells (59), and elevated expression of BTK or BMX correlates with poor prognosis. Ibrutinib is a blood-barrier-penetrable TKI approved for the therapy of blood cancers. Single-cell RNA sequencing has shown that TAMs also highly express BTK. Moreover, activation of BTK and its downstream effector, WASp, is involved in branched actin nucleation, favoring cytoskeletal organization (60) (Figure 2). Mechanistically, BTK activated in TAMs interacts with and phosphorylates WASp, organizing the actin cytoskeleton, which is essential for phagocytosis. Treatment with ibrutinib disrupts BTK-mediated WASp activation, perturbing TAM anticancer phagocytic efficacy. Activation of WASp with the selective small molecule activator EG-011 restores ibrutinib impaired TAM engulfment of tumor cells, improving ibrutinib efficacy in mice bearing glioblastomas, primary central nervous system lymphomas, and lung cancer brain metastases (61).

BMX was also expressed in melanoma cells and BTX by single-cell RNA sequencing was highly expressed in TAMs. Immunofluorescence analysis disclosed that the abundance of ionized calcium-binding adapter molecule 1-positive (IBA1+) TAMs was reduced in ibrutinib-treated tumors (61). Ibrutinib treatment impaired WASp phosphorylation, which was restored with EG-011 and correlates with recovery of the phagocytic capacity in TAMs. Immunofluorescence analysis of human glioblastoma tissue showed specific co-localization of both p-BTK and p-WASp with IBA+ TAMs, and their expression was associated with prolonged overall survival (61). In short, pharmacological inhibition of BTK activity by ibrutinib attenuated branched actin network formation in TAMs, suppressing phagocytosis, whereas reactivating WASp by EG-011 restored the phagocytic capacity (Figure 2).

Brain acid-soluble protein 1 (BASP1)

BASP1 gene encodes the brain acid-soluble protein 1, abundant in nerve terminals during brain development, involved in neurite outgrowth, maturation of the actin cytoskeleton, and organization of the plasma membrane, but expressed in several tissues. BASP1 is downregulated in several primary cancers, including acute and chronic lymphocytic leukemia and melanoma, and tumors with downregulated BASP1 display elevated Myc expression. Tumor-suppressive functions of BASP1 have been reported in acute myeloid leukemia, breast cancer, pancreatic cancer, colorectal cancer, and others (62,63). However, BASP1 is up-regulated and associated with poor prognosis in lung adenocarcinoma (64). Its protein levels also correlate with tumor progression in this disease. The reciprocal interaction between BASP1 and EGFR promotes EGFR signaling in brain metastasis in lung cancer. Moreover, synergistic effects of EGFR tyrosine kinase inhibitor and arsenic trioxide reduce the level of BASP1 in lung cancer cells with acquired resistance to EGFR inhibitors (65). It is suggested that treatment with arsenic trioxide and osimertinib could benefit lung cancer patients with brain metastases. Arsenic trioxide has been used in the treatment of patients with central nervous system relapses (65).

The complete function of BASP1 in melanoma merits further research. Originally, BASP1 expression was found to be suppressed in melanoma compared with benign nevi (66), while long noncoding RNA BASP1-AS1 promotes proliferation, migration, and invasion in melanoma cells. Mechanistically, BASP1-AS1 interacted with Y-box binding protein 1 (YBX1) and activated NOTCH3 signaling and Myc activation (67).

Consensus, controversies and future directions

Several concepts have emerged as areas of broad consensus in melanoma brain metastasis research. First, immune checkpoint inhibitors have substantially improved survival outcomes and remain the cornerstone of treatment. Second, the brain microenvironment actively participates in metastatic progression rather than serving as a passive site of tumor growth. Third, microglia and other myeloid populations are increasingly recognized as key regulators of immune responses within brain metastases.

However, several important questions remain unresolved. The biological mechanisms underlying the superiority of upfront immunotherapy over targeted therapy in BRAF-mutant melanoma are still unclear. Likewise, the precise role of TBK1 signaling, the contribution of innate immune sensing pathways, and the significance of metabolic regulators such as TCTN1 and CPT1A in melanoma brain metastasis require further validation. The extent to which findings from breast cancer, lung cancer, and other tumor models can be extrapolated to melanoma also remains uncertain.

Future studies should focus on defining the interactions between neutrophils, microglia, and tumor cells within the brain metastatic niche; validating TBK1 as a predictive biomarker and therapeutic target; elucidating the role of mitochondrial dysfunction and innate immune signaling; and exploring metabolic vulnerabilities associated with fatty acid oxidation. A better understanding of these interconnected mechanisms may facilitate the development of more effective therapeutic strategies for patients with melanoma brain metastases.

Conclusions

The outlook of metastatic melanoma has substantially improved with immunotherapy, principally with the impressive 10-year outcomes that are attained with the combination of nivolumab (PD-1 inhibitors) plus ipilimumab (CTLA-4 antibody) in advanced melanoma (30). The fact that the benefit of nivolumab plus ipilimumab is superior than dabrafenib plus trametinib as first-line therapy in advanced BRAF-mutant melanoma has not yet been biologically deciphered. Presumably, this could occur through mechanisms similar to those observed in KRAS-mutant lung cancer cell lines, in which sotorasib induces IREIα (20). In a separate study, IRE1α repairs double stranded DNA in triple negative breast cancer, causing a cold tumor that is refractory to immunotherapy (21). Therefore, although the biological basis underlying the superior clinical benefit of immunotherapy over targeted therapy in BRAF-mutant melanoma remains unclear, some clues are emerging. Better understanding of microglia and brain metastasis is warranted with particular attention to TBK1 signaling, and in vitro microglia culture experiments could further help elucidate biological mechanisms in melanoma and brain metastasis. Src and TBK1 are emerging as important therapeutic targets, and overproduction of interferon channeled through TBK1 is of particular interest for further clinical validation. Multiple upstream regulators of TBK1 are emerging (Table 1). Metabolism reprogramming is an area that merits further investigation, particularly the roles of TCTN1 and carnitine palmitoyltransferase 1A (CPT1A).

Other therapeutical aspects, such as neutrophil-rich microenvironment, have not been dealt with in this manuscript and are of enormous interest. For example, blocking C-X-C-motif chemokine receptor 2 (CXCR2) signaling with reparixin plus PD-L1 inhibition restores CD8+ T cell function and suppresses tumor growth in lung cancer models (68). Also missing is the field of sensory neurons that promote cancer and melanoma (69). In triple negative breast cancer, a high concentration of nerve growth factor (NGF) in tumor microenvironment triggers sensory neurons to secrete the neuropeptide calcitonin gene-related peptide (CGRP) that activates cancer-associated fibroblasts to secrete collagen. CGRP binds to its receptor, receptor activity modifying protein 1 (RAMP1), which is expressed in cancer associated fibroblasts, and subsequently activates cyclic AMP (cAMP)/protein kinase A (PKA)/c-AMP-response element binding protein 1 (CREB1) signaling to increase collagen deposition. CGPR has been found to be a predictor of immunotherapy response. Nerves induce T cell exhaustion in melanoma and pancreatic cancer, and, in combination with anti-PD-1 treatment, the CGRP antagonist rimegepant, approved by the Food and Drug Administration (FDA) to treat migraine, has been shown to increase tumor immunotherapy in patients with CGRP-positive tumors (70). Physical barriers with cancer cell- expressing collagens in non-small cell lung cancers prevent T cell infiltration and cause resistance to immunotherapy. Mechanistically, TGFβ is upregulated upon immunotherapy, and treatment with TGFβ increased the expression of collagens (71). It is of utmost interest that CGRP-RAMP1 signaling activates cancer-associated fibroblasts to secrete collagen (70).

Although the pathways discussed in this review may initially appear diverse, they can be integrated into a common biological framework. Src-EZH2 signaling promotes the establishment of an immunosuppressive metastatic niche through neutrophil recruitment, whereas microglial activation and innate immune sensing pathways converge on TBK1 signaling. In turn, TBK1 coordinates inflammatory responses, interferon production, and interactions between tumor cells and the brain microenvironment. These processes may ultimately facilitate metabolic reprogramming through CPT1A-dependent fatty acid oxidation and TCTN1-mediated enhancement of metastatic fitness. Therefore, TBK1 may represent a central signaling hub connecting immune regulation, neuroinflammation, and metabolic adaptation in melanoma brain metastasis.

Acknowledgments

None.

Provenance and Peer Review: This article was commissioned by the editorial office, Annals of Translational Medicine for the series “Advances in Treatment for Melanoma”. The article has undergone external peer review.

Peer Review File: Available at https://atm.amegroups.com/article/view/10.21037/atm-2026-0127/prf

Funding: This work was supported by generous funding from Julián Santamaría Valiño to the IOR Foundation .

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://atm.amegroups.com/article/view/10.21037/atm-2026-0127/coif). The series “Advances in Treatment for Melanoma” was commissioned by the editorial office without any funding or sponsorship. M.G.C. served as the unpaid Guest Editor of the series. R.R. serves as an unpaid Honorary Editor-in-Chief of Annals of Translational Medicine from June 2026 to May 2028. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

Weigelin B, Bakker GJ, Friedl P. Intravital third harmonic generation microscopy of collective melanoma cell invasion: Principles of interface guidance and microvesicle dynamics. Intravital 2012;1:32-43. [Crossref] [PubMed] Davies H, Bignell GR, Cox C, et al. Mutations of the BRAF gene in human cancer. Nature 2002;417:949-54. [Crossref] [PubMed] Ascierto PA, Kirkwood JM, Grob JJ, et al. The role of BRAF V600 mutation in melanoma. J Transl Med 2012;10:85. [Crossref] [PubMed] Smith MP, Brunton H, Rowling EJ, et al. Inhibiting Drivers of Non-mutational Drug Tolerance Is a Salvage Strategy for Targeted Melanoma Therapy. Cancer Cell 2016;29:270-84. [Crossref] [PubMed] Miller AJ, Mihm MC Jr. Melanoma. N Engl J Med 2006;355:51-65. [Crossref] [PubMed] Ascierto PA, Del Vecchio M, Merelli B, et al. Nivolumab for Resected Stage III or IV Melanoma at 9 Years. N Engl J Med 2026;394:333-42. [Crossref] [PubMed] Li J, Ouyang S, Lin Z, et al. The IGF2BP3-FASN axis drives lipid metabolic reprogramming to promote brain colonization in non-small cell lung cancer. Cell Death Dis 2025;16:684. [Crossref] [PubMed] Chen P, Zuo Y, Wang M, et al. Lung cancer cells upregulate stearoyl-CoA desaturase 1 in microglia by activating the STAT3 pathway to change microglial inflammatory response in lung-to-brain metastases. Cell Death Dis 2025;16:702. [Crossref] [PubMed] Bidany-Mizrahi T, Maroun K, Mancini M, et al. WWOX maintains epidermal identity and suppresses EMT to prevent aggressive cutaneous squamous cell carcinoma. Proc Natl Acad Sci U S A 2026;123:e2534844123. [Crossref] [PubMed] Yang T, Xu R, Huo J, et al. WWOX activation by toosendanin suppresses hepatocellular carcinoma metastasis through JAK2/Stat3 and Wnt/β-catenin signaling. Cancer Lett 2021;513:50-62. [Crossref] [PubMed] Goldberg SB, Gettinger SN, Mahajan A, et al. Pembrolizumab for patients with melanoma or non-small-cell lung cancer and untreated brain metastases: early analysis of a non-randomised, open-label, phase 2 trial. Lancet Oncol 2016;17:976-83. [Crossref] [PubMed] Bonzano E, Barruscotti S, Chiellino S, et al. Current Treatment Paradigms for Advanced Melanoma with Brain Metastases. Int J Mol Sci 2025;26:3828. [Crossref] [PubMed] Redmer T. Deciphering mechanisms of brain metastasis in melanoma - the gist of the matter. Mol Cancer 2018;17:106. [Crossref] [PubMed] Amaral T, Ottaviano M, Arance A, et al. Cutaneous melanoma: ESMO Clinical Practice Guideline for diagnosis, treatment and follow-up. Ann Oncol 2025;36:10-30. [Crossref] [PubMed] Garbe C, Amaral T, Peris K, et al. European consensus-based interdisciplinary guideline for melanoma. Part 1: Diagnostics - Update 2024. Eur J Cancer 2025;215:115152. Atkins MB, Lee SJ, Chmielowski B, et al. Combination Dabrafenib and Trametinib Versus Combination Nivolumab and Ipilimumab for Patients With Advanced BRAF-Mutant Melanoma: The DREAMseq Trial-ECOG-ACRIN EA6134. J Clin Oncol 2023;41:186-97. [Crossref] [PubMed] Ascierto PA, Casula M, Bulgarelli J, et al. Sequential immunotherapy and targeted therapy for metastatic BRAF V600 mutated melanoma: 4-year survival and biomarkers evaluation from the phase II SECOMBIT trial. Nat Commun 2024;15:146. [Crossref] [PubMed] Ascierto PA, Mandalà M, Ferrucci PF, et al. Sequencing of Checkpoint or BRAF/MEK Inhibitors on Brain Metastases in Melanoma. NEJM Evid 2024;3:EVIDoa2400087. Robert C, Kicinski M, Dutriaux C, et al. Combination of encorafenib and binimetinib followed by ipilimumab and nivolumab versus ipilimumab and nivolumab in patients with advanced melanoma with BRAF(V600E) or BRAF(V600K) mutations (EBIN): an international, open-label, randomised, controlled, phase 2 study. Lancet Oncol 2025;26:781-94. [Crossref] [PubMed] Lv X, Lu X, Cao J, et al. Modulation of the proteostasis network promotes tumor resistance to oncogenic KRAS inhibitors. Science 2023;381:eabn4180. [Crossref] [PubMed] Xu L, Peng F, Luo Q, et al. IRE1α silences dsRNA to prevent taxane-induced pyroptosis in triple-negative breast cancer. Cell 2024;187:7248-7266.e34. [Crossref] [PubMed] Kircher DA, Silvis MR, Cho JH, et al. Melanoma Brain Metastasis: Mechanisms, Models, and Medicine. Int J Mol Sci 2016;17:1468. [Crossref] [PubMed] Phadke M, Ozgun A, Eroglu Z, et al. Melanoma brain metastases: Biological basis and novel therapeutic strategies. Exp Dermatol 2022;31:31-42. [Crossref] [PubMed] Diaz MJ, Mark I, Rodriguez D, et al. Melanoma Brain Metastases: A Systematic Review of Opportunities for Earlier Detection, Diagnosis, and Treatment. Life (Basel) 2023;13:828. [Crossref] [PubMed] O’Toole KT, Roan BM, Hardman TM, et al. Unraveling the Enigma of Melanoma Brain Metastasis: New Molecular Insights and Therapeutic Directions. Int J Mol Sci 2026;27:2019. Mitsuya K, Nakasu Y, Kurakane T, et al. Elevated preoperative neutrophil-to-lymphocyte ratio as a predictor of worse survival after resection in patients with brain metastasis. J Neurosurg 2017;127:433-7. [Crossref] [PubMed] Zhang L, Yao J, Wei Y, et al. Blocking immunosuppressive neutrophils deters pY696-EZH2-driven brain metastases. Sci Transl Med 2020;12:eaaz5387. [Crossref] [PubMed] Tian H, Zhao D, Zhou Z, et al. SRC at the crossroads of KRAS inhibitor resistance: Mechanisms and therapeutic opportunities. Cancer Lett 2026;639:218194. [Crossref] [PubMed] Snyder A, Makarov V, Merghoub T, et al. Genetic basis for clinical response to CTLA-4 blockade in melanoma. N Engl J Med 2014;371:2189-99. [Crossref] [PubMed] Wolchok JD, Chiarion-Sileni V, Rutkowski P, et al. Final, 10-Year Outcomes with Nivolumab plus Ipilimumab in Advanced Melanoma. N Engl J Med 2025;392:11-22. [Crossref] [PubMed] Gutmann DH, Kettenmann H. Microglia/Brain Macrophages as Central Drivers of Brain Tumor Pathobiology. Neuron 2019;104:442-9. [Crossref] [PubMed] Soto MS, Sibson NR. The Multifarious Role of Microglia in Brain Metastasis. Front Cell Neurosci 2018;12:414. [Crossref] [PubMed] Vinnakota JM, Adams RC, Athanassopoulos D, et al. Anti-PD-1 cancer immunotherapy induces central nervous system immune-related adverse events by microglia activation. Sci Transl Med 2024;16:eadj9672. [Crossref] [PubMed] Jiang J, Zuo M, Zhao K, et al. mtDNA leakage promotes neuron-glia crosstalk to induce epilepsy by cGAS-STING-driven neuroinflammation and serine metabolic reprogramming. Proc Natl Acad Sci U S A 2026;123:e2522313123. [Crossref] [PubMed] Milcarek AT, Yeon M, Esposito C, et al. Mitochondrial double-stranded RNA fuels pancreatic cancer growth via RIG-I/TLR3 inflammation. Proc Natl Acad Sci U S A 2026;123:e2528281123. [Crossref] [PubMed] Ghosh JC, Perego M, Agarwal E, et al. Ghost mitochondria drive metastasis through adaptive GCN2/Akt therapeutic vulnerability. Proc Natl Acad Sci U S A 2022;119:e2115624119. [Crossref] [PubMed] Santarpia M, Aguilar A, Chaib I, et al. Non-Small-Cell Lung Cancer Signaling Pathways, Metabolism, and PD-1/PD-L1 Antibodies. Cancers (Basel) 2020;12:1475. [Crossref] [PubMed] Benci JL, Xu B, Qiu Y, et al. Tumor Interferon Signaling Regulates a Multigenic Resistance Program to Immune Checkpoint Blockade. Cell 2016;167:1540-1554.e12. [Crossref] [PubMed] Gong K, Guo G, Panchani N, et al. EGFR inhibition triggers an adaptive response by co-opting antiviral signaling pathways in lung cancer. Nat Cancer 2020;1:394-409. [Crossref] [PubMed] Liu Z, Liu W, Wang W, et al. CPT1A-mediated fatty acid oxidation confers cancer cell resistance to immune-mediated cytolytic killing. Proc Natl Acad Sci U S A 2023;120:e2302878120. [Crossref] [PubMed] Huang Y, Wang F, Lin X, et al. Nuclear VCP drives colorectal cancer progression by promoting fatty acid oxidation. Proc Natl Acad Sci U S A 2023;120:e2221653120. [Crossref] [PubMed] Wang Y, Huang ZJ, Yu NN, et al. MLKL/retromer axis controls PD-L1 recycling to compromise antitumor immunity during VCP inhibition-induced necroptosis. Proc Natl Acad Sci U S A 2025;122:e2518675122. [Crossref] [PubMed] Gong K, Guo G, Gerber DE, et al. TNF-driven adaptive response mediates resistance to EGFR inhibition in lung cancer. J Clin Invest 2018;128:2500-18. [Crossref] [PubMed] Perez-Ruiz E, Minute L, Otano I, et al. Prophylactic TNF blockade uncouples efficacy and toxicity in dual CTLA-4 and PD-1 immunotherapy. Nature 2019;569:428-32. [Crossref] [PubMed] Li Z, Wang Y, Chen Z, et al. WFDC21P is essential for G3BP1-mediated RIG-I activation and antitumor immunity in triple-negative breast cancer. Proc Natl Acad Sci U S A 2026;123:e2532576123. [Crossref] [PubMed] Khan F, Petrosyan E, Liu Y, et al. Microglial TBK1 Signaling Promotes Breast Cancer Brain Metastasis. Cancer Res 2026;86:12-21. [Crossref] [PubMed] Biermann J, Melms JC, Amin AD, et al. Dissecting the treatment-naive ecosystem of human melanoma brain metastasis. Cell 2022;185:2591-2608.e30. [Crossref] [PubMed] Sun Y, Revach OY, Anderson S, et al. Targeting TBK1 to overcome resistance to cancer immunotherapy. Nature 2023;615:158-67. [Crossref] [PubMed] Barbie DA, Tamayo P, Boehm JS, et al. Systematic RNA interference reveals that oncogenic KRAS-driven cancers require TBK1. Nature 2009;462:108-12. [Crossref] [PubMed] Beel S, Kolloch L, Apken LH, et al. κB-Ras and Ral GTPases regulate acinar to ductal metaplasia during pancreatic adenocarcinoma development and pancreatitis. Nat Commun 2020;11:3409. [Crossref] [PubMed] Vu HL, Aplin AE. Targeting mutant NRAS signaling pathways in melanoma. Pharmacol Res 2016;107:111-6. [Crossref] [PubMed] Vu HL, Aplin AE. Targeting TBK1 inhibits migration and resistance to MEK inhibitors in mutant NRAS melanoma. Mol Cancer Res 2014;12:1509-19. [Crossref] [PubMed] Miranda A, Shirley CA, Jenkins RW. Emerging roles of TBK1 in cancer immunobiology. Trends Cancer 2024;10:531-40. [Crossref] [PubMed] Revach OY, Liu S, Jenkins RW. Targeting TANK-binding kinase 1 (TBK1) in cancer. Expert Opin Ther Targets 2020;24:1065-78. [Crossref] [PubMed] Maan M, Jaiswal N, Liu M, et al. TBK1 Reprograms Metabolism in Breast Cancer: An Integrated Omics Approach. J Proteome Res 2025;24:121-33. [Crossref] [PubMed] Ma Y, Temkin SM, Hawkridge AM, et al. Fatty acid oxidation: An emerging facet of metabolic transformation in cancer. Cancer Lett 2018;435:92-100. [Crossref] [PubMed] Houten SM, Violante S, Ventura FV, et al. The Biochemistry and Physiology of Mitochondrial Fatty Acid β-Oxidation and Its Genetic Disorders. Annu Rev Physiol 2016;78:23-44. [Crossref] [PubMed] Li Y, Ming R, Zhang T, et al. TCTN1 Induces Fatty Acid Oxidation to Promote Melanoma Metastasis. Cancer Res 2025;85:84-100. [Crossref] [PubMed] Pal Singh S, Dammeijer F, Hendriks RW. Role of Bruton’s tyrosine kinase in B cells and malignancies. Mol Cancer 2018;17:57. [Crossref] [PubMed] Cory GO, Garg R, Cramer R, et al. Phosphorylation of tyrosine 291 enhances the ability of WASp to stimulate actin polymerization and filopodium formation. Wiskott-Aldrich Syndrome protein. J Biol Chem 2002;277:45115-21. Cheng Y, Li T, Wang M, et al. Pharmacological activation of WASp potentiates macrophage phagocytosis and enhances ibrutinib efficacy against mouse models of brain tumors. Sci Transl Med 2026;18:eaed1155. [Crossref] [PubMed] Weber LI, Timpen LE, Egger-Hörschinger AS, et al. Reactivation of the silenced BASP1 gene suppresses oncogenic WNT signaling in human colorectal cancer cells. Proc Natl Acad Sci U S A 2026;123:e2524159123. [Crossref] [PubMed] Zhou Q, Andersson R, Hu D, et al. Quantitative proteomics identifies brain acid soluble protein 1 (BASP1) as a prognostic biomarker candidate in pancreatic cancer tissue. EBioMedicine 2019;43:282-94. [Crossref] [PubMed] Wang X, Cao Y. High BASP1 Expression is Associated with Poor Prognosis and Promotes Tumor Progression in Human Lung Adenocarcinoma. Cancer Invest 2021;39:409-22. [Crossref] [PubMed] Lin CC, Huang YK, Cho CF, et al. Targeting positive feedback between BASP1 and EGFR as a therapeutic strategy for lung cancer progression. Theranostics 2020;10:10925-39. [Crossref] [PubMed] Ransohoff KJ, Wu W, Cho HG, et al. Two-stage genome-wide association study identifies a novel susceptibility locus associated with melanoma. Oncotarget 2017;8:17586-92. [Crossref] [PubMed] Li Y, Gao Y, Niu X, et al. LncRNA BASP1-AS1 interacts with YBX1 to regulate Notch transcription and drives the malignancy of melanoma. Cancer Sci 2021;112:4526-42. [Crossref] [PubMed] Qian W Wei X Barros AJ Respiratory viral infections prime accelerated lung cancer growth. bioRxiv 2025 ; 2025.09.02.672566 . Magnon C, Hondermarck H. The neural addiction of cancer. Nat Rev Cancer 2023;23:317-34. [Crossref] [PubMed] Zhang SW, Wang H, Xiao Y, et al. Sensory neurons drive immune exclusion by stimulating a dense extracellular matrix in the breast cancer tumor microenvironment. Cell 2026;189:1039-1055.e20. [Crossref] [PubMed] Wang M, Wang Y, Pan X, et al. Acquired resistance to immunotherapy by physical barriers with cancer cell-expressing collagens in non-small cell lung cancer. Proc Natl Acad Sci U S A 2025;122:e2500019122. [Crossref] [PubMed]

Cite this article as: Rosell R, González-Cao M, Olmo-González D, Hold E, Molina-Vila MÁ. Tumor microenvironment and signaling pathways in melanoma brain metastasis. Ann Transl Med 2026;14(4):52. doi: 10.21037/atm-2026-0127