Melanoma is one of the most deadly and therapy-resistant cancers. Here we show that N⁶-methyladenosine (m⁶A) mRNA demethylation by fat mass and obesity-associated protein (FTO) increases melanoma growth and decreases response to anti-PD-1 blockade immunotherapy. FTO level is increased in human melanoma and enhances melanoma tumorigenesis in mice. FTO is induced by metabolic starvation stress through the autophagy and NF-κB pathway. Knockdown of FTO increases m⁶A methylation in the critical protumorigenic melanoma cell-intrinsic genes including PD-1 (PDCD1), CXCR4, and SOX10, leading to increased RNA decay through the m⁶A reader YTHDF2. Knockdown of FTO sensitizes melanoma cells to interferon gamma (IFNγ) and sensitizes melanoma to anti-PD-1 treatment in mice, depending on adaptive immunity. Our findings demonstrate a crucial role of FTO as an m⁶A demethylase in promoting melanoma tumorigenesis and anti-PD-1 resistance, and suggest that the combination of FTO inhibition with anti-PD-1 blockade may reduce the resistance to immunotherapy in melanoma.

Combinations of BRAF inhibitors and MEK inhibitors (BRAFi + MEKi) are FDA-approved to treat BRAF^V600E/K-mutant melanoma. Efficacy of BRAFi + MEKi associates with cancer cell death and alterations in the tumor immune microenvironment; however, the links are poorly understood. We show that BRAFi + MEKi caused durable melanoma regression in an immune-mediated manner. BRAFi + MEKi treatment promoted cleavage of gasdermin E (GSDME) and release of HMGB1, markers of pyroptotic cell death. GSDME-deficient melanoma showed defective HMGB1 release, reduced tumor-associated T cell and activated dendritic cell infiltrates in response to BRAFi + MEKi, and more frequent tumor regrowth after drug removal. Importantly, BRAFi + MEKi–resistant disease lacked pyroptosis markers and showed decreased intratumoral T-cell infiltration but was sensitive to pyroptosis-inducing chemotherapy. These data implicate BRAFi + MEKi–induced pyroptosis in antitumor immune responses and highlight new therapeutic strategies for resistant melanoma. Significance: Targeted inhibitors and immune checkpoint agents have advanced the care of patients with melanoma; however, detailed knowledge of the intersection between these two research areas is lacking. We describe a molecular mechanism of targeted inhibitor regulation of an immune-stimulatory form of cell death and provide a proof-of-principle salvage therapy concept for inhibitor-resistant melanoma. See related commentary by Smalley, p. 176. This article is highlighted in the In This Issue feature, p. 161

The mechanisms underlying adaptive resistance of melanoma to targeted therapies remain unclear. By combining ChIP sequencing with microarray-based gene profiling, we determined that ERBB3 is upregulated by FOXD3, a transcription factor that promotes resistance to RAF inhibitors in melanoma. Enhanced ERBB3 signaling promoted resistance to RAF pathway inhibitors in cultured melanoma cell lines and in mouse xenograft models. ERBB3 signaling was dependent on ERBB2; targeting ERBB2 with lapatinib in combination with the RAF inhibitor PLX4720 reduced tumor burden and extended latency of tumor regrowth in vivo versus PLX4720 alone. These results suggest that enhanced ERBB3 signaling may serve as a mechanism of adaptive resistance to RAF and MEK inhibitors in melanoma and that cotargeting this pathway may enhance the clinical efficacy and extend the therapeutic duration of RAF inhibitors.

Unleashing the immune system with immune checkpoint inhibitors (ICI) has significantly improved overall survival for subsets of patients with stage III/IV cancer. However, many tumors are nonresponsive to ICIs, in part due to a lack of tumor-infiltrating lymphocytes (TIL). Converting these immune “cold” tumors to “hot” tumors that are thus more likely to respond to ICIs is a major obstacle for cancer treatment. Triggering inflammatory forms of cell death, such as necroptosis and pyroptosis, may alter the tumor immune microenvironment and the influx of TILs. We present an emerging view that promoting tumor-localized necroptosis and pyroptosis may ultimately enhance responses to ICI. Significance: Many tumor types respond poorly to ICIs or respond but subsequently acquire resistance. Effective therapies for ICI-nonresponsive tumors are lacking and should be guided by evidence from preclinical studies. Promoting inflammatory cell death mechanisms within the tumor may alter the local immune microenvironment toward an ICI-responsive state.

Cellular plasticity contributes to intra-tumoral heterogeneity and phenotype switching, which enable adaptation to metastatic microenvironments and resistance to therapies. Mechanisms underlying tumor cell plasticity remain poorly understood. SOX10, a neural crest lineage transcription factor, is heterogeneously expressed in melanomas. Loss of SOX10 reduces proliferation, leads to invasive properties, including the expression of mesenchymal genes and extracellular matrix, and promotes tolerance to BRAF and/or MEK inhibitors. We identify the class of cellular inhibitor of apoptosis protein-1/2 (cIAP1/2) inhibitors as inducing cell death selectively in SOX10-deficient cells. Targeted therapy selects for SOX10 knockout cells underscoring their drug tolerant properties. Combining cIAP1/2 inhibitor with BRAF/MEK inhibitors delays the onset of acquired resistance in melanomas in vivo. These data suggest that SOX10 mediates phenotypic switching in cutaneous melanoma to produce a targeted inhibitor tolerant state that is likely a prelude to the acquisition of resistance. Furthermore, we provide a therapeutic strategy to selectively eliminate SOX10-deficient cells.

The uncontrolled proliferation of cancer cells has led to the development of small-molecule inhibitors to target cell-cycle progression. Palbociclib, ribociclib, and abemaciclib are ATP-competitive inhibitors of cyclin-dependent kinases 4/6 (CDK4/6), which function early within the G1 phase of the cell cycle. Recently, CDK4/6 inhibitors have gained FDA approval in postmenopausal estrogen receptor (ER)–positive/human epidermal growth factor receptor 2 (HER2)–negative breast cancer and testing in other cancer types is underway. However, resistance to CDK4/6 inhibitors frequently develops. In addition, targeting CDK4/6 may augment the action of immune checkpoint blockade agents. Here, we review recent studies that provide the preclinical rationale for treatment combinations and schedules that include CDK4/6 inhibitors. Furthermore, we discuss inhibitor effects on tumor-infiltrating lymphocytes as a preclinical rationale for targeting CDK4/6 in combination with anti–PD-1 or anti–CTLA-4 antibodies.