Melanoma Plasticity: Promoter of Metastasis and Resistance to Therapy

Abstract

Melanoma is the deadliest form of skin cancer. Although targeted therapies and immunotherapies have revolutionized the treatment of metastatic melanoma, most patients are not cured. Therapy resistance remains a significant clinical challenge. Melanoma comprises phenotypically distinct subpopulations of cells, exhibiting distinct gene signatures leading to tumor heterogeneity and favoring therapeutic resistance. Cellular plasticity in melanoma is referred to as phenotype switching. Regardless of their genomic classification, melanomas switch from a proliferative and differentiated phenotype to an invasive, dedifferentiated and often therapy-resistant state. In this review we discuss potential mechanisms underpinning melanoma phenotype switching, how this cellular plasticity contributes to resistance to both targeted therapies and immunotherapies. Finally, we highlight novel strategies to target plasticity and their potential clinical impact in melanoma.

Keywords: immunotherapy; melanoma; phenotype switching; targeted therapy; therapy resistance.

Figure 1

Melanoma cell states. At least 6 different melanoma cell states have been thus far characterized, including a MAPKi-induced hyperdifferentiated/pigmented state, a MITF^high/AXL^low melanocytic/differentiated state, an intermediate/transitory state, a CD36⁺ starved-like melanoma cell (SMC) state, a MITF^low/AXL^high dedifferentiated/undifferentiated state, and a MITF^low/NGFR^high neural crest stem cell (NCSC)-like state (, –16). While the hyperdifferentiated state is induced by MAPK-targeted therapy and the intermediate state exhibit both proliferative and invasive phenotypes, the melanocytic state generally corresponds to the “proliferative” state, and the SMC state, the dedifferentiated state, and the NCSC-like state together make up the “invasive” state (–16). Notably, in many cases, an “invasive” phenotype is used to describe the MITF^low/AXL^high dedifferentiated population, while in some other cases, the “invasive” state refers to both AXL^high/dedifferentiated and NGFR^high/dedifferentiated populations. To avoid confusion, cell state-specific markers and MITF activity are often combined to define each state. For example, it is generally accepted that the melanocyte state is marked as MITF^high/AXL^low, the SMC state is marked with CD36⁺ and medium activity of MITF, the dedifferentiated state is defined as MITF^low/AXL^high and the NCSC-like state is defined as MITF^low/NGFR^high/SOX10⁺/GFRA2⁺ (, –16).

Figure 2

The BRN2 and AP1/TEADs transcriptional networks in melanoma phenotype switching. Left: BRN2 signaling is activated by various pathways, including RAS/RAF/MAPK (106, 107), PI3K/PAX3 (108, 109), Wnt/β-catenin (110), and TNFα/MYC (111, 112) pathways. BRN2 mediates melanoma cell dedifferentiation by inhibiting MITF transcription (41), while MITF in turn represses BRN2 through miR-211 (44). BRN2 promotes melanoma cell invasion through transcriptional repression of the PDE5A, resulting in accumulated levels of cGMP and ultimately increased cell contractility (106, 107). BRN2 also induces NFIB (113), Notch1, and DLL1 (45), which together promote melanoma phenotype switching through EZH2 (72, 113) and subsequent activation of the WNT/β-catenin signaling (113, 114). Middle: The AP-1 complex composed of c-Jun and Fra1 is activated downstream of the MAPK pathway (49, 115, 116) and several extracellular ligands, such as TGFβ, TNFα and WNT5A (, , –120). AP-1 suppresses MITF through the Fra1 transcriptional target HMGA1 (53), while MITF binds to the JUN promoter and blocks its transcription (48, 50). AP-1 also upregulates the expression of AXL (48, 53), NGFR (48), and CD73 (104), driving phenotype switching to the invasive and therapy resistant state (49, 121). Right: Downstream of Hippo, TGFβ, EGFR, and Wnt/β-catenin pathways (122), the YAP/TAZ-TEAD complex cooperates with AP-1 to drive melanoma phenotype switching and therapy resistance through AXL and actin remodeling (–128).

Figure 3

Proposed translational regulation of melanoma phenotype switching. In response to environmental stress such as nutrient starvation, p-eIF2α mediates ISR and inhibits eIF2B activity, which diminishes global translation whilst increasing the translation of ATF4 (9, 57, 147, 148). ATF4 cooperates with the p-eIF2α-mediated translational reprogramming to drive phenotype switching (9, 57). In a nutrient sufficient environment, eIF2α is not phosphorylated and cap-dependent translation is on. Efficient global translation enables melanoma cells to sustain a proliferative state (9). Hyperactivated MAPK and PI3K-AKT pathways lead to MNK1/2-mediated eIF4E phosphorylation, enhancing the translation of a selected subset of mRNAs, including NGFR, MMP3, MMP9, and SNAI1 (146, 149, 150). Consequently, these oncogenes promote melanoma cell invasion, metastasis, and therapy resistance (37, 70, 146, 149, 151, 152).

Figure 4

Melanoma phenotype switching in acquired resistance to MAPK-targeted therapy and immunotherapies. Top: During initial response to MAPK-targeted therapy, while melanocytic-state cells are susceptible to MAPKi-induced cell death, a subset of cells switch to a drug-tolerant hyperdifferentiated state mediated by the PAX3-MITF axis. Hyperactivated MITF affords these hyperdifferentiated cells with a survival advantage, enabling them to quickly become the dominant population during the early drug-tolerant phase (, –187). In parallel, remaining cells undergo a fatty acid oxidation-dependent metabolic shift, resulting in the emergence of drug-tolerant CD36⁺ SMC populations (77). These cells undergo a continuous dedifferentiation during prolonged MAPKi treatment resistance (14, 54, 80, 188), resulting in the co-emergence and increase of MITF^low/AXL^high invasive and MITF^low/NGFR^high NCSC-like cells (16). These cells express increased levels of cellular receptors, allowing them to grow, bypassing the MAPK signaling and thereby permitting MAPKi resistance (10, 14, 16, 39, 54, 71, 75, 76, 78, 80, 189, 190). Consequently, at the acquired resistance phase, melanomas show a predominant expression of AXL and NGFR (54, 137). Bottom: During initial response to immunotherapy, activated CD8⁺ T cells recognize melan-A and GP100 antigens expressed on melanocytic-state cells and subsequently eliminate them (7, 31). Meanwhile, immunotherapy- and tumor microenvironment (TME)-induced inflammation drives melanoma phenotype switching, leading to decreased expression of melanocytic antigens and increased levels of pro-inflammatory cytokines (7, 14, 31, 48). Prolonged inflammation leads to increased infiltration of MDSCs (, , –195), which further promote phenotype switching via secretion of WNT5A (191, 192). In the acquired resistance phase, melanomas are enriched for the MITF^low/AXL^high invasive and MITF^low/NGFR^high NCSC-like populations (–105). Consequently, these highly dedifferentiated cells escape immune cell recognition (7, 14, 31, 48) and attract high numbers of MDSCs, which further mediate immune suppression (193). In addition, MITF^low/AXL^high invasive cells are associated with high expression of CD73 (104), and MITF^low/NGFR^high NCSC-like cells are found to have increased levels of PD-L1 (105, 196), which ultimately contribute to immunotherapy resistance.