Myeloid-Derived Suppressor Cells Are a Major Source of Wnt5A in the Melanoma Microenvironment and Depend on Wnt5A for Full Suppressive Activity.

Metastatic dissemination remains a significant barrier to successful therapy for melanoma. Wnt5A is a potent driver of invasion in melanoma and is believed to be secreted from the tumor microenvironment (TME). Our data suggest that myeloid-derived suppressor cells (MDSC) in the TME are a major source of Wnt5A and are reliant upon Wnt5A for multiple actions. Knockdown of Wnt5A specifically in the myeloid cells demonstrated a clear decrease in Wnt5A expression within the TME in vivo as well as a decrease in intratumoral MDSC and regulatory T cell (Treg). Wnt5A knockdown also decreased the immunosuppressive nature of MDSC and decreased expression of TGFß1 and arginase 1. In the presence of Wnt5A-depleted MDSC, tumor-infiltrating lymphocytes expressed decreased PD-1 and LAG3, suggesting a less exhausted phenotype. Myeloid-specific Wnt5A knockdown also led to decreased lung metastasis. Tumor-infiltrating MDSC from control animals showed a strong positive correlation with Treg, which was completely ablated in animals with Wnt5A-negative MDSC. Overall, our data suggest that while MDSC contribute to an immunosuppressive and less immunogenic environment, they exhibit an additional function as the major source of Wnt5A in the TME. SIGNIFICANCE: These findings demonstrate that myeloid cells provide a major source of Wnt5A to facilitate metastatic potential in melanoma cells and rely on Wnt5A for their immunosuppressive function.

Changes in Aged Fibroblast Lipid Metabolism Induce Age-Dependent Melanoma Cell Resistance to Targeted Therapy via the Fatty Acid Transporter FATP2.

Older patients with melanoma (>50 years old) have poorer prognoses and response rates to targeted therapy compared with young patients (<50 years old), which can be driven, in part, by the aged microenvironment. Here, we show that aged dermal fibroblasts increase the secretion of neutral lipids, especially ceramides. When melanoma cells are exposed to the aged fibroblast lipid secretome, or cocultured with aged fibroblasts, they increase the uptake of lipids via the fatty acid transporter FATP2, which is upregulated in melanoma cells in the aged microenvironment and known to play roles in lipid synthesis and accumulation. We show that blocking FATP2 in melanoma cells in an aged microenvironment inhibits their accumulation of lipids and disrupts their mitochondrial metabolism. Inhibiting FATP2 overcomes age-related resistance to BRAF/MEK inhibition in animal models, ablates tumor relapse, and significantly extends survival time in older animals. SIGNIFICANCE: These data show that melanoma cells take up lipids from aged fibroblasts, via FATP2, and use them to resist targeted therapy. The response to targeted therapy is altered in aged individuals because of the influences of the aged microenvironment, and these data suggest FATP2 as a target to overcome resistance.See related commentary by Montal and White, p. 1255.This article is highlighted in the In This Issue feature, p. 1241.

Paradoxical Role for Wild-Type p53 in Driving Therapy Resistance in Melanoma.

Metastatic melanoma is an aggressive disease, despite recent improvements in therapy. Eradicating all melanoma cells even in drug-sensitive tumors is unsuccessful in patients because a subset of cells can transition to a slow-cycling state, rendering them resistant to most targeted therapy. It is still unclear what pathways define these subpopulations and promote this resistant phenotype. In the current study, we show that Wnt5A, a non-canonical Wnt ligand that drives a metastatic, therapy-resistant phenotype, stabilizes the half-life of p53 and uses p53 to initiate a slow-cycling state following stress (DNA damage, targeted therapy, and aging). Inhibiting p53 blocks the slow-cycling phenotype and sensitizes melanoma cells to BRAF/MEK inhibition. In vivo, this can be accomplished with a single dose of p53 inhibitor at the commencement of BRAF/MEK inhibitor therapy. These data suggest that taking the paradoxical approach of inhibiting rather than activating wild-type p53 may sensitize previously resistant metastatic melanoma cells to therapy.

CADM1 is a TWIST1-regulated suppressor of invasion and survival.

Metastatic cancer remains a clinical challenge; however, patients diagnosed prior to metastatic dissemination have a good prognosis. The transcription factor, TWIST1 has been implicated in enhancing the migration and invasion steps within the metastatic cascade, but the range of TWIST1-regulated targets is poorly described. In this study, we performed expression profiling to identify the TWIST1-regulated transcriptome of melanoma cells. Gene ontology pathway analysis revealed that TWIST1 and epithelial to mesenchymal transition (EMT) were inversely correlated with levels of cell adhesion molecule 1 (CADM1). Chromatin immunoprecipitation (ChIP) studies and promoter assays demonstrated that TWIST1 physically interacts with the CADM1 promoter, suggesting TWIST1 directly represses CADM1 levels. Increased expression of CADM1 resulted in significant inhibition of motility and invasiveness of melanoma cells. In addition, elevated CADM1 elicited caspase-independent cell death in non-adherent conditions. Expression array analysis suggests that CADM1 directed non-adherent cell death is associated with loss of mitochondrial membrane potential and subsequent failure of oxidative phosphorylation pathways. Importantly, tissue microarray analysis and clinical data from TCGA indicate that CADM1 expression is inversely associated with melanoma progression and positively correlated with better overall survival in patients. Together, these data suggest that CADM1 exerts tumor suppressive functions in melanoma by reducing invasive potential and may be considered a biomarker for favorable prognosis.

Remodeling of the Collagen Matrix in Aging Skin Promotes Melanoma Metastasis and Affects Immune Cell Motility.

Physical changes in skin are among the most visible signs of aging. We found that young dermal fibroblasts secrete high levels of extracellular matrix (ECM) constituents, including proteoglycans, glycoproteins, and cartilage-linking proteins. The most abundantly secreted was HAPLN1, a hyaluronic and proteoglycan link protein. HAPLN1 was lost in aged fibroblasts, resulting in a more aligned ECM that promoted metastasis of melanoma cells. Reconstituting HAPLN1 inhibited metastasis in an aged microenvironment, in 3-D skin reconstruction models, and in vivo. Intriguingly, aged fibroblast-derived matrices had the opposite effect on the migration of T cells, inhibiting their motility. HAPLN1 treatment of aged fibroblasts restored motility of mononuclear immune cells, while impeding that of polymorphonuclear immune cells, which in turn affected regulatory T-cell recruitment. These data suggest that although age-related physical changes in the ECM can promote tumor cell motility, they may adversely affect the motility of some immune cells, resulting in an overall change in the immune microenvironment. Understanding the physical changes in aging skin may provide avenues for more effective therapy for older patients with melanoma. SIGNIFICANCE: These data shed light on the mechanochemical interactions that occur between aged skin, tumor, and immune cell populations, which may affect tumor metastasis and immune cell infiltration, with implications for the efficacy of current therapies for melanoma.See related commentary by Marie and Merlino, p. 19.This article is highlighted in the In This Issue feature, p. 1.

Age Correlates with Response to Anti-PD1, Reflecting Age-Related Differences in Intratumoral Effector and Regulatory T-Cell Populations.

Purpose: We have shown that the aged microenvironment increases melanoma metastasis, and decreases response to targeted therapy, and here we queried response to anti-PD1.Experimental Design: We analyzed the relationship between age, response to anti-PD1, and prior therapy in 538 patients. We used mouse models of melanoma, to analyze the intratumoral immune microenvironment in young versus aged mice and confirmed our findings in human melanoma biopsies.Results: Patients over the age of 60 responded more efficiently to anti-PD-1, and likelihood of response to anti-PD-1 increased with age, even when we controlled for prior MAPKi therapy. Placing genetically identical tumors in aged mice (52 weeks) significantly increased their response to anti-PD1 as compared with the same tumors in young mice (8 weeks). These data suggest that this increased response in aged patients occurs even in the absence of a more complex mutational landscape. Next, we found that young mice had a significantly higher population of regulatory T cells (Tregs), skewing the CD8+:Treg ratio. FOXP3 staining of human melanoma biopsies revealed similar increases in Tregs in young patients. Depletion of Tregs using anti-CD25 increased the response to anti-PD1 in young mice.Conclusions: While there are obvious limitations to our study, including our inability to conduct a meta-analysis due to a lack of available data, and our inability to control for mutational burden, there is a remarkable consistency in these data from over 500 patients across 8 different institutes worldwide. These results stress the importance of considering age as a factor for immunotherapy response. Clin Cancer Res; 24(21); 5347-56. ©2018 AACR See related commentary by Pawelec, p. 5193.

Response and Resistance to Paradox-Breaking BRAF Inhibitor in Melanomas In Vivo and Ex Vivo.

FDA-approved BRAF inhibitors produce high response rates and improve overall survival in patients with BRAF V600E/K-mutant melanoma, but are linked to pathologies associated with paradoxical ERK1/2 activation in wild-type BRAF cells. To overcome this limitation, a next-generation paradox-breaking RAF inhibitor (PLX8394) has been designed. Here, we show that by using a quantitative reporter assay, PLX8394 rapidly suppressed ERK1/2 reporter activity and growth of mutant BRAF melanoma xenografts. Ex vivo treatment of xenografts and use of a patient-derived explant system (PDeX) revealed that PLX8394 suppressed ERK1/2 signaling and elicited apoptosis more effectively than the FDA-approved BRAF inhibitor, vemurafenib. Furthermore, PLX8394 was efficacious against vemurafenib-resistant BRAF splice variant-expressing tumors and reduced splice variant homodimerization. Importantly, PLX8394 did not induce paradoxical activation of ERK1/2 in wild-type BRAF cell lines or PDeX. Continued in vivo dosing of xenografts with PLX8394 led to the development of acquired resistance via ERK1/2 reactivation through heterogeneous mechanisms; however, resistant cells were found to have differential sensitivity to ERK1/2 inhibitor. These findings highlight the efficacy of a paradox-breaking selective BRAF inhibitor and the use of PDeX system to test the efficacy of therapeutic agents. Mol Cancer Ther; 17(1); 84-95. ©2017 AACR.

ATG5 Mediates a Positive Feedback Loop between Wnt Signaling and Autophagy in Melanoma.

Autophagy mediates resistance to various anticancer agents. In melanoma, resistance to targeted therapy has been linked to expression of Wnt5A, an intrinsic inhibitor of ß-catenin, which also promotes invasion. In this study, we assessed the interplay between Wnt5A and autophagy by combining expression studies in human clinical biopsies with functional analyses in cell lines and mouse models. Melanoma cells with high Wnt5A and low ß-catenin displayed increased basal autophagy. Genetic blockade of autophagy revealed an unexpected feedback loop whereby knocking down the autophagy factor ATG5 in Wnt5Ahigh cells decreased Wnt5A and increased ß-catenin. To define the physiologic relevance of this loop, melanoma cells with different Wnt status were treated in vitro and in vivo with the potent lysosomotropic compound Lys05. Wnt5Ahigh cells were less sensitive to Lys05 and could be reverted by inducing ß-catenin activity. Our results suggest the efficacy of autophagy inhibitors might be improved by taking the Wnt signature of melanoma cells into account. Cancer Res; 77(21); 5873-85. ©2017 AACR.

Inhibition of Age-Related Therapy Resistance in Melanoma by Rosiglitazone-Mediated Induction of Klotho.

Purpose: Aging is a poor prognostic factor for melanoma. We have shown that melanoma cells in an aged microenvironment are more resistant to targeted therapy than identical cells in a young microenvironment. This is dependent on age-related secreted factors. Klotho is an age-related protein whose serum levels decrease dramatically by age 40. Most studies on klotho in cancer have focused on the expression of klotho in the tumor cell. We have shown that exogenous klotho inhibits internalization and signaling of Wnt5A, which drives melanoma metastasis and resistance to targeted therapy. We investigate here whether increasing klotho in the aged microenvironment could be an effective strategy for the treatment of melanoma.Experimental Design: PPARγ increases klotho levels and is increased by glitazones. Using rosiglitazone, we queried the effects of rosiglitazone on Klotho/Wnt5A cross-talk, in vitro and in vivo, and the implications of that for targeted therapy in young versus aged animals.Results: We show that rosiglitazone increases klotho and decreases Wnt5A in tumor cells, reducing the burden of both BRAF inhibitor-sensitive and BRAF inhibitor-resistant tumors in aged, but not young mice. However, when used in combination with PLX4720, tumor burden was reduced in both young and aged mice, even in resistant tumors.Conclusions: Using glitazones as adjuvant therapy for melanoma may provide a new treatment strategy for older melanoma patients who have developed resistance to vemurafenib. As klotho has been shown to play a role in other cancers too, our results may have wide relevance for multiple tumor types. Clin Cancer Res; 23(12); 3181-90. ©2017 AACR.

sFRP2 in the aged microenvironment drives melanoma metastasis and therapy resistance.

Cancer is a disease of ageing. Clinically, aged cancer patients tend to have a poorer prognosis than young. This may be due to accumulated cellular damage, decreases in adaptive immunity, and chronic inflammation. However, the effects of the aged microenvironment on tumour progression have been largely unexplored. Since dermal fibroblasts can have profound impacts on melanoma progression, we examined whether age-related changes in dermal fibroblasts could drive melanoma metastasis and response to targeted therapy. Here we find that aged fibroblasts secrete a Wnt antagonist, sFRP2, which activates a multi-step signalling cascade in melanoma cells that results in a decrease in ß-catenin and microphthalmia-associated transcription factor (MITF), and ultimately the loss of a key redox effector, APE1. Loss of APE1 attenuates the response of melanoma cells to DNA damage induced by reactive oxygen species, rendering the cells more resistant to targeted therapy (vemurafenib). Age-related increases in sFRP2 also augment both angiogenesis and metastasis of melanoma cells. These data provide an integrated view of how fibroblasts in the aged microenvironment contribute to tumour progression, offering new possibilities for the design of therapy for the elderly.

The Wnts of change: How Wnts regulate phenotype switching in melanoma.

The outgrowth of metastatic and therapy-resistant subpopulations in cancer remains a critical barrier for the successful treatment of this disease. In melanoma, invasion and proliferation are uncoupled, such that highly proliferative melanoma cells are less likely to be invasive, and vice versa. The transition between each state is likely a dynamic rather than a static, permanent change. This is referred to as "phenotype switching". Wnt signaling pathways drive phenotypic changes and promote therapy resistance in melanoma, as well as play roles in the modulation of the immune microenvironment. Three Wnt signaling pathways play a role in melanoma progression, canonical (ß-catenin dependent), polar cell polarity (PCP), and the Wnt/Ca²âº pathway. Here we summarize phenotype plasticity and its role in therapy resistance and immune evasion. Targeting the Wnt signaling pathways may be an effective way to overcome tumor plasticity in melanoma.

Function-blocking ERBB3 antibody inhibits the adaptive response to RAF inhibitor.

ERBB3/HER3 expression and signaling are upregulated in mutant BRAF melanoma as an adaptive, prosurvival response to FDA-approved RAF inhibitors. Because compensatory ERBB3 signaling counteracts the effects of RAF inhibitors, cotargeting ERBB3 may increase the efficacy of RAF inhibitors in mutant BRAF models of melanoma. Here, we corroborate this concept by showing that the ERBB3 function-blocking monoclonal antibody huHER3-8 can inhibit neuregulin-1 activation of ERBB3 and downstream signaling in RAF-inhibited melanoma cells. Targeting mutant BRAF in combination with huHER3-8 decreased cell proliferation and increased cell death in vitro, and decreased tumor burden in vivo, compared with targeting either mutant BRAF or ERBB3 alone. Furthermore, the likelihood of a durable tumor response in vivo was increased when huHER3-8 was combined with RAF inhibitor PLX4720. Together, these results offer a preclinical proof of concept for the application of ERBB3-neutralizing antibodies to enhance the efficacy of RAF inhibitors in melanoma to delay or prevent tumor regrowth. As ERBB3 is often upregulated in response to other kinase-targeted therapeutics, these findings may have implications for other cancers as well.

Adaptive resistance to RAF inhibitors in melanoma.

The discovery of activating mutations in BRAF at high frequency in cutaneous melanoma opened the door to new treatment options, which have resulted in significantly better patient outcomes. Treatments such as the FDA-approved RAF inhibitor vemurafenib and the more recently approved dabrafenib and trametinib combination therapy are designed to target the ERK1/2 pathway. Initial success in targeting this pathway is evidenced by the high percentage of melanoma patients who undergo tumor remission. However, the beneficial effects of these targeted therapies are usually short-lived due to the development of resistance, which leads to disease progression. As a result, studies have focused on the acquired forms of resistance that develop following continued exposure to therapy. Conversely, far fewer studies have investigated the adaptive forms of resistance, which activate rapidly, promote cell survival, and may underlie the development of acquired resistance by providing melanoma cells the time to develop additional mutations. We provide a detailed review of the known mechanisms of adaptive resistance in melanoma and relate them to similar responses to targeted therapies in other tumor types.

Melanoma adapts to RAF/MEK inhibitors through FOXD3-mediated upregulation of ERBB3.

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.

SHOC2 and CRAF mediate ERK1/2 reactivation in mutant NRAS-mediated resistance to RAF inhibitor.

ERK1/2 signaling is frequently dysregulated in tumors through BRAF mutation. Targeting mutant BRAF with vemurafenib frequently elicits therapeutic responses; however, durable effects are often limited by ERK1/2 pathway reactivation via poorly defined mechanisms. We generated mutant BRAF(V600E) melanoma cells that exhibit resistance to PLX4720, the tool compound for vemurafenib, that co-expressed mutant (Q61K) NRAS. In these BRAF(V600E)/NRAS(Q61K) co-expressing cells, re-activation of the ERK1/2 pathway during PLX4720 treatment was dependent on NRAS. Expression of mutant NRAS in parental BRAF(V600) cells was sufficient to by-pass PLX4720 effects on ERK1/2 signaling, entry into S phase and susceptibility to apoptosis in a manner dependent on the RAF binding site in NRAS. ERK1/2 activation in BRAF(V600E)/NRAS(Q61K) cells required CRAF only in the presence of PLX4720, indicating a switch in RAF isoform requirement. Both ERK1/2 activation and resistance to apoptosis of BRAF(V600E)/NRAS(Q61K) cells in the presence of PLX4720 was modulated by SHOC-2/Sur-8 expression, a RAS-RAF scaffold protein. These data show that NRAS mutations confer resistance to RAF inhibitors in mutant BRAF cells and alter RAF isoform and scaffold molecule requirements to re-activate the ERK1/2 pathway.