Paracrine Activin-A Signaling Promotes Melanoma Growth and Metastasis through Immune Evasion

The secreted growth factor Activin-A of the transforming growth factor beta family and its receptors can promote or inhibit several cancer hallmarks including tumor cell proliferation and differentiation, vascularization, lymphangiogenesis and inflammation. However, a role in immune evasion and its relationship with tumor-induced muscle wasting and tumor vascularization, and the relative contributions of autocrine versus paracrine Activin signaling remain to be evaluated. To address this, we compared the effects of truncated soluble Activin receptor IIB as a ligand trap, or constitutively active mutant type IB receptor versus secreted Activin-A or the related ligand Nodal in mouse and human melanoma cell lines and tumor grafts. We found that although cell-autonomous receptor activation arrested tumor cell proliferation, Activin-A secretion stimulated melanoma cell dedifferentiation and tumor vascularization by functional blood vessels, and it increased primary and metastatic tumor burden and muscle wasting. Importantly, in mice with impaired adaptive immunity, the tumor-promoting effect of Activin-A was lost despite sustained vascularization and cachexia, suggesting that Activin-A promotes melanoma progression by inhibiting antitumor immunity. Paracrine Activin-A signaling emerges as a potential target for personalized therapies, both to reduce cachexia and to enhance the efficacy of immunotherapies.

Immunosuppressive roles of Activin-A in melanoma and characterization of its precursor cleavage

Katarina Pinjusic

Activin-A, a transforming growth factor êµ family member, is a pleiotropic cytokine with diverse functions in development, fertility, adult tissue homeostasis, and aging. Accordingly, deregulation of Activin-A signaling has been associated with many pathological conditions, including cancer and cancer-associated muscle wasting (cachexia). Activin-A was shown to signal in endocrine, but also local autocrine and paracrine manner. Its secretion in many cancer types is associated with a poor prognosis. Previous studies suggest that Activin-A expression in melanoma promotes tumor growth and metastasis indirectly by inhibiting adaptive anti-tumor immunity, leading to a decrease in the intratumoral frequency of cytotoxic CD8+ T cells (CTLs). However, the underlying mechanisms and the relevant direct target cells of Activin-A and their significance for cancer therapies remained unknown. Similar to other TGFêµ-related ligands, proActivin-A is cleaved by proprotein convertases (PCs) to release mature ligands. However, unlike other family members whose signaling is tightly regulated, Activin-A apparently does not have latency.In this thesis, I characterize proActivin-A cleavage and show that one prodomain can be cleaved independently of known PCs. The resulting hemi-cleaved Activin-A can bind activin type II receptors and induce SMAD3 signaling in reporter cells. A protein interactome screen and analysis of cleavage intermediates in plasma revealed that the hemi-cleaved Activin-A likely interacts with extracellular matrix proteins via the remaining prodomain where it promotes local signaling and regulates trafficking. Luciferase reporter assay showed that hemi-cleaved Activin-A can avoid, at least partially, inhibition by endogenous inhibitors FSTL3 and FST, and enable parallel activation of BMP and Activin signaling pathways. Finally, analysis of B16-F1 melanoma models revealed that only fully cleaved Activin-A in source cells increased tumor growth by acting locally in the tumor microenvironment suggesting that the context of Activin-A precursor processing regulates its tumor growth-promoting effect.The second part of this thesis addresses the mechanisms of Activin-induced immunosuppression in melanoma. We found that Activin-A impairs CTL function indirectly and independently of CD4+ T cell, CSF1R+ macrophages, and IL4 signaling. At least in part, anti-tumor CTL responses are impaired by the inhibitory effects of Activin-A on the secretion of critical CTL-recruiting chemokines CXCL9 and CXCL10 by macrophages and dendritic cells (DCs). Furthermore, Activin-A conferred resistance to immune checkpoint blockade therapy. Characterization of Activin-induced changes in the gene expression of tumor-infiltrating cells by Single cell RNA sequencing revealed that INHBA expression induced the most changes in DCs and least in T cells. Interestingly, the most prominently induced hallmark in Activin-secreting tumors across cell types was the IFN signature. In vitro analyses showed that Activin-A increases IFNÎ³-induced activation of STAT1 and confers resistance to cytostatic IFN signaling in cancer cells.Altogether, the findings of this thesis expand our knowledge on the regulation of Activin-A signaling by precursor cleavage and offer insights into mechanisms of Activin-induced immunosuppression in melanoma. Inhibition of Activin-A maturation thus emerges as a potential new strategy to improve responses to immunotherapies in melanoma and likely other cancers

EPFL2022

Role of Activin-A signaling in melanoma

Olivier Andreas Dubey

Cutaneous melanomas arise from transformed skin melanocytes. While accounting for only 1% of skin malignancies, melanoma is responsible for the majority of skin cancer-related deaths. Owing to lifestyle changes and increased exposure to sunlight, melanoma occurrence is on the rise in developed countries. Identification of the molecular mechanisms driving melanoma formation led to the development of new classes of therapeutics which revolutionized the standard of care. Among these, small molecule inhibitors targeting the MAPK pathway and immune checkpoint inhibitors resulted in unprecedented clinical success. Nevertheless, many patients still do not benefit from these therapies. Activin-A, a secreted ligand of the TGF-Î² superfamily, was recently found to be overexpressed in melanoma. However, whether Activin-A signaling promotes melanoma progression or therapy resistance is unknown. In this thesis, the contributions of autocrine and paracrine Activin-A signaling to melanoma growth are analyzed using lentiviral expression in melanoma cells of a constitutively active mutant type I Activin receptor (caALK4), of secreted Activin-A (gain-of-function), or of a ligand trap (loss-of-function). Paradoxically, while forced ALK4 signaling in melanoma grafts restored tumor suppression by inhibiting tumor cell survival and proliferation, transgenic Activin-A accelerated tumor growth. Conversely, expression of a ligand trap comprising the extracellular domain of Activin receptor IIB fused to human Fc (AIIB-Fc) slowed melanoma growth. Importantly, in immunodeficient Rag1-/- mice, effects of paracrine Activin-A gain and loss-of-function were abrogated, suggesting that Activin-A promotes melanoma progression by inhibiting the adaptive anti-tumor response. Flow cytometry analysis of tumor immune infiltrates combined with antibody-mediated cell depletion experiments confirmed that Activin-A promotes melanoma progression by inhibiting CD8+ tumor-infiltrating lymphocytes (TILs). Furthermore, inhibition of endogenous Activin-A sensitized a treatment-resistant melanoma model to immune checkpoint inhibition by anti-PD1 and anti-CTLA4 antibodies. Activin-A therefore emerges as a promising target to improve immunotherapy outcome in melanoma.

EPFL2019

Immunosuppressive roles of Activin-A in melanoma and characterization of its precursor cleavage

Katarina Pinjusic

EPFL2022

Role of Activin-A signaling in melanoma

Olivier Andreas Dubey

EPFL2019