Lipid accumulation confers pro-fibrotic functions to Tumor-Associated Macrophages in melanoma

Macrophages represent the primary infiltrating immune subset in most cancer types and tumor-associated macrophages (TAMs) sustain tumor growth, promote invasion and mediate immune suppression. Cellular metabolism is implicated in the education of macrophages and dictates their phenotype. Interestingly, macrophages engulfed with lipids have been recently described in various cancers, including prostate, gastric cancer and melanoma. However, the mechanisms that drive lipid accumulation in macrophages remain uncovered. In this study, we applied high-throughput techniques, including single cell RNA sequencing and proteomic analysis to unravel the mechanisms responsible for lipid loading in macrophages and to explore the implications of lipid accumulation on functional activity of TAMs in melanoma. Our investigation confirmed that lipid accumulation is a distinct feature of tumor-infiltrating macrophages in melanoma. Notably, we observed that lipid-accumulation in TAMs correlates with tumor size and with the expression, by macrophages, of markers associated with immunosuppression, including Arginase-1 and PD-L1. Bulk RNA sequencing revealed that the CLEAR signaling pathway is deregulated in lipid-loaded macrophages compared to the lipid-deprived counterpart. Accordingly, we observed that the transcription factor EB (TFEB), master regulator of the CLEAR network, is dysfunctional in tumor-conditioned macrophages. In addition, autophagic flux is impaired in macrophages exposed to tumor cells, thus indicating a potential role of TFEB in promoting autophagy deficiency and lipid accumulation in TAMs. In the second part of the project, we explored the mechanisms triggered by lipid accumulation in TAMs. Notably, we discovered that lipid loaded macrophages can sustain a pro-fibrotic program and directly contribute to fibrosis, producing collagen and extracellular matrix (ECM)-related proteins. Furthermore, our findings demonstrated that transcription regulator YAP is activated upon lipid accumulation in TAMs and promotes collagen synthesis and ECM deposition. Importantly, inhibition of YAP effectively prevented expression of genes related to ECM in macrophages. In conclusion, this project sheds light on mechanisms that contribute to lipid accumulation in TAMs and introduces innovative approaches to target lipid-loaded macrophages in melanoma. Based on our findings, we propose to target lipid accumulation in TAMs as a strategy to improve efficacy of current available immunotherapies, thus representing an opportunity for advancing cancer therapy.