Metabolic reprogramming of tumor-associated macrophages by collagen turnover promotes fibrosis in pancreatic cancer

A hallmark of pancreatic tumors is their highly desmoplastic stroma composed of fibroblasts, immune cells, and a dense network of collagen fibers. Tumor-associated macrophages are one of the most abundant immune cell populations in the pancreatic tumor stroma. Their protumorigenic function has been attributed predominantly to their capacity to promote immune evasion and metastasis. Tumor-assoc iated macrophages are also well known for their role in the remodeling of the stroma via collagen production and degradation, with the latter being mediated by mannose receptor (MRC1)-dependent endocytosis of collagen. Here we show that MRC1-mediated collagen internalization and subsequent lysosomal degradation by macrophages harboring a tumor-associated phenotype are accompanied by the accumulation of collagen-derived intracellular free amino acids and increased arginine biosynthesis. The resulting increase in intracellular arginine levels leads to the up-regulation of inducible nitric oxide synthase and the production of reactive nitrogen species. Furthermore, reactive nitrogen species derived from internalized and degraded collagen promotes a profibrotic phenotype in pancreatic stellate cells resulting in enhanced intratumoral collagen deposition. Overall, our findings identify a role for extracellular matrix remodeling in the functional modulation of tumor-associated macrophages via metabolic rewiring.