Distinct regulation of collagen biosynthesis by hypoxia and vitamin C uncovers an essential role for CXCL12-CXCR4 signaling

Collagen biosynthesis is a complex multistep process, essential for functional tissue formation and repair. It is highly active during embryogenesis and wound healing, leading to physiological fibrogenesis, while its dysregulation induces pathologies affecting connective tissues, namely fibrosis. There are several key cellular microenvironmental factors regulating collagen biosynthesis, including vitamin C, oxygen tension and growth factors. Whilst the function of these distinct factors has previously been studied individually, how they operate together, synergistically or not, is not fully characterized. Using primary human dermal fibroblasts, we demonstrated that ascorbic acid (AA) and hypoxia both induced collagen I deposition when applied individually and had additive effects when combined together. Their effects were associated to the expected increase in collagen prolyl hydroxylation. However, AA induced a much stronger collagen secretion compared to hypoxia, while hypoxia leads to a higher collagen I hydroxylation. This observation challenged the common view of collagen prolyl hydroxylation being the central mechanism of AA-induced collagen secretion. Using proteomic analysis of the secretome, biochemical and cell imaging approaches, we further showed that increased CXCL12 extracellular levels and activation of its cognate CXCR4 receptor are at the core of AA-induced collagen intracellular trafficking and secretion. This mechanism is specific to AA and is not involved under hypoxia. The existence of distinct mechanisms underlying the effects of AA and hypoxia is further demonstrated by the different ultrastructure of collagen fibrils in the two conditions. Together, our results contribute to explain how these two triggers drive collagen biosynthesis and pave the way for potential pro- or anti-fibrotic strategies aimed at improving tissue repair.