Hypoxia-induced secretory autophagy in cancer-associated fibroblasts promotes ECM remodelling through serglycin secretion in oral squamous cell carcinoma.

BACKGROUND: The worst pattern of invasion (WPOI) is a critical histological prognostic indicator in oral squamous cell carcinoma (OSCC), yet the underlying mechanisms driving high WPOI remain poorly understood. While cancer-associated fibroblasts (CAFs) and their secreted factor serglycin (SRGN) are implicated in tumour progression, the regulation of SRGN secretion within the hypoxic tumour microenvironment is unknown. METHODS: We performed single-cell RNA sequencing (scRNA-seq) on 6 OSCC samples (3 each of WPOI 1-3 and 4-5) to identify subgroups of CAFs and their characteristic gene expression profiles. Using Western blot, qRT-PCR, and immunofluorescence, we investigated hypoxia-induced SRGN secretion pathways. Complementary CRISPR-Cas9 knockout, Co-IP assays, and xenograft models elucidated SRGN's role in ECM remodelling. RESULTS: ScRNA-seq revealed significant enrichment of CAFs, particularly an SRGN-expressing myCAF subpopulation, in high-WPOI (4-5) OSCC tissues. Under hypoxia, CAFs switched SRGN secretion from the conventional ER-Golgi pathway to an unconventional secretory autophagy pathway, dependent on autophagosome formation but independent of lysosomal degradation. Secreted SRGN directly interacted with matrix metalloproteinase 2 (MMP2) and matrix metalloproteinase 9 (MMP9) in the extracellular matrix (ECM), enhancing ECM remodelling and OSCC invasion and migration. In vivo, either genetic ablation of SRGN in CAFs or pharmacological inhibition of autophagy significantly suppressed tumour growth, inhibited collagen I degradation, and restored E-cadherin expression. CONCLUSION: Our study identifies a novel mechanism whereby hypoxia induces CAFs to secrete SRGN via secretory autophagy. This SRGN-MMP2/9 axis drives ECM remodelling and promotes OSCC invasion, which histologically manifests as high WPOI. Targeting secretory autophagy or SRGN represents a promising therapeutic strategy for aggressive OSCC. KEY POINTS: Under normoxia, SRGN enters the conventional secretory pathway via the endoplasmic reticulum (ER) and Golgi apparatus for extracellular release. Under hypoxia, elevated autophagy levels in CAFs facilitate the release of SRGN into the ECM through secretory autophagy-mediated plasma membrane fusion. Within the ECM, SRGN interacts with MMP2 and MMP9, enhancing ECM remodelling and OSCC invasion.