Background:
Osteopontin (OPN), a secreted phosphoprotein, contributes to tumor progression in various cancers. However, the molecular mechanisms underlying OPN secretion remain unclear. We hypothesized that Rab37, a small GTPase previously implicated in exocytosis and tumor modulation, regulates OPN secretion through both conventional and autophagy-based secretory pathways.
Materials and Methods:
Single-cell RNA sequencing (scRNA-seq) was performed on tumor-infiltrating immune cells from wild-type (WT) and Rab37 knockout (KO) mice. THP-1-derived macrophages and RAW264.7 cells were used to study Rab37-mediated OPN secretion. Confocal microscopy, western blotting, ELISA, GTP-agarose pulldown, vesicle isolation, and autophagosome purification were employed to assess Rab37-OPN interactions and secretory mechanisms. Clinical relevance was evaluated using multiplex immunohistochemistry on lung cancer tissues.
Results:
Rab37 KO reduced OPN secretion. Rab37-associated vesicles contained OPN cargo. OPN co-localized with Rab37, and GTP-bound Rab37 promoted its secretion in a GTP-dependent manner in tumor-associated macrophages (TAMs). In tumor-educated macrophages, Rab37 also facilitated unconventional OPN secretion via secretory autophagy, evidenced by increased LC3-II expression, reduced OPN secretion upon ATG5 KO, and enhanced secretion with chloroquine treatment. GTP-bound Rab37 inhibited Rab7-mediated autolysosome formation, diverting autophagosomes toward secretion. Functionally, Rab37-mediated OPN secretion activated STAT3 signaling and enhanced lung cancer cell proliferation and migration. Clinically, OPN-High/Rab37+/LC3-II+/CD163+ TAMs were associated with poor prognosis in lung cancer patients.
Conclusion:
Rab37 regulates OPN secretion via both conventional exocytosis and secretory autophagy in TAMs, thereby promoting a pro-tumorigenic microenvironment. Targeting the Rab37–OPN axis may offer a novel therapeutic strategy for lung cancer.