179 C1q stimulates hyaluronic acid degradation via gC1qR/HABP1/p32 in malignant pleural mesothelioma

Background: C1q exerts pro-tumorigenic functions in the tumour microenvironment (TME), independently of complement activation [1]. In malignant pleural mesothelioma (MPM), HA-bound C1q is able to increase tumor cell adhesion, migration and proliferation, but also to enhance the production of pro-inflammatory and pro-metastatic HA fragments due to HAS3 upregulation [2]. An increasingly activated HA metabolism is associated with cancer progression. Here, we investigated HA-bound C1q contribution in HA degradation via modulation of hyaluronidases (HYAL1, HYAL2) and the involvement of globular C1q receptor/HABP1/p32 (gC1qR), as a receptor of both HA and C1q. Methods: Primary cell isolation, zymography, immunohistochemistry, Real-Time quantitative PCR (RT-qPCR), immunofluorescence, Western blot, surface biotinylation assay, flow cytometry, proximity ligation assay (PLA), RNA interference, bioinformatics analysis. Results: After initial characterization of HYALs in MPM primary cells, we focused on HYAL2, since GEPIA bioinformatics analysis revealed an unfavorable prognostic index in MPM patients with higher HYAL2 mRNA levels. Upon seeding MPM cells onto HA-bound C1q, HYAL2 upregulation was highlighted by RT-qPCR, flow cytometry and Western blot. In the attempt to determine receptors involved in HA-C1q signaling, a striking membrane and intracellular co-localization between HYAL2 and gC1qR was found by immunofluorescence, surface biotinylation assay and PLA. RNA interference of C1QBP (gene for gC1qR) unveiled a promising regulatory effect of gC1qR on HYAL2 expression, determining an unexpected HYAL2 downregulation. Moreover, functional blockage of gC1qR by a specific antibody hampered HA-C1q signaling and impeded HYAL2 upregulation. Conclusions: HA-C1q interplay can act as a tumor-promoting signaling complex by enhancing HYAL2 expression, suggesting a consequent higher rate of HA catabolism and the release of pro-inflammatory and pro-tumorigenic HA fragments. Furthermore, we demonstrated a novel regulatory function of gC1qR in the TME due to its involvement in the modulation of HA metabolism. The regulation of HA metabolism is crucial in view of its key role in tumor progression and its connection with most of the hallmarks of cancer.