Protein arginine methylation of BAG5 by PRMT6 regulates autophagy in hepatocellular carcinoma tumours

Abstract

Autophagy is a major catabolism to maintain cellular homeostasis in response to environmental changes by removing harmful intracellular aggregates and damaged organelles. Meanwhile, this catabolic process provides a route for cancer cells to adapt and survive from a complicated and stressful tumour microenvironment. As a result, extensive studies were conducted to focus on the potential of autophagy inhibition to enhance efficacy of conventional treatment regimen in treating cancers. However, current clinical trials failed to deliver any promising anticancer outcomes of targeting autophagy, including hepatocellular carcinoma (HCC). In order to exploit this vulnerability as anticancer therapy, our full understanding of the precise mechanism is necessary. Our recent study found out a key post-translational modification enzyme, protein arginine methyltransferase 6 (PRMT6), was frequently downregulated in HCC and this downregulation exerted its gain-offunctions in HCC tumourigenesis (Chan et al. Cell Rep 2018). Our current study characterised an inhibitory role of PRMT6 in autophagy via protein arginine methylation in HCC. Mechanistically, we found out Bcl-2 associated athanogene 5 (BAG5) was an interacting partner of PRMT6 by mass spectrometry. We further demonstrated that PRMT6 methylated BAG5 on the 15th and 24th arginine residues of its N-terminus to recruit and to degrade its interacting partner, heat shock cognate 70 (HSPA8/HSC70), a well-known autophagy player. Besides, we demonstrated that BAG5 was important in promoting HCC properties and itself was required to govern the stability of HSC70 for autophagy induction. By in vitro and in vivo experiments, we also demonstrated that BAG5 was a downstream effector of PRMT6 in regulating autophagy. Our results displayed that HCC cells with PRMT6 downregulation were dependent on autophagy for cell survival and targeting autophagy sensitised them to sorafenib treatment. Taken together, our findings in this study suggest PRMT6 downregulation in HCC favoured autophagy induction and provide support for HCC tumourigenesis through modulating BAG5 functionalities. Most importantly, PRMT6 can be used as a prognostic marker for autophagy inhibition as a combination therapy with sorafenib, suggesting targeting this mechanism may provide novel therapeutic insights for this deadly disease.