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

Abstract

Inhibition of autophagy has been extensively studied as an anti-cancer therapy. Autophagy is a critical survival factor for cancer cells, whereby it maintains cellular homeostasis by degrading damaged organelles and unwanted proteins, and supports cellular biosynthesis in response to stress. Cancer cells are located in a very complex and stressful microenvironment where tumour cells are able to adapt and survive. However, the mechanism underlying this is not well defined. Our group has recently reported the post-translational modification enzyme, protein arginine methyltransferase 6 (PRMT6) to be frequently down-regulated and to exhibit a tumour suppressive role in hepatocellular carcinoma (HCC) (Chan et al. Cell Rep 2018). Findings from our current study identified protein arginine methylation by PRMT6 to inhibit autophagy in HCC. We found enhanced autophagic flux in HCC cells to negatively correlate with PRMT6 expression under various stress conditions that is commonly observed in the HCC tumour mass including nutrient deprivation, hypoxia and genotoxic/oxidative stress induced by drugs. By mass spectrometry-based proteomics, we identified a number of autophagyrelated proteins as potential interacting partners of PRMT6, with Bcl-2 associated athanogene 5 (BAG5) identified as the ‘top hit’. Binding between PRMT6 and BAG5 was validated by immunoprecipitation assays. In addition, we further demonstrated that PRMT6 methylated BAG5 on its N-terminus arginine residues and as a result recruited and degraded its chaperone partner heat shock protein (HSPA8/HSC70), a well-known autophagic player. Consistently, we also found BAG5 to also play a pivotal role in promoting HCC tumourigenesis through modulating autophagic events in vitro and in vivo. By functional rescue experiments, we demonstrated that inhibition of autophagy via BAG5 suppression reversed the effect of PRMT6 down-regulation on HCC tumorigenesis and sensitivity to sorafenib. In sum, our findings suggest PRMT6 down-regulation in HCC tumours to promote tumourigenicity and sorafenib resistance through autophagic flux de-regulation and anti-apoptotic activity. Mechanistically, this process involves modulating arginine methylation on BAG5, subsequently leading to altered stability of HSC70. Targeting this mechanism of stress response may provide novel therapeutic insights for this deadly disease.