Functional and Mechanistic Characterization of PRMT6-Regulated Autophagy in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a major health burden worldwide. Tumor recurrence and therapy resistance represent major obstacles in the treatment of the disease, with their mechanism of action largely uncharacterized. Autophagy is a critical survival factor for cancer cells, whereby it maintains cellular homeostasis including degradation of damaged organelles and unwanted proteins as well as the support of cellular biosynthesis in response to environmental stress, preventing cells from undergoing apoptosis. Our group has recently reported on the regulatory role of protein arginine methyltransferase 6 (PRMT6) down-regulation in maintenance of HCC, particularly potentiating resistance to sorafenib and chemotherapy. By tandem affinity purification/mass spectrometry analysis of cells with PRMT6 stably overexpressed, we identified a number of autophagy related proteins as potential interacting partners of PRMT6, suggesting a possible role of PRMT6 in modulating autophagy in HCC. Treatment of HCC cells with stress induced conditions, including sorafenib, nutrient deprivation and hypoxic condition, resulted in a marked reduction of PRMT6 expression, concomitant with elevated levels of LC3BII. Further, under these various stimuli, knockdown of PRMT6 in HCC cells would also result in enhanced expression of LC3BII, accumulation of GFP-LC3B positive puncta staining as well as an increase in autophagosome formation. Consistently, treatment of HCC cells with PRMT6 overexpressed would leading to an opposing effect. Further studies by immunoprecipitation analysis validated PRMT6 to physically interact with AMBRA1 (activating molecule in BECN1-regulated autophagy protein 1), a well-known autophagy initiative component, which takes part in autophagy vesicle nucleation, a critical step for autophagic initiation. Our findings suggest PRMT6 down-regulation in HCC tumors to promote tumorigenicity through autophagic flux de-regulation. Targeting the mechanisms of stress response may provide novel therapeutic insights for the disease. Citation Format: Noelia Che, Man Tong, Kai Yu Ng, Phillis WF Kau, Xin Yuan Guan, Michael SY Huen, Stephanie Ma. Functional and mechanistic characterization of PRMT6 regulated autophagy in hepatocellular carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1349.