TMEM166 negatively regulates unfolded protein response to affect hepatocellular carcinoma cell growth and sorafenib resistance.

Transmembrane protein 166 (TMEM166), an endoplasmic reticulum (ER)-resident membrane protein, exerts anticancer effects by inducing autophagy and apoptosis. Although tissues of various cancers downregulate its expression, the biological function of TMEM166 in hepatocellular carcinoma (HCC) remains unclear. Herein, we report that TMEM166 negatively regulates unfolded protein response (UPR) in HCC. TMEM166 was noted to interact with ACSL3 to maintain ACSL3 stability and facilitate lipid storage. TMEM166 deletion reduced ACSL3 expression and increased lipid utilisation in the mitochondria through fatty acid β-oxidation (FAO), ultimately boosting ATP production. Moreover, TMEM166-knockout (KO) cells demonstrated accelerated protein synthesis via the AMPK-mTOR axis. These effects induced sublethal ER stress and UPR activation in TMEM166-KO cells. Furthermore, TMEM166 KO promoted HCC cell proliferation and sorafenib resistance via UPR activity upregulation. We analysed the clinical significance of TMEM166-regulated UPR in human HCC cells and noted that TMEM166 expression was negatively correlated with the activities of UPR-related transcriptional factors such as ATF4, ATF6 and XBP1s in the cells. This study is the first to elucidate the relationship among TMEM166, ER stress, and HCC and may provide and indicate newer avenues for TMEM166-targeted gene therapy strategies for HCC treatment.