Mechanism of metformin inhibiting malignant progression of hepatocellular carcinoma by promoting degradation of aldo-keto reductase AKR1C3

This study aimed to elucidate the mechanism of action of metformin (MET) in inhibiting the malignant progression of hepatocellular carcinoma (HCC) by regulating the degradation of aldo-keto reductase family 1 member C3 (AKR1C3). The correlation between the sensitivity of different hepatocellular carcinoma cell lines to MET and their basal expression levels of AKR1C3 was firstly evaluated. MET was found to significantly reduce the level and accelerate the degradation rate of AKR1C3 protein by Western blot. The interaction between MET and AKR1C3 protein was confirmed by cellular thermal shift assay (CETSA). Proteasome inhibitor MG132 and the lysosomal inhibitor chloroquine (CQ) were used to screen the degradation pathway, and confirm, in combination with the HBSS starvation-induced autophagy model, that MET mediated the degradation of AKR1C3 through the autophagy lysosome pathway. Ubiquitylation assay showed that MET specifically enhanced the K63-linked polyubiquitylation modification of AKR1C3. Sequestosome 1 (SQSTM1/p62) knockdown, immunoprecipitation, and immunofluorescence co-localization analyses confirmed that the autophagy receptor p62 plays a key role in mediating MET-induced degradation of AKR1C3. The adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK) inhibitor compound C was used to demonstrate that the regulatory effect of MET on AKR1C3 is independent of the classical AMPK signaling pathway. The experimental results showed that metformin promoted the ubiquitination modification of AKR1C3 by targeting AKR1C3, enhanced the binding of AKR1C3 to autophagy receptor p62, then degraded the AKR1C3 protein through selective autophagy-like pathway, and ultimately inhibited the malignant phenotypes of hepatocellular carcinoma cells, which is a regulatory mechanism free of the classical AMPK activation pathway of metformin.