Suppression of ACADM-mediated fatty acid oxidation by caveolin-1 contributes to hepatocellular carcinoma progression

Abstract

In hepatocellular carcinoma (HCC), one of the metabolic changes is the way that fatty acids are metabolised in cancer cells. Fatty acids are essential building blocks of membranes and signalling molecules, which also provide energy for the growth and aggressive activities of cancer cells. Caveolin-1 (Cav1), a caveolae protein responsible for signal transduction across the cell membrane, has been reported to be a metastasis promoter in HCC. Caveolae has been shown to play critical roles in lipid uptake and supply to the lipid droplets in adipocytes. Based on the emerging roles of Cav1 in cancer metabolism, we endeavoured to explore whether and how Cav1 regulates metabolism in HCC.

In metastatic HCC cells, in which Cav1 is highly expressed, knockdown of Cav1 reduced cellular lipid contents in cells, and inhibited HCC cell growth, motility and tumorigenesis. Treatment using an inhibitor of mitochondrial fatty acid β-oxidation elevated fatty acid content and restored the attenuated aggressiveness of Cav1 knockdown cells. This suggests the novel role of Cav1 in suppressing β-oxidation, leading to the acquisition of increased aggressive behaviour of HCC cells. Among the β-oxidation regulators, acyl co-enzyme A dehydrogenase, medium chain (ACADM), which is an enzyme that catalyses the first step of β-oxidation, was found to negatively correlate with Cav1 in HCC cell lines, in-house HCC tissues and the TCGA database of HCC. Indeed, the level of ACADM was upregulated in Cav1 knockdown cells; suppression of ACADM in these cells restored HCC cell aggressiveness and cellular lipid content. Furthermore, knockdown of ACADM recapitulated the functional effect of Cav1 overexpression by displaying elevated levels of triglycerides and phospholipids, with enhanced HCC cell growth and motility. The hydrodynamic injection mouse model demonstrated that ACADM knockout accelerated N-RasV12/TP53-knockoutinduced HCC development. Clinically, downregulation of ACADM was observed in more than 64% of HCC clinical samples, correlating to several aggressive clinicopathological features such as bigger tumour size, advanced tumour stage, venous invasion and poor cell differentiation. Further investigation confirmed sterol regulatory element-binding protein-1c (SREBP-1c) as an upstream regulator of ACADM, which was in turn regulated by Cav1. The overexpression of SREBP-1c in Cav1 knockdown cells augmented their cell aggressiveness; inhibition of Cav1 resulted in the suppression of SREBP-1c nuclear translocation from the cytoplasm. Treatment of HCC tumours with the combinational administration of sorafenib and eicosapentaenoic acid, an inhibitor of SREBP-1c, significantly reduced tumour growth, revealing a potential novel therapy for HCC management.

Taken together, our current findings suggest that the deregulation of β-oxidation mediated by the Cav1/SREBP-1c/ACADM axis results in HCC progression. Our study implicates the limitation of fatty acid availability to cancer cells as a therapeutic strategy in HCC.