Hepatocellular carcinoma redirects to ketolysis for progression under nutrition deprivation stress

Abstract

Cancer cells are known for their capacity to rewire metabolic pathways to support survival and proliferation under various stress conditions. Ketone bodies, though produced in the liver, are not consumed in normal adult liver cells. We find here that ketone catabolism or ketolysis is re-activated in hepatocellular carcinoma (HCC) cells under nutrition deprivation conditions. Mechanistically, 3-oxoacid CoA-transferase 1 (OXCT1), a rate-limiting ketolytic enzyme whose expression is suppressed in normal adult liver tissues, is re-induced by serum starvation-triggered mTORC2-AKT-SP1 signaling in HCC cells. Moreover, we observe that enhanced ketolysis in HCC is critical for repression of AMPK activation and protects HCC cells from excessive autophagy, thereby enhancing tumor growth. Importantly, analysis of clinical HCC samples reveals that increased OXCT1 expression predicts higher patient mortality. Taken together, we uncover here a novel metabolic adaptation by which nutrition-deprived HCC cells employ ketone bodies for energy supply and cancer progression.

Aberrant ketolysis fuels hepatocellular cancer progression

“…Similar results were obtained with other human HCC cell lines, including Hep3B and PLC cells, and OXCT1 upregulation driven by serum withdrawal was confirmed by immunoblotting [4]. In line with this finding, exogenously supplied β-hydroxybutyrate (a ketone body) was converted by HCC cells subjected to starvation (but not by non-transformed human liver THLE3 cells, nor by HCC cells maintained in control conditions) into various intermediates of the tricyclic acid cycle, including citrate, succinate, fumarate, malate, glutamate and aspartate, supporting a time-dependent recovery in intracellular ATP levels and proliferation rates [4]. Importantly, the stable downregulation of OXCT1 by two distinct shRNA-coding constructs virtually abolished the ability of HepG2 cells undergoing serum deprivation to catabolize exogenous β-hydroxybutyrate.…”

“…Surprisingly, HepG2 cells subjected to serum (but not glucose or glutamine) deprivation also exhibited increased levels of 3-oxoacid CoA-transferase 1 (OXCT1), which catalyzes the rate-limiting reaction of ketolysis and is normally not expressed in the adult liver [7]. Similar results were obtained with other human HCC cell lines, including Hep3B and PLC cells, and OXCT1 upregulation driven by serum withdrawal was confirmed by immunoblotting [4]. In line with this finding, exogenously supplied β-hydroxybutyrate (a ketone body) was converted by HCC cells subjected to starvation (but not by non-transformed human liver THLE3 cells, nor by HCC cells maintained in control conditions) into various intermediates of the tricyclic acid cycle, including citrate, succinate, fumarate, malate, glutamate and aspartate, supporting a time-dependent recovery in intracellular ATP levels and proliferation rates [4].…”

“…Of note, β-hydroxybutyrate was not exogenously supplied in this last set of experiments, suggesting that HCC can employ endogenously produced ketone bodies to proliferate despite serum deprivation. Corroborating this hypothesis, the depletion of HMGCS2 virtually abolished the proliferative advantage conferred to serum-depleted HepG2 cells by OXCT1 overexpression, unless exogenous β-hydroxybutyrate was added to the culture medium [4].…”

“…Moreover, the ability of mTORC2-activated AKT1 to drive the upregulation of OXCT1 was mechanistically ascribed to the transcription factor SP1. Thus, similar to their AKT1-deprived counterparts, SP1-depleted HepG2 cells were unable to catabolize exogenous β-hydroxybutyrate [4]. Interestingly, the ability of HepG2 cells to upregulate OXCT1 was associated with limited activation of the energy sensor AMP-activated protein kinase (AMPK) upon serum withdrawal [9], presumably reflecting an improved capacity to preserve bioenergetic homeostasis via ketolysis.…”

“…Although less universal and less specific than initially thought (meaning that different cancers can exhibit quite distinct metabolic shifts, and that at least some of the metabolic alterations that accompany malignancy are also found in highly proliferating non-transformed cells), such a rewiring process provides putative targets for the development of novel anticancer agents [3]. Recent work from Huafeng Zhang's group identifies a novel metabolic circuitry based on ketolysis through which hepatocellular carcinoma (HCC) cells proliferate in spite of adverse microenvironmental conditions [4].…”