Abstract
Metabolic changes are associated with cancer, but whether they are just bystander effects of deregulated oncogenic signaling pathways or characterize early phases of tumorigenesis remains unclear. Here we show in a rat model of hepatocarcinogenesis that early preneoplastic foci and nodules that progress towards hepatocellular carcinoma (HCC) are characterized both by inhibition of oxidative phosphorylation (OXPHOS) and by enhanced glucose utilization to fuel the pentose phosphate pathway (PPP). These changes respectively require increased expression of the mitochondrial chaperone TRAP1 and of the transcription factor NRF2 that induces the expression of the rate-limiting PPP enzyme glucose-6-phosphate dehydrogenase (G6PD), following miR-1 inhibition. Such metabolic rewiring exclusively identifies a subset of aggressive cytokeratin-19 positive preneoplastic hepatocytes and not slowly growing lesions. No such metabolic changes were observed during non-neoplastic liver regeneration occurring after two/third partial hepatectomy. TRAP1 silencing inhibited the colony forming ability of HCC cells while NRF2 silencing decreased G6PD expression and concomitantly increased miR-1; conversely, transfection with miR-1 mimic abolished G6PD expression. Finally, in human HCC patients increased G6PD expression levels correlates with grading, metastasis and poor prognosis. Our results demonstrate that the metabolic deregulation orchestrated by TRAP1 and NRF2 is an early event restricted to the more aggressive preneoplastic lesions.
Highlights
Most neoplastic cells increase glucose utilization and uncouple it from oxygen availability to face hypoxic conditions that might occur in the core of a fast growing tumor mass [1]
We investigated the metabolic features of early preneoplastic foci (EPF) in the RH model, characterized by a synchronous expansion of carcinogen-initiated cells that can be identified by preneoplastic markers, such as the glutathione S-transferase, placental form (GST-P), as early as 3-7 days after partial hepatectomy (PH)
Compared to RNT cells RH cells expressed higher protein levels of TRAP1 (Figure 3C), and a lower succinate-coenzyme Q reductase (SQR) activity of succinate dehydrogenase (SDH), that could be enhanced by 17-AAG, which was ineffective in RNT cells (Supplementary Figure S3F). in accord with a central role played by TRAP1 in energy metabolism of tumor cells, we found that knocking-down TRAP1 increased both basal and maximal oxygen consumption rate of RH cells (Supplementary Figure S4A)
Summary
Most neoplastic cells increase glucose utilization and uncouple it from oxygen availability to face hypoxic conditions that might occur in the core of a fast growing tumor mass [1]. This metabolic shift, termed aerobic glycolysis or Warburg effect [2, 3, 4], is required to sustain the anabolic needs of tumor cells [5], mainly by conveying metabolites into the pentose phosphate pathway (PPP) that provides both biosynthetic building blocks and anti-oxidant defenses [6]. Succinate acts as an oncometabolite [22], implying that metabolic changes elicited by TRAP1 directly contribute to tumor growth [18]
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