Orexins are hypothalamic
neuropeptides that regulate feeding, reward, wakefulness and energy homeostasis. The present study sought to characterize the involvement of
orexin A in
glucose metabolism in HepG2 human
hepatocellular carcinoma cells, and investigated the role of
hypoxia-inducible factor-1α (HIF-1α) in the response. HepG2 cells were exposed to different concentrations of
orexin A (10-9 to 10-7 M) in vitro, without or with the
orexin receptor 1 (OX1R) inhibitor (SB334867), HIF-1α inhibitor (YC-1) or a combination of both inhibitors. Subsequently, OX1R, HIF-1α expression and localization,
glucose uptake,
glucose transporter 1 (GLUT1) expression and
ATP content were measured. We further investigated the intracellular fate of
glucose by measuring the gene expression of
pyruvate dehydrogenase kinase 1 (PDK1),
lactate dehydrogenase (LDHA) and
pyruvate dehydrogenase B (PDHB), as well as metabolite levels including
lactate generation and mitochondrial
pyruvate dehydrogenase (PDH) activity. The activity of
phosphoinositide 3-kinase (PI3K)/Akt/
mammalian target of rapamycin (mTOR) pathway was also assessed. Our results showed that the expression of OX1R was predominantly located in the nucleus in HepG2 cells.
Orexin A oxygen-independently promoted the
mRNA and
protein expression of HIF-1α as well as its nuclear accumulation in HepG2 cells and the elevated HIF-1α
protein was associated, at least partly, with the activation of the PI3K/Akt/mTOR pathway.
Orexin A stimulated GLUT1 expression,
glucose uptake as well as
ATP generation in HepG2 cells via OX1R acting through the HIF-1α pathway. Moreover,
orexin A inhibited LDHA, PDK1 expression and
lactate production, stimulated PDHB expression and PDH
enzyme activity independent of HIF-1α. Our results indicated that
orexin signaling facilitated the
glucose flux into mitochondrial oxidative metabolism rather than glycolysis in HepG2 cells. These findings provide new insight into the regulation of
glucose metabolism by
orexin A in
hepatocellular carcinoma cells.