Modulation of cellular energetics by galactose and pioglitazone

The Warburg effect is ameliorated by culturing transformed cells in the presence of galactose instead of glucose as the primary carbon source. However, metabolic consequences may occur in addition to sensitizing the cells to mitochondrial toxins. The screening of pharmaceutical agents against transformed cells while using galactose must therefore be carefully evaluated. Pioglitazone is employed in clinical applications to treat type-2 diabetes but clearly has other off-target effects. Human hepatocellular carcinoma cells (HepG2) were cultured in glucose or galactose-containing medium to investigate the role of pioglitazone on cellular bioenergetics by calorimetry and respirometry. Compared with cells cultured in 10 mM glucose, HepG2 cells cultured in the presence of 10 mM galactose showed decreased metabolic activity as measured by cellular heat flow. Interestingly, cellular heat flow increased after the addition of pioglitazone for cells cultured in glucose, but not for cells cultured in galactose. Our calorimetric data indicated that a reduction in cellular capacity for glycolysis was the mechanism responsible for the increase in sensitivity to pioglitazone, and possibly to mitochondrial toxins in general, for cells cultured in galactose. Furthermore, oxygen consumption rates were decreased after the addition of pioglitazone to cells grown in glucose but remained unchanged for cells grown in the presence of galactose. We have demonstrated that pioglitazone induces a reduction in mitochondrial activity that is partially compensated via an increase in glycolysis in the presence of glucose.