Fructose metabolism in hepatocytes: primary cells or cell lines?

Abstract

Aims: The beneficial action of D-fructose in parenteral nutrition was attributed to the fact that this hexose is metabolized more rapidly than glucose, and that its metabolization is independent of insulin (1). Since it became known that a high intravenous dose of fructose reduces the intracellular ATP levels in human and murine liver, use of fructose in parenteral nutrition was discontinued. Recently, we reported that effective anti-neoplastic therapy with melphalan requires sufficient intracellular ATP levels (2,3). It still remains open, whether or not fructose-mediated ATP depletion has any significance on cell death or viability of healthy liver parenchymal cells. Aims: We compared the activity of fructose metabolizing enzymes in primary hepatocytes and commonly used cell lines to determine whether cell lines can model hepatocyte fructose metabolism. Results: The specific activity of fructokinase and aldolase b was investigated in the cell lines HepG2, Huh7 and in primary hepatocytes. The specific activity of fructokinase was similar in all three cell types. We detected a lower activity of aldolase b in the two cell lines in comparison with the primary hepatocytes. Additionally, the activity of hexokinases, which catalyze the first step of the fructose metabolism in muscle and other tissue, was determined. Compared to the primary cells, the specific activity of hexokinases was greater in HepG2, Huh7 and also in the murine cell lines Hepa 1–6 and AML–12. Western blot analysis revealed a stronger expression level of Hexokinase II (HK II) in all cell lines. These results indicated that the high activity of HK II might result in a shift from liver-type to muscle-type fructose metabolism in the cell-lines. Therefore, the HK II was inhibited with 3-bromopyruvate in HepG2 cells to study the resulting fructose metabolism. Under HK II inhibition, fructose resulted in a decrease of the ATP-levels in HepG2-cells comparable to that in primary hepatocytes in the absence of 3-bromopyruvate. Conclusions: Our data provide evidence that commonly used cell lines are not suitable to study fructose metabolism in hepatocytes, due to specific changes in enzyme activity in the cell lines. This appears to result in a shift from liver-type to muscle-type fructose metabolism in HepG2-cells. These findings suggest that in the presence of fructose, primary hepatocytes could be less susceptible to chemotherapeutics such as melphalan than neoplastically transformed cells.