Modulation of Cellular Proliferation Alters Glutamine Transport and Metabolism in Human Hepatoma Cells

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Abstract
The authors determined the effects of growth inhibition on glutamine transport and metabolism in human hepatoma cells. Hepatoma cells exhibit markedly higher (10- to 30-fold) glutamine uptake than normal human hepatocytes, via a disparate transporter protein with a higher affinity for glutamine. Currently, little is known about the effects of growth arrest on glutamine transport and metabolism in hepatoma cells. The authors determined proliferation rates, glutamine transport, and glutaminase activities in the human hepatoma cell lines HepG2, Huh-7, and SK-Hep, both in the presence and absence of the chemotherapeutic agents novobiocin and sodium butyrate. The transport activities for alanine, arginine, and leucine also were determined in both treated and untreated cells. Glutaminase activity was determined in normal human liver tissue and compared with that present in hepatoma cells. Glutaminase activities were similar in all three cell lines studied, despite differences in proliferation rates, and were sixfold higher than the activity in normal human liver. In contrast to normal hepatocytes, which expressed the liver-specific glutaminase, hepatomas expressed the kidney-type isoform. Sodium butyrate (1 mmol/L) and novobiocin (0.1 mmol/L) inhibited cellular proliferation and reduced both glutamine transport and glutaminase activity by more than 50% after 48 hours in the faster-growing, less differentiated SK-Hep cells. In contrast, the agents required 72 hours to attenuate glutamine uptake by 30% and 50% in the slower-growing, more differentiated HepG2 and Huh-7 cell lines, respectively. Treatment of all three cell lines with novobiocin/butyrate also resulted in a 30% to 60% attenuation of the transport of alanine, arginine, and leucine, and glutamine, indicating that inhibition of cellular proliferation similarly affects disparate amino acid transporters. Hepatocellular transformation is characterized by a marked increase in glutamine transport and metabolism. Inhibition of cellular proliferation attenuates glutamine transport and metabolism, especially in fast-growing, relatively undifferentiated hepatoma cells. Because the uptake of other amino acids is similarly reduced under cytostatic conditions, plasma membrane amino acid transport activity in hepatoma cells is regulated by the proliferation state of the cells.