Glutamine transport in isolated human hepatocytes and transformed liver cells

The transport of L-glutamine was examined in isolated adult and fetal human hepatocytes as well as in the human hepatoma cell lines HepG2 and SK-Hep. In all cells studied, glutamine uptake was at least 85% Na(+)-dependent. Kinetic analysis of the Na(+)-dependent component indicated mediation by a single transporter in three human hepatocyte preparations and in SK-Hep cells, whereas two transporters appeared to be responsible for glutamine uptake in HepG2 cells and in hepatocytes from the liver of one male patient. Amino acid inhibition analysis showed primary mediation by System N in fetal and adult hepatocytes, whereas System ASC was principally responsible for glutamine uptake in transformed cells. Similar to the rat transporter, human System N was pH-sensitive, stereospecific, and responsive to treatment with steroid hormones. Although the human carrier was less tolerant of Li(+)- for Na+ substitution, glutamine transport in primary human hepatocytes was stimulated by treatment with hypotonic buffer (cell swelling), as reported in rat parenchymal cells. In contrast, glutamine transport in hepatoma cells was relatively insensitive to changes in extracellular pH and failed to show enhanced activity in response to hypoosmotic challenge. Collectively, the data suggest that markedly distinct plasma membrane transporters mediate the concentrative uptake of glutamine in normal and transformed human hepatocytes, and that the salient properties of System N have been largely conserved from rat to man.