The rat hepatoma cell line H4-II-E was found to express much higher activities of Na+-dependent glutamine and aspartate transport than those observed in normal cultured hepatocytes, in agreement with previous work of others on human hepatocytes. Na+-dependent glutamine transport in rat hepatoma cells could be resolved into two components. One was pH-dependent, tolerated Li+ for Na+ substitution and was inhibited only by asparagine and histidine; characteristics similar to those of transport System N in hepatocytes. The other transport system had a similar Km for glutamine but was pH independent, did not accept Li+ ions and was completely inhibited by excess concentrations of lysine, histidine, leucine, serine and cysteine, but not by methyl-aminoisobutyrate or phenylalanine. This pattern of inhibition is distinct from that of any transporter occurring in normal hepatocytes and may indicate the presence of a new transporter isoform. Similar results were obtained with the cell line HTC. Na+-dependent aspartate transport in H4 hepatoma cells was mediated by a high-affinity system (Km 5 μM) and was inhibited by D-aspartate and L-glutamate but not by D-glutamate - properties characteristic of the high-affinity glutamate transporter EAAC1. C-terminal antibodies to the EAAC1 protein recognized a single band of 58 kDa in hepatocyte membranes, but an additional strong band of 60 kDa was present in H4 hepatoma cells. These results provide further evidence for the view that tumour cells may express additional isoforms of amino acid transport systems which are not present in non-transformed cells.
The high affinity insulin receptor mediates growth stimulation in rat hepatoma cells.