Aspects of glutamine metabolism in human tumour cells

The importance of the non-essential amino acid, glutamine, to the proliferation of human tumour cells was investigated. All of the cells studied had a high activity of phosphate-dependent glutaminase and were found to utilise glutamine from the culture medium during long term culture. The rate of cell proliferation, determined by [6-3H]-thymidine incorporation, was dependent on glutamine concentration with the exception of Hs578T and MOLT 4 cells. The glutamine concentration giving half maximal growth (ED50) ranged from 0.02-0.24 mM. The glutamine analogue, acivicin [L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid], markedly inhibited cell proliferation in the absence of glutamine. However, in the presence of glutamine acivicin only caused inhibition of proliferation in certain cell lines. Replacement of glutamine in the culture medium by glutamate resulted in an increase in the rate of cell proliferation when compared with rates in the absence of glutamine with no glutamate supplement. In addition, cells grown in the presence of the glutamine synthetase inhibitor, methionine sulphoximine, showed a marked decrease in proliferation. These data suggested the presence of glutamine synthetase in human tumour cells, which was confirmed by radiochemical assay of maximal glutamine synthetase activity. The breast tumour cells Hs578T, with high glutamine synthetase activity, were able to proliferate at rates similar to that in the presence of glutamine, when glutamine-deficient medium was supplemented with purine nucleosides. However, the breast tumour cells MCF7, with low glutamine synthetase activity, were unable to proliferate at comparable rates in the presence of either purine or pyrimidine nucleoside supplements.