Selective guanosine phosphate deficiency in hepatoma cells induced by inhibitors of IMP dehydrogenase.

Inhibition of IMP dehydrogenase in AS-30D hepatoma cells in suspension culture resulted in a pronounced and selective reduction of guanine nucleotide pools. Total acid-soluble guanine nucleotides decreased to 40% and the content of GTP and GDP dropped to about 20% of control within 4 h when mycophenolate or ribavirin were used as the inhibitors. Induction of GTP deficiency was associated with a 50% rise in UTP and other uracil nucleotides. Guanosine rapidly reversed both the reduction of guanine nucleotide pools and the elevation of cellular UTP contents. Enzymatic nucleotide analyses in cell and tissue extracts after treatment with ribavirin indicated that ribavirin 5'-triphosphate was an effective substrate for yeast hexokinase, yeast phosphoglycerate kinase, and nucleosidediphosphate kinase from yeast or bovine liver. These results were confirmed in detail by the use of synthetic ribavirin 5'-triphosphate and 5'-diphosphate. The latter nucleotide analog was also a substrate of pyruvate kinase from muscle. Mycophenolate-induced GTP deficiency was associated with an arrest of hepatoma cell growth in suspension culture. Ribavirin, at an equimolar concentration, was much less effective in this respect. None of the two inhibitors had a detectable effect, however, in vivo when guanine or uracil nucleotides were assayed in liver. This indicated that an inhibition of de novo guanylate synthesis in vivo can be compensated by salvage pathway synthesis.