Differences in GTPase-activating protein activity between liver tumors and normal liver tissue in mice

The intrinsic GTPase activity of the cellular protein p21ras is strongly increased by two cytosolic proteins, the GTPase-activating protein (GAP) produced by the neurofibromatosis type 1 gene (NF1-GAP) and a GAP of 120 kDa molecular mass (p120-GAP). The GAP-mediated stimulation of p21ras GTPase activity was measured in cytosol obtained from carcinogen-induced liver tumors and normal liver tissues of mice of two strains, namely C3H/He and C57BL/6J. For this purpose, cytosolic extracts were incubated with recombinant human p21ras complexed to [gamma-32P]GTP and the time-dependent decrease in p21ras bound radioactivity was measured. Liver cytosolic extracts mediated an increase in the GTPase activity of wild-type p21ras. There were great differences between tumor and normal tissues in the maximal velocity (Vmax) and in the apparent Michaelis constant (KM) of the p21ras GTPase reaction. Both Vmax and apparent KM were decreased in the liver tumors. Cytosolic extracts isolated from liver tumors that harbored point mutations in codon 61 of the c-H-ras gene did not differ in their activity from extracts obtained from non-mutated liver tumors. Since both GAP proteins are important cellular regulators of the ras signaling pathway and probably also effectors of p21ras, the observed differences in GAP activity may be of relevance for the tumorigenic process in mouse liver.