RAS TRANSFORMATION RESULTS IN AN ELEVATED LEVEL OF CYCLIN D1 AND ACCELERATION OF G(1) PROGRESSION IN NIH 3T3 CELLS

Ectopic overexpression of v-H-Ras protein in NIH 3T3 cells resulted in cellular transformation and an acceleration of G(1) progression of th ese cells. A shortened G(1) phase was found to be associated with an i ncreased level of cyclin D1 but not cyclin E protein. Using an antisen se blocking method, reduced synthesis of cyclin D1 in v-H-Ras transfor mants resulted in a slower G(1) progression rate of these cells. Altho ugh constitutive overexpression of cyclin D1 in NIH 3T3 cells accelera ted G(1) progression, cells remained untransformed. Furthermore, inhib ition of cyclin D1 synthesis greatly impaired the soft-agar cloning ef ficiency of v-H-Ras transformants. These results suggest that increase d expression of cyclin D1 is necessary but not sufficient for the tran sforming activity of v-H-Ras. Similar effect on cell cycle progression was also observed in Raf-transformed cells, In addition to cyclin D1, cyclin E protein was found to be elevated in Src transformants, This may account for the further shortening of the G(1) phase of these cell s. Activation of an additional Ras-independent pathway was suggested t o be responsible for the further acceleration of the G(1) phase in Src transformants.