Serum starved v-mos-transformed cells are unable to appropriately downregulate cyclins and CDKs

Abstract

Serum deprived v-mos-transformed NIH3T3 cells are unable to enter a true quiescent state, but instead, arrest in the early G1 phase of the cell cycle. We have analysed several cell cycle regulatory proteins in these G1 arrested cells and show altered regulation in the expression and activity of certain cyclins and cyclin-dependent kinases. In particular, p34^cdc2, cyclin A, cyclin D and cyclin E are not appropriately down-regulated in serum starved, G1 arrested, v-mos-transformed cells as compared with quiescent NIH3T3 cells. Furthermore, serum starved v-mos-transformed cells have elevated histone H1 kinase activity associated with cyclin A, cyclin E, p33^cdk2, and p34^cdc2. Using a metallothionein-inducible c-mos^mu expression system, we show that c-mos^mu induction in quiescent NIH3T3 cells causes elevated expression of p34^cdc2. However, this induction of c-mos^mu and subsequent expression of p34^cdc2 was not sufficient to promote significant entry of cells into S phase. Analysis of extracts from serum starved v-H-ras, v-src, and tpr-met transformed NIH3T3 cells demonstrates that these oncogene-transformed cells also contain elevated levels of p34^cdc2. We propose that the altered regulation of these critical cell cycle regulatory molecules, and specifically the inability to fully downregulate their activity, contributes significantly to neoplastic transformation and subsequent unregulated growth of tumor cells.