[Function of an inhibitor of cyclin-dependent kinase p27/Kip in cells transformed by E1A + E1B19 kDa + E1A + cHa-Ras, differing in their ability to realize a G1-block during serum starvation].

We studied the capability of E1A + cHa-ras and E1A + E1B19kDa transformants to undergo the G1/S arrest of the cell cycle following depletion of serum growth factors. It has been shown that serum starvation induced the G1/S arrest both in normal rat embryo fibroblasts (REF) and in E1A + E1B19kDa transformants, whereas E1A + cHa-ras transformed cells lost this feature. To analyse the mechanisms underlying these differences, we studied the expression of p27/KIP, its intracellular distribution and association with E1A oncoproducts. The content of the p27/KIP inhibitor of cyclin-dependent kinases was found to change a little upon transformation by two complementary oncogene pairs. However, serum starvation for 24 h led to a significant increase in the content of p27/KIP in E1A + E1B19kDa transformants, while E1A + cHa-ras cells accumulated p27/KIP less markedly. According to the immunofluorescence study, the p27/KIP inhibitor is located in the nucleus of both normal and transformed cells. Moreover, serum starvation did not lead to its inhibition due to redistribution to the cytoplasm in both cell lines. Also, we were unable to detect association of p27/KIP with E1A oncoproducts in immunoprecipitated complexes. The obtained data indicate that, in contrast to E1A + cHa-ras transformants, in E1A + E1B19kDa cells the p27/KIP inhibitor is functional and it is capable of inducing the G1/S block after serum starvation.