STAUROSPORINE-INDUCED G(1) ARREST IS ASSOCIATED WITH THE INDUCTION AND ACCUMULATION OF CYCLIN-DEPENDENT KINASE INHIBITORS

The addition of 10 nM staurosporine (ST) to MDA 361 breast carcinoma c ells induces a G(1) arrest, which correlates with the loss of the cata lytic activity of the G(1)-associated cyclin-dependent kinases (cdks) and increased levels of underphosphorylated retinoblastoma protein. Th is treatment resulted in a slight but detectable reduction in the prot ein levels of cdk6 but did not reduce the levels of cdk2, cdk4, or the D cyclins. The level of cyclin E declined initially but returned to n ormal levels 24 h after exposure to 10 nM ST. Because the levels of th e G(1) cdks and cyclins did not correlate with loss of kinase activity , the role of the cdk inhibitors involved in regulating the activity o f the G(1)-associated cdks was investigated. The significant reduction in cdk activity observed in MDA 361 cells treated with ST for 24 h co rrelated with increased levels of p18 and p27(Kip). The inhibition of kinase activity of preformed cdk2 complexes by lysates of MDA 361 cell s that had been treated with 10 nM ST for 24 h was shown to be due to p27(Kip). The reduction in the level of the active phosphorylated form of cdk2 also correlated with an increase in the level of p27(Kip), wh ich has been shown to inhibit the phosphorylation of the activating Th r-160 residue of cdk2. These results indicate that treatment of MDA 36 1 cells with 10 nM ST induces a significant increase in the levels of several cdk inhibitors that appear to be responsible for the observed G(1) arrest.