THE KINASE INHIBITOR STAUROSPORINE INDUCES G(1) ARREST AT 2 POINTS - EFFECT ON RETINOBLASTOMA PROTEIN-PHOSPHORYLATION AND CYCLIN-DEPENDENT KINASE-2 IN NORMAL AND TRANSFORMED-CELLS

Staurosporine (ST), a protein kinase inhibitor, at a concentration of 20 nM arrests normal diploid fibroblasts 3 h into G(1), (H. A. Crissma n et al., Proc. Natl. Acad. Sci. USA, 88: 7580-7584, 1991; K. Abe et a l, Exp. Cell Res., 192: 122-127, 1991). ST (2 nM) induces a new G(1) a rrest point at 6 h into G(1). Partial phosphorylation of the retinobla stoma protein was observed at the 2 nM ST arrest point, whereas the re tinoblastoma protein was unphosphorylated or underphosphorylated at th e 20 nM arrest point. This correlated with the activity of the cyclin- dependent kinase 2 (CDK2) and the phosphorylation of the Thr160 residu e of p33(CDK2). The cyclin E and cyclin D1/2 levels were reduced at th e 20 nM ST arrest point. In HeLa cells that do not arrest in G(1) in r esponse to 2 or 20 nM ST, the retinoblastoma protein and CDK2 phosphor ylations and CDK2 activity were not affected by ST. These results sugg est that ST inhibits one or more G1-regulating protein kinases, which lie upstream of CDK2.