The kinase inhibitor staurosporine induces G1 arrest at two 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 G1 (H. A. Crissman et al., Proc. Natl. Acad. Sci. USA, 88: 7580-7584, 1991; K. Abe et al., Exp. Cell Res., 192: 122-127, 1991). ST (2 nM) induces a new G1 arrest point at 6 h into G1. Partial phosphorylation of the retinoblastoma protein was observed at the 2 nM ST arrest point, whereas the retinoblastoma protein was unphosphorylated or underphosphorylated at the 20 nM arrest point. This correlated with the activity of the cyclin-dependent kinase 2 (CDK2) and the phosphorylation of the Thr160 residue of p33CDK2. The cyclin E and cyclin D1/2 levels were reduced at the 20 nM ST arrest point. In HeLa cells that do not arrest in G1 in response to 2 or 20 nM ST, the retinoblastoma protein and CDK2 phosphorylations and CDK2 activity were not affected by ST. These results suggest that ST inhibits one or more G1-regulating protein kinases, which lie upstream of CDK2.