The calmodulin-dependent protein kinase-II (CaMK-II) inhibitor KN-93 has been 
shown to reversibly arrest mouse and human cells in the G1 phase of the cell 
cycle [Tombes, R. M., Westin, E., Grant. S., and Krystal, G. (1995) Cell Growth 
Differ. 6, 1073-1070; Rasmussen, G., and Rasmussen, C. (1995) Biochem. Cell 
Biol. 71, 201-207]. The stimulation of Ca(2+)-independent (autonomous) CaMK-II 
enzymatic activity, a barometer of in situ activated CaMK-II, was prevented by 
the same KN-93 concentrations that cause G1 phase arrest. KN-93 caused the 
retinoblastoma protein pRB to become dephosphorylated and the activity of both 
cdk2 and cdk4, two potential pRb kinases, to decrease. Neither the activity of 
p42MAP kinase, an early response G1 signaling molecule, nor the phosphorylation 
status or DNA-binding capability of the transcription factors serum response 
factor and cAMP responsive element-binding protein was altered during this G1 
arrest. The protein levels of cyclin-dependent kinase 2 (cdk2) and cdk4 were 
unaffected during this G1 arrest and the total cellular levels of the cdk 
inhibitors p21cip1 and p27kip1 were not increased. Instead, the cdk4 activity 
decreases resulting from KN-93 were the result of a 75% decrease in cyclin D1 
levels. In contrast, cyclin A and E levels were relatively constant. Cdk2 
activity decreases were primarily the result of enhanced p27kip1 association 
with cdk2/cyclin E. All of these phenomena were unaffected by KN-93's inactive 
analog, KN-92, and were reversible upon KN-93 washout. The kinetics of recovery 
from cell cycle arrest were similar to those reported for other G1 phase 
blockers. These results suggest a mechanism by which G1 Ca2+ signals could be 
linked via calmodulin-dependent phosphorylations to the cell cycle-controlling 
machinery through cyclins and cdk inhibitors.