GLUCOCORTICOID REGULATION OF G(1) CYCLIN-DEPENDENT KINASE GENES IN LYMPHOID-CELLS

These experiments were undertaken to study cell cycle-dependent regulation of expression of genes encoding cyclin-dependent kinases (Cdks).(3) P1798 T-lymphoma cells were studied as a model system, since these cells undergo reversible G(0), arrest within 24 h after addition of 0.1 mu M dexamethasone to mid log phase cultures. G(0) arrest is associated with inhibition of expression of several Cdks. The mRNAs encoding Cdk1 and Cdk4 decreased by 80-90% within 24 h. Fifty % inhibition of Cdk4 mRNA occurred within about 4 h, and 50% inhibition of Cdk1 mRNA was observed within 12-14 h. There was a slight decrease (<50%) in the abundance of the mRNAs encoding Cdk2 and Cdk5. Cdk6 mRNA did not decrease in glucocorticoid-treated cells. Cdk1 and Cdk2 protein revels were reduced by no more than 50-70% within 24 h after the addition of dexamethasone, and the amounts of Cdk5 and Cdk6 protein did not change. However, the amount of Cdk4 protein decreased by >90% under these circumstances. P1798 cells enter S phase in a synchronous fashion within 16-20 h after removal of dexamethasone. Cdk1, Cdk2, and Cdk5 mRNAs and proteins increased at or after the time that cells began to enter S phase. The mRNA encoding Cdk4 increased much more rapidly after removal of glucocorticoids, and a 5-fold increase in Cdk4 mRNA abundance was observed within 8 h after removal of the steroid. A corresponding increase in Cdk4 protein was observed, indicating that inhibition of Cdk4 expression is more proximal to the glucocorticoid-induced blockade in G(1) progression. Glucocorticoids also inhibited Cdk4 expression in cells that were arrested at the G(1) S boundary by thymidine block, but expression of Cdk1 was not inhibited in such cells. This observation indicates that glucocorticoid regulation of Cdk4 is not dependent on cell cycle progression, whereas inhibition of Cdk1 is probably secondary to G, arrest. Nuclear run-on data indicate that dexamethasone inhibits transcription of Cdk4 in P1798 cells. Furthermore, glucocorticoids inhibited expression of Cdk4 in activated splenocytes, with no significant effect on Cdk6 expression. Glucocorticoids regulate expression of several G(1) cyclin-dependent kinase genes. Some of these (such as Cdk1) are inhibited as cells withdraw into G(0). Cdk4 appears to be directly regulated by glucocorticoids, and inhibition of this G(1) cyclin-dependent kinase may play a role in glucocorticoid-mediated G(0) arrest of lymphoid cells.