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

These experiments were undertaken to study cell cycle-dependent regula tion 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 associa ted with inhibition of expression of several Cdks. The mRNAs encoding Cdk1 and Cdk4 decreased by 80-90% within 24 h. Fifty % inhibition of C dk4 mRNA occurred within about 4 h, and 50% inhibition of Cdk1 mRNA wa s observed within 12-14 h. There was a slight decrease (<50%) in the a bundance of the mRNAs encoding Cdk2 and Cdk5. Cdk6 mRNA did not decrea se in glucocorticoid-treated cells. Cdk1 and Cdk2 protein revels were reduced by no more than 50-70% within 24 h after the addition of dexam ethasone, and the amounts of Cdk5 and Cdk6 protein did not change. How ever, the amount of Cdk4 protein decreased by >90% under these circums tances. P1798 cells enter S phase in a synchronous fashion within 16-2 0 h after removal of dexamethasone. Cdk1, Cdk2, and Cdk5 mRNAs and pro teins 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 obse rved within 8 h after removal of the steroid. A corresponding increase in Cdk4 protein was observed, indicating that inhibition of Cdk4 expr ession 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 expr ession of Cdk1 was not inhibited in such cells. This observation indic ates that glucocorticoid regulation of Cdk4 is not dependent on cell c ycle progression, whereas inhibition of Cdk1 is probably secondary to G, arrest. Nuclear run-on data indicate that dexamethasone inhibits tr anscription of Cdk4 in P1798 cells. Furthermore, glucocorticoids inhib ited expression of Cdk4 in activated splenocytes, with no significant effect on Cdk6 expression. Glucocorticoids regulate expression of seve ral G(1) cyclin-dependent kinase genes. Some of these (such as Cdk1) a re inhibited as cells withdraw into G(0). Cdk4 appears to be directly regulated by glucocorticoids, and inhibition of this G(1) cyclin-depen dent kinase may play a role in glucocorticoid-mediated G(0) arrest of lymphoid cells.