GLUCOCORTICOIDS INDUCE G1 AS WELL AS S-PHASE LENGTHENING IN NORMAL HUMAN STIMULATED LYMPHOCYTES - DIFFERENTIAL-EFFECTS ON CELL-CYCLE REGULATORY PROTEINS

In order to analyze dexamethasone effects on peripheral blood lymphocy te proliferation, we defined various experimental conditions: dexameth asone introduced (i) at the time of phytohemagglutinin stimulation, (i i) 48 h after the beginning of phytohemagglutinin stimulation, and (ii i) on unstimulated lymphocytes. In stimulated lymphocytes, we observed an early G1 accumulation (P < 0.005), a delayed increase in the durat ion of S-phase (P < 0.03), and a consequent increase in cell-cycle dur ation. The expression of several cyclins, cyclin-dependent kinases (CD Ks), and CDK inhibitors (CKIs) was modified. Cyclin D3, CDK4, and CDK6 involved in G1-phase control were significantly decreased under dexam ethasone treatment whatever the level of stimulation of lymphocytes (s timulated or unstimulated PBL). Cyclin E and CDK2, acting in G1/S-phas e transition and S-phase regulation, decreased in stimulated lymphocyt es before any modification of S-phase (P < 0.002). The expression of C KIs, mainly of p27(Kip1), appeared to vary with the degree of cell sti mulation: a decrease was observed on treated unstimulated lymphocytes, while p27(Kip1) increased in dexamethasone-treated cells during stimu lation. Our results indicate sequential modifications of the cell-cycl e regulation by dexamethasone starting with an action on G1 followed b y S-phase control modifications. The protein analysis pinpoints the ma jor complexes concerned: CDK4 and CDK6/cyclin D are mainly involved in G1-phase modifications, while CDK2 and its partner, cyclin E, might b e specifically involved in the lengthening of S-phase. The variations observed for p27(Kip1) might amplify the functional effects of dexamet hasone on kinasic complexes. (C) 1998 Academic Press.