cAMP-mediated growth inhibition of lymphoid cells in G1: rapid down-regulation of cyclin D3 at the level of translation

cAMP is an important physiological mediator of lymphoid growth inhibition. The purpose of the present study was to establish the link between cAMP and the cell cycle machinery leading to inhibition of G1/S transition in human peripheral blood lymphocytes (PBL). To unravel immediate effects of cAMP o n this part of the cell cycle machinery, lymphocytes were synchronized in m id to late G1 after stimulation with phytohemaglutenin (PHA) for 32 h. We r eport that addition of forskolin or cAMP analogues to the cells resulted in dephosphorylation of retinoblastoma protein commencing as early as 30 min. A rapid effect of forskolin was noted on the activity of cyclin-dependent kinase (cdk) 4, which decreased significantly within 30 min of treatment. T he decrease in cdk4 activity was concurrent with reduced levels of cyclin D 3 protein and a decrease in the fraction of cdk4 associated with cyclin D3. The down-regulation of cyclin D3 was at the level of translation, and this event was preceded by a pronounced inhibition of Akt/protein kinase B phos phorylation at Ser 473. Taken together, our data imply that cyclin D3 is a major effector of cAMP-mediated inhibition of cell cycle progression in PBL , and that cAMP exerts its effect on cyclin D3 expression at the level of t ranslation.