ACTIVIN A INDUCTION OF CELL-CYCLE ARREST INVOLVES MODULATION OF CYCLIN D2 AND P21(CIP1 WAF1) IN PLASMACYTIC CELLS/

Activins, members of the transforming growth factor-beta family, have been implicated in the regulation of growth and differentiation of var ious types of cells. We have recently found that activin A induces apo ptotic cell death of plasmacytic cells including B cell hybridoma cell s and myeloma cells. In the present study, we demonstrated that activi n A caused cell-cycle arrest in the G1 phase before appearance of apop totic cells in mouse B cell hybridoma cells. Phosphorylation of retino blastoma protein (Rb) and in vitro, Rb kinase activity of cyclin-depen dent kinase (CDK)4 was inhibited in activin A-treated cells. Analysis of expression of genes regulating Rb phosphorylation revealed that act ivin A suppressed cyclin D2, the sole D-type cyclin gene expressed in the hybridoma cells, and activated p21(CIP1/WAF1) but had no effect on expression of cyclin-dependent kinases (CDK2, CDK4, CDK6) and other C DK inhibitors (p27(KIP1), p16(INK4a), p15(INK4b)). Modulation of cycli n D2 and p21(CIP1/WAF1) expression resulted in a decrease in level of cyclin D2-CDK4 complex and an increase in level of CDK4 complexed with p21(CIP1/WAF1). Moreover, overexpression of cyclin D2 partially abrog ated inhibition of Rb phosphorylation and G1 arrest in the hybridoma c ells.