Early induction of cyclin D2 expression in phorbol ester-responsive B-1 lymphocytes

B-l lymphocytes represent a distinct B cell subset with characteristic feat ures that include self-renewing capacity and unusual mitogenic responses. B -l cells differ from conventional B cells in terms of the consequences of p horbol eater treatment: B-l cells rapidly enter S phase in response to phor bol eater alone, whereas B-2 cells require a calcium ionophore in addition to phorbol ester to trigger cell cycle progression. To address the mechanis m underlying the varied proliferative responses of B-l and B-2 cells, we ev aluated the expression and activity of the G1 cell cycle regulator, cyclin D2, and its associated cyclin-dependent kinases (Cdks). Cyclin D2 expressio n was upregulated rapidly, within 2-4 h, in phorbol ester-stimulated B-l ce lls, in a manner dependent on intact transcription/translation, but was not increased in phorbol ester-stimulated B-2 cells. Phorbol ester-stimulated cyclin D2 expression was accompanied by the formation of cyclin D2-Cdk4, an d, to a lesser extent, cyclin D2-Cdk6, complexes; cyclin D2-containing comp lexes were found to be catalytically functional, in terms gf their ability to phosphorylate exogenous Rb in vitro and to specifically phosphorylate en dogenous Rb on serine(780) in vivo. These results strongly suggest that the rapid induction of cyclin D2 by a normally nonmitogenic phorbol ester stim ulus is responsible for B-l cell progression through G1 phase. The ease and rapidity with which cyclin D2 responds in B-l cells may contribute to the proliferative features of this subset.