REGULATION OF THE CATALYTIC SUBUNIT (P34(PSK-J3) CDK4) FOR THE MAJOR D-TYPE CYCLIN IN MATURE B-LYMPHOCYTES/

We examined the expression of the cyclin-dependent kinase 4, p34(PSK-J 3)/Cdk4 protein, in small dense, activated, and proliferating primary B lymphocytes. A small steady state level of cdk4 synthesis was detect ed in resting B cells, Stimulation of resting B cells with mitogenic a mounts of F(ab')(2) fragments of goat anti-mouse IgM (anti-Ig) resulte d in increased synthesis of cdk4 protein during the mid to late G(1) p hase of the cell cycle; LPS or the combination of phorbol ester and ca lcium ionophore also elevated cdk4 levels, Resting B cells that were r endered competent by treatment with IL 4 or low doses of anti-Ig or, a lternatively, were activated by phorbol ester or ionomycin alone also exhibited heightened cdk4 protein levels, Subsequent analysis of poten tial cdk4 regulatory subunit D-type cyclins revealed that cyclin D2, n ot cyclin D1 or D3, is expressed in primary mature B lymphocytes. The induction of cyclin D2 synthesis in response to mitogenic anti-Ig para lleled cdk4 expression; however, IL-4 or low dose anti-Ig alone did no t increase the rate of de novo cyclin D2 synthesis above that of resti ng & cells, The significance of the lack of cyclin D2 regulation by co mpetence-inducing growth factors was demonstrated, in that only mitoge nic factors that stimulated DNA synthesis 1) led to the formation of s table cyclin D2/cdk4 holoenzyme complexes during G(1) phase progressio n, and 2) afforded the isolation of anti-cyclin D2 or anti-cdk4 immuno precipitates that phosphorylated retinoblastoma. These findings sugges t a role for these proteins during the mid to late G(1) phase progress ion and possibly the G(1)/S phase transition in primary mature B lymph ocytes.