IMMUNOBIOLOGY Retinoic Acid Prevents Phosphorylation of pRB in Normal Human B Lymphocytes: Regulation of Cyclin E, Cyclin A, and p21Cip1

The mechanisms underlying the growth-inhibitory effect of retinoids on normal human B lymphocytes are not well understood. We addressed this issue by examining the effect of retinoic acid on the cell cycle machinery involved in G1/S transition. When retinoic acid was administered to B cells stimulated into mid to late G1 by anti-IgM antibodies (anti-μ) and Staphylococcus aureus crude cell suspension (SAC), the phosphorylation of pRB required for S-phase entry was prevented in a timeand dose-dependent manner. Thus, 2-hour treatment with retinoic acid at the optimal concentration of 1 μmol/L prevented phosphorylation of pRB, and effects were noted at concentrations as low as 10 nmol/L. Based on our results, we suggest that the rapid effect of retinoic acid on pRB phosphorylation is due primarily to the reduced expression of cyclin E and cyclin A in late G1. This could lead to the diminished cyclin E– and cyclin A–associated kinase activities noted as early as 2 hours after addition of retinoic acid. Furthermore, our results imply that the transient induction of p21Cip1 could also be involved. Thus, retinoic acid induced a rapid, but transient increased binding of p21Cip1 to CDK2. The retinoic acid receptor (RAR) agonist TTNPB mimicked the key events affected by retinoic acid, such as pRB phosphorylation, cyclin E expression, and expression of p21Cip1, whereas the RAR-selective antagonist Ro 41-5253 counteracted the effects of retinoic acid. This implies that retinoic acid mediates its growth-inhibitory effect on B lymphocytes via the nuclear receptors. r 1999 by The American Society of Hematology.