Inhibition of myeloma cell growth by all-trans retinoic acid is associatedwith upregulation of p21(WAF1) and dephosphorylation of the retinoblastomaprotein

Retinoic acid and dexamethasone, in combination, inhibit the growth of huma n myeloma cell lines in a synergistic manner. Previously, we observed that all-trans retinoic acid (ATRA) caused G1 arrest and inhibited clonogenic gr owth of the OPM-2 human myeloma cell line. This was associated with downreg ulation of the IL-6 receptor (IL-6R) gp80 protein, while autocrine IL-6 pro duction and gp130 were not affected. Growth inhibition was not reversed by the addition of exogenous IL-6 or forced, constitutive expression of the IL -6 receptor gp80 protein, suggesting that the mechanism of action of ATRA m ay be due to effects on the post-receptor pathway. Therefore, in this study we have investigated whether growth arrest was associated with changes in the level of phosphorylation of the RE protein. ATRA decreased the level of phosphorylation of the RE protein at doses > 5 x 10(-9) M and also induced a five fold increase in p21(WAF1), while levels of p27(KIP1) and CDK2 were unchanged. The ATRA-mediated increase in p21 preceded the change in RE pho sphorylation and G1 arrest and was not reversed by the addition of exogenou s IL-6. The levels of CDK2 activity were inhibited approximately 60% in ATR A-treated cells, suggesting that the increased p21 levels were sufficient t o inhibit CDK activity and cause RE hypophosphorylation. Increased levels o f p21 have recently been observed in human myeloma cells exposed to dexamet hasone, and we suggest that the common ability of these two agents to inhib it myeloma cell growth depends on their induction of p21.