Growth inhibition of a human myeloma cell line by all-trans retinoic acid is not mediated through downregulation of interleukin-6 receptors but through upregulation of p21(WAF1)

All-trans retinoic acid (ATRA) has previously been shown to inhibit the growth of OPM-2 human myeloma cells. The growth inhibition was postulated to result from a transcriptional downregulation of interleukin-6 receptor α (IL-6Rα) with IL-6Rβ (gp130) unaffected. To formally test this hypothesis, an expression vector designed for constitutive IL-6Rα expression was constructed and used for transfection of OPM-2 cells. Six stable transfectants were cloned. The expression of IL-6Rα was shown by immunofluorescence with anti-IL-6Rα antibody and ¹²⁵I-IL-6 binding. In five of six transfectant clones, cellular IL-6Rα was 1.5- to 6-fold higher than the parental cells, with the ligand binding affinity unchanged. While ATRA reduced IL-6Rα expression in the parental OPM-2 cells, it enhanced its expression in these five transfectants. The clonogenic growth of these transfectants, however, remained strongly inhibited by ATRA. Further analysis, comparing the parental OPM-2 cells and a representative transfectant, clone C5, showed that IL-6 caused rapid tyrosine phosphorylation of gp130 in both OPM-2 and C5 clones. Pretreatment with ATRA greatly reduced IL-6-induced gp130 phosphorylation in OPM-2 cells, reflecting a reduction in cellular IL-6Rα. In contrast, IL-6-induced gp130 phosphorylation was not reduced by ATRA pretreatment in C5 cells, indicating that the expressed IL-6Rα was functional. Similar to OPM-2 cells, C5 cells were sensitive to growth inhibition by dexamethasone, which was entirely reversed by exogenous IL-6, suggesting that the IL-6 postreceptor signal transduction remained intact. ATRA was further shown to upregulate p21(WAF1) expression and cause dephosphorylation of the retinoblastoma protein (pRB) in both OPM-2 and C5 cells. Exogenous IL-6 also failed to reverse these effects of ATRA. Thus, the growth inhibitory activity of ATRA is not mediated through cellular IL-6Rα downregulation and is likely to result from a direct upregulation of p21(WAF1) and consequent dephosphorylation of pRB.