Involvement of nuclear steroid/thyroid/retinoid receptors and of protein kinases in the regulation of growth and of c-erbB and retinoic acid receptor expression in MCF-7 breast cancer cells

Nuclear steroid/thyroid/retinoid receptors and c‐erbB membrane receptor tyrosine kinases control epithelial growth and differentiation. Retinoid receptors can dimerize with the vitamin D receptor, the glucocorticoid receptor or the thyroid receptor. Furthermore, multiple c‐erbB receptor dimers have been identified. It has been shown that some of these receptor pathways communicate with each other via cross‐connected regulatory networks. Molecular interactions between retinoid receptors or estrogen receptors (ER) and c‐erbB‐2, and between ER and retinoic acid receptor(RAR)‐α have been reported. Here, we demonstrate the effects of steroids/thyroids/retinoids and of activators of protein kinase A (forskolin, Forsk) and C (12‐O‐tetradecanoylphorbol‐13‐acetate, TPA), on growth and expression of c‐erbB and RARs in MCF‐7 breast cancer cells, which contain high levels of RAR‐α and ‐γ, and which express significant amounts of c‐erbB‐2 and ‐3. All trans‐retinoic acid (tRA), the anti‐estrogen ICI 182 780 (ICI), Forsk and TPA reduced, whereas triiodothyronine and 17β‐estradiol (E2) stimulated cell growth. Flow cytometry revealed that tRA and E2 reduced c‐erbB‐2 and ‐3, whereas tamoxifen, Forsk and TPA up‐regulatedc‐erbB‐2. c‐erbB‐3 was co‐regulated with c‐erbB‐2. Northern analysis demonstrated that RAR‐α was down‐regulated by dexamethasone, ICI, and TPA, whereas vitamin D3 and E2 up‐regulated RAR‐α. RAR‐γ expression was less responsive to such treatment, being reduced only by ICI and Forsk. These data indicate that nuclear receptor and protein kinase signaling communicate with each other and control the expression of RARs and c‐erbB receptors. Efficient growth control requires the coordinated interplay of both receptor systems.