Retinoic Acid Receptor α Expression Correlates with Retinoid-induced Growth Inhibition of Human Breast Cancer Cells Regardless of Estrogen Receptor Status

Retinoic acid receptor (RAR) α has been shown to play a role in retinoid-induced growth inhibition of human breast cancer cell lines that express the estrogen receptor (ER). The dogma in the field has been that ER-positive breast cancer cell lines respond to retinoid treatment because they express RARα, whereas ER-negative breast cancer cell lines are refractory to retinoid treatment and have been thought to express little or no RARα. We set out to test several ER-negative breast cancer cell lines for expression of RARα protein and responsiveness to retinoids in growth inhibition assays. Of six ER-negative breast cancer cell lines that were tested, one (SK-BR-3) had high levels of RARα protein as measured by ligand-binding immunoprecipitation (∼55 fmol/mg protein) and also displayed sensitivity to growth inhibition by retinoids (9- cis -retinoic acid; EC50, ≈3 nm). These cells were more sensitive than an ER-positive cell line, T-47D, which expressed ∼35 fmol RARα/mg total protein (9- cis retinoic acid; EC50, ≈50–100 nm). Another ER-negative cell line, Hs578T, also expressed RARα (∼23 fmol/mg) and was sensitive to retinoid-induced growth inhibition, albeit to a lesser extent than SK-BR-3 or T-47D cells. In contrast, the other ER-negative cell lines tested expressed low (<10 fmol/mg) or no detectable levels of RARα protein and also did not respond to retinoids in growth inhibition assays. A RARα agonist displayed 100 times greater potency than a RARγ agonist in growth inhibition of both T-47D and SK-BR-3 cells, suggesting RARα involvement in the process. Furthermore, a RARα antagonist completely abolished the growth inhibition induced by RAR agonist, implying that the activity of the agonists is exerted solely through RARα, not RARγ, which is also expressed in both cell lines. Additionally, although retinoid X receptor (RXR) compounds are weakly active in growth inhibition of the RARα-positive cell lines, they markedly increased the growth-inhibitory activity of RAR lingands. RXR compounds also potentiated the action of the antiestrogen 4-hydroxytamoxifen to inhibit the growth of T-47D cells. These findings have clinical ramifications in that patients with ER-negative tumors that are RARα positive may be candidates for retinoid therapy. Additionally, combinations of RXR ligands with RAR lines (especially RARα agonists) and/or antiestrogens may have utility in the treatment of breast cancer.