ER-negative breast cancer cell lines for expression of RARa protein and responsiveness to retinoids in growth

Retinoic acid receptor (RAR) a 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 RARa, whereas ER-negative breast cancer cell lines are refractory to retinold treatment and have been thought to express little or no RARa. We set out to test several ER-negative breast cancer cell lines for expression of RARa 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 RARer protein as measured by ligand-binding immunoprecipitation (—55 finol/mg protein) and also dis played sensitivity to growth inhibition by retinoids (9-cis-retinoic acid; EC,.,, “3 nM). These cells were more sensitive than an ER-positive cell line, T-47D, which expressed --35 fmol RARa/mg total protein (9-d.c retinoic acid; EC@, @5O—1OO vms). Another ER-negative cell line, HsS78T, also expressed RARa (—23fmol/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 RARa protein and also did not respond to retinoids in growth Inhibition assays. A RARa agonist displayed 100 times greater potency than a RARy agonist in growth inhibition of both T-47D and SK-BR-3 cells, suggesting RARa involvement in the process. Fur thermore, a RARa antagonist completely abolished the growth inhibition Induced by RAR agonists, implying that the activity of the agonists is exerted solely through RARa, not RAR@ which i.calso expressed in both cell lines. Additionally, although retinoid X receptor (RXR) compounds are weakly active in growth inhibition ofthe RARa-positive cell lines, they markedly Increased the growth-inhibitory activity of PAR ligands. RXR compounds also potentiated the action of the antiestrogen 4-hydroxyta moxifen to inhibit the growth of T-47D cells. These findings have clinical ramifications in that patients with ER-negative tumors that are RARa positive may be candidates for retinoid therapy. Additionally, combina tions of RXR ligands with RAR ligands (especially RARa agonists) and/or antiestrogens may have utility in the treatment of breast cancer.