Fitzgerald P, Teng M, Chandraratna R A, Heyman R A, Allegretto E A
Department of Retinoid Research, Ligand Pharmaceuticals, Inc., San Diego, California 92121, USA.
Cancer Res. 1997 Jul 1;57(13):2642-50.
Retinoic acid receptor (RAR) alpha 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 RAR alpha, whereas ER-negative breast cancer cell lines are refractory to retinoid treatment and have been thought to express little or no RAR alpha. We set out to test several ER-negative breast cancer cell lines for expression of RAR alpha 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 RAR alpha protein as measured by ligand-binding immunoprecipitation (approximately 55 fmol/mg protein) and also displayed sensitivity to growth inhibition by retinoids (9-cis-retinoic acid; EC50, approximately 3 nM). These cells were more sensitive than an ER-positive cell line, T-47D, which expressed approximately 35 fmol RAR alpha/mg total protein (9-cis retinoic acid; EC50, approximately 50-100 nM). Another ER-negative cell line, Hs578T, also expressed RAR alpha (approximately 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 RAR alpha protein and also did not respond to retinoids in growth inhibition assays. A RAR alpha agonist displayed 100 times greater potency than a RARgamma agonist in growth inhibition of both T-47D and SK-BR-3 cells, suggesting RAR alpha involvement in the process. Furthermore, a RAR alpha antagonist completely abolished the growth inhibition induced by RAR agonists, implying that the activity of the agonists is exerted solely through RAR alpha, not RARgamma, which is also expressed in both cell lines. Additionally, although retinoid X receptor (RXR) compounds are weakly active in growth inhibition of the RAR alpha-positive cell lines, they markedly increased the growth-inhibitory activity of RAR ligands. 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 RAR alpha positive may be candidates for retinoid therapy. Additionally, combinations of RXR ligands with RAR ligands (especially RAR alpha agonists) and/or antiestrogens may have utility in the treatment of breast cancer.
维甲酸受体（RAR）α已被证明在维甲酸诱导的对表达雌激素受体（ER）的人乳腺癌细胞系的生长抑制中发挥作用。该领域的传统观念认为，ER阳性乳腺癌细胞系对维甲酸治疗有反应是因为它们表达RARα，而ER阴性乳腺癌细胞系对维甲酸治疗不敏感，并且一直被认为很少或不表达RARα。我们着手在生长抑制试验中检测几种ER阴性乳腺癌细胞系中RARα蛋白的表达以及对维甲酸的反应性。在检测的6种ER阴性乳腺癌细胞系中，一种（SK-BR-3）通过配体结合免疫沉淀法测得具有高水平的RARα蛋白（约55 fmol/mg蛋白），并且对维甲酸（9-顺式维甲酸；EC50，约3 nM）诱导的生长抑制也表现出敏感性。这些细胞比表达约35 fmol RARα/mg总蛋白的ER阳性细胞系T-47D更敏感（9-顺式维甲酸；EC50，约50 - 100 nM）。另一种ER阴性细胞系Hs578T也表达RARα（约23 fmol/mg），并且对维甲酸诱导的生长抑制敏感，尽管程度低于SK-BR-3或T-47D细胞。相比之下，检测的其他ER阴性细胞系表达低水平（<10 fmol/mg）或未检测到RARα蛋白，并且在生长抑制试验中对维甲酸也无反应。一种RARα激动剂在T-47D和SK-BR-3细胞的生长抑制中显示出比RARγ激动剂高100倍的效力，表明RARα参与了该过程。此外，一种RARα拮抗剂完全消除了RAR激动剂诱导的生长抑制，这意味着激动剂的活性仅通过RARα发挥，而非RARγ，这两种细胞系中也都表达RARγ。另外，尽管类视黄醇X受体（RXR）化合物在RARα阳性细胞系的生长抑制中活性较弱，但它们显著增加了RAR配体的生长抑制活性。RXR化合物还增强了抗雌激素4-羟基他莫昔芬抑制T-47D细胞生长的作用。这些发现具有临床意义，即RARα阳性的ER阴性肿瘤患者可能是维甲酸治疗的候选对象。此外，RXR配体与RAR配体（尤其是RARα激动剂）和/或抗雌激素的联合使用可能在乳腺癌治疗中具有应用价值。
