Rao U S, Fine R L, Scarborough G A
Department of Pharmacology, University of North Carolina, Chapel Hill 27599.
Biochem Pharmacol. 1994 Jul 19;48(2):287-92. doi: 10.1016/0006-2952(94)90099-x.
Multidrug-resistant (MDR) tumor cells reduce the toxicity of antineoplastic drugs by an energy-dependent active efflux mechanism mediated by the MDR1 gene product, the P-glycoprotein (Pgp). Pgp expressed in cultured Sf9 insect cells has been shown to exhibit a high capacity ATPase activity in the presence of a variety of drugs known to be transported by the Pgp (Sarkadi et al., J Biol Chem 267: 4854-4858, 1992). The strict dependence of the Pgp ATPase activity on the presence of transport substrates indicates that the drug-stimulated ATPase activity is a direct reflection of the drug transport function of the Pgp. In the present study, this system has been utilized to investigate the possibility that antiestrogens and steroid hormones are transported by the Pgp. Antiestrogens such as tamoxifen, metabolites of tamoxifen (4-hydroxytamoxifen and N-desmethyltamoxifen), droloxifen, and toremifene stimulated the Pgp ATPase activity, and the maximum stimulation obtained with these agents equalled the maximal stimulation obtained by the best known MDR chemosensitizer, verapamil. Clomifene, nafoxidine and diethylstilbestrol also stimulated the Pgp ATPase activity, with maximal activations 75, 60 and 45% of the verapamil stimulation, respectively. Different degrees of stimulation of the Pgp ATPase activity were also obtained in the presence of steroid hormones such as progesterone, beta-estradiol, hydrocortisone, and corticosterone. Among these, progesterone is a potent inducer of the Pgp ATPase activity; at 50 microM, this hormone stimulated the Pgp ATPase activity as effectively as verapamil. These results suggest that the antiestrogens and steroid hormones that are known to reverse the multidrug-resistant phenotype do so by directly interacting with Pgp, thus interfering with its anticancer drug-extruding activity.
多药耐药（MDR）肿瘤细胞通过由MDR1基因产物P-糖蛋白（Pgp）介导的能量依赖性主动外排机制降低抗肿瘤药物的毒性。在培养的Sf9昆虫细胞中表达的Pgp已被证明在存在多种已知由Pgp转运的药物时表现出高容量ATP酶活性（Sarkadi等人，《生物化学杂志》267: 4854 - 4858，1992）。Pgp ATP酶活性对转运底物存在的严格依赖性表明药物刺激的ATP酶活性是Pgp药物转运功能的直接反映。在本研究中，该系统已被用于研究抗雌激素和类固醇激素是否由Pgp转运的可能性。他莫昔芬等抗雌激素、他莫昔芬的代谢产物（4-羟基他莫昔芬和N-去甲基他莫昔芬）、屈洛昔芬和托瑞米芬刺激了Pgp ATP酶活性，用这些药物获得的最大刺激等于最著名的MDR化学增敏剂维拉帕米获得的最大刺激。克罗米芬、萘福昔定和己烯雌酚也刺激了Pgp ATP酶活性，最大激活分别为维拉帕米刺激的75%、60%和45%。在存在孕酮、β-雌二醇、氢化可的松和皮质酮等类固醇激素的情况下，也获得了不同程度的Pgp ATP酶活性刺激。其中，孕酮是Pgp ATP酶活性的有效诱导剂；在50微摩尔时，这种激素刺激Pgp ATP酶活性的效果与维拉帕米一样有效。这些结果表明，已知能逆转多药耐药表型的抗雌激素和类固醇激素是通过直接与Pgp相互作用来实现的，从而干扰其抗癌药物外排活性。
