Molecular Mechanisms by which Rexinoids Prevent BRCA

描述（由申请人提供）：类维生素A是动物和人类中有前途的化学预防剂，但由于天然类维生素A毒性相对较大，因此通常不用于癌症预防。在之前的资助期间，我们证明了9-顺式视黄酸（9 cRA），结合PAR和RXR类维生素A受体，抑制转基因小鼠乳腺肿瘤的发生，但毒性太大，无法进一步开发为化学预防剂。然后，我们证明了选择性结合RXR的类维生素A如LGD 1069（“rexinoids”）仍然以最小的毒性抑制乳腺细胞生长和肿瘤发生。这些临床前研究使我们能够设计和开放第一个使用rexinoids的人类癌症预防临床试验，目前正在NIH的单独资助下进行。我们还表明，rexinoids抑制乳腺细胞生长主要通过G1细胞周期阻滞。9 eRA和LGD 1069抑制肿瘤发生的能力取决于特定的致癌途径-两者在通过过度表达ferbB 2而患乳腺癌的小鼠中比在通过表达SV 40 T抗原而使Rb和P53失活的小鼠中更有效-但与他莫昔芬不同，两者都成功地预防了ER阴性和ER阳性肿瘤。我们还发现，小鼠乳腺肿瘤发生的成功抑制与乳腺上皮组织中增殖减少和考克斯-2表达减少有关，这表明预防的生物标志物和未来治疗的潜在靶点。我们在此假设类Rexinoids通过调节生长调节抑制乳腺癌的发生。研究人员发现，通过RXR异二聚体与特异性结合伴侣在正常乳腺细胞中的蛋白质，以及将rexinoids与选定的互补化学预防剂组合将更有效地预防乳腺癌。为了检验这些假设，我们将：（1）阐明类Rexinoids抑制正常乳腺细胞生长的机制，通过定义G1细胞周期阻滞的分子基础和确定类Rexinoids如何调节关键的生长调节因子（细胞周期蛋白D1、IGFBP-6和考克斯-2）。(2)使用免疫共沉淀和酵母双杂交技术鉴定介导类rexinoids的肿瘤抑制和生长抑制作用的关键RXR二聚化伴侣。然后在乳腺细胞中选择性抑制单个RXR伴侣，以确认它们在rexinoids生长抑制中的作用。(3)确定在发生ER阴性乳腺癌的转基因小鼠中，Cox 2抑制剂与rexinoid组合是否比单独使用任何一种药物更有效地预防乳腺癌，以及这些药物的组合是否增加了特定生物标志物的调节和增殖抑制。这些研究将为制定预防人类ER阴性乳腺癌的有效策略提供基础。

DESCRIPTION (provided by applicant): Retinoids are promising chemopreventive agents in animals and in humans, but because natural retinoids are relatively toxic, they are not generally used for cancer prevention. During the previous funding period we demonstrated that 9-cis retinoic acid (9cRA), which binds both the PAR and RXR retinoid receptors, suppresses mammary tumorigenesis in transgenic mice, but is too toxic for further development as a chemopreventive agent. We then demonstrated that retinoids like LGD1069 that selectively bind RXR ("rexinoids") still inhibit breast cell growth and tumorigenesis, with minimal toxicity. These preclinical studies enabled us to design and open the first human cancer prevention clinical trial using rexinoids, now ongoing under a separate NIH grant. We also showed that rexinoids suppress breast cell growth predominantly via a G1 cell cycle block. The ability of both 9eRA and LGD1069 to suppress tumorigenesis was dependent on the specific oncogenic pathway--both were more effective in mice that develop breast cancer through overexpression oferbB2 than in mice that have Rb and P53 inactivated by expression of SV40 T antigen--but, unlike tamoxifen, both successfully prevented ER-negative as well as ER-positive tumors. We have also found that successful suppression of mammary tumorigenesis in the mice is associated with reduced proliferation and reduced Cox-2 expression in the mammary epithelial tissues, suggesting both biomarkers of prevention and potential targets for future therapies. We here hypothesize that rexinoids suppress breast carcinogenesis by modulating growth regulatory. proteins in normal breast cells, via RXR heterodimers with specific binding partners, and that combining rexinoids with selected complementary chemopreventive agents will more effectively prevent breast cancer. To test these hypotheses, we will: (1) Elucidate the mechanism by which rexinoids suppress the growth of normal breast cells, by defining the molecular basis for the G l cell cycle block and determining how rexinoids modulate critical growth regulators (cyclin D1, IGFBP-6, and Cox-2). (2) identify the critical RXR dimerization partners that mediate the tumor suppressive and growth suppressive effects of rexinoids, using co-immunoprecipitation and yeast-two-hybrid techniques. Individual RXR partners will then be selectively suppressed in breast cells to confirm their role in growth inhibition by rexinoids. (3) Determine whether combining a Cox2 inhibitor with a rexinoid more effectively prevents breast cancer than either agent alone in transgenic mice that develop ER-negative mammary cancer, and whether the combination of these agents increases modulation of specific biomarkers and inhibition of proliferation. These investigations will provide a foundation for development of an effective strategy to prevent ER-negative breast cancer in humans.