Rexinoids prevent the mammary tumorigenesis in MMTV-erbB2 mice by down-regulating cyclin D1 expression.

A107

Rexinoids are RXR-selective ligands that have been shown to prevent carcinogenesis with less toxicity than naturally occurring retinoids. Our previous studies demonstrated that LGD1069 (bexarotene) and LG100268, two most extensively studied rexinoids, prevent ER-negative mammary tumorigenesis in animal models. However, the mechanism by which rexinoids suppress carcinogenesis is not fully understood. Mammary tumorigenesis is a multi-stage process that proceeds through hyperplasia, ductal carcinoma in situ and invasive breast cancer. To determine whether rexinoids prevent tumor development at the early stages, MMTV-erbB2 mice were treated daily by oral gavage with vehicle, LGD1069 or LG100268 for 2 to 4 months (from 3 months to 7 months of age), to let premalignant lesions develop. Significant amounts of hyperplasia and ductal carcinoma in situ lesions were observed in mammary glands from vehicle-treated mice at 7 months of age. Both LGD1069 and LG100268 dramatically suppressed the development of premalignant lesions at all stages. This reduction in premalignant lesions was associated with reduced proliferation and down-regulation of a key regulator of cell cycle progression, cyclin D1, at the RNA and protein levels. Cycloheximide experiments indicated that down-regulation of cyclin D1 by rexinoids requires new protein synthesis. To investigate which protein is responsible for the cyclin D1 down-regulation, we examined the expression of rexinoid target genes, and found that growth inhibiting protein DEC2 was induced by both LGD1069 and LG100268 at the protein level. Knock-down of DEC2 in normal human mammary epithelial cells eliminated the repression of cyclin D1 by the rexinoids. Moreover, forced expression of DEC2 in normal and malignant breast cell significantly inhibited cell proliferation and repressed the transcription of cyclin D1. These data suggest that LGD1069 and LG100268 prevent mammary tumorigenesis by suppressing mammary epithelial proliferation at a point early in the progression to cancer in part due to down-regulation of cyclin D1. Our results also show that the induction of DEC2 causes down-regulation of cyclin D1. Thus, both cyclin D1 and DEC2 play a role in the cancer prevention induced by rexinoids. These studies were supported by the funding from the NCI (U19 CA086809 and R01 CA078480) the Susan G. Komen Breast Cancer Foundation.