Reversal by RARα agonist Am580 of c-Myc-induced imbalance in RARα/RARγ expression during MMTV-Myc tumorigenesis

Introduction: Retinoic acid signaling plays key roles in embryonic development and in maintaining the differentiated status of adult tissues. Recently, the nuclear retinoic acid receptor (RAR) isotypes alpha, beta and gamma were found to play specific functions in the expansion and differentiation of the stem compartments of various tissues. For instance, RAR gamma appears to be involved in stem cell compartment expansion, while RAR alpha and RAR beta are implicated in the subsequent cell differentiation. We found that over-expressing c-Myc in normal mouse mammary epithelium and in a c-Myc-driven transgenic model of mammary cancer, disrupts the balance between RARg and RAR alpha/beta in favor of RAR gamma. Methods: The effects of c-Myc on RAR isotype expression were evaluated in normal mouse mammary epithelium, mammary tumor cells obtained from the MMTV-Myc transgenic mouse model as well as human normal immortalized breast epithelial and breast cancer cell lines. The in vivo effect of the RAR alpha-selective agonist 4-[( 5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthyl) carboxamido] benzoic acid (Am580) was examined in the MMTVMyc mouse model of mammary tumorigenesis. Results: Modulation of the RAR alpha/beta to RAR gamma expression in mammary glands of normal mice, oncomice, and human mammary cell lines through the alteration of RAR-target gene expression affected cell proliferation, survival and tumor growth. Treatment of MMTV-Myc mice with the RAR alpha-selective agonist Am580 led to significant inhibition of mammary tumor growth (similar to 90%, P<0.001), lung metastasis (P< 0.01) and extended tumor latency in 63% of mice. Immunocytochemical analysis showed that in these mice, RAR alpha responsive genes such as Cyp26A1, E-cadherin, cellular retinol-binding protein 1 (CRBP1) and p27, were up-regulated. In contrast, the mammary gland tumors of mice that responded poorly to Am580 treatment (37%) expressed significantly higher levels of RAR gamma. In vitro experiments indicated that the rise in RAR gamma was functionally linked to promotion of tumor growth and inhibition of differentiation. Thus, activation of the RAR alpha pathway is linked to tumor growth inhibition, differentiation and cell death. Conclusions: The functional consequence of the interplay between c-Myc oncogene expression and the RAR gamma to RAR alpha/beta balance suggests that prevalence of RAR gamma over-RAR alpha/beta expression levels in breast cancer accompanied by c-Myc amplification or over-expression in breast cancer should be predictive of response to treatment with RAR alpha-isotype- specific agonists and warrant monitoring during clinical trials. See related editorial by Garattini et al http://breast-cancer-research. com/content/14/5/111