SAT-332 ERRα Modulates Genes of Importance to Macrophage Biology and Breast Cancer Progression

Breast cancer is a leading cause of death among women in the US with mortality most often due to metastasis. Bone is a common primary site of metastasis, and it is known that local bone quality dictates the susceptibility of this tissue to colonization. Estrogen-related receptor alpha (ERRα) is a nuclear receptor expressed in a variety of tissues and is involved in development, metabolic regulation, bone physiology, and immune cell differentiation. ERRα is expressed in macrophages, and there is a growing body of literature suggesting that macrophages are involved in both bone physiology as well as metastatic progression. Therefore, we hypothesized that the activity of this receptor within macrophages would have important patho-physiological consequences. Using RAW264.7 cells as a model, we first explored the expression of genes key to macrophage function. RAW264.7 cells polarized into the ‘M1’ state with lipopolysaccharide and IFNγ for 24hrs. Interestingly, when ERRα was overexpressed in these cells, we observed a significant reduction in interleukin 6 (IL-6) and RANKL expression, but an increase in the classic M1 markers, NOS2 and CD82. Conversely, when cells were treated with XCT-790, a small molecule antagonist and degrader of ERRα, IL-6 mRNA was increased, while NOS2 and CD86 were reduced. IL-6 and RANKL are known to be involved in bone resorption, and IL-6 has been implicated in cancer cell migration and the promotion of tumor progression. Furthermore, the production of nitric oxide by NOS2 can impair T cell expansion thus facilitating immune-escape by cancer cells. Thus, we speculated that macrophage activation of ERRα may promote breast cancer metastasis to the bone. We first confirmed that the bone-tropic MDA-MB-231 breast cancer cells migrate towards macrophages. We found that this migration was attenuated when IL-6 was depleted. Cholesterol has recently been identified as an agonist for ERRα. Therefore, we placed mice on a normal or high cholesterol diet and grafted with MDA-MB-231 cells. Contrary to our data indicating that ERRα activation would be protective, mice on a high cholesterol diet developed significantly more bone lesions. Hence, the effects of cholesterol on promoting breast cancer metastasis to the bone could not be explained solely by its ERRα activity. In conclusion, we have found that ERRα functions to attenuate cytokine release and decrease NOS2 expression in M1 polarized macrophages. Although cholesterol has been described as an ERRα agonist, a high cholesterol diet increased metastasis to the bone, contrary to what would be expected based on the ERRα inhibition of IL-6 and NOS2. Our data suggest that further studies are required to better elucidate the role of ERRα, prior to developing this nuclear receptor as a therapeutic target.