The involvement of small heterodimer partner (SHP) in the inhibition of hepatic bile acid synthesis from cholesterol has been established. However, extrahepatic expression of SHP implies that SHP may have regulatory functions other than those in the liver. Here, we find that SHP mRNA expression is high in murine bone marrow cells, suggesting a physiological role within macrophages. Indeed, expression of SHP in macrophages decreases the transcriptional activity and nuclear localization of nuclear factor κB, whereas downregulation of SHP has the opposite effects. Expression of genes associated with macrophage–T cell crosstalk were altered by overexpression or downregulation of SHP. Intriguingly, increasing SHP expression in macrophages resulted in decreased T cell expansion, a hallmark of T cell activation, whereas knockdown of SHP resulted in increased expansion. Analyses of the expanded T cells revealed a dichotomous skewing between effector T cells and regulatory T cells (Tregs), with SHP overexpression reducing Tregs and downregulation of SHP increasing their expansion. The expanded Tregs were confirmed to be suppressive via adoptive transfers. IL-2 and TGF-β, known inducers of Treg differentiation, were found to be regulated by SHP. Furthermore, SHP occupancy at the promoter region of IL-2 was increased after macrophages were challenged with lipopolysaccharide. Neutralizing antibodies to IL-2 and TGF-β inhibited the expansion of Tregs mediated by downregulation of SHP. This study demonstrates that expression and activity of SHP within macrophages can alter T cell fate and identifies SHP as a potential therapeutic target for autoimmune diseases or solid cancers.

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.