Activation of ERα Signaling Differentially Modulates IFN-γ Induced HLA-Class II Expression in Breast Cancer Cells

The coordinate regulation of HLA class II (HLA-II) is controlled by the class II transactivator, CIITA, and is crucial for the development of anti-tumor immunity. HLA-II in breast carcinoma is associated with increased IFN-γ levels, reduced expression of the estrogen receptor (ER) and reduced age at diagnosis. Here, we tested the hypothesis that estradiol (E 2 ) and ERα signaling contribute to the regulation of IFN-γ inducible HLA-II in breast cancer cells. Using a panel of established ER − and ER + breast cancer cell lines, we showed that E 2 attenuated HLA-DR in two ER + lines (MCF-7 and BT-474), but not in T47D, while it augmented expression in ER − lines, SK-BR-3 and MDA-MB-231. To further study the mechanism(s), we used paired transfectants: ERα + MC2 (MDA-MB-231 c10A transfected with the wild type ERα gene) and ERα − VC5 (MDA-MB-231 c10A transfected with the empty vector), treated or not with E 2 and IFN-γ. HLA-II and CIITA were severely reduced in MC2 compared to VC5 and were further exacerbated by E 2 treatment. Reduced expression occurred at the level of the IFN-γ inducible CIITA promoter IV. The anti-estrogen ICI 182,780 and gene silencing with ESR1 siRNA reversed the E 2 inhibitory effects, signifying an antagonistic role for activated ERα on CIITA pIV activity. Moreover, STAT1 signaling, necessary for CIITA pIV activation, and selected STAT1 regulated genes were variably downregulated by E 2 in transfected and endogenous ERα positive breast cancer cells, whereas STAT1 signaling was noticeably augmented in ERα − breast cancer cells. Collectively, these results imply immune escape mechanisms in ERα + breast cancer may be facilitated through an ERα suppressive mechanism on IFN-γ signaling.