Abstract B58: Inflammatory signaling regulates osteoprotegerin expression in breast cancer cells

Osteoprotegerin (OPG) is a secreted member of the Tumor Necrosis Factor family. In addition to the functional roles of OPG in bone metabolism and apoptosis, there is growing evidence of the tumor promoting role of OPG in breast cancer. We have previously shown OPG can promote the invasion and metastasis of triple negative breast cancer cells. To understand the regulation of OPG, we are investigating the molecular mechanisms that control OPG expression in breast cancer cells. Breast cancer cell lines MDA-MB-231 and MDA-MB-436 (triple negative); SKBR-3 and HCC1954 (Human Epidermal Growth Factor Receptor 2 positive; HER2+); ZR75-1 and MCF-7 (Estrogen Receptor positive; ER+) were treated with Interleukin1beta (IL1-beta) for 24 hours. Levels of OPG mRNA and secreted OPG protein were analyzed by real time RT-PCR and ELISA, respectively. IL1-beta induced OPG expression regardless of basal OPG levels, with the exception of MDA-MB-231 which exhibited relatively moderate OPG induction. To explore the signaling mechanisms downstream of IL1-beta, we treated breast cancer cells with MAP kinase inhibitors in the presence of IL1-beta and examined the effects on OPG protein induction. Both p38 MAPK inhibitors, SB203580 and SB202190, were observed to partially blocked OPG induction by IL1-beta. Western blot analysis confirmed IL1-beta induced p38 phosphorylation. Additionally, we examined the effects of NFkappaB p65 binding to potential NFKappaB binding sites within the OPG promoter upon IL1-beta treatment by ChIP assay in SKBR3 and MDA-MB-436 cells. The highest fold increased binding was observed to occur within the -188 and -4675bp regions upon cytokine treatment in SKBR3 cells. No detectable binding was observed in MDA-MB-436 cells. We also looked at whether OPG expression levels could be altered by incubating breast cancer cells with macrophages, a cell type found in the breast tumor microenvironment. We co-cultured cells from each subtype with THP-1 human macrophages for 8 hours and extracted mRNA from the breast cancer cells or incubated the breast cancer cells for an additional 16 hours alone to measure secreted OPG protein. We found that co-culture with macrophages significantly increased OPG mRNA and protein expression in MCF-7 (ER+) and SKBR-3 (HER2+) cells but there was no increase in the MDA-MB-231 cells (triple negative). Overall, we show that IL1-beta can induce OPG expression in breast cancer cells and this signaling may be mediated by activating p38 MAP kin