Pro-inflammatory cytokines IL-6 and CCL2 suppress expression of circadian gene Period2 in mammary epithelial cells

Chronic inflammation is known to contribute to tumor initiation and cancer progression. In breast tissue, the core circadian gene Period (PER)2 plays a critical role in mammary gland development and possesses tumor suppressor function. Interleukin (IL)-6 and C-C motif chemokine ligand (CCL) 2 are among the most abundant cytokines in the inflammatory microenvironment. We found that acute stimulation by IL-6/CCL2 reduced PER2 expression in non-tumorigenic breast epithelial cells. Longer term exposure to IL-6/CCL2 suppressed PER2 to an even lower level. IL -6 activated STAT3/NF kappa B p50 signaling to recruit HDAC1 to the PER2 promoter. CCL2 activated the PI3K/AKT pathway to promote ELK-1 cytoplasm-to-nucleus translocation, recruit HDAC1 to the proximal PER2 promoter and facilitate DNMT3-EZH2-PER2 promoter association. Ectopic expression of PER2 inhibited IL-6 or CCL2 induced mammosphere forming ability and reduced sphere size indicating that PER2 repression in breast epithelial cells can be crucial to activate tumorigenesis in an inflammatory microenvironment. The diminished expression of PER2 can be observed over a time scale of hours to weeks following IL-6/CCL2 stimulation suggesting that PER2 suppression occurs in the early stage of the interaction between an inflammatory microenvironment and normal breast epithelial cells. These data show new mechanisms by which mammary cells interact with a cancerous microenvironment and provide additional evidence that PER2 expression contributes to breast tumorigenesis.