PPARδ is pro-tumorigenic in a mouse model of COX-2-induced mammary cancer

Cyclooxygenase-2 (COX-2), overexpressed in inflammatory conditions and cancer, regulates angiogenesis and tumorigenesis via the production of biologically active prostanoids. Previously, we showed that COX-2 over-expression in the mammary gland of transgenic mice induces an angiogenic switch and transforms the mammary epithelium into invasive mammary carcinoma. Since COX-2-derived prostanoids can activate the nuclear receptor PPARδ, we crossed Pparδ^-/- mice with COX-2 transgenic mice in the FVB/N background. PPARδ was expressed constitutively in the mammary gland of virgin, pregnant and lactating mice. Mammary hyperplasia and tumorigenesis in the COX-2 transgenic mice was markedly reduced in the Pparδ^-/- mice compared to their wild type counterparts. Analysis of the mammary tissues indicated that immunoreactive Ki-67, cyclin D1 and phosphorylated histone 3 (Phospho H3) were reduced in Pparδ^-/- mice, suggesting that PPARδ activation regulates cell proliferation in the mammary gland. We postulate that activation of the nuclear receptor PPARδ by COX-2-derived prostanoids may be involved in the proliferation of mammary epithelial cells and therefore contribute to mammary cancer development.