Peroxisome Proliferator-Activated Receptor-δ Induces Cell Proliferation by a Cyclin E1–Dependent Mechanism and Is Up-regulated in Thyroid Tumors

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Abstract
Peroxisome proliferator-activated receptors (PPARs) are lipid-sensing nuclear receptors that have been implicated in multiple physiologic processes including cancer. Here, we determine that PPARδ induces cell proliferation through a novel cyclin E1–dependent mechanism and is up-regulated in many human thyroid tumors. The expression of PPARδ was induced coordinately with proliferation in primary human thyroid cells by the activation of serum, thyroid-stimulating hormone/cyclic AMP, or epidermal growth factor/mitogen-activated protein kinase mitogenic signaling pathways. Engineered overexpression of PPARδ increased thyroid cell number, the incorporation of bromodeoxyuridine, and the phosphorylation of retinoblastoma protein by 40% to 45% in just 2 days, one usual cell population doubling. The synthetic PPARδ agonist GW501516 augmented these PPARδ proliferation effects in a dose-dependent manner. Overexpression of PPARδ increased cyclin E1 protein by 9-fold, whereas knockdown of PPARδ by small inhibitory RNA reduced both cyclin E1 protein and cell proliferation by 2-fold. Induction of proliferation by PPARδ was abrogated by knockdown of cyclin E1 by small inhibitory RNA in primary thyroid cells and by knockout of cyclin E1 in mouse embryo fibroblasts, confirming a cyclin E1 dependence for this PPARδ pathway. In addition, the mean expression of native PPARδ was increased by 2-fold to 5-fold (P < 0.0001) and correlated with that of the in situ proliferation marker Ki67 (R = 0.8571; P = 0.02381) in six different classes of benign and malignant human thyroid tumors. Our experiments identify a PPARδ mechanism that induces cell proliferation through cyclin E1 and is regulated by growth factor and lipid signals. The data argue for systematic investigation of PPARδ antagonists as antineoplastic agents and implicate altered PPARδ–cyclin E1 signaling in thyroid and other carcinomas. [Cancer Res 2008;68(16):6578–86]