15-Deoxy- (cid:1) 12,14 Prostaglandin J 2 Up-Regulates Kru¨ppel-Like Factor 4 Expression Independently of Peroxisome Proliferator-Activated Receptor (cid:1) by Activating the Mitogen-Activated Protein Kinase Kinase/Extracellular Signal-Regulated Kinase Signal Transduction Pathway in HT-29 Colon Canc

15-Deoxy- (cid:1) 12,14 prostaglandin J 2 (15d-PGJ 2 ) is a natural ligand for the peroxisome proliferator-activated receptor (cid:1) (PPAR (cid:1) ) that exhibits antiproliferative activity in colon cancer cells, but its mechanism of action is still poorly understood. In this study, we showed that Kru¨ppel-like factor 4 (KLF4) is one of the downstream effectors of 15d-PGJ 2 . Treatment of HT-29 cells with 15d-PGJ 2 resulted in up-regulation of both KLF4 mRNA and protein expression, and these increases were also observed in other colon cancer cell lines. Down-regulation of KLF4 expression by small interfering RNA (siRNA) targeting KLF4 reduced 15d-PGJ 2 -mediated G 1 phase arrest, suggest- ing that KLF4-mediated function of 15d-PGJ 2 . The effect of 15d-PGJ 2 on KLF4 expression seems not to involve its nuclear receptor PPAR (cid:1) , in that our data show that:1) KLF4 gene promoter does not contain putative PPRE sequence, 2) 15d- PGJ 2 rapidly activates extracellular signal-regulated kinase (ERK) and induces KLF4 mRNA expression, 3) KLF4 is induced by 15d-PGJ 2 but not by rosiglitazone, a synthetic PPAR (cid:1) li- gand, and 4) 15d-PGJ 2 is unable to stimulate PPAR-dependent promoter activity in the absence of cotransfected PPAR (cid:1) In this study, we examined the effect of 15d-PGJ 2 on KLF4 expression in HT-29 colon cancer cells. Our results show that that 15d-PGJ 2 inhibits proliferation of HT-29 cells and induces up-regulation of KLF4 mRNA and protein levels. The induction of KLF4 by 15d-PGJ 2 seems not to involve in nuclear receptor PPAR (cid:1) but is dependent on the activation of MEK/ERK pathway. These data may provide a novel mechanism governing the antiproliferative property of 15d-PGJ 2 in colon cancer cells.

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