Homeodomain-interacting protein kinase-2 restrains cytosolic phospholipase A2-dependent prostaglandin E2 generation in human colorectal cancer cells.

Purpose: Homeodomain-interacting protein kinase-2 (HIPK2), a corepressor for homeodomain transcription factors, is amultifunctional kinasewhose role in tumor cell survival is not completely clarified.We addressed whether HIPK2 restrains colon tumorigenesis by turning off cytosolic phospholipase A2 (cPLA2)-dependent prostaglandin E2 (PGE2) generation in the light of overwhelming evidence suggesting the contribution of this prostanoid in a variety of cancers. Experimental Design: In the human colorectal cancer cell line, RKO, we studied the effect of RNA interference for HIPK2 (HIPK2i) on prostanoid biosynthesis, both in the absence and in the presence of the cPLA2 inhibitor arachidonyl trifluoromethyl ketone.We evaluated the role ofHIPK2 in the cPLA2 gene regulation by reverse transcriptase-PCR, transcriptional activity, and chromatin immunoprecipitation analyses.The involvement of HIPK2 in tumorigenicity in vivo was studied by tumor growthofHIPK2i cells innudemice.We compared the gene expressionofHIPK2 and cPLA2 in human colorectal cancer specimens by reverse transcriptase-PCR. Results: HIPK2 silencing was associated with rousing PGE2 biosynthesis that was profoundly suppressed by the cPLA2 inhibitor. HIPK2 overexpression, along with histone deacetylase-1, inhibited the cPLA2-luc promoter that is strongly acetylated in HIPK2i cells. The tumors derived from HIPK2i cells injected in nude mice showed noticeably increased growth compared with parental cells. HIPK2 mRNA levels were significantly higher in colorectal cancers of patients with familial adenomatous polyposis,which showed undetectable cPLA2 levels comparedwith sporadic colorectal cancer expressing cPLA2. Conclusions:Our findings reveal thenovelmechanismofHIPK2 to restrainprogressionof human colon tumorigenesis, at least in part, by turning off cPLA2-dependent PGE2 generation.