Homeodomain-interacting Protein Kinase 2 Overexpression Research Articles

Homeodomain-Interacting Protein Kinase-2 Restrains Cytosolic Phospholipase A2–Dependent Prostaglandin E2 Generation in Human Colorectal Cancer Cells

Homeodomain-interacting protein kinase-2 (HIPK2), a corepressor for homeodomain transcription factors, is a multifunctional kinase whose 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. 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 of HIPK2 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 growth of HIPK2i cells in nude mice. We compared the gene expression of HIPK2 and cPLA2 in human colorectal cancer specimens by reverse transcriptase-PCR. 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 compared with sporadic colorectal cancer expressing cPLA2. Our findings reveal the novel mechanism of HIPK2 to restrain progression of human colon tumorigenesis, at least in part, by turning off cPLA2-dependent PGE2 generation.

Interaction of Brn3a and HIPK2 mediates transcriptional repression of sensory neuron survival

The Pit1-Oct1-Unc86 domain (POU domain) transcription factor Brn3a controls sensory neuron survival by regulating the expression of Trk receptors and members of the Bcl-2 family. Loss of Brn3a leads to a dramatic increase in apoptosis and severe loss of neurons in sensory ganglia. Although recent evidence suggests that Brn3a-mediated transcription can be modified by additional cofactors, the exact mechanisms are not known. Here, we report that homeodomain interacting protein kinase 2 (HIPK2) is a pro-apoptotic transcriptional cofactor that suppresses Brn3a-mediated gene expression. HIPK2 interacts with Brn3a, promotes Brn3a binding to DNA, but suppresses Brn3a-dependent transcription of brn3a, trkA, and bcl-x L. Overexpression of HIPK2 induces apoptosis in cultured sensory neurons. Conversely, targeted deletion of HIPK2 leads to increased expression of Brn3a, TrkA, and Bcl-xL, reduced apoptosis and increases in neuron numbers in the trigeminal ganglion. Together, these data indicate that HIPK2, through regulation of Brn3a-dependent gene expression, is a critical component in the transcriptional machinery that controls sensory neuron survival.

HIPK2 overexpression leads to stabilization of p53 protein and increased p53 transcriptional activity by decreasing Mdm2 protein levels

BackgroundHIPK2 (homeodomain-interacting protein kinase 2) has been identified as a nuclear serine/threonine kinase. A central function of HIPK2 is repressing transcription of homeodomain containing transcription factors.Results and ConclusionsWe show here that HIPK2 activates transcription mediated by tumor suppressor p53 responsive promoter elements. Overexpression of HIPK2 leads to an increase of p53 protein expression or stability, which becomes enhanced further in the presence of the DNA damaging drug doxorubicin. The effects of HIPK2 on p53 are not observed with kinase deficient HIPK2 mutants. However, HIPK2 is not sufficient for phosphorylation of three crucial serine residues of p53, suggesting that HIPK2-induced p53 activation does not involve phosphorylation of p53. Instead, HIPK2 leads to a downregulation of p53-induced Mdm2 protein and this may lead to stabilization of p53. Overexpression of HIPK2 does not lead to a change of Mdm2 mRNA expression. The data suggest that HIPK2 plays a critical role in p53 mediated cellular responses by removing the p53 inhibitor protein Mdm2 via modification of the protein itself or its intracellular movement.