Regulation of vascular endothelial growth factor expression by homeodomain-interacting protein kinase-2

Homeodomain-interacting protein kinase-2 (HIPK2) plays an essential role in restraining tumor progression as it may regulate, by itself or within multiprotein complexes, many proteins (mainly transcription factors) involved in cell growth and apoptosis. This study takes advantage of the recent finding that HIPK2 may repress the beta-catenin transcription activity. Thus, we investigated whether HIPK2 overexpression may down-regulate vascular endothelial growth factor (VEGF) levels (a beta-catenin target gene) and the role of beta-catenin in this regulation, in order to consider HIPK2 as a tool for novel anti-tumoral therapeutical approaches.

The regulation of VEGF expression by HIPK2 was evaluated by using luciferase assay with VEGF reporter construct, after overexpression of the beta-catenin transcription factor. Relative quantification of VEGF and beta-catenin mRNAs were assessed by reverse-transcriptase-PCR (RT-PCR) analyses, following HIPK2 overexpression, while beta-catenin protein levels were evaluated by western immunoblotting.

HIPK2 overexpression in tumor cells downregulated VEGF mRNA levels and VEGF promoter activity. The VEGF downregulation was partly depending on HIPK2-mediated beta-catenin regulation. Thus, HIPK2 could induce beta-catenin protein degradation that was prevented by cell treatment with proteasome inhibitor MG132. The beta-catenin degradation was dependent on HIPK2 catalytic activity and independent of p53 and glycogen synthase kinase 3beta (GSK-3beta) activities.

These results suggest that VEGF might be a target of HIPK2, at least in part, through regulation of beta-catenin activity. These findings support the function of HIPK2 as tumor suppressor and hypothesise a role for HIPK2 as antiangiogenic tool in tumor therapy approaches.