The Homeodomain-Interacting Protein Kinase 2 Regulates Insulin Promoter Factor-1/Pancreatic Duodenal Homeobox-1 Transcriptional Activity

The homeodomain transcription factor insulin promoter factor (IPF)-1/pancreatic duodenal homeobox (PDX)-1 plays a crucial role in both pancreas development and maintenance of β-cell function. Targeted disruption of the Ipf1/Pdx1 gene in β-cells of mice leads to overt diabetes and reduced Ipf1/Pdx1 gene expression results in decreased insulin expression and secretion. In humans, mutations in the IPF1 gene have been linked to diabetes. Hence, the identification of molecular mechanisms regulating the transcriptional activity of this key transcription factor is of great interest. Herein we analyzed homeodomain-interacting protein kinase (Hipk) 2 expression in the embryonic and adult pancreas by in situ hybridization and RT-PCR. Moreover, we functionally characterized the role of HIPK2 in regulating IPF1/PDX1 transcriptional activity by performing transient transfection experiments and RNA interference. We show that Hipk2 is expressed in the developing pancreatic epithelium from embryonic d 12–15 but that the expression becomes preferentially confined to pancreatic endocrine cells at later developmental stages. Moreover, we show that HIPK2 positively influences IPF1/PDX1 transcriptional activity and that the kinase activity of HIPK2 is required for this effect. We also demonstrate that HIPK2 directly phosphorylates the C-terminal portion of IPF1/PDX1. Taken together, our data provide evidence for a new mechanism by which IPF1/PDX1 transcriptional activity, and thus possibly pancreas development and/or β-cell function, is regulated. Homeodomain-interacting protein kinase 2 phosphorylates the C-terminal portion of IPF1/PDX1 as well as positively regulating IPF1/PDX1 transcriptional activity.