Hepatocyte Nuclear Factor 4 Alpha Ligand Binding and F Domains Mediate Interaction and Transcriptional Synergy with the Pancreatic Islet LIM HD Transcription Factor Isl1

The orphan nuclear receptor HNF4α and the LIM homeodomain factor Isl1 are co-expressed in pancreatic β-cells and are required for the differentiation and function of these endocrine cells. HNF4α activates numerous genes and mutations in its gene are associated with maturity onset diabetes of the young. Cofactors and transcription factors that interact with HNF4α are crucial to modulate its transcriptional activity, since the latter is not regulated by conventional ligands. These transcriptional partners interact mainly through the HNF4α AF-1 module and the ligand binding domain, which contains the AF-2 module. Here, we showed that Isl1 could enhance the HNF4α–mediated activation of transcription of the HNF1α, PPARα and insulin I promoters. Isl1 interacted with the HNF4α AF-2 but also required the HNF4α carboxy-terminal F domain for optimal interaction and transcriptional synergy. More specifically, we found that naturally occurring HNF4α isoforms, differing only in their F domain, exhibited different abilities to interact and synergize with Isl1, extending the crucial transcriptional modulatory role of the HNF4α F domain. HNF4α interacted with both the homeodomain and the first LIM domain of Isl1. We found that the transcriptional synergy between HNF4α and Isl1 involved an increase in HNF4α loading on promoter. The effect was more pronounced on the rat insulin I promoter containing binding sites for both HNF4α and Isl1 than on the human HNF1α promoter lacking an Isl1 binding site. Moreover, Isl1 could mediate the recruitment of the cofactor CLIM2 resulting in a further transcriptional enhancement of the HNF1α promoter activity.