Mechanisms of mutual functional interactions between HNF-4α and HNF-1α revealed by mutations that cause maturity onset diabetes of the young

Hepatic nuclear factor (HNF)-4α and HNF-1α are key endodermal transcriptional regulators that physically and functionally interact. HNF-4α and HNF-1α cooperatively activate genes with binding sites for both factors, whereas suppressive interactions occur at regulatory sequences with a binding site for only one factor. The liver fatty acid binding protein gene ( Fabp1) has binding sites for both factors, and chromatin precipitation assays were utilized to demonstrate that HNF-4α increased HNF-1α Fabp1 promoter occupancy during cooperative transcriptional activation. The HNF4 P2 promoter contains a HNF-1 but not HNF-4 binding site, and HNF-4α suppressed HNF-1α HNF4 P2 activation and decreased promoter HNF-1α occupancy. The apolipoprotein C III ( APOC3) promoter contains a HNF-4 but not HNF-1 binding site, and HNF-1α suppressed HNF-4α APOC3 activation and decreased HNF-4α promoter occupancy. Maturity onset diabetes of the young (MODY) as well as defects in hepatic lipid metabolism result from mutations in either HNF-4α or HNF-1α. We found that MODY missense mutant R127W HNF-4α retained wild-type individual Fabp1 activation and bound to HNF-1α better than wild-type HNF-4α, yet did not cooperate with HNF-1α or increase HNF-1α Fabp1 promoter occupancy. The R127W mutant was also defective in both suppressing HNF-1α activation of HNF4 P2 and decreasing HNF-1α promoter occupancy. The HNF-1α R131Q MODY mutant also retained wild-type Fabp1 activation and bound to HNF-4α as well as the wild type but was defective in both suppressing HNF-4α APOC3 activation and decreasing HNF-4α promoter occupancy. These results suggest HNF-1α-HNF-4α functional interactions are accomplished by regulating factor promoter occupancy and that defective factor-factor interactions may contribute to the MODY phenotype.