Liver-specific reactivation of the inactivated Hnf-1alpha gene: elimination of liver dysfunction to establish a mouse MODY3 model

Mice deficient in hepatocyte nuclear factor 1 alpha (HNF-1alpha) develop dwarfism, liver dysfunction, and type 2 diabetes mellitus. Liver dysfunction in HNF-1alpha-null mice includes severe hepatic glycogen accumulation and dyslipidemia. The liver dysfunction may appear as soon as 2 weeks after birth. Since the HNF-1alpha-null mice become diabetic 2 weeks after birth, the early onset of the liver dysfunction is unlikely to be due to the diabetic status of the mice. More likely, it is due directly to the deficiency of HNF-1alpha in liver. Although the HNF-1alpha-null mice have an average life span of 1 year, the severe liver phenotype has thwarted attempts to study the pathogenesis of maturity-onset diabetes of the young type 3 (MODY3) and to examine therapeutic strategies for diabetes prevention and treatment in these mice. To circumvent this problem, we have generated a new Hnf-1alpha mutant mouse line, Hnf-1alpha(kin/kin), using gene targeting to inactivate the Hnf-1alpha gene and at the same time, to incorporate the Cre-loxP DNA recombination system into the locus for later revival of the Hnf-1alpha gene in tissues by tissue-specifically expressed Cre recombinase. The Hnf-1alpha(kin/kin) mice in which the expression of HNF-1alpha was inactivated in germ line cells were indistinguishable from the HNF-1alpha-null mice with regard to both the diabetes and liver phenotypes. Intriguingly, when the inactivated Hnf-1alpha gene was revived in liver (hepatic Hnf-1alpha revived) by the Cre recombinase driven by an albumin promoter, the Hnf-1alpha(kin/kin) mice, although severely diabetic, grew normally and did not develop any of the liver dysfunctions. In addition, we showed that the expression of numerous genes in pancreas, including a marker gene for pancreas injury, was affected by liver dysfunction but not by the deficiency of HNF-1alpha in pancreas. Thus, our hepatic-Hnf-1alpha-revived mice may serve as a useful mouse model to study the human MODY3 disorder.     

Functional inhibitory cross-talk between constitutive androstane receptor and hepatic nuclear factor-4 in hepatic lipid/glucose metabolism is mediated by competition for binding to the DR1 motif and to the common coactivators, GRIP-1 and PGC-1alpha

The role of the constitutive androstane receptor (CAR) in xenobiotic metabolism by inducing expression of cytochromes P450 is well known, but CAR has also been implicated in the down-regulation of key genes involved in bile acid synthesis, gluconeogenesis, and fatty acid beta-oxidation by largely unknown mechanisms. Because a key hepatic factor, hepatic nuclear factor-4 (HNF-4), is crucial for the expression of many of these genes, we examined whether CAR could suppress HNF-4 transactivation. Expression of CAR inhibited HNF-4 transactivation of CYP7A1, a key gene in bile acid synthesis, in HepG2 cells, and mutation of the DNA binding domain of CAR impaired this inhibition. Gel shift assays revealed that CAR competes with HNF-4 for binding to the DR1 motif in the CYP7A1 promoter. TCPOBOP, a CAR agonist that increases the interaction of CAR with coactivators, potentiated CAR inhibition of HNF-4 transactivation. Furthermore, inhibition by CAR was reversed by expression of increasing amounts of GRIP-1 or PGC-1alpha, indicating that CAR competes with HNF-4 for these coactivators. Treatment of mice with phenobarbital or TCPOBOP resulted in decreased hepatic mRNA levels of the reported genes down-regulated by CAR, including Cyp7a1 and Pepck. In vivo recruitment of endogenous CAR to the promoters of Cyp7a1 and Pepck was detected in mouse liver after phenobarbital treatment, whereas association of HNF-4 and coactivators, GRIP-1, p300, and PGC-1alpha, with these promoters was significantly decreased. Our data suggest that CAR inhibits HNF-4 activity by competing with HNF-4 for binding to the DR1 motif and to the common coactivators, GRIP-1 and PGC-1alpha, which may be a general mechanism by which CAR down-regulates key genes in hepatic lipid and glucose metabolism.