Hepatocyte nuclear factor 4alpha (HNF4alpha) plays a critical role in regulating 
the expression of many genes essential for normal functioning of liver, gut, 
kidney, and pancreatic islets. A nonsense mutation (Q268X) in exon 7 of the 
HNF4alpha gene is responsible for an autosomal dominant, early-onset form of 
non-insulin-dependent diabetes mellitus (maturity-onset diabetes of the young; 
gene named MODY1). Although this mutation is predicted to delete 187 C-terminal 
amino acids of the HNF4alpha protein the molecular mechanism by which it causes 
diabetes is unknown. To address this, we first studied the functional properties 
of the MODY1 mutant protein. We show that it has lost its transcriptional 
transactivation activity, fails to dimerize and bind DNA, implying that the 
MODY1 phenotype is because of a loss of HNF4alpha function. The effect of loss 
of function on HNF4alpha target gene expression was investigated further in 
embryonic stem cells, which are amenable to genetic manipulation and can be 
induced to form visceral endoderm. Because the visceral endoderm shares many 
properties with the liver and pancreatic beta-cells, including expression of 
genes for glucose transport and metabolism, it offers an ideal system to 
investigate HNF4-dependent gene regulation in glucose homeostasis. By exploiting 
this system we have identified several genes encoding components of the 
glucose-dependent insulin secretion pathway whose expression is dependent upon 
HNF4alpha. These include glucose transporter 2, and the glycolytic enzymes 
aldolase B and glyceraldehyde-3-phosphate dehydrogenase, and liver pyruvate 
kinase. In addition we have found that expression of the fatty acid binding 
proteins and cellular retinol binding protein also are down-regulated in the 
absence of HNF4alpha. These data provide direct evidence that HNF4alpha is 
critical for regulating glucose transport and glycolysis and in doing so is 
crucial for maintaining glucose homeostasis.