THE MATURITY-ONSET DIABETES OF THE YOUNG (MODY1) TRANSCRIPTION FACTORHNF4-ALPHA REGULATES EXPRESSION OF GENES REQUIRED FOR GLUCOSE-TRANSPORT AND METABOLISM

Hepatocyte nuclear factor 4 alpha (HNF4 alpha) plays a critical role i n regulating the expression of many genes essential for normal functio ning of liver, gut, kidney, and pancreatic islets. A nonsense mutation (Q268X) in exon 7 of the HNF4 alpha gene is responsible for an autoso mal dominant, early-onset form of non-insulin-dependent diabetes melli tus (maturity-onset diabetes of the young; gene named MODY1). Although this mutation is predicted to delete 187 C-terminal amino acids of th e HNF4 alpha protein the molecular mechanism by which it causes diabet es is unknown. To address this, we first studied the functional proper ties of the MODY1 mutant protein. We show that it has lost its transcr iptional transactivation activity, fails to dimerize and bind DNA, imp lying that the MODY1 phenotype is because of a loss of HNF4 alpha func tion. The effect of loss of function on HNF4 alpha target gene express ion was investigated further in embryonic stem cells, which are amenab le to genetic manipulation and can be induced to form visceral endoder m. 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 HNF 4-dependent gene regulation in glucose homeostasis. By exploiting this system we have identified several genes encoding components of the gl ucose-dependent insulin secretion pathway whose expression is dependen t upon HNF4 alpha. These include glucose transporter 2, and the glycol ytic enzymes aldolase B and glyceraldehyde-3-phosphate dehydrogenase, and liver pyruvate kinase. In addition we have found that expression o f the fatty acid binding proteins and cellular retinol binding protein also are down-regulated in the absence of HNF4 alpha. These data prov ide direct evidence that HNF4 alpha is critical for regulating glucose transport and glycolysis and in doing so is crucial for maintaining g lucose homeostasis.