Analysis of upstream glucokinase promoter activity in transgenic mice and identification of glucokinase in rare neuroendocrine cells in the brain and gut.

A transgene consisting of an upstream glucokinase (GK) promoter fragment linked to coding sequences of the human growth hormone gene was expressed in certain neuroendocrine cells of the pancreas, pituitary, brain, gut, thyroid, and lungs of mice. In pancreas, the transgene was expressed in a nonuniform manner among beta cells and in a variable but substantial fraction of the other islet cell types. In pituitary, it was expressed in corticotropes, and in brain, it was expressed in cells of the medial hypothalamus. Within the gut transgene expression was detected in a subset of enteroendocrine cells of the stomach and duodenal epithelium, some of which also exhibited glucagon-like polypeptide-1 immunoreactivity. In thyroid, transgene expression was observed in C cells of neonatal animals, whereas in the lung, it was expressed among rare endocrine cells of the bronchopulmonary mucosa. RNA polymerase chain reaction analysis of human growth hormone mRNA corroborated the tissue-specific transgene expression pattern. Prompted by the finding of transgene expression in specific neuroendocrine cells, we sought to determine whether GK mRNA and GK itself was also expressed in the brain and gut, tissues not previously associated with the expression of this enzyme. Using rat tissues, GK mRNA was detected by RNA polymerase chain reaction in both the brain and intestine and was localized to specific cells in the hypothalamus and enteric mucosa by in situ hybridization. A high Km glucose phosphorylating activity was detected from isolated rat jejunal enterocytes that displayed a chromatographic elution profile identical to hepatic GK. GK immunoreactivity was detected in cells of the medial hypothalamus with many of the same cells also displaying GLUT2 immunoreactivity. Together, these studies provide evidence for upstream GK promoter activity, GK mRNA, and GK itself in certain neuroendocrine cells outside the pancreatic islet and lead us to suggest that GK may play a broader role in glucose sensing by neuroendocrine cells than was thought previously.