| Glucose-6-phosphatase (G6Pase) plays an important role in glucose metabolism by catalyzing the terminal step of both glycogenolysis and gluconeogenesis. Although G6Pase is proposed to be a multifunctional and multicomponent system residing in the membrane of endoplasmic reticulum, until now neither the structure of its components nor the function of each protein has been totally understood. So far two components of the G6Pase system have been cloned, including the G6Pase catalytic subunit (p36) and the putative glucose-6-phosphate translocase (p46). Genetic deficiency of G6Pase leads to glycogen storage disease type-I (GSD-I), while mutations in p36 and p46 genes account for GSD-Ia and most of GSD-I non a respectively. Furthermore, diabetes mellitus is associated with increased G6Pase activity, which may contribute to the enhanced hepatic glucose production.;Previous studies have shown that p36 gene expression is under nutritional and hormonal regulation. In this work, the gene regulation of newly cloned p46 was investigated and compared with that of p36 gene. We found that under the conditions like increased glucose concentration, dietary phosphate deprivation or streptozotocin-induced diabetes, p36 and p46 genes were similarly up-regulated. However, the sensitivities of these two genes to different hormones or reagents were found to be quite different as shown in HepG2 hepatoma cells. Insulin has dominant negative effects on both p36 and p46 gene expression, but compared to p36, p46 gene has a much lower sensitivity to insulin. Glucagon, cAMP and thapsigargin significantly increase p36 gene transcription but barely affect p46 gene, while glucocorticoids remarkably and sensitively induce both genes. Based on the distinct hormonal regulation of p36 and p46 gene expression, their possible roles in glucose metabolism were proposed.;We explored in two ways to study the yet unclear p46 function: (1) On the one hand, in order to study the p46 function in hepatic G6Pase system, we performed p46 overexpression in hepatocytes via recombinant adenovirus mediated gene transfer, which resulted in induced p36 transcription and increased G6Pase activity. In addition, overexpression of p46 led to significant metabolic impacts in primary hepatocytes, including decreased glycogen synthesis, increased glycogen degradation and decreased glycolysis; (2) On the other hand, we studied p46 gene transcription in leucocytes, where p36 is absent, and identified four different p46 transcripts, three of which are not present in liver. We hypothesize that mutated p46 gene might be responsible for neutropenia and neutrophil dysfunctions seen in GSD-Ib and Ic; p46 may bear other functions in leucocytes by differential mRNA splicing.;In conclusion, we characterized the gene regulation of newly cloned p46 gene, investigated effects of adenovirus mediated overexpression of p46 on glucose and glycogen metabolisms and discovered different transcripts of p46 gene in leucocytes. |
