Expression of the glucose-6-phosphatase system in endocrine cells

Abstract

Glucose-6-phosphatase (G6pase) is a multi-component enzyme system, which regulates the catalysis of gluco se-6 -phosphate (G6P) to glucose and inorganic phosphate. This reaction constitutes the final steps of both gluconeogenesis and glycogenolysis, processes that are critical in the maintenance of glucose homeostasis. While glucokinase, the rate limiting enzyme in glucose metabolism in liver and pancreatic islets, phosphorylates glucose to G6P, G6pase dephosphorylates G6P to glucose. In islets, glucose metabolism controls the rate of hormonal secretion. G6pase activity has been shown to be increased in islets isolated from animal models of type 2 diabetes.

Using RT-PCR with hepatic G6pase catalytic subunit (G6PC) primers, we have demonstrated that the sizes of PCR-amplified products from ob/ob mouse islets are identical to those from liver cDNA. This was confirmed by cloning and sequencing of the G6PC from islet cDNA. The expression of the G6P transporter, G6PT1, was also demonstrated, suggesting that all of the identified hepatic G6pase system genes are expressed in pancreatic islets. Finally, the expression of IGRP (islet-specific G6pase-related protein; G6PC2) in pancreatic islets was confirmed, and its expression in liver was also observed. Expression of UGRP (ubiquitous ly-expres sed G6pase-related protein; G6PC3) in pancreatic islets was also demonstrated.

Although both pancreatic alpha-cells and intestinal L-cells express the proglucagon gene, the propeptide is post-translationally modified in a cell-specific manner. Whilst glucagon is the secreted product from pancreatic alpha-cells, glucagon-like peptide-1 (GLP-1) is secreted by the intestinal L-cells. These hormonal peptides act on major organs to maintain glucose homeostasis after and between meals. Because of the technical difficulty of studying purified populations of a- and L-cells, we have used the hamster alpha-cell line InR 1 G9, and the mouse intestinal L-cell cell line, GLUTag, to study the expression of G6pase system genes in these cell types. InR1G9 cells have been shown to release glucagon in response to chronically elevated glucose levels, whilst GLP-1 secretion by GLUTag cells is acutely controlled by the ambient glucose concentration.

Both G6PT1 and G6PC3 were expressed in both cell lines, whilst G6PC and G6PC2 were expressed in InR1G9 and GLUTag cells, respectively. Thus, various components of the G6pase system are expressed in pancreatic islets and endocrine cell lines. The functional relevance of these observations has yet to be demonstrated.