Subcellular localization of G-protein alpha subunits in adult rat islets

Decreased ADP-ribosylation of the Galpha(olf) and Galpha(s) subunits by high glucose in pancreatic B-cells

In HIT-T15 insulinoma B-cells incubated in presence of [(32)P]NAD, we identified by autoradiography and immunoblotting ADP-ribosylation (ADP-R) of the trimeric G-protein Galpha(s) and Galpha(olf) subunits (45 kDa) induced by cholera toxin in M1 (120,000g) and M2 (70,000g) subcellular fractions containing plasma membranes, insulin granules, and mitochondria. This ADP-R indicates that these two fractions contain functionally competent Galpha subunits for adenylyl cyclase activation. Prolonged exposure of HIT-T15 cells to high glucose (25 mM instead of 6 mM) specifically reduced the ADP-R in Galpha(s) and Galpha(olf) subunits in the M1 fraction only, despite the clear increase of their accumulation in this compartment. A similar alteration in the ADP-R of the M1-associated Galpha(s) and Galpha(olf) subunits was observed in pancreatic islets isolated from fasted and fed rats. These results may explain, at least in part, the undesirable effects of sustained hyperglycemia on the cAMP-dependent process of insulin secretion in diabetes.

Cellular and subcellular expression of Golf/Gs and Gq/G11 alpha-subunits in rat pancreatic endocrine cells

We studied the cellular and subcellular localization of Galpha-subunits in pancreas by immunocytochemistry. Golfalpha and G11alpha were specifically localized in islet insulin B-cells and glucagon A-cells, respectively. Gsalpha and Gqalpha labeling was more abundant in B-cells. The presence of Golfalpha in B-cells was confirmed by in situ hybridization. In B-cells, Golfalpha and Gsalpha were found in the Golgi apparatus, plasma membrane (PM) and, remarkably, in mature and immature insulin secretory granules, mainly at the periphery of the insulin grains. Gqalpha was detected on the rough endoplasmic reticulum (RER) near the Golgi apparatus. In A-cells, the Galpha-subunits were mostly within the glucagon granules: G11alpha gave the strongest signal, Gsalpha less strong, Gq was scarce, and Golf was practically absent. Gqalpha and Gsalpha immunoreactivity was detected in acinar cells, although it was much weaker than that in islet cells. The cell-dependent distribution of the Galpha-subunits indicates that the stimulatory pathways for pancreatic function differ in acinar and in islet B- and A-cells. Furthermore, the G-protein subunits in islet cell secretory granules might be functional and participate in granule trafficking and hormone secretion.

Galphaolf identification by RT-PCR in purified normal pancreatic B cells and in islets from rat models of non-insulin-dependent diabetes

Recent reports using immunohistochemistry have shown that Galphaolf which shares 88% homology with Galphas was expressed in pancreatic islets. To test the specificity of the expression of this G protein isotype in rat islet cells, B and non-B cells were separated by flow cytometry. The expression of Galphaolf and adenylyl cyclases (AC) of types II, III, V, and VI was evaluated by reverse-transcriptase polymerase chain reaction (RT-PCR). Since alterations in the expression of AC III were recently reported in the GK rat (a model of non-insulin-dependent diabetes mellitus, NIDDM), we also have analyzed the mRNA expression of Galphaolf and AC isoforms in pancreatic islets from GK rats and from adult rats neonatally treated by streptozotocin (nSTZ rats), another model of NIDDM. Southern blots of amplicons generated with specific primers of Galphaolf revealed the presence of a 540-bp band only in B cells. AC of types II, III, V, and VI were expressed both in B and non-B cells. However, AC III mRNA was clearly more abundant in non-B than in B cells. Moreover, in B cells the expression of AC VI was higher than that of AC V, whereas equal expressions of AC V and AC VI were found in non-B cells. In GK rat islets, the mRNA expressions of Galphaolf, AC II, and AC III were clearly increased and no change in AC V and AC VI was found. In nSTZ rat islets, Galphaolf expression was barely detectable, but AC II and AC III mRNA levels were higher than those observed in controls. In conclusion, Galphaolf mRNA appeared specifically expressed in islet B cells and was increased in GK islets. The steady-state mRNA levels of AC II and AC III were clearly increased in the islets of the two rat models of NIDDM. Thus, alterations in the expression of G protein isotypes and AC isoforms could contribute to the diabetic phenotype.

GLP-1 receptors in golden Syrian hamster islets: identification and functional characterization

This study aims at the identification and functional characterization of glucagon-like peptide 1 (7-36) amide (GLP-1) receptor in islets from Golden Syrian hamsters. Using a polyclonal antibody against rat GLP-1 receptors, Western blotting of the islet proteins revealed two major bands of 44 and 70 kDa, similar to those found in rat islets, RINm5F cells, and HIT-T15 cells. In Northern blots, transcripts of 2.7, 3.6 and 3.7 kb were observed in rat islets and RINm5F cells after hybridization with rat GLP-1 receptor cDNA probes of either 21 9 bp or 1.5 kb. Such was not the case in either hamster islets or HIT-T15 cells. However, a single 3.6-kb transcript was observed in the latter two cases when a human GLP-1 receptor cDNA probe of 1.6 kb was used for hybridization. In the isolated perfused pancreas of Golden Syrian hamsters, a rise in D-glucose concentration from 3.3 to 8.3 mM caused a biphasic stimulation of insulin release, which was further increased by either GLP-1 or glucagon (10(-9)M each). The enhancing action of GLP-1 on glucose-stimulated insulin secretion was much more marked than that of glucagon. The rise in D-glucose concentration decreased by 46+/-4% the release of glucagon, but GLP-1 failed to exert any obvious effect on glucagon secretion in the presence of 8.3 mM D-glucose. These results indicate that GLP-1 receptors are expressed in islets of Golden Syrian hamsters with an extracellular part possessing the same immunoreactivity as the rat islet GLP-1 receptors. The expression of the mRNA for the GLP-1 receptor differs, however, from that found in rat or human islets.