Necessity of endogenous GTP derived from glucose-6-phosphate for insulin secretion augmented by glucose under protein kinase A activation

Triggering of insulin release by a combination of cAMP signal and nutrients: an ATP-sensitive K+ channel-independent phenomenon

Nutrient augmentation of Ca(2+)-triggered insulin release occurs in an ATP-sensitive K(+) (K(ATP)) channel--independent manner. Here, using rat islets, we explored the possibility of the K(ATP) channel-independent nutrient triggering of insulin release. In the presence of 250 micromol/l diazoxide, simultaneous application of forskolin and 16.7 mmol/l glucose strongly stimulated insulin release: fourfold and eightfold increases with 1 and 30 micromol/l forskolin, respectively. alpha-Ketoisocaproate (KIC) and 3-isobutylmethylxanthine (IBMX) could be used in place of glucose and forskolin, respectively, to trigger insulin release in the presence of diazoxide. Triggering of insulin release by a combination of nutrients and forskolin was not attenuated by 10 micromol/l nifedipine (a blocker of voltage-dependent Ca(2+) channels) and 2 micromol/l thapsigargin (an inhibitor of intracellular Ca(2+)-ATPase), ascertaining independence of this phenomenon from Ca(2+) entry and from intracellular Ca(2+) liberation. As anticipated, the action of glucose and KIC was greatly (>80%) suppressed by inhibition of mitochondrial metabolism by 2 mmol/l sodium azide (NaN(3)). A combination of palmitate and dimethyl glutamate (a cell-permeable glutamate donor), but not either one alone, weakly but unequivocally triggered insulin release when applied simultaneously with forskolin. In this case, however, mitochondrial poisoning by azide was without effect. The finding suggests that a combination of induced palmitoylation and cytosolic glutamate accumulation partially reconstituted signaling beyond mitochondrial metabolism in the beta-cell upon glucose stimulation. In conclusion, a combination of cAMP signal and nutrients potently triggers insulin release under full activation of the K(ATP) channel, indicating the multiplicity of driving force for insulin exocytosis.

Distinct effect of diazoxide on insulin secretion stimulated by protein kinase A and protein kinase C in rat pancreatic islets

Protein kinase activation is known to stimulate glucose-induced insulin secretion in the presence of diazoxide. Diazoxide opens the ATP-sensitive K(+) channel and inhibits FAD-linked glycerophosphate dehydrogenase activity in a concentration-dependent manner. In the present study, we examined the effect of lower (100 microM) and higher (250 microM) concentrations of diazoxide on insulin release by protein kinase A (PKA) and protein kinase C (PKC) activation. Forced depolarization by a high potassium concentration, augmented the intracellular Ca(2+) concentration ([Ca(2+)](i)) similarly in the presence of both concentrations of diazoxide. Under this condition, 250 microM diazoxide inhibited insulin release enhanced by PKA activation but not that by PKC. Under a basal concentration of [Ca(2+)](i), PKC activation elicited glucose-induced insulin secretion at 100 and 250 microM diazoxide, while PKA activation did so only at 100 microM. These augmentations were completely inhibited by mannoheptulose, a glucokinase inhibitor. Glyceraldehyde, in place of glucose, enhanced insulin secretion by PKC activation under both concentrations of diazoxide. On the other hand, it did not affect PKA-stimulated insulin release under either conditions, but in the case of 100 microM, glucose augmented the insulin secretion in the presence of glyceraldehyde and db-cAMP concentration-dependently. These data suggest that insulin release stimulated by PKA and PKC activation under diazoxide is dependent on glucose metabolism, and that a signal derived from proximal steps in glycolysis may be necessary for the secretion by PKA activation.

Inosine monophosphate dehydrogenase: A molecular switch integrating pleiotropic GTP-dependent beta-cell functions

Studies of pancreatic islet function in the pathogenesis of type 2 diabetes mellitus have tended to focus on the short-term control of insulin secretion. However, the long-term control of beta-cell mass is also relevant to diabetes, since this parameter is reduced substantially even in non-insulin-dependent diabetes in humans. In animal models of type 2 diabetes, the normal balance between beta-cell proliferation and programmed cell death is perturbed. We take the perspective in this overview that inosine monophosphate dehydrogenase (IMPDH; EC 1.1.1. 205) may represent a previously neglected molecular integrator or sensor that exerts both functional (secretory) and anatomical (proliferative) effects within beta-cells. These properties reflect the fact that IMPDH is a rate-limiting enzyme in the new synthesis of the purine guanosine triphosphate (GTP), which modulates both exocytotic insulin secretion and DNA synthesis, as well as a number of other critical cellular functions within the beta-cell. Alterations in the expression or activity of IMPDH may be central to beta-cell replication, cell cycle progression, differentiation, and maintenance of adequate islet mass, effects that are probably mediated both by GTP directly, and indirectly via low molecular mass GTPases. If GTP becomes depleted, a hierarchy of beta-cell functions becomes progressively paralyzed, until eventually the effete cell is removed via apoptosis.