Effects of amino acids, glucagon, insulin and acetylcholine on cyclic nucleotide metabolism and amylase secretion in isolated mouse pancreatic fragments

Signaling Proteins Associated with Diabetic-Induced Exocrine Pancreatic Insufficiency in Rats

Diabetes mellitus (DM) is associated with pancreatic atrophy and compromised digestion of carbohydrates as a result of exocrine pancreatic insufficiency and lower alpha-amylase synthesis and secretion. The reduced production of digestive enzymes is likely to be caused by deregulated protein metabolism. The relative concentrations and phosphorylation of signaling proteins associated with protein translation, such as PKB, p70S6K1, 4E-BP1, ERK1/2, and also some of those implicated in protein breakdown, such as ubiquitin and NF-kappaB, in the pancreas of streptozotocin (STZ)-induced type I diabetic pancreas were measured using Western blotting. There were significant decreases in the levels of total PKB, p70S6K, 4E-BP1, ERK1/2, and NF-kappaB in the diabetic pancreas compared to control. In contrast, the phosphorylation of p70S6K1, 4E-BP1, ERK1/2, and protein ubiquitination increased significantly compared to controls. Together, these results indicate that STZ-induced DM leads to reduced levels of enzymes mediating protein synthesis while their phosphorylation is actually increased, perhaps in an attempt to maintain protein homeostasis, which is further compromised by heightened ubiquitin-dependent protein breakdown. It is likely that these factors are responsible for pancreatic atrophy, enzyme synthesis, and net protein loss in DM.

EGF receptor activation stimulates endogenous gastrin gene expression in canine G cells and human gastric cell cultures

Gastrin release from the antral gastrin-expressing cell (G cell) is regulated by bombesin and luminal factors. Yet, these same extracellular regulators do not stimulate expression of the gene. Since the gastric mucosa expresses large quantities of EGF receptor ligands such as TGFalpha, we examined whether EGF receptor ligands stimulate gastrin gene expression in gastrin-expressing cell cultures. EGF receptor activation of primary cultures stimulated gastrin gene expression about twofold; whereas bombesin treatment of antral G cell cultures stimulated gastrin release but not gene expression. EGF and TGFalpha were weak stimulants of gastrin release. EGF receptor activation of AGS human gastric adenocarcinoma cell line stimulated gastrin gene expression nearly fourfold; and gastrin reporter constructs transfected into AGS cells were stimulated more than fourfold by EGF. EGF induction was conferred by the previously defined GC-rich gastrin EGF response element (gERE) element located at -68 to -53 bp upstream from the cap site since a mutation of the gERE element abolished both basal and EGF induction. Moreover, EGF treatment of AGS cells stimulated binding of the transcription factor Sp1 to this element. Collectively, these results demonstrate that gastrin gene expression and gastrin release are regulated by different signaling pathways: gene expression by EGF receptor activation and gastrin secretion by neuropeptides and luminal factors.

Interactions of islet hormones with acetylcholine in the isolated rat pancreas

This study investigates the effects of the islet hormones, insulin (INS), glucagon (GLU) and somatostatin (SOM) on acetylcholine (ACh)-evoked amylase secretion and calcium (Ca2+) mobilization in the isolated rat pancreas. Stimulation of pancreatic segments and acini with either INS, GLU or SOM resulted in small increases of amylase output compared to much large increases in enzyme output with ACh. Combinations of the peptide hormones with ACh resulted in enhanced secretory responses compared to the effects obtained with either ACh or each of the islet hormone alone. Genistein, the tyrosine kinase inhibitor, evoked a decrease in amylase output from pancreatic segments. It had no effect on the ACh evoke secretory response but it markedly inhibited the potentiation of the islet hormones with ACh. In pancreatic acinar cells either INS, GLU or SOM elicited moderate increases in amylase output compared to much larger responses with ACh. Furthermore, the islet hormones failed to potentiate the secretory effect of ACh in pancreatic acini. In fura-2 AM loaded acinar cells both INS and GLU evoked small increases in intracellular free calcium concentration [Ca2+]i compared to a much larger elevation with ACh. Both INS and GLU enhanced the ACh-evoked [Ca2+]i. Genistein elicited a decrease in [Ca2+]i both in the absence and presence of both INS and GLU. It also decreased the rise in [Ca2+]i resulting from the combined presence of ACh with both INS and GLU. SOM had no significant effect on the ACh-induced [Ca2+]i. When genistein was combined with ACh and SOM there was a decrease in [Ca2+]i compared to the response obtained with SOM and ACh alone. The results indicate that both tyrosine kinase and cellular Ca2+ seem to be the intracellular mediators associated with the enhanced secretory responses obtained with a combination of the islet hormones with ACh. Finally, our results using immunohistochemical techniques confirm the presence of INS-, GLU- SOM- and ACh-immunoreactive cells in the endocrine and neural elements of the rat pancreas.

Biochemical aspects of the phosphoinositide signalling system with special reference to the formation of inositol cyclic phosphates and arachidonic acid and metabolites on agonist stimulation

Several aspects of the phosphoinositide signalling system recently studied in our Laboratory are considered here. 1. The formation of inositol 1:2-cyclic-4,5-trisphosphate (IcP3) and inositol 1:2-cyclic-4-bisphosphate (IcP2) have been shown here to occur in pancreatic minilobules stimulated with carbamylcholine. Identification is based on mobility on ionophoresis on paper and on HPLC, acid lability, and conversion of the inositol cyclic phosphates to their respective non-cyclic inositol phosphates on treatment with acid. The levels of inositol 1:2-cyclic phosphate (IcP), IcP2, and IcP3 were 0.7%, 6.8%, and 29.8% of their respective non-cyclic inositol phosphates. The level of IcP3 is sufficient to evoke release of calcium from the endoplasmic reticulum. 2. In a previous study, we demonstrated that on agonist stimulation of pancreatic minilobules prelabelled with [14C]arachidonate, [14C]stearate, or [3H]glycerol, there was a substantial release of all three of these compounds, amounting to approximately 50% of the total PI loss, which was up to 70% of the total cellular PI (7). It was shown that this loss in PI was due to the sequential actions of phospholipase C and diacylglycerol (DG) lipase. Evidence against the phospholipase A2 pathway was no formation of lysophosphatidylinositol. Further evidence against the phospholipase A2 pathway shown here is the lack of stimulation by agonist of glycerophosphorylinositol formation. We also show here that the stimulation of PI loss in guinea pig brain cortex slices is likely also to be via the sequential actions of phospholipase C and DG-lipase, i.e., there was an increase in the steady-state level of monoacylglycerol and a rise in free arachidonate on stimulation with acetylcholine. The formation of prostaglandin E and prostaglandin F was also increased in brain cortex, corpus striatum, and hippocampus. The effects of acetylcholine were abolished by atropine. 3. Previous studies showed that the DG-lipase inhibitor, RHC 80267, inhibited agonist-stimulated formation of glycerol and fatty acids and raised the steady-state level of DG (7). We have now used RHC 80267 as a tool to elevate the level of DG and to lower the level of arachidonate to see if either of these products might modulate the carbamylcholine-stimulated cGMP levels in pancreatic minilobules. RHC 80267 inhibited formation of cGMP. Addition of arachidonate did not affect this inhibition, nor did addition of free arachidonate to control minilobules have any effect, thus suggesting that liberation of free arachidonate by carbamylcholine was not responsible for the carbamylcholine-induced rise in cGMP.(ABSTRACT TRUNCATED AT 400 WORDS)