Effects of prednisolone and dexamethasone on (pro)insulin biosynthesis and insulin release by rat islets of Langerhans

Administration of either prednisolone or dexamethasone (10 mg/kg body wt/day/oral) to rats for 21 days resulted in inhibition of (pro)insulin biosynthesis and immunoreactive insulin release by isolated islets. A gradual reversal of prednisolone's effect was obtained after exposing islets to increasing glucose concentrations but glucose challenges failed to influence dexamethasone's effect.     

Influence of pilocarpine pretreatment on (pro)insulin biosynthesis of isolated islets of rats.

3[H]-Leucine incorporation into (pro)insulin and insulin secretion was investigated using islets prepared by collagenase digestion from pancreata of rats pretreated with the cholimimetic agent pilocarpine or with saline (controls). Under the influence of pilocarpine pretreatment the [3H]-leucine incorporation into islet proteins with insulin immunoreactivity is enhanced at 6 mM glucose in the incubation medium of the islets but the incorporated radioactivity at 18 mM glucose is independent of the pretreatment of the animals. Only small or no changes were found regarding insulin secretion. It is concluded that an influence of pilocarpine pretreatment should be taken into consideration using such islets for studies on the regulation of (pro)insulin biosynthesis.     

Studies on the (pro) insulin biosynthesis of islets of Langerhans of sand rats (Psammomys obesus).

By feeding a regular laboratory chow, sand rats (Psammomys obesus) from our breeding colony gained different body weights, though they received approximately the same quantity of calories. Sand rats, reaching a body weight above 160 g (group B) showed significantly increased blood glucose values in contrast to the animals with a body weight under 160 g (group A). Isolated pancreatic islets of these two groups of sand rats were incubated with [3H]-leucine to study the incorporation of this amino acid into proinsulin and insulin. The incorporation into proteins of pancreatic islets of sand rats of group B was stimulated by 0.45 mg and 3.0 mg/ml glucose. In group A there was no further stimulation from 0.45 mg to 3.0 mg/ml glucose. Insulin secretion could be stimulated by glucose in both groups, but the stimulation was stronger in group B than in group A.     

Studies on (pro)insulin biosynthesis and secretion of pancreatic islets of sand rats (Psammomys obesus) after different feeding conditions.

It was possible to induce different metabolic states in sand rats of our breeding colony or in newly caught Egyptian sand rats, respectively, by feeding a pellet diet or vegetable diet (green cabbage). Newly captured sand rats fed only on native food were used as reference group (group C). Plasma IRI-level and glucose in vivo and [3H]-leucine incorporation into proinsulin and insulin, insulin secretion and insulin content in vitro were investigated. Sand rats fed on pellet chow and ad libitum (group B) developed a hyperinsulinism and showed higher sensitivity of [3H]-leucine incorporation into proinsulin and insulin to glucose (maximal stimulation at 3 mM) and increased incorporation rates in vitro. Restriction of pellet food to 35-40 kcal/animal/day (group A) lead to changes of all parameters, which were investigated in the same direction as in group B, but to a much smaller extent. Newly captured sand rats, which were fed green cabbage for 4 to 6 weeks divided into two groups: One group (group D1) was comparable to the normal group (C) in IRI levels, glucose levels, glucose sensitivity and amount of [3H]-leucine incorporation. The other group (group D2) tended to group A. Marked changes in insulin content and insulin secretion of isolated pancreatic islets could not be found in any group.     

The effect of l-ascorbate on catecholamine biosynthesis (Short Communication)

l-Ascorbate stimulates the enzymic hydroxylation of phenylalanine in vitro by recycling tetrahydrobiopterin, which reduces O₂ utilized in the reaction. It is suggested that ascorbate might have a similar function in vivo; this would explain the apparent regulation of tyrosine hydroxylase and tryptophan hydroxylase activities by this vitamin.     

Ganglioside biosynthesis in rat liver: effect of UDP-amino sugars on individual transfer reactions

Several glycosyltransferases participating in ganglioside biosynthesis were measured in Golgi-rich fractions from rat liver. Addition of those UDP-amino sugars to the enzyme assays which accumulate in liver after treatment of rats with D-galactosamine inhibited the transferases to different degrees. The simultaneous presence of UDP-GalN, UDP-GalNAc, UDP-GlcN, and UDP-GlcNAc in concentrations resembling their overall content in livers 6 h after D-galactosamine administration led to an inhibition of the glycolipid galactosyltransferases, GL2 and GM1 synthases of 44 and 64%, respectively. GM2 synthase was moderately inhibited whereas the sialyltransferases (GM3, GD3, and GD1a synthases) were almost unimpaired. Induction of liver cell damage by D-galactosamine did not cause any change of glycosyltransferase activities as determined in rat liver homogenates and Golgi-rich fractions. These results indicate a possible role for UDP-amino sugars in the depression of ganglioside biosynthesis observed in vivo after GalN administration.     

The effect of N-acylglucosamines on the biosynthesis and secretion of insulin in the rat.

The effects of N-acylglucosamines on insulin release have been studied. N-Acylglucosamines stimulated insulin release from rat islets in vitro only if a sub-stimulatory concentration of glucose was also present, and this secretory response was abolished by mannoheptulose. In perifused islets the rapidity of the secreotry response to N-acetyl-D-glucosamine was similar to that observed with D-glucose. Increasing acyl-chain length from N-acetyl- to N-hexanoyl-D-glucosamine impaired the secretory response; however, N-dichloroacetyl-D-glucosamine was a more potent stimulator of release than was N-acetyl-D-glucosamine. Polymers of N-acetyl-D-glucosamine containing two to six monomers linked alpha1-4 did not stimulate insulin release; glucosamine linked to dextran via a propionyl or hexanoyl spacer group was also without insulin-releasing ability. N-Acylglucosamines were also effective in eliciting insulin release in vivo when injected into conscious rats. At the dose used (86 mumol), N-acetylgucosamine elicited a rapid rise in plasma-insulin concentration; N-butyrylglucosamine was less effective, and there was little or no response to N-hexanoylglucosamine. The response to N-dichloroacetyl-glucosamine was greater than that to N-acetylglucosamine; an increase in plasma insulin concentration could be elicited by N-dichloroacetylglucosamine at a dose (17 mumol) at which neither glucose nor N-acetylglucosamine was effective. The secretory response to acetylglucosamine is not mediated by conversion into glucose. Rates of (pro)-insulin biosynthesis by rat islets have been measured (Pro)-insulin biosynthesis was stimulated by glucose, and this response was abolished by mannoheptulose. N-Acetylglucosamine also stimulated (pro)-insulin biosynthesis; this effect of N-acetylglucosamine did not require the presence of glucose, and was not abolished by mannoheptulose. It is concluded that there are differences in signal reception and/or transduction for the processes of insulin biosynthesis and release.     

In vivo regulation of cell death by embryonic (pro)insulin and the insulin receptor during early retinal neurogenesis

Programmed cell death is an established developmental process in the nervous system. Whereas the regulation and the developmental role of neuronal cell death have been widely demonstrated, the relevance of cell death during early neurogenesis, the cells affected and the identity of regulatory local growth factors remain poorly characterized. We have previously described specific in vivo patterns of apoptosis during early retinal neurogenesis, and that exogenous insulin acts as survival factor (Díaz, B., Pimentel, B., De Pablo, F. and de la Rosa, E. J. (1999) Eur. J. Neurosci. 11, 1624-1632). Proinsulin mRNA was found to be expressed broadly in the early embryonic chick retina, and decreased later between days 6 and 8 of embryonic development, when there was increased expression of insulin-like growth factor I mRNA, absent or very scarce at earlier stages. Consequently, we studied whether proinsulin and/or insulin ((pro)insulin) action in prevention of cell death has physiological relevance during early neural development. In ovo treatment at day 2 of embryonic development with specific antibodies against (pro)insulin or the insulin receptor induced apoptosis in the neuroretina. The distribution of apoptotic cells two days after the blockade was similar to naturally occurring cell death, as visualized by TdT-mediated dUTP nick end labeling. The apoptosis induced by the insulin receptor blockade preferentially affected to the Islet1/2 positive cells, that is, the differentiated retinal ganglion cells. In parallel, the insulin survival effect on cultured retinas correlated with the activation of Akt to a greater extent than with the activation of MAP kinase. These results suggest that the physiological cell death occurring in early stages of retinal development is regulated by locally produced (pro)insulin through the activation of the Akt survival pathway.     

Enzymes required for the biosynthesis of N-formylated sugars

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