Effects of streptozotocin in vitro on proinsulin biosynthesis, insulin release and ATP content of isolated rat islets of Langerhans

Abstract

Summary. Proinsulin synthesis, insulin release and intracellular ATP concentrations were measured in isolated rat islets of Langerhans under control conditions of in vitro incubation and after treatment with several concentrations of streptozotocin for different periods of time. It was found that streptozotocin inhibited proinsulin synthesis, as well as insulin release, in a time and concentration dependent manner. The characteristics of the inhibition of these two processes were similar in general terms, but one dissimilarity was noted, i.e. after 60 min exposure to a high concentration of streptozotocin, proinsulin synthesis was inhibited more than insulin release. ATP content was reduced by high concentrations of streptozotocin, but it was found that proinsulin synthesis and insulin release could be inhibited without any effect on ATP content by a low (0.22 mM) concentration of streptozotocin. The effect of streptozotocin on proinsulin synthesis was judged to be the result of a target specificity for the B-cell rather than a specific effect on proinsulin relative to total protein synthesis. [4] activity, has been structurally characterized as 2-deoxy-2-(3'-methyl-3'-nitrosoureido)-D-glucopyranose [5]. The compound has been used extensively for the induction of experimental diabetes in laboratory animals and may, because of the reproducibility of its effects, be the agent of choice for this purpose [6][7][8][9][10][11][12][13][14][15][16].The mechanism by which streptozotocin selectively destroys ttie B-cells of the islets of Langerhans is not known, although lowering of intracellular nicotinamide adenine dinucleotide (NAD) levels may be involved. Certainly, the administration of nicotinamide, a precursor of NAD in many tissues, protects against the diabetogenic effects of subsequently administered streptozotocin [17][18][19][20][21][22][23][24][25]. This protection may, however, unmask a long term oncogenic effect of the drug [26,27].Studies of the effects of streptozotocin on islets of Langerhans have shown a correlation between the extent of B-cell destruction and the decrease in islet NAD [25]; decreased oxidation of glucose by islets isolated after in vivo administration of a diabetogenic dose of streptozotocin [28]; and a dose-related inhibition of glucose-induced insulin secretion during two hours of incubation, when the drug was added in vitro to isolated islets [29]. The purpose of the present study was that of investigating the effects of streptozotocin on proinsulin synthesis, while simultaneously measuring its effects upon insulin release. Only one preliminary report dealing with proinsulin synthesis and streptozotocin has appeared [30].

Regulation of proinsulin synthesis in pancreatic islets and a new aspect to insulin-dependent diabetes

“…A protecting effect of glucose against the action of alloxan has been shownboth in vivo (81) and in vitro (82), whereas the action of streptozotocin has been described not to be affected by glucose (61,62). HoweVer, our recent study also revealed that the inhibitory effects on proinsulin synthesis not 0oo) only of alloxan but also of streptozotocin are (47) significantly lessened by glucose.…”

“…On the other hand, it is known that streptozotocin, a typical diabetogenic agent, depresses NAD content in pancreatic islets and inhibits proinsulin synthesis (60,61). As nicotinamide prevents these effects of streptozotocin (62)(63)(64), it has been so far considered that streptozotocin impairs NAD synthesis in islets to cause diabetes (65,66).…”

Poly(ADP-ribose) polymerase gene disruption conferred mice resistant to streptozotocin-induced diabetes

“…STZ-treated Parp Ϫ/Ϫ mice showed a delayed onset of mild but transient hyperglycemia. Because exposure of isolated islets to STZ in vitro also results in a decrease of glucose oxidation to CO 2 (9) and insulin synthesis (25), in addition to the direct DNA damage, these effects may contribute to the delayed and slight ␤-cell dysfunction also seen in Parp Ϫ/Ϫ mice. Blood insulin levels (data not shown) and glucose concentrations were similar in untreated Parp Ϫ/Ϫ and Parp ϩ/ϩ mice.…”

Evidence for the presence of insulin binding sites in isolated rat intestinal epithelial cells

“…Pork monocomponent insulin was iodinated by the chloramine T method to a specific activity of 150-180 ~tCi/~g (0.4-0.5 atom of iodine per molecule, purified on Sephadex G 50 [30], and tested prior to use for its ability to bind to rat liver plasma membranes [12]. Pork monocomponent insulin, crystalline beef insulin, pork proinsulin were obtained from Novo, Novoall6, DK-2880 Bagsvaerdt (Denmark).…”