Differential target molecules for toxicity induced by streptozotocin and alloxan in pancreatic islets of mice in vitro

Streptozotocin (STZ) and alloxan (ALX) are potent diabetogens in different species of laboratory animals. Here, we describe differential in vitro effects of STZ and ALX on beta-cell molecules that are essential for glucose transport and metabolism, the glucose transporter 2 (GLUT2) and glucokinase (GK), respectively. Incubation of isolated pancreatic islets of C57 BL/6 mice with STZ or ALX for 30 min resulted in a concentration-dependent gradual loss of beta-cell function as determined by basal and D-glucose (D-G)-stimulated insulin release. ALX concentration-dependently reduced the mRNA expression of GLUT2 and GK and the effect on GLUT2 was more marked. STZ, in contrast, did not affect the mRNA expression of GLUT2 and GK, but concentration-dependently reduced the GLUT2 protein expression. Both STZ and ALX failed to affect the mRNA expression of proinsulin and of beta-actin. The deleterious effects of STZ and ALX were not due to beta-cell loss, because the total RNA yields and protein contents as well as the proinsulin mRNA expression in isolated islets of the differentially treated islets did not differ significantly from controls. Furthermore, islets that had been exposed to STZ or ALX responded to the non-glucose secretagogue arginine in a pattern comparable to that of solvent-treated cultures. When preincubating islet cultures with either D-G or its chemically closely related analogue 5-thio-D-glucose (5-T-G), different effects were obtained after treatment with either ALX or STZ. Thus, preincubation with 5-T-G protected the cultures from STZ-induced GLUT2 protein reduction, whereas D-G failed to do so. Preincubation with D-G, however, protected the cultures from ALX-induced reduction of GLUT2 and GK mRNA expression, whereas 5-T-G, at best, exerted a modest protection against ALX at a concentration of 1 mmol/l. Apparently, in vitro, GLUT2 protein is a key target molecule for STZ, while GLUT2 mRNA and GK mRNA are target molecules for ALX.