A role for polyamines in glucose-stimulated insulin-gene expression

The aim of the present study was to evaluate the possible role for polyamines in the glucose regulation of the metabolism of insulin mRNA of pancreatic islet cells. For this purpose islets were prepared from adult mice and cultured for 2 days in culture medium RPMI 1640 containing 3.3 mM- or 16.7 mM-glucose with or without the addition of the inhibitors of polyamine biosynthesis difluoromethylornithine (DFMO) and ethylglyoxal bis(guanylhydrazone) (EGBG). Culture at the high glucose concentration increased the islet contents of both insulin mRNA and polyamines. The synthesis of total RNA, total islet polyamines and polyamines associated with islet nuclei was also increased. When the combination of DFMO and EGBG was added in the presence of 16.7 mM-glucose, low contents of insulin mRNA, spermine and spermidine were observed. Total islet polyamine synthesis was also depressed by DFMO + EGBG, unlike islet biosynthesis of polyamines associated with nuclei, which was not equally decreased by the polyamine-synthesis inhibitors. Total RNA synthesis and turnover was not affected by DFMO + EGBG. Finally, actinomycin D attenuated the glucose-induced enhancement of insulin mRNA, and cycloheximide counteracted the insulin-mRNA attenuation induced by inhibition of polyamine synthesis. It is concluded that the glucose-induced increase in insulin mRNA is paralleled by increased contents and rates of polyamine biosynthesis and that an attenuation of the increase in polyamines prevents the increase in insulin mRNA. In addition, the results are compatible with the view that polyamines exert their effects on insulin mRNA mainly by increasing the stability of this messenger.