Glucose-induced binding of the polypyrimidine tract-binding protein (PTB) to the 3′-untranslated region of the insulin mRNA (ins-PRS) is inhibited by rapamycin

Despite considerable knowledge on the regulation of insulin gene transcription, little is known about the post-transcriptional control mechanisms of this gene. We have recently reported glucose- and hypoxia-regulated binding of the polypyrimidine tract-binding protein (PTB) to the pyrimidine-rich sequence of the 3'-untranslated insulin mRNA (ins-PRS), an event which may control insulin mRNA stability. The present aim was to probe for the signaling pathways that control this binding activity. Rat islets were exposed to pharmacological inhibitors against several molecules, previously shown to be involved in glucose signaling. The inhibitors used were; LY 294002 (PI3 kinase), Rp-cAMP triatylamine ( the cAMP-dependent protein kinase PKA), bisindolylmaleimide I hydrochloride (PKC), PD 098059 (ERK1/ERK2), SB 203580 (p38/SAPK2a), rapamycin ( mTOR) and okadaic acid (PP1/2A). PTB-binding activity to the ins-PRS was then analyzed by elecrophoretic mobility shift assay ( EMSA). The glucose- induced PTB-binding was only inhibited by the mTOR inhibitor rapamycin. Rapamycin also reduced glucose-induced insulin mRNA expression. Thus, our results suggest an involvement of mTOR in glucose- induced PTB/ins-PRS binding and insulin mRNA stability.