Different signaling pathways utilized by insulin to regulate the expression of ENT2, CNT1, CNT2 nucleoside transporters in rat cardiac fibroblasts

In cardiac fibroblasts (CFs), insulin was shown to affect the expression of ENT2, CNT1, and CNT2 transporter. In the present study, we determined the signaling pathways utilized by insulin to regulate the expression of these nucleoside transporters. In the primary culture of rat CFs, insulin increased the mRNA level of ENT2 and suppressed the CNT1 and CNT2 mRNA levels. The insulin-induced increase of the ENT2 mRNA level was blocked by rapamycin (an inhibitor of mTOR) and by cycloheximide (an inhibitor of protein synthesis), whereas neither wortmannin (an inhibitor of PI3K) nor PD98059 (an inhibitor of MEK) affected the insulin action on the ENT2 transcript level. PD98059 completely blocked the insulin-induced decrease of the CNT1 and CNT2 mRNAs levels. Wortmannin prevented the insulin-induced change of the CNT1 mRNA level, but had no effect on the CNT2 mRNA. Rapamycin abolished the insulin effect on the CNT1 mRNA level, but not on the CNT2 mRNA. Cycloheximide prevented the insulin-induced decrease of CNT2 mRNA, but had no effect on the CNT1 mRNA level. Overall, our results demonstrate that the expression level of ENT2, CNT1, and CNT2 transporters in CFs is differentially regulated by insulin. Moreover, in this cell type insulin employs a distinct signaling pathway to regulate the expression of each transporter.