Palmitate-Induced Inhibition of Insulin Gene Expression in Rat Islet β-Cells Involves the Ceramide Transport Protein

Endogenous ceramide plays an important role in the palmitate (Palm) impairment of proinsulin gene expression in pancreatic islet β-cells. Changes in the liposoluble ceramide levels not only depend on metabolic enzymes but also on its transport to subcellular sites in response to Palm stimuli. In this study, we show that suppression of ceramide transport protein (CERT) mRNA with small interfering RNA contributed to intracellular ceramide accumulation in response to chronic Palm exposure and impairment of proinsulin gene expression, similar to the effect of inhibiting ceramide scavenging enzyme sphingomyelin synthase (SMS). High dose Palm treatment increased protein kinase D (PKD)-induced phosphorylation of CERT and its dysfunction. Intracellular accumulation of ceramide was associated with reduction of PDX-1 nuclear localization and MafA protein levels and stimulation of CCAAT/enhancer binding protein β (C/EBP β) expression. These conditions also corresponded with a reduction of PDX-1 and MafA and an increase of C/EBP β binding to the insulin promoter. Furthermore, down-regulation of C/EBP β could block ceramide impairment of proinsulin gene expression. The results reveal that Palm-mediated dysfunction of ceramide transport may contribute to intracellular ceramide accumulation and result in dysfunction of pancreatic beta cells by affecting binding of transcription factors to the insulin promoter.