Abstract
Current evidence suggests that uncoupling protein-2 (UCP2) is a regulator of insulin secretion. It is also known that chronic exposure of pancreatic islets to free fatty acids (FFAs) blunts glucose-stimulated insulin secretion and is accompanied by elevated levels of UCP2. However, the mechanisms regulating expression of UCP2 in beta-cells are unknown. Here, we show that UCP2 mRNA and protein levels were increased after a 48-h exposure of INS-1(832/13) beta-cells to oleic acid (0.5 mm) by activation of the UCP2 promoter. Furthermore, progressive deletions of the mouse UCP2 promoter (from -7.3 kb to +12 bp) indicated that an enhancer region (-86/-44) was responsible for both basal and FFA-stimulated UCP2 gene transcription. This enhancer contains tightly clustered Sp1, sterol regulatory element (SRE), and double E-Box elements. While all three sequence motifs were required for basal activity of the UCP2 promoter, the mutations in either the SRE or the E-Box elements eliminated the response to FFAs. The SRE and sterol regulatory element binding protein-1 (SREBP1) appear to be crucial for the response of the UCP2 gene to FFAs, since overexpression of the nuclear forms of the SREBPs increased UCP2 promoter activity by 7-10-fold and restored the ability of E-Box mutants to respond to oleic acid. These data support a model in which SREBP is the major modulator of UCP2 gene transcription by FFA, while E-Box binding factors play a supportive role.
Highlights
Uncoupling protein-2 (UCP2)1 is expressed in many tissues that are important for regulating carbohydrate and lipid metabolism, most notably pancreatic -cells, white and brown adipocytes, skeletal muscle, and hypothalamus
Fatty Acids Increase Expression of uncoupling protein-2 (UCP2) in INS-1 Cells through the Ϫ86/Ϫ44 Enhancer Region—Since UCP2 mRNA levels are increased in pancreatic -cells after prolonged exposure to free fatty acids (FFAs) [6, 7], we tested whether this response resulted from transcriptional regulation of the UCP2 gene
Based on these findings we searched for the regulatory regions in the UCP2 promoter that serve as the molecular sensor of FFAs
Summary
Uncoupling protein-2 (UCP2)1 is expressed in many tissues that are important for regulating carbohydrate and lipid metabolism, most notably pancreatic -cells, white and brown adipocytes, skeletal muscle, and hypothalamus. We show that UCP2 mRNA and protein levels were increased after a 48-h exposure of INS-1(832/13) -cells to oleic acid (0.5 mM) by activation of the UCP2 promoter.
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