GLUT4 repression in response to oxidative stress is associated with reciprocal alterations in C/EBP alpha and delta isoforms in 3T3-L1 adipocytes

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Abstract
Insulin responsiveness of adipocytes is acquired during normal adipogenesis, and is essential for maintaining whole-body insulin sensitivity. Differentiated adipocytes exposed to oxidative stress become insulin resistant, exhibiting decreased expression of genes like the insulin-responsive glucose transporter GLUT4. Here we assessed the effect of oxidative stress on DNA binding capacity of C/EBP isoforms known to participate in adipocyte differentiation, and determine the relevance for GLUT4 gene regulation. By electrophoretic mobility shift assay, nuclear proteins from oxidized adipocytes exhibited decreased binding of C/EBPα-containing dimers to a DNA oligonucleotide harboring the C/EBP binding sequence from the murine GLUT4 promoter. C/EBPδ-containing dimers were increased, while C/EBPβ-dimers were unchanged. These alterations were mirrored by a 50% decrease and a 2-fold increase in the protein content of C/EBPα and C/EBPδ, respectively. In oxidized cells, GLUT4 protein and mRNA levels were decreased, and a GLUT4 promoter segment containing the C/EBP binding site partially mediated oxidative stress-induced repression of a reported gene. The antioxidant lipoic acid protected against oxidation-induced decrease in GLUT4 and C/EBPα mRNA, but did not prevent the increase in C/EBPδ mRNA. We propose that oxidative stress induces adipocyte insulin resistance partially by affecting the expression of C/EBPα and δ, resulting in altered C/EBP-dimer composition potentially occupying the GLUT4 promoter.

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