Hepatic phosphorylation of transcription factor SREBP-1a interferes with gene regulation and peroxisomal function

The key lipid metabolism transcription factor sterol regulatory element-binding protein (SREBP)-1a integrates gene regulatory effects of hormones, cytokines, nutrition and metabolites as lipids, glucose or cholesterol via stimuli specific phosphorylation by different MAPK cascades. We have formerly reported the systemic impact of phosphorylation in transgenic mouse models with liver-specific overexpression of the N-terminal transcriptional active domain of SREBP-1a (alb-SREBP-1a) or a MAPK kinase phosphorylation sites deficient variant (alb-SREBP-1aP; (S63A, S117A, T426V)), respectively. Here we investigated the molecular basis of the systemic observation in holistic hepatic gene expression analyses and lipid degrading organelles involved in the pathogenesis of metabolic syndrome, i.e. peroxisomes, by 2D-DIGE and mass spectrometry analyses. Although alb-SREBP-1a mice develop a severe phenotype with visceral adipositas and hepatic lipid accumulation featuring a fatty liver, the hepatic differential gene expression and alterations in peroxisomal protein patterns compared to control mice were surprisingly relative low. In contrast, phosphorylation site deficient alb-SREBP-1aP mice, protected from hepatic lipid accumulation phenotype, showed gross alteration in hepatic gene expression and peroxisomal proteome. Further knowledge based analyzes revealed that overexpression of SREBP-1a favored mainly acceleration in lipid metabolism and indicated a regular insulin signaling, whereas disruption of SREBP-1a phosphorylation resulted in massive alteration of cellular processes including signs for loss of lipid metabolic targets. These results could be the link to a disturbed lipid metabolism that overall resembles a state of insulin resistance.

24

Knebel B, Hartwig S, Jacob S, Kettel U, Schiller M, Passlack W, Koellmer C, Lehr S, Müller-Wieland D, Kotzka J