Preventing Phosphorylation of Sterol Regulatory Element-Binding Protein 1a by MAP-Kinases Protects Mice from Fatty Liver and Visceral Obesity

Jorg Kotzka,Ulrike Nitzgen,Ulrike Nitzgen,Birgit Knebel,Sonja Hartwig,Lorena Kremer,Ulrike Nitzgen,Ulrike Nitzgen,Lorena Kremer,Lorena Kremer,Sylvia Jacob,Sonja Hartwig,Lorena Kremer,Jutta Haas,Birgit Knebel,Dirk Muller–Wieland

doi:10.1371/journal.pone.0032609

Abstract

The transcription factor sterol regulatory element binding protein (SREBP)-1a plays a pivotal role in lipid metabolism. Using the SREBP-1a expressing human hepatoma cell line HepG2 we have shown previously that human SREBP-1a is phosphorylated at serine 117 by ERK-mitogen-activated protein kinases (MAPK). Using a combination of cell biology and protein chemistry approach we show that SREBP-1a is also target of other MAPK-families, i.e. c-JUN N-terminal protein kinases (JNK) or p38 stress activated MAP kinases. Serine 117 is also the major phosphorylation site in SREBP-1a for JNK. In contrast to that the major phosphorylation sites of p38 MAPK family are serine 63 and threonine 426. Functional analyses reveal that phosphorylation of SREBP-1a does not alter protein/DNA interaction. The identified phosphorylation sites are specific for both kinase families also in cellular context. To provide direct evidence that phosphorylation of SREBP-1a is a regulatory principle of biological and clinical relevance, we generated transgenic mice expressing mature transcriptionally active N-terminal domain of human SREBP–1a variant lacking all identified phosphorylaton sites designed as alb-SREBP-1aΔP and wild type SREBP-1a designed as alb-SREBP-1a liver specific under control of the albumin promoter and a liver specific enhancer. In contrast to alb-SREBP–1a mice the phosphorylation–deficient mice develop no enlarged fatty livers under normocaloric conditions. Phenotypical examination reveales a massive accumulation of adipose tissue in alb-SREBP-1a but not in the phosphorylation deficient alb-SREBP-1aΔP mice. Moreover, preventing phosphorylation of SREBP-1a protects mice also from dyslipidemia. In conclusion, phosphorylation of SREBP-1a by ERK, JNK and p38 MAPK-families resembles a biological principle and plays a significant role, in vivo.

Highlights

Sterol regulatory element binding proteins (SREBPs) are the predominant transcription factors controlling the synthesis of cholesterol and fatty acids in liver [1]
The results showed that sterol regulatory element binding protein (SREBP)-1a-NT was an efficient substrate for all kinases with an average of 0.9 mole phosphates per mole protein for JUN N-terminal protein kinases (JNK) isoforms and 1.5 mole phosphates per mole protein for the p38 isoforms
The phosphorylation efficiency of JNK1 or JNK2 on SREBP-1a S117A was reduced by 90%, whereas phosphorylation by the p38 kinase isoforms were not altered compared to wild type (Figure 1A)

Summary

Introduction

Sterol regulatory element binding proteins (SREBPs) are the predominant transcription factors controlling the synthesis of cholesterol and fatty acids in liver [1]. The different functions for the SREBP-1 isoforms are mainly thought to be casued by the different length of the N-terminal transactivation domain. Whereas this domain of SREBP-2 and SREBP-1a has a comparable length the respective N-Terminal domain of SREBP-1c is much shorter and a much weaker transcriptional activator. Whereas SREBP-1a and -2 is active to a similar degree on genes with a promoter with the classical sre-1element SREBP-1c is not. On the other hand SREBP-1a and SREBP-1c act comparable on genes with a promoter containing E-box element, but not SREBP-2 [8,9,11,12]

Methods

Results

Conclusion