Tuesday, October 2, 2012

How is SREBP Regulated by Insulin?

SREBP is a transcription factor which integrates anabolic signals and drives transcription of several important lipogenic genes such as Fatty Acid Synthase, Acetyl-CoA Carboxylase and the LDL Receptor.  In liver tissue ,this serves to enhance the uptake, synthesis and storage of lipid in the postprandial state.  The biochemical mechanisms by which this happens is unclear but a recent paper from the Brown and Goldstein laboratory has shed some light on this.

Figure: SREBP Processing and Translocation (from http://en.wikipedia.org/wiki/Srebp)
Previously it had been known that insulin causes both the cleavage and nuclear translocation of SREBP in hepatocytes (see Figure). Confusing the issue was the fact that SREBP was also increased transcriptionally. Owing to the presence of a SRE element in the promoter, I thought that the transcriptional effects were likely due to a positive feedback loop where insulin causes SREBP processing, which in turn causes more transcription of the mRNA.  Adding credence to this hypothesis was the fact that inhibitors of the PI3K->mTORC1 pathways (Wortmannin and Rapamycin) inhibited both transcription and processing of SREBP1.

In the Owen et al. paper, a transgenic rat is generated which puts SREBP1c under the control of a non-insulin responsive promoter, allowing for examination of the processing of SREBP1c independent of the SREBP1c promoter.  Consistent with previous findings, they show that both Wortmannin and Rapamycin block processing and mRNA synthesis, but that another inhibitor LYS6K2 which is specific for S6K (a target of mTORC1) blocks only processing and not mRNA levels.

ResearchBlogging.orgThis not only suggests that S6K is the proximal effector of the PI3K-mTORC1 pathway with respect to processing, but that S6K plays no role in the transcriptional regulation.  This also, for the most part, excludes a role for the SREBP -> SRE positive feedback loop, since under LYS6K conditions, SREBP cleavage is blocked but mRNA levels are unchanged.  Put another way, if the SREBP positive feedback loop was important, then this would suggest that mRNA of SREBP would be reduced under all conditions in which SREBP processing is blocked.

Owen JL, Zhang Y, Bae SH, Farooqi MS, Liang G, Hammer RE, Goldstein JL, & Brown MS (2012). Insulin stimulation of SREBP-1c processing in transgenic rat hepatocytes requires p70 S6-kinase. Proceedings of the National Academy of Sciences of the United States of America PMID: 22927400

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How is SREBP Regulated by Insulin? by Dave Bridges is licensed under a Creative Commons Attribution 3.0 Unported License.