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.



This 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


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

Rab5 Knockdown In Vivo

This paper from the Zerial Laboratory describes a study in which the three mammalian isoforms of Rab5 are reduced in whole livers or cultured hepatocytes by a lipid nanoparticle mediated knockdown treatment. They then analyse images from these cells and liver sections and determine that there are reduced early endosomes, late endosomes and lysosomes in cells depleted of Rab5.

They also identify two functional defects in these tissues; reduced LDL uptake and impaired apical polarity of the hepatocytes. They also generate a simplified model of Rab5/early endosomal dynamics and suggest a role for Rab5 in vesicle fission as well as fusion. Although this model fits the available data quite well, it ignores several other key components of endosomal formation and maturation.

The authors also present the curious finding that while Rab5 depletion reduces both LDL uptake and endosome numbers, dynamin depletion only affects LDL uptake but not the number of endosomes. One possible explanation for this defect is that they quantify the presence of endosomes in this assay largely by EEA1 puncta. Since EEA1 is a Rab5 effector, it is possible that in the Rab5 depleted cells there is still a population of early endosomes, but that these are not detected by EEA1 staining.

Together this study presents a convincing picture for the role of Rab5 in endosomal dynamics, and highlight the important role of Rab5 in both endocytic pathways and in the establishment of polarity in vivo.

Citation

Zeigerer A, Gilleron J, Bogorad RL, Marsico G, Nonaka H, Seifert S, Epstein-Barash H, Kuchimanchi S, Peng CG, Ruda VM, Del Conte-Zerial P, Hengstler JG, Kalaidzidis Y, Koteliansky V, & Zerial M (2012). Rab5 is necessary for the biogenesis of the endolysosomal system in vivo. Nature, 485 (7399), 465-70 PMID: 22622570

Disclosure: I have received constructs and yeast strains from the Zerial laboratory in the past.



Rab5 Knockdown In Vivo by Dave Bridges is licensed under a Creative Commons Attribution 3.0 Unported License.