Medizinische Universität Graz Austria/Österreich - Forschungsportal - Medical University of Graz
Gewählte Publikation:
Sahu-Osen, A.
Regulation of intracellular lipid storage and mobilization
PhD-Studium (Doctor of Philosophy); Humanmedizin; [ Dissertation ] Graz Medical University; 2015. pp.
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- Autor*innen der Med Uni Graz:
- Sahu-Osen Anita
- Betreuer*innen:
- Birner-Grünberger Ruth
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- Abstract:
- Lipid metabolism has gained much attention in the recent years as it has been implicated with the metabolic syndrome and associated pathophysiological complications. The metabolic syndrome is a cluster of medical conditions associated with energy utilization and storage. Therefore, an in-depth understanding of lipid metabolism and the mutations/modifications of its components has become a center of attention for researchers to resolve diseases like diabetes, obesity and lipidoses. Recent work in the intracellular lipid metabolism field has identified several new enzymes and their regulators involved in the regulation of storage, breakdown and mobilization of intracellular neutral lipids. This Ph.D. thesis is mainly focused on the inves-tigation of breakdown of neutral lipids (lipolysis). Adipose tissue is the organ where excess neutral lipids are stored in intra-cellular lipid droplets. It has been studied mostly for lipolysis, which involves mobilization of stored triacylglycerides (TAG) in a regulated manner. ß-adrenergic signaling pathway via protein kinase A (PKA) is the major and most studied signaling pathway which controls intracellular lipolysis in adi-pocytes. It targets the lipolytic enzyme hormone sensitive lipase (HSL) and lipid droplet coating protein perilipins, which can control the activity of lipases. However, there are a few important aspects of intracellular lipolysis which have not been investigated such as whether; I) PKA can also regulate modulators of lipolysis, e.g. Comparative gene identification 58 (CGI-58), by phosphorylation similar to HSL and perilipin and II) if the presence/absence of perilipins affects protein factors which are involved in the regulation of lipid droplet size and structural homeostasis. Fundamentally, this thesis consists of two different, yet related parts which shed light on these two missing links in the regulation of intracellular lipolysis. The first part of the thesis focuses on the regulation of CGI-58 by phos-phorylation on serine 239, a newly identified phosphosite, by protein kinase A (PKA) and the second part focuses on protein factors involved in lipid droplet size regulation. CGI-58/ABHD5 and G0/G1 switch gene 2 (G0S2) regulate the major TAG-lipase Adipose triglyceride lipase (ATGL) either by activating or inhibiting it, respectively. Under basal conditions in adipocytes CGI-58 prevents ATGL interaction with perilipin1, thus preventing access to TAG. However, during PKA activated lipolysis CGI-58 disperses into the cytosol allowing ATGL to access the TAG store. Nonetheless, CGI-58 regulation by PKA is still blurred. Since CGI-58 amino acid sequence contains a PKA consensus sequence RKYS239, we explored the effect of CGI-58 phosphorylation and its contribution in the lipolytic process in part 1 of this thesis. Prof. Ronald P. Kühnlein’s laboratory has previously investigated the effect of two different perilipins on the size of lipid droplets in Drosophila melanogaster. They have clearly demonstrated that loss of perilipin1 (LSD1) results in a gain in lipid droplet size. Additionally, they have shown perilipin 2 (Plin2/LSD2) knock out as well as a perilipin free system (Plin1/Plin2 double knock out) show normal lipid droplet size. These findings motivated us to study the proteome present on lipid droplets from single and complete perilipin knock out D. melanogaster fat bodies in part 2 of this thesis. In summary, findings from this thesis firstly provide insight into effects of CGI-58 phosphorylation on its subcellular localization and secondly identified cyclophilin 1 (Cyp1) as a novel protein factor being involved in lipid droplet size regulation. Together all this adds information to the regulation of intracellular lipid storage and mobilization that is available to date.