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Triacylglycerol is synthesized in a specific subclass of caveolae in primary adipocytes
Linköping University, Department of Clinical and Experimental Medicine, Cell Biology. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Clinical and Experimental Medicine, Cell Biology. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Clinical and Experimental Medicine, Clinical Chemistry. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Medical and Health Sciences, Internal Medicine. Linköping University, Faculty of Health Sciences.ORCID iD: 0000-0002-1680-1000
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2005 (English)In: Journal of Biological Chemistry, ISSN 0021-9258, E-ISSN 1083-351X, Vol. 280, no 1, 5-8 p.Article in journal (Refereed) Published
Abstract [en]

A principal metabolic function of adipocytes is to synthesize triacylglycerol (TG) from exogenous fatty acids. The level of fatty acids has to be tightly controlled in the adipocyte, as they can act as detergents that rapidly dissolve the plasma membrane, causing cell lysis if allowed to accumulate. Fatty acids therefore have to be efficiently converted to TG and stored in the central lipid droplet. We report that in intact primary adipocytes exogenous oleic acid was taken up and directly converted to TG in the plasma membrane, in a novel subclass of caveolae that specifically contains the protein perilipin. Isolated caveolae catalyzed de novo TG synthesis from oleic acid and glycerol 3-phosphate. Electron microscopy revealed the presence of caveolin and perilipin in caveolae and in lipid-laden bulbs in the plasma membrane, and fluorescence microscopy demonstrated colocalization of fatty acids/TG with caveolin and perilipin at the plasma membrane. A second caveolae fraction was isolated, which lacked perilipin and the triacylglycerol synthesizing enzymes. Both caveolae fractions contained caveolin-1 and the insulin receptor. The findings demonstrate that specific subclasses of caveolae carry out specific functions in cell metabolism. In particular, triacylglycerol is synthesized at the site of fatty acid entry in one of these caveolae classes.

Place, publisher, year, edition, pages
2005. Vol. 280, no 1, 5-8 p.
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-20650DOI: 10.1074/jbc.C400429200PubMedID: 15537657OAI: oai:DiVA.org:liu-20650DiVA: diva2:235439
Available from: 2009-09-16 Created: 2009-09-16 Last updated: 2017-12-13Bibliographically approved
In thesis
1. Lipid Metabolism andInsulin Signalling in Adipocytes: enhanced autophagy in type 2 diabetes
Open this publication in new window or tab >>Lipid Metabolism andInsulin Signalling in Adipocytes: enhanced autophagy in type 2 diabetes
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Energy storage in the adipose tissue, to an extent leading to obesity, is associated with local as well assystemic insulin resistance. When insulin-producing beta-cells in the pancreas gradually fail tocompensate, plasma levels of glucose rise and overt type 2 diabetes is diagnosed. Adipocytes are largecells, mostly consisting of one big central lipid droplet, with the surrounding plasma membrane full ofsmall invaginations called caveolae. As caveolae contain the insulin receptor and several other insulinsignallingproteins, we have investigated several aspects of caveolae. We have also mapped mechanismsand defects in the insulin-signalling network in adipocytes from type 2 diabetic patients.

In paper I, we show that a subtype of caveolae has the capability to synthesize triglycerides from fattyacids and glycerol-3-phosphate. The triglyceride-synthesizing caveolae subtype also contains perilipin,suggesting the existence of a mechanism to protect newly made triglycerides from hydrolysis.

In paper II, we demonstrate that adipocytes from patients with type 2 diabetes have an attenuated insulinstimulatedphosphorylation of IRS-1 at Ser-307 (human sequence), which correlates with reduced insulinstimulatedphosphorylation of IRS-1 at tyrosine residues. Insulin-stimulated phosphorylation of IRS-1 atSer-307 is dependent on the nutrient sensor TORC1. This finding indicates that adipocytes from type 2diabetic patients have reduced TORC1 activity.

In paper III, we focus on the mechanisms for RBP4-induced insulin resistance. We also continue ourmapping of insulin-resistance in adipocytes from type 2 diabetes. These cells exhibit, in addition toimpaired insulin-stimulated glucose uptake and the defects presented in paper I, impaired insulinstimulatedphosphorylation of ERK. We do, however, not see any defects in PKB signalling. Neither dowe se any enhanced insulin-stimulated phosphorylation of IRS-1 at Ser-312 (human sequence), a site thatin mice is hyper-stimulated in response to high-fat feeding. Incubation with RBP4 recapitulates all defectswe so far have seen in type 2 diabetes except reduced insulin-stimulated glucose uptake. These results aremirrored by blockade of endogenously produced RBP4 in the incubations with adipocytes from type 2diabetic patients. In other words, RBP4-blocking antibodies restore all insulin-signalling defects we havefound in adipocytes from type 2 diabetic patients, except insulin-stimulated glucose uptake.

In paper IV we show by several approaches that TORC1 activation is down-regulated in adipocytes fromtype 2 diabetic patients. The main finding is that there is enhanced autophagy in those adipocytes.Interestingly, autophagy may be a mechanism to enhance the breakdown of stored triglycerides in theadipocyte.

In conclusion, our data suggest that caveolae, in addition to being micro-domains for insulin-signallingare metabolic platforms. We describe defects in insulin-signalling in adipocytes from type 2 diabeticpatients where the main finding is enhanced autophagy in these obese patients. The perceived starvationin adipose tissue might via secretion of adipokines, such as RBP4, have implications for local as well assystemic insulin-resistance.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2009. 71 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1138
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-20656 (URN)978-91-7393-575-3 (ISBN)
Public defence
2009-10-09, Berzeliussalen, Hälsouniversitetet, Campus US, Linköpings Universitet, Linköping, 09:00 (English)
Opponent
Supervisors
Available from: 2009-09-16 Created: 2009-09-16 Last updated: 2009-09-17Bibliographically approved
2. Central adipocyte functions in specialized caveolae
Open this publication in new window or tab >>Central adipocyte functions in specialized caveolae
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Obesity and overweight is an increasing health problem in the world. Besides reducing quality of life obesity is a great risk factor for development of diseases such as type 2 diabetes, cardiovascular disease, and stroke. The adipose tissue is a flexible organ that grows in size if the body is provided with excess energy. Adipocytes store the excess energy in the form of triacylglycerol. The incorporation of fat is stimulated by insulin, while catecholamines promote hydrolysis of triacylglycerol. In the membrane of adipocytes there are cave-like invaginations, caveolae, where proteins involved in insulin signalling, such as the insulin receptor are located. The focus of this thesis is on the function, and thereby the importance, of adipocytes and caveolae in metabolism.

The lipid content of the plasma membrane and caveolae was analyzed and compared to lipid phases determined in model membranes. The amount of different membrane lipids indicated heterogeneity of the caveolae, since there were not enough of some specific molecules to be present in every caveola. Using density gradient ultra centrifugation three different subclasses of caveolae were identified. These subclasses were named according to their relative densities: very high density caveolae, high density caveolae and low density caveolae. There were clear differences between the classes with regard to the distribution of specific proteins in them. In low density caveolae proteins linked to cholesterol metabolism, scavenger receptor class B type I and CD36, were abundant. Proteins involved in fatty acid metabolism, fatty acid transporter protein 1 and 4, acyl CoA-syntethase, perilipin and hormone sensitive lipase, were localized to high density caveolae. Cholesteryl ester uptake from HDL particles and hydrolysis to cholesterol was found in low density caveolae and the uptake of fatty acids was restricted to high density caveolae. Remarkably, the fatty acids were not only taken up via high density caveolae, but also incorporated into triacylglycerol, diacylglycerol, and phosphatidylcholine. This indicates that the whole machinery for lipid synthesis is located in these caveolae.

In addition to being an energy store the adipose tissue is also an endocrine organ. Several of the secreted proteins affect the metabolism of the body, for instance adiponectin that affects insulin sensitivity. Here adiponectin was located at the plasma membrane to the caveolae, both by biochemical isolation of caveolae and by electron microscopy. Together with earlier findings this indicates that caveolae may be involved in the secretion of adiponectin.

In conclusion, caveolae are involved in central processes such as glucose metabolism and signalling. Herein it is shown that caveolae is involved in the metabolism of fatty acids and cholesterol and may also be involved in the secretion of the metabolic hormone adiponectin. The findings in this thesis together with previous research establish caveolae as metabolic platforms.

Place, publisher, year, edition, pages
Linköping: Institutionen för biomedicin och kirurgi, 2006. 50 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 971
Keyword
Obesity, Overweight, Diabetes, Stroke, Cholesterol metabolism, Fatty acids
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-8244 (URN)91-85643-37-8 (ISBN)
Public defence
2006-12-08, Berzeliussalen, Campus US, Linköpings Universitet, Linköping, 13:00 (English)
Opponent
Available from: 2007-02-01 Created: 2007-02-01 Last updated: 2012-10-24Bibliographically approved

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Öst, AnitaGustavsson, JohannaNyström, Fredrik HStrålfors, Peter

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