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Localization of adiponectin at the plasma membrane in caveolae of human adipocytes
Linköping University, Department of Biomedicine and Surgery, Cell biology. Linköping University, Faculty of Health Sciences.
Albert Einstein College of Medicine, New York, NY, USA.
Linköping University, Department of Biomedicine and Surgery, Cell biology. Linköping University, Faculty of Health Sciences.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The hormone adiponectin is exclusively secreted by adipocytes and it regulates whole body energy homeostasis primarily by affecting liver and muscles. The circulating level of adiponectin inversely correlates with insulin resistance and type 2 diabetes. Little is known about the secretion of adiponectin by the adipocyte, but in caveolin-1 knockout mice the amount of secreted adiponectin is severely reduced. Here we show that in human adipocytes adiponectin was associated with the plasma membrane and to a specific class of high-density-caveolae. Substantial amounts of adiponectin were also found in the microsomal and cytosolic fractions of adipocytes, while very little was associated with mitochondria, nuclei, or the fat. Adiponectin in isolated caveolae was readily degraded by added trypsin, demonstrating that adiponectin was largely bound to the cytosolic face of the caveolae membrane. The findings indicate that caveolae may have a critical role in the secretion of adiponectin by human adipocytes.

National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-84835OAI: oai:DiVA.org:liu-84835DiVA: diva2:562342
Available from: 2012-10-24 Created: 2012-10-24 Last updated: 2012-10-24Bibliographically approved
In thesis
1. 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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Örtegren, UnnStrålfors, Peter

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