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J. Biol. Chem., Vol. 280, Issue 1, 5-8, January 7, 2005

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Triacylglycerol Is Synthesized in a Specific Subclass of Caveolae in Primary Adipocytes^*

Anita Öst, Unn Örtegren, Johanna Gustavsson, Fredrik H. Nystrom¶, and Peter Strålfors||

From the Department of Cell Biology and Diabetes Research Centre, ^¶Department of Medicine and Care, Linköping University, SE58185 Linköping, Sweden

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.

Received for publication, September 13, 2004 , and in revised form, October 26, 2004.

^* This work was supported by the Åke Wiberg Foundation, the Lions Foundation, Linköping University Hospital Research Funds, the Östergötland County Council, the Swedish Foundation for Strategic Research (Glycoconjugates in Biological Systems), the Swedish Diabetes Association, and the Swedish Research Council. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

These authors contributed equally to this work.

^|| To whom correspondence should be addressed. Tel.: 46-13-224315; Fax: 46-13-224314; E-mail: peter.stralfors{at}ibk.liu.se.

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