Separation and characterization of caveolae subclasses in the plasma membrane of primary adipocytes; segregation of specific proteins and functions

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Abstract
Caveolae are nearly ubiquitous plasma membrane domains that in adipocytes vary in size between 25 and 150 nm. They constitute sites of entry into the cell as well as platforms for cell signalling. We have previously reported that plasma membrane‐associated caveolae that lack cell surface access can be identified by electron microscopy. We now report the identification, after density gradient ultracentrifugation, of a subclass of very high‐density apparently closed caveolae that were not labelled by cell surface protein labelling of intact cells. These caveolae contained caveolin‐1 and caveolin‐2. Another class of high‐density caveolae contained caveolin‐1, caveolin‐2 and specifically fatty acid transport protein‐1, fatty acid transport protein‐4, fatty acyl‐CoA synthetase, hormone‐sensitive lipase, perilipin, and insulin‐regulated glucose transporter‐4. This class of caveolae was specialized in fatty acid uptake and conversion to triacylglycerol. A third class of low‐density caveolae contained the insulin receptor, class B scavenger receptor‐1, and insulin‐regulated glucose transporter‐4. Small amounts of these proteins were also detected in the high‐density caveolae. In response to insulin, the insulin receptor autophosphorylation and the amount of insulin‐regulated glucose transporter‐4 increased in these caveolae. The molar ratio of cholesterol to phospholipid in the three caveolae classes varied considerably, from 0.4 in very high‐density caveolae to 0.9 in low‐density caveolae. There was no correlation between the caveolar contents of caveolin and cholesterol. The low‐density caveolae, with the highest cholesterol concentration, were particularly enriched with the cholesterol‐rich lipoprotein receptor class B scavenger receptor‐1, which mediated cholesteryl ester uptake from high‐density lipoprotein and generation of free cholesterol in these caveolae, suggesting a specific role in cholesterol uptake/metabolism. These findings demonstrate a segregation of functions in caveolae subclasses.