Caveolin expression and activation in retroperitoneal and subcutaneous adipocytes: influence of a high-fat diet.

Theeffect ofahigh-fatdiet onthe expressionofthethreemainisoformsofcaveolinsin adipocytesisolated fromratretroperitoneal andsubcutaneouswhiteadiposetissuewasinvestigated.Twodistinctphasescanbedistinguishedonatime-dependentresponseinadipocytesfrom both locations. The early stage affects only to retroperitoneal adipocytes and implies caveolin-1 activation and caveolin-2inactivation,togetherwithincreasedexpressionofinsulinsignalingintermediaries.Thisinitialresponsewouldbeaimedtocounterbalancetheenergyoverload.Continuedexposuretothehigh-fatdietproducesanincreaseincirculatingglucoseandinsulinlevels,inducingalatestageinwhichadipocytesfrombothlocationsareaffected.Thislatestageischaracterized bygeneralincreasedcaveolin-1andcaveolin-2expression; while on the other hand, the insulin signaling intermediaries are downregulated, with the noticeable exception of GLUT-4,whose expression remains high. Therefore, it seems that at this stage caveolins and GLUT-4 are regulated independently of the insulinpathway,throughamechanismthatcouldbemediatedbyinflammationandoxidativestressassociatedwithobesity.AlthoughthisGLUT-4upregulationsuggestsaresponseagainsttheraiseincirculatingglucose,thismightnotbethecase,sincethedevelopinginsulinresistanceat this stage indicates a prediabetic state. We have also found that the high-fat diet is able to induce the expression of muscle-specificcaveolin-3 in retroperitoneal adipocytes since the initial phase. This observation is similar to what we reported previously in skeletalmuscle (Go´mez-Ruiz et al., 2009, FEBS Lett 583:3259–3264), suggesting a similar regulatory mechanism for this isoform.J. Cell. Physiol. 225: 206–213, 2010. 2010 Wiley-Liss, Inc.Obesity is the outcome of a long-term imbalance of energyintakeoverexpenditure,whichischaracterizedbyanexcessfatdeposition in white adipose tissue (WAT) (Campio´n et al.,2006).WATnotonlyservesasthemainsiteforenergystoragein the form of triglycerides but also contributes to systemicglucoseandlipidregulationthroughitsfunctionasanendocrineorgan (Fischer-Posovszky et al., 2007). Different hormones, inparticular insulin and catecholamines, govern the storage andutilization of energy in the fat depots (Boden and Hoeldtke,2003). In obese individuals adipose tissue contributessignificantly to disease, being linked to cardiovascular risk andmetabolic disorders such as type 2 diabetes and dyslipidemias(Rothetal.,2004).Moreover,theriskforsuchillnessesismorestrongly associated with abdominal and in particular visceraladiposities, than with subcutaneous (SC) fat depots. TheseWAT deposits are different not only because of their bodydistribution, but mainly because they exhibit distinctphysiologicalandmetabolicproperties(Gestaetal.,2007;Tranet al., 2008).In fact, the obese individuals who most likely develop insulinresistance are those with inflammation of visceral adiposetissue, ectopic fat deposition, and adipose tissue dysfunction(Bluher, 2009). In addition, visceral fat is known to exhibitmorphological differences in comparison with SC, whose fatcells are approximately 50% larger than omental adipocytes(Van Harmelen et al., 1998). There are also functionaldifferences between both depots, including lipolysis regulationby insulin (Giorgino et al., 2005) or macrophage infiltration(Harman-Boehm et al., 2007). In regard to this, it has beenreported that adipose tissue in obesity is associated withchronic inflammation and hypoxia, which are conditions thatwould affect gene expression through the associated reactiveoxygen species (ROS) production and oxidative stress (Fabriset al., 2001; Wood et al., 2009).Caveolae are a specialized type of lipid rafts enriched incholesterolandsphingolipidsthatform50–100nmflask-shapedinvaginations in the plasma membrane (Yamada, 1955). Thesestructures are present in most cell types, being particularlyabundant in adipocytes, where they account for 30% of theplasma membrane surface (Fan et al., 1983). Caveolins are themain structural proteins of caveolae and three major isoformswith molecular weights between 18 and 24kDa have beenidentified: Cav-1, Cav-2, and Cav-3 (Williams and Lisanti,2004a,b). Cav-1 is most abundantly expressed in terminallydifferentiatedcellssuchasfibroblasts,epithelialandendothelialcells, and adipocytes, where is responsible for caveolaeformation (Scherer et al., 1994). Cav-2 is coexpressed withCav-1,whileCav-3isthespecificisoformofmuscletissue,beingresponsibleforcaveolaeformationinmyocytes,althoughithasalso been found in chondrocytes and astrocytes (Schwab et al.,1999; Shin et al., 2005).Caveolae have been implicated in a number of functions,including endocytosis, transport across plasma membrane, andsignaltransduction(VanDeursetal.,2003).Withregardtothis,