Caveolae and adipocyte lipid metabolism Caveolae are prominent features of the adipocyte plasma membrane comprising ca. 50% of the cell surface area, thus begging the question of why they are so abundant and what their functions are in this metabolically important cell type. Caveolar structures are 50-100 nm sac-like invaginations projecting from the plasma membrane into the cytosol. They are comprised of small integral membrane proteins, caveolins 1 &2, and peripheral membrane proteins of the cavin family (cavin-1- 3). They concentrate high levels of cholesterol and sphingolipids which are needed to maintain their structural integrity. Loss of adipocyte caveolae components, in vitro and in vivo, by gene manipulation/inactivation results in abnormalities of adipocyte metabolism. It is the long-term goal of this proposal to understand how the relevant proteins and lipids interact to form caveolae, and how caveolae impact the metabolic and biochemical functions of the fat cell, which has phenotypic consequences for the organism.
Specific Aim 1 will address the mechanism(s) by which lipid metabolism is dysregulated in vitro and in vivo in adipocytes deficient in caveolae. Preliminary data support the hypothesis that cavin-1 and/or cavin-2 serve as regulatory factor(s) in hormonally- stimulated lipolysis, and we will experimentally address this hypothesis in animals and in cell culture models lacking specific caveolae components. Cholesterol depletion collapses caveolae concomitantly with remodeling of the cortical cytoskeleton and degradation of cavin-2 by the proteosome.
In Aim 2, we will manipulate caveolae by this and other protocols to determine how caveolae sense cholesterol, and how caveolae influence plasma membrane organization and function. We will test the hypothesis that ubiquitination is requisite for caveolae formation.
In Aim 3, we propose to phenotypically analyze mice lacking cavin-1 in adipocytes to assess the contribution of this tissue to the metabolic abnormalities seen in the global cavin-1 knockout, and we will metabolically characterize mice lacking cavin-3, which have a reduced adipose mass. We will also phenotype and analyze cavin-2 knockout mice. The multi-organ phenotype of caveolae-deficient mice appears identical to that of humans harboring inactivating mutations in caveolae components, and therefore the physiological relevance of these studies to human biology is considerable. The adipocyte studies will also inform the research community interested in caveolae in other relevant systems and tissues, for example, in cardiovascular biology.
Lipodystrophies (abnormal fat accumulation) are relatively common in the human population, and although many have not been attributed to a abnormalities in a specific gene, very recent work (2009, 2010) identified forms of congenital generalized lipodystrophy together with cardiac and muscle abnormalities that are caused by inactivating mutations in the genes for caveolin-1 and cavin-1, the proteins we propose to study that form characteristic cell surface membrane structures in a variety of cells. These proteins are also involved in cholesterol metabolism and thus our work will address important but poorly understood aspects of lipid metabolism of direct relevance to human disease.
