Adipose tissue plays a central role in the control of energy homeostasis through the storage and turnover of triglycerides and through the secretion of factors that affect satiety and fuel utilization. Agents that enhance insulin sensitivity, such as rosiglitazone, appear to exert their therapeutic effect through adipose tissue, but the precise mechanisms of their actions are unclear. Rosiglitazone changes the morphological features and protein profiles of mitochondria in 3T3-L1 adipocytes. To examine the relevance of these effects in vivo, we studied white adipocytes from ob/ob mice during the development of obesity and after treatment with rosiglitazone. The levels of approximately 50% of gene transcripts encoding mitochondrial proteins were decreased with the onset of obesity. About half of those genes were upregulated after treatment with rosiglitazone, and this was accompanied by an increase in mitochondrial mass and changes in mitochondrial structure. Functionally, adipocytes from rosiglitazone-treated mice displayed markedly enhanced oxygen consumption and significantly increased palmitate oxidation. These data reveal mitochondrial remodeling and increased energy expenditure in white fat in response to rosiglitazone treatment in vivo and suggest that enhanced lipid utilization in this tissue may affect whole-body energy homeostasis and insulin sensitivity.

Mice with a fat-specific insulin receptor knock-out (FIRKO) exhibit a polarization of white adipose tissue into two populations of cells, one small (diameter <50 μm) and one large (diameter >100 μm), accompanied by changes in insulin-stimulated glucose uptake, triglyceride synthesis, and lipolysis. To characterize these subclasses of adipocytes, we have used a proteomics approach in which isolated adipocytes from FIRKO and control (IR lox/lox) mice were separated by size, fractionated into cytosolic and membrane subfractions, and analyzed by sucrose gradient, SDS-PAGE, and mass spectrometry. A total of 27 alterations in protein expression at key steps in lipid and energy metabolism could be defined, which were coordinately regulated by adipocyte cell size, impaired insulin signaling, or both. Nine proteins, including vimentin, EH-domain-containing protein 2, elongation factor 2, glucose-regulated protein 78, transketolase, and succinyl-CoA transferase were primarily affected by presence or absence of insulin signaling, whereas 21 proteins, including myosin non-muscle form A, annexin 2, annexin A6, and Hsp47 were regulated in relation to adipocyte size. Of these 27 alterations in protein expression, 14 changes correlated with altered levels of mRNA, whereas the remaining 13 were the result of changes in protein translation or turnover. These data suggest an intrinsic heterogeneity in adipocytes with differences in protein expression patterns caused by transcriptional and post-transcriptional alterations related to insulin action and cellular lipid accumulation.

White adipose tissue is an important endocrine organ involved in the control of whole-body metabolism, insulin sensitivity, and food intake. To better understand these functions, 3T3-L1 cell differentiation was studied by using combined proteomic and genomic strategies. The proteomics approach developed here exploits velocity gradient centrifugation as an alternative to isoelectric focusing for protein separation in the first dimension. A 20- to 30-fold increase in the concentration of numerous mitochondrial proteins was observed during adipogenesis, as determined by mass spectrometry and database correlation analysis. Light and electron microscopy confirmed a large increase in the number of mitochondrion profiles with differentiation. Furthermore, mRNA profiles obtained by using Affymetrix GeneChips revealed statistically significant increases in the expression of many nucleus-encoded mitochondrial genes during adipogenesis. Qualitative changes in mitochondrial composition also occur during adipose differentiation, as exemplified by increases in expression of proteins involved in fatty acid metabolism and of mitochondrial chaperones. Furthermore, the insulin sensitizer rosiglitazone caused striking changes in mitochondrial shape and expression of selective mitochondrial proteins. Thus, although mitochondrial biogenesis has classically been associated with brown adipocyte differentiation and thermogenesis, our results reveal that mitochondrial biogenesis and remodeling are inherent to adipose differentiation per se and are influenced by the actions of insulin sensitizers.

Apoptosis is a regulated cell death program controlled by extrinsic and intrinsic signaling pathways. The intrinsic pathway involves stress signals that activate pro-apoptotic members of the Bcl-2 family, inducing permeabilization of mitochondria and release of apoptogenic factors. These proteins localize to the outer mitochondrial membrane. Ian4, a mitochondrial outer membrane protein with GTP-binding activity, is normally present in thymocytes, T cells, and B cells. We and others have recently discovered that a mutation in the rat Ian4 gene results in severe T cell lymphopenia that is associated with the expression of autoimmune diabetes. The mechanism by which Ian4 controls T cell homeostasis is unknown. Here we show that the absence of Ian4 in T cells causes mitochondrial dysfunction, increased mitochondrial levels of stress-inducible chaperonins and a leucine-rich protein, and T cell-specific spontaneous apoptosis. T cell activation and caspase 8 inhibition both prevented apoptosis, whereas transfection of T cells with Ian4-specific small interfering RNA recapitulated the apoptotic phenotype. The findings establish Ian4 as a tissue-specific regulator of mitochondrial integrity.