Marrow adipose tissue (MAT) accumulates in diverse clinical conditions but remains poorly understood. Here we show region-specific variation in MAT adipocyte development, regulation, size, lipid composition, gene expression, and genetic determinants. Early MAT formation in mice is conserved, while later development is strain dependent. Proximal, but not distal, MAT is lost with 21-day cold exposure. Rat MAT adipocytes from distal sites have an increased proportion of monounsaturated fatty acids and expression of Scd1/Scd2, Cebpa and Cebpb. Humans also have increased distal marrow fat unsaturation. We define proximal ‘regulated’ MAT (rMAT) as single adipocytes interspersed with active hematopoiesis, whereas distal ‘constitutive’ MAT (cMAT) has low hematopoiesis, contains larger adipocytes, develops earlier, and remains preserved upon systemic challenges. Loss of rMAT occurs in mice with congenital generalized lipodystrophy type 4, whereas both rMAT and cMAT are preserved in mice with congenital generalized lipodystrophy type 3. Consideration of these MAT subpopulations may be important for future studies linking MAT to bone biology, hematopoiesis and whole-body metabolism.

At least three different glucose transport systems exist in mammalian cells. These are: (1) the constitutively active, facilitative carrier characteristic of human erythrocytes, Hep G2 (ref. 2) cells and rat brain; (2) the Na-dependent active transporter of kidney and small intestine; and (3) the facilitative carrier of rat liver (B. Thorens and H. F. Lodish, personal communication). A fourth possible glucose transport system is the insulin-dependent carrier that may be specific to muscle and adipose tissue. This transporter resides primarily in an intracellular compartment in resting cells from where it translocates to the cell surface upon cellular insulin exposure. This raises the question of whether hormonal regulation of glucose transport is conferred by virtue of a tissue-specific signalling mechanism or a tissue-specific glucose transporter. Here we present data supporting the latter concept based upon a monoclonal antibody against the fat cell glucose transporter that identifies a unique, insulin-regulatable glucose transport protein in muscle and adipose tissue.

SUMMARY Caveolae are specialized invaginations of the plasma membrane found in numerous cell types. They have been implicated as playing a role in a variety of physiological processes and are typically characterized by their association with the caveolin family of proteins. We show here by means of targeted gene disruption in mice, that a distinct caveolae-associated protein, Cavin/PTRF, is an essential component of caveolae. Animals lacking Cavin have no morphologically detectable caveolae in any cell type examined and have markedly diminished protein expression of all three caveolin isoforms whilst retaining normal or above normal caveolin mRNA expression. Cavin knockout mice are viable and of normal weight but have higher circulating triglyceride levels, significantly reduced adipose tissue mass, glucose intolerance and hyperinsulinemia, which characteristics constitute a lipodystrophic phenotype. Our results underscore the multi-organ role of caveolae in metabolic regulation and the obligate presence of Cavin for caveolae formation.

A number of studies have demonstrated that tumor necrosis factor-␣ (TNF-␣) is associated with profound insulin resistance in adipocytes and may also play a critical role in the insulin resistance of obesity and noninsulin-dependent diabetes mellitus. Reports on the mechanism of TNF-␣ action have been somewhat contradictory. GLUT4 down-regulation has been implicated as a possible cause of insulin resistance as has been the reduced kinase function of the insulin receptor. Here we examine the effects of tumor necrosis factor on the protein components thought to be involved in insulin-stimulated glucose transport in adipocytes, namely the insulin receptor, its major substrate IRS-1, and the insulin responsive glucose transporter GLUT4. Prolonged exposure (72-96 h) of 3T3-L1 adipocytes to TNF-␣ causes a substantial reduction (>80%) in IRS-1 and GLUT4 mRNA and protein as well as a lesser reduction (>50%) in the amount of the insulin receptor. Nevertheless, the remaining proteins appear to be biochemically indistinguishable from those in untreated adipocytes. Both the insulin receptor and IRS-1 are tyrosine-phosphorylated to the same extent in response to acute insulin stimulation following cellular TNF-␣ exposure. Furthermore, the ability of the insulin receptor to phosphorylate exogenous substrate in the test tube is also normal following its isolation from TNF-␣-treated cells. These results are confirmed by the reduced but obvious level of insulindependent glucose transport and GLUT4 translocation observed in TNF-␣-treated adipocytes. We conclude that the insulin resistance of glucose transport in 3T3-L1 adipocytes exposed to TNF-␣ for 72-96 h results from a reduced amount in requisite proteins involved in insulin action. These results are consistent with earlier studies indicating that TNF-␣ reduces the transcriptional activity of the GLUT4 gene in murine adipocytes, and reduced mRNA transcription of a number of relevant genes may be the general mechanism by which TNF-␣ causes insulin resistance in adipocytes.

Quantitative ultrastructural analysis and proteomics detail CLIC structure, composition, and function.