Adipose triglyceride lipase (ATGL) is a recently described adipose-enriched protein with triglyceride-specific lipase activity. ATGL shares the greatest sequence homology with adiponutrin, a nutritionally regulated protein of unclear biological function. Here we present a functional analysis of ATGL and adiponutrin and describe their regulation by insulin. Retroviral-mediated overexpression of ATGL in 3T3-L1 adipocytes increased basal and isoproterenol-stimulated glycerol and nonesterified fatty acid (NEFA) release, whereas siRNA-mediated knockdown of ATGL had the opposite effect. In contrast, siRNA-mediated knockdown of adiponutrin in 3T3-L1 adipocytes had no effect on glycerol or NEFA release. In mice, both ATGL and adiponutrin are nutritionally regulated in adipose tissue, with ATGL being upregulated and adiponutrin being downregulated by fasting. In 3T3-L1 adipocytes, insulin decreased ATGL and increased adiponutrin expression in a dose-and time-dependent manner, suggesting that insulin directly mediates this nutritional regulation. In addition, adipose expression of ATGL was increased by insulin deficiency and decreased by insulin replacement in streptozotocininduced diabetic mice and was increased in fat-specific insulin receptor knockout mice, whereas adiponutrin showed the opposite pattern. These data suggest that murine ATGL but not adiponutrin contributes to net adipocyte lipolysis and that ATGL and adiponutrin are oppositely regulated by insulin both in vitro and in vivo. Diabetes 55: 148 -157, 2006 A dipose tissue triglycerides are the predominant form of energy storage in animals. The ability to store and release this energy in response to variable energy availability is advantageous to survival and requires a carefully regulated balance between triglyceride synthesis and hydrolysis. Dysregulation of these processes may result in metabolic disorders, such as obesity and lipodystrophy, which are associated with dyslipidemia, insulin resistance, and overt diabetes. Hence understanding the regulatory mechanisms underlying the storage and mobilization of triglycerides is essential to understanding the pathophysiology of obesity, diabetes, and related metabolic disorders.Hormone-sensitive lipase (HSL) has traditionally been considered the key lipolytic enzyme in adipocytes (1-4). Lipolytic hormones such as catecholamines stimulate lipolysis primarily via cAMP-mediated activation of protein kinase (PK) A. PKA then phosphorylates HSL and perilipin A. Phosphorylation of perilipin A alleviates the barrier function of this protein and prompts its active participation in the lipolytic process. Phosphorylation of HSL results in translocation of HSL from the cytosol to the lipid droplet where it catalyzes the hydrolysis of tri-, di-, and monoglycerides; cholesteryl esters; and other substrates. Insulin acutely inhibits lipolysis, at least in part, via inhibition of the above cAMP-dependent pathway by PKBdependent phosphorylation and activation of phosphodiesterase 3B, which in turn lowers cAMP levels (5). How...

There are many theories of aging and parameters that influence lifespan, including genetic instability, telomerase activity and oxidative stress. The role of caloric restriction, metabolism and insulin and insulin-like growth factor-1 signaling in the process of aging is especially well conserved throughout evolution. These latter factors interact with each other, the former factors and histone deacetylases of the SIR family in a complex interaction to influence lifespan.

SummaryCaloric restriction, leanness and decreased activity of insulin/insulin-like growth factor 1 (IGF-1) receptor signaling are associated with increased longevity in a wide range of organisms from Caenorhabditis elegans to humans. Fatspecific insulin receptor knock-out (FIRKO) mice represent an interesting dichotomy, with leanness and increased lifespan, despite normal or increased food intake. To determine the mechanisms by which a lack of insulin signaling in adipose tissue might exert this effect, we performed physiological and gene expression studies in FIRKO and control mice as they aged. At the whole body level, FIRKO mice demonstrated an increase in basal metabolic rate and respiratory exchange ratio. Analysis of gene expression in white adipose tissue (WAT) of FIRKO mice from 6 to 36 months of age revealed persistently high expression of the nuclear-encoded mitochondrial genes involved in glycolysis, tricarboxylic acid cycle, β β β β -oxidation and oxidative phosphorylation as compared to expression of the same genes in WAT from controls that showed a tendency to decline in expression with age. These changes in gene expression were correlated with increased cytochrome c and cytochrome c oxidase subunit IV at the protein level, increased citrate synthase activity, increased expression of peroxisome proliferator-activated receptor γ γ γ γ coactivator 1 α α α α (PGC-1 α α α α ) and PGC-1 β β β β , and an increase in mitochondrial DNA in WAT of FIRKO mice. Together, these data suggest that maintenance of mitochondrial activity and metabolic rates in adipose tissue may be important contributors to the increased lifespan of the FIRKO mouse.