Five mouse and human leptin receptors (Ob-R) have recently been identified, a long isoform (Ob-Rb), preferentially expressed in hypothalamus, and 4 short isoforms, Ob-Ra, ObRe, Ob-Rd, and Ob-Re. We have identified a new short isoform in the rat, r-OB-Rf, with 6 C-terminal amino acids and a 3' untranslated region without homology to other Ob-R isoforms. Its higher expression in rat liver and spleen compared to brain, stomach, kidney, thymus, heart, lung and hypothalamus, contrasts with Ob-Ra and Ob-Rb homologues and raises l~Jssibilities of as yet unidentified roles for members of the growing Ob-R gene family.

Hyperlipidemia appears to play an integral role in loss of glucose-stimulated insulin secretion (GSIS) in type 2 diabetes. This impairment can be simulated in vitro by chronic culture of 832/13 insulinoma cells with high concentrations of free fatty acids, or by study of lipid-laden islets from Zucker diabetic fatty rats. Here we show that impaired GSIS is not a simple result of saturation of lipid storage pathways, as adenovirus-mediated overexpression of a cytosolically localized variant of malonylCoA decarboxylase in either cellular model results in dramatic lowering of cellular triglyceride stores but no improvement in GSIS. Instead, the glucose-induced increment in "pyruvate cycling" activity (pyruvate exchange with tricarboxylic acid cycle intermediates measured by 13 C NMR), previously shown to play an important role in GSIS, is completely ablated in concert with profound suppression of GSIS in lipid-cultured 832/13 cells, whereas glucose oxidation is unaffected. Moreover, GSIS is partially restored in both lipid-cultured 832/13 cells and islets from Zucker diabetic fatty rats by addition of a membrane permeant ester of a pyruvate cycling intermediate (dimethyl malate). We conclude that chronic exposure of islet ␤-cells to fatty acids grossly alters a mitochondrial pathway of pyruvate metabolism that is important for normal GSIS.A major contributing factor to the development of type 2 diabetes is inadequate insulin secretion to compensate for insulin resistance. A hallmark of this ␤-cell dysfunction is the impairment and eventual complete loss of glucose-stimulated insulin secretion (GSIS).1 Hyperlipidemia, and the consequent accumulation of triglycerides (TG) and other lipid-derived intermediates in ␤-cells, is now well recognized as a variable that correlates with development of impaired insulin secretion (1-6). Furthermore, culture of pancreatic islets (3,7,8) or insulinoma cell lines (9) with elevated levels of free fatty acids in vitro results in loss of GSIS, and glucose sensing is also dramatically impaired in fat-laden islets from Zucker diabetic fatty (ZDF) rats (2, 3). However, a biochemical mechanism linking chronic exposure of islet cells to high levels of free fatty acids and impairment of GSIS has not emerged.To gain more insight into this important issue, two independent model systems were exploited. First, we have described recently (10) stable subclones of the rat insulinoma INS-1 cell line with robust GSIS, such as cell line 832/13. As shown here, chronic culture of these cells in 1 mM oleate/palmitate (2:1) causes profound impairment of GSIS. Second, islets from ZDF rats are both lipid-laden and poorly glucose-responsive (3). By using these model systems, two hypotheses about the mechanism of lipid-induced impairment of GSIS were tested. The first is that accumulation of lipid-derived metabolites caused by chronic exposure of ␤-cells to fatty acids plays a direct role in the functional impairment. To test this idea, we have employed a recombinant adenovirus encoding a variant, cytosolic...

To determine whether the antilipogenic actions of insulin-induced gene 1 (insig-1) demonstrated in cultured preadipocytes also occur in vivo, we infected Zucker diabetic fatty (ZDF) (fa͞fa) rats, with recombinant adenovirus containing insig-1 or -2 cDNA. An increase of both proteins appeared in their livers. In control ZDF (fa͞fa) rats infected with adenovirus containing the ␤-galactosidase (␤-gal) cDNA, triacylglycerols in the liver and plasma rose steeply whereas the insig-infected rats exhibited substantial attenuation of the increase in hepatic steatosis and hyperlipidemia. Insig overexpression was associated with a striking reduction in the elevated level of nuclear sterol regulatory element-binding protein (SREBP)-1c, the activated form of the transcription factor. The mRNA of SREBP-1c lipogenic target enzymes also fell. The mRNA of endogenous insig-1, but not -2a and -2b, was higher in the fatty livers of untreated obese ZDF (fa͞fa) rats compared with controls, but the elevation was not sufficient to block the Ϸ3-fold increase in SREBP-1c expression and activity. In normal animals, adenovirusinduced overexpression of the insigs reduced the increase in SREBP-1c mRNA and its target enzymes caused by refeeding. The findings demonstrated that both insigs have antilipogenic action when transgenically overexpressed in livers with increased SREBP1c-mediated lipogenesis. However, the increase in endogenous insig-1 expression associated with augmented lipogenesis may limit it, but is insufficient to prevent it. Insulin-induced gene 1 (insig-1) was originally cloned by Peng et al.(1) in regenerating liver and was subsequently shown to be dramatically elevated in the fat tissue of rats at the onset of diet-induced obesity (DIO) (2). Its function was unknown until Yang et al. (3) demonstrated that it binds sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP), thereby preventing it from leaving the endoplasmic reticulum to escort SREBPs to the Golgi. SREBP proteins are inactive until converted by proteolytic processing in the Golgi into active transcription factors (4). In other words, insig-1 effectively blocks the activation of SREBPs.Although this seminal discovery was directed at the liver's cholesterogenic transcription factor, SREBP-2, it obviously was relevant to other members of the SREBP family. It was subsequently shown that transfection of insig-1 into preadipocytes completely blocks glucose-derived lipogenesis in 3T3-L1 adipocytes (5). This finding led to the proposal that the rise in insig-1 in the expanding adipocytes of diet-induced obesity ''brakes'' the lipogenesis so as to avoid an overaccumulation of fat that would exceed their storage capacity and damage them.This antilipogenic effect of insig-1 had been demonstrated in vitro without any corroborating evidence that the same antilipogenic effect also occurred in vivo in adult animals. To obtain such evidence, we chose to determine whether overexpression of insig-1 [and͞or insig-2, a second SCAP-binding protein discov...

Adiponectin is essential for the regulation of tissue substrate utilization and systemic insulin sensitivity. Clinical studies have suggested a positive association of circulating adiponectin with healthspan and lifespan. However, the direct effects of adiponectin on promoting healthspan and lifespan remain unexplored. Here, we are using an adiponectin null mouse and a transgenic adiponectin overexpression model. We directly assessed the effects of circulating adiponectin on the aging process and found that adiponectin null mice display exacerbated age-related glucose and lipid metabolism disorders. Moreover, adiponectin null mice have a significantly shortened lifespan on both chow and high-fat diet. In contrast, a transgenic mouse model with elevated circulating adiponectin levels has a dramatically improved systemic insulin sensitivity, reduced age-related tissue inflammation and fibrosis, and a prolonged healthspan and median lifespan. These results support a role of adiponectin as an essential regulator for healthspan and lifespan.