Comparison of proteomic and metabolomic profiles of mutants of the mitochondrial respiratory chain in Caenorhabditis elegans

Single-gene mutations that disrupt mitochondrial respiratory chain function in Caenorhabditis elegans change patterns of protein expression and metabolites. Our goal was to develop useful molecular fingerprints employing adaptable techniques to recognize mitochondrial defects in the electron transport chain. We analyzed mutations affecting complex I, complex II, or ubiquinone synthesis and discovered overarching patterns in the response of C. elegans to mitochondrial dysfunction across all of the mutations studied. These patterns are in KEGG pathways conserved from C. elegans to mammals, verifying that the nematode can serve as a model for mammalian disease. In addition, specific differences exist between mutants that may be useful in diagnosing specific mitochondrial diseases in patients.

Isoprenylcysteine carboxylmethyltransferase regulates mitochondrial respiration and cancer cell metabolism

Isoprenylcysteine carboxylmethyltransferase (Icmt) catalyzes the last of the three-step posttranslational protein prenylation process for the so-called CaaX proteins, which includes many signaling proteins, such as most small GTPases. Despite extensive studies on Icmt and its regulation of cell functions, the mechanisms of much of the impact of Icmt on cellular functions remain unclear. Our recent studies demonstrated that suppression of Icmt results in induction of autophagy, inhibition of cell growth and inhibition of proliferation in various cancer cell types, prompting this investigation of potential metabolic regulation by Icmt. We report here the findings that Icmt inhibition reduces the function of mitochondrial oxidative phosphorylation in multiple cancer cell lines. In-depth oximetry analysis demonstrated that functions of mitochondrial complex I, II and III are subject to Icmt regulation. Consistently, Icmt inhibition decreased cellular ATP and depleted critical tricarboxylic acid cycle metabolites, leading to suppression of cell anabolism and growth, and marked autophagy. Several different approaches demonstrated that the impact of Icmt inhibition on cell proliferation and viability was largely mediated by its effect on mitochondrial respiration. This previously unappreciated function of Icmt, which can be therapeutically exploited, likely has a significant role in the impact of Icmt on tumorigenic processes.

Branched-chain amino acid levels are associated with improvement in insulin resistance with weight loss

AIMS/HYPOTHESIS

Insulin resistance (IR) improves with weight loss, but this response is heterogeneous. We hypothesised that metabolomic profiling would identify biomarkers predicting changes in IR with weight loss.

METHODS

Targeted mass spectrometry-based profiling of 60 metabolites, plus biochemical assays of NEFA, β-hydroxybutyrate, ketones, insulin and glucose were performed in baseline and 6 month plasma samples from 500 participants who had lost ≥4 kg during Phase I of the Weight Loss Maintenance (WLM) trial. Homeostatic model assessment of insulin resistance (HOMA-IR) and change in HOMA-IR with weight loss (∆HOMA-IR) were calculated. Principal components analysis (PCA) and mixed models adjusted for race, sex, baseline weight, and amount of weight loss were used; findings were validated in an independent cohort of patients (n = 22).

RESULTS

Mean weight loss was 8.67 ± 4.28 kg; mean ∆HOMA-IR was -0.80 ± 1.73, range -28.9 to 4.82). Baseline PCA-derived factor 3 (branched chain amino acids [BCAAs] and associated catabolites) correlated with baseline HOMA-IR (r = 0.50, p < 0.0001) and independently associated with ∆HOMA-IR (p < 0.0001). ∆HOMA-IR increased in a linear fashion with increasing baseline factor 3 quartiles. Amount of weight loss was only modestly correlated with ∆HOMA-IR (r = 0.24). These findings were validated in the independent cohort, with a factor composed of BCAAs and related metabolites predicting ∆HOMA-IR (p = 0.007).

CONCLUSIONS/INTERPRETATION

A cluster of metabolites comprising BCAAs and related analytes predicts improvement in HOMA-IR independent of the amount of weight lost. These results may help identify individuals most likely to benefit from moderate weight loss and elucidate novel mechanisms of IR in obesity.

Insulin resistance is associated with a metabolic profile of altered protein metabolism in Chinese and Asian-Indian men

AIMS/HYPOTHESIS

Insulin resistance (IR) is associated with obesity, but can also develop in individuals with normal body weight. We employed comprehensive profiling methods to identify metabolic events associated with IR, while controlling for obesity.

METHODS

We selected 263 non-obese (BMI approximately 24 kg/m2) Asian-Indian and Chinese men from a large cross-sectional study carried out in Singapore. Individuals taking medication for diabetes or hyperlipidaemia were excluded. Participants were separated into lower and upper tertiles of IR based on HOMA indices of < or =1.06 or > or =1.93, respectively. MS-based metabolic profiling of acylcarnitines, amino acids and organic acids was combined with hormonal and cytokine profiling in all participants.

RESULTS

After controlling for BMI, commonly accepted risk factors for IR, including circulating fatty acids and inflammatory cytokines, did not discriminate the upper and lower quartiles of insulin sensitivity in either Asian- Indian or Chinese men. Instead, IR was correlated with increased levels of alanine, proline, valine, leucine/isoleucine, phenylalanine, tyrosine, glutamate/glutamine and ornithine, and a cluster of branched-chain and related amino acids identified by principal components analysis. These changes were not due to increased protein intake by individuals in the upper quartile of IR. Increased abdominal adiposity and leptin, and decreased adiponectin and IGF-binding protein 1 were also correlated with IR.

CONCLUSIONS/INTERPRETATION

These findings demonstrate that perturbations in amino acid homeostasis, but not inflammatory markers or NEFAs, are associated with IR in individuals of relatively low body mass.

Adenovirus-mediated leptin expression normalises hypertension associated with diet-induced obesity

In our previous study, moderate increases in plasma leptin levels achieved via administration of recombinant adenovirus containing the rat leptin cDNA were shown to correct the abnormal metabolic profile in rats with diet-induced obesity, suggesting that these animals had developed resistance to the metabolic effects of leptin, which could be reversed by leptin gene over-expression. However, the effect of this therapeutic strategy on blood pressure was not investigated. The present study aimed to determine whether a moderate increase of endogenous plasma leptin levels affected arterial blood pressure in rats with diet-induced obesity and hypertension. The major finding from the present study was that the natural rise in plasma leptin with weight-gain is insufficient to counterbalance high blood pressure associated with obesity, additional increases of circulating leptin levels with adenoviral leptin gene therapy led to normalisation of blood pressure in high-fat diet-induced obese and hypertensive rats. Mechanistically, the reduction of blood pressure by leptin in obese rats was likely independent of alpha-adrenergic and acetylcholinergic receptor mediation. This is the first study to demonstrate that further increases in circulating leptin levels by leptin gene transfer during obesity could reduce blood pressure.

Inflammatory mechanisms in diabetes: lessons from the beta-cell

Inflammation plays an important role in the destruction of pancreatic islet beta-cells that leads to type I diabetes. This involves infiltration of T-cells and macrophages into the islets and local production of inflammatory cytokines such as interleukin (IL)-1 beta, tumor necrosis factor (TNF)-alpha, and interferon (IFN)-gamma. Our laboratory has developed several strategies for protecting beta-cells against oxidative stress and cytokine-induced cytotoxicity. These include a cytokine selection strategy that results in cell lines that are resistant to the combined effects of IL-1 beta+IFN-gamma. More recently, we have combined the cytokine selection procedure with overexpression of the antiapoptotic gene bcl-2, resulting in cell lines with greater resistance to oxidative stress and cytokine-induced damage than achieved with either procedure alone. This article summarizes this work and the remarkably divergent mechanisms by which protection is achieved in the different model systems. We also discuss the potential relevance of insights gained from these approaches for enhancing islet cell survival and function in both major forms of diabetes.

Control of hepatic gluconeogenesis through the transcriptional coactivator PGC-1

Blood glucose levels are maintained by the balance between glucose uptake by peripheral tissues and glucose secretion by the liver. Gluconeogenesis is strongly stimulated during fasting and is aberrantly activated in diabetes mellitus. Here we show that the transcriptional coactivator PGC-1 is strongly induced in liver in fasting mice and in three mouse models of insulin action deficiency: streptozotocin-induced diabetes, ob/ob genotype and liver insulin-receptor knockout. PGC-1 is induced synergistically in primary liver cultures by cyclic AMP and glucocorticoids. Adenoviral-mediated expression of PGC-1 in hepatocytes in culture or in vivo strongly activates an entire programme of key gluconeogenic enzymes, including phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase, leading to increased glucose output. Full transcriptional activation of the PEPCK promoter requires coactivation of the glucocorticoid receptor and the liver-enriched transcription factor HNF-4alpha (hepatic nuclear factor-4alpha) by PGC-1. These results implicate PGC-1 as a key modulator of hepatic gluconeogenesis and as a central target of the insulin-cAMP axis in liver.

Protein synthesis. The perks of balancing glucose

What do the regulation of translation initiation and glucose metabolism have to do with each other? Quite a lot, it seems, according to Sonenberg and Newgard in their Perspective. They discuss new findings that identify the kinase responsible for inactivating eIF2--a factor that is required for translation initiation (and hence protein synthesis)--when the endoplasmic reticulum is under stress. Loss of this kinase results in destruction of insulin-producing b cells in the pancreas and dysregulation of glucose homeostasis.

Overexpression of the P46 (T1) translocase component of the glucose-6-phosphatase complex in hepatocytes impairs glycogen accumulation via hydrolysis of glucose 1-phosphate

The final step of gluconeogenesis and glycogenolysis is catalyzed by the glucose-6-phosphatase (Glc-6-Pase) enzyme complex, located in the endoplasmic reticulum. The complex consists of a 36-kDa catalytic subunit (P36), a 46-kDa glucose 6-phosphate translocase (P46), and putative glucose and inorganic phosphate transporters. Mutations in the genes encoding P36 or P46 have been linked to glycogen storage diseases type Ia and type Ib, respectively. However, the relative roles of these two proteins in control of the rate of glucose 6-phosphate hydrolysis have not been defined. To gain insight into this area, we have constructed a recombinant adenovirus containing the cDNA encoding human P46 (AdCMV-P46) and treated rat hepatocytes with this virus, or a virus encoding P36 (AdCMV-P36), or the combination of both viruses, resulting in large and equivalent increases in expression of the transgenes within 8-24 h of viral treatment. The overexpressed P46 protein was appropriately targeted to hepatocyte microsomes and caused a 58% increase in glucose 6-phosphate hydrolysis in nondetergent-treated (intact) microsomal preparations relative to controls, whereas overexpression of P36 caused a 3.6-fold increase. Overexpression of P46 caused a 50% inhibition of glycogen accumulation in hepatocytes from fasted rats incubated at 25 mm glucose relative to cells treated with a control virus (AdCMV-betaGAL). Furthermore, in hepatocytes from fed rats cultured at 25 mm glucose and then exposed to 15 mm glucose, AdCMV-P46 treatment activated glycogenolysis, as indicated by a 50% reduction in glycogen content relative to AdCMV-betaGAL-treated controls. In contrast, overexpression of P46 had only small effects on glycolysis, whereas overexpression of P36 had large effects on both glycogen metabolism and glycolysis, even in the presence of co-overexpressed glucokinase. Finally, P46 overexpression enhanced glucose 1-phosphate but not fructose 6-phosphate hydrolysis in intact microsomes, providing a mechanism by which P46 overexpression may exert its preferential effects on glycogen metabolism.

Overexpression of a modified human malonyl-CoA decarboxylase blocks the glucose-induced increase in malonyl-CoA level but has no impact on insulin secretion in INS-1-derived (832/13) beta-cells

The long-chain acyl-CoA (LC-CoA) model of glucose-stimulated insulin secretion (GSIS) holds that secretion is linked to a glucose-induced increase in malonyl-CoA level and accumulation of LC-CoA in the cytosol. We have previously tested the validity of this proposal by overexpressing goose malonyl-CoA decarboxylase (MCD) in INS-1 cells, but these studies have been criticized due to: 1) the small insulin secretion response (2-4-fold) of the INS-1 cells used; 2) unknown contribution of the ATP-sensitive K(+) (K(ATP)) channel-independent pathway of GSIS in INS-1 cells, which has been implicated as the site at which lipids regulate insulin granule exocytosis; and 3) deletion of the N-terminal mitochondrial targeting sequence, but not the C-terminal peroxisomal targeting sequence in the goose MCD construct, raising the possibility that a significant fraction of the overexpressed enzyme was localized to peroxisomes. To address these outstanding concerns, INS-1-derived 832/13 cells, which exhibit robust K(ATP) channel-dependent and -independent pathways of GSIS, were treated with a new adenovirus encoding human MCD lacking both its mitochondrial and peroxisomal targeting sequences (AdCMV-MCD Delta 5), resulting in large increases in cytosolic MCD activity. Treatment of 832/13 cells with AdCMV-MCD Delta 5 completely blocked the glucose-induced rise in malonyl-CoA and attenuated the inhibitory effect of glucose on fatty acid oxidation. However, MCD overexpression had no effect on K(ATP) channel-dependent or -independent GSIS in 832/13 cells. Furthermore, combined treatment of 832/13 cells with AdCMV-MCD Delta 5 and triacsin C, an inhibitor of long chain acyl-CoA synthetase that reduces LC-CoA levels, did not impair GSIS. These findings extend our previous observations and are not consistent with the LC-CoA hypothesis as originally set forth.

Expression of the transcription factor STAT-1 alpha in insulinoma cells protects against cytotoxic effects of multiple cytokines

Destruction of pancreatic islet beta-cells in type 1 diabetes appears to result from direct contact with infiltrating T-cells and macrophages and exposure to inflammatory cytokines such as interferon (IFN)-gamma, interleukin (IL)-1 beta, and tumor necrosis factor TNF-alpha that such cells produce. We recently reported on a method for selection of insulinoma cells that are resistant to the cytotoxic effects of inflammatory cytokines (INS-1(res)), involving their growth in progressively increasing concentrations of IL-1 beta plus IFN-gamma, and selection of surviving cells. In the current study, we have investigated the molecular mechanism of cytokine resistance in INS-1(res) cells. By focusing on the known components of the IFN-gamma receptor signaling pathway, we have discovered that expression levels of signal transducer and activator of transcription (STAT)-1 alpha are closely correlated with the cytokine-resistant and -sensitive phenotypes. That STAT-1 alpha is directly involved in development of cytokine resistance is demonstrated by an increase of viability from 10 +/- 2% in control cells to 50 +/- 6% in cells with adenovirus-mediated overexpression of STAT-1 alpha (p < 0.001) after culture of both cell groups in the presence of 100 units/ml IFN-gamma plus 10 ng/ml IL-1 beta for 48 h. The resistance to IL-1 beta plus IFN-gamma in STAT-1 alpha-expressing cells is due in part to interference with IL-1 beta-mediated stimulation of inducible nitric-oxide synthase expression and nitric oxide production. Furthermore, overexpression of STAT-1 alpha does not impair robust glucose-stimulated insulin secretion in the INS-1-derived cell line 832/13. We conclude that expression of STAT-1 alpha may be a means of protecting insulin-producing cell lines from cytokine damage, which, in conjunction with appropriate cell-impermeant macroencapsulation devices, may allow such cells to be used for insulin replacement in type 1 diabetes.

Overexpression of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase in mouse liver lowers blood glucose by suppressing hepatic glucose production

Hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase is an important regulatory enzyme of glucose metabolism. By controlling the level of fructose-2,6-bisphosphate, an allosteric activator of the glycolytic enzyme 6-phosphofructo-1-kinase and an inhibitor of the gluconeogenic enzyme fructose-1,6-bisphosphatase, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase regulates hepatic glucose output. We studied the effects of adenovirus-mediated overexpression of this enzyme on hepatic glucose metabolism in normal or diabetic mice. These animals were treated with virus encoding either wild-type or bisphosphatase activity-deficient 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase. Seven days after virus injection, hepatic fructose-2,6-bisphosphate levels increased significantly in both normal and diabetic mice, with larger increases observed in animals with overexpression of the mutant enzyme. Blood glucose levels in normal mice overexpressing either enzyme were lowered, accompanied by increased plasma lactate, triglycerides, and FFAs. Blood glucose levels were markedly reduced in diabetic mice overexpressing the wild-type enzyme, and still more so in mice overexpressing the mutant form of the enzyme. The lower blood glucose levels in diabetic mice were accompanied by partially normalized plasma triglycerides and FFAs, increased plasma lactate, and increased liver glycogen levels, relative to diabetic mice treated with a control adenovirus. Our findings underscore the critical role played by hepatic 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in control of fuel homeostasis and suggest that this enzyme may be considered as a therapeutic target in diabetes.

Glucose activates mitogen-activated protein kinase (extracellular signal-regulated kinase) through proline-rich tyrosine kinase-2 and the Glut1 glucose transporter

Glucose serves as both a nutrient and regulator of physiological and pathological processes. Presently, we found that glucose and certain sugars rapidly activated extracellular signal-regulated kinase (ERK) by a mechanism that was: (a) independent of glucose uptake/metabolism and protein kinase C but nevertheless cytochalasin B-inhibitable; (b) dependent upon proline-rich tyrosine kinase-2 (PYK2), GRB2, SOS, RAS, RAF, and MEK1; and (c) amplified by overexpression of the Glut1, but not Glut2, Glut3, or Glut4, glucose transporter. This amplifying effect was independent of glucose uptake but dependent on residues 463-468, IASGFR, in the Glut1 C terminus. Accordingly, glucose effects on ERK were amplified by expression of Glut4/Glut1 or Glut2/Glut1 chimeras containing IASGFR but not by Glut1/Glut4 or Glut1/Glut2 chimeras lacking these residues. Also, deletion of Glut1 residues 469-492 was without effect, but mutations involving serine 465 or arginine 468 yielded dominant-negative forms that inhibited glucose-dependent ERK activation. Glucose stimulated the phosphorylation of tyrosine residues 402 and 881 in PYK2 and binding of PYK2 to Myc-Glut1. Our findings suggest that: (a) glucose activates the GRB2/SOS/RAS/RAF/MEK1/ERK pathway by a mechanism that requires PYK2 and residues 463-468, IASGFR, in the Glut1 C terminus and (b) Glut1 serves as a sensor, transducer, and amplifier for glucose signaling to PYK2 and ERK.

Overexpression of protein targeting to glycogen in cultured human muscle cells stimulates glycogen synthesis independent of glycogen and glucose 6-phosphate levels

There is growing evidence that glycogen targeting subunits of protein phosphatase-1 play a critical role in regulation of glycogen metabolism. In the current study, we have investigated the effects of adenovirus-mediated overexpression of a specific glycogen targeting subunit known as protein targeting to glycogen (PTG) in cultured human muscle cells. PTG was overexpressed both in muscle cells cultured at high glucose (glycogen replete) or in cells incubated for 18 h in the absence of glucose and then incubated in high glucose (glycogen re-synthesizing). In both glycogen replete and glycogen resynthesizing cells, PTG overexpression caused glycogen to be synthesized at a linear rate 1-5 days after viral treatment, while in cells treated with a virus lacking a cDNA insert (control virus), glycogen content reached a plateau at day 1 with no further increase. In the glycogen replete PTG overexpressing cells, glycogen content was 20 times that in controls at day 5. Furthermore, in cells undergoing glycogen resynthesis, PTG overexpression caused a doubling of the initial rate of glycogen synthesis over the first 24 h relative to cells treated with control virus. In both sets of experiments, the effects of PTG on glycogen synthesis were correlated with a 2-3-fold increase in glycogen synthase activity state, with no changes in glycogen phosphorylase activity. The alterations in glycogen synthase activity were not accompanied by changes in the intracellular concentration of glucose 6-phosphate. We conclude that PTG overexpression activates glycogen synthesis in a glucose 6-phosphate-independent manner in human muscle cells while overriding glycogen-mediated inhibition. Our findings suggest that modulation of PTG expression in muscle may be a mechanism for enhancing muscle glucose disposal and improving glucose tolerance in diabetes.

Organizing glucose disposal: emerging roles of the glycogen targeting subunits of protein phosphatase-1

Glucose is stored in mammalian tissues in the form of glycogen. Glycogen levels are markedly reduced in liver or muscle cells of patients with insulin-resistant or insulin-deficient forms of diabetes, suggesting that impaired glycogen synthesis may contribute to development of hyperglycemia. Recently, interest in this area has been further stimulated by new insights into the spatial organization of metabolic enzymes within cells and the importance of such organization in regulation of glycogen metabolism. It is now clear that a four-member family of glycogen targeting subunits of protein phosphatase-1 (PP1) plays a major role in coordinating these events. These proteins target PP1 to the glycogen particle and also bind differentially to glycogen synthase, glycogen phosphorylase, and phosphorylase kinase, thereby serving as molecular scaffolds. Moreover, the various glycogen-targeting subunits have distinct tissue expression patterns and can influence regulation of glycogen metabolism in response to glycogenic and glycogenolytic signals. The purpose of this article is to summarize new insights into the structure, function, regulation, and metabolic effects of the glycogen-targeting subunits of PP1 and to evaluate the possibility that these molecules could serve as therapeutic targets for lowering of blood glucose in diabetes.

Glucose down-regulates the expression of the peroxisome proliferator-activated receptor-alpha gene in the pancreatic beta -cell

To better understand the action of glucose on fatty acid metabolism in the beta-cell and the link between chronically elevated glucose or fatty acids and beta-cell decompensation in adipogenic diabetes, we investigated whether glucose regulates peroxisomal proliferator-activated receptor (PPAR) gene expression in the beta-cell. Islets or INS(832/13) beta-cells exposed to high glucose show a 60-80% reduction in PPARalpha mRNA expression. Oleate, either in the absence or presence of glucose, has no effect. The action of glucose is dose-dependent in the 6-20 mm range and maximal after 6 h. Glucose also causes quantitatively similar reductions in PPARalpha protein and DNA binding activity of this transcription factor. The effect of glucose is blocked by the glucokinase inhibitor mannoheptulose, is partially mimicked by 2-deoxyglucose, and is not blocked by the 3-O-methyl or the 6-deoxy analogues of the sugar that are not phosphorylated. Chronic elevated glucose reduces the expression levels of the PPAR target genes, uncoupling protein 2 and acyl-CoA oxidase, which are involved in fat oxidation and lipid detoxification. A 3-day exposure of INS-1 cells to elevated glucose results in a permanent rise in malonyl-CoA, the inhibition of fat oxidation, and the promotion of fatty acid esterification processes and causes elevated insulin secretion at low glucose. The results suggest that a reduction in PPARalpha gene expression together with a rise in malonyl-CoA plays a role in the coordinated adaptation of beta-cell glucose and lipid metabolism to hyperglycemia and may be implicated in the mechanism of beta-cell "glucolipotoxicity."

Distinctive regulatory and metabolic properties of glycogen-targeting subunits of protein phosphatase-1 (PTG, GL, GM/RGl) expressed in hepatocytes

Glycogen-targeting subunits of protein phosphatase-1 facilitate interaction of the phosphatase with enzymes of glycogen metabolism. We have shown that overexpression of one member of the family, protein targeting to glycogen (PTG), causes large increases in glycogen storage in isolated hepatocytes or intact rat liver. In the current study, we have compared the metabolic and regulatory properties of PTG (expressed in many tissues), with two other members of the gene family, G(L) (expressed primarily in liver) and G(M)/R(Gl) (expressed primarily in striated muscle). Adenovirus-mediated expression of these proteins in hepatocytes led to the following key observations. 1) G(L) has the highest glycogenic potency among the three forms studied. 2) Glycogen synthase activity ratio is much higher in G(L)-overexpressing cells than in PTG or G(M)/R(Gl)-overexpressing cells. Thus, at moderate levels of G(L) overexpression, glycogen synthase activity is increased by insulin treatment, but at higher levels of G(L) expression, insulin is no longer required to achieve maximal synthase activity. In contrast, cells with high levels of PTG overexpression retain dose-dependent regulation of glycogen synthesis and glycogen synthase enzyme activity by insulin. 3) G(L)- and G(M)/R(Gl)-overexpressing cells exhibit a strong glycogenolytic response to forskolin, whereas PTG-overexpressing cells are less responsive. This difference may be explained in part by a lesser forskolin-induced increase in glycogen phosphorylase activity in PTG-overexpressing cells. Based on these results, we suggest that expression of either G(L) or G(M)/R(Gl) in liver of diabetic animals may represent a strategy for lowering of blood glucose levels in diabetes.

Selection of insulinoma cell lines with resistance to interleukin-1beta- and gamma-interferon-induced cytotoxicity

Engineered insulinoma cell lines may represent an alternative to isolated islets for transplantation therapy of type 1 diabetes. Success of this approach may require development of cell lines that can withstand cytokine-mediated damage. To this end, we have cultured INS-1 insulinoma cells in increasing concentrations of interleukin-1beta (IL-1beta) + gamma-interferon (IFN-gamma), with approximate weekly iterations over an 8-week period. Based on the C,N diphenyl-N'-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium+ ++ bromide (MTT) viability assay, the selected cells, termed INS-1res, were 100% viable after 5 days of treatment with 10 ng/ml of IL-1beta. These cells were also 78 +/- 1.2% viable after 5 days of exposure to the combination of 10 ng/ml IL-1beta and 100 U/ml IFN-gamma, whereas parental INS-1 cells treated in the same manner were only 0.3 +/- 0.03% viable. INS-1res cells were also resistant to treatment with supernatants from activated rat peripheral blood mononuclear cells, whereas only 20% of parental INS-1 cells survived such treatment. The resistance to IL-1beta conferred by this procedure was stable, whereas the partial resistance to IFN-gamma was transient but reinducible by culture in the presence of cytokines. Stable transfection of INS-1res cells with a plasmid containing the human insulin cDNA and expansion of the transfected colonies in the absence of cytokines produced cell lines that were on average more resistant to IL-1beta + IFN-gamma (53 +/- 11%) than similarly transfected clones derived from parental INS-1 cells (15 +/- 7%). Importantly, several INS-1res-derived clones retained the capacity to secrete insulin in response to glucose concentrations over the normal physiological range. With regard to the mechanism by which selection was conferred, we found normal levels of IFN-gamma receptor mRNA, but a 60% reduction in expression of the IL-1 receptor type I (IL-1RI) in INS-1res cells compared with parental INS-1 cells. IL-1beta signaling through p38 MAP kinase was found to be normal in INS-1res cells, suggesting that their expression of IL-1RI is sufficient to maintain cytokine action. However, normal IL-1beta-mediated translocation of NF-kappaB and induction of inducible nitric oxide synthase expression and nitric oxide production was severely impaired in the INS-1res cell lines, suggesting a mechanism for the IL-1beta resistance. In sum, this study defines a strategy for isolation of cytokine-resistant beta-cell lines and provides a new system for studying the mechanisms by which such resistance can be achieved.

Correction of diet-induced hyperglycemia, hyperinsulinemia, and skeletal muscle insulin resistance by moderate hyperleptinemia

Human obesity and high fat feeding in rats are associated with the development of insulin resistance and perturbed carbohydrate and lipid metabolism. It has been proposed that these metabolic abnormalities may be reversible by interventions that increase plasma leptin. Up to now, studies in nongenetic animal models of obesity and in human obesity have concentrated on multiple injection therapy with mixed results. Our study sought to determine whether a sustained, moderate increase in plasma leptin, achieved by administration of a recombinant adenovirus containing the leptin cDNA (AdCMV-leptin) would be effective in reversing the metabolic abnormalities of the obese phenotype. Wistar rats fed a high-fat diet (HF) were heavier (P < 0.05), had increased fat mass and intramuscular triglycerides (mTG), and had elevated plasma glucose, insulin, triglyceride, and free fatty acids compared with standard chow-fed (SC) control animals (all P < 0.01). HF rats also had impaired glucose tolerance and were markedly insulin resistant, as demonstrated by a 40% reduction in insulin-stimulated muscle glucose uptake (P < 0.001). Increasing plasma leptin levels to 29.0 +/- 1.5 ng/ml (from 7.0 +/- 1.4 ng/ml, P < 0.001) for a period of 6 days decreased adipose mass by 40% and normalized plasma glucose and insulin levels. In addition, insulin-stimulated skeletal muscle glucose uptake was normalized in hyperleptinemic rats, an effect that correlated closely with a 60% (P < 0.001) decrease in mTG. Importantly, HF rats that received a control adenovirus containing the beta-galactosidase cDNA and were calorically matched to AdCMV-leptin-treated animals remained hyperglycemic, hyperinsulinemic, insulin resistant, and maintained elevated mTG. We conclude that a gene-therapeutic intervention that elevates plasma leptin moderately for a sustained period reverses diet-induced hyperglycemia, hyperinsulinemia, and skeletal muscle insulin resistance, and that these improvements are tightly linked to leptin-induced reductions in mTG.

Isolation of INS-1-derived cell lines with robust ATP-sensitive K+ channel-dependent and -independent glucose-stimulated insulin secretion

The biochemical mechanisms involved in regulation of insulin secretion are not completely understood. The rat INS-1 cell line has been used to gain insight in this area because it secretes insulin in response to glucose concentrations in the physiological range. However, the magnitude of the response is far less than that seen in freshly isolated rat islets. In the current study, we have stably transfected INS-1 cells with a plasmid containing the human proinsulin gene. After antibiotic selection and clonal expansion, 67% of the resultant clones were found to be poorly responsive to glucose in terms of insulin secretion (< or =2-fold stimulation by 15 mmol/l compared with 3 mmol/l glucose), 17% of the clones were moderately responsive (2- to 5-fold stimulation), and 16% were strongly responsive (5- to 13-fold stimulation). The differences in responsiveness could not be ascribed to differences in insulin content. Detailed analysis of one of the strongly responsive lines (832/13) revealed that its potent response to glucose (average of 10-fold) was stable over 66 population doublings (approximately 7.5 months of tissue culture) with half-maximal stimulation at 6 mmol/l glucose. Furthermore, in the presence of 15 mmol/l glucose, insulin secretion was potentiated significantly by 100 pmol/l isobutylmethylxanthine (320%), 1 mmol/l oleate/palmitate (77%), and 50 nmol/l glucagon-like peptide 1 (60%), whereas carbachol had no effect. Glucose-stimulated insulin secretion was also potentiated by the sulfonylurea tolbutamide (threefold at 3 mmol/l glucose and 50% at 15 mmol/l glucose) and was abolished by diazoxide, which demonstrates the operation of the ATP-sensitive K+ channel (K(ATP)) in 832/13 cells. Moreover, when the K(ATP) channel was bypassed by incubation of cells in depolarizing K+ (35 mmol/l), insulin secretion was more effectively stimulated by glucose in 832/13 cells than in parental INS-1 cells, which demonstrates the presence of a K(ATP) channel-independent pathway of glucose sensing. We conclude that clonal selection of INS-1 cells allows isolation of cell lines that exhibit markedly enhanced and stable responsiveness to glucose and several of its known potentiators. These lines may be attractive new vehicles for studies of beta-cell function.

Activation of direct and indirect pathways of glycogen synthesis by hepatic overexpression of protein targeting to glycogen

Glycogen-targeting subunits of protein phosphatase-1, such as protein targeting to glycogen (PTG), direct the phosphatase to the glycogen particle, where it stimulates glycogenesis. We have investigated the metabolic impact of overexpressing PTG in liver of normal rats. After administration of PTG cDNA in a recombinant adenovirus, animals were fasted or allowed to continue feeding for 24 hours. Liver glycogen was nearly completely depleted in fasted control animals, whereas glycogen levels in fasted or fed PTG-overexpressing animals were 70% higher than in fed controls. Nevertheless, transgenic animals regulated plasma glucose, triglycerides, FFAs, ketones, and insulin normally in the fasted and fed states. Fasted PTG-overexpressing animals receiving an oral bolus of [U-(13)C]glucose exhibited a large increase in hepatic glycogen content and a 70% increase in incorporation of [(13)C]glucose into glycogen. However, incorporation of labeled glucose accounted for only a small portion of the glycogen synthesized in PTG-overexpressing animals, consistent with our earlier finding that PTG promotes glycogen synthesis from gluconeogenic precursors. We conclude that hepatic PTG overexpression activates both direct and indirect pathways of glycogen synthesis. Because of its ability to enhance glucose storage without affecting other metabolic indicators, the glycogen-targeting subunit may prove valuable in controlling blood glucose levels in diabetes.

Increased glucose disposal induced by adenovirus-mediated transfer of glucokinase to skeletal muscle in vivo

In non-insulin-dependent diabetes mellitus, insulin-stimulated glucose uptake is impaired in muscle, contributing in a major way to development of hyperglycemia. We previously showed that expression of the glucose phosphorylating enzyme glucokinase (GK) in cultured human myocytes improved glucose storage and disposal, suggesting that GK delivery to muscle in situ could potentially enhance glucose clearance. Here we have tested this idea directly by intramuscular delivery of an adenovirus containing the liver GK cDNA (AdCMV-GKL) into one hind limb. We injected an adenovirus containing the beta-galactosidase gene (AdCMV-lacZ) into the hind limb of newborn rats. beta-Galactosidase activity was localized in muscle for as long as 1 month after delivery, with a large percentage of fibers staining positive in the gastrocnemius. Using the same approach with AdCMV-GKL, GK protein content was increased from zero to 50-400% of the GK in normal liver sample, and total glucose phosphorylating activity was increased in GK-expressing muscles relative to the counterpart uninfected muscle. Expression of GK in muscle improved glucose tolerance rather than changing basal glycemic control. Glucose levels were reduced by approximately 35% 10 min after administration of a glucose bolus to fed animals treated with AdCMV-GKL relative to AdCMV-lacZ-treated controls. The enhanced rate of glucose clearance was reflected in increases in muscle 2-deoxy glucose uptake and blood lactate levels. We conclude that restricted expression of GK in muscle leads to an enhanced capacity for muscle glucose disposal and whole body glucose tolerance under conditions of maximal glucose-insulin stimulation, suggesting that under these conditions glucose phosphorylation becomes rate-limiting. Our findings also show that gene delivery to a fraction of the whole body is sufficient to improve glucose disposal, providing a rationale for the development of new therapeutic strategies for treatment of diabetes.-Jiménez-Chillarón, J. C., Newgard, C. B., Gómez-Foix, A. M. Increased glucose disposal induced by adenovirus-mediated transfer of glucokinase to skeletal muscle in vivo.

Glucokinase overexpression restores glucose utilization and storage in cultured hepatocytes from male Zucker diabetic fatty rats

Zucker diabetic fatty rats develop type 2 diabetes concomitantly with peripheral insulin resistance. Hepatocytes from these rats and their control lean counterparts have been cultured, and a number of key parameters of glucose metabolism have been determined. Glucokinase activity was 4.5-fold lower in hepatocytes from diabetic rats than in hepatocytes from healthy ones. In contrast, hexokinase activity was about 2-fold higher in hepatocytes from diabetic animals than in healthy ones. Glucose-6-phosphatase activity was not significantly different. Despite the altered ratios of glucokinase to hexokinase activity, intracellular glucose 6-phosphate concentrations were similar in the two types of cells when they where incubated with 1-25 mM glucose. However, glycogen levels and glycogen synthase activity ratio were lower in hepatocytes from diabetic animals. Total pyruvate kinase activity and its activity ratio as well as fructose 2,6-bisphosphate concentration and lactate production were also lower in cells from diabetic animals. All of these data indicate that glucose metabolism is clearly impaired in hepatocytes from Zucker diabetic fatty rats. Glucokinase overexpression using adenovirus restored glucose metabolism in diabetic hepatocytes. In glucokinase-overexpressing cells, glucose 6-phosphate levels increased. Moreover, glycogen deposition was greatly enhanced due to the activation of glycogen synthase. Pyruvate kinase was also activated, and fructose-2,6-bisphosphate concentration and lactate production were increased in glucokinase-overexpressing diabetic hepatocytes. Overexpression of hexokinase I did not increase glycogen deposition. In conclusion, hepatocytes from Zucker diabetic fatty rats showed depressed glycogen and glycolytic metabolism, but glucokinase overexpression improved their glucose utilization and storage.

Metabolic impact of glucokinase overexpression in liver: lowering of blood glucose in fed rats is accompanied by hyperlipidemia

The balance between hepatic glucose uptake and production is perturbed in both major forms of diabetes. It has been suggested that pharmacologic or genetic methods for enhancing glucokinase (GK) enzymatic activity in liver might be a means of increasing glucose disposal and lowering blood glucose in diabetic patients. To better evaluate this possibility, we used a recombinant adenovirus containing the cDNA encoding GK (AdCMV-GKL) to achieve overexpression of the enzyme at different levels in liver of normal rats. In a first set of experiments, in rats fasted for 18 h, AdCMV-GKL infusion caused a 211% increase in hepatic GK activity relative to animals infused with a control virus (AdCMV-betaGAL). AdCMV-GKL-treated fasted rats exhibited no significant changes in circulating glucose, free fatty acids (FFAs), lactate, beta-hydroxybutyrate, or insulin levels relative to controls, whereas triglyceride (TG) levels were slightly increased (53%). In a second set of studies, in rats fed ad libitum, GK was overexpressed in liver by 3- and 6.4-fold. Animals with the lower degree of GK overexpression exhibited no significant changes in circulating glucose, FFAs, insulin, TG, or lactate levels relative to controls that received a virus encoding a catalytically inactive mutant GK (AdCMV-GK203), but did show a modest increase in lactate (58%) relative to AdCMV-betaGAL-infused controls. In contrast, the higher level of GK overexpression caused a 38% decrease in blood glucose levels and a 67% decrease in circulating insulin levels relative to AdCMV-GK203-infused animals. The decline in glucose levels was accompanied by a 190% increase in circulating TG and a 310% increase in circulating FFAs; total plasma cholesterol was unaffected. Finally, fasted animals treated with AdCMV-GKL had 5.4 times as much liver glycogen as AdCMV-betaGAL-treated controls; no significant increases in liver glycogen were observed at either level of GK overexpression in ad libitum-fed rats relative to fed controls. In sum, levels of hepatic GK overexpression associated with a decline in blood glucose are accompanied by equally dramatic increases in FFAs and TG, raising concerns about manipulation of liver GK activity as a viable strategy for treatment of diabetes.

Sparing effect of leptin on liver glycogen stores in rats during the fed-to-fasted transition

The effect of moderate hyperleptinemia ( approximately 20 ng/ml) on liver and skeletal muscle glycogen metabolism was examined in Wistar rats. Animals were studied approximately 90 h after receiving recombinant adenoviruses encoding rat leptin (AdCMV-leptin) or beta-galactosidase (AdCMV-betaGal). Liver and skeletal muscle glycogen levels in the fed and fasted (18 h) states were similar in AdCMV-leptin- and AdCMV-betaGal-treated rats. However, after delivery of a glucose bolus, liver glycogen levels were significantly greater in AdCMV-leptin compared with AdCMV-betaGal rats (P < 0.05). To investigate the mechanism(s) of these differences, glycogen levels were measured immediately after the cessation of a 3- or 6-h glucose infusion or 3, 6, and 9 h after the cessation of a 6-h glucose infusion. Similar increases in liver and skeletal muscle glycogen occurred in hyperleptinemic and control rats in response to glucose infusions. However, 3 and 6 h after the cessation of a glucose infusion, liver glycogen levels were approximately twofold greater (P < 0.05) in AdCMV-leptin-treated compared with AdCMV-betaGal-treated animals. Skeletal muscle glycogen levels were similar in AdCMV-leptin-treated and AdCMV-betaGal-treated animals at the same time points. Glycogen phosphorylase, phosphodiesterase 3B, and glycogen synthase activities were unaltered by hyperleptinemia. We conclude that moderate increases in plasma leptin levels decrease liver glycogen degradation during the fed-to-fasted transition.

Glucose-6-phosphatase overexpression lowers glucose 6-phosphate and inhibits glycogen synthesis and glycolysis in hepatocytes without affecting glucokinase translocation. Evidence against feedback inhibition of glucokinase

In hepatocytes glucokinase (GK) and glucose-6-phosphatase (Glc-6-Pase)(1) have converse effects on glucose 6-phosphate (and fructose 6-phosphate) levels. To establish whether hexose 6-phosphate regulates GK binding to its regulatory protein, we determined the effects of Glc-6-Pase overexpression on glucose metabolism and GK compartmentation. Glc-6-Pase overexpression (4-fold) decreased glucose 6-phosphate levels by 50% and inhibited glycogen synthesis and glycolysis with a greater negative control coefficient on glycogen synthesis than on glycolysis, but it did not affect the response coefficients of glycogen synthesis or glycolysis to glucose, and it did not increase the control coefficient of GK or cause dissociation of GK from its regulatory protein, indicating that in hepatocytes fructose 6-phosphate does not regulate GK translocation by feedback inhibition. GK overexpression increases glycolysis and glycogen synthesis with a greater control coefficient on glycogen synthesis than on glycolysis. On the basis of the similar relative control coefficients of GK and Glc-6-Pase on glycogen synthesis compared with glycolysis, and the lack of effect of Glc-6-Pase overexpression on GK translocation or the control coefficient of GK, it is concluded that the main regulatory function of Glc-6-Pase is to buffer the glucose 6-phosphate concentration. This is consistent with recent findings that hyperglycemia stimulates Glc-6-Pase gene transcription.

Metabolic engineering with recombinant adenoviruses

Fuel homeostasis in mammals is accomplished by the interplay between tissues and organs with distinct metabolic roles. These regulatory mechanisms are disrupted in obesity and diabetes, leading to a renewed emphasis on discovery of molecular and pharmacologic methods for reversing metabolic disorders. In this chapter, we review the use of recombinant adenoviral vectors as tools for delivering metabolic regulatory genes to cells in culture and to tissues of intact animals. Included are studies on the use of these vectors for gaining insights into the biochemical mechanisms that regulate glucose-stimulated insulin secretion from pancreatic islet beta-cells. We also highlight their use for understanding the function of newly discovered genes that regulate glycogen metabolism in liver and other tissues, and for evaluating "candidate" genes such as glucose-6-phosphatase, which may contribute to development of metabolic dysfunction in pancreatic islets and liver. Finally, we discuss the use of adenoviral and related vectors for causing chronic increases in the levels of circulating hormones. These examples serve to highlight the power of viral gene transfer vectors as tools for understanding metabolic regulatory mechanisms.

Snare protein expression and adenoviral transfection of amphicrine AR42J

The amphicrine AR42J acinar cell line is an excellent model to study both exocrine and neuroendocrine exocytotic mechanisms. As a first step toward this goal, we determined the specific isoforms of the v- and t-SNARE and Munc18 families expressed in these cells. In addition, we show that dexamethasone-induced differentiation toward the exocrine phenotype causes an upregulation of several of these proteins. AR42J is notoriously difficult to transfect, limiting its usefulness as a model. However, we have now overcome this obstacle by acheiving high efficiency expression of a beta-galactosidase reporter gene and truncated SNAP-25 gene using adenoviral infection techniques. The AR42J cells can now be used to pursue and elucidate the distinct functions of individual SNARE isoforms used in endocrine and exocrine secretion within a single cell line.

Increase in PDX-1 levels suppresses insulin gene expression in RIN 1046-38 cells

RIN1046-38 cells (RIN-38) exhibit a passage-dependent reduction in both basal and glucose-regulated insulin secretion, accompanied by decreased insulin content. In an attempt to explain the mechanism of the gradual decrease in insulin production in cultured cells, we analyzed the insulin promoter activity and the levels of an important trans-activator of the insulin gene, PDX-1, as a function of aging in culture. We demonstrate that the decrease in insulin content and secretion is reflected in decreased promoter activity and is associated with a decrease in E47 and BETA2 nuclear factors, but with a paradoxical 3-fold increase in PDX-1 protein levels. To dissect the effect of increased PDX-1 from the decrease in the additional transcription factors on insulin promoter activity, we overexpressed PDX-1 protein in low passage RIN-38 cells by recombinant adenovirus technology. PDX-1 overexpression did not reduce E47 and BETA2 levels, but was sufficient to suppress rat insulin promoter activity in a dose-dependent manner. The fact that PDX-1 levels participate in trans-activation of insulin promoter activity was demonstrated in HIT-T15 cells. Treating HIT-T15 cells with 1-2 multiplicity of infection of AdCMV-PDX-1 increased rat insulin promoter activity, whereas higher doses repressed insulin promoter activity in these cells as in RIN-38 cells. Our data demonstrate that PDX-1 regulates transcription of the insulin gene in a dose-dependent manner. Depending on its nuclear dosage and the levels of additional cooperating transcription factors, PDX-1 may act as an activator or a repressor of insulin gene expression, such that low as well as high doses may be deleterious to insulin production.

Adenovirus-mediated overexpression of uncoupling protein-2 in pancreatic islets of Zucker diabetic rats increases oxidative activity and improves beta-cell function

The discovery of uncoupling protein (UCP)-2, a ubiquitously expressed protein homologous to UCP-1, has raised the possibility that energy balance of cells might be regulated in tissues other than brown adipocytes. In normal pancreatic islets, UCP-2 is upregulated by leptin and is low in leptin-resistant islets of ZDF rats. To determine whether UCP-2 does, in fact, have uncoupling activity and, if so, whether such activity would favorably influence the abnormalities in leptin-unresponsive UCP-2-underexpressing islets of diabetic ZDF rats, we transferred the UCP-2 gene to the islets of diabetic ZDF rats and lean (+/+) ZDF control rats. Although ATP was reduced by 23% in both groups of islets, the ATP:ADP ratio increased by 42 and 141%, respectively. [3H]palmitate oxidation was increased by 50%, and [3H]glucose oxidation was 42-63% higher. Preproinsulin mRNA was 2.9-fold above control levels, and glucose-stimulated insulin secretion, which was negligible in control ZDF rat islets, was improved in UCP-2-overexpressing islets. The high fat content of the islets was not reduced, however. We conclude that UCP-2 has uncoupling function when overexpressed in leptin-insensitive islets and that its overexpression corrects the underexpression of the insulin gene and ameliorates glucose-stimulated insulin secretion, possibly by increasing the ATP:ADP ratio.

Overexpression of G11alpha and isoforms of phospholipase C in islet beta-cells reveals a lack of correlation between inositol phosphate accumulation and insulin secretion

It has been suggested that insulin secretion from pancreatic islets may be mediated in part by activation of phospholipases C (PLCs) and phosphoinositide hydrolysis. The purpose of this study was to determine whether the relatively modest fuel-stimulated insulin secretion responses of rodent beta-cell lines might be explained by inadequate expression or activation of PLC isoforms. We have found that two insulinoma cell lines, INS-1 and betaG 40/110, completely lack PLC-delta1 expression but have levels of expression of PLC-beta1, -beta2, -beta3, -delta2, and -gamma1 that are similar to or slightly reduced from those found in fresh rat islets. Adenovirus-mediated overexpression of PLC-delta1, -beta1, or -beta3 in INS-1 or betaG 40/110 cells results in little or no enhancement in inositol phosphate (IP) accumulation and no improvement in insulin secretion when the cells are stimulated with glucose or carbachol, despite the fact that the overexpressed proteins are fully active in cell extracts. Overexpression of PLC-beta1 or -beta3 in normal rat islets elicits a larger increase in IP accumulation but, again, has no effect on insulin secretion. Because the effect of carbachol on insulin secretion is thought to be mediated through muscarinic receptors that link to the Gq/11 class of heterotrimeric G proteins, we also overexpressed G11alpha in INS-1 cells, either alone or in concert with overexpression of PLC-beta1 or -beta3. Overexpression of G11alpha enhances IP accumulation, an effect slightly potentiated by co-overexpression of PLC-beta1 or -beta3, but these maneuvers do not affect glucose or carbachol-stimulated insulin secretion. In sum, our studies show a lack of correlation between IP accumulation and insulin secretion in INS-1 cells, betaG 40/110 cells, or cultured rat islets. We conclude that overexpression of PLC isoforms and/or G11alpha is not an effective means of enhancing fuel responsiveness in the insulinoma cell lines studied.

Reversing adipocyte differentiation: implications for treatment of obesity

Conventional treatment of obesity reduces fat in mature adipocytes but leaves them with lipogenic enzymes capable of rapid resynthesis of fat, a likely factor in treatment failure. Adenovirus-induced hyperleptinemia in normal rats results in rapid nonketotic fat loss that persists after hyperleptinemia disappears, whereas pair-fed controls regain their weight in 2 weeks. We report here that the hyperleptinemia depletes adipocyte fat while profoundly down-regulating lipogenic enzymes and their transcription factor, peroxisome proliferator-activated receptor (PPAR)gamma in epididymal fat; enzymes of fatty acid oxidation and their transcription factor, PPARalpha, normally low in adipocytes, are up-regulated, as are uncoupling proteins 1 and 2. This transformation of adipocytes from cells that store triglycerides to fatty acid-oxidizing cells is accompanied by loss of the adipocyte markers, adipocyte fatty acid-binding protein 2, tumor necrosis factor alpha, and leptin, and by the appearance of the preadipocyte marker Pref-1. These findings suggest a strategy for the treatment of obesity by alteration of the adipocyte phenotype.

Lipoapoptosis in beta-cells of obese prediabetic fa/fa rats. Role of serine palmitoyltransferase overexpression

We reported that the lipoapoptosis of beta-cells observed in fat-laden islets of obese fa/fa Zucker Diabetic Fatty (ZDF) rats results from overproduction of ceramide, an initiator of the apoptotic cascade and is induced by long-chain fatty acids (FA). Whereas the ceramide of cytokine-induced apoptosis may be derived from sphingomyelin hydrolysis, FA-induced ceramide overproduction seems to be derived from FA. We therefore semiquantified mRNA of serine palmitoyltransferase (SPT), which catalyzes the first step in ceramide synthesis. It was 2-3-fold higher in fa/fa islets than in +/+ controls. [3H]Ceramide formation from [3H]serine was 2.2-4. 5-fold higher in fa/fa islets. Triacsin-C, which blocks palmitoyl-CoA synthesis, and L-cycloserine, which blocks SPT activity, completely blocked [3H]ceramide formation from [3H]serine. Islets of fa/fa rats are unresponsive to the lipopenic action of leptin, which normally depletes fat and prevents FA up-regulation of SPT. To determine the role of leptin unresponsiveness in the SPT overexpression, we transferred wild type OB-Rb cDNA to their islets; now leptin completely blocked the exaggerated FA-induced increase of SPT mRNA while reducing the fat content. Beta-cell lipoapoptosis was partially prevented in vivo by treating prediabetic ZDF rats with L-cycloserine for 2 weeks. Ceramide content and DNA fragmentation both declined 40-50%. We conclude that lipoapoptosis of ZDF rats is mediated by enhanced ceramide synthesis from FA and that blockade by SPT inhibitors prevents lipoapoptosis.

Perturbation of fuel homeostasis caused by overexpression of the glucose-6-phosphatase catalytic subunit in liver of normal rats

The terminal step in hepatic gluconeogenesis is catalyzed by glucose-6-phosphatase, an enzyme activity residing in the endoplasmic reticulum and consisting of a catalytic subunit (glucose-6-phosphatase (G6Pase)) and putative accessory transport proteins. We show that Zucker diabetic fatty rats (fa/fa), which are known to exhibit impaired suppression of hepatic glucose output, have 2.4-fold more glucose-6-phosphatase activity in liver than lean controls. To define the potential contribution of increased hepatic G6Pase to development of diabetes, we infused recombinant adenoviruses containing the G6Pase cDNA (AdCMV-G6Pase) or the beta-galactosidase gene into normal rats. Animals were studied by one of three protocols as follows: protocol 1, fed ad libitum for 7 days; protocol 2, fed ad libitum for 5 days, fasted overnight, and subjected to an oral glucose tolerance test; protocol 3, fed ad libitum for 4 days, fasted for 48 h, subjected to oral glucose tolerance test, and then allowed to refeed overnight. Hepatic glucose-6-phosphatase enzymatic activity was increased by 1.6-3-fold in microsomes isolated from AdCMV-G6Pase-treated animals in all three protocols, and the resultant metabolic profile was similar in each case. AdCMV-G6Pase-treated animals exhibited several of the abnormalities associated with early stage non-insulin-dependent diabetes mellitus, including glucose intolerance, hyperinsulinemia, decreased hepatic glycogen content, and increased peripheral (muscle) triglyceride stores. These animals also exhibited significant decreases in circulating free fatty acids and triglycerides, changes not normally associated with the disease. Our studies show that overexpression of G6Pase in liver is sufficient to perturb whole animal glucose and lipid homeostasis, possibly contributing to the development of metabolic abnormalities associated with diabetes.

Overexpression of protein targeting to glycogen (PTG) in rat hepatocytes causes profound activation of glycogen synthesis independent of normal hormone- and substrate-mediated regulatory mechanisms

Protein targeting to glycogen (PTG), also known as PPP1R5, is a widely expressed member of a growing family of proteins that target protein phosphatase-1 (PP-1) to glycogen particles. Because PTG also binds to glycogen synthase and phosphorylase kinase, it has been suggested that it serves as a "scaffold" for efficient activation of glycogen synthesis. However, very little is known about the metabolic effects of PTG. In this study, we have used recombinant adenovirus to overexpress PTG in primary rat hepatocytes, a cell type with high glycogenic capacity. We find that overexpression of PTG potently activates glycogen synthesis in cultured hepatocytes. Surprisingly, the glycogenic effect of PTG is observed even in the complete absence of carbohydrates or insulin in the culture medium. Furthermore, glycogenolytic agents such as forskolin or glucagon are largely ineffective at activating glycogen degradation in PTG overexpressing hepatocytes, even though large increases in cAMP levels are demonstrated. These metabolic effects of PTG overexpression are accompanied by a 3.6-fold increase in glycogen synthase activation state and a 40% decrease in glycogen phosphorylase activity. Our results are consistent with a model in which PTG overexpression "locks" the hepatocyte in a glycogenic mode, presumably via its ability to promote interaction of enzymes of glycogen metabolism with PP-1.

Overexpression of leptin receptors in pancreatic islets of Zucker diabetic fatty rats restores GLUT-2, glucokinase, and glucose-stimulated insulin secretion

The high-Km glucose transporter, GLUT-2, and the high-Km hexokinase of beta cells, glucokinase (GK), are required for glucose-stimulated insulin secretion (GSIS). GLUT-2 expression in beta cells of Zucker diabetic fatty (ZDF) rats is profoundly reduced at the onset of beta-cell dysfunction of diabetes. Because ZDF rats are homozygous for a mutation in their leptin receptor (OB-R) gene and are therefore leptin-insensitive, we expressed the wild-type OB-R gene in diabetic islets by infusing a recombinant adenovirus (AdCMV-OB-Rb) to determine whether this reversed the abnormalities. Leptin induced a rise in phosphorylated STAT3, indicating that the transferred wild-type OB-R was functional. GLUT-2 protein rose 17-fold in AdCMV-OB-Rb-treated ZDF islets without leptin, and leptin caused no further rise. GK protein rose 7-fold without and 12-fold with leptin. Preproinsulin mRNA increased 64% without leptin and rose no further with leptin, but leptin was required to restore GSIS. Clofibrate and 9-cis-retinoic acid, the partner ligands for binding to peroxisome proliferator-activator receptor alpha (PPARalpha) and retinoid X receptor, up-regulated GLUT-2 expression in islets of normal rats, but not in ZDF rats, in which PPARalpha is very low. Because the fat content of islets of diabetic ZDF rats remains high unless they are treated with leptin, it appears that restoration of GSIS requires normalization of intracellular nutrient homeostasis, whereas up-regulation of GLUT-2 and GK is leptin-independent, requiring only high expression of OB-Rb.

The repression of hormone-activated PEPCK gene expression by glucose is insulin-independent but requires glucose metabolism

Phosphoenolpyruvate carboxykinase (PEPCK) is a rate-controlling enzyme in hepatic gluconeogenesis, and it therefore plays a central role in glucose homeostasis. The rate of transcription of the PEPCK gene is increased by glucagon (via cAMP) and glucocorticoids and is inhibited by insulin. Under certain circumstances glucose also decreases PEPCK gene expression, but the mechanism of this effect is poorly understood. The glucose-mediated stimulation of a number of glycolytic and lipogenic genes requires the expression of glucokinase (GK) and increased glucose metabolism. HL1C rat hepatoma cells are a stably transfected line of H4IIE rat hepatoma cells that express a PEPCK promoter-chloramphenicol acetyltransferase fusion gene that is regulated in the same manner as the endogenous PEPCK gene. These cells do not express GK and do not normally exhibit a response of either the endogenous PEPCK gene, or of the trans-gene, to glucose. A recombinant adenovirus that directs the expression of glucokinase (AdCMV-GK) was used to increase glucose metabolism in HL1C cells to test whether increased glucose flux is also required for the repression of PEPCK gene expression. In AdCMV-GK-treated cells glucose strongly inhibits hormone-activated transcription of the endogenous PEPCK gene and of the expressed fusion gene. The glucose effect on PEPCK gene promoter activity is blocked by 5 mM mannoheptulose, a specific inhibitor of GK activity. The glucose analog, 2-deoxyglucose mimics the glucose response, but this effect does not require GK expression. 3-O-methylglucose is ineffective. Glucose exerts its effect on the PEPCK gene within 4 h, at physiologic concentrations, and with an EC50 of 6.5 mM, which approximates the Km of glucokinase. The effects of glucose and insulin on PEPCK gene expression are additive, but only at suboptimal concentrations of both agents. The results of these studies demonstrate that, by inhibiting PEPCK gene transcription, glucose participates in a feedback control loop that governs its production from gluconeogenesis.

Expression of glucokinase in cultured human muscle cells confers insulin-independent and glucose concentration-dependent increases in glucose disposal and storage

Insulin resistance, as is found in skeletal muscle of individuals with obesity and NIDDM, appears to involve a reduced capacity of the hormone to stimulate glucose uptake and/or phosphorylation. The glucose phosphorylation step, as catalyzed by hexokinase II, has been described as rate limiting for glucose disposal in muscle, but overexpression of this enzyme under control of a muscle-specific promoter in transgenic mice has had limited metabolic impact. In the current study, we investigated in a cultured muscle model whether expression of glucokinase, which in contrast to hexokinase II is not inhibited by glucose-6-phosphate (G-6-P), would have a pronounced metabolic impact. We used a recombinant adenovirus containing the cDNA-encoding rat liver glucokinase (AdCMV-GKL) to increase the glucose phosphorylating activity in cultured human muscle cells by fourfold. G-6-P levels increased in AdCMV-GKL-treated cells in a glucose concentration-dependent manner over the range of 1-30 mmol/l, whereas the much smaller increases in G-6-P in control cells were maximal at glucose concentrations <5 mmol/l. Further, cells expressing glucokinase accumulated 17 times more 2-deoxyglucose-6-phosphate than control cells. In AdCMV-GKL-treated cells, the time-dependent rise in G-6-P correlated with an increase in the activity ratio of glycogen synthase. AdCMV-GKL-treated cells also exhibited a 2.5- to 3-fold increase in glycogen content and a four- to fivefold increase in glycolytic flux, proportional to the increase in glucose phosphorylating capacity. All of these observations were made in the absence of insulin. Thus we concluded that expression of glucokinase in cultured human muscle cells results in proportional increases in insulin-independent glucose disposal, and that muscle glucose storage and utilization becomes controlled in a glucose concentration-dependent manner in AdCMV-GKL-treated cells. These results encourage testing whether delivery of glucokinase to muscle in vivo has an impact on glycemic control, which could be a method for circumventing the failure of insulin to stimulate glucose uptake and/or phosphorylation in muscle normally in insulin-resistant subjects.

Resistance to adenovirally induced hyperleptinemia in rats. Comparison of ventromedial hypothalamic lesions and mutated leptin receptors

Leptin regulates appetite and body weight via hypothalamic targets, but it can act directly on cultured pancreatic islets to regulate their fat metabolism. To obtain in vivo evidence that leptin may act peripherally as well as centrally, we compared the effect of adenovirally induced hyperleptinemia on food intake, body weight, and islet fat content in ventromedial hypothalamic-lesioned (VMHL) rats, sham-lesioned (SL) controls, and Zucker Diabetic Fatty (ZDF) rats in which the leptin receptor is mutated. Infusion with recombinant adenovirus containing the rat leptin cDNA increased plasma leptin by approximately 20 ng/ml in VMHL and ZDF rats but had no effect on their food intake, body weight, or fat tissue weight. Caloric matching of hyperphagic VMHL rats to SL controls did not reduce their resistance to hyperleptinemia. Whereas prediabetic ZDF rats had a fourfold elevation in islet fat, in VMHL rats islet fat was normal and none of them became diabetic. Isolated islets from ZDF rats were completely resistant to the lipopenic action of leptin, while VMHL islets exhibited 50% of the normal response; caloric matching of VMHL rats to SL controls increased leptin responsiveness of their islets to 92% of controls. We conclude that leptin regulation of adipocyte fat requires an intact VMH but that islet fat content is regulated independently of the VMH.

Protection against lipoapoptosis of beta cells through leptin-dependent maintenance of Bcl-2 expression

Obesity causes its complications through functional and morphologic damage to remotely situated tissues via undetermined mechanisms. In one rodent model of obesity, the Zucker diabetic fatty fa/fa rat, overaccumulation of triglycerides in the pancreatic islets may be responsible for a gradual depletion of beta cells, leading to the most common complication of obesity, non-insulin-dependent diabetes mellitus. At the onset of non-insulin-dependent diabetes mellitus, the islets from fa/fa rats contain up to 100 times the fat content of islets from normal lean rats. Ultimately, about 75% of the beta cells disappear from these fat-laden islets as a consequence of apoptosis induced by long-chain fatty acids (FA). Here we quantify Bcl-2, the anti-apoptosis factor in these islets, and find that Bcl-2 mRNA and protein are, respectively, 85% and 70% below controls. In normal islets cultured in 1 mM FA, Bcl-2 mRNA declined by 68% and completely disappeared in fa/fa islets cultured in FA. In both groups, suppression was completely blocked by the fatty acyl-CoA synthetase inhibitor, triacsin C, evidence of its mediation by fatty acyl-CoA. To determine whether leptin action blocked FA-induced apoptosis, we cultured normal and fa/fa islets in 1 mM FA with or without leptin. Leptin completely blocked FA-induced Bcl-2 suppression in normal islets but had no effect on islets from fa/fa rats, which are unresponsive to leptin because of a mutation in their leptin receptors (OB-R). However, when wild-type OB-R is overexpressed in fa/fa islets, leptin completely prevented FA-induced Bcl-2 suppression and DNA fragmentation.

Role of peroxisome proliferator-activated receptor alpha in disease of pancreatic beta cells

Expression of peroxisome proliferator-activated receptor alpha (PPARalpha) and enzymes of fatty acid (FA) oxidation is markedly reduced in the fat-laden, dysfunctional islets of obese, prediabetic Zucker diabetic fatty (fa/fa) rats with mutated leptin receptors (OB-R). Leptin, PPARalpha/retinoid x receptor ligands, and FA all up-regulate PPARalpha and enzymes of FA oxidation and stimulate [3H]-palmitate oxidation in normal islets but not in islets from fa/fa rats. Overexpression of normal OB-R in islets of fa/fa rats corrects all of the foregoing abnormalities and reverses the diabetic phenotype. PPARalpha is a OB-R-dependent factor required for normal fat homeostasis in islet cells.

Molecular or pharmacologic perturbation of the link between glucose and lipid metabolism is without effect on glucose-stimulated insulin secretion. A re-evaluation of the long-chain acyl-CoA hypothesis

The mechanism by which glucose stimulates insulin secretion from the pancreatic islets of Langerhans is incompletely understood. It has been suggested that malonyl-CoA plays a regulatory role by inhibiting fatty acid oxidation and promoting accumulation of cytosolic long-chain acyl-CoA (LC-CoA). In the current study, we have re-evaluated this "long-chain acyl-CoA hypothesis" by using molecular and pharmacologic methods to perturb lipid metabolism in INS-1 insulinoma cells or rat islets during glucose stimulation. First, we constructed a recombinant adenovirus containing the cDNA encoding malonyl-CoA decarboxylase (AdCMV-MCD), an enzyme that decarboxylates malonyl-CoA to acetyl-CoA. INS-1 cells treated with AdCMV-MCD had dramatically lowered intracellular malonyl CoA levels compared with AdCMV-betaGal-treated cells at both 3 and 20 mM glucose. Further, at 20 mM glucose, AdCMV-MCD-treated cells were less effective at suppressing [1-14C]palmitate oxidation and incorporated 43% less labeled palmitate and 50% less labeled glucose into cellular lipids than either AdCMV-betaGAL-treated or untreated INS-1 cells. Despite the large metabolic changes caused by expression of MCD, insulin secretion in response to glucose was unaltered relative to controls. The alternative, pharmacologic approach for perturbing lipid metabolism was to use triacsin C to inhibit long-chain acyl-CoA synthetase. This agent caused potent attenuation of palmitate oxidation and glucose or palmitate incorporation into cellular lipids and also caused a 47% decrease in total LC-CoA. Despite this, the drug had no effect on glucose-stimulated insulin secretion in islets or INS-1 cells. We conclude that significant disruption of the link between glucose and lipid metabolism does not impair glucose-stimulated insulin secretion in pancreatic islets or INS-1 cells.

Stable expression of manganese superoxide dismutase (MnSOD) in insulinoma cells prevents IL-1beta- induced cytotoxicity and reduces nitric oxide production

The fact that insulin-producing islet beta-cells are susceptible to the cytotoxic effects of inflammatory cytokines represents a potential hinderance to the use of such cells for transplantation therapy of insulin-dependent diabetes mellitus (IDDM). In the current study, we show that IL-1beta induces destruction of INS-1 insulinoma cells, while having no effect on a second insulinoma cell line RIN1046-38 and its engineered derivatives, and that this difference is correlated with a higher level of expression of manganese superoxide dismutase (MnSOD) in the latter cells. Stable overexpression of MnSOD in INS-1 cells provides complete protection against IL-1beta-mediated cytotoxicity, and also results in markedly reduced killing when such cells are exposed to conditioned media from activated human or rat PBMC. Further, overexpression of MnSOD in either RIN- or INS-1-derived lines results in a sharp reduction in IL-1beta-induced nitric oxide (NO) production, a finding that correlates with reduced levels of the inducible form of nitric oxide synthase (iNOS). Treatment of INS-1 cells with L-NMMA, an inhibitor of iNOS, provides the same degree of protection against IL-1beta or supernatants from LPS-activated rat PBMC as MnSOD overexpression, supporting the idea that MnSOD protects INS-1 cells by interfering with the normal IL-1beta-mediated increase in iNOS. Because NO and its derivatives have been implicated as critical mediators of beta-cell destruction in IDDM, we conclude that well regulated insulinoma cell lines engineered for MnSOD overexpression may be an attractive alternative to isolated islets as vehicles for insulin replacement in autoimmune diabetes.

Fundamental metabolic differences between hepatocytes and islet beta-cells revealed by glucokinase overexpression

Adenovirus-mediated overexpression of the glucose phosphorylating enzyme glucokinase causes large changes in glycolytic flux and glucose storage in isolated rat hepatocytes, but not in pancreatic islets. We have used the well-differentiated insulinoma cell line INS-1 to investigate the basis for these apparent cell-type specific differences. We find that 2- or 5-[3H]glucose usage is increased at low (</=5 mM) but not high glucose concentrations in INS-1 cells treated with a recombinant adenovirus containing the glucokinase cDNA (AdCMV-GKI), while glucose usage is increased at both low and high glucose concentrations in similarly treated hepatocytes. Utilization of 2-[3H]glucose in INS-1 cells is suppressed in glucokinase overexpressing INS-1 cells in a rapid, glucose concentration-dependent, and reversible fashion, while such regulation is largely absent in hepatocytes. Levels of hexose phosphates (glucose-6-phosphate, fructose-6-phosphate, and fructose-1,6-bisphosphate) were profoundly and rapidly elevated following the switch to high glucose in either AdCMV-GKI-treated INS-1 cells or hepatocytes relative to controls. In contrast, triose phosphate levels (glyceraldehyde-3-phosphate + dihydroxyacetone phosphate) were much higher in AdCMV-GKI-treated INS-1 cells than in similarly treated hepatocytes, suggesting limited flux throught the glyceraldehyde-3-phosphate dehydrogenase (G3PDH) step in the former cells. Hepatocytes were found to contain approximately 62 times more lactate dehydrogenase (LDH) activity than INS-1 cells, and this was reflected in a 3-fold increase in lactate production in AdCMV-GKI-treated hepatocytes relative to similarly treated INS-1 cells. Since the amounts of G3PDH activity in INS-1 and hepatocyte extracts are similar, we suggest that flux through this step in INS-1 cells is limited by failure to regenerate NAD in the LDH reaction and that a fundamental difference between hepatocytes and islet beta-cells is the limited capacity of the latter to metabolize glycolytic intermediates beyond the G3PDH step.

OB-Rb gene transfer to leptin-resistant islets reverses diabetogenic phenotype

In obese Zucker diabetic fatty (ZDF) rats with mutant leptin receptors, pancreatic islets have an approximately 50-fold increase in fat (TG), overproduce nitric oxide (NO), and lack a normal proinsulin mRNA response to fatty acids. We overexpressed the wild-type full-length "b" isoform of the leptin receptor (OB-Rb) in ZDF islets by perfusing ZDF pancreata with recombinant adenovirus containing the cDNA encoding OB-Rb. In cultured islets isolated from these animals, leptin lowered islet TG by 87% and completely blocked TG formation from free fatty acids. Overproduction of NO was reduced, and the preproinsulin mRNA response to free fatty acids was restored. This establishes defective leptin action as the proximate cause of lipotoxic diabetes in ZDF rats.

Metabolic impact of adenovirus-mediated overexpression of the glucose-6-phosphatase catalytic subunit in hepatocytes

Glucose-6-phosphatase (G6Pase) catalyzes the hydrolysis of glucose 6-phosphate (Glu-6-P) to free glucose and, as the last step in gluconeogenesis and glycogenolysis in liver, is thought to play an important role in glucose homeostasis. G6Pase activity appears to be conferred by a set of proteins localized to the endoplasmic reticulum, including a glucose-6-phosphate translocase, a G6Pase phosphohydrolase or catalytic subunit, and glucose and inorganic phosphate transporters in the endoplasmic reticulum membrane. In the current study, we used a recombinant adenovirus containing the cDNA encoding the G6Pase catalytic subunit (AdCMV-G6Pase) to evaluate the metabolic impact of overexpression of the enzyme in primary hepatocytes. We found that AdCMV-G6Pase-treated liver cells contain significantly less glycogen and Glu-6-P, but unchanged UDP-glucose levels, relative to control cells. Further, the glycogen synthase activity state was closely correlated with Glu-6-P levels over a wide range of glucose concentrations in both G6Pase-overexpressing and control cells. The reduction in glycogen synthesis in AdCMV-G6Pase-treated hepatocytes is therefore not a function of decreased substrate availability but rather occurs because of the regulatory effects of Glu-6-P on glycogen synthase activity. We also found that AdCMV-G6Pase-treated-cells had significantly lower rates of lactate production and [3-3H]glucose usage, coupled with enhanced rates of gluconeogenesis and Glu-6-P hydrolysis. We conclude that overexpression of the G6Pase catalytic subunit alone is sufficient to activate flux through the G6Pase system in liver cells. Further, hepatocytes treated with AdCMV-G6Pase exhibit a metabolic profile resembling that of liver cells from patients or animals with non-insulin-dependent diabetes mellitus, suggesting that dysregulation of the catalytic subunit of G6Pase could contribute to the etiology of the disease.

Adenovirus-mediated expression of the catalytic subunit of glucose-6-phosphatase in INS-1 cells. Effects on glucose cycling, glucose usage, and insulin secretion

Glucose-6-phosphatase (Glu-6-Pase) catalyzes the terminal step of gluconeogenesis, the conversion of glucose 6-phosphate (Glu-6-P) to free glucose. This enzyme activity is thought to be conferred by a complex of proteins residing in the endoplasmic reticulum (ER), including a Glu-6-P translocase that transports Glu-6-P into the lumen of the ER, a phosphohydrolase catalytic subunit residing in the lumen, and putative glucose and inorganic phosphate transporters that allow exit of the products of the reaction. In this study, we have investigated the effect of adenovirus-mediated overexpression of the Glu-6-Pase catalytic subunit on glucose metabolism and insulin secretion, using a well differentiated insulinoma cell line, INS-1. We found that the overexpressed Glu-6-Pase catalytic subunit was normally glycosylated, correctly sorted to the ER, and caused a 10-fold increase in Glu-6-Pase enzymatic activity in in vitro assays. Consistent with these findings, a 4.2-fold increase in 3H2O incorporation into glucose was observed in INS-1 cells treated with the recombinant adenovirus containing the Glu-6-Pase catalytic subunit cDNA (AdCMV-Glu-6-Pase). 3-[3H]Glucose usage was decreased by 32% in AdCMV-Glu-6-Pase-treated cells relative to controls, resulting in a proportional 30% decrease in glucose-stimulated insulin secretion. Our findings indicate that overexpression of the Glu-6-Pase catalytic subunit significantly impacts glucose metabolism and insulin secretion in islet beta-cells. However, INS-1 cells treated with AdCMV-Glu-6-Pase do not exhibit the severe alterations of beta-cell function and metabolism associated with islets from rodent models of obesity and non-insulin-dependent diabetes mellitus, suggesting the involvement of genes in addition to the catalytic subunit of Glu-6-Pase in the etiology of such beta-cell dysfunction.

beta-cell function in normal rats made chronically hyperleptinemic by adenovirus-leptin gene therapy

Leptin was overexpressed in the liver of normal Wistar rats by infusing recombinant adenovirus containing the cDNA encoding leptin. Plasma leptin levels rose to 12-24 ng/ml (vs. <2 ng/ml in control rats), and food intake and body weight fell. Visible fat disappeared within 7 days. Plasma insulin fell to <50% of normal in association with hypoglycemia, suggesting enhanced insulin sensitivity. Although beta-cells appeared histologically normal, the pancreases were unresponsive to perfusion with stimulatory levels of glucose and arginine. Since islet triglyceride content was 0, compared with 14 ng/islet in pair-fed control rats, we coperfused a 2:1 oleate:palmitate mixture (0.5 mmol/l). This restored insulin responses to supranormal levels. When normal islets were cultured with 20 ng/ml of leptin, they too became triglyceride-depleted and failed to respond when perifused with glucose or arginine. Perifusion of fatty acids restored both responses. We conclude that in normal rats, hyperleptinemia for 2 weeks causes reversible beta-cell dysfunction by depleting tissue lipids, thereby depriving beta-cells of a lipid-derived signal required for the insulin response to other fuels.

Engineering of glycerol-stimulated insulin secretion in islet beta cells. Differential metabolic fates of glucose and glycerol provide insight into mechanisms of stimulus-secretion coupling

Insulin secretion from beta cells in the islets of Langerhans can be stimulated by a number of metabolic fuels, including glucose and glyceraldehyde, and is thought to be mediated by metabolism of the secretagogues and an attendant increase in the ATP:ADP ratio. Curiously, glycerol fails to stimulate insulin secretion, even though it has been reported that islets contain abundant glycerol kinase activity and oxidize glycerol efficiently. We have reinvestigated this point and find that rat islets and the well differentiated insulinoma cell line INS-1 contain negligible glycerol kinase activity. A recombinant adenovirus containing the bacterial glycerol kinase gene (AdCMV-GlpK) was constructed and used to express the enzyme in islets and INS-1 cells, resulting in insulin secretion in response to glycerol. In AdCMV-GlpK-treated INS-1 cells a greater proportion of glycerol is converted to lactate and a lesser proportion is oxidized compared with glucose. The two fuels are equally potent as insulin secretagogues, despite the fact that oxidation of glycerol at its maximally effective dose (2-5 mM) occurs at a rate that is similar to the rate of glucose oxidation at its basal, nonstimulatory concentration (3 mM). We also investigated the possibility that glycerol may signal via expansion of the glycerol phosphate pool to allow enhanced fatty acid esterification and formation of complex lipids. Addition of Triacsin-C, an inhibitor of long-chain acyl-CoA synthetase, to AdCMV-GlpK-treated INS-1 cells did not inhibit glycerol-stimulated insulin secretion despite the fact that it blocked glycerol incorporation into cellular lipids. We conclude from these studies that glycerol kinase expression is sufficient to activate glycerol signaling in beta cells, showing that the failure of normal islets to respond to this substrate is due to a lack of this enzyme activity. Further, our studies show that glycerol signaling is not linked to esterification or oxidation of the substrate, but is likely mediated by its metabolism in the glycerol phosphate shuttle and/or the distal portion of the glycolytic pathway, either of which can lead to production of ATP and an increased ATP:ADP ratio.

Engineered cell lines for insulin replacement in diabetes: current status and future prospects

The recently completed diabetes complications and control trial has highlighted the need for improvement of insulin delivery systems for treatment of insulin-dependent diabetes mellitus. Despite steady improvement in methods for islet and whole pancreas transplantation over the past three decades, the broad-scale applicability of these approaches remains uncertain due in part to the difficulty and expense associated with procurement of functional tissue. To address this concern, we and others have been using the tools of molecular biology to develop cell lines with regulated insulin secretion that might serve as a surrogate for primary islets or pancreas tissue in transplantation therapy. This article seeks to provide a brief summary of the current status of this growing field, with a particular emphasis on progress in producing cell lines with appropriate glucose-stimulated insulin secretion.