p38 MAPK activation upregulates proinflammatory pathways in skeletal muscle cells from insulin-resistant type 2 diabetic patients

Skeletal muscle is the key site of peripheral insulin resistance in type 2 diabetes. Insulin-stimulated glucose uptake is decreased in differentiated diabetic cultured myotubes, which is in keeping with a retained genetic/epigenetic defect of insulin action. We investigated differences in gene expression during differentiation between diabetic and control muscle cell cultures. Microarray analysis was performed using skeletal muscle cell cultures established from type 2 diabetic patients with a family history of type 2 diabetes and clinical evidence of marked insulin resistance and nondiabetic control subjects with no family history of diabetes. Genes and pathways upregulated with differentiation in the diabetic cultures, compared with controls, were identified using Gene Spring and Gene Set Enrichment Analysis. Gene sets upregulated in diabetic myotubes were associated predominantly with inflammation. p38 MAPK was identified as a key regulator of the expression of these proinflammatory gene sets, and p38 MAPK activation was found to be increased in the diabetic vs. control myotubes. Although inhibition of p38 MAPK activity decreased cytokine gene expression from the cultured diabetic myotubes significantly, it did not improve insulin-stimulated glucose uptake. Increased cytokine expression driven by increased p38 MAPK activation is a key feature of cultured myotubes derived from insulin-resistant type 2 diabetic patients. p38 MAPK inhibition decreased cytokine expression but did not affect the retained defect of impaired insulin action in the diabetic muscle cells.

Xenobiotic incorporation into pyruvate dehydrogenase complex can occur via the exogenous lipoylation pathway

Lipoylated enzymes such as the E2 component of pyruvate dehydrogenase complex (PDC-E2) are targets for autoreactive immune responses in primary biliary cirrhosis, with lipoic acid itself forming a component of the dominant auto-epitopes. A candidate mechanism for the initiation of tolerance breakdown in this disease is immune recognition of neo-antigens formed by xenobiotic substitution of normal proteins. Importantly, sensitization with proteins artificially substituted with the lipoic acid analogue xenobiotic 6-bromohexanoic acid (6BH) can induce an immune response that cross-reacts with PDC-E2. This study investigated the potential of recombinant lipoylation enzymes lipoate activating enzyme and lipoyl-AMP(GMP):N-lysine lipoyl transferase to aberrantly incorporate xenobiotics into PDC-E2. It was found that these enzymes could incorporate lipoic acid analogues including octanoic and hexanoic acids and the xenobiotic 6BH into PDC-E2. The efficiency of incorporation of these analogues showed a variable dependence on activation by adenosine triphosphate (ATP) or guanosine triphosphate (GTP), with ATP favoring the incorporation of hexanoic acid and 6BH whereas GTP enhanced substitution by octanoic acid. Importantly, competition studies showed that the relative incorporation of both 6BH and lipoic acid could be regulated by the balance between ATP and GTP, with the formation of 6BH-substituted PDC-E2 predominating in an ATP-rich environment.

CONCLUSION

Using a well-defined system in vitro we have shown that an important xenobiotic can be incorporated into PDC in place of lipoic acid by the exogenous lipoylation system; the relative levels of lipoic acid and xenobiotic incorporation may be determined by the balance between ATP and GTP. These observations suggest a clear mechanism for the generation of an auto-immunogenic neo-antigen of relevance for the pathogenesis of primary biliary cirrhosis.

Protein phosphorylation and hormone action

Although the scheme hormone leads to raised cyclic AMP levels leads to activated protein kinase leads to phosphorylated protein leads to physiological response may represent an outline for the action of several hormones, in the best understood example, namely regulation of glucogen metabolism in mammalian muscle, the picture is more complex. Modification of phosphorylase kinase by cyclic AMP-dependent protein kinase, after stimulation by adrenaline, leads to phosphorylation of the enzyme at two sites. Activation is associated exclusively with the phosphorylation of the primary site, but the secondary phosphorylation indirectly antagonizes the primary phosphorylation in that it is necessary to render the primary site susceptible to dephosphorylation. The recent separation of two distinct phosphorylase kinase phosphatases specific for the two sites shows that reversal of the hormonal stimulation is controlled by the relative activities of two enzymes with opposing functions. Glycogen synthetase, which is phosphorylated and inactivated by cyclic AMP-dependent protein kinase, is also under the control of insulin. Although insulin appears to stimulate glycogen synthetase by reversal of the inactivation catalysed by the cyclic AMP-dependent protein kinase, tissue cyclic AMP concentrations do not alter. The recent identification of a second glycogen synthetase kinase, unaffected by cyclic AMP, therefore raises the possibility that insulin action may also be mediated through phosphorylation-dephosphorylation mechanisms, which antagonize those mediated through cyclic AMP-dependent protein kinase.

Does impaired mitochondrial function affect insulin signaling and action in cultured human skeletal muscle cells?

Insulin-resistant type 2 diabetic patients have been reported to have impaired skeletal muscle mitochondrial respiratory function. A key question is whether decreased mitochondrial respiration contributes directly to the decreased insulin action. To address this, a model of impaired cellular respiratory function was established by incubating human skeletal muscle cell cultures with the mitochondrial inhibitor sodium azide and examining the effects on insulin action. Incubation of human skeletal muscle cells with 50 and 75 microM azide resulted in 48 +/- 3% and 56 +/- 1% decreases, respectively, in respiration compared with untreated cells mimicking the level of impairment seen in type 2 diabetes. Under conditions of decreased respiratory chain function, insulin-independent (basal) glucose uptake was significantly increased. Basal glucose uptake was 325 +/- 39 pmol/min/mg (mean +/- SE) in untreated cells. This increased to 669 +/- 69 and 823 +/- 83 pmol/min/mg in cells treated with 50 and 75 microM azide, respectively (vs. untreated, both P < 0.0001). Azide treatment was also accompanied by an increase in basal glycogen synthesis and phosphorylation of AMP-activated protein kinase. However, there was no decrease in glucose uptake following insulin exposure, and insulin-stimulated phosphorylation of Akt was normal under these conditions. GLUT1 mRNA expression remained unchanged, whereas GLUT4 mRNA expression increased following azide treatment. In conclusion, under conditions of impaired mitochondrial respiration there was no evidence of impaired insulin signaling or glucose uptake following insulin exposure in this model system.

Targeted suppression of calpain-10 expression impairs insulin-stimulated glucose uptake in cultured primary human skeletal muscle cells

Calpain-10 was identified as a novel type 2 diabetes susceptibility gene, although the mechanisms by which it increases susceptibility to type 2 diabetes remain unclear. As skeletal muscle is the principal site of the peripheral insulin resistance for glucose disposal in type 2 diabetes, we investigated whether targeted suppression of calpain-10 expression directly affects insulin action in cultured human skeletal muscle cells. Short interfering RNAs (siRNAs) were employed to specifically suppress CAPN10 gene expression. Suppression was seen at both the transcript and protein level, as assessed by quantitative PCR and Western blotting. Suppression of CAPN10 mRNA expression (75% decrease compared to untransfected myotubes) was associated with a significant decrease (p=0.04) in insulin-stimulated glucose uptake (1.03+/-0.06 [mean+/-SEM]-fold increase over basal) compared to the untransfected myotubes (1.43+/-0.16-fold increase). In contrast, decreased suppression of calpain-10 expression did not affect insulin-stimulated glycogen synthesis nor insulin-stimulated phosphorylation of protein kinase B, a key component of the insulin-signalling pathway. This study confirms that calpain-10 plays a role in insulin-stimulated glucose uptake in human skeletal muscle cells. Suppression of calpain-10 expression did not affect insulin-stimulated glycogen synthesis nor insulin-signalling via PKB, suggesting that calpain-10 may exert a direct regulatory effect upon the glucose uptake mechanism.

Role of glycogen content in insulin resistance in human muscle cells

We have used primary human muscle cell cultures to investigate the role of glycogen loading in cellular insulin resistance. Insulin pre-treatment for 2 h markedly impaired insulin signaling, as assessed by protein kinase B (PKB) phosphorylation. In contrast, insulin-dependent glycogen synthesis, glycogen synthase (GS) activation, and GS sites 3 de-phosphorylation were impaired only after 5 h of insulin pre-treatment, whereas 2-deoxyglucose transport was only decreased after 18 h pre-treatment. Insulin-resistant glycogen synthesis was associated closely with maximal glycogen loading. Both glucose limitation and 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR) treatment during insulin pre-treatment curtailed glycogen accumulation, and concomitantly restored insulin-sensitive glycogen synthesis and GS activation, although GS de-phosphorylation and PKB phosphorylation remained impaired. Conversely, glycogen super-compensation diminished insulin-sensitive glycogen synthesis and GS activity. Insulin acutely promoted GS translocation to particulate subcellular fractions; this was abolished by insulin pre-treatment, as was GS dephosphorylation therein. Limiting glycogen accumulation during insulin pre-treatment re-instated GS dephosphorylation in particulate fractions, whereas glycogen super-compensation prevented insulin-stimulated GS translocation and dephosphorylation. Our data suggest that diminished insulin signaling alone is insufficient to impair glucose disposal, and indicate a role for glycogen accumulation in inducing insulin resistance in human muscle cells.

Key role for ceramides in mediating insulin resistance in human muscle cells

Elevated non-esterified fatty acids, triglyceride, diacylglycerol, and ceramide have all been associated with insulin resistance in muscle. We set out to investigate the role of intramyocellular lipid metabolites in the induction of insulin resistance in human primary myoblast cultures. Muscle cells were subjected to adenovirus-mediated expression of perilipin or incubated with fatty acids for 18 h, prior to insulin stimulation and measurement of lipid metabolites and rates of glycogen synthesis. Adenovirus-driven perilipin expression lead to significant accumulation of triacylglycerol in myoblasts, without any detectable effect on insulin sensitivity, as judged by the ability of insulin to stimulate glycogen synthesis. Similarly, incubation of cells with the monounsaturated fatty acid oleate resulted in triacylglycerol accumulation without inhibiting insulin action. By contrast, the saturated fatty acid palmitate induced insulin resistance. Palmitate treatment caused less accumulation of triacylglycerol than did oleate but also induced significant accumulation of both diacylglycerol and ceramide. Insulin resistance was also caused by cell-permeable analogues of ceramide, and palmitate-induced resistance was blocked in the presence of inhibitors of de novo ceramide synthesis. Oleate co-incubation completely prevented the insulin resistance induced by palmitate. Our data are consistent with ceramide being the agent responsible for insulin resistance caused by palmitate exposure. Furthermore, the triacylglycerol derived from oleate was able to exert a protective role in sequestering palmitate, thus preventing its conversion to ceramide.

Diverging regulation of pyruvate dehydrogenase kinase isoform gene expression in cultured human muscle cells

The pyruvate dehydrogenase complex occupies a central and strategic position in muscle intermediary metabolism and is primarily regulated by phosphorylation/dephosphorylation. The identification of multiple isoforms of pyruvate dehydrogenase kinase (PDK1-4) and pyruvate dehydrogenase phosphatase (PDP1-2) has raised intriguing new possibilities for chronic pyruvate dehydrogenase complex control. Experiments to date suggest that PDK4 is the major isoenzyme responsible for changes in pyruvate dehydrogenase complex activity in response to various different metabolic conditions. Using a cultured human skeletal muscle cell model system, we found that expression of both PDK2 and PDK4 mRNA is upregulated in response to glucose deprivation and fatty acid supplementation, the effects of which are reversed by insulin treatment. In addition, insulin directly downregulates PDK2 and PDK4 mRNA transcript abundance via a phosphatidylinositol 3-kinase-dependent pathway, which may involve glycogen synthase kinase-3 but does not utilize the mammalian target of rapamycin or mitogen-activated protein kinase signalling pathways. In order to further elucidate the regulation of PDK, the role of the peroxisome proliferators-activated receptors (PPAR) was investigated using highly potent subtype selective agonists. PPARalpha and PPARdelta agonists were found to specifically upregulate PDK4 mRNA expression, whereas PPARgamma activation selectively decreased PDK2 mRNA transcript abundance. PDP1 mRNA expression was unaffected by all conditions analysed. These results suggest that in human muscle, hormonal and nutritional conditions may control PDK2 and PDK4 mRNA expression via a common signalling mechanism. In addition, PPARs appear to independently regulate specific PDK isoform transcipt levels, which are likely to impart important metabolic mediation of fuel utilization by the muscle.

Relationship between the inhibition of phosphatidic acid phosphohydrolase-1 by oleate and oleoyl-CoA ester and its apparent translocation

Phosphatidic acid phosphohydrolase-1 (PAP-1) activity is reversibly inhibited by fatty acids and their acyl-CoA esters and it appears paradoxical that these effectors have been reported to increase the liver's esterification capacity by translocating the rate-limiting enzyme PAP-1 from cytosol to the endoplasmic reticulum. Therefore, we have examined the effect of oleate, oleoyl-CoA, and spermine on the activation and translocation of PAP-1 of rat liver. PAP-1 activity is directly inhibited by oleic acid and oleoyl-CoA ester in an allosteric manner, resulting in the formation of inactive PAP-1-fatty acid (or -acyl-CoA) complex, even in the absence of any subcellular structures. Such association/aggregation of PAP-1 can be easily collected by centrifugation and may explain the apparent translocation phenomenon of this enzyme to a particular structure in the presence of fatty acids or acyl-CoA esters as reported in many works. Indeed, incubation of cytosol fraction alone with oleate or oleoyl-CoA at 37 degrees C, followed by centrifugation, induces a significant increase (sevenfold) in PAP-1 activity in the pellet fraction. This displacement is accompanied by an increase in the specific activity of PAP-1 in the pellet fraction. Spermine is less effective than oleate in inducing the displacement of PAP-1 activity from cytosol to the pellet fraction in the absence of any membrane structures. This apparent translocation of PAP-1 is also promoted when homogenate fraction was incubated with oleate prior to the preparation of cytosol and microsomal fraction. Thus, many of the announced factors, including fatty acids, would promote the in vitro association/aggregation of PAP-1 enzyme rather than its translocation, and therefore, re-evaluation of the reported effects on PAP-1 translocation phenomenon is required. It is proposed that fatty acids and their esters would favour beta-oxidation over esterification by promoting the forming of inactive associated PAP-1 in situations such as starvation and metabolic stress in which there is an increased supply of fatty acids to the liver.

Calpain inhibition and insulin action in cultured human muscle cells

Variation in the calpain 10 gene has been reported to increase susceptibility to type 2 diabetes. Part of this susceptibility appears to be mediated by a decrease in whole body insulin sensitivity. As skeletal muscle is the primary tissue site of the peripheral insulin resistance in type 2 diabetes, the aim of this study was to use a human skeletal muscle cell culture system to explore the effects of calpain inhibition on insulin action. Calpain 10 mRNA and protein expression was examined in cultured myoblasts, myotubes, and whole skeletal muscle from non-diabetic subjects using RT-PCR and Western blotting. Changes in insulin-stimulated glucose uptake and glycogen synthesis in response to the calpain inhibitors ALLN and ALLM were measured. Calpain 10 expression was confirmed in cultured human myoblasts, myotubes, and native skeletal muscle. Insulin-stimulated glucose uptake was significantly decreased following preincubation with ALLN [404+/-40 vs 505+/-55 (mean+/-SEM)pmol/mg/min; with vs without ALLN: p = 0.04] and ALLM [455+/-38 vs 550+/-50 pmol/mg/min; with vs without ALLM: p = 0.025] in day 7 fused myotubes, but not in myoblasts. Neither ALLN nor ALLM affected insulin-stimulated glycogen synthesis in myoblasts or myotubes. These studies confirm calpain 10 expression in cultured human muscle cells and support a role for calpains in insulin-stimulated glucose uptake in human skeletal muscle cells that may be relevant to the pathogenesis of the peripheral insulin resistance in type 2 diabetes.

Hormone-sensitive lipase--new roles for an old enzyme

Although described initially as an intracellular adipocyte-specific triacylglycerol lipase, it is now clear that HSL (hormone-sensitive lipase) is expressed in multiple tissues and plays a number of roles in lipid metabolism, including that of a neutral cholesteryl ester hydrolase. The major isoform is a single polypeptide with a molecular mass of approx. 84 kDa and which comprises three major domains: a catalytic domain, a regulatory domain encoding several phosphorylation sites and an N-terminal domain involved in protein-protein and protein-lipid interactions. The activity of HSL is regulated acutely by several mechanisms, including reversible phosphorylation by a number of different protein kinases, translocation to different sites within the cell and interaction with a number of proteins, some of which may serve to direct the inhibitory products of HSL away from the protein. It is also apparent from work with HSL null mice that more than one enzyme species may be classified as a hormone-sensitive lipase. The possible presence of HSL in macrophages remains controversial, and the role of the protein in pancreatic beta-cells has yet to be fully elucidated. Altered expression of HSL in different cell types may be associated with a number of pathological states, including obesity, atherosclerosis and Type II diabetes.

Cultured muscle cells from insulin-resistant type 2 diabetes patients have impaired insulin, but normal 5-amino-4-imidazolecarboxamide riboside-stimulated, glucose uptake

Impaired insulin action is a characteristic feature of type 2 diabetes. The study aims were to investigate whether after prolonged culture skeletal muscle cultures from insulin-resistant, type 2 diabetic patients (taking >100 U insulin/d) displayed impaired insulin signaling effects compared with cultures from nondiabetic controls and to determine whether retained abnormalities were limited to insulin action by studying an alternative pathway of stimulated glucose uptake. Studies were performed on myotubes differentiated for 7 d between passages 4 and 6. Insulin-stimulated glucose uptake (100 nm; P < 0.05) and insulin-stimulated glycogen synthesis (1 nm; P < 0.01) were significantly impaired in the diabetic vs. control cultures. Protein kinase B (PKB) expression and phosphorylated PKB levels in response to insulin stimulation (20 nm) were comparable in the diabetic and control cultures. 5-Amino-4-imidazolecarboxamide riboside (AICAR) mimics the effect of exercise on glucose uptake by activating AMP-activated protein kinase. There was no difference in AICAR (2 mm)-stimulated glucose uptake between diabetic vs. control myotube cultures (P = not significant). In conclusion, diabetic muscle cultures retain signaling defects after prolonged culture that appear specific to the insulin signaling pathway, but not involving PKB. This supports an intrinsic abnormality of the diabetic muscle cells that is most likely to have a genetic basis.

Regulation of glycogen synthase by glucose and glycogen: a possible role for AMP-activated protein kinase

We report here use of human myoblasts in culture to study the relationships between cellular glycogen concentrations and the activities of glycogen synthase (GS) and AMP-activated protein kinase (AMPK). Incubation of cells for 2 h in the absence of glucose led to a 25% decrease in glycogen content and a significant decrease in the fractional activity of GS. This was accompanied by stimulation of both the alpha1 and alpha2 isoforms of AMPK, without significant alterations in the ratios of adenine nucleotides. When glucose was added to glycogen-depleted cells, a rapid and substantial increase in GS activity was accompanied by inactivation of AMPK back to basal values. Inclusion of the glycogen phosphorylase inhibitor, CP-91149, prevented the loss of glycogen during glucose deprivation but not the activation of AMPK. However, in the absence of prior glycogen breakdown, glucose treatment failed to activate GS above control values, indicating the crucial role of glycogen content. Activation of AMPK by either 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR) or hydrogen peroxide was also associated with a decrease in the activity ratio of GS. AICAR treatment had no effect on total cellular glycogen content but led to a modest increase in glucose uptake. These data support a role for AMPK in both stimulating glucose uptake and inhibiting GS in intact cells, thus promoting glucose flux through glycolysis.

Glucose-dependent regulation of cholesterol ester metabolism in macrophages by insulin and leptin

Insulin resistance, obesity, and diabetes are characterized by hyperglycemia, hyperinsulinemia, and hyperleptinemia and are associated with increased risk of atherosclerosis. In an effort to understand how this occurs, we have investigated whether these factors cause disregulation of cholesterol ester metabolism in J774.2 macrophages. Raising glucose levels alone was sufficient to increase uptake of acetylated low density lipoprotein but did not stimulate synthesis of cholesterol esters. In the presence of high glucose, both insulin and leptin increased the rate of cholesterol ester synthesis, although they did not further increase uptake of acetylated low density lipoprotein. However, in the presence of high glucose both insulin and leptin caused a significant increase in the activity of acyl-CoA: cholesterol O-acyltransferase (ACAT) combined with a significant reduction in the level of hormone-sensitive lipase (HSL). Because ACAT is the main enzyme responsible for cholesterol ester synthesis and HSL contributes significantly to neutral cholesterol ester hydrolase activity, this suggests that glucose primes the J774.2 cells so that in the presence of high insulin or leptin they will store cholesterol esters. This contrasts with 3T3-L1 adipocytes, where HSL activity and expression are increased by insulin in high glucose conditions. These findings may provide an explanation for the observation that in conditions characterized by hyperglycemia, hyperleptinemia, and hyperinsulinemia, triglyceride lipolysis in adipocytes is increased while hydrolysis of cholesterol esters in macrophages is decreased, contributing to foam cell formation.

Conditionally immortalized cell lines as model systems for high-throughput biology in drug discovery

There is an increasing emphasis on the need for high-quality biological data much earlier in the drug-discovery process. This has led to the development of high-throughput approaches to biology, many of which rely on the use of cell-culture models. Unfortunately, available cell-culture models often reflect poorly the characteristics of the tissue they are supposed to represent. However, the conditional-immortalization approach as applied by Xcellsyz offers the possibility of producing human cell lines on demand, which are truly representative of the tissue from which they derive.

Investigation of a mechanism for accelerated breakdown of immune tolerance to the primary biliary cirrhosis-associated autoantigen, pyruvate dehydrogenase complex

Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by autoreactive T- and B-cell responses to the highly conserved enzyme pyruvate dehydrogenase complex (PDC). In this study we have examined the breakdown of T-cell tolerance to self-PDC using a mouse model. Female SJL/J mice were sensitized intraperitoneally with foreign-PDC (bovine) and/or self-PDC (murine) in complete Freund's adjuvant, and serum, spleen, and liver tissue was taken 8 weeks later. Animals sensitized with foreign-PDC produced IgG antibodies that were reactive with both foreign and self-PDC, but splenic T cells from these animals only responded to stimulation with foreign PDC. Sensitization with self-PDC elicited neither antibodies nor reactive T cells. Significantly, cosensitization with mixed self-PDC and foreign-PDC resulted in a full breakdown of self-tolerance, with generation of both antibody and T-cell responses to self-PDC of the type seen exclusively in human PBC patients. Mild bile duct lesions deficient in CD8(+) T cells were seen 8 weeks after sensitization with either foreign or self-PDC. However, after sensitization with mixed self-PDC and foreign-PDC, these lesions were significantly larger and heavily infiltrated by CD8(+) T cells. Liver-infiltrating T cells derived from the self-PDC and foreign-PDC cosensitized but not from control animals showed reactivity with self-PDC, suggesting a possible role for autoreactive PDC-specific T-cell responses in the pathogenesis of the observed histologic changes. It is likely that B-cell cross-reactivity between foreign and self-PDC enhances the potential for breakdown of T-cell self-tolerance by allowing efficient presentation of self-antigens in the inoculum. This model may provide a useful system for investigating the etiology and treatment of PBC.

Regulation of phosphatidic acid phosphohydrolase 1 by fatty acids

In the starved state and during metabolic stress, free fatty acids (FFA) are the principal hepatic energy supply, undergoing beta-oxidation. Accordingly, it appears paradoxical that FFA have been reported to increase the liver's esterification capacity by translocating the rate-limiting enzyme phosphatidic acid phosphohydrolase (PAP-1) from the cytosol to the endoplasmic reticulum. We have therefore investigated the regulation of rat liver PAP-1. Oleic acid inhibited PAP activity in all subcellular fractions, with PAP-1 activity in cytosol being the most sensitive. Inhibition was also observed with oleoyl-CoA, linoleate, and palmitate. Fatty acids and their derivatives show detergent effects at high concentrations, and such effects can lead to enzyme inhibition. Inhibition by oleate, however, was reversed by phosphatidic acid and albumin and exhibited sigmoidal kinetics. These results demonstrate that PAP-1 is reversibly inhibited by FFA and their CoA esters, which may play a role in directing hepatic FFA to beta-oxidation during times of increased energy demand.

Translocation of hormone-sensitive lipase and perilipin upon lipolytic stimulation during the lactation cycle of the rat

The removal of the litter from lactating rats results in a decrease in the lipolytic response to catecholamines in maternal adipocytes; this effect can be prevented by concomitant treatment of the rats with growth hormone. The decrease in response to catecholamines following litter removal was not due to a change in the amount of either hormone-sensitive lipase (HSL) or perilipin per adipocyte or in the proportion of either of these proteins associated with the fat droplet. Incubation in vitro with isoproterenol did not cause any apparent net translocation of HSL to the fat droplet in adipocytes from the mature female rats in any state used in this study, but isoproterenol did cause a movement of perlipin away from the fat droplet. This translocation of perilipin was not altered by litter removal. Thus, the decrease in response to catecholamines found on litter removal from lactating rats appears to be due to a diminished ability to activate HSL associated with fat droplet.

Regulation of glycogen synthesis in human muscle cells

Glucose uptake into muscle and its subsequent storage as glycogen is a crucial factor in energy homeostasis in skeletal muscle. This process is stimulated acutely by insulin and is impaired in both insulin-resistant states and in type 2 diabetes mellitus. A signalling pathway involving protein kinase B and glycogen synthase kinase 3 seems certain to have a key role in stimulating glycogen synthesis but other signalling pathways also contribute, including a rapamycin-sensitive pathway stimulated by amino acids. Although glycogen synthesis is one of the classical insulin-regulated pathways, it is also regulated in an insulin-independent manner; for example glycogen synthesis in muscle is stimulated significantly after strenuous exercise, with much of this stimulation being independent of the involvement of insulin. Evidence suggests that glucose and the glycogen content of the muscle have a key role in this stimulation but the molecular mechanism has yet to be fully explained.

Effects of tumor necrosis factor-alpha on insulin action in cultured human muscle cells

Reported discrepancies in the effects of tumor necrosis factor (TNF)-alpha in modulating insulin sensitivity of cultured cells may relate both to cell types studied and to the time course of exposure to the cytokine. Additionally, the relationship of effects on glucose metabolism to changes in the insulin signaling pathway cannot be assumed. For in vitro study, the cell type most relevant to insulin resistance in humans is the cultured human muscle cell. In the present study, TNF brought about no change in the rate of glycogen synthesis in cultured human muscle cells unless present during differentiation. The presence of TNF (5 ng/ml) during the process of differentiation of myoblasts into mature myotubes diminished the response of glycogen synthesis to acute insulin stimulation. This finding was associated with an impairment of differentiation-dependent increases in total cellular glycogen synthase (GS) activity. Under the same conditions of TNF exposure, there was no effect on the response to acute insulin stimulation of the fractional activity of GS. Similarly, there was no effect on the insulin stimulation of protein kinase B (PKB) and inhibition of glycogen synthase kinase 3 (GSK-3). Acute insulin stimulation brought about a 4.08 +/- 0.44-fold stimulation of activity of PKB in the absence of TNF, with 4.81 +/- 0.70-fold stimulation in cells exposed to TNF. GSK-3 activity decreased to 74.0 +/- 5.8% of basal after insulin stimulation without TNF and 78.3 +/- 5.0% after TNF exposure. However, differentiation of myocytes, as defined by an increase in the acetylcholine receptor, myogenin, and mature creatine kinase isoform expression, was impaired in TNF-treated cells. These studies demonstrate that TNF, if present during differentiation, decreases insulin-stimulated rates of storage of glucose as glycogen and total GS activity but does not downregulate the insulin-signaling system to GS. More generally, TNF also inhibits differentiation of human muscle cells in culture.

Insulin and leptin acutely regulate cholesterol ester metabolism in macrophages by novel signaling pathways

Leptin is produced in adipose tissue and acts in the hypothalamus to regulate food intake. However, recent evidence also indicates a potential for direct roles for leptin in peripheral tissues, including those of the immune system. In this study, we provide direct evidence that macrophages are a target tissue for leptin. We found that J774.2 macrophages express the functional long form of the leptin receptor (ObRb) and that this becomes tyrosine-phosphorylated after stimulation with low doses of leptin. Leptin also stimulates both phosphoinositide 3-kinase (PI 3-kinase) activity and tyrosine phosphorylation of JAK2 and STAT3 in these cells. We investigated the effects of leptin on hormone-sensitive lipase (HSL), which acts as a neutral cholesterol esterase in macrophages and is a rate-limiting step in cholesterol ester breakdown. Leptin significantly increased HSL activity in J774.2 macrophages, and these effects were additive with the effects of cAMP and were blocked by PI 3-kinase inhibitors. Conversely, insulin inhibited HSL in macrophages, but unlike adipocytes, this effect did not require PI 3-kinase. These results indicate that leptin and insulin regulate cholesterol-ester homeostasis in macrophages and, therefore, defects in this process caused by leptin and/or insulin resistance could contribute to the increased incidence of atherosclerosis found associated with obesity and type 2 diabetes.

Control of glycogen synthesis by glucose, glycogen, and insulin in cultured human muscle cells

A key feature of type 2 diabetes is impairment in the stimulation of glycogen synthesis in skeletal muscle by insulin. Glycogen synthesis and the activity of the enzyme glycogen synthase (GS) have been studied in human myoblasts in culture under a variety of experimental conditions. Incubation in the absence of glucose for up to 6 h caused an approximately 50% decrease in glycogen content, which was associated with a small decrease in the fractional activity of GS. Subsequent reincubation with physiological concentrations of glucose led to a dramatic increase in the rate of glycogen synthesis and in the fractional activity of GS, an effect which was both time- and glucose concentration-dependent and essentially additive with the effects of insulin. This effect was seen only after glycogen depletion. Inhibitors of signaling pathways involved in the stimulation of glycogen synthesis by insulin were without significant effect on the stimulatory action of glucose. These results indicate that at least two distinct mechanisms exist to stimulate glycogen synthesis in human muscle: one acting in response to insulin and the other acting in response to glucose after glycogen depletion, such as that which results from exercise or starvation.

Regulation of glycogen synthesis by amino acids in cultured human muscle cells

Insulin and a number of metabolic factors stimulate glycogen synthesis and the enzyme glycogen synthase. Using human muscle cells we find that glycogen synthesis is stimulated by treatment of the cells with lithium ions, which inhibit glycogen synthase kinase 3. Insulin further stimulates glycogen synthesis in the presence of lithium ions, an effect abolished by wortmannin and rapamycin. We report also that amino acids stimulate glycogen synthesis and glycogen synthase, these effects also being blocked by rapamycin and wortmannin. Amino acids stimulate p70(s6k) and transiently inhibit glycogen synthase kinase 3 without effects on the activity of protein kinase B or the mitogen-activated protein kinase pathway. Thus, the work reported here demonstrates that amino acid availability can regulate glycogen synthesis. Furthermore, it demonstrates that glycogen synthase kinase 3 can be inactivated within cells independent of activation of protein kinase B and p90(rsk).

Secretory autoantibodies in primary biliary cirrhosis (PBC)

It is unclear how breakdown in immune tolerance to the ubiquitous self-antigen pyruvate dehydrogenase complex (PDC), seen in the autoimmune liver disease PBC, gives rise to tissue damage with such a limited distribution (restricted to the liver and salivary and lachrymal glands). One property shared by these tissues is the ability to export secretory IgA by the process of transcytosis. The aim of this study was to address whether active transcytosis of anti-PDC IgA occurs across epithelial surfaces in PBC, a finding that might implicate mucosal specific immune mechanisms in the pathogenesis of this disease. Parotid saliva was collected from PBC patients (n = 44), normal controls (n = 28) and PBC patients post-liver transplantation (n = 11). IgA and secretory component-positive antibodies specific for human PDC were quantified by ELISA and immunoblotting. PBC patients (but not control subjects) had anti-PDC IgA in their saliva. The strong correlation seen between titres detected using anti-IgA and anti-secretory component antibodies suggests that this is predominantly secretory IgA reaching the saliva by the active process of epithelial transcytosis. Titres of anti-PDC IgA remain high in PBC patients saliva post-liver transplant. Findings from studies of IgA in viral infection models raise the possibility that anti-PDC IgA could, whilst undergoing transcytosis, bind to newly translated PDC components in the cytoplasm of the epithelial cells transporting them out of the cell and inducing metabolic damage. This model would, if correct, help to explain the mechanism and tropism of tissue damage in PBC and the aberrant pattern of expression of PDC on the apical surface of biliary and salivary epithelial cells reported in this disease.

Experimental autoimmune cholangitis: a mouse model of immune-mediated cholangiopathy

BACKGROUND

Primary biliary cirrhosis (PBC) is characterised by intra-hepatic immune-mediated cholangiopathy (non-suppurative destructive cholangitis (NSDC)). Although auto-reactive immune responses against pyruvate dehydrogenase complex (PDC) have been characterised in PBC, the lack of an animal model of the disease has limited study of the mechanisms of disease induction and the development of novel approaches to therapy.

AIMS

To develop and validate a mouse model of immune-mediated cholangiopathy relevant for future use in the study of the aetio-pathogenesis and therapy of PBC.

METHODS

Female SJL/J, C57BL/6, NOD and BALB/c mice were sensitised with PDC, its purified E2/E3BP component, and a PDC-E2 derived peptide p163 (a dominant T-cell epitope in humans) in complete Freund's adjuvant (CFA). Morphological changes were assessed under light microscopy by a hepatic histopathologist blinded to the experimental details. Antibody responses to PDC were studied by ELISA and PDC inhibition assay.

RESULTS

An initial series of experiments was performed to survey the susceptibility of female mice of a range of strains to the induction of NSDC by i.p. sensitisation with PDC, PDC-E2/E3BP or p163 in CFA. Although each animal showed a specific antibody response following sensitisation, it was found that NSDC development (assessed at 30 weeks post-sensitisation) was restricted to SJL/J mice following sensitisation with any of the mitochondrial antigen preparations. A subsequent series of experiments was performed to examine the specificity and aetiology of this disease. Significant bile duct lesions were only seen in SJL/J animals following sensitisation with CFA containing PDC, and were absent from CFA only and un-sensitised controls. Kinetic analysis revealed that this pathology developed slowly, but a high incidence of animals with severe lesions was observed after 30 weeks.

CONCLUSIONS

We have described a model of experimental autoimmune cholangitis (EAC) with immunological (anti-PDC antibodies) and histological (immune-mediated cholangiopathy) features suggestive of PBC. This model may be useful in further defining the role of self-tolerance breakdown in the development of this condition.

Decreased insulin responsiveness of glucose uptake in cultured human skeletal muscle cells from insulin-resistant nondiabetic relatives of type 2 diabetic families

To investigate the contribution of inherited biochemical defects to the peripheral insulin resistance of type 2 diabetes, we studied cultured skeletal muscle from 10 insulin-resistant nondiabetic first-degree relatives of type 2 diabetic families and 6 control subjects. Insulin stimulation of glucose uptake and glycogen synthesis was maximal in myoblasts. Insulin-stimulated glucose uptake (fold-stimulation over basal uptake) was decreased in relative compared with control myoblasts at 0.001 micromol/l (0.93 +/- 0.05 [mean +/- SE] vs. 1.15 +/- 0.06, P < 0.05) and 0.1 micromol/l (1.38 +/- 0.10 vs. 1.69 +/- 0.08, P = 0.025) insulin. Insulin responsiveness was markedly impaired in 5 of the relative myoblast cultures, and in 4 of these, there was an associated increase in basal glucose uptake (76.7 +/- 7.0 vs. 47.4 +/- 5.5 pmol x min(-1) x mg(-1) protein, relative vs. control; P < 0.02). Expression of insulin receptor substrate 1, phosphatidylinositol 3-kinase, protein kinase B, and glycogen synthase was normal in the relative cultures with impaired insulin responsiveness. Glycogen synthesis was also normal in the relative cultures. We conclude that the persistence of impaired insulin responsiveness in some of the relative cultures supports the role of inherited factors in the insulin resistance of type 2 diabetes and that the association with increased basal glucose uptake suggests that the 2 abnormalities may be linked.

Specificity determinants for the pyruvate dehydrogenase component reaction mapped with mutated and prosthetic group modified lipoyl domains

Efficient catalysis in the second step of the pyruvate dehydrogenase (E1) component reaction requires a lipoyl group to be attached to a lipoyl domain that displays appropriately positioned specificity residues. As substrates, the human dihydrolipoyl acetyltransferase provides an N-terminal (L1) and an inner (L2) lipoyl domain. We evaluated the specificity requirements for the E1 reaction with 27 mutant L2 (including four substitutions for the lipoylated lysine, Lys(173)), with three analogs substituted for the lipoyl group on Lys(173), and with selected L1 mutants. Besides Lys(173) mutants, only E170Q mutation prevented lipoylation. Based on analysis of the structural stability of mutants by differential scanning calorimetry, alanine substitutions of residues with aromatic side chains in terminal regions outside the folded portion of the L2 domain significantly decreased the stability of mutant L2, suggesting specific interactions of these terminal regions with the folded domain. E1 reaction rates were markedly reduced by the following substitutions in the L2 domain (equivalent site-L1): L140A, S141A (S14A-L1), T143A, E162A, D172N, and E179A (E52A-L1). These mutants gave diverse changes in kinetic parameters. These residues are spread over >24 A on one side of the L2 structure, supporting extensive contact between E1 and L2 domain. Alignment of over 40 lipoyl domain sequences supports Ser(141), Thr(143), and Glu(179) serving as specificity residues for use by E1 from eukaryotic sources. Extensive interactions of the lipoyl-lysine prosthetic group within the active site are supported by the limited inhibition of E1 acetylation of native L2 by L2 domains altered either by mutation of Lys(173) or enzymatic addition of lipoate analogs to Lys(173). Thus, efficient use by mammalian E1 of cognate lipoyl domains derives from unique surface residues with critical interactions contributed by the universal lipoyl-lysine prosthetic group, key specificity residues, and some conserved residues, particularly Asp(172) adjacent to Lys(173).

Autoreactive responses to pyruvate dehydrogenase complex in the pathogenesis of primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is a cholestatic liver disease characterised by immune-mediated destruction of the biliary epithelial cells (BEC) lining the intrahepatic bile ducts (non-suppurative destructive cholangitis (NSDC)). Autoantibody and autoreactive T-cell responses specific for the self-antigen pyruvate dehydrogenase complex (PDC) are almost ubiquitous in PBC patients, leading to the view that the disease has an autoimmune aetiology. Autoreactive responses in PBC appear to be directed at the E2 and at the E3-binding protein (E3BP) (protein X) components of PDC, with the dominant B-cell and T-cell epitopes in E2 (fewer data are available for E3BP) spanning the inner (of two) lipoic acid-binding domains. The causal link between the breakdown of self-tolerance to PDC (particularly at the T-cell level) and the development of NSDC has been emphasised by the demonstration, in a murine model (experimental autoimmune cholangitis), that sensitisation with PDC of mammalian origin results in a breakdown of both B-cell and T-cell tolerance to murine PDC accompanied by the development of NSDC. An increasing understanding of the role played by PDC-specific autoreactive T cells in the pathogenesis of PBC has led us to examine the role played by the target cells in PBC (BEC) in both the inducer and effector mechanisms responsible for PBC.

Translocation of hormone-sensitive lipase and perilipin upon lipolytic stimulation of rat adipocytes

Adipocyte lipolysis was compared with hormone-sensitive lipase (HSL)/perilipin subcellular distribution and perilipin phosphorylation using Western blot analysis. Under basal conditions, HSL resided predominantly in the cytosol and unphosphorylated perilipin upon the lipid droplet. Upon lipolytic stimulation of adipocytes isolated from young rats with the beta-adrenergic agonist, isoproterenol, HSL translocated from the cytosol to the lipid droplet, but there was no movement of perilipin from the droplet to the cytosol; however, perilipin phosphorylation was observed. By contrast, upon lipolytic stimulation and perilipin phosphorylation in cells from more mature rats, there was no HSL translocation but a significant movement of perilipin away from the lipid droplet. Adipocytes from younger rats had markedly greater rates of lipolysis than those from the older rats. Thus high rates of lipolysis require translocation of HSL to the lipid droplet and translocation of HSL and perilipin can occur independently of each other. A loss of the ability to translocate HSL to the lipid droplet probably contributes to the diminished lipolytic response to catecholamines with age.

Breakdown of tolerance to pyruvate dehydrogenase complex in experimental autoimmune cholangitis: a mouse model of primary biliary cirrhosis

The autoimmune liver disease primary biliary cirrhosis (PBC) is characterized by autoreactive responses to a highly conserved self-antigen, pyruvate dehydrogenase complex (PDC). We recently reported the development of PBC-like lesions in SJL mice sensitized with PDC and have named this model disease experimental autoimmune cholangitis (EAC). In the present study, the breakdown of tolerance to PDC has been investigated in animals sensitized for EAC. Splenic mononuclear cells from SJL mice sensitized with bovine heart PDC (bPDC) in adjuvant showed T-cell proliferative and mixed Th1/Th2 cytokine secretory responses following in vitro stimulation with bPDC. Despite the likelihood of extensive sequence homology with mouse PDC (there is a greater than 95% sequence identity between rat and human PDC-E2 subunits), bPDC was highly immunogenic inducing significant T- and B-cell responses in the absence of any form of adjuvant. The multi-subunit quaternary structure of intact PDC was critical for this immunostimulatory activity because no response was produced by sensitization with monomeric recombinant PDC-E2 inner lipoyl domain. Mice sensitized with bPDC and CFA developed, within 2 weeks of sensitization, high-titer antibody responses reactive with bPDC that were fully cross-reactive with the murine homologue. Breakdown of T-cell tolerance to self-PDC took significantly longer, not being seen until 20 weeks postsensitization; a similar length of time to that previously shown to be required for EAC lesion development. Conclusions drawn from these data may have important implications for our understanding, and therapeutic manipulation, of PBC in humans.

Characterization of the autoantibody responses to recombinant E3 binding protein (protein X) of pyruvate dehydrogenase in primary biliary cirrhosis

Autoantibodies to the pyruvate dehydrogenase complex (PDC) are present in the serum of more than 95% of patients with primary biliary cirrhosis (PBC), the major epitope being the inner lipoyl domain of the E2 component. Immunoblotting suggests a similar prevalence of antibodies to a tightly associated lipoic acid-containing protein, E3 binding protein (E3BP). Attempts to resolve E3BP from E2 have been unsuccessful, restricting study of the nature and significance of antibody responses to the individual proteins. In particular, it is unclear (1) whether there is true cross-reactivity between E3BP and E2 and, if so, which is the originating response and (2) whether autoantibodies preferentially bind a lipoylated epitope on E3BP as is the case with PDC-E2. In this study, complementary DNAs encoding rE2, full-length rE3BP, its single lipoyl domain (rLip), and core domain (rE3BPCore) were cloned, and the proteins were expressed in Escherichia coli. Sera from 47 PBC patients were studied by immunoblotting and enzyme-linked immunosorbent assay (ELISA) against rE2, rE3BP, rE3BPCore, and both unlipoylated (U) and lipoylated (L) rLip. All sera were reactive by ELISA to some degree with all recombinant proteins except rE3BPCore, to which only 6 of 47 showed any reactivity. Significant correlations (P <.0001) were observed when comparing absorbance values for rE3BP with both rLip (U) (r = 0.793) and (L) (r = 0.963). The mean absorbance for rLip (U, 0.26 +/- 0.05) was, however, significantly lower than the absorbance for rLip (L) (0.78 +/- 0.12; P <.0001). After probing by immunoblotting and elution of antibodies from rE2 and rE3BP, subsequent reprobing against the components in whole PDC revealed true cross-reactivity. In summary, the response to E3BP is primarily directed against the lipoylated domain of the protein. It still remains unclear, however, whether the initial breakdown of tolerance is to E2 or E3BP.

Inhibition of rat hepatocyte proliferation by transforming growth factor beta and glucagon is associated with inhibition of ERK2 and p70 S6 kinase

Stimulation of hepatocyte proliferation by epidermal growth factor (EGF) and insulin is inhibited by transforming growth factor beta (TGF-beta) and by glucagon. It is also suppressed by inhibitors of various protein kinases, including rapamycin, which blocks activation of p70 S6 kinase (p70(S6k)), PD98059, which inhibits the activation of extracellular-regulated kinase (ERK), and SB 203580, an inhibitor of the p38 mitogen-activated protein kinase (p38 MAPK). In this study, we investigated whether the inhibition of proliferation by TGF-beta involves these protein kinase cascades. Culture of hepatocytes with TGF-beta for 16 hours decreased the stimulation by EGF of ERK2 and p70(S6k) (by 50% and 35%, respectively), but did not affect the stimulation of either p38 MAPK, c-jun NH2-terminal kinase (JNK), or protein kinase B (PKB). Culture of hepatocytes with glucagon for 16 hours also inhibited the stimulation by EGF of activation of ERK2 and p70(S6k) (by approximately 50%). The inhibitory effects of glucagon were observed when the hormone was added either 10 minutes or 60 minutes before EGF addition, whereas no effects of TGF-beta were observed after 10-minute or 60-minute incubation. These results suggest that the inhibition of hepatocyte proliferation by TGF-beta may be in part mediated by inhibition of ERK2 and p70(S6k), but does not involve PKB, JNK, or p38 MAPK. Unlike glucagon, the effects of TGF-beta are not elicited in response to short-term treatment.

T cell responses to the putative dominant autoepitope in primary biliary cirrhosis (PBC)

PBC is characterized by T cell-mediated destruction of the biliary epithelial cells lining the small intrahepatic bile ducts. The E2 and E3 binding protein (E3BP (protein X)) components of pyruvate dehydrogenase complex (PDC) are disease-specific autoantigens in PBC. Attempts to localize the T cell autoepitopes within PDC-E2 have, however, generated contradictory results. One study has suggested the presence of T cell epitopes throughout PDC-E2, whilst another has identified a single dominant 14 amino acid T cell epitope (p163) spanning the lipoic acid binding lysine residue in the inner lipoyl domain (ILD) of PDC-E2. The aim of the current study was to determine the prevalence of T cell responses to p163 and PDC-E2 ILD, and the role played by lipoylation of these antigens in their immunogenicity, in a UK PBC population. We found that the majority of the PBC patients showing a 6-day peripheral blood T cell proliferative response to native human PDC also responded, in a MHC class II-restricted fashion, to biochemically purified PDC-E2 and E3BP (which co-purify) (9/10 positive (SI > 2.76), mean SI 5.74 +/- 5.04 (PDC-E2/E3BP) versus 6.67 +/- 3.84 (PDC), P = NS), implying that the important PBC-specific T cell epitopes are contained within the PDC-E2 or E3BP components of PDC. Only a minority of patients responsive to PDC, however, responded to either lipoylated recombinant PDC-E2 ILD (4/10 positive, mean SI 1.98 +/- 1.24, P < 0.005 versus PDC response) or lipoylated p163 (4/12 positive, mean SI 1.90 +/- 1.58, P < 0.001). The lipoylation state did not affect the T cell response to either ILD or p163. Our findings suggest that in some UK patients with PBC there are immunodominant T cell autoepitopes within PDC-E2/E3BP which are outside the ILD of PDC-E2.

Enhanced ultrasensitive detection of structurally diverse antigens using a single immuno-PCR assay protocol

Our studies of DNA damage and repair in autoimmune disease, lymphomagenesis, and carcinogenesis, require identification of an immunoassay approach that is capable of ultrasensitive detection in a routine human tissue biopsy of several physicochemically diverse antigens, some of which will be present at very low level. Immuno-polymerase chain reaction (immuno-PCR) is a recently described method for ultrasensitive antigen detection that combines the amplification power of PCR with a method similar to a standard antibody capture, enzyme-linked immunosorbent assay (ELISA). As a test of the universality of immuno-PCR, and as an assessment of the suitability of this method for our studies, we used a single immuno-PCR protocol to assay purified forms of the following physicochemically diverse antigens: oligomeric pyruvate dehydrogenase complex (PDC; Mr 8.5 x 10(6)), the promutagenic DNA base adduct O(6)-methylguanosine (Mr 298) and its monomeric repair enzyme, O(6)-methylguanine-DNA methyltransferase (MGMT; Mr 22,000), and a peptide from the N-terminus of MGMT (Mr 2310). We found that all antigens could be ultrasensitively assayed using the single immuno-PCR protocol. Assay limits observed using antigen-specific (primary) antibodies at 1 microg/ml, were in the approximate range of 10(2)-10(9) molecules, with O(6)-methylguanosine being detected most sensitively. Sensitivity of the antigen assay appeared to positively correlate with primary antibody titres determined by ELISA. Furthermore, we observed a substantial increase in detection sensitivity for all antigens by the use of primary antibodies at the higher level of 10 microg/ml. The latter approach permitted antigen assay within the approximate range of 10(0)-10(7) molecules. The combination of higher titre primary antibodies and their use at higher input level, produced an increase of immuno-PCR assay sensitivity of up to four orders of magnitude greater than those previously reported through the use of this assay to measure other antigens. This represents up to a nine order of magnitude increase in immunoassay sensitivity compared to ELISA. Our findings provide compelling evidence that immuno-PCR is indeed a universal ultrasensitive antigen detection method. Using the indicated assay enhancements. immuno-PCR performed as detailed here can offer greatly increased sensitivity for antigen measurement compared to other methods. Thus, our findings suggest that parallel quantitation of several different antigens in very small samples of human tissue will be readily attainable using immuno-PCR.

Control of glycogen synthesis in cultured human muscle cells

The regulation of glycogen synthesis and associated enzymes was studied in human myoblasts and myotubes maintained in culture. Both epidermal growth factor (EGF) and insulin stimulated glycogen synthesis approximately 2-fold, this stimulation being accompanied by a rapid and stable activation of the controlling enzyme glycogen synthase (GS). EGF also caused inhibition of glycogen synthase kinase 3 (GSK-3) and activation of the alpha isoform of protein kinase B (PKB) with the time-course and magnitude of its effects being similar to those induced by insulin. An inhibitor of the mitogen-activated protein (MAP) kinase pathway did not prevent stimulation of GS by EGF, suggesting that this pathway is not essential for the effect. A partial decrease in the fold activation of GS was, however, observed when p70(S6k) activation was blocked with rapamycin, suggesting a contribution of this pathway to the control of GS by either hormone. Wortmannin, a selective inhibitor of phosphatidylinositol 3'-kinase (PI-3 kinase) completely blocked the effects of both EGF and insulin in these cells. These results demonstrate that EGF, like insulin, activates glycogen synthesis in muscle, acting principally via the PKB/GSK-3 pathway but with a contribution from a rapamycin-sensitive component that lies downstream of PI-3 kinase.

Dephosphorylation of perilipin by protein phosphatases present in rat adipocytes

By incubating 32P-labelled adipocytes, and extracts from these cells, in the presence or absence of specific inhibitors, we evaluated the contribution of protein phosphatases PP1, PP2A and PP2C, to the dephosphorylation of perilipin, an acutely hormone-regulated adipocyte phosphoprotein. Under conditions to completely inhibit PP2A activity, perilipin phosphatase activity in extracts remain unaffected, but PP1 inhibition results in abolition of perilipin phosphatase activity. Inhibition of PP1 (and 2A) in intact adipocytes stimulated lipolysis and increased phosphorylation of perilipin. No involvement of PP2C was found. Hence, PP1 constitutes the predominant if not sole perilipin phosphatase in adipocytes.

Three-dimensional structure of the major autoantigen in primary biliary cirrhosis

BACKGROUND & AIMS

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by the presence of antimitochondrial autoantibodies in patients' serum. The major autoantigen, recognized by antibodies from > 95% of patients with PBC, has been identified as the E2 component (E2p) of the pyruvate dehydrogenase multienzyme complex. Immunodominant sites on E2p have been localized to the inner of the two lipoyl domains, where the essential cofactor lipoic acid is attached covalently. The aim of this study was to determine the three-dimensional structure of the inner lipoyl domain of human E2p.

METHODS

The domain was expressed in Escherichia coli; after purification, its structure was analyzed using nuclear magnetic resonance spectroscopy.

RESULTS

The structure of the lipoyl domain from human E2p was determined, and the implications of the structure for autoimmune recognition were assessed.

CONCLUSIONS

Knowledge of the structure further defines the major epitope and may help in the design of antigen-specific immunotherapy for treatment of PBC.

Immunogenetic analysis of a panel of monoclonal IgG and IgM anti-PDC-E2/X antibodies derived from patients with primary biliary cirrhosis

BACKGROUND/AIMS

Autoantibodies with specificity for the E2 component of the pyruvate dehydrogenase complex (PDC-E2) are commonly present in primary biliary cirrhosis. The aim of this study was to generate and characterise human anti-PDC-E2 monoclonal antibodies and analyse immunoglobulin gene usage and mutation for clues to pathogenesis.

METHODS

Peripheral B-lymphocytes from two patients with primary biliary cirrhosis were used to generate heterohybridomas secreting PDC-E2 specific monoclonal antibodies. The antibodies were characterised by ELISA, immunoblotting, indirect immunofluorescence and enzyme inhibition techniques, and their encoding immunoglobulin genes were amplified, cloned and sequenced.

RESULTS

Four IgGlambda and one IgMlambda monoclonal antibodies specific for PDC-E2 were generated: all gave bands at 74 kD and 52 kD on PDC immunoblots, two clones were specific for the lipoylated inner lipoyl domain, and all inhibited target enzyme function. Sequence analysis suggested unrestricted VH gene usage, but a strong preference for lambda light chains. The extent of somatic mutation was high (3-20%), with evidence for antigen selection in 3/5 VH sequences.

CONCLUSIONS

These monoclonal antibodies closely resemble the hallmark autoantibodies of primary biliary cirrhosis. Their specificities demonstrate true cross reactivity between an epitope on PDC-E2 and Protein X, and the existence of a subset of B cells that recognise only the lipoylated form of the antigen. The pattern of immunoglobulin gene mutations suggests an antigen-driven selection of high affinity IgG autoantibodies, supporting a possible role for exogenous antigen in the pathogenesis of primary biliary cirrhosis.

Signalling pathways involved in the stimulation of glycogen synthesis by insulin in rat hepatocytes

In hepatocytes glycogen storage is stimulated by insulin and this effect of insulin is counteracted by epidermal growth factor (EGF). The mechanism by which insulin stimulates glycogen synthesis in liver is unknown. We investigated the involvement of candidate protein kinases in insulin signalling in hepatocytes. Both insulin and EGF activated extracellular regulated kinase 2 (ERK-2), p70rsk and protein kinase B (PKB) and inactivated glycogen synthase kinase-3 (GSK-3). Whereas EGF caused a greater activation of ERK-2 than insulin, the converse was true for PKB. The stimulation by insulin of ERK-2 was blocked by a mitogen-activated protein (MEK) inhibitor (PD 98059) and of p70rsk by rapamycin. However, these inhibitors, separately or in combination, did not block the stimulation of glycogen synthesis by insulin, indicating that activation of these kinases is not essential for the stimulation of glycogen synthesis by insulin. Mono Q fractionation of hepatocyte extracts resolved a single myelin basic protein (MBP) kinase peak from extracts of EGF-treated cells (peak 1, eluting at 200 mmol/l NaCl) and two peaks from insulin-treated cells (peak 1 eluting at 200 mmol/l NaCl and peak 2 eluting at 400 mmol/l NaCl). In the combined presence of insulin and EGF, activation of peak 2 was abolished. In situ MBP kinase assays and immunoblotting established that peak 1 coincides with ERK-2 and peak 2 is not an activated form of ERK-1 or ERK-2. It is concluded that PKB, which is activated to a greater extent by insulin than EGF, and peak 2, which is activated by insulin and counteracted by EGF, are possible candidates in mediating the stimulation of glycogen synthesis by insulin.

The human biliary epithelial cell plasma membrane antigen in primary biliary cirrhosis: pyruvate dehydrogenase X?

BACKGROUND & AIMS

Patients with primary biliary cirrhosis (PBC) have autoantibodies that react with components of mitochondrial multienzyme complexes. In addition to binding to mitochondria, patients' autoantibodies to the assumed major autoantigen pyruvate dehydrogenase complex (PDC) dihydrolipoamide acetyltransferase (E2) bind to the plasma membrane of biliary epithelial cells (BECs) specifically in PBC. The aim of this study was to characterize BEC plasma membrane antigens recognized by patients' autoantibodies in PBC.

METHODS

Antigens prepared from intracellular and plasma membrane-enriched fractions of BECs purified from PBC and control liver were immunoblotted with anti-PDC.

RESULTS

In the intracellular fraction, anti-PDC recognized BEC protein bands corresponding to the molecular weight value of E2 and X components of human heart PDC on Western blots. No difference was observed between PDC-E2 in BECs from PBC and controls. However, in PBC but not controls, a 50-kilodalton antigen was detected in the plasma membrane-enriched fraction. This antigen comigrated with component X of purified human heart PDC and was recognized by antibodies specific for PDC-X.

CONCLUSIONS

The data suggest that PDC-X or a cross-reactive 50-kilodalton antigen is the BEC plasma membrane antigen recognized by patients' autoantibodies in PBC. Furthermore, this antigen, rather than PDC-E2, may be a major B-cell target antigen in PBC.

T cell responses to natural human proteins in primary biliary cirrhosis

The study of T cell responses to autoantigens in human autoimmunity has been hampered by difficulties, firstly in identifying significant autoantigens, and secondly in the purification of authentic human proteins in sufficient quantities to allow characterization of antigen-specific T cell responses. In this study we have purified a human autoantigen, pyruvate dehydrogenase, retaining its enzymatic activity, and characterized autoreactive T cell responses to it in a human autoimmune disease, primary biliary cirrhosis. T cell responses to a mixture of the E2 and protein X subunits of human pyruvate dehydrogenase complex are seen in most affected patients, but in only a small minority of normal and chronic liver disease controls. By contrast, responses to whole pyruvate dehydrogenase complex occur with equal frequency in both groups. This suggests that responses to the E2 component/protein X of pyruvate dehydrogenase complex play a role in the pathogenesis of primary biliary cirrhosis. The availability of significant quantities of the human autoantigen in primary biliary cirrhosis makes this condition an interesting model in which to study true autoreactive human T cell responses.

T cell responses to tuberculin purified protein derivative in primary biliary cirrhosis: evidence for defective T cell function

BACKGROUND

Primary biliary cirrhosis (PBC) has an autoimmune aetiology, although little is known regarding the mechanisms of breakdown of self tolerance. One postulated mechanism of control of self tolerance is through interacting T cell subsets, a phenomenon explored in this study.

AIMS

To characterise and compare T cell subset responses to an antigen (tuberculin purified protein derivative derived from mycobacteria) in PBC patients and controls. Cross reactive responses to mycobacteria have recently been implicated in the aetiology of PBC.

SUBJECTS

58 PBC patients, 25 normal controls, and 34 chronic liver disease controls.

METHODS

Responses to antigen were measured in terms of primary T cell proliferation and cytokine secretion (by ELISA). Responding cells were phenotyped by FACS analysis.

RESULTS

Similar CD4+ T cell proliferative responses were seen in PBC patients (mean (SD) stimulation index (SI) 22.6 (27.2), 42 of 58 (72.4%) positive response), normal controls (46.5 (88.0), 17 of 25 (68%) positive), and chronic liver disease controls (24.8 (49.8), 27 of 34 (79.4%) positive)). Secretion of both interferon gamma and IL10 was significantly lower in PBC patients than controls (IFN gamma: PBC 822.7 (1100) pg/ml, controls 2929 (3402) pg/ml, p < 0.05: IL10: PBC 11.1 (15.6) pg/ml, controls 34.7 (63.4) pg/ml, p < 0.05).

CONCLUSIONS

In PBC unimpaired T cell proliferation is seen with reduced secretion of both Th-1 (interferon gamma) and Th-2 type (IL10) cytokines. These findings may result from differential subset responses and may help explain the defects of functional immunity seen in PBC.

Biochemistry and autoimmune response to the 2-oxoacid dehydrogenase complexes in primary biliary cirrhosis

Pyruvate dehydrogenase complex (PDC), 2-oxo-glutarate dehydrogenase complex (OGDC), and the branched-chain 2-oxoacid dehydrogenase complex (BCOADC) constitute the 2-oxoacid dehydrogenase family of multienzyme complexes. These complexes, which are larger than ribosomes and which consist of multiple copies of E1, E2, and E3 subunits together with regulatory kinases and phosphatases and, in the case of PDC, an E3-binding protein (protein X), each play an important role in oxidative metabolism in mitochondria. Primary biliary cirrhosis (PBC) is associated with a high incidence of autoantibodies directed at mitochondrial autoantigens (the antimito-chondrial antibodies), identified as the E2 components of PDC, OGDC, and BCOADC, together with protein X and the E1 alpha and E1 beta subunits of PDC. The dominant B-cell autoepitope in PBC has been identified as the inner lipoic acid binding domain of PDC-E2, with the lipoic acid co-factor, which plays a critical role in E2 enzymatic activity, playing a role in autoantibody binding to antigen. Autoreactive CD4+ T cells specific for human PDC-E2 are also present in both the peripheral blood and liver mononuclear cell infiltrates of PBC patients. The mechanism of break-down of B-cell and T-cell self-tolerance to these ubiquitous mitochondrial antigens in such an organ-specific manner remains unclear. The apparent importance of autoreactive responses to these self-antigens does, however, raise the possibility that antigen-specific immunotherapy may offer a novel route to therapy in PBC.

Transforming growth factor beta increases the activity of phosphatidate phosphohydrolase-1 in rat hepatocytes

Phosphatidic acid (PA) is a potent second messenger arising from growth factor-induced stimulation of phospholipase D which hydrolyses phosphatidylcholine. PA is hydrolysed to diacylglycerol by PA phosphohydrolase (PAP) which exists in two forms: PAP-1 and PAP-2. In rat hepatocyte cultures, overnight (20h) incubation with transforming growth factor (TGF) beta (1 ng/ml) increased PAP-1 activity two-fold. This effect was concentration and time dependent and was greatest at low cell density. The TGFbeta effect on PAP-1 was additive to stimulation induced by dexamethasone but not by glucagon and it reversed the inhibition by insulin. Epidermal growth factor had no effect on PAP-1 activity. None of the above hormones or growth factors affected the subcellular distribution of PAP-1. Stimulation of PAP-1 by TGFbeta may be involved in mediating some of its biological effects.

Insulin action in cultured human myoblasts: contribution of different signalling pathways to regulation of glycogen synthesis

A key metabolic action of insulin is the stimulation of non-oxidative glucose utilization in skeletal muscle, by increasing both glucose uptake and glycogen synthesis. The molecular mechanism underlying this process has been investigated using a variety of experimental systems. We report here the use of cultured human myoblasts to study insulin control of glycogen synthesis in humans. In these cells insulin stimulates glycogen synthesis approx. 2.2-fold, associated with a similar activation of glycogen synthase (GS) which occurs within 5-10 min of the addition of insulin. Insulin also causes inactivation of glycogen synthase kinase-3 (GSK-3) and activation of protein kinase B, both processes being sufficiently rapid to account for the effects of insulin on GS. Activation by insulin of the protein kinases p70s6K, p90s6K and extracellular signal-regulated kinase 2 (ERK2) is observed, but is significantly slower than the activation of GS. Selective inhibitors of the p70s6K pathway (rapamycin), the ERK2/p90s6K pathway (PD98059) and phosphatidylinositol 3-kinase (wortmannin) have been used to probe the contribution of these components to insulin signalling in human muscle. Wortmannin blocks activation of both glycogen synthesis and GS and inactivation of GSK-3. PD98059 is without effect on these events, while rapamycin is without effect on inactivation of GSK-3 but partially blocks activation of glycogen synthesis and GS. Taken together, these findings suggest that protein kinase B is responsible for the inactivation of GSK-3, but that an additional rapamycin-sensitive mechanism may contribute to the activation of GS and stimulation of glycogen synthesis.

Evidence for a multi-domain structure for hormone-sensitive lipase

Hormone-sensitive lipase (HSL) is a multi-functional enzyme involved in several aspects of lipid metabolism. Limited tryptic digestion of HSL led to selective loss of activity against lipid substrates but not against the water-soluble substrate, p-nitrophenyl butyrate. Following labelling of the active site of HSL with either [3H]di-isopropylfluorophosphate or [14C]orlistat, tryptic digestion of HSL generated a stable radiolabelled domain of molecular mass approx. 17.6 kDa, consistent with this representing a catalytic domain of HSL capable of hydrolysing water-soluble but not lipid substrates. Following phosphorylation of HSL by cyclic AMP-dependent protein kinase, limited tryptic digestion produced a stable phosphorylated domain of molecular mass 11.5 kDa. Based on these experimental data a model for a domain structure of HSL is proposed.

Effects of novel synthetic sterol probes on enzymes of cholesterol metabolism in cell-free and cellular systems

A series of novel sterols was synthesised as probes for the enzymatic and cellular functions of two important enzymes of intracellular cholesterol metabolism, acyl-CoA:cholesterol acyltransferase (ACAT) and cholesterol 7 alpha-hydroxylase. The compounds were 6-fluoro-5-cholesten-3 beta-ol (6-fluorocholesterol), 7-cholesten-3 beta-ol (7-cholestenol), 6 beta-fluorocholestan-3 beta-ol (6 beta-fluorocholestanol), 3-acetoxy-6-fluorocholestan-3 beta-ol (3-acetoxy-6-fluorocholestanol) and 7-methoxy-5-cholesten-3 beta-ol (7-methoxycholesterol). They were designed to reveal the effect of small changes in sterol structure, particularly reactivity of certain parts of the ring structure and polarity, on enzyme activity and intracellular cholesterol metabolism. The 3 beta-hydroxy group was essential for interaction with both enzymes since 3-acetoxy-6-fluorocholestanol did not affect any of the enzyme-catalysed reactions. 6-Fluorocholesterol and 7-cholestenol had no effect on cholesterol esterification but did inhibit the hydroxylation of cholesterol, as did the other compounds with groups that could influence the 7 position, namely 6 beta-fluorocholestanol and 7-methoxycholesterol. The fluorocholestanols were all competitive substrates for cholesterol esterification in cell-free and cellular assays of ACAT activity. 7-Methoxycholesterol was a surprisingly effective inhibitor of ACAT for a simple sterol. However, 6-fluorocholesterol did not have any effect on ACAT, suggesting that interactions between the enzyme and the region around C-6 and C-7 of the sterol are important. These results show that it is possible to dissect components of cholesterol metabolism using simple, specifically substituted sterols and thus define a new approach to studying the relationships between the various enzymes that catalyse intracellular cholesterol metabolism.

Down-regulation of hormone-sensitive lipase in sterol ester-laden J774.2 macrophages

The development of atherosclerotic plaques in arteries is a key step in atherogenesis, with cholesterol ester accumulation in macrophage-derived foam cells being recognized as a major pathogenic event in this process. In this study, the mouse macrophage cell line J774.2 was induced to accumulate intracellular sterol esters by incubation with 25-hydroxycholesterol in the presence of oleic acid. The accumulation of sterol esters in these cells was found to be accompanied by a marked decrease in the activity of the enzyme responsible for their hydrolysis, namely hormone-sensitive lipase (HSL); Western blotting studies revealed a corresponding decrease in the levels of the HSL polypeptide. Similar findings were obtained after incubation with oxidized low-density lipoprotein or very-low-density lipoprotein. These findings suggest that down-regulation of the expression of HSL is important in cholesterol ester accumulation in macrophages.