Stephen J. Yeaman

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.

Hormone‐sensitive lipase is responsible for the neutral cholesterol ester hydrolase activity in macrophages

Anti‐hormone‐sensitive lipase (HSL) immunoglobulin selectively immunoprecipitates a single 84 kDa 32P‐phosphoprotein from macrophage homogenates previously phosphorylated by cyclic AMP‐dependent protein kinase in the presence of [γ‐32P]ATP‐Mg. This immunoglobulin also completely removes the neutral cholesterol ester hydrolase activity from macrophage homogenates. These data demonstrate that HSL is responsible for the neutral cholesterol ester hydrolase activity in macrophages and hence plays a key role in cholesterol metabolism in these cells.

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. PPARα and PPARδ agonists were found to specifically upregulate PDK4 mRNA expression, whereas PPARγ 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.

Inhibition of hormone‐sensitive lipase by intermediary lipid metabolites

Hormone‐sensitive lipase (HSL) is inhibited in a non‐competitive manner by oleoyl CoA, oleic acid and 2‐monopalmitoylglycerol, 50% inhibition being observed at concentrations of approx. 0.1 μM, 0.5 μM and 500 μM, respectively. HSL is a key enzyme in lipid metabolism, mobilising triacylglycerol and cholesterol ester stores in several tissues. Feedback inhibition of HSL by oleoyl CoA and oleic acid may therefore prevent accumulation of free fatty acids and cholesterol in the cell, whereas 2‐monoacylglycerol may act as a feedback inhibitor if the capacity of monoacylglycerol lipase is exceeded.

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 α 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 p70S6k 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.

Cytosolic cholesterol ester hydrolase from bovine corpus luteum: Its purification, identification, and relationship to hormone-sensitive lipase

The cytosolic cholesterol ester hydrolase from bovine corpus luteum has been purified 760-fold, using isoelectric precipitation and gel filtration chromatography, followed by ion-exchange and adsorption chromatographies in the presence of non-ionic detergent. Further purification was achieved by affinity chromatography on triacylglycerol-containing polyacrylamide-agarose. The partially purified enzyme was inhibited by NaF, HgCl2 and DFP. Incubation with [3H]DFP resulted in specific labelling of a polypeptide of Mr = 84000, the same subunit molecular weight as that of the enzyme from adrenal cortex. This Mr 84000 polypeptide from corpus luteum was phosphorylated by the catalytic subunit of cyclic AMP-dependent protein kinase, phosphorylation causing greater than 2-fold activation of the enzyme. Several properties of the cholesterol ester hydrolase from corpus luteum show striking similarities to those of hormone-sensitive lipase from adipose tissue. This provides further evidence that hormone-sensitive lipase, in addition to its role in adipose tissue lipolysis, has a key role in steroidogenic tissues, namely catalysing the supply of free cholesterol from the cholesterol ester stores.

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

Summary 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 98 059) 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. [Diabetologia (1998) 41: 16–25]

Hormone-sensitive lipase is involved in the hydrolysis of lipoidal derivatives of estrogens and other steroid hormones

Long-chain fatty acid esters of 17 beta-estradiol and other steroid hormones, which are formed in hormone-sensitive tissues, can be regenerated to the free hormone by the action of an esterase present in the cytosol. This esterase has now been examined in bovine placenta cotyledons. Activity towards steroid fatty acid esters was accompanied by activity towards a diacylglycerol analogue and cholesteryl oleate. During purification procedures, the ratio of activities towards the diacylglycerol analogue and estradiol 17 beta-oleate remained approximately constant. Activity towards these two substrates was inhibited by increasing concentrations of HgCl2 and phenylmethanesulfonyl fluoride in a parallel manner. Upon treatment with [3H]diisopropyl fluorophosphate, a major labelled species of Mr approx. 84,000 was formed. Activation by ATP and the catalytic subunit of cAMP-dependent protein kinase occurred. These properties were very similar to those of the hormone-sensitive lipase of bovine adipose tissue previously reported and run in parallel in this study. A highly purified preparation of this latter enzyme was found to hydrolyse steroid fatty acid esters and relative activities towards such substrates, diacylglycerol analogue and cholesteryl oleate, were similar to the placenta esterase. When the two esterases were phosphorylated with [gamma-32P]ATP, a labelled species of Mr 84,000 was isolated in both cases by use of an antibody raised against purified hormone-sensitive lipase of bovine adipose tissue. It is concluded that hormone-sensitive lipase is very likely the enzyme responsible for hydrolysis of steroid fatty acid esters in bovine placenta and possibly steroid hormone target tissues in general.

The multifunctional role of hormone-sensitive lipase in lipid metabolism

Hormone sensitive lipase (HSL) is an enzyme of relatively broad specificity, having the ability to hydrolyze tri-, di- and mono-acylglycerols as well as cholesterol esters and small water-soluble substrates. This broad specificity allows HSL to perform a variety of functions in several tissues. A key feature of HSL is its ability to be activated via phosphorylation by cyclic AMP-dependent protein kinase. In addition it is phosphorylated at a second site by several kinases, notably AMP-activated protein kinase. Phosphorylation of this site apparently plays a role in rendering the enzyme hormone-insensitive, in that prior phosphorylation at site 2 prevents phosphorylation and activation at site 1 by cyclic AMP-dependent protein kinase. Investigation of the protein phosphatases responsible for dephosphorylation of these sites has indicated that phosphatase 2A plays a predominant role but also that protein phosphatase 2C is a significant phosphatase targeted against both phosphorylation sites. Evidence indicates that HSL has at least three functional domains which contain (a) the phosphorylation sites which control activity, (b) the active site responsible for the catalytic activity and (c) a lipid binding site responsible for anchoring the lipase at the water-lipid interface. Using limited proteolytic studies we have found that it is possible to cleave HSL into several fragments including a stable domain of M(r) approximately 17.6 kDa which contains the active site serine residue. Digestion under similar conditions also generates a stable domain of M(r) approximately 11.5 kDa containing both phosphorylation sites. Furthermore, under appropriate conditions it is possible to digest HSL and retain activity against water-soluble substrates but with the concomitant loss of activity against triacylglycerol, implying that a lipid binding domain is lost during this procedure. HSL is responsible for the neutral cholesterol esterase activity in macrophages and it may play a role in the accumulation of cholesterol esters which occur during the development of foam cells. HSL activity is reduced in macrophage foam cells, at least partly due to increased activity of a cytosolic HSL inhibitor protein. A finding unexplained for many years has been that, although lipolysis can be stimulated 50-100-fold in adipocytes by lipolytic hormones, HSL can apparently only be activated 2-3-fold via phosphorylation in vitro by cyclic AMP-dependent protein kinase. One possibility to explain this discrepancy is that an additional anchoring protein is missing from the in vitro system and indirect evidence is now accumulating for such a protein.

Characterisation of the reactivity of autoantibodies in primary biliary cirrhosis

Autoantibodies in the sera of patients with primary biliary cirrhosis, shown previously to recognise the E2 polypeptide of the mammalian pyruvate dehydrogenase complex (PDC), have been demonstrated to react with the E2 component of PDC from bacteria (E. coli) and yeast (S. cerevisiae). Limited tryptic digestion, which cleaves E2 into well‐characterised domains, followed by Western blotting indicates that the main immunodominant region of PDC E2 lies within the lipoic acid‐containing domains of the polypeptide.