Protein Kinase C Epsilon Deletion in Adipose Tissue, but Not in Liver, Improves Glucose Tolerance

Protein kinase C epsilon (PKCɛ) activation in the liver is proposed to inhibit insulin action through phosphorylation of the insulin receptor. Here, however, we demonstrated that global, but not liver-specific, deletion of PKCɛ in mice protected against diet-induced glucose intolerance and insulin resistance. Furthermore, PKCɛ-dependent alterations in insulin receptor phosphorylation were not detected. Adipose-tissue-specific knockout mice did exhibit improved glucose tolerance, but phosphoproteomics revealed no PKCɛ-dependent effect on the activation of insulin signaling pathways. Altered phosphorylation of adipocyte proteins associated with cell junctions and endosomes was associated with changes in hepatic expression of several genes linked to glucose homeostasis and lipid metabolism. The primary effect of PKCɛ on glucose homeostasis is, therefore, not exerted directly in the liver as currently posited, and PKCɛ activation in this tissue should be interpreted with caution. However, PKCɛ activity in adipose tissue modulates glucose tolerance and is involved in crosstalk with the liver.

Proteomic analysis of livers from fat-fed mice deficient in either PKCδ or PKCε identifies Htatip2 as a regulator of lipid metabolism

Insulin resistance contributes to the development of Type 2 diabetes, and is associated with lipid oversupply. Deletion of isoforms of the lipid-activated protein kinase C (PKC) family, PKCδ or PKCε, improves insulin action in fat-fed mice, but differentially affects hepatic lipid metabolism. To investigate the mechanisms involved, we employed an in vivo adaptation of SILAC to examine the effects of a fat diet together with deletion of PKCδ or PKCε on the expression of liver proteins. We identified a total of 3359 and 3488 proteins from the PKCδ and PKCε knockout study groups, respectively, and showed that several enzymes of lipid metabolism were affected by the fat diet. In fat-fed mice, 23 proteins showed changes upon PKCδ deletion while 19 proteins were affected by PKCε deletion. Enzymes of retinol metabolism were affected by the absence of either PKC. Pathway analysis indicated that monosaccharide metabolism was affected only upon PKCδ deletion, while isoprenoid biosynthesis was affected in a PKCε-specific manner. Certain proteins were regulated inversely, including HIV-1 tat interactive protein 2 (Htatip2). Overexpression or knockdown of Htatip2 in hepatocytes affected fatty acid storage and oxidation, consistent with a novel role in mediating the differential effects of PKC isoforms on lipid metabolism. All MS data have been deposited in the ProteomeXchange with identifier PXD000971 (http://proteomecentral.proteomexchange.org/dataset/PXD000971).

Deletion of protein kinase Cε in mice has limited effects on liver metabolite levels but alters fasting ketogenesis and gluconeogenesis

AIMS/HYPOTHESIS

Protein kinase Cε (PKCε) is emerging as a key mediator of lipid-induced insulin resistance in liver and hepatic lipid metabolism itself. We investigated whether PKCε plays a role in other metabolic processes, to further examine its suitability as a therapeutic target.

METHODS

We measured amino acid, organic acid and sugar levels by liquid and gas chromatography-mass spectrometry of liver extracts from chow and fat-fed wild-type (WT) and PKCε-deficient (Prkce(-/-)) mice. Fed and fasting glucose, ketone and fatty acid levels were measured in blood. Triacylglycerol levels and gluconeogenic and ketogenic enzyme expression were measured in liver. The effect of fasting on epididymal fat pad mass was also determined.

RESULTS

Metabolomic analysis indicated that the short-term high-fat diet affected over 20 compounds, including a 50% reduction in the glucogenic amino acid alanine. Prkce deletion resulted only in a reduction of 4-hydroxyproline and aspartate and an increase in glutamate. However, upon fasting, Prkce(-/-) mice were better able to maintain blood glucose levels and also exhibited lower levels of the ketone β-hydroxybutyrate compared with WT mice. Upon fasting, Prkce deletion also resulted in lower liver and plasma lipids and a smaller reduction in fat pad mass.

CONCLUSIONS/INTERPRETATION

Metabolomic analysis provided new insights into the effects of a high-fat diet on liver metabolite levels. Glucose homeostasis under fasting conditions is improved in Prkce(-/-) mice, which, in turn, may reduce the mobilisation of lipid from adipose tissue, reducing the availability of ketogenic substrate in the liver. Together with the protection against fat-diet-induced glucose intolerance previously observed in the fed state, these findings indicate PKCε as a unique therapeutic target for the improvement of glucose homeostasis.

Time-dependent effects of Prkce deletion on glucose homeostasis and hepatic lipid metabolism on dietary lipid oversupply in mice

AIMS/HYPOTHESIS

We examined the time-dependent effects of deletion of the gene encoding protein kinase C epsilon (Prkce) on glucose homeostasis, insulin secretion and hepatic lipid metabolism in fat-fed mice.

METHODS

Prkce(-/-) and wild-type (WT) mice were fed a high-fat diet for 1 to 16 weeks and subjected to i.p. glucose tolerance tests (ipGTT) and indirect calorimetry. We also investigated gene expression and protein levels by RT-PCR, quantitative protein profiling (isobaric tag for relative and absolute quantification; iTRAQ) and immunoblotting. Lipid levels, mitochondrial oxidative capacity and lipid metabolism were assessed in liver and primary hepatocytes.

RESULTS

While fat-fed WT mice became glucose intolerant after 1 week, Prkce(-/-) mice exhibited normal glucose and insulin levels. iTRAQ suggested differences in lipid metabolism and oxidative phosphorylation between fat-fed WT and Prkce(-/-) animals. Liver triacylglycerols were increased in fat-fed Prkce(-/-) mice, resulting from altered lipid partitioning which promoted esterification of fatty acids in hepatocytes. In WT mice, fat feeding elevated oxygen consumption in vivo and in isolated liver mitochondria, but these increases were not seen in Prkce(-/-) mice. Prkce(-/-) hepatocytes also exhibited reduced production of reactive oxygen species (ROS) in the presence of palmitate. After 16 weeks of fat feeding, however, the improved glucose tolerance in fat-fed Prkce(-/-) mice was instead associated with increased insulin secretion during ipGTT, as we have previously reported.

CONCLUSIONS/INTERPRETATION

Prkce deletion ameliorates diet-induced glucose intolerance via two temporally distinct phenotypes. Protection against insulin resistance is associated with changes in hepatic lipid partitioning, which may reduce the acute inhibitory effects of fatty acid catabolism, such as ROS generation. In the longer term, enhancement of glucose-stimulated insulin secretion prevails.

Saturated- and n-6 polyunsaturated-fat diets each induce ceramide accumulation in mouse skeletal muscle: reversal and improvement of glucose tolerance by lipid metabolism inhibitors

Lipid-induced insulin resistance is associated with intracellular accumulation of inhibitory intermediates depending on the prevalent fatty acid (FA) species. In cultured myotubes, ceramide and phosphatidic acid (PA) mediate the effects of the saturated FA palmitate and the unsaturated FA linoleate, respectively. We hypothesized that myriocin (MYR), an inhibitor of de novo ceramide synthesis, would protect against glucose intolerance in saturated fat-fed mice, while lisofylline (LSF), a functional inhibitor of PA synthesis, would protect unsaturated fat-fed mice. Mice were fed diets enriched in saturated fat, n-6 polyunsaturated fat, or chow for 6 wk. Saline, LSF (25 mg/kg x d), or MYR (0.3 mg/kg x d) were administered by mini-pumps in the final 4 wk. Glucose homeostasis was examined by glucose tolerance test. Muscle ceramide and PA were analyzed by mass spectrometry. Expression of LASS isoforms (ceramide synthases) was evaluated by immunoblotting. Both saturated and polyunsaturated fat diets increased muscle ceramide and induced glucose intolerance. MYR and LSF reduced ceramide levels in saturated and unsaturated fat-fed mice. Both inhibitors also improved glucose tolerance in unsaturated fat-fed mice, but only LSF was effective in saturated fat-fed mice. The discrepancy between ceramide and glucose tolerance suggests these improvements may not be related directly to changes in muscle ceramide and may involve other insulin-responsive tissues. Changes in the expression of LASS1 were, however, inversely correlated with alterations in glucose tolerance. The demonstration that LSF can ameliorate glucose intolerance in vivo independent of the dietary FA type indicates it may be a novel intervention for the treatment of insulin resistance.

A proteome map of murine heart and skeletal muscle

The balance of hypertrophy and atrophy is critical for the adaptation of cardiac and skeletal muscle mass to the demands of the environment and when deregulated can cause disease. Here we have used a proteomics approach to generate protein reference maps for the mouse heart and skeletal muscle, which provide a molecular basis for future functional and pathophysiological studies. The reference map provides information on molecular mass, pI, and literature data on function and localization, to facilitate the identification of proteins based on their migration in 2-D gels. In total, we have identified 351 cardiac and 284 skeletal muscle protein spots, representing 249 and 214 different proteins, respectively. In addition, we have visualized the protein pattern of mouse heart and skeletal muscle at defined conditions comparing knockout (KO) animals deficient in the sarcomeric protein titin (a genetic atrophy model) and control littermates. We found 20 proteins that were differently expressed linking titin's kinase region to the heat-shock- and proteasomal stress response. Taken together, the established reference maps should provide a suitable tool to relate protein expression and PTM to cardiovascular and skeletal muscle disease using the mouse as an animal model.

Cardiac hypertrophy and reduced contractility in hearts deficient in the titin kinase region

BACKGROUND

Titin is a giant protein crucial for the assembly and elasticity of the sarcomere. Recently, titin has been linked to signal transduction through its kinase domain, which has been proposed to sense mechanical load. We developed a knockout in which expression of M-line-deficient titin can be induced in adult mice and investigated the role of the titin kinase region in cardiac function.

METHODS AND RESULTS

Isolated heart experiments revealed that in titin M-line-deficient mice, the contractile response to beta-adrenergic agonists and extracellular calcium is reduced. However, the Ca2+ sensitivity and cooperativity of activation of skinned cardiac muscle were unchanged. In knockout mice, calcium transients showed a reduced rate of calcium uptake, and expression analysis showed reduced levels of calmodulin, phospholamban, and SERCA2. Ultimately, knockout mice developed cardiac hypertrophy and heart failure, which involves protein kinase C signal transduction but not the mitogen-activated protein kinase pathway.

CONCLUSIONS

The titin kinase region emerges as a regulator of contractile function through effects on calcium handling and hypertrophy through protein kinase signal transduction. These novel functions of titin might provide a rationale for future therapeutic approaches to attenuate or reverse symptoms of heart failure.

Muscle atrophy in Titin M-line deficient mice

We investigated the response to deletion of the titin M-line region in striated muscle, using a titin knockout model and a range of techniques that include histology, in situ hybridization, electron microscopy, and 2D gel analysis. We found that the loss of titin's kinase domain and binding sites for myomesin and MURF-1 causes structural changes in the sarcomere that proceed from the M-line to the Z-disc and ultimately result in disassembly of the sarcomere. Disassembly goes along with central localization of nuclei (a hallmark for muscular dystrophy), up-regulation of heat-shock proteins, and induction of proteasome activity. While fiber type composition does not change in soleus and extensor digitorum longus muscle, fiber size is reduced. Animals die from complications of muscle atrophy at five weeks of age. In addition to the structural importance of the titin M-line region in any striated muscle, our data show how differences in M-line composition between heart and skeletal muscle affect sarcomere stability and function.

DNA-damage detection in man after radiation exposure--the comet assay--its possible application for human biomonitoring

The exposure of human beings to ionizing radiation is still of great concern to occupational and environmental medicine. The goal of this workshop is to identify a panel of biological markers that could be used in humans after exposure to ionizing radiation. The comet assay or single cell gel (SCG) assay is a new method that allows efficient determination of single-strand breaks (SSB) and double-strand breaks (DSB), as well as alkali-labile sites in the DNA of single cells. In order to demonstrate the practicability of the comet assay for the detection of DNA damage caused by low doses of ionizing radiation, we exposed human peripheral blood cells to radiation in vitro. The extent of DNA damage in blood cells irradiated with x-rays (0.05-1 Gy) was significantly increased above the control values even at 0.05 Gy and shows a clear dose-relationship. To investigate the repair kinetics for x-ray-induced DNA damage following acute and chronic (fractionated) irradiation, we exposed peripheral blood to 1 Gy and examined the tail moment at different time intervals. The effect of one acute dose is repaired within two h, whereas the effect of fractionated irradiation gives a totally different result. The tail moment of the initial damage increased indicating an accumulation of the damage, and the repair activity clearly decreased. Until now, there was no data available concerning DNA damage in vivo. For this reason, we explored patients subjected to radioiodine therapy as well as a Chernobyl liquidator.(ABSTRACT TRUNCATED AT 250 WORDS)

Early effects of benzene exposure in mice. Hematological versus genotoxic effects

Female BDF1 mice were exposed to 100, 300 and 900 ppm benzene 6 h/day, 5 days/week, up to 8 weeks. Hematological studies included peripheral blood data, T4 and T8 lymphocyte counts in the blood and the spleen, hemopoietic stem and progenitor cell assays in the marrow (CFU-S, CFU-C, BFU-E, CFU-E). The single cell gel assay ("comet assay") was applied in parallel with cells from the peripheral blood, bone marrow, spleen and liver. The results showed minor changes in the stem and progenitor cells and the development of a slight anemia at 4 and 8 weeks, in agreement with reported data. New was the increase of the T4/T8 ratio in the peripheral blood (not in the spleen) at the end of the first week of exposure to 300 and 900 ppm. The results of the "comet assay" indicate a much higher sensitivity to this test system (strand breaks and alkali labile sites of DNA). The tail moment indicative of the damage to DNA increased as early as 3 days with 300 ppm in the peripheral blood cells. Furthermore, the liver cells did react to a much higher extent than the other cells tested. With 100 ppm significant changes were seen in the liver after 5 days, but not in the blood. The repair, studied 24 and 48 h after the end of the exposure, was almost complete after 5-day exposure period in the blood and the liver, but not after 4 weeks of exposure with 300 ppm in the blood, and 100 and 300 ppm in the liver.

Does physical activity induce DNA damage?

The single cell gel electrophoresis (SCG) assay (comet assay) is a sensitive technique for detecting the presence of DNA strand-breaks and alkali-labile damage in individual cells. This technique was used to study peripheral blood cells from three volunteers after physical activity. The test subjects had to run on a treadmill and were checked for blood pressure and ECG, lactate concentration and creatine kinase activity. Blood was taken before and several times during and after the run. In a first multiple step test, the volunteers ran as long as possible with increasing speed. In a second test they had to run for 45 min with a fixed individual speed which was defined to ensure an aerobic metabolism. In the first test, the white blood cells of all subjects showed increased DNA migration in the SCG assay. The effect was seen 6 h after the end of the exercise and reached its maximum 24 h later. After 72 h, DNA migration decreased to about control level. The distribution of DNA migration among cells clearly demonstrated that the majority of white blood cells exhibited increased DNA migration and that the effect was not only due to a small fraction of damaged cells. From the same blood samples, blood cultures were set up to study a possible effect on the frequency of sister chromatid exchanges (SCE), another indicator for genotoxic effects. However, there was no significant increase in SCE in any of the cultures. In the second exercise, during aerobic metabolism, the effect on DNA migration was not seen.(ABSTRACT TRUNCATED AT 250 WORDS)