Characterization of expression of phosphofructokinase isoforms in isolated rat pancreatic islets and purified beta cells and cloning and expression of the rat phosphofructokinase-A isoform

Hypoxia inducible factor -1 regulates WSSV-induced glycolytic genes in the white shrimp Litopenaeus vannamei

Hypoxia-inducible factor -1 (HIF-1) is a transcriptional factor that regulates the expression of several glycolytic genes. The white spot syndrome virus (WSSV) induces a shift in glycolysis that favors viral replication in white shrimp Litopenaeus vannamei. HIF-1 is related to the pathogenesis of the WSSV infection through the induction of metabolic changes in infected white shrimp. Although the WSSV infection is associated with metabolic changes, the role of HIF-1 on key glycolytic genes during the WSSV infection has not been examined. In this work, we evaluated the effect of HIF-1α silencing on expression and activity of glycolytic enzymes (Hexokinase-HK, phosphofructokinase-PFK and pyruvate kinase-PK) along with the glucose transporter 1 (Glut1), regulatory enzymes (glucose-6-phosphate dehydrogenase-G6PDH and pyruvate dehydrogenase-PDH), and metabolic intermediates of glycolysis (glucose-6-phosphate-G6P and pyruvate). The expression of Glut1 increased in each tissue evaluated after WSSV infection, while HK, PFK and PK gene expression and enzyme activities increased in a tissue-specific manner. G6PDH activity increased during WSSV infection, and its substrate G6P decreased, while PDH activity decreased and its substrate pyruvate increased. Silencing of HIF-1α blocked the WSSV-induced Glut1 and glycolytic genes upregulation and enzyme activity in a tissue-specific manner. We conclude that HIF-1 regulates the WSSV-induced glycolysis through induction of glycolytic genes contributing to glucose metabolism in tissues of infected shrimp. Also, the inhibition, and activation of regulatory genes are likely to decrease the availability of the raw materials essential for WSSV replication and increase oxidative metabolism.

Streptozotocin diabetes increases mRNA expression of ketogenic enzymes in the rat heart

BACKGROUND

Diabetic cardiomyopathy develops in insulin-dependent diabetic patients who have no hypertension, cardiac hypertrophy or vascular disease. Diabetes increases cardiac fatty acid oxidation, but cardiac hypertrophy limits fatty acid oxidation. Here we examined effects of diabetes on gene expression in rat hearts.

METHODS

We used oligonucleotide microarrays to examine effects of insulindependent diabetes in the rat heart. RTQ PCR confirmed results of microarrays. Specific antibodies were used to examine changes in the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2).

RESULTS

A surprising result of diabetes was increased mRNA encoding all enzymes of the ketone body synthesis pathway. Increased mRNA expression for these enzymes was confirmed by RTQ PCR. The mRNA encoding HMGCS2, the rate-controlling enzyme, was 27 times greater in diabetic hearts. Total HMGCS2 protein increased 8-fold in diabetic hearts, but no difference was found in HMGCS2 protein in control vs. diabetic liver.

CONCLUSIONS

Insulin-dependent diabetes induced the enzymes of ketone body synthesis in the heart, including HMGCS2, as well as increasing enzymes of fatty acid oxidation.

GENERAL SIGNIFICANCE

The mammalian heart does not export ketone bodies to other tissues, but rather is a major consumer of ketone bodies. Induction of HMGCS2, which is normally expressed only in the fetal and newborn heart, may indicate an adaptation by the heart to combat "metabolic inflexibility" by shifting the flux of excess intramitochondrial acetyl-CoA derived from elevated fatty acid oxidation into ketone bodies, liberating free CoA to balance the acetyl-CoA/CoA ratio in favor of increased glucose oxidation through the pyruvate dehydrogenase complex.

Synchronization of pancreatic islet oscillations by intrapancreatic ganglia: a modeling study

Plasma insulin measurements from mice, rats, dogs, and humans indicate that insulin levels are oscillatory, reflecting pulsatile insulin secretion from individual islets. An unanswered question, however, is how the activity of a population of islets is coordinated to yield coherent oscillations in plasma insulin. Here, using mathematical modeling, we investigate the feasibility of a potential islet synchronization mechanism, cholinergic signaling. This hypothesis is based on well-established experimental evidence demonstrating intrapancreatic parasympathetic (cholinergic) ganglia and recent in vitro evidence that a brief application of a muscarinic agonist can transiently synchronize islets. We demonstrate using mathematical modeling that periodic pulses of acetylcholine released from cholinergic neurons is indeed able to coordinate the activity of a population of simulated islets, even if only a fraction of these are innervated. The role of islet-to-islet heterogeneity is also considered. The results suggest that the existence of cholinergic input to the pancreas may serve as a regulator of endogenous insulin pulsatility in vivo.

Molecular cloning of equine muscle-type phosphofructokinase cDNA

The complete coding region sequence of equine muscle-type phosphofructokinase (ePFKM) was obtained from skeletal muscle of a thoroughbred horse. The deduced amino acid sequence of ePFKM showed 97%, 96%, 96%, 96% and 95% identity to canine, human, mouse, rabbit and rat PFKM, respectively. The amino and carboxyl terminal halves of ePFKM presented a structure of tandem repeat, as other mammalian PFKMs. As the amino acid residues constituting various ligand-binding sites were also conserved, it is thought that ePFKM has enzymatic activity similar to PFKM in other mammals.

Cardiac Hypertrophy by Hypertension and Exercise Training Exhibits Different Gene Expression of Enzymes in Energy Metabolism

Hypertension-induced pathological cardiac hypertrophy (hypertensive heart) and exercise training-induced physiological cardiac hypertrophy (athletic heart) have differences in cardiac properties. We hypothesized that gene expression of energy metabolic enzymes differs between these two types of cardiac hypertrophy. To investigate whether mRNA expression of key enzymes in the long-chain fatty acid (FA), glucose, and lactic acid metabolic pathways differs between these two types of cardiac hypertrophy, we used the hearts of spontaneously hypertensive rats (SHR; 19 weeks old) as a model of the hypertensive heart, swim-trained rats (Trained; 19 weeks old, swimming training for 15 weeks) as a model of the athletic heart, and sedentary Wistar-Kyoto rats (Control; 19 weeks old). SHR developed hypertensive cardiac hypertrophy, of which cardiac function was deteriorated, whereas Trained rats developed an athletic heart, of which cardiac function was enhanced. The mRNA expression of CD36, which involved in uptake of long-chain FA, in the heart was almost never detected in the SHR group. Furthermore, the mRNA expression of key enzymes in the long-chain FA metabolic pathway (acyl CoA synthase [ACoAS], carnitine palmitoyl transferase [CPT]-I, CPT-II, and isocitrate dehydrogenase [ISCD]) in the heart was significantly higher in the SHR group compared with the Control group. The mRNA expression of ACoAS, CPT-I, ISCD, and CD36 in the heart did not differ between Trained group and Control group, whereas that of CPT-II in the Trained group was significantly higher compared with the Control group. The mRNA expression of key enzymes (phosphofructokinase and lactate dehydrogenase) in glycolytic metabolic pathway in the heart was markedly higher in the SHR group compared with the Control group, whereas these mRNA expressions did not differ between Trained group and Control group. These findings suggest that the molecular phenotypes in the energy metabolic system differ in hypertension-induced pathological and exercise training-induced physiological cardiac hypertrophy, and these differences may participate in the differences in cardiac function.

Fatty acid-induced beta cell hypersensitivity to glucose. Increased phosphofructokinase activity and lowered glucose-6-phosphate content

Diabetic states are characterized by a raised serum/islet level of long chain fatty acids and a lowered ED50 for glucose-induced insulin secretion. Prolonged culture (> 6 h) of islets with long chain fatty acids replicates the basal insulin hypersecretion. We examined this effect in rat islets cultured for 24 h with 0.25 mM oleate. Insulin secretion at 2.8 mM glucose was doubled in combination with a 60% lowered islet content of glucose-6-phosphate (G6P). Investigation of the lowered G6P showed: (a) increased glucose usage from 0.5 to 100 mM glucose with identical values measured by [2-3H]glucose and [5-3H]glucose, (c) indicating little glucose- 6-phosphatase activity, (b) unchanged low pentose phosphate shunt activity, (c) 50% increased phosphofructokinase (PFK) Vmax, (d) a normal ATP/ADP ratio, and (e) unchanged fructose 2,6 bisphosphate content. Triacsin C, an inhibitor of fatty acyl-CoA synthetase, prevented the increase in PFK activity and the lowered G6P content. These results suggest that long chain acyl-CoA mediates the rise in PFK activity, which in turn lowers the G6P level. We speculate that the inhibition of hexokinase by G6P is thus attenuated, thereby causing the basal insulin hypersecretion.

Cloning and expression of a group IV cytosolic Ca2+-dependent phospholipase A2 from rat pancreatic islets. Comparison of the expressed activity with that of an islet group VI cytosolic Ca2+-independent phospholipase A2

Stimulation of pancreatic islets with glucose induces phospholipid hydrolysis and accumulation of nonesterified arachidonic acid, which may play signaling or effector roles in insulin secretion. Of enzymes that catalyze phospholipid hydrolysis, islet beta-cells express low molecular weight secretory phospholipases A2 (PLA2) and a Group VI, Ca2+-independent PLA2 (iPLA2). Previous studies indicate that islets also express a protein recognized by antibodies against a Group IV, cytosolic, Ca2+-dependent PLA2 (cPLA2). To further examine the possible expression of cPLA2 by islets, we screened a rat islet cDNA library with a probe that recognizes cPLA2 sequence, and isolated a full-length cPLA2 cDNA. The rat islet cPLA2-deduced amino acid sequence is 96% identical to those of human and mouse cPLA2. Transfection of COS-7 cells with cPLA2 cDNA in an expression vector induced expression of Ca2+-dependent PLA2 activity and of a protein recognized by anti-cPLA2 antibody. Comparison of recombinant islet cPLA2 and iPLA2 activities expressed in transfected COS-7 cells indicated that iPLA2 but not cPLA2 is stimulated by ATP. Both activities are similarly sensitive to inhibition by arachidonyltrifluoromethyl ketone, but iPLA2 is more effectively inhibited by a haloenol lactone suicide substrate than cPLA2. RT-PCR experiments with RNA from purified islet beta-cells and from an alpha-cell-enriched population prepared by fluorescence-activated cell-sorting indicated that cPLA2 mRNA is more abundant in the beta-cell population. Immunoblotting analyses indicate that islets express cPLA2-immunoreactive protein, and that interleukin-1 does not affect its expression. The cPLA2 is thus one of at least three classes of PLA2 enzymes with distinct properties expressed in beta-cells.