Subcellular shifts in cyclic AMP phosphodiesterase and its calcium-dependent regulation in liver: role of diabetes

Effect of vitamin D3 in reducing metabolic and oxidative stress in the liver of streptozotocin-induced diabetic rats

Diabetes mellitus is a growing health problem worldwide and is associated with severe liver complications. The aim of the present study is to analyse the status of metabolic and free-radical-scavenging enzymes and second messengers in the liver of streptozotocin (STZ)-induced diabetic rats, and to determine the hepatoprotective role of vitamin D(3). All studies were performed using the liver of adult male Wistar rats. Gene expression studies were carried out using real-time PCR with specific probes. Second messenger levels were determined using (3)H-labelled Biotrak assay kits, and glucose uptake assay with D-[(14)C]glucose. The present results show that there was a decrease in hepatic glucose uptake, malate dehydrogenase activity, glycogen content, inositol triphosphate (IP(3)) and cyclic GMP levels, and superoxide dismutase, glutathione peroxidase, phospholipase C, cyclic AMP-responsive element-binding protein, vitamin D receptor (VDR) and insulin receptor (INSR) gene expression in the diabetic rats when compared with the controls (all P < 0·05), while cyclic AMP levels and GLUT2 expression were increased (P < 0·05). Treatment of the diabetic rats with vitamin D(3) and insulin reversed the altered parameters to near control values. In conclusion, the data suggest a novel role of vitamin D(3) in restoring impaired liver metabolism in STZ-induced diabetic rats by regulating glucose uptake, storage and metabolism. We demonstrated that the restoring effect of vitamin D(3) is mediated through VDR modulation, thereby improving signal transduction and controlling free radicals in the liver of diabetic rats. These data suggest a potential role for vitamin D(3) in the treatment of diabetes-associated hepatic complications.

O-glycosylation of Sp1 and transcriptional regulation of the calmodulin gene by insulin and glucagon

Both insulin and glucagon stimulate steady-state levels of Sp1 transcription factor, but only insulin stimulates transcription of the calmodulin (CaM) gene in liver. Because O-glycosylation of Sp1 by O-linked N-acetylglucosamine (O-GlcNAc) is thought to regulate its ability to activate transcription, we assayed the levels of Sp1 with anti-Sp1 and anti-O-GlcNAc antibodies in Western blots by use of extracts of H-411E liver cells treated with insulin (10,000 microU/ml) or glucagon (1.5 x 10(-5) M). We also assessed subcellular localization of the native and glycosylated Sp1 in H411E cells treated with either hormone in the presence of deoxynorleucine (DON, an indirect inhibitor of O-glycosylation) or streptozotocin (STZ, an indirect stimulator of O-glycosylation). Insulin stimulated both total and O-GlcNAc-modified Sp1 primarily in the nucleus and induced CaM gene transcription (P < 0.0001). In contrast, glucagon promoted accumulation of Sp1 in the cytoplasm but not the nucleus, without significantly stimulating (P = not significant) either its O-glycosylation or transcription of the CaM gene. DON inhibited O-glycosylation of Sp1 and its ability to migrate to the nucleus and transactivate CaM gene transcription. In contrast, cotreatment of cells with STZ and glucagon enhanced O-glycosylation of Sp1, promoting its migration to the nucleus and resulting in increased CaM gene transcription. Thus O-glycosylation of Sp1 by insulin, but not glucagon, apparently enhances its (Sp1) nuclear recruitment and results in activation of CaM gene transcription.

Effect of insulin treatment on smooth muscle calmodulin levels in rats with long-term streptozotocin-diabetes

Altered responses to several agonists have been reported in various smooth muscles from experimentally-diabetic animals suggesting a defective contractile process of smooth muscle. Recently, decreased smooth muscle calmodulin levels have been reported in streptozotocin-diabetic rats. However, the effectiveness of insulin on the decreased calmodulin levels in diabetic rats has not been questioned. Therefore, the present study was designed to examine the effect of insulin on smooth muscle calmodulin levels from streptozotocin-diabetic rats. Calmodulin levels of the smooth muscle were measured by a radioimmunoassay technique. Streptozotocin diabetes caused a significant decrease in tissue calmodulin levels of smooth muscles. Insulin therapy for 20 days did not correct the changes in calmodulin levels of rat smooth muscles, although it normalised blood glucose in streptozotocin-diabetic rats. These findings suggest that the altered smooth muscle calmodulin may contribute the defective contractile responses in diabetes and these changes may be resistant to insulin therapy.

Effect of short and long term streptozotocin diabetes on smooth muscle calmodulin levels in the rat

Diabetes mellitus is a metabolic disease associated with certain complications which have also been demonstrated in the experimental models of this disease. Altered responses to several agonists have been reported in various smooth muscles from alloxan or streptozotocin diabetic animals. Since these reports revealed a defect in the contractile process of smooth muscles from experimentally-induced diabetes, short and long term effects of diabetes on calmodulin levels in the smooth muscles of aorta, trachea, vas deferens and duodenum were investigated using streptozotocin diabetic rats. In spite of the fact that most of the reports have demonstrated the defective contractions in long term diabetic rats, short term effect (for 1 week) of diabetes on calmodulin levels in the smooth muscles of aorta, trachea, vas deferens and duodenum was also investigated in the present study using streptozotocin diabetic rats to understand whether the changes in calmodulin dependent contractile process begin at an earlier stage of the disease. Tissue calmodulin levels of the smooth muscles were measured by the radioimmunoassay technique using a [125I]-labeled kit. Although rats injected with streptozotocin exerted the characteristics of diabetes such as polyuria, polydipsy, polyphagy and elevated blood glucose levels, unchanged calmodulin levels were found in the rats with short term streptozotocin diabetes. In contrast, long term streptozotocin diabetes (for 8 weeks) was found to cause a significant decrease in tissue calmodulin levels of these four smooth muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Demonstration of inhibitory guanine nucleotide regulatory protein (Gi) function in liver and hepatocyte membranes from streptozotocin-treated rats

By using a defined plasma-membrane preparation, functional inhibition of adenylate cyclase activity by the inhibitory G-protein (Gi) was observed in liver and hepatocyte membranes from rats made diabetic by streptozotocin. These observations contrast with previous reports which have shown a defect in Gi in this diabetic animal model. These results suggest that Gi function is not impaired in the livers of streptozotocin-treated rats and that plasma-membrane preparation procedures should be clearly defined before ascribing Gi defects to a pathological state such as diabetes.

The influence of glucose concentration on angiotensin II-induced hypertrophy of proximal tubular cells in culture

Incubation of cultured murine proximal tubular cells in serum-free media containing 450 mg/dl of glucose resulted in cellular hypertrophy as defined by an increase in cell size, total protein content, and synthesis after 72 h. 10 nM angiotensin II further increased this hypertrophy, but failed to have any effect on cells grown in 100 mg/dl glucose. This enhancement by angiotensin II was blocked by treatment with 1 microM of the angiotensin-receptor antagonist DuP 753. Although cells incubated in either glucose media exhibited similar high-affinity angiotensin II-receptors, the receptor density was elevated only in cells grown in the presence of high glucose. Stimulation of cells in high glucose for 60 min with 10 nM angiotensin II also reduced significantly intracellular cAMP concentrations. This was not the case for proximal tubular cells cultured in normal glucose. Our results indicate that high glucose and angiotensin II have additive effects on the induction of hypertrophy in renal tubular cells.

Insulin control of cyclic AMP phosphodiesterase

Cyclic AMP phosphodiesterase (PDE) is an enzyme involved in cellular homeostasis of cyclic AMP. It exists as multiple isozymes in cells, but only the high affinity, membrane-bound isozyme is sensitive to hormonal modulation. Several isozymes or isoforms of the low Km PDE have been detected. Data suggest that several mechanisms exist for hormonal modulation of PDE. Activity of the low Km PDE species may be modulated by phosphorylation/dephosphorylation, phospholipid substrate concentration, insulin second messenger, cyclic GMP, guanine nucleotide binding proteins, calmodulin, or aggregation/disaggregation of monomeric forms. Modulation of PDE isoforms by different hormones may be through different regulatory components or mechanisms.

Calmodulin distribution in peripheral nerve: an EM immunocytochemical study

We used electron microscopic immunocytochemistry to study the distribution of calmodulin in rat sciatic nerve. Calmodulin immunoreactivity was found throughout the axoplasmic matrix, but particularly along microtubules. Schwann cell cytoplasm and nuclei demonstrated immunoreactivity, while compact myelin did not. There was particularly intense immuno-gold deposition within Schmidt Lanterman clefts. At the nodes of Ranvier, calmodulin appeared preferentially in the paranodal region, along the apposition of the axolemma to the paranodal loops of myelin and extending into the paranodal loops. The presence of calmodulin immunoreactivity along microtubules supports biochemical and pharmacological evidence of calmodulin involvement in regulating the assembly and phosphorylation of microtubules, and in fast axonal transport along microtubules. The co-localization of paranodal calmodulin immunoreactivity with Ca-ATPase activity demonstrated cytochemically (Mata et al., Brain Research, in press) supports the notion that the paranodal Ca-ATPase activity may be regulated by calmodulin, and agrees with the in vitro biochemical evidence for Ca-ATPase of other cells.

Lack of effect of insulin in hepatocytes isolated from streptozotocin-diabetic male rats

Diabetes mellitus is known to affect drug and steroid metabolism in the rat liver. Recently we have demonstrated that in-vitro insulin addition to hepatocytes obtained from normal male rats showed a significant dose-related increase in androstenedione metabolism. We have extended our study this time by using 3- and 21-days streptozotocin (STZ) diabetic and insulin-treated STZ-diabetic male rats. Hepatocytes from 3- and 21-days STZ-diabetic rats were resistant to the effect of insulin while insulin-treated diabetic rats indicated partial restoration of insulin effect. Since insulin resistance is a characteristic feature of type II diabetes mellitus, we would like to suggest that STZ-diabetic rats may be a model for type II diabetes mellitus.

Properties and characterization of low molecular weight inhibitor of calmodulin-dependent cAMP phosphodiesterase from rat liver

We have identified two different species of inhibitors of calmodulin-dependent cAMP phosphodiesterase: 1) a low molecular weight (LMW) and 2) a high molecular weight (HMW) form. These inhibitors are extracted from rat liver. Both LMW and HMW inhibitors are heat-stable, acidic in nature and lose activity with prolonged storage and/or repeated freezing and thawing. The low molecular weight inhibitor has been purified to about 7,000-fold with 300% recovery. LMW inhibits calmodulin-dependent cAMP phosphodiesterase regardless of the source of calmodulin (e.g. fat, brain, heart, erythrocytes). LMW appears to be lipid in nature with a molecular weight of 1,500-5,000. The role of these inhibitors in diabetes and mechanism of action of insulin is presented.

Cyclic nucleotide phosphodiesterase and aggregation in platelets from diabetic rats

Platelet aggregation and cyclic nucleotide (cNCL) phosphodiesterase (PDE) have been studied in a new strain of insulin-dependent spontaneously diabetic rat (SDR). The rate of aggregation of washed platelets induced by ADP or ionophore A23187 was decreased in SDR as compared to asymptomatic littermates. The activity of soluble cGMP-PDE was increased in SDR, while no significant difference was observed between SDR and control in soluble and particulate cAMP-PDE activities nor in particulates cGMP-PDE activity. Furthermore, a kinetic study of soluble cGMP-PDE in platelets demonstrated that the apparent Km was lower while the Vmax was higher in SDR. Increases were also observed in the activities of particulate cAMP-PDE and cGMP-PDE at low and high substrate concentrations in liver and heart of SDR. These anomalies of platelet aggregation and cNCL-PDE in SDR were partially correctable by insulin. For comparison, a similar study was performed in streptozotocin-induced diabetic rats (STZ). In contrast to SDR, the rate of platelet aggregation induced by ADP was increased in STZ, and the activity of soluble cGMP-PDE in platelets was decreased in STZ. A similar decrease in the activities of cAMP-PDE in liver was also observed in STZ. This study confirms observations concerning the decrease of cGMP-PDE in tissues of STZ diabetic rats. However, since opposite anomalies in PDE activity as well as a platelet function were observed in another model of diabetes (SDR), the significance of these anomalies in the pathophysiology of diabetes requires further investigation.