Inhibitory effect of hyperglycemia on insulin-induced Akt/protein kinase B activation in skeletal muscle

To determine the molecular mechanism underlying hyperglycemia-induced insulin resistance in skeletal muscles, postreceptor insulin-signaling events were assessed in skeletal muscles of neonatally streptozotocin-treated diabetic rats. In isolated soleus muscle of the diabetic rats, insulin-stimulated 2-deoxyglucose uptake, glucose oxidation, and lactate release were all significantly decreased compared with normal rats. Similarly, insulin-induced phosphorylation and activation of Akt/protein kinase B (PKB) and GLUT-4 translocation were severely impaired. However, the upstream signal, including phosphorylation of the insulin receptor (IR) and insulin receptor substrate (IRS)-1 and -2 and activity of phosphatidylinositol (PI) 3-kinase associated with IRS-1/2, was enhanced. The amelioration of hyperglycemia by T-1095, a Na(+)-glucose transporter inhibitor, normalized the reduced insulin sensitivity in the soleus muscle and the impaired insulin-stimulated Akt/PKB phosphorylation and activity. In addition, the enhanced PI 3-kinase activation and phosphorylation of IR and IRS-1 and -2 were reduced to normal levels. These results suggest that sustained hyperglycemia impairs the insulin-signaling steps between PI 3-kinase and Akt/PKB, and that impaired Akt/PKB activity underlies hyperglycemia-induced insulin resistance in skeletal muscle.

Insulin prevents cardiomyocytes from oxidative stress-induced apoptosis through activation of PI3 kinase/Akt

BACKGROUND

Loss of cardiomyocytes by apoptosis is proposed to cause heart failure. Reactive oxygen species induce apoptosis in many types of cells including cardiomyocytes. Because insulin has been reported to have protective effects, we examined whether insulin prevents cardiomyocytes from oxidative stress-induced apoptotic death.

METHODS AND RESULTS

Cultured cardiomyocytes of neonatal rats were stimulated by hydrogen peroxide (H(2)O(2)). Apoptosis was evaluated by means of the TUNEL method and DNA laddering. Incubation with 100 micromol/L H(2)O(2) for 24 hours increased the number of TUNEL-positive cardiac myocytes (control, approximately 4% versus H(2)O(2), approximately 23%). Pretreatment with 10(-)(6) mol/L insulin significantly decreased the number of H(2)O(2)-induced TUNEL-positive cardiac myocytes (approximately 12%) and DNA fragmentation induced by H(2)O(2). Pretreatment with a specific phosphatidylinositol 3 kinase (PI3K) inhibitor, wortmannin, and overexpression of dominant negative mutant of PI3K abolished the cytoprotective effect of insulin. Insulin strongly activated both PI3K and the putative downstream effector AKT: Moreover, a proapoptotic protein, BAD:, was significantly phosphorylated and inactivated by insulin through PI3K.

CONCLUSIONS

These results suggest that insulin protects cardiomyocytes from oxidative stress-induced apoptosis through the PI3K pathway.

p110beta is up-regulated during differentiation of 3T3-L1 cells and contributes to the highly insulin-responsive glucose transport activity

Activation of p85/p110 type phosphatidylinositol kinase is essential for aspects of insulin-induced glucose metabolism, including translocation of GLUT4 to the cell surface and glycogen synthesis. The enzyme exists as a heterodimer containing a regulatory subunit (e.g. p85alpha) and one of two widely distributed isoforms of the p110 catalytic subunit: p110alpha or p110beta. In the present study, we compared the two isoforms in the regulation of insulin action. During differentiation of 3T3-L1 cells into adipocytes, p110beta was up-regulated approximately 10-fold, whereas expression of p110alpha was unaltered. The effects of the increased p110 expression were further assessed by expressing epitope tagged p110beta and p110alpha in 3T3-L1 cells using adenovirus transduction systems, respectively. In vitro, the basal lipid kinase activity of p110beta was lower than that of p110alpha. When p110alpha and p110beta were overexpressed in 3T3-L1 adipocytes, exposing cells to insulin induced each of the subunits to form complexes with p85alpha and tyrosine-phosphorylated IRS-1 with similar efficiency. However, whereas the kinase activity of p110beta, either endogenous or exogeneous, was markedly enhanced by insulin stimulation, only very small increases of the activity of p110alpha were observed. Interestingly, overexpression of p110beta increased insulin-induced glucose uptake by 3T3-L1 cells without significantly affecting basal glucose transport, whereas overexpression of p110alpha increased both basal and insulin-stimulated glucose uptake. Finally, microinjection of anti-p110beta neutralizing antibody into 3T3-L1 adipocytes abolished insulin-induced translocation of GLUT4 to the cell surface almost completely, whereas anti-p110alpha neutralizing antibody did only slightly. Together, these findings suggest that p110beta plays a crucial role in cellular activities evoked acutely by insulin.

Antidiabetic effect of T-1095, an inhibitor of Na(+)-glucose cotransporter, in neonatally streptozotocin-treated rats

3-(Benzo[b]furan-5-yl)-2', 6'-dihydroxy-4'-methylpropiophenone-2'-O-(6-O-methoxycarbonyl)-bet a-D -glucopyranoside (T-1095) is a derivative of phlorizin, a potent inhibitor of Na(+)-glucose cotransporters. We determined the antidiabetic effect of T-1095 in neonatally streptozotocin-treated diabetic rats. Orally administered T-1095 is metabolized to an active form, 3-(benzo[b]furan-5-yl)-2', 6'-dihydroxy-4'-methylpropiophenone-2'-O-beta-D-glucopyranoside (T-1095A), which inhibits renal Na(+)-glucose cotransporters as potently as phlorizin in vitro. A single oral administration of T-1095 (30 and 100 mg/kg, p.o.) markedly lowered blood glucose levels with a concomitant increase in urinary glucose excretion; whereas the effect on blood glucose levels in non-diabetic rats was minimal. Continuous administration of T-1095 to diabetic rats for 6 weeks (0.1% in diet) improved not only hyperglycemia, but also the elevation of plasma free fatty acid and plasma ketone body levels. In addition, oral glucose tolerance testing clearly illustrated the improvement of glucose tolerance and insulin secretion with T-1095. In fact, amelioration of impaired insulin sensitivity in diabetic rats was demonstrated by the increase of whole-body and skeletal-muscle insulin-mediated glucose utilization with normalization of muscle glucose transporter (GLUT)4 content, and decrease of the hepatic glucose production rate. Consequently, polyuria and glucosuria were also improved in the T-1095-treated group. Therefore, T-1095 has a therapeutic potential as a means of ameliorating abnormal glucose metabolism via diminished glucose toxicity.

The N-terminal 34 residues of the 55 kDa regulatory subunits of phosphoinositide 3-kinase interact with tubulin

There are five regulatory subunit isoforms of phosphoinositide 3-kinase (PI 3-kinase), which are classified into three groups: proteins of 85 kDa (p85alpha and p85beta), 55 kDa (p55alpha and p55gamma) and 50 kDa (p50alpha). Structural differences between the three groups reside in the N-terminus. To elucidate the unique functional role of the 55 kDa regulatory subunits, GST (glutathione S-transferase) fusion proteins containing a unique N-terminal portion consisting of a 34-amino-acid sequence of p55alpha or p55gamma (GST-p55alpha/gammaN(1-34)) were used as affinity matrices to screen rat brain cell extracts for proteins to which this portion binds specifically. A protein that bound was identified as beta-tubulin by protein sequencing. In addition, not only the beta isoform of tubulin, but also the alpha and gamma isoforms, were detected in the protein absorbed from cell lysates with GST-p55gammaN(1-34) and GST-p55alphaN(1-34) by immunoblotting. Indeed, the only regulatory subunit present in the purified microtubule assembly from rat brain was the 55 kDa isoform; neither 85 kDa nor 50 kDa subunits were detected. These results indicate endogenous binding of 55 kDa regulatory subunits of PI 3-kinase to tubulin in the brain. Finally, we measured tubulin-associated PI 3-kinase activity in CHO/IR cells overexpressing each of the five regulatory subunit isoforms. Only in cells expressing p55alpha or p55gamma was there a significant elevation of tubulin-associated PI 3-kinase activity in response to insulin. These results suggest that the p55alpha and p55gamma regulatory subunits have important roles in regulating PI 3-kinase activity, particularly for microtubules at the cell periphery.

Hyperglycemia contributes insulin resistance in hepatic and adipose tissue but not skeletal muscle of ZDF rats

To determine the contribution of hyperglycemia to the insulin resistance in various insulin-sensitive tissues of Zucker diabetic fatty (ZDF) rats, T-1095, an oral sodium-dependent glucose transporter (SGLT) inhibitor, was administered by being mixed into food. Long-term treatment with T-1095 lowered both fed and fasting blood glucose levels to near normal ranges. A hyperinsulinemic euglycemic clamp study that was performed after 4 wk of T-1095 treatment demonstrated partial recovery of the reduced glucose infusion rate (GIR) in the T-1095-treated group. In the livers of T-1095-treated ZDF rats, hepatic glucose production rate (HGP) and glucose utilization rate (GUR) showed marked recovery, with almost complete normalization of reduced glucokinase/glucose-6-phosphatase (G-6-Pase) activities ratio. In adipose tissues, decreased GUR was also shown to be significantly improved with a normalization of insulin-induced GLUT-4 translocation. In contrast, in skeletal muscles, the reduced GUR was not significantly improved in response to amelioration of hyperglycemia by T-1095 treatment. These results suggest that the contribution of hyperglycemia to insulin resistance in ZDF rats is very high in the liver and considerably elevated in adipose tissues, although it is very low in skeletal muscle.

Hyperglycemia impairs the insulin signaling step between PI 3-kinase and Akt/PKB activations in ZDF rat liver

Akt/PKB activation is reportedly essential for insulin-induced glucose metabolism in the liver. During the hypoinsulinemic and hyperglycemic phase in the Zucker diabetic fatty (ZDF) rat liver, insulin-induced phosphorylations of the insulin receptor (IR) and insulin receptor substrate (IRS)-1/2 were significantly enhanced. Similarly, phosphatidylinositol (PI) 3-kinase activities associated with IRS-1/2 were markedly increased in ZDF rat liver compared with those in the control lean rat liver. However, interestingly, insulin-induced phosphorylation and kinase activation of Akt/PKB were severely suppressed. The restoration of normoglycemia by sodium-dependent glucose transporter (SGLT) inhibitor to ZDF rats normalized elevated PI 3-kinase activation and phosphorylation of IR and IRS-1/2 to lean control rat levels. In addition, impaired insulin-induced Akt/PKB activation was also normalized. These results suggest that chronic hyperglycemia reduces the efficiency of the activation step from PI 3-kinase to Akt/PKB kinase and that this impairment is the molecular mechanism underlying hyperglycemia-induced insulin resistance in the liver.

Localization of the histamine H(2) receptor, a target for antiulcer drugs, in gastric parietal cells

BACKGROUND/AIMS

Histamine H(2) receptor antagonists are widely used for the treatment of peptic ulcer disorders. However, whether the H(2) receptor is present in parietal or immune cells in the lamina propria remains controversial. This study is designed to determine the H(2) receptor localization immunohistochemically using an antibody against the newly cloned mouse histamine H(2) receptor.

METHODS

We cloned the mouse histamine H(2) receptor gene and generated a specific antipeptide antibody against the C terminus. Immunohistochemical studies were performed with this antibody and with a monoclonal antibody against H(+)/K(+) adenosine triphosphatase (ATPase).

RESULTS

Histamine H(2) receptors were localized on the plasma membrane and on the cytoplasm just beneath the plasma membrane on the basolateral sides of gastric cells. Confocal microscopy of double-stained sections using the monoclonal antibody against H(+)/K(+) ATPase, a specific parietal cell marker, showed that histamine H(2) receptors colocalized with H(+)/K(+) ATPase. No specific histamine H(2) receptor immunoreactivities were observed in the submucosal regions.

CONCLUSION

The H(2) receptor is localized in the gastric parietal cell.

Imidapril, an angiotensin-converting enzyme inhibitor, improves insulin sensitivity by enhancing signal transduction via insulin receptor substrate proteins and improving vascular resistance in the Zucker fatty rat

Angiotensin-converting enzyme (ACE) inhibitors are antihypertensive agents, that inhibit the conversion of angiotensin I to angiotensin II, resulting in smooth-muscle relaxation and a reduction of vascular resistance. Recently, it has been suggested that ACE inhibitors improve insulin resistance in diabetic patients. To investigate the effect of an ACE inhibitor on insulin sensitivity, insulin signaling, and circulation, imidapril was administered orally or intraduodenally to Zucker fatty rats. Oral administration of imidapril improved insulin sensitivity based on the results of an oral glucose tolerance test (OGTT) and a decrease in urinary glucose secretion. Phosphatidylinositol 3-kinase (PI 3-kinase) activity associated with hepatic insulin receptor substrate-1 (IRS-1) in the insulin-stimulated condition was significantly enhanced 110% without a significant alteration in tyrosine phosphorylation of IRS-1 in the imidapril-treated group. In muscle, IRS-1 tyrosine phosphorylation and PI 3-kinase activity associated with IRS-1 in the insulin-stimulated condition were enhanced 70% and 20%, respectively, in the imidapril-treated group. In contrast, an alteration of the IRS-2 pathway was observed only in liver; a significant insulin-induced increase in the IRS-2-associated PI 3-kinase over the basal level was observed in the imidapril-treated group but not in the control. In addition, treatment with imidapril was shown to significantly reduce blood pressure and increase blood flow in the liver and muscle. These results suggest that the ACE inhibitor imidapril may improve insulin sensitivity not only by acting directly on the insulin signaling pathway but also by increasing blood flow in tissues via normalization of vascular resistance, a major cause of hypertension.

T-1095, an inhibitor of renal Na+-glucose cotransporters, may provide a novel approach to treating diabetes

T-1095A and T-1095 are synthetic agents derived from phlorizin, a specific inhibitor of Na+-glucose cotransporters (SGLTs). Unlike phlorizin, T-1095 is absorbed into the circulation via oral administration, is metabolized to the active form, T-1095A, and suppresses the activity of SGLTs in the kidney. Orally administered T-1095 increases urinary glucose excretion in diabetic animals, thereby decreasing blood glucose levels. Indeed, the postprandial hyperglycemia after a meal load was shown to be suppressed by this compound in streptozotocin (STZ)-induced diabetic rats. With long-term T-1095 treatment, both blood glucose and HbA1c levels were reduced in STZ-induced diabetic rats and yellow KK mice. In addition, there was amelioration of abnormal carbohydrate metabolism, i.e., hyperinsulinemia and hypertriglyceridemia, and of the development of microalbuminuria, in yellow KK mice. Thus, T-1095 may be a useful antidiabetic drug, providing a novel therapeutic approach for diabetes.

p85/p110-type phosphatidylinositol kinase phosphorylates not only the D-3, but also the D-4 position of the inositol ring

Activation of p85/p110-type phosphatidylinositol (PI) kinase has been implicated in various cellular activities. This PI kinase phosphorylates the D-4 position with a similar or higher efficiency than the D-3 position when trichloroacetic acid-treated cell membrane is used as a substrate, although it phosphorylates almost exclusively the D-3 position of the inositol ring in phosphoinositides when purified PI is used as a substrate. Furthermore, the lipid kinase activities of p110 for both the D-3 and D-4 positions were completely abolished by introducing kinase-dead point mutations in their lipid kinase domains (DeltaKinalpha and DeltaKinbeta, respectively). In addition, both PI 3- and PI 4-kinase activities of p110alpha and p110beta immunoprecipitates were similarly inhibited by either wortmannin or LY294002, specific inhibitors of p110. Insulin induced phosphorylation of not only the D-3 position, but also the D-4 position. Indeed, overexpression of p110 in Sf9 or 3T3-L1 cells induced marked phosphorylation of the D-4 position to a level comparable to or much greater than that of D-3, whereas inhibition of endogenous p85/p110-type PI kinase via overexpression of dominant-negative p85alpha (Deltap85alpha) in 3T3-L1 adipocytes abolished insulin-induced synthesis of both. Thus, p85/p110-type PI kinase phosphorylates the D-4 position of phosphoinositides more efficiently than the D-3 position in vivo, and each of the D-3- or D-4-phosphorylated phosphoinositides may transmit signals downstream.

Enhanced insulin-stimulated activation of phosphatidylinositol 3-kinase in the liver of high-fat-fed rats

Insulin receptor substrate (IRS)-1 and IRS-2, which mediate phosphatidylinositol (PI) 3-kinase activation, play essential roles in insulin-induced translocation of GLUT4 and in glycogen synthesis. In this study, we investigated the process of PI 3-kinase activation via binding with IRS-1 and -2 in liver, muscle, and fat of high-fat-fed rats, a model of insulin-resistant diabetes. In the liver of high-fat-fed rats, insulin increased the PI 3-kinase regulatory subunit p85alpha and the PI 3-kinase activities associated with IRS-1 3.6- and 2.4-fold, and with IRS-2, 4.7- and 3.0-fold, respectively, compared with those in control rats. The tyrosine phosphorylation levels of IRS-1 and IRS-2 were not significantly altered, however. In contrast with the liver, tyrosine phosphorylation levels and associated PI 3-kinase proteins and activities were decreased in the muscle and adipose tissue of high-fat-fed rats. Thus, high-fat feeding appears to cause insulin resistance in the liver by a mechanism different from the impaired PI 3-kinase activation observed in muscle and adipose tissue. Taking into consideration that hepatic PI 3-kinase activation is severely impaired in obese diabetic models such as Zucker fatty rats, it is possible that the mechanism by which a high-fat diet causes insulin resistance is quite different from that associated with obesity and overeating due to abnormality in the leptin system. This is the first report to show increased PI 3-kinase activation by insulin in an insulin-resistant diabetic animal model. These findings may be important for understanding the mechanism of insulin resistance in human NIDDM, since a high-fat diet is considered to be one of the major factors exacerbating insulin insensitivity in humans.

Varicella pneumonia in a healthy adult presenting with severe respiratory failure

We describe a case of varicella pneumonia in a 24-year-old healthy man presenting with severe respiratory failure. A chest radiograph showed diffuse, bilateral airspace consolidation; additional complications included liver dysfunction and thrombocytopenia. However, treatment with intravenous acyclovir and gamma-globulin improved his clinical symptoms and signs. A greater than four-fold change in paired titers of the varicella-zoster virus antibody was observed. Bronchoalveolar lavage performed during the recovery phase revealed increased total cell and lymphocyte counts and a decreased CD4:CD8 ratio of T lymphocytes. Transbronchial lung biopsy findings were compatible with a diagnosis of interstitial pneumonia.

Factors affecting quality of ulcer healing after lansoprazole treatment

To evaluate endogenous and exogenous factors affecting the quality of ulcer healing produced by proton pump inhibitors, gastric acid pH, serum gastrin, and serum pepsinogen (PG) I and II were measured in peptic ulcer patients before and after treatment with lansoprazole 30 mg once daily. Lansoprazole achieved more rapid scarring in duodenal ulcer (n = 34), with a healing rate of 97.1% after 6 weeks, than in gastric ulcer (n = 56), with a healing rate of 92.8% after 8 weeks. Scarring was the most rapid in gastroduodenal ulcer (n = 8), with a healing rate of 100% after 8 weeks, but the rate of complete scarring was the lowest (37.5%). Lower gastric acidity and lower PG I:II ratio were associated with poor quality ulcer scarring in patients with gastric ulcers, but the opposite was true for those with duodenal and gastroduodenal ulcers. For gastric ulcers, not only ulcer size but also mucosal atrophy was an important factor in ulcer healing. Smoking and alcohol consumption had little effect on the quality of ulcer healing during treatment. These results suggest that there are a number of differences between gastric ulcers and duodenal ulcers in terms of the quality of ulcer healing after lansoprazole treatment.

Neural inhibition of gastrointestinal smooth muscle motility in guinea-pigs

The neural inhibition of the gastrointestinal (GI) smooth muscle motility was studied by means of electrical transmural nerve stimulation (ETNS) in guinea-pigs. In untreated muscle strips, ETNS induced four types of response, consisting of three basic components, i.e., contraction and relaxation during ETNS, and after-contraction. Following atropinization, all the responses changed to only one type, i.e., relaxation during ETNS followed by after-contraction. The relaxation of the muscle strips induced by ETNS after atropinization was not significantly reduced under superimposing guanethidine treatment in any site of the GI tract. The maximal relaxation of the muscle strips induced by ETNS after atropine and guanethidine treatments was not uniform throughout the GI tract. The relaxation of the gastric body and colon was greater than that of the jejunum and ileum. The extent of the relaxation was significantly different even in the colon. Theophylline and phentolamine did not reduce the ETNS-induced relaxation following atropine and guanethidine treatments in the distal colon and taenia coli. The findings suggest the following: the non-adrenergic inhibitory (NAI) nerve is the main postganglionic nerve to inhibit the GI smooth muscle motility; and the role of adrenergic nerve is of little importance in respect to postganglionic inhibition. The innervation of the NAI nerve seems not to be uniform throughout the GI tract. Theophylline and phentolamine are not the specific antagonists of the NAI nerve.

Ulcerative colitis complicated by idiopathic central serous chorioretinopathy with bullous retinal detachment

A 44-year-old woman, hospitalized with severe diarrhea due to ulcerative colitis, was found to have a rare ocular complication, idiopathic central serous chorioretinopathy with bullous retinal detachment. The ocular symptom started with the flare-up of the ulcerative colitis. Steroid therapy was not effective on the ocular disorder, while her ulcerative colitis became dormant. Relationship between the two disorders is discussed.