Boswellic extracts and 11-keto-ß-boswellic acids prevent type 1 and type 2 diabetes mellitus by suppressing the expression of proinflammatory cytokines.

BACKGROUND: Type 1 diabetes is an autoimmune disease directed to the pancreatic islets where inflammation leads to the death of insulin-producing ß cells and insulin deficiency. Type 2 diabetes, which is closely related to overweight, is characterized by insulin resistance. In both cases, proinflammatory cytokines play an important role by causing insulitis and insulin resistance. The gum resin of Boswellia species and its pharmacologically active compounds, including 11-keto-ß-boswellic acids have been shown to suppress the expression of proinflammatory cytokines in various immune-competent cells. PURPOSE: To review the present evidence of the therapeutic effects of boswellic extracts (BE) and/or 11-keto-ß-boswellic acids in the prevention/treatment of diabetes mellitus and to provide comprehensive insights into the underlying molecular mechanisms. METHODS: This review considers all available informations from preclinical and clinical studies concerning BEs, 11-keto-ß-boswellic acids, proinflammatory cytokines and diabetes mellitus collected via electronic search (PubMed) and related publications of the author. RESULTS: Type 1 diabetes: Studies in mice with autoimmune diabetes revealed that in the model of multiple injections of low doses of streptozotocin (MLD-STZ), an extract of the gum resin of Boswellia serrata and 11-keto-ß-boswellic acid (KBA) suppressed the increase in proinflammatory cytokines in the blood, infiltration of lymphocytes into pancreatic islets and increase in blood glucose. In a second model, i.e. the nonobese diabetic (NOD) mouse, KBA prevented the infiltration of lymphocytes into pancreatic islets. Regarding the clinical effects, a case report provided evidence that BE suppressed the blood levels of tyrosine phosphatase antibody (IA2-A), a marker for insulitis, in a patient with late-onset autoimmune diabetes of the adult (LADA). Type 2 diabetes: In a preclinical study in rats where obesity was alimentary induced, the administration of BE significantly reduced food intake, overweight, proinflammatory cytokines such as interleukin-1ß (IL-1ß), and tumor necrosis factor-α (TNF-α) and ameliorated the parameters of glucose and lipid metabolism. Similar results were obtained in a second animal study, where type 2 diabetes was induced by a combination of a high-fat/high-fructose diet and a single dose of streptozotocin. Two clinical trials with patients with type 2 diabetes receiving the resin of Boswellia serrata demonstrated improvement in the blood glucose, HbA1c and lipid parameters. CONCLUSION: Preclinical and clinical data suggest that BE and/or 11-keto-ß-boswellic acids by inhibiting the expression of proinflammatory cytokines from immune-competent cells, may prevent insulitis and insulin resistance in type 1 and type 2 diabetes, respectively, and therefore may be an option in the treatment/prevention of type 1 and type 2 diabetes. It is hypothesized that molecularly, BE and 11-keto-ß-boswellic acids act via interference with the IκB kinase/Nuclear Transcription Factor-κB (IKK/NF-κB) signaling pathway through inhibition of the phosphorylation activity of IKK. However, further investigations and well-designed clinical studies are required.

Boswellic Acids and Their Role in Chronic Inflammatory Diseases.

Boswellic acids, which are pentacyclic triterpenes belong to the active pharmacological compounds of the oleogum resin of different Boswellia species. In the resin, more than 12 different boswellic acids have been identified but only KBA and AKBA received significant pharmacological interest. Biological Activity: In an extract of the resin of Boswellia species multiple factors are responsible for the final outcome of a therapeutic effect, be it synergistic or antagonistic. Moreover, the anti-inflammatory actions of BAs are caused by different mechanisms of action. They include inhibition of leukotriene synthesis and to a less extend prostaglandin synthesis. Furthermore inhibition of the complement system at the level of conversion of C3 into C3a and C3b. A major target of BAs is the immune system. Here, BEs as well as BAs including KBA and AKBA, have been shown to decrease production of proinflammatory cytokines including IL-1, IL-2, IL-6, IFN-γ and TNF-α which finally are directed to destroy tissues such as cartilage, insulin producing cells, bronchial, intestinal and other tissues. NFĸB is considered to be the target of AKBA. The complex actions of BEs and BAs in inflamed areas may be completed by some effects that are localized behind the inflammatory process as such tissue destruction. In this case, in vitro- and animal studies have shown that BAs and BEs suppress proteolytic activity of cathepsin G, human leucocyte elastase, formation of oxygen radicals and lysosomal enzymes. PHARMACOKINETICS: Whereas KBA is absorbed reaching blood levels being close to in vitro IC50, AKBA which is more active in in vitro studies than KBA, but undergoes much less absorption than KBA. However, absorption of both is increased more than twice when taken together with a high-fat meal.Clinical Studies There are a variety of chronic inflammatory diseases which respond to treatment with extracts from the resin of Boswellia species. Though, the number of cases is small in related clinical studies, their results are convincing and supported by the preclinical data. These studies include rheumatoid arthritis, osteoarthritis, chronic colitis, ulcerative colitis, collagenous colitis, Crohn's disease and bronchial asthma. It can not be expected that there is cure from these diseases but at least improvement of symptoms in about 60-70 % of the cases. Side Effects The number and severity of side effects is extremely low. The most reported complaints are gastrointestinal symptoms. Allergic reactions are rare. And most authors report, that treatment with BEs is well tolerated and the registered side effects in BE- and placebo groups are similar.

11-Keto-ß-Boswellic Acids Prevent Development of Autoimmune Reactions, Insulitis and Reduce Hyperglycemia During Induction of Multiple Low-Dose Streptozotocin (MLD-STZ) Diabetes in Mice.

The aim of the work was to study whether or not 11-keto-ß-boswellic acids prevent induction of autoimmune reactions, insulitis, and hyperglycemia in the model of multiple low-dose streptozotocin (MLD-STZ) diabetes. Using male mice (n = 6) diabetes was induced by daily i.p. injections of 40 mg/kg STZ for 5 days. In a second series together with STZ, daily i. p. injections of 11-keto-ß-boswellic acid (KBA) and O-acetyl-11-keto-ß-boswellic acid (AKBA) (7.5 and 15.0 mg/kg) were applied for 10 days. Thereafter, pro-and anti-inflammatory cytokines in the blood, histochemistry of pancreatic islets, and blood glucose levels were assayed. Five days after the last injection of STZ, a significant burst of pro-and anti-inflammatory cytokines in the blood, infiltration of lymphocytes (CD3) into pancreatic islets, and appearance of peri-insular apoptotic cells were observed. Plasma glucose increased significantly (124.4 ± 6.65 vs. 240.2 ± 27.36 mg/dl, p <0.05). Simultaneous treatment with KBA and AKBA significantly reduced pro-and anti-inflammatory cytokines (IFN-γ p < 0.01, p < 0.01; IL-1A p < 0.001, p < 0.001; IL-1B p < 0.001, p < 0.001; IL-2 p < 0.001, p < 0.001; IL-6 p < 0.01, p < 0.001; TNF-α p < 0.05, p < 0.001; IL-4 p < 0.01, p < 0.001; IL-10 p < 0.001, p < 0.001) in the blood. No infiltration of lymphocytes into pancreatic islets and appearance of peri-insular cells were detected. Moreover, KBA and AKBA reduced STZ-mediated increase of blood glucose on day 10 to 163.25 ± 16.6 (p < 0.05) and 187.6 ± 19.5 mg/dl (p < 0.05), respectively. In the model of MLD-STZ induced diabetes KBA and AKBA prevent cytokine burst, development of insulitis and reduce increase of blood glucose through "silencing" a forced-up immune reaction.

Prevention of multiple low-dose streptozotocin (MLD-STZ) diabetes in mice by an extract from gum resin of Boswellia serrata (BE).

Type 1-diabetes is an autoimmune disease, where a chronic inflammatory process finally causes ß-cell death and insulin deficiency. Extracts from gum resin of Boswellia serrata (BE) have been shown to posses anti-inflammatory properties especially by targeting factors/mediators related to autoimmune diseases. Multiple low dose-streptozotocin (MLD-STZ) treatment is a method to induce diabetes in animals similar to Type 1 diabetes in humans. It was aimed to study whether or not a BE could prevent hyperglycemia, inflammation of pancreatic islets and increase of proinflammatory cytokines in the blood in MLD-STZ treated mice. In BK+/+ wild type mice, 5 days of daily treatment with 40 mg/kg STZ i.p. produced permanent increase of blood glucose, infiltration of lymphocytes into pancreatic islets (CD3-stain), apoptosis of periinsular cells (staining for activated caspase 3) after 10 days as well as shrinking of islet tissue after 35 days (H&E staining). This was associated with an increase of granulocyte colony stimulating factor (G-CSF), granulocyte/macrophage colony stimulating factor (GM-CSF) and proinflammatory cytokines (IL-1A, IL-1B, IL-2, IL-6, IFN-γ, TNF-α) in the blood. Whereas BE alone did not affect blood glucose in non diabetic mice, in STZ treated mice simultaneous i.p. injection of 150 mg/kg of BE over 10 days prevented animals from increase of blood glucose levels. Histochemical studies showed, that i.p. injection of 150 mg/kg BE for 10 days starting with STZ treatment, avoided lymphocyte infiltration into islets, apoptosis of periinsular cells and shrinking of islet size 35 days after STZ. As far as the cytokines tested are concerned, there was a significant inhibition of the increase of G-CSF and GM-CSF. BE also significantly prevented the increase of IL-1A, IL-1B, IL-2, IL-6, IFN-γ and TNF-α. It is concluded that extracts from the gum resin of Boswellia serrata prevent islet destruction and consequent hyperglycemia in an animal model of type 1 diabetes probably by inhibition of the production/action of cytokines related to induction of islet inflammation in an autoimmune process.

Modulation of the immune system by Boswellia serrata extracts and boswellic acids.

Extracts from the gum resin of Boswellia serrata and some of is constituents including boswellic acids affect the immune system in different ways. Among the various boswellic acids 11-keto-beta-boswellic acid (KBA) and acetyl-11-keto-beta-boswellic acid have been observed to be active. However, also other boswellic acids may exhibit actions in the immune system. In the humoral defence system a mixture of boswellic acis at higher doses reduced primary antibody titres; on the other hand lower doses enhanced secondary antibody titres following treatment with sheep erythrocytes. In the cellular defence boswellic acides appear to increase lymphocyte proliferation whereas higher concentrations are even inhibitory. Moreover, BAs increase phagocytosis of macrophages. BAs affect the cellular defence system by interaction with production/release of cytokines. Thus, BAs inhibit activation of NFkappaB which is a product of neutrophile granulocytes. Consequently a down regulation of TNF-alpha and decrease of IL-1, IL-2, IL-4, IL-6 and IFN-gamma, which are proinflammatory cytokines by BEs and BAs has been reported. Suppressions of the classic way of the complement system was found to be due to inhibition of the conversion of C3 into C3a and C3b. However, which of these pharmacological actions contribute to the therapeutic effects and which is finally the best dosage of a standardized extract needs further examination. And it is also a question whether or not a single BA will have the same therapeutic effect as a standardized extract. Among the mediators of inflammatory reaction, mast cell stabilisation has been described by a BE. Inhibition of prostaglandin synthesis appears to play only a minor role as far as the anti-inflammatory effect is concerned. On the other hand the inhibitory action of BAs on 5-LO leading to a decreased production of leukotrienes has received high attention by the scientific community since a variety of chronic inflammatory diseases is associatied with increased leukotriene activity. At the end of the cascade of events in the cellular immune system as far as it directs to various tissues of the body - i.e. autoimmune diseases - formation of oxygen radicals and proteases (for example elastase) play an important destructive role. Here, BEs as well as BAs have been found to be inhibitory. From the pharmacological properties of BEs and BAs it is not surprising that positive effects of BEs in some chronic inflammatory diseases including rheumatoid arthritis, bronchial asthma, osteoarthritis, ulcerative colitis and Crohn's disease have been reported.

[Involving the pharmacist in the management and care of diabetics]. / Diabetiker-Versorgung in Deutschland. Soil man die Apotheker mit einbinden, und wenn ja, wie?

Diabetes mellitus is an endemic disease in all countries of the world, with the type 2 form reaching epidemic proportions. In addition to treatment by the physician, it necessitates a considerable effort in terms of providing victims with training, care and information. Within this concept, pharmaceutical care by the pharmacist in the area of prevention must be regarded as multifaceted. In this connection his activities must be strictly differentiated from those of the physician. With regard to the qualifications needed, the pharmacist must supplement the knowledge gained during his studies by intensive courses in the field of diabetology. These are organized by the regional Chamber of Pharmacists and the regional offices of the German Diabetes Association (DDG). Thanks to the fact that he is readily accessible and well qualified, the pharmacist can thus make a major contribution to the individual management of the diabetic.

Boswellic acids in chronic inflammatory diseases.

Oleogum resins from BOSWELLIA species are used in traditional medicine in India and African countries for the treatment of a variety of diseases. Animal experiments showed anti-inflammatory activity of the extract. The mechanism of this action is due to some boswellic acids. It is different from that of NSAID and is related to components of the immune system. The most evident action is the inhibition of 5-lipoxygenase. However, other factors such as cytokines (interleukins and TNF-alpha) and the complement system are also candidates. Moreover, leukocyte elastase and oxygen radicals are targets. Clinical studies, so far with pilot character, suggest efficacy in some autoimmune diseases including rheumatoid arthritis, Crohn's disease, ulcerative colitis and bronchial asthma. Side effects are not severe when compared to modern drugs used for the treatment of these diseases.

Spasmolytic and tonic effect of Iberogast (STW 5) in intestinal smooth muscle.

STW 5 (Iberogast) is a fixed combination of nine medicinal plant extracts effective in the treatment of functional dyspepsia and irritable bowel syndrome. The effects of STW 5, a combination of Iberis amara fresh plant extract, and other eight plant extracts as well as single extract components including extracts from Menthae piperitae folium, Matricariae flos and Liquiritiae radix, were assayed in guinea pig ileum with or without stimulation with acetylcholine or histamine, in order to find a possible effect on the contractility of intestinal smooth muscle. STW 5 decreased acetylcholine- and histamine-induced contraction of guinea pig ileum. This was also true for extracts of Menthae piperitae folium, Matricariae flos and L. radix. Extract from I. amara, however, showed no spasmolytic action; in contrary, it increased the basal resting tone and contraction of atonic ileal segments. This was also true when STW 5 was employed. A spasmolytic action of STW 5 could also be observed in duodenum, jejunum and colon. These data are the first to show not only the spasmolytic effects of STW 5 and its component extracts in intestinal muscle but also the tonicising effects of STW 5 through its component Iberis amara extract in relaxed intestinal muscle. Thus, pharmacological evidence suggests a dual-action principle and may explain, at least in part, the clinically observed therapeutic efficacy of STW 5 (Iberogast) in both hypotonic and spastic dysmotility symptoms of functional dyspepsia and irritable bowel syndrome.

Encapsulation of islets in rough surface, hydroxymethylated polysulfone capillaries stimulates VEGF release and promotes vascularization after transplantation.

The transplantation of encapsulated islets of Langerhans is one approach to treat type 1 diabetes without the need of lifelong immunosuppression. Capillaries have been used for macroencapsulation because they have a favorable surface-to-volume ratio and because they can be refilled. It is unclear at present whether the outer surface of such capillaries should be smooth to prevent, or rough to promote, cell adhesions. In this study we tested a new capillary made of modified polysulfone (MWCO: 50 kDa) with a rough, open-porous outer surface for islet transplantation. Compared with free-floating islets, encapsulation of freshly isolated rat islets affected neither the kinetics nor the efficiency of glucose-induced insulin release in perifusion experiments. Free-floating islets maintained insulin secretion during cell culture but encapsulated islets gradually lost their glucose responsiveness and released VEGF. This indicated hypoxia in the capillary lumen. Transplantation of encapsulated rat islets into diabetic rats significantly reduced blood glucose concentrations from the first week of implantation. This hypoglycaemic effect persisted until explantation 4 weeks later. Transplantation of encapsulated porcine islets into diabetic rats reduced blood glucose concentrations depending on the islet purity. With semipurified islets a transient reduction of blood glucose concentrations was observed (2, 8, 18, 18 days) whereas with highly purified islets a sustained normoglycaemia was achieved (more than 28 days). Explanted capillaries containing rat islets were covered with blood vessels. Vascularization was also observed on capillaries containing porcine islets that were explanted from normoglycaemic rats. In contrast, on capillaries containing porcine islets that were explanted from hyperglycemic rats a fibrous capsule and lymphocyte accumulations were observed. No vascularization on the surface of transplanted capillaries was observed in the absence of islets. In conclusion, encapsulated islets can release VEGF, which appears to be an important signal for the vascularization of the capillary material. The rough, open-porous outer surface of the polysulfone capillary provides a site well suited for vascular tissue formation and may allow a prolonged islet function after transplantation.

Encapsulation of Islets in Rough Surface, Hydroxymethylated Polysulfone Capillaries Stimulates VEGF Release and Promotes Vascularization after Transplantation.

The transplantation of encapsulated islets of Langerhans is one approach to treat type 1 diabetes without the need of lifelong immunosuppression. Capillaries have been used for macroencapsulation because they have a favorable surface-to-volume ratio and because they can be refilled. It is unclear at present whether the outer surface of such capillaries should be smooth to prevent, or rough to promote, cell adhesions. In this study we tested a new capillary made of modified polysulfone (MWCO: 50 kDa) with a rough, open-porous outer surface for islet transplantation. Compared with free-floating islets, encapsulation of freshly isolated rat islets affected neither the kinetics nor the efficiency of glucose-induced insulin release in perifusion experiments. Free-floating islets maintained insulin secretion during cell culture but encapsulated islets gradually lost their glucose responsiveness and released VEGF. This indicated hypoxia in the capillary lumen. Transplantation of encapsulated rat islets into diabetic rats significantly reduced blood glucose concentrations from the first week of implantation. This hypoglycaemic effect persisted until explantation 4 weeks later. Transplantation of encapsulated porcine islets into diabetic rats reduced blood glucose concentrations depending on the islet purity. With semipurified islets a transient reduction of blood glucose concentrations was observed (2, 8, 18, 18 days) whereas with highly purified islets a sustained normoglycaemia was achieved (more than 28 days). Explanted capillaries containing rat islets were covered with blood vessels. Vascularization was also observed on capillaries containing porcine islets that were explanted from normoglycaemic rats. In contrast, on capillaries containing porcine islets that were explanted from hyperglycemic rats a fibrous capsule and lymphocyte accumulations were observed. No vascularization on the surface of transplanted capillaries was observed in the absence of islets. In conclusion, encapsulated islets can release VEGF, which appears to be an important signal for the vascularization of the capillary material. The rough, open-porous outer surface of the polysulfone capillary provides a site well suited for vascular tissue formation and may allow a prolonged islet function after transplantation.

Areal density measurement is a convenient method for the determination of porcine islet equivalents without counting and sizing individual islets.

The determination of islet mass is important for the normalization of islet experiments in the laboratory and for the precise dosing of islets for transplantation. The common microscopical analysis is based on individual islet sizing, calculation of the frequency distribution, and conversion into islet equivalents (IEQ), which is the volume of a spherical islet with a diameter of 150 microm. However, islets are of irregular form, which makes this determination user dependent, and the analysis is irreproducible once the original sample is discarded. This routine technique of islet quantification was compared with the analysis of areal density measurements. It was assumed that the entire area occupied by islets can be expressed in IEQ without sizing and counting individual islets. Porcine islets were isolated by continuous digestion/filtration and purified by gradient centrifugation. Purified islets were stained with dithizone and were repeatedly pictured under the microscope with random area selection. A total of 51 pictures was taken from 11 different purifications and stained islets were detected by digital image analysis. The correlation coefficient (r) between bothanalyses was 0.977 with an underestimation of islet yield by areal density detection (slope: 0.75 +/- 0.03). Areal density analysis per picture took about 1 min, which is about 10 times faster than the traditional method without increasing the method error (CV 2.1% vs. 2.7%). In summary, areal density measurements allow a rapid and reproducible estimation of IEQ without counting individual islets. It can be performed in a single step analysis without computer programming and is valuable for online determinations of islet yield preceding transplantation.

Glucose oversupply increases Delta9-desaturase expression and its metabolites in rat skeletal muscle.

AIM/HYPOTHESIS: Previous studies have shown that prolonged glucose infusion causes insulin resistance and triglyceride accumulation in rat skeletal muscle. In this study, we investigated a possible relationship between insulin resistance and the composition of different accumulated lipid fractions in rat skeletal muscle. METHODS: Continuous glucose infusion was carried out in rats for 7 days. Lipids were extracted from skeletal muscle, separated by thin layer chromatography and fatty acid composition of phospholipids, triglycerides, diglycerides, free fatty acids and cholesterol esters fractions was analysed by gas chromatography. Delta9-Desaturase mRNA was measured by real time polymerase chain reaction. The enzyme activity was measured in the microsomal fractions. RESULTS: Prolonged glucose infusion (5 days) increased the relative content of palmitoleic acid (16:1 N7) several-fold (2.3- to 5.8-fold) in four out of five lipid fractions and enhanced oleic acid (18:1 N9) two-fold in three lipid fractions suggesting increased Delta9-desaturase activity while the content of several polyunsaturated fatty acids was reduced. In parallel, Delta9-Desaturase mRNA contents and enzyme activities in skeletal muscle were increased 10-fold, 75-fold, 2.6-fold and 7.7-fold after 2 and 5 days of glucose infusion, respectively. CONCLUSION/INTERPRETATION: Our results suggest that long-term glucose oversupply induces a rapid increase in Delta9-desaturase expression and enzyme activity in skeletal muscle which leads to fast and specific changes in fatty acid metabolism possibly contributing to the insulin resistance in this animal model.

[Boswellic acids (components of frankincense) as the active principle in treatment of chronic inflammatory diseases]. / Boswelliasäuren (Inhaltsstoffe des Weihrauchs) als wirksame Prinzipien zur Behandlung chronisch entzündlicher Erkrankungen.

Preparations from the gum resin of Boswellia serrata have been used as a traditional remedy in Ayurvedic medicine in India for the treatment of inflammatory diseases. Compounds from the gum with genuine antiinflammatory effects are pentacyclic triterpenes of the boswellic acid type. Boswellic acids inhibit the leukotriene biosynthesis in neutrophilic granulocytes by a non-redox, noncompetitive inhibition of 5-lipoxygenase. The effect is triggered by boswellic acids binding to the enzyme. Moreover certain boswellic acids have been described to inhibit elastase in leukocytes, to inhibit proliferation, induce apoptosis and to inhibit topoisomerases of leukoma- and glioma cell lines. A series of chronic inflammatory diseases are thought to be perpetuated by leukotrienes. In clinical trials promising results were observed in patients with rheumatoid arthritis, chronic colitis, ulcerative colitis, Crohn's disease, bronchial asthma und peritumoral brains edemas.

The hypoglycemic effect of Nigella sativa oil is mediated by extrapancreatic actions.

A plant mixture containing extracts of Nigella sativa possesses blood glucose lowering effects, but the direct antidiabetic effect of Nigella sativa is not yet established. Therefore, the effect of Nigella sativa oil (NSO) on blood glucose concentrations was studied in streptozotocin diabetic rats. In addition, the effect of NSO, nigellone and thymoquinone were studied on insulin secretion of isolated rat pancreatic islets in the presence of 3, 5.6 or 11.1 mM glucose. NSO significantly lowered blood glucose concentrations in diabetic rats after 2, 4 and 6 weeks. The blood lowering effect of NSO was, however, not paralleled by a stimulation of insulin release in the presence of NSO, nigellone or thymoquinone. The data indicate that the hypoglycemic effect of NSO may be mediated by extrapancreatic actions rather than by stimulated insulin release.

Nigella sativa oil, nigellone and derived thymoquinone inhibit synthesis of 5-lipoxygenase products in polymorphonuclear leukocytes from rats.

In the present study, Nigella sativa oil (NSO), nigellone (polythymoquinone) and derived thymoquinone were studied to evaluate their effect on the formation of 5-lipoxygenase (5-LO) products from polymorphonuclear leukocytes (PMNL).NSO produced a concentration dependent inhibition of 5-LO products and 5-hydroxy-eicosa-tetra-enoic acid (5-HETE) production with half maximal effects (IC(50)) at 25+/-1 micro g/ml, respectively 24+/-1 micro g/ml. Nigellone caused a concentration-related inhibition of 5-HETE production (IC(50): 11.9+/-0.3 micro g/ml). Moreover thymoquinone, the active principle of NSO inhibited the production of 5-LO products (IC(50): 0.26+/-0.02 micro g/ml) and 5-HETE production (IC(50): 0.36+/-0.02 micro g/ml) in a similar way. The effects are probably due to an antioxidative action. The data may in part explain the effect of the oil, its derived thymoquinone and nigellone in ameliorating inflammatory diseases.

Prolonged glucose infusion into conscious rats inhibits early steps in insulin signalling and induces translocation of GLUT4 and protein kinase C in skeletal muscle.

AIMS/HYPOTHESIS: Previous studies on diabetic patients have shown that hyperglycaemia increases glucose uptake in an apparently insulin-independent manner. However, the molecular mechanism has not been clarified. METHODS: We studied rats receiving continuous glucose infusion to address this question. In this animal model, rats accommodate systemic glucose oversupply and rapidly develop insulin resistance. RESULTS: Glucose infusion increased both plasma glucose and insulin concentrations to peak after one day. In spite of continuous glucose infusion normoglycaemia was reached after 5 days while insulin concentrations remained higher. Focusing our studies in day 2 (hyperglycaemia/hyperinsulinaemia) and day 5 (normoglycaemia/hyperinsulinaemia) we found, particularly in day 5, that the early steps of the insulin signalling cascade in skeletal muscle of glucose-infused rats were not more activated when compared to control animals as assessed by a comparable phosphorylation of the insulin receptor, IRS-1 and PKB and by an unaltered IRS-1-associated Ptd(Ins) 3' kinase activity. Continuous glucose infusion induced GLUT4 protein expression and translocation to the plasma membrane while neither expression nor translocation of GLUT1 was affected. Translocation of PKC- betaI, - betaII (> threefold) and -alpha, -theta (to a lesser extent) to the plasma membrane was significantly induced after 2 days but not after 5 days of glucose infusion when normoglycaemia was reached. CONCLUSIONS/INTERPRETATION: Our data support the hypothesis that continuous glucose infusion induces translocation of GLUT4 while the early steps of the insulin signalling cascade were not increased. These effects could be mediated by activation of PKC.