Absence of the β1 subunit of AMP‐activated protein kinase reduces myofibroblast infiltration of the kidneys in early diabetes

Activation of the heterotrimeric energy‐sensing kinase AMP‐activated protein kinase (AMPK) has been reported to improve experimental diabetic kidney disease. We examined the effect of type 1 diabetes in wild‐type (WT) mice and mice lacking the β1 subunit of AMPK (AMPK β1^?/? mice), which have reduced AMPK activity in kidneys and other organs. Diabetes was induced using streptozotocin (STZ) and the animals followed up for 4 weeks. Hyperglycaemia was more severe in diabetic AMPK β1^?/? mice, despite the absence of any difference in serum levels of insulin, adiponectin and leptin. There was no change in AMPK activity in the kidneys of diabetic WT mice by AMPK activity assay, or phosphorylation of either the αT172 activation site on the α catalytic subunit of AMPK or the AMPK‐specific phosphosite S79 on acetyl CoA carboxylase 1 (ACC1). Phosphorylation of the inhibitory αS485 site on the α subunit of AMPK was significantly increased in the WT diabetic mice compared to non‐diabetic controls. Despite increased plasma glucose levels in the diabetic AMPK β1^?/? mice, there were fewer myofibroblasts in the kidneys compared to diabetic WT mice, as evidenced by reduced α‐smooth muscle actin (α‐SMA) protein by Western blot, mRNA by qRT‐PCR and fewer α‐SMA‐positive cells by immunohistochemical staining. Albuminuria was also reduced in the AMPK β1^?/? mice. In contrast to previous studies, therefore, myofibroblasts were reduced in the kidneys of AMPK β1^?/? diabetic mice compared to diabetic WT mice, despite increased circulating glucose, suggesting that AMPK can worsen renal fibrosis in type 1 diabetes.     

B‐1 cells produce insulin and abrogate experimental streptozotocin‐induced diabetes

The participation of B‐1 cells in a murine model of spontaneous diabetes has been recently reported. Here, we describe the role of B‐1 cells in streptozotocin (STZ) induced diabetes in mice. We demonstrated that XID (B‐1 cell‐deficient) mice are more susceptible to STZ treatment than WT mice, as evidenced by their higher blood glucose level in response to STZ. Unexpectedly, the XID mice that were i.p. transferred with purified B‐1 cells, either before or after the STZ treatment, did not develop diabetes. These cell transfers provided long‐lasting protection for the XID mice against STZ‐induced diabetes, suggesting that B‐1 cells play an important role in the experimental diabetes pathobiology. We also showed that B‐1 cell culture supernatants were able to regulate the blood glucose level of the diabetic XID mice, and we identified insulin‐producing cells when B‐1 cells were differentiated in B‐1 cell‐derived phagocyte in vitro. These findings provide a novel role for B‐1 cells in metabolic processes, presenting a new mechanism to explain the pathogenesis of diabetes and a possible therapeutical target.     

Insulin binding and glucose metabolism in adipocytes of streptozotocin-diabetic rats.

To investigate the mechanism of the cellular insulin insensitivity of diabetic rats, insulin binding, glucose transport, and glucose oxidation were studied in adipocytes from streptozotocin-diabetic rats. Increased insulin binding was found in cells from diabetic rats, and this was due to an increased number of insulin receptors rather than a change in receptor affinity. Basal and insulin-stimulated glucose oxidation was decreased in adipocytes from diabetic rats when the data are expressed in absolute terms or as percent increased above basal. Although the absolute rate of basal and insulin-stimulated glucose transport was decreased in adipocytes from diabetic rats, the percent increase above basal of insulin-stimulated glucose transport was not decreased. In conclusion, although the cellular insulin insensitivity exists in adipocytes from diabetic rats, the number of insulin receptors was increased, coupling between insulin receptors and the glucose transport system is intact in adipocytes from diabetic rats, and a defect in intracellular glucose metabolism rather than glucose transport plays a major role in the insulin insensitivity of adipocytes from diabetic rats.     

Reduced tetanus antibody titers in overweight children

The adipokine, leptin, regulates blood glucose and the insulin secretory function of beta cells, while also modulating immune cell function. We hypothesized that the dual effects of leptin may prevent or suppress the autoreactive destruction of beta cells in a virally induced rodent model of type 1 diabetes. Nearly 100% of weanling BBDR rats treated with the combination of an innate immune system activator, polyinosinic:polycytidylic acid (pIC), and Kilham rat virus (KRV) become diabetic within a predictable time frame. We utilized this model to test the efficacy of leptin in preventing diabetes onset, remitting new onset disease, and preventing autoimmune recurrence in diabetic rats transplanted with syngeneic islet grafts. High doses of leptin delivered via an adenovirus vector (AdLeptin) or alzet pump prevented diabetes in>90% of rats treated with pIC+KRV. The serum hyperleptinemia generated by this treatment was associated with decreased body weight, decreased non-fasting serum insulin levels, and lack of islet insulitis in leptin-treated rats. In new onset diabetics, hyperleptinemia prevented rapid weight loss and diabetic ketoacidosis, and temporarily restored euglycemia. Leptin treatment also prolonged the survival of syngeneic islets transplanted into diabetic BBDR rats. In diverse therapeutic settings, we found leptin treatment to have significant beneficial effects in modulating virally induced diabetes. These findings merit further evaluation of leptin as a potential adjunct therapeutic agent for treatment of human type 1 diabetes.     

Carnosine decreased oxidation and glycation products in serum and liver of high‐fat diet and low‐dose streptozotocin‐induced diabetic rats

High‐fat diet (HFD) and low‐dose streptozotocin (STZ)‐treated rats provide useful animal model for type II diabetes mellitus. Oxidative stress and advanced glycation end products (AGEs) play a role in the development of diabetic complications. Carnosine (CAR) has anti‐oxidant and anti‐glycating properties. We investigated the effects of CAR on oxidation and glycation products in HFD+STZ rats. Rats were fed with HFD (60% of total calories from fat) for 4 weeks, and then a single dose of STZ (40 mg/kg; i.p.) was applied. Rats with blood glucose levels above 200 mg/dl were fed with HFD until the end of the 12th week. CAR (250 mg/kg body weight; i.p.; five times a week) was administered to the rats for the last four weeks. CAR significantly decreased serum triglyceride (TG) (57.7%), cholesterol (35.6%) levels and hepatic marker enzyme activities of HFD+STZ rats. It significantly reduced serum reactive oxygen species (ROS) (23.7%), AGEs (13.4%) and advanced oxidized protein products (AOPP) (35.9%) and hepatic TG (59%), ROS (26%), malondialdehyde (MDA) (11.5%), protein carbonyl (PC) (19.2%) and AGE (20.2%) levels. Liver steatosis and hepatocyte ballooning were also significantly reduced. However, CAR treatment did not alter serum glucose and blood glycated haemoglobin and hepatic anti‐oxidant enzyme activities/mRNA expressions in HFD+STZ rats. Our results indicate that CAR decreased accumulation of oxidation and glycation products, such as MDA, AGE, AOPP and PC in the serum and liver and ameliorated hepatic dysfunction in HFD+STZ rats. This effect may be related to its anti‐oxidative, anti‐glycating, and anti‐lipogenic potential.     

Effects of three‐month streptozotocin‐induced diabetes in mice on blood platelet reactivity,COX‐1 expression and adhesion potential

Diabetes is associated with an increased risk of cardiovascular disease. This is partially attributed to an altered activation status of blood platelets in this disease. Previously, alterations have been shown in COX‐1 and protease activated receptor (PAR)‐3 receptor expression in platelets in two animal models of diabetes, there have not been studies which address expression of these proteins in mice with long‐term streptozotocin (STZ)‐induced diabetes. We have also addressed the effect of diabetes on platelet adhesion under flow conditions. With the use of flow cytometry, we have shown that certain markers of platelet basal activation, such as active form of α_IIbβ₃ and of CD40L were increased in STZ‐induced diabetic mice. Platelets from STZ‐induced diabetic mice were also more reactive when stimulated with PAR‐4 activating peptide as revealed by higher expression of active form of α_IIbβ₃, membrane‐bound on vWillebrand Factor and binding of exogenous fluorescein isothyanate‐labelled fibrinogen. Expression of COX‐1 and production of thromboxane A₂ in platelets of STZ‐induced diabetic mice were higher than in control animals. We observed no effect of diabetes on ability of platelets to form stable adhesions with fibrinogen in flow conditions. We conclude that although certain similarities exist between patterns of activation of platelets in animal models of diabetes, the differences should also be taken into account.     

A preliminary study of the use of human adipose tissue-derived stem cells for the treatment of streptozotocin-induced diabetes mellitus in a rat model

The purpose of this study is to determine if intraventricular injection of human adipose tissue-derived stem cells (hADSC) are effective in treating streptozotocin (STZ)-induced diabetes mellitus (DM) in nude rats. Twenty-two adult, male nude rats (strain Crl:NIH-Fox1^RNU) were used to induce diabetes using streptozotocin. A single, 150 mg/kg STZ was injected intraperitoneally. Severity of the induced diabetic state was assessed by daily monitoring of body weight, clinical signs, and blood glucose levels. C-peptide was assessed before ADSC injection (T0) and at 3, 5, and 21 days after ADSC injection. Eight rats (40%) developed DM within 24 h after STZ injection. Of the eight rats that developed DM, five were given 2 million freshly prepared ADSC intraventricularly under echocardiography guidance 10 days after STZ injection and three were only given sterile saline for comparison. Surviving rats were humanly sacrificed 21 days after ADSC injection. The average weight of diabetic rats decreased significantly after STZ injection. ADSC injection had no effect on the body weight of rats. Non-fasting serum glucose levels increased significantly in both groups. In diabetic rats, C-peptide decreased significantly before ADSC injection and seemed to return to normal 21 days after ADSC administration. Results of this preliminary study might suggest a beneficial effect of using hADSC for the treatment of STZ-induced diabetes in adult nude rats.     

Ascorbic acid oral treatment modifies lipolytic response and behavioural activity but not glucocorticoid metabolism in cafeteria diet‐fed rats

Aim: To analyse the effects of vitamin C (VC), a potent dietary antioxidant, oral supplementation on body weight gain, behavioural activity, lipolytic response and glucocorticoid metabolism in the early stages of diet‐induced overweight in rats. Methods: Food intake, locomotive activity and faecal corticosterone were assessed during the 14 day trial period. After 2 weeks, the animals were sacrificed and the body composition, biochemical markers and lipolytic response from isolated adipocytes from retroperitoneal white adipose tissue were examined. Results: The intake of a high‐fat diet by rats induced a significant increase in body weight, adiposity and insulin resistance markers as well as a decrease in faecal corticosterone levels compared with standard diet‐fed rats. Interestingly, the animals fed on the cafeteria diet showed a significant increase in the isoproterenol‐induced lipolytic response in isolated adipocytes. Furthermore, this cafeteria‐fed group showed a reduced locomotive behaviour than the control rats. On the other hand, oral VC supplementation in animals receiving the high‐fat diet restored the cafeteria diet effect in some of the analysed variables such as final body weight and plasma insulin to control group levels. Remarkably, increases in locomotive behaviour and a significant decrease in the lipolytic response induced by isoproterenol on isolated adipocytes from animals treated with VC were observed. Conclusion: This work demonstrates that an oral ascorbic acid supplementation has direct effects on behavioural activity and on adipocyte lipolysis in early obesity stages in rats, which could indicate a protective short‐term role of this vitamin against adiposity induced by chronic high‐fat diet consumption.     

DHEA improves impaired activation of Akt and PKC ζ/λ‐GLUT4 pathway in skeletal muscle and improves hyperglycaemia in streptozotocin‐induced diabetes rats

Aim: Addition of dehydroepiandrosterone (DHEA) to a cultured skeletal muscle locally synthesizes 5α‐dihydrotestosterone (DHT). It induced activation of glucose metabolism‐related signalling pathway via protein kinase B (Akt) and protein kinase C zeta/lambda (PKC ζ/λ)‐glucose transporter‐4 (GLUT4) proteins. However, such an effect of DHEA in vivo remains unclear. Methods: Using streptozotocin (STZ)‐induced rats with type 1 diabetes mellitus, we tested the hypothesis that a single bout of DHEA injection in the rats improves hyperglycaemia and muscle GLUT4‐regulated signalling pathway. After 1 week of STZ injection (55 mg kg^?1) with male Wistar rats, fasting glucose concentrations were determined in a blood sample taken from the tail vein. Blood glucose levels were then monitored for 180 min after DHEA or sesame oil (control) was injected (n = 10 for each group). Results: Blood glucose levels decreased significantly for 30–150 min after 2 mg DHEA injection in the STZ rats. In the skeletal muscle, expression and translocation of GLUT4 protein, phosphorylation of Akt and PKC ζ/λ, and phosphofructokinase and hexokinase enzyme activities increased significantly by DHEA injection. However, DHEA‐induced improvements in Akt and PKC ζ/λ‐GLUT4 pathways were blocked by a DHT inhibitor. Conclusion: These results suggest that a single bout of DHEA injection can improve hyperglycaemia and activate the glucose metabolism‐related signalling pathway via Akt and PKC ζ/λ‐GLUT4 proteins of skeletal muscles in rats. Moreover, these results show that a DHEA‐induced increase in muscle glucose uptake and utilization might contribute to improvement in hyperglycaemia in type 1 diabetes mellitus.     

Low carbohydrate ketogenic diet prevents the induction of diabetes using streptozotocin in rats

Diabetes continues to be an overwhelmingly prevalent endocrine disorder that leads to several micro- and macrocomplications. It has been widely accepted that changes in dietary habits could induce or prevent the onset of diabetes. It is shown that low carbohydrate ketogenic diet (LCKD) is effective in the amelioration of many of the deleterious consequences of diabetes. However, its role in preventing the onset of diabetes is not understood. Therefore, this study is focused on the effect of LCKD in preventing the induction of diabetes using streptozotocin (STZ) in rats by biochemical and histological methods. Forty-two Wistar rats weighing 150-250 g were used in this study. The animals were divided into three groups: normal diet (ND), low carbohydrate ketogenic diet (LCKD), and high carbohydrate diet (HCD). Specific diets ad libitum were given to each group of animals for a period of 8 weeks. Each group was further subdivided into normal control, sham control and diabetic groups. Animals in the diabetic group were given a single intraperitoneal injection of STZ (55 mg/kg). All the animals were sacrificed 4 weeks after the injection of STZ. Daily measurements of food and water intake as well as weekly measurement of body weight were taken during the whole 12 weeks of the experiment. After injecting with STZ, the blood glucose level of all the groups increased significantly except for the group fed on LCKD (p value<0.01). Also, food intake, water intake and urine output were significantly increased in all groups except for the LCKD group (p value<0.01). There was also a significant decrease in the weight gain of the animals that were fed on a LCKD as compared to other groups (p value<0.05). Although, substantial decrease in the number of β cells was noticed in diabetic rats, there were no change in the number of β cells in the LCKD treated diabetic animals as compared to LCKD control group. The results presented in this study, therefore, suggests that LCKD prevents the development of diabetes using streptozotocin in rats.     

Ultrastructural changes in Leydig cells of streptozotocin-induced diabetic rats.

Hyperglycemia (experimental diabetes) was induced in adult male rats by destruction of the pancreatic beta cells with a single intravenous injection of streptozotocin (STZ). Testes from diabetic, from insulin-treated diabetic, and from sham-injected normal rats were fxed by vascular perfusion. The fine structure of Leydig cells was examined at two, three, and four weeks after the STZ injection in the untreated diabetic animals, and at four weeks in the controls and insulin-treated diabetic rats. A number of morphological changes was observed in Leydig cells of untreated diabetic animals. Most obvious of these was an accumulation of lipid droplets, not normally present in Leydig cells in adults of this species. Smooth endoplasmic reticulum (SER) was markedly reduced in Leydig cells of the hyperglycemic rats. Several types of intracellular bodies were seen exclusively in Leydig cells of the untreated diabetic animals. Many resembled secondary lysosomes or dense bodies, while others appeared to be autophagic vacuoles. In addition, a small, granule-containing lamellar structure was seen either within a typical dense body or free in the cytoplasm. Myelin-like structures were commonly observed within the cytoplasm of the Leydig cell or within mitochondria. The appearance of the mitochondria in diabetic rats was otherwise normal. The extracellular spaces surrounding Leydig cells from untreated hyperglycemic rats also contained large accumulations of myelin-like material. These structural changes appear to be direct consequences of the diabetic state of the animals, since the ultrastructure of insulin-treated diabetic rats did not differ from that of the controls. These findings may reflect an alteration or breakdown of Leydig cell components normally involved in the synthesis of androgen, and correlate with previous reports of lowered circulating levels of testosterone in diabetic rats.     

Patterns of nutrient selection in rats with streptozotocin-induced diabetes

The effects of experimental diabetes on energy intake, patterns of nutrient selection, water intake, body weight and body composition were examined in male Sprague-Dawley rats given ground Purina Chow or a dietary self-selection regime. Following adaptation to dietary conditions, a portion of the animals in each diet group were made diabetic by the administration of 45 mg/kg streptozotocin (STZ). The remaining animals in each group served as vehicle-injected controls. STZ reliably produced diabetes in rats on both dietary regimes. Immediately after the induction of diabetes, rats on the self-selection regime increased carbohydrate and protein intakes and decreased fat intake. Approximately three weeks after STZ administration, diabetic rats reduced carbohydrate intake and increased fat intake. Diabetic animals in both diet groups were hyperphagic and polydipsic relative to non-diabetic controls. During the first three weeks following STZ injections, energy and water intakes of diabetic animals in the two dietary conditions were similar. However, after this initial period, energy and water intakes of diabetic rats given the self-selection regime were significantly lower than those of diabetic animals given Purina Chow.     

Metabolism of D-[3-3H]glucose, D-[5-3H]glucose, D-[U-14C]glucose, D-[1-14C]glucose and D-[6-14C]glucose in pancreatic islets in an animal model of type-2 diabetes

This study aims at exploring specific aspects of D-glucose metabolism, so far not yet investigated, in pancreatic islets from adult control rats and animals (STZ rats) injected with streptozotocin during the neonatal period. The latter animals, which represent a current model of type-2 diabetes, displayed a lower body weight, higher plasma D-glucose concentration and lower insulinogenic index than control rats. The protein, DNA and insulin content were all also lower in islets prepared from STZ, rather than control rats. In the presence of 10.0 mM D-glucose, the paired ratio between D-[U-14C]glucose oxidation and D-[5-3H]glucose utilization was also decreased in the islets from STZ rats. No significant difference between control and STZ rats was observed, however, in terms of the ratios between D-[3-3H]glucose and D-[5-3H]glucose utilization, between the generation of radioactive lactate from 14C-labelled D-glucose and tritiated D-glucose utilization and between D-[1-14C]glucose and D-[6-14C]glucose oxidation. These findings reinforce the view that the previously documented preferential impairment of the oxidative modality of glycolysis in islets from STZ rats contrasts with the absence of any major anomaly in other variables of D-glucose catabolism.     

Cyclic AMP-dependent and independent inhibition of lipolysis by adenosine and decreased pH.

NA-stimulated lipolysis and cAMP formation in isolated rat fat cells is inhibited by acidosis. In the present report we have examined the quantitative relationship between lipolysis and cAMP formation at normal and reduced pH and the possible involvement of adenosine, an endogenous inhibitor of cAMP formation. Adenosine antagonized cAMP accumulation and to a considerably lower degree lipolysis, effects potentiated by acidosis. Theophylline, an antagonist of adenosine effects, stimulated lipolysis and cAMP-accumulation, and potentiated responses to NA. Adenosine deaminase (ADA) had theophylline-like effects. Acidosis inhibited lipolysis and cAMP accumulation induced by ADA and theophylline to a larger extent than those induced by NA. It is suggested that adenosine modulates fat cell cAMP production and may contribute to the antilipolytic effect of acidosis. There was a curvilinear relationship between cAMP elevation and glycerol production in fat cell suspensions, which was different at pH 7.4 and at pH 6.6. The amount of cAMP needed for half-maximal activation of lipolysis increased from 1.3 (pH 7.4) to 3.1 pMol X 10(-5) cells (pH 6.6). The maximal glycerol production was reduced from 1 300 to 900 nMol X 10(-5) cells. The antilipolytic effect of acidosis is apparently due partly to an inhibition of cAMP formation and partly to inhibition of subsequent step(s) in the activation sequence.     

Resveratrol attenuates reproductive alterations in type 1 diabetes‐induced rats

The progression of diabetes mellitus leads to several complications including overproduction of reactive oxygen species and reproductive alterations. As resveratrol (RES) is a powerful anti‐oxidant and an anti‐apoptotic compound, we hypothesized that side effects of type‐1 diabetes (DM1) on male reproduction could be reduced by the RES treatment. Eighty‐four prepubertal male rats were distributed into seven groups: sham‐control (SC), RES‐treated (R), resveratrol‐vehicle‐treated (RV), diabetic (D), diabetic‐insulin‐treated (DI), diabetic‐RES‐treated (DR), diabetic‐insulin and RES‐treated (DIR). DM1 was induced by a single intraperitoneal streptozotocin (STZ) injection (65 mg/kg) on the 30th day postpartum (dpp). Animals of DR, DIR and R groups received 150 mg/day of RES by gavage for 43 consecutive days (from the 33 to 75 dpp). DI and DIR rats received subcutaneous injections of insulin (1 U/100 g b.w./day) from 5th day after the DM1 induction. The blood glucose level was monitored. At 75 dpp, the euthanasia was performed for morphometric and biometric testicular analyses, spermatic evaluation and hormonal doses. In the D group, the blood glucose level was higher than in the DR, DI and DIR groups. Besides morphometric testicular measurements, testosterone and estradiol doses were lower in D group than in DR and DIR groups; LH dose was also lower than in DR. The preputial separation age was delayed in diabetes‐induced groups. The DR and DIR groups showed an improvement in sperm mitochondrial activity, epididymal sperm counts and the frequency of morphologically normal sperms. RES treatment improved glycaemic level, sperm quantitative and qualitative parameters and the hormonal profile in DM1‐induced rats and seems to be a good reproductive protector.     

Fat gain with physical detraining is correlated with increased glucose transport and oxidation in periepididymal white adipose tissue in rats

As it is a common observation that obesity tends to occur after discontinuation of exercise, we investigated how white adipocytes isolated from the periepididymal fat of animals with interrupted physical training transport and oxidize glucose, and whether these adaptations support the weight regain seen after 4 weeks of physical detraining. Male Wistar rats (45 days old, weighing 200 g) were divided into two groups (n=10): group D (detrained), trained for 8 weeks and detrained for 4 weeks; and group S (sedentary). The physical exercise was carried out on a treadmill for 60 min/day, 5 days/week for 8 weeks, at 50-60% of the maximum running capacity. After the training protocol, adipocytes isolated from the periepididymal adipose tissue were submitted to glucose uptake and oxidation tests. Adipocytes from detrained animals increased their glucose uptake capacity by 18.5% compared with those from sedentary animals (P<0.05). The same cells also showed a greater glucose oxidation capacity in response to insulin stimulation (34.55%) compared with those from the S group (P<0.05). We hypothesize that, owing to the more intense glucose entrance into adipose cells from detrained rats, more substrate became available for triacylglycerol synthesis. Furthermore, this increased glucose oxidation rate allowed an increase in energy supply for triacylglycerol synthesis. Thus, physical detraining might play a role as a possible obesogenic factor for increasing glucose uptake and oxidation by adipocytes.     

Induction of receptor for advanced glycation end products by insufficient leptin action triggers pancreatic β‐cell failure in type 2 diabetes

Glucolipotoxicity, which is exerted by free fatty acids (FFA) and prolonged hyperglycemia, is implicated in pancreatic β‐cell failure in diabetes. Pattern recognition receptors such as receptor for advanced glycation end products (RAGE) and toll‐like receptors 2 and 4 could mediate danger signals in β‐cells. We examined whether RAGE contributes to β‐cell failure in a type 2 diabetes mouse model. Pancreatic islets were isolated from ob/ob, db/db, diet‐induced obesity (DIO), RAGE‐null (RAGE^−/−), and RAGE^+/+ wild‐type (WT) control mice and dispersed into single cells for flow cytometry. RAGE expression was detected in insulin‐positive β‐cells of ob/ob and db/db mice, but not of WT, DIO, or RAGE^−/− mice: thus, inadequate leptin receptor signaling and RAGE expression may be linked. Compared with RAGE^+/+ db/db mice, RAGE^−/− db/db mice showed higher β‐cell number and mass with less apoptosis as well as glucose tolerance with higher insulin secretion without any differences in serum levels of FFA and adiponectin. Palmitate or oleate pretreatment combined with a leptin antagonist induced RAGE expression, AGE‐elicited apoptosis, and impaired glucose‐stimulated insulin secretion by advanced glycation end products (AGE) in MIN6 cells. FFA elevation with concomitant AGE formation during prolonged hyperglycemia could cause β‐cell damage through insufficient leptin action and subsequent RAGE induction in type 2 diabetes.     

Cyclic AMP-Dependent and Independent Inhibition of Lipolysis by Adenosine and Decreased pH

NA-stimulated lipolysis and cAMP formation in isolated rat fat cells is inhibited by acidosis. In the present report we have examined the quantitative relationship between lipolysis and cAMP formation at normal and reduced pH and the possible involvement of adenosine, an endogenous inhibitor of cAMP formation. Adenosine antagonized cAMP accumulation and to a considerably lower degree lipolysis, effects potentiated by acidosis. Theophylline, an antagonist of adenosine effects, stimulated lipolysis and cAMP-accumulation, and potentiated responses to NA. Adenosine deaminase (ADA) had theophylline-like effects. Acidosis inhibited lipolysis and cAMP accumulation induced by ADA and theophylline to a larger extent than those induced by NA. It is suggested that adenosine modulates fat cell cAMP production and may contribute to the antilipolytic effect of acidosis. There was a curvilinear relationship between cAMP elevation and glycerol production in fat cell suspensions, which was different at pH 7.4 and at pH 6.6. The amount of cAMP needed for half-maximal activation of lipolysis increased from 1.3 (pH 7.4) to 3.1 pMol × 10^-5 cells (pH 6.6). The maximal glycerol production was reduced from 1 300 to 900 nMol × 10^-5 cells. The antilipolytic effect of acidosis is apparently due partly to an inhibition of cAMP formation and partly to inhibition of subsequent step(s) in the activation sequence.     

Ultrastructural changes in Leydig cells of streptozotocininduced diabetic rats

Hyperglycemia (experimental diabetes) was induced in adult male rats by destruction of the pancreatic beta cells with a single intravenous injection of streptozotocin (STZ). Testes from diabetic, from insulin-treated diabetic, and from sham-injected normal rats were fixed by vascular perfusion. The fine structure of Leydig cells was examined at two, three, and four weeks after the STZ injection in the untreated diabetic animals, and at four weeks in the controls and insulin-treated diabetic rats. A number of morphological changes was observed in Leydig cells of untreated diabetic animals. Most obvious of these was an accumulation of lipid droplets, not normally present in Leydig cells in adults of this species. Smooth endoplasmic reticulum (SER) was markedly reduced in Leydig cells of the hyperglycemic rats. Several types of intracellular bodies were seen exclusively in Leydig cells of the untreated diabetic animals. Many resembled secondary lysosomes or dense bodies, while others appeared to be autophagic vacuoles. In addition, a small, granule-containing lamellar structure was seen either within a typical dense body or free in the cytoplasm. Myelin-like structures were commonly observed within the cytoplasm of the Leydig cell or within mitochondria. The appearance of the mitochondria in diabetic rats was otherwise normal. The extracellular spaces surrounding Leydig cells from untreated hyperglycemic rats also contained large accumulations of myelin-like material. These structural changes appear to be direct consequences of the diabetic state of the animals, since the ultrastructure of insulin-treated diabetic rats did not differ from that of the controls. These findings may reflect an alteration or breakdown of Leydig cell components normally involved in the synthesis of androgen, and correlate with previous reports of lowered circulating levels of testosterone in diabetic rats.     

Neu1 sialidase interacts with perilipin 1 on lipid droplets and inhibits lipolysis in 3T3‐L1 adipocytes

Fatty acids are stored within adipocytes in lipid droplets (LDs) as triacylglycerol (TG), which is converted to free fatty acid (FFA) and glycerol via lipolysis. Increased plasma FFA levels in obesity are associated with several clinical conditions. We previously found that Neu1 activity is aberrant in the epididymal fat and liver of obese and diabetic mice. Here, we examined involvement of Neu1 in lipolysis in 3T3‐L1 adipocytes. Small interfering RNA against Neu1 was introduced into adipocytes, and glycerol concentrations were measured in the culture medium. We then assessed the effects of Neu1 knockdown on lipolytic protein expression and phosphorylation, as well as interactions between perilipin 1 (Plin1) and hormone‐sensitive lipase (HSL) after isoproterenol (IS) stimulation. Interactions between Neu1 and Plin1 were analyzed by immunoprecipitation and immunofluorescent imaging using adipocytes transfected with pCMV6‐mNeu1‐myc‐DYKDDDDK (mNeu1DDK). Neu1 knockdown increased glycerol concentrations in culture media and Plin1 phosphorylation in whole lysates of IS‐stimulated cells. Neu1 knockdown increased interaction between Plin1 and HSL after IS stimulation whereas that between Neu1 and Plin1 on LD observed under basal conditions was lost. These results suggest that Neu1 inhibits lipolysis induced by β‐adrenergic stimulation in adipocytes via interactions with Plin1 on LD.