Stimulation of glucose transport by semicarbazide-sensitive amine oxidase activity in adipocytes from diabetic rats

Semicarbazide-sensitive amine oxidase (SSAO) is highly expressed in adipose cells, and substrates of SSAO such as benzylamine in combination with low concentrations of vanadate strongly stimulate glucose transport and GLUT4 recruitment in mouse 3T3-L1 adipocytes and in isolated rat adipocytes. Here we examined whether this combination of molecules also stimulates glucose transport in adipocytes from streptozotocin-induced diabetic rats and from Goto-Kakizaki diabetic rats. As previously reported, adipocytes obtained from streptozotocin-induced diabetic rats, showed a reduced stimulation of glucose transport in response to insulin. Under these conditions, the combination of benzylamine and vanadate caused a marked stimulation of glucose transport that was similar to the stimulation detected in control adipocytes. Adipocytes isolated from Goto-Kakizaki diabetic rats also showed a defective response to insulin; however, acute incubation in the presence of benzylamine and vanadate stimulated glucose transport in these cells to the same extent than in adipocytes from non-diabetic rats. These data indicate that adipocytes obtained from two different models of animal diabetes do not show resistance to the activation of glucose transport by SSAO activity, which is in contrast to the well reported resistance to insulin action. It seems to suggest that SSAO activity in combination with vanadate triggers a glucose transport-activating intracellular pathway that remains intact in the diabetic state. Further, our data support the view that the combination of benzylamine and vanadate could be an effective therapy in diabetes.     

In vivo effects of vanadate on hepatic glycogen metabolizing and lipogenic enzymes in insulin-dependent and insulin-resistant diabetic animals

The insulin-mimetic action of vanadate is well established but the exact mechanism by which it exerts this effect is still not clearly understood. The role of insulin in the regulation of hepatic glycogen metabolizing and lipogenic enzymes is well known. In our study, we have, therefore, examined the effects of vanadate on these hepatic enzymes using four different models of diabetic and insulin-resistant animals. Vanadate normalized the blood glucose levels in all animal models. In streptozotocin-induced diabetic rats, the amount of liver glycogen and the activities of the active-form of glycogen synthase, both active and inactive-forms of phosphorylase, and lipogenic enzymes like glucose 6-phosphate dehydrogenase and malic enzyme were decreased and vanadate treatment normalized all of these to near normal levels. The other three animal models (db/db mouse, sucrose-fed rats and fa/fa obese Zucker rats) were characterized by hyperinsulinemia, hypertriglyceridemia, increases in activities of lipogenic enzymes, and marginal changes in glycogen metabolizing enzymes. Vanadate treatment brought all of these values towards normal levels. It should be noted that vanadate shows differential effects in the modulation of lipogenic enzymes activities in type I and type II diabetic animals. It increases the activities of lipogenic enzymes in streptozotocin-induced diabetic animals and prevents the elevation of activities of these enzymes in hyperinsulinemic animals. The insulin-stimulated phosphorylation of insulin receptor subunit and its tyrosine kinase activity was increased in streptozotocin-induced diabetic rats after treatment with vanadate. Our results support the view that insulin receptor is one of the sites involved in the insulin-mimetic actions of vanadate.     

Lipid-induced beta-cell dysfunction in vivo in models of progressive beta-cell failure

We determined the effect of 48-h elevation of plasma free fatty acids (FFA) on insulin secretion during hyperglycemic clamps in control female Wistar rats (group a) and in the following female rat models of progressive beta-cell dysfunction: lean Zucker diabetic fatty (ZDF) rats, both wild-type (group b) and heterozygous for the fa mutation in the leptin receptor gene (group c); obese (fa/fa) Zucker rats (nonprediabetic; group d); obese prediabetic (fa/fa) ZDF rats (group e); and obese (fa/fa) diabetic ZDF rats (group f). FFA induced insulin resistance in all groups but increased C-peptide levels (index of absolute insulin secretion) only in obese prediabetic ZDF rats. Insulin secretion corrected for insulin sensitivity using a hyperbolic or power relationship (disposition index or compensation index, respectively, both indexes of beta-cell function) was decreased by FFA. The decrease was greater in normoglycemic heterozygous lean ZDF rats than in Wistar controls. In obese "prediabetic" ZDF rats with mild hyperglycemia, the FFA-induced decrease in beta-cell function was no greater than that in obese Zucker rats. However, in overtly diabetic obese ZDF rats, FFA further impaired beta-cell function. In conclusion, 1) the FFA-induced impairment in beta-cell function is accentuated in the presence of a single copy of a mutated leptin receptor gene, independent of hyperglycemia. 2) In prediabetic ZDF rats with mild hyperglycemia, lipotoxicity is not accentuated, as the beta-cell mounts a partial compensatory response for FFA-induced insulin resistance. 3) This compensation is lost in diabetic rats with more marked hyperglycemia and loss of glucose sensing.     

Novel form of lipolysis induced by leptin.

Hyperleptinemia causes disappearance of body fat without a rise in free fatty acids (FFA) or ketones, suggesting that leptin can deplete adipocytes of fat without releasing FFA. To test this, we measured FFA and glycerol released from adipocytes obtained from normal lean Zucker diabetic fatty rats (+/+) and incubated for 0, 3, 6, or 24 h in either 20 ng/ml recombinant leptin or 100 nM norepinephrine (NE). Whereas NE increased both FFA and glycerol release from adipocytes of +/+ rats, leptin increased glycerol release in +/+ adipocytes without a parallel increase in FFA release. In adipocytes of obese Zucker diabetic fatty rats (fa/fa) with defective leptin receptors, NE increased both FFA and glycerol release, but leptin had no effect on either. Leptin significantly lowered the mRNA of leptin and fatty acid synthase of adipocytes (FAS) (p < 0.05), and up-regulated the mRNA of peroxisome proliferator-activated receptor (PPAR)-alpha, carnitine palmitoyl transferase-1, (CPT-1), and acyl CoA oxidase (ACO) (p < 0.05). NE (100 nM) also lowered leptin mRNA (p < 0.05) but did not affect FAS, PPARalpha, ACO, or CPT-1 expression. We conclude that in normal adipocytes leptin directly decreases FAS expression, increases PPARalpha and the enzymes of FFA oxidation, and stimulates a novel form of lipolysis in which glycerol is released without a proportional release of FFA.     

In Zucker diabetic fatty rats plasma leptin levels are correlated with plasma insulin levels rather than with body weight.

The obese (ob) gene product leptin, secreted from adipose tissue, acts in the hypothalamus to regulate body energy stores. In vitro experiments showed that insulin increases both leptin mRNA expression and leptin secretion by adipocytes. Here, we report on the relationship between plasma insulin and plasma leptin in a longitudinal in vivo study. In Zucker diabetic fatty (ZDF) rats, an animal model for non-insulin-dependent diabetes mellitus (NIDDM), and in ZDF control rats, blood glucose, body weight, plasma insulin and plasma leptin levels were measured from 10 to 25 weeks of age. In ZDF control rats, body weight, plasma leptin and plasma insulin levels increased gradually during the study period. In ZDF rats, the time course of plasma leptin was similar to that of plasma insulin, but did not parallel that of body weight. Calculation of partial correlation coefficients revealed that in ZDF control rats plasma leptin correlated with body weight rather than with plasma insulin. However, in ZDF rats, plasma leptin correlated with plasma insulin rather than with body weight, suggesting an important role for insulin in the modulation of leptin secretion in this animal model for NIDDM.     

Downregulation of prolactin-releasing peptide gene expression in the hypothalamus and brainstem of diabetic rats

We investigated the prolactin-releasing peptide (PrRP) mRNA levels in the hypothalamus and brainstem of streptozotocin (STZ)-induced diabetic rats and fa/fa Zucker diabetic rats, using in situ hybridization histochemistry. PrRP mRNA levels in the hypothalamus and brainstem of STZ-induced diabetic rats were significantly reduced in comparison with those of control rats. PrRP mRNA levels in the diabetic rats were reversed by both insulin and leptin. PrRP mRNA levels in the fa/fa diabetic rats were significantly reduced in comparison with those of Fa/? rats. PrRP mRNA levels in the fa/fa diabetic rats were significantly increased by insulin-treatment, but did not reach control levels in the Fa/? rats. We also investigated the effect of restraint stress on PrRP mRNA levels in STZ-induced diabetic rats. The PrRP mRNA levels in the control and the STZ-induced diabetic rats increased significantly after restraint stress. The diabetic condition and insulin-treatment may affect the regulation of PrRP gene expression via leptin and other factors, such as plasma glucose level. The diabetic condition may not impair the role of PrRP as a stress mediator.     

Epinephrine-induced hyperpolarization of islet cells without KATP channels

ID-1101 (4-hydroxyisoleucine), an amino acid extracted from fenugreek seeds, exhibits an interesting glucose-dependent insulin-stimulating activity. The present study was undertaken to investigate a possible extrapancreatic effect of ID-1101 on insulin signaling and action besides its previously described insulinotropic action. Insulin-sensitizing effects of ID-1101 were investigated in rat in vivo by three different approaches: 1) using euglycemic hyperinsulinemic clamps in two different rat models of insulin resistance, i.e., Zucker fa/fa rats and rats fed a sucrose-lipid diet; 2) measuring liver and muscle phosphatidylinositol (PI) 3-kinase activity after an acute injection of ID-1101 in normal and insulin-resistant diabetic rats; and 3) after chronic treatment in two rat models of insulin resistance. Euglycemic hyperinsulinemic clamp experiments revealed that ID-1101 can improve insulin resistance through an increase of peripheral glucose utilization rate in sucrose-lipid-fed rats and by decreasing hepatic glucose production in Zucker fa/fa rats. Moreover, we demonstrated that a single injection of ID-1101 activates the PI 3-kinase activity in liver and muscle from normal rats but also in muscle from diabetic rats. Finally, chronic ID-1101 treatment significantly reduced insulinemia in type 2 diabetic rats and reduced the progression of hyperinsulinemia in insulin-resistant obese Zucker fa/fa rats. These findings clearly demonstrate that ID-1101 can reduce insulin resistance through activation of the early steps of insulin signaling in peripheral tissues and in liver. In summary, ID-1101, besides its insulinotropic effect, directly improves insulin sensitivity, making it a potentially very valuable therapeutic agent for diabetes treatment.     

Vanadate induces normolipidemia and a reduction in the levels of hepatic lipogenic enzymes in obese Zucker rat

The effects of vanadate administration on the plasma lipids and hepatic lipogenic enzymes were investigated in Zucker (fa/fa) rat, a model for obesity and non insulin-dependent diabetes. These animals were administered sodium orthovanadate through drinking water for a period of four months. The plasma levels of insulin, triacylglycerols and total cholesterol were significantly (p<0.001) elevated in untreated obese control rats as compared to the lean animals. In the livers of obese rats, the number of insulin receptors decreased by 60% and the activities of lipogenic enzymes acetyl-CoA carboxylase and ATP-citrate lyase increased by 4.7- and 5.6-folds, respectively. The messenger RNA for ATP-citrate lyase as measured by Northern blot analysis showed a parallel increase in obese control rats. Treatment of these rats with vanadate caused 56–77% decreases in the plasma levels of insulin, triacylglycerols and total cholesterol. The insulin receptor numbers in vanadate-treated obese rats increased (119%) compared to levels in untreated obese animals. The elevated activities of acetyl-CoA carboxylase and ATP-citrate lyase observed in livers of obese rats were significantly reduced by vanadate. The messenger RNA for ATP-citrate lyase also decreased in vanadate-treated obese rats back to the lean control levels. This study demonstrates that vanadate exerts potent actions on lipid metabolism in diabetic animals in addition to the recognized effects on glucose homeostasis.     

Leptin receptor-deficient Zucker (fa/fa) rat retards the development of pig serum-induced liver fibrosis with Kupffer cell dysfunction

The aim of this study was to investigate the role of leptin in the development of liver fibrosis with Kupffer cell function using leptin receptor deficient rats. Male Zucker (fa/fa) and control (fa/-) rats received pig serum for 8 weeks. Animals were sacrificed to estimate the degree of liver fibrosis and stellate cell activation with the expression of alpha smooth muscle actin (alphaSMA). Microarray analysis was performed. Isolated Kuppfer cells of Zucker and control rats were treated with LPS. LPS uptake and TNF-alpha production were examined. Stellate cells were also isolated from Zucker and control rats. The expression of procollagen type I mRNAs was examined. Control rats developed liver fibrosis 8 weeks after injection of pig serum and showed an increased liver hydroxyproline content of 348 +/- 34 microg/g (n = 10) compared with Zucker rats (225 +/- 13, n = 10, P < 0.01). The procollagen type I mRNA level and alphaSMA expression of Zucker rats were also significantly reduced. Microarray analysis indicated significantly reduced expression of TNF-alpha, LPS-binding protein, urokinase-type plasminogen activator (uPA), IGF, IGF-binding protein (IGFBP)-3,5, and increased expression of apolipoprotein IV. Isolated Kupffer cells of Zucker rats showed significantly reduced LPS uptake as well as TNF-alpha production compared with control rats. However, no significant change was observed in procollagen type I mRNA levels of isolated stellate cells after 4 days of culture on plastic dishes. These results suggest that leptin receptor deficiency retards the development of liver fibrosis due to the dysfunction of Kuppfer cells.     

Leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes.

The aim of the present study was to gain insight into the signaling pathway used by leptin to stimulate lipolysis. The lipolytic rate of white adipocytes from sex- and age-matched lean (+/+) and fa/fa rats was determined in the absence or presence of leptin together with a number of agents acting at different levels of the signaling cascade. Leptin did not modify FSK-, dbcAMP-, and IBMX-stimulated lipolysis. Lipolysis can also be maximally stimulated by lowering media adenosine levels with adenosine deaminase (ADA), i.e., in the ligand-free state. Although ADA produced near maximal lipolysis in adipocytes of lean animals, only half of the maximal lipolytic rate (50.9+/-3.2%) was achieved in fat cells from fa/fa rats (P=0.0034). In adipocytes from lean animals preincubated with ADA, leptin caused a concentration-related stimulation of lipolysis (P=0.0001). However, leptin had no effect on the lipolytic activity of adipocytes in the ligand-free state from fa/fa rats. The adenosine A1 receptor agonist CPA effectively inhibited basal lipolysis in both lean and obese adipocytes (P=0.0001 and P=0.0090, respectively). Leptin had no effect on the lipolytic rate of adipocytes isolated from fa/fa rats and preincubated with CPA. When adipocytes were incubated with the A1 receptor antagonist DPCPX, a significant increase in glycerol release was observed in fa/fa fat cells (P=0.009), whereas cells isolated from lean rats showed no differences to ADA-stimulated lipolysis. After pretreatment with PTX, which inactivates receptor-mediated Gi function, adipocytes of obese rats became as responsive to the stimulatory actions of ISO as cells from lean rats (P=0.0090 vs. ISO in fa/fa rats; P=0.2416 vs. lean rats, respectively). PTX treatment of lean cells, however, did not alter their response to this lipolytic agent. It can be concluded that the lipolytic effect of leptin is located at the adenylate cyclase/Gi proteins level and that leptin-induced lipolysis opposes the tonic inhibition of endogenous adenosine in white adipocytes.     

Adaptation of ghrelin levels to limit body weight gain in the obese Zucker rat

In this study, we measured the ghrelin, leptin, and insulin variations in lean and obese Zucker fa/fa rats during the acute phase of body weight gain. At 2 months of age, plasma insulin and leptin concentrations in fa/fa rats were, respectively, 470% and 3700% higher than in lean rats (p <0.0001). Plasma ghrelin was significantly lower (-24.6%; p <0.02) than in lean rats. At 6 months of age, ghrelin increased in both genotypes but the difference was no more significant. The inverse correlations existing between ghrelin and either body weight (BW), insulin or leptin at 2 months of age were no more observable in 6-month-old rats. At 6 months of age, the lean rats had the same body weight as the 2-month-old obese rats. In these body weight-matched rats, ghrelin was not correlated with BW but it remained negatively correlated with insulin and leptin. At the same body weight, obese rats had a much lower plasma ghrelin than lean rats (717+/-42 vs. 1754+/-83 pg/ml; p <0.0001). These data indicate that body composition rather than body weight is the primary factor for the down-regulation of the ghrelin system. This down-regulation constitutes a mechanism of defense of the organism against the development of obesity at least during the first part of life.     

Leptin enhances TNF-alpha production via p38 and JNK MAPK in LPS-stimulated Kupffer cells

Leptin is now recognized as a proinflammatory cytokine and thought to be a progressive factor for non-alcoholic steatohepatitis (NASH). Here we showed the effects of leptin on the production of TNF-alpha (tumor necrosis factor-alpha) by Kupffer cells (KCs) with signal transduction. Leptin enhanced TNF-alpha production accompanied by a dose-dependent increase of MAPK activity in lipopolysaccharide (LPS)-stimulated KCs. SB203580 and JNK inhibitor I, specific inhibitors of P38 and JNK, inhibited TNF-alpha production in KCs but PD98059, an inhibitor of the ERK pathway, did not affect TNF-alpha production by KCs. Recombinant constitutively active adenovirus (Ad)-MKK6 and-MKK7 increased TNF-alpha production in KCs with activation of P38 and JNK without any change by Ad-MEK1 delivery. On the other hand, KCs isolated from the Zucker rat (fa/fa), a leptin receptor-deficient rat, showed reduced production of TNF-alpha on stimulation with LPS. The delivery of Ad-MKK6 and-MKK7, but not Ad-MEK1, increased TNF-alpha production in KCs of Zucker rats with activation of P38 and JNK. Addition of leptin to normal rats increased LPS-induced hepatic TNF-alpha production in vivo and leptin receptor-deficient Zucker rats showed reduced hepatic TNF-alpha production on addition of LPS in vivo. These findings indicate that P38 and JNK pathways are involved in the signal transduction of leptin enhancement of LPS-induced TNF-alpha production.     

Effect of prolonged treatment with tyramine on glucose tolerance in streptozotocin-induced diabetic rats

The biogenic amine tyramine has been reported to stimulate in vitro glucose transport in adipocytes, cardiomyocytes and skeletal muscle, and to improve in vivo glucose utilization in rats. These effects were dependent on amine oxidation, since they were blocked by inhibitors of monoamine oxidase (MAO) and semicarbazide-sensitive amine oxidase (SSAO). We thus tested in this work whether a prolonged treatment with tyramine could improve glucose tolerance in streptozotocin-induced diabetic rats. First, tyramine content of standard rodent chow was determined by HPLC and daily tyramine intake of control rats was estimated to be around 26 micromol/kg body weight. Then, tyramine was administred during 3 weeks in streptozotocin-induced diabetic rats at 29 micromol/kg by daily i.p. injection alone or together with vanadate 0.02 micromol/kg. In another group of diabetic rats, tyramine was subcutaneously delivered at 116 micromol/kg/day by osmotic minipumps. All tyramine treatments resulted in a decrease of the hyperglycemic responses to an i.p. glucose load. Adipocytes isolated from either untreated or treated diabetic rats were sensitive to the stimulation of glucose uptake by tyramine. However, diabetic animals receiving tyramine for three weeks did not recover from their hyperglycemia, hypoinsulinemia and glucosuria. These results show that the improvement of glucose tolerance induced by prolonged tyramine administration occurs in an insulin-depleted model and probably results from peripheral insulin-like actions of the oxidation of MAO/SSAO substrates, such as the stimulation of glucose uptake into adipocytes.     

Effects of Irbesartan on the Growth and Differentiation of Adipocytes in Obese Zucker Rats

Objective: The aim of this study was to evaluate the effects of the selective angiotensin receptor 1 antagonist irbesartan on the growth and differentiation of the adipocytes in obese Zucker fa/fa rats. Research Methods and Procedures: Obese Zucker fa/fa rats were treated by oral route for 3 weeks with irbesartan at doses of 3–10‐30 mg/kg per day. The adipocyte differentiation was evaluated by analyzing tissue samples of white (retroperitoneal) or brown (interscapular) adipose tissue for the presence of peroxisome proliferator activated receptor γ, leptin, and the activity of glycerol‐3‐phosphate dehydrogenase. Results: This study showed that the treatment of obese Zucker fa/fa with irbesartan effectively reduced the differentiation of adipocytes within brown (interscapular) and white (retroperitoneal) adipose tissue. In fact, irbesartan significantly (p < 0.01) and dose‐dependently reduced the tissue levels of leptin, peroxisome proliferator activated receptor γ, and the activity of the enzyme glycerol‐3‐phoshate dehydrogenase accepted markers of adipocyte differentiation. None of the tested doses of irbesartan affected these markers in non‐obese rats. Discussion: The antagonism of the angiotensin receptor 1 receptors with irbesartan reduces the adipogenic activity of angiotensin II in obese Zucker rats, with the endpoint being reduction of the growth and differentiation of the adipocytes within the adipose tissue.     

Multiple defects occur in the guanine nucleotide regulatory protein system in liver plasma membranes of obese (fa/fa) but not lean (Fa/Fa) Zucker rats: loss of functional Gi and abnormal Gs function

Hepatocyte membranes from both lean and obese Zucker rats exhibited adenylate cyclase activity that could be stimulated by glucagon, forskolin, NaF and elevated concentrations of p[NH]ppG. In membranes from lean animals, functional Gi was detected by the ability of low concentrations of p[NH]ppG to inhibit forskolin-activated adenylate cyclase. This activity was abolished by treatment of hepatocytes with either pertussis toxin or the phorbol ester TPA, prior to making membranes for assay of adenylate cyclase activity. In hepatocyte membranes from obese animals no functional Gi activity was detected. Quantitative immunoblotting, using an antibody able to detect the alpha subunit of Gi, showed that hepatocyte plasma membranes from both lean and obese Zucker rats had similar amounts of Gi-alpha subunit. This was 6.2 pmol/mg plasma membrane for lean and 6.5 pmol/mg plasma membrane for obese animals. Using thiol pre-activated pertussis toxin and [32P]-NAD+, similar degrees of labelling of the 40 kDa alpha subunit of Gi were found using plasma membranes of both lean and obese Zucker rats. We suggest that liver plasma membranes from obese Zucker rats express an inactive Gi alpha subunit. Thus lesions in liver Gi functioning are seen in insulin-resistant obese rats and in alloxan- and streptozotocin-induced diabetic rats which also show resistance as regards the acute actions of insulin. Liver plasma membranes of obese animals also showed an impairment in the coupling of glucagon receptors to Gs-controlled adenylate cyclase, with the Kd values for activation by glucagon being 17.3 and 126 nM for lean and obese animals respectively. Membranes from obese animals also showed a reduced ability for high concentration of p[NH]ppG to activate adenylate cyclase. The use of [32P]-NAD+ and thiol-preactivated cholera toxin to label the 43 kDa and 52 kDa forms of the alpha-subunit of Gs showed that a reduced labelling occurred using liver plasma membranes from obese animals. It is suggested that abnormalities in the levels of expression of primarily the 52 kDa form of alpha-Gs may give rise to the abnormal coupling between glucagon receptors and adenylate cyclase in liver membranes from obese (fa/fa) Zucker rats.     

Semicarbazide-sensitive amine oxidase activity exerts insulin-like effects on glucose metabolism and insulin-signaling pathways in adipose cells

Semicarbazide-sensitive amine oxidase (SSAO) is very abundant at the plasma membrane in adipocytes. The combination of SSAO substrates and low concentrations of vanadate markedly stimulates glucose transport and GLUT4 glucose transporter recruitment to the cell surface in rat adipocytes by a mechanism that requires SSAO activity and hydrogen peroxide formation. Substrates of SSAO such as benzylamine or tyramine in combination with vanadate potently stimulate tyrosine phosphorylation of both insulin-receptor substrates 1 (IRS-1) and 3 (IRS-3) and phosphatidylinositol 3-kinase (PI 3-kinase) activity in adipose cells, which occurs in the presence of a weak stimulation of insulin-receptor kinase. Moreover, the acute administration of benzylamine and vanadate in vivo enhances glucose tolerance in non-diabetic and streptozotocin-induced diabetic rats and reduces hyperglycemia after chronic treatment in streptozotocin-diabetic rats. Based on these observations, we propose that SSAO activity and vanadate potently mimic insulin effects in adipose cells and exert an anti-diabetic action in an animal model of type 1 diabetes mellitus.     

The BBZDR/Wor rat model for investigating the complications of type 2 diabetes mellitus

Congenic and inbred strains of rats offer researchers invaluable insight into the etiopathogenesis of diabetes and associated complications. The inbred Bio-Breeding Zucker diabetic rat (BBZDR)/Wor rat strain is a relatively new and emerging model of type 2 diabetes. This strain was created by classical breeding methods used to introgress the defective leptin receptor gene (Lepr(fa)) from insulin-resistant Zucker fatty rats into the inbred BBDR/Wor strain background. The diabetic male BBZDR/Wor rat is homozygous for the fatty mutation and shares the genetic background of the original BB strain. Although lean littermates are phenotypically normal, obese juvenile BBZDR/Wor rats are hyperlipidemic and hyperleptinemic, become insulin resistant, and ultimately develop hyperglycemia. Furthermore, the BBZDR/Wor rat is immune competent and does not develop autoimmunity. Similar to patients with clinical diabetes, the BBZDR/Wor rat develops complications associated with hyperglycemia. The BBZDR/Wor rat is a model system that fully encompasses the ability to study the complications that affect human type 2 diabetic patients. In this review, recent work that has evaluated type 2 diabetic complications in BBZDR/Wor rats is discussed, including the authors' preliminary unpublished studies on cardiovascular disease.     

Single intracerebroventricular bolus injection of a recombinant adenovirus expressing leptin results in reduction of food intake and body weight in both lean and obese Zucker fa/fa rats

Leptin acts as a satiety factor within the central nervous system by binding to its receptor located in the hypothalamus. A missense mutation of the leptin receptor induces hyperphagia and obesity in the obese Zucker fa/fa rat. Since the CNS is an important target of leptin action, we hypothesized that leptin gene transfer into the lateral cerebral ventricle could efficiently lead to inhibition of food intake and reduction of body weight in obese fa/fa rats as well as in lean animals. A single intracerebroventricular injection of an adenoviral vector containing a cDNA encoding leptin resulted in the expression of leptin in the ependymal cells lining the ventricle and the secretion of leptin into the cerebrospinal fluid (CSF). During the first week after injection, when high concentrations of leptin were produced in the CSF, the reducing effects of leptin on food intake and body weight were comparable in lean and in obese fa/fa rats. The subsequent decline in CSF leptin levels, that was similar in lean and obese fa/fa rats, resulted in the faster resumption of food intake and body weight gain in obese than in lean animals, confirming a reduced sensitivity to leptin in the obese group. The results of this study show that leptin gene delivery into the cerebral ventricles allows for the production of elevated leptin concentrations in CSF, and they support the hypothesis that the impaired sensitivity to leptin may be overcome in obese fa/fa rats.     

Urinary chiro-inositol and myo-inositol excretion is elevated in the diabetic db/db mouse and streptozotocin diabetic rat

Inositol phosphoglycan molecules containing either D-chiro-inositol or myo-inositol have been isolated from various mammalian tissues and are putative mediators of insulin action. Urinary excretion of inositols appears to be altered in diabetes mellitus; however, the relationships with different types of diabetes are unclear. The objective of this study was to determine the urinary excretion of chiro- and myo-inositol in diabetic animal models, including streptozotocin (STZ) rats, db/db mice, and fa/fa Zucker rats. In STZ rats (type 1 diabetes), 12-hr urinary excretion of chiro-inositol was elevated 336-fold and myo-inositol excretion was elevated 47-fold compared with their nondiabetic counterparts. When corrected for creatinine, chiro-inositol excretion was 259-fold higher and myo-inositol excretion was 36-fold higher in STZ rats than in normal rats. The same pattern was observed in db/db mice (type 2 diabetes), where 12-hr urinary chiro-inositol excretion was elevated 247-fold compared with normal mice. When corrected for creatinine, chiro-inositol excretion was 2455-fold higher and urinary myo-inositol excretion was elevated 8.5-fold in db/db mice compared with normal mice. The fa/fa Zucker rats (impaired glucose tolerance) had a pattern of urinary inositol excretion that was similar to the nondiabetic animals (lean Zucker rats, C57BL/6 mice, and Sprague-Dawley rats). In summary, urinary chiro-inositol and myo-inositol excretion was elevated in animal models of type 1 and type 2 diabetes mellitus, concomitant with hyperglycemia and glucosuria.     

Correlation Between Pancreatic Islet Uncoupling Protein-2 (UCP2) mRNA Concentration And Insulin Status in Rats

Hypothesizing that UCP2 may influence insulin secretion by modifying the ATP/ADP ratio within pancreatic islets, we have investigated the expression of intraislet UCP2 gene in rats showing insulin oversecretion (non-diabetic Zucker fa/fa obese rats, glucose-infused Wistar rats) or insulin undersecretion (fasting and mildly diabetic rats). We found that in Zucker fa/fa obese rats, hyperinsulinemia (1222 ± 98 pmol/1 vs. 128 ± 22 pmol/1 in lean Zucker rats) was accompanied by a significant increase in UCP2 mRNA levels. In rat submitted to a 5 day infusion with glucose, hyperinsulinemia (1126 ± 101 pmol/l vs. 215 ± 25 pmol/1 in Wistar control rats), coincided with an enhanced intraislet UCP2 gene expression, whereas a 8h or a 2 day-infusion did not induce significant changes in UCP2 mRNA expression. In rats made hypoinsulinemic and mildly diabetic by the injection of a low dose of streptozotocin, and in 4-day-fasting rats (plasma insulin 28 ± 5 pmol/1) UCP2 gene expression was sharply decreased. A 3-day-fast was ineffective. The data show the existence of a time-dependent correlation between islet mRNA UCP2 and insulin that may be interpreted as an adaptative response to prolonged insulin excess.