Decreased incorporation of glucose into lipids and increased lactate production by adipose tissue after long-term treatment of rats with D-fenfluramine

Cell-permeable ceramides increase basal glucose incorporation into triacylglycerols but decrease the stimulation by insulin in 3T3-L1 adipocytes

OBJECTIVE

To investigate mechanisms for the regulation of glucose incorporation into triacylgycerols in adipocytes by ceramides, which mediate some actions of tumour necrosis factor-alpha (TNFalpha).

DESIGN

The effects of C(2)- and C(6)-ceramides (N-acetyl- and N-hexanoyl-sphingosines, respectively) on glucose uptake and incorporation into triacylglycerols and pathways of signal tansduction were measured in 3T3-L1 adipocytes.

RESULTS

C(6)-ceramide increased basal 2-deooxyglucose uptake but decreased insulin-stimulated uptake without changing the EC(50) for insulin. Incubating 3T3-L1 adipocytes from 2 to 24 h with C(2)-ceramide progressively increased glucose incorporation into the fatty acid and especially the glycerol moieties of triacylglycerol. These effects were accompanied by increased GLUT1 synthesis resulting from ceramide-induced activation phosphatidylinositol 3-kinase, ribosomal S6 kinase and mitogen-activated protein kinase. C(2)-ceramide also increased p21-activated kinase and protein kinase B activities. However, C(2)-ceramide decreased the insulin-stimulated component of these signalling pathways and also glucose incorporation into triacylglycerol after 2 h.

CONCLUSIONS

Cell-permeable ceramides can mimic some effects of TNFalpha in producing insulin resistance. However, ceramides also mediate long-term effects that enable 3T3 L1 adipocytes to take up glucose and store triacylglycerols in the absence of insulin. These observations help to explain part of the nature and consequence of TNFalpha-induced insulin resistance and the control of fat accumulation in adipocytes in insulin resistance and obesity.

In situ metabolic and hemodynamic response to dexfenfluramine in white adipose tissue of rats

Serotonergic neurons are included in the regulation of eating behavior and energy metabolism. Dexfenfluramine (DF), a serotonin releaser and reuptake inhibitor, is known to reduce food intake and body weight and to improve the metabolic profile of obese subjects with and without metabolic complications such as type 2 diabetes. Due to cases of valvular heart diseases, DF was withdrawn from the market in 1997. However, serotonergic drugs are still used in clinical practice. We studied the hemodynamic and metabolic changes induced by in situ perfusion of inguinal subcutaneous adipose tissue (SAT) of normal-weight rats with either 1 microM isoproterenol (IP) or 5 microM DF using the microdialysis technique. Perfusion of SAT with IP resulted in an increase in blood flow (+25%) and lipolysis (+35%) when compared to baseline. In contrast to that, perfusion of SAT with DF resulted in a decrease in blood flow (-25%) and lipolysis (-35%). Additionally, dialysate glucose was decreased and dialysate lactate was increased during perfusion with DF, indicating stimulation of glucose uptake and the glycolytic pathway. It is concluded that DF reduces blood flow and lipolysis whereas it stimulates the glycolytic pathway in SAT and that this could contribute to the positive metabolic outcome, i.e., lowered blood lipids and fat mass of DF-treated obese subjects.

Exaggerated stress-induced release of nonesterified fatty acids in JCR:LA-corpulent rats

Studies were performed to test the hypothesis that the stress response is exaggerated in obesity and to identify which component of the response is modified. Chronically cannulated lean (+/+) and obese (cp/cp) JCR:LA rats were subjected to mild restraint stress for 15 minutes. Blood pressure and serum glucose, insulin, and corticosterone responses did not differ significantly between genotypes before, during, or after restraint. Lean rats had a significantly greater plasma epinephrine (EPI) response but a similar norepinephrine (NE) response compared with obese rats. Serum nonesterified fatty acid (NEFA) concentrations were unchanged in lean rats, but increased from 0.86 to a mean of 1.48 mmol/L in obese rats within 10 minutes of restraint. All animals recovered to prestress values by 45 minutes postrestraint. In obese rats, handling increased NEFAs to greater than 2 mmol/L before or at 165 minutes after restraint. In lean rats, NEFAs increased when handling occurred at 165 minutes after restraint, but there was no significant NEFA response at the prerestraint point. The sensitivity of adipose tissue to NE-induced lipolysis was not significantly different between genotypes. It is concluded that the exaggerated accumulation of NEFAs in the blood of obese rats results from increased adipose tissue mass. These increases in NEFAs in obese rats resulting from mild stress reached levels normally associated with gross pathology such as ketotic diabetes.

Chronic d-fenfluramine treatment reduces fat intake independent of macronutrient preference

We investigated the effect of chronic dexfenfluramine (DFEN) treatment on macronutrient selection in a three-choice diet paradigm using Sprague-Dawley rats. Baseline macronutrient intakes were measured for several days before the initiation of treatment. In Experiment 1, daily intraperitoneal injections of DFEN (1.5 mg/kg) or saline were administered 60 min before dark onset for 12 consecutive days and 24 h macronutrient intakes were measured. DFEN significantly reduced absolute fat intake (kcal) by 30% and relative fat intake (% of total energy) by 14% in animals that received dexfenfluramine treatment compared to controls over the 12-day period. Absolute carbohydrate intake was increased 24% compared to controls, but this difference was not significant. These changes in food intake resulted in a 10% lower total energy intake. Upon discontinuation of the drug, fat intake of the DFEN-treated rats rebounded to control levels within 24 h. In Experiment 2, rats were assigned to carbohydrate- or fat-preferring groups based on the ratio of their average daily carbohydrate to fat intake (kcal). All animals then received DFEN. During DFEN treatment, fat-preferring rats reduced their daily fat intake from 62 to 53% of total energy. The low baseline fat intake of carbohydrate-preferring rats was reduced even further by DFEN (from 24 to 15% of total energy). These corresponding effects of DFEN on macronutrient selection in both fat- and carbohydrate-preferring rats indicate that chronic DFEN treatment selectively suppressed fat intake independent of the preferred macronutrient diet.

Modulation of glucagon-induced glucose production by dexfenfluramine in rat hepatocytes

The mechanism of the antihyperglycaemic action of dexfenfluramine (DEXF) was investigated in isolated rat hepatocytes exposed to glucagon. Preincubation of hepatocytes with DEXF caused a dose-dependent inhibition of cyclic AMP formation by 100 nM glucagon (Ki = 0.29 mM) that was almost complete at 1 mM DEXF. Surprisingly, glucagon-induced phosphorylase activation was not affected by DEXF despite the significant drop in cyclic AMP levels. Glucose production stimulated by glucagon was inhibited by up to 48% by 1 mM DEXF, and the rate of glucose production correlated positively with the steady-state concentration of glucose 6-phosphate. DEXF also partially restored lactate + pyruvate production which was abolished by an optimal concentration of glucagon. Although DEXF was not able to prevent the inactivation of pyruvate kinase by glucagon, the lack of further accumulation of phosphoenolpyruvate in DEXF-treated cells supports the conclusion that the flux through pyruvate kinase is stimulated, probably via the increase in fructose 2,6-bisphosphate, thereby increasing glycolysis. Our results thus indicate that DEXF counteracts the inhibition of glycolysis by glucagon and that this property might contribute to the antihyperglycaemic effect of this drug. Furthermore, this study shows that, in the presence of the drug, glucagon caused phosphorylase activation and pyruvate kinase inactivation without a significant increase in cyclic AMP levels.

Potentiation by caffeine of the frequencies of chromosomal aberrations induced by chronic exposure to fenfluramine in mice

Fenfluramine (Fen), an amphetamine-derivative widely used in the treatment of obesity, has been evaluated in vivo in the bone marrow cells of Swiss albino mice for assessing its clastogenic potentials. Concentrations of 0.75, 1.50 and 5.0 mg Fen/kg body weight (b.w.) were administered orally for the study. Long-term treatment for 21 days showed dose-dependent significant increase in chromosomal aberrations on the 8th day. A significant decrease in aberration levels was seen in the late treatment period. Caffeine alone produced dose- and duration-dependent clastogenicity at doses of 2.0, 4.0 and 6.0 mg/kg b.w. when given by gavage. Using caffeine post-treatment (4.0 and 6.0 mg/kg b.w.) 2 h after Fen application, a strong synergism could be seen in the late treatment period as shown by the dose-response curves and by statistical analysis using the principle of least squares. The results support the hypothesis that prolonged Fen application induces dose-dependent increase in post-replication repair and caffeine enhanced toxicity by inhibiting repair process(es). The study suggests that Fen is a clastogen and since caffeine may have a synergistic effect, it should be avoided during treatment.

Dexfenfluramine. A review of its pharmacological properties and therapeutic potential in obesity

Dexfenfluramine stimulates serotoninergic activity by inhibiting serotonin reuptake into presynaptic neurons and by enhancing its release into brain synapses. Based on the serotonin hypothesis of appetite control these effects would be expected to reduce food intake and thus body-weight. Studies in animal models and severely overweight patients have confirmed the effectiveness of dexfenfluramine as a weight-reducing agent which appears to be well tolerated. Permanent weight loss is the goal of weight-reducing strategies and, based on current clinical evidence, dexfenfluramine appears to exert a weight reducing effect over periods of up to 12 months without development of tolerance, a problem that has limited the long term use of other pharmacological agents used in the treatment of this disorder. Dexfenfluramine facilitated weight loss in patients who had not responded satisfactorily to other weight-reducing strategies, prevented relapse in those patients who had achieved weight reduction by other methods, and corrected disturbed eating patterns (and therefore reduced weight gain) in small studies involving patients with premenstrual syndrome, seasonal affective disorder and nicotine withdrawal syndrome. Follow-up of the longest study reported with dexfenfluramine suggests that continued therapy is required in severely overweight patients if weight loss is to be maintained. Dexfenfluramine has not been directly compared with nonpharmacological measures of weight control such as behaviour modification or exercise programmes. The decision that pharmacological means are indicated in overweight patients must be highly individualised, and must consider the many complex factors that often contribute to overweight states, as well as the anticipated magnitude of drug effect. Despite such a cautionary note, and the expected need (at this stage of its development) for an expanded clinical study programme in certain areas, dexfenfluramine is a clear advance in the pharmacological approach to improved management of overweight individuals.

Sustained decreases in weight and serum insulin, glucose, triacylglycerol and cholesterol in JCR:LA-corpulent rats treated with D-fenfluramine

1. The effects of D-fenfluramine were studied in the JCR:LA-corpulent rat that is grossly obese, hyperphagic, hyperlipidaemic, hyperinsulinaemic and atherosclerosis-prone. 2. Daily doses of 1, 2.5 and 5 mg kg-1 of D-fenfluramine produced sustained decreases in body weight and food intake over a period of 30 days in 6 month old female rats fed ad libitum. This was accompanied by decreases in the circulating concentrations of glucose, triacylglycerol, free cholesterol and insulin. 3. Food restriction imposed by meal feeding also decreased circulating glucose, triacylglycerols, cholesterol and insulin and diminished the effect of D-fenfluramine on these parameters in male and female rats. 4. Addition of D-fenfluramine to drinking water to give a dose of about 0.25 mg kg-1 daily produced a sustained decrease in body weight and food intake of male and female rats over a nine week period. 5. The results show that the JCR:LA-corpulent rat is very sensitive to the pharmacological effects of D-fenfluramine. These rats should provide an appropriate animal model for determining the mechanisms of action of this anti-obesity agent and whether apparently beneficial changes in metabolism translate into long-term protection against premature atherosclerosis.

Decreased serum lipids, serum insulin and triacylglycerol synthesis in adipose tissue of JCR:LA-corpulent rats treated with benfluorex

Rats of the JCR:LA-corpulent strain were treated with benfluorex daily at a dose of 25 mg/kg body weight. This strain of rat, if homozygous for the cp gene (cp/cp), is hyperphagous, obese, hypertriglyceridemic, insulin resistant and in the case of male rats, atherosclerosis prone. The benfluorex treatment produced a sharp reduction in food intake which remained suppressed despite recovery toward normal after 2 weeks of treatment. This was accompanied by sustained decreases in body weight and adipose tissue mass. The ability of adipose tissue from female rats to take up glucose and convert it to lactate, glyceride-glycerol and fatty acids was decreased. This decrease was largely due to decreased adipose tissue mass. The serum concentrations of glucose, lactate, triacylglycerol, cholesterol, phospholipids and insulin were decreased in both sexes. The treatment also improved glucose tolerance and decreased corticosterone concentrations in male rats only. While reduction of food consumption contributes to the effects seen, benfluorex clearly had significant direct metabolic effects. The effects are consistent with an improved insulin sensitivity leading to a decrease in circulating triacylglycerol. The changes produced by benfluorex are all in directions that should inhibit atherogenesis in this animal model for the human obesity/hypertriglyceridemia/insulin resistant syndrome.

Stimulation of apolipoprotein secretion in very-low-density and high-density lipoproteins from cultured rat hepatocytes by dexamethasone

The effects of dexamethasone (a synthetic glucocorticoid) and insulin on the secretion of very-low-density lipoprotein (VLDL) and high-density lipoprotein (HDL) were investigated. Rat hepatocytes in monolayer culture were preincubated for 15 h in the presence or absence of combinations of 100 nM-dexamethasone and 2 nM-, 10 nM- or 50 nM-insulin. Dexamethasone increased [3H]oleate incorporation into secreted triacylglycerol by 2.7-fold and the mass of triacylglycerol secreted by 1.5-fold. Insulin alone decreased these parameters and antagonized the effect of dexamethasone. Dexamethasone increased the secretion of [3H]leucine in apolipoprotein (apo) E, and in the large (BH) and small (BI) forms of apo B in VLDL by about 7.1-, 3.6- and 4.0-fold respectively. Insulin alone decreased the secretion of these 3H-labelled apolipoproteins in VLDL. However, 2 nM-insulin with dexamethasone increased the secretion of 3H-labelled apo BH and apo BL by a further 0.8- and 3.2-fold respectively; 50 nM-insulin decreased the secretions of apo E, apo BH and apo BL in VLDL. Similar effects for dexamethasone or insulin alone were also obtained for the masses of apo E and apo BL + H secreted in VLDL. Albumin secretion was not significantly altered by either dexamethasone or insulin alone, but in combination they stimulated by 2.1-2.6-fold. Insulin or dexamethasone alone had little effect on the secretion of apolipoproteins in the HDL fraction. However, dexamethasone plus 2 nM-insulin increased the incorporation of [3H]leucine into apo AI, apo AH plus apo C, apo AIV and apo E of HDL by about 1.8-, 1.6-, 1.7- and 2.0-fold respectively. The apo E in the bottom fraction represented about 69% of the total 3H-labelled apo E secreted. The responses in the total secretion of apo E from the hepatocytes resembled those seen in HDL. The interactions of insulin and dexamethasone are discussed in relation to the general regulation of lipoprotein metabolism, the development of hyperlipidaemias and the predisposition to premature atherosclerosis.