Effects of Trecadrine, a beta3-adrenergic agonist, on leptin secretion, glucose and lipid metabolism in isolated rat adipocytes

Is the β3-Adrenoceptor a Valid Target for the Treatment of Obesity and/or Type 2 Diabetes?

β3-Adrenoceptors mediate several functions in rodents that could be beneficial for the treatment of obesity and type 2 diabetes. This includes promotion of insulin release from the pancreas, cellular glucose uptake, lipolysis, and thermogenesis in brown adipose tissue. In combination, they lead to a reduction of body weight in several rodent models including ob/ob mice and Zucker diabetic fatty rats. These findings stimulated drug development programs in various pharmaceutical companies, and at least nine β3-adrenoceptor agonists have been tested in clinical trials. However, all of these projects were discontinued due to the lack of clinically relevant changes in body weight. Following a concise historical account of discoveries leading to such drug development programs we discuss species differences that explain why β3-adrenoceptors are not a meaningful drug target for the treatment of obesity and type 2 diabetes in humans.

Effects of lipoic acid on lipolysis in 3T3-L1 adipocytes

Lipoic acid (LA) is a naturally occurring compound with beneficial effects on obesity. The aim of this study was to evaluate its effects on lipolysis in 3T3-L1 adipocytes and the mechanisms involved. Our results revealed that LA induced a dose- and time-dependent lipolytic action, which was reversed by pretreatment with the c-Jun N-terminal kinase inhibitor SP600125, the PKA inhibitor H89, and the AMP-activated protein kinase activator AICAR. In contrast, the PI3K/Akt inhibitor LY294002 and the PDE3B antagonist cilostamide enhanced LA-induced lipolysis. LA treatment for 1 h did not modify total protein content of hormone-sensitive lipase (HSL) but significantly increased the phosphorylation of HSL at Ser(563) and at Ser(660), which was reversed by H89. LA treatment also induced a marked increase in PKA-mediated perilipin phosphorylation. LA did not significantly modify the protein levels of adipose triglyceride lipase or its activator comparative gene identification 58 (CGI-58) and inhibitor G(0)/G(1) switch gene 2 (G0S2). Furthermore, LA caused a significant inhibition of adipose-specific phospholipase A2 (AdPLA) protein and mRNA levels in parallel with a decrease in the amount of prostaglandin E(2) released and an increase in cAMP content. Together, these data suggest that the lipolytic actions of LA are mainly mediated by phosphorylation of HSL through cAMP-mediated activation of protein kinase A probably through the inhibition of AdPLA and prostaglandin E(2).

Clock genes and clock-controlled genes in the regulation of metabolic rhythms

Daily rotation of the Earth on its axis and yearly revolution around the Sun impose to living organisms adaptation to nyctohemeral and seasonal periodicity. Terrestrial life forms have developed endogenous molecular circadian clocks to synchronize their behavioral, biological, and metabolic rhythms to environmental cues, with the aim to perform at their best over a 24-h span. The coordinated circadian regulation of sleep/wake, rest/activity, fasting/feeding, and catabolic/anabolic cycles is crucial for optimal health. Circadian rhythms in gene expression synchronize biochemical processes and metabolic fluxes with the external environment, allowing the organism to function effectively in response to predictable physiological challenges. In mammals, this daily timekeeping is driven by the biological clocks of the circadian timing system, composed of master molecular oscillators within the suprachiasmatic nuclei of the hypothalamus, pacing self-sustained and cell-autonomous molecular oscillators in peripheral tissues through neural and humoral signals. Nutritional status is sensed by nuclear receptors and coreceptors, transcriptional regulatory proteins, and protein kinases, which synchronize metabolic gene expression and epigenetic modification, as well as energy production and expenditure, with behavioral and light-dark alternance. Physiological rhythmicity characterizes these biological processes and body functions, and multiple rhythms coexist presenting different phases, which may determine different ways of coordination among the circadian patterns, at both the cellular and whole-body levels. A complete loss of rhythmicity or a change of phase may alter the physiological array of rhythms, with the onset of chronodisruption or internal desynchronization, leading to metabolic derangement and disease, i.e., chronopathology.

A new organotypic culture of adipose tissue fragments maintains viable mature adipocytes for a long term, together with development of immature adipocytes and mesenchymal stem cell-like cells

Adipose tissue that consists of mature and immature adipocytes is suggested to contain mesenchymal stem cells (MSCs), but a culture system for analyzing their cell types within the tissue has not been established. Here we show that three-dimensional collagen gel culture of rat sc adipose tissue fragments maintained viable mature adipocytes for a long term, producing immature adipocytes and MSC-like cells from the fragments, using immunohistochemistry, ELISA, and real time RT-PCR. Bromodeoxyuridine uptake of mature adipocytes was detected. Adiponectin and leptin, and adipocyte-specific genes of adiponectin, leptin, and PPAR-gamma were detected in culture assembly, whereas the lipogenesis factor insulin (20 mU/ml) and inflammation-related agent TNF-alpha (2 nm) increased and decreased, respectively, all of their displays. Both spindle-shaped cell types with oil red O-positive lipid droplets and those with expression of MSC markers (CD105 and CD44) developed around the fragments. The data indicate that adipose tissue-organotypic culture retains unilocular structure, proliferative ability, and some functions of mature adipocytes, generating both immature adipocytes and CD105+/CD44+ MSC-like cells. This suggests that our method will open up a new way for studying both multiple cell types within adipose tissue and the cell-based mechanisms of obesity and metabolic syndrome.

Linoleic acid decreases leptin and adiponectin secretion from primary rat adipocytes in the presence of insulin

Obesity rates have dramatically increased over the last few decades and, at the same time, major changes in the type of fatty acid intake have occurred. Linoleic acid, an n-6 polyunsaturated fatty acid, is an essential fatty acid occurring in high amounts in several western diets. A potential role of this fatty acid on obesity has been suggested. Controversial effects of linoleic acid on insulin sensitivity have also been reported. Thus, the aim of this study was to examine the direct effects of linoleic acid on leptin and adiponectin production, two adipokines known to influence weight gain and insulin sensitivity. Because insulin-stimulated glucose metabolism is an important regulator of leptin production, the effects of linoleic acid on adipocyte metabolism were also examined. For this purpose, isolated rat adipocytes were incubated with linoleic acid (1-200 microM) in the absence or presence of insulin. Linoleic acid (1-200 microM) significantly decreased insulin-stimulated leptin secretion and expression (P < 0.05), however, no changes in basal leptin production were observed. Linoleic acid also induced a significant decrease (approximately 20%) in adiponectin secretion (P < 0.05), but only in the presence of insulin and at the highest concentration tested (200 microM). This fatty acid did not modify either glucose uptake or lactate production and the percentage of glucose metabolized to lactate was not changed either. Together, these results suggest that linoleic acid seems to interfere with other insulin signalling pathway different from those controlling glucose uptake and metabolism, but involved in the regulation of leptin and adiponectin production.

Conjugated linoleic acid inhibits glucose metabolism, leptin and adiponectin secretion in primary cultured rat adipocytes

Conjugated linoleic acid (CLA) supplementation has been reported to induce insulin resistance in animals and humans, however, the underlying mechanisms remain unclear. The aim of this study was to examine the direct effects of CLA on leptin and adiponectin secretion, two hormones with actions known to influence insulin sensitivity. Isolated rat adipocytes were incubated with CLA (1-200microM) in the absence and presence of insulin (1.6nM). CLA inhibited both basal and insulin-stimulated leptin gene expression and secretion (-30 to -40%, P<0.05-0.01). CLA also inhibited basal adiponectin production (-20 to -40%, P<0.05-0.01), but not in the presence of insulin. CLA (50-200muM) decreased basal glucose uptake (P<0.05-0.01) and significantly increased the proportion of glucose metabolized to lactate (P<0.01). Insulin treatment partially prevented the inhibitory effects of CLA on glucose uptake and induced a significant increase (P<0.05-0.01) in the percentage of glucose metabolized to lactate. A strong inverse relationship was observed between the increase in the anaerobic utilization of glucose and the decreases of both leptin and adiponectin secretion. In addition, lipolysis and the expression of the adipogenic transcription factor PPARgamma were decreased by CLA. These results indicate that CLA inhibits leptin and adiponectin secretion and suggest that increased anaerobic metabolism of glucose may be involved in these effects. The inhibition of PPARgamma could also mediate the inhibition of adiponectin induced by CLA. Furthermore, the inhibition of leptin and adiponectin production induced by CLA may contribute to insulin resistance observed in CLA-treated animals and humans.

Intermittent and rhythmic exposure to melatonin in primary cultured adipocytes enhances the insulin and dexamethasone effects on leptin expression

Considering the cyclic characteristic of production and secretion of pineal melatonin, it is reasonable to assume that this oscillation might be important in determining the variety of its circadian and seasonal effects. To simulate this physiological condition in vitro, isolated adipocytes were exposed to melatonin in a circadian-like pattern by adding the hormone to the incubating medium during 12 hr (mimicking the night), followed by an equal period without melatonin (mimicking the day). This intermittent procedure was interrupted when three cycles with melatonin were fulfilled (60-hr incubation). Here, we report the effects of melatonin (1 nM) added intermittently or continuously to the incubating medium alone or in combination with insulin (5 nM) and/or dexamethasone (7 nM) on leptin release and expression by rat adipocytes. After acute 12-hr incubation neither melatonin nor insulin alone affected leptin expression, but together they increased it by 105%. Dexamethasone increased leptin mRNA content and release (70%) but this effect was not enhanced by melatonin. Nevertheless, after 60 hr under intermittent melatonin, we observed a synergism between melatonin and dexamethasone. This interaction promoted an increment (75% compared with dexamethasone alone) in leptin release and expression. Our results suggest that circadian-like exposure to melatonin potentiates the dexamethasone action and is important to the effects promoted by insulin on leptin expression. Based on an in vitro approach, this work helps to clarify the physiological relevance and the repercussions of the in vivo circadian pattern of melatonin secretion.

Benzo[a]pyrene impairs beta-adrenergic stimulation of adipose tissue lipolysis and causes weight gain in mice. A novel molecular mechanism of toxicity for a common food pollutant

Benzo[a]pyrene (B[a]P) is a common food pollutant that causes DNA adduct formation and is carcinogenic. The report of a positive correlation between human plasma B[a]P levels and body mass index, together with B[a]P's lipophilicity, led us to test for possible adverse effects of B[a]P on adipose tissue. In ex vivo experiments using primary murine adipocytes, B[a]P rapidly (within minutes) and directly inhibited epinephrine-induced lipolysis (up to 75%) in a dose-dependent manner. Half-maximum inhibition was obtained with a B[a]P concentration of 0.9 mg.L(-1) (3.5 microm). Lipolysis induced by beta(1)-, beta(2)- and beta(3)-adrenoreceptor-specific agonists, as well as ACTH, were also significantly inhibited by B[a]P, whereas forskolin-induced lipolysis was not B[a]P-sensitive. Similar inhibition of catecholamine-induced lipolysis by B[a]P was also seen in isolated human adipocytes; half-maximum inhibition of lipolysis was achieved with a B[a]P concentration of 0.02 mg.L(-1) (0.08 microm). In vivo treatment of C57Bl/6J mice with 0.4 mg.kg(-1) B[a]P inhibited epinephrine-induced release of free fatty acids by 70%. Chronic exposure of mice to B[a]P (0.5 mg.kg(-1) injected i.p. every 48 h) for 15 days also decreased lipolytic response to epinephrine and induced a 43% higher weight gain compared with controls (B[a]P: 2.23 +/- 0.12 g versus control: 1.56 +/- 0.18 g, P < 0.01) due to increased fat mass. The weight gain occurred consistently without detectable changes in food intake. These results reveal a novel molecular mechanism of toxicity for the environmental pollutant B[a]P and introduce the notion that chronic exposure of human population to B[a]P and possibly other polycyclic aromatic hydrocarbons could have an impact on metabolic disorders, such as obesity.

Eicosapentaenoic fatty acid increases leptin secretion from primary cultured rat adipocytes: role of glucose metabolism

Eicosapentaenoic acid (EPA), one of the n-3 polyunsaturated fatty acids, has been shown to stimulate leptin mRNA expression and secretion in 3T3-L1 cells. However, other studies have reported inhibitory effects of EPA on leptin expression and secretion in vivo and in vitro. To determine the direct effects of EPA on basal and insulin-stimulated leptin secretion, isolated rat adipocytes were incubated with EPA in the absence and presence of insulin. EPA (10, 100, and 200 μM) increased basal leptin gene expression and secretion (+43.8%, P < 0.05; +71.1%, P < 0.01; and +73.7%, P < 0.01, respectively). EPA also increased leptin secretion in the presence of 1.6 nM insulin; however, the effect was less pronounced than in the absence of it. Because adipocyte glucose and lipid metabolism are involved in the regulation of leptin production, the metabolic effects of this fatty acid were also examined. EPA (200 μM) increased basal glucose uptake in isolated adipocytes (+50%, P < 0.05). Anaerobic metabolism of glucose, as assessed by lactate production and proportion of glucose metabolized to lactate, has been shown to be inversely correlated to leptin secretion and was decreased by EPA in both the absence and presence of insulin. EPA increased basal glucose oxidation as determined by the proportion of14C-labeled glucose metabolized to CO2. Lipogenesis (14C-labeled glucose incorporation into triglyceride) was decreased by EPA in the absence of insulin, whereas lipolysis (glycerol release) was unaffected. The EPA-induced increase of basal leptin secretion was highly correlated with increased glucose utilization ( r = +0.89, P < 0.01) and inversely related to the anaerobic glucose metabolism to lactate. EPA’s effect on insulin-stimulated leptin secretion was not related to increased glucose utilization but was inversely correlated with anaerobic glucose metabolism to lactate ( r = −0.84, P < 0.01). Together, the results suggest that EPA, like insulin, stimulates leptin production by increasing the nonanaerobic/oxidative metabolism of glucose.

Isoproterenol decreases leptin release from rat and human adipose tissue through posttranscriptional mechanisms

In vivo and in vitro studies indicate that beta-adrenergic receptor agonists decrease leptin release from fat cells in as little as 30 min. Our objective was to determine whether alterations in leptin biosynthesis or secretion were involved in the short-term adrenergic regulation of leptin in human and rat adipose tissue. Isoproterenol (Iso) decreased leptin release from incubated adipose tissue of both nonobese and obese subjects to similar extent (-28 vs. -21% after 3 h). Inhibition of protein synthesis with cycloheximide did not block the effect of Iso on leptin release from human adipose tissue, suggesting that the Iso effect is independent of leptin synthesis. Iso also tended to increase tissue leptin content at the end of the 3-h incubation, as expected from the observed inhibition of release. Consistent with a posttranslational mechanism, Iso treatment did not affect leptin mRNA levels or relative rate of leptin biosynthesis as directly assessed by [35S]methionine incorporation into immunoprecipitable leptin. In contrast to these results in human adipose tissues, Iso did not decrease basal leptin release from rat adipose tissue. However, Iso did decrease insulin-stimulated leptin release by inhibiting the ability of insulin to increase leptin biosynthesis without detectably affecting leptin mRNA levels. Thus, in both human and rat, adrenergic regulation of posttranscriptional events (secretion in humans, translation in rats) may contribute to the rapid decline in circulating leptin that occurs when the sympathetic nervous system is activated, such as during fasting and cold exposure. Furthermore, the rat does not provide an ideal model to study mechanisms of cellular leptin regulation in humans.

Melatonin enhances leptin expression by rat adipocytes in the presence of insulin

Leptin and melatonin play an important role in the regulation of body mass and energy balance. Both hormones show a circadian rhythm, with increasing values at night. In addition, melatonin receptors were recently described in adipocytes, where leptin is synthesized. Here, we investigated the influence of melatonin and its interaction with insulin and dexamethasone on leptin expression. Isolated rat adipocytes were incubated with melatonin (1 nM) alone or in combination with insulin (5 nM) and/or dexamethasone (7 nM) for 6 h. Melatonin or insulin alone did not affect leptin expression, but together they increased it by 120%. Dexamethasone increased leptin mRNA content (105%), and this effect was not enhanced by melatonin. Simultaneous treatment with the three hormones provoked a further increase in leptin release (250%) and leptin mRNA (100%). Melatonin prevented the forskolin-induced inhibition (95%) of leptin expression. In addition, melatonin's ability to stimulate leptin release (in the presence of insulin) was completely blocked by pertussis toxin and luzindole. To gain further insight into the molecular basis of melatonin and insulin synergism, the insulin-signaling pathway was investigated. Melatonin increased the insulin-induced insulin receptor-beta tyrosine phosphorylation, which led to an increased serine phosphorylation of the downstream convergent protein Akt. We concluded that melatonin interacts with insulin and upregulates insulin-stimulated leptin expression. These effects are caused by melatonin binding to the pertussis toxin-sensitive G(i) protein-coupled membrane receptor (MT1 subtype) and the cross talk with insulin, since insulin receptor and its convergent target Akt are coactivated by melatonin.

Effects of arachidonic acid on leptin secretion and expression in primary cultured rat adipocytes

Leptin, a hormone produced in adipocytes, is a key signal in the regulation of food intake and energy expenditure. Several studies have suggested that leptin can be regulated by macronutrients intake. Arachidonic acid is a dietary fatty acid known to affect cell metabolism. Controversial effects of this fatty acid on leptin have been reported. The aim of this experimental trial was to evaluate the effect of the arachidonic acid on basal and insulin-stimulated leptin secretion and expression in isolated rat adipocytes. Because insulin-stimulated glucose metabolism is an important regulator of leptin expression and secretion by the adipocytes, the effects of the arachidonic acid on indices of adipocyte metabolism were also examined. Isolated adipocytes were incubated with arachidonic acid (1-200 microM) in the absence and presence of insulin (1.6 nM). Leptin secretion and expression, glucose utilization and lactate production were determined at 96 h. The arachidonic acid (200 microM) inhibited both the basal and insulin stimulated leptin secretion and expression. Glucose utilization was not affected by the acid. Basal lactate production was increased by the fatty acid at the highest concentration used (200 microM), however lactate production in presence of insulin was not modified. Finally, the percentage of glucose carbon released as lactate was significantly increased (200 microM). These results suggest that the inhibitory effect of the arachidonic acid on leptin secretion and expression may be due, al least in part, to the increase in the anaerobic utilization of glucose.

Effects of a beta3-adrenergic agonist on glucose uptake and leptin expression and secretion in cultured adipocytes from lean and overweight (cafeteria) rats

The increase in body and white adipose tissue weights induced by a high-fat diet were prevented by treatment with the beta3-adrenergic agonist Trecadrine. Plasma insulin levels were slightly elevated in overweight rats, while a decrease was observed in Trecadrine-treated groups. Insulin-dependent glucose uptake was impaired in adipocytes of the overweight rats in relation to lean animals. The beta3-adrenergic agonist induced an increase in insulin-stimulated glucose uptake by adipocytes as compared to the nontreated animals. In fact, Trecadrine treatment was able to restore to control values the impairment in insulin-mediated glucose uptake induced by the cafeteria diet, suggesting that Trecadrine prevents the development of insulin resistance in overweight animals. Basal leptin secretion was increased in adipocytes of the overweight rats in relation to lean animals. Trecadrine treatment induced a decrease in basal leptin secretion compared to the untreated animals. Insulin-stimulated leptin secretion reached similar levels in adipocytes of the overweight rats as in lean animals. There was a trend for insulin-induced leptin secretion to be lower at 24 h in Trecadrine-treated rats, but it did not reach statistical significance. In conclusion, adipocytes of diet-induced overweight animals have a higher basal leptin secretion, which is reduced by treatment with Trecadrine. However, neither the cafeteria diet nor the Trecadrine treatment significantly alters the ability of adipocytes to increase leptin secretion in response to insulin.