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M B Zemel

Publications and source records attributed to M B Zemel.

At least 37 records · Page 2Linked to original sources
PubMed1998

Agouti/melanocortin interactions with leptin pathways in obesity.

The cloning of mouse obesity genes and their human homologues provides unique opportunities to identify novel cellular targets for therapeutic intervention. The first of these to be cloned, agouti, antagonizes central nervous system melanocortin receptor (MCR) binding, resulting in hyperphagia and an obesity/hyperinsulinemia syndrome. There appears to be significant cross-talk between the agouti and leptin signaling systems. Agouti antagonism of central nervous system (CNS) MCR binding inhibits the anorexic effects of leptin, whereas agouti up-regulates adipocyte leptin expression, serving to limit the magnitude of agouti-induced obesity. The effects of agouti and leptin mutations on obesity, however, are independent and additive. Agouti also regulates adipocyte lipid metabolism, functioning both to increase the expression and activity of lipogenic genes and to inhibit lipolysis. Both of these actions occur via a Ca(2+)-dependent mechanism, suggesting that modulation of adipocyte Ca2+ transport may be a key target for further investigation.

Agouti Signaling Protein↗
PubMed1998

The agouti gene product inhibits lipolysis in human adipocytes via a Ca2+-dependent mechanism.

Overexpression of the murine agouti gene results in obesity. The human homologue of agouti is expressed primarily in human adipocytes, and we have shown recombinant agouti protein to increase adipocyte intracellular Ca2+([Ca2+]i) and thereby stimulate lipogenesis. However, since recent data demonstrate that increasing adipocyte [Ca2+]i may also inhibit lipolysis, we have investigated the role of agouti-induced [Ca2+]i increases in regulating lipolysis in human adipocytes. Short-term (1 h) exposure to recombinant agouti (100 nM) protein had no effect on basal lipolysis, although longer term treatment (24 h) caused a 60% decrease in basal lipolysis (P<0.0001). Short-term agouti treatment totally inhibited ACTH-induced lipolysis (P<0.05). Since melanocortin receptors (MCR) are involved in some actions of agouti, we next determined whether agouti's antilipolytic effect is exerted through competitive antagonism of the ACTH receptor (MCR-2). Forskolin (1 microM), an adenylate cyclase activator, induced a 48% increase in lipolysis in human adipocytes (P<0.05); this effect was reversed by 100 nM agouti (P<005), demonstrating that the antilipolytic effect of agouti is distal to the ACTH receptor. To determine the role of [Ca2+]i in the antilipolytic effect of agouti, human adipocytes were treated with KCl or arginine vasopressin to stimulate voltage- and receptor-stimulated Ca2+ influx, respectively. Both agents caused inhibition of forskolin-induced lipolysis (P<0.005). Furthermore, agouti's antilipolytic effect was also blocked by the Ca2+ channel blocker nitrendipine. These data demonstrate that agouti exerts a potent antilipolytic effect in human adipocytes via a Ca2+-dependent mechanism. This effect, combined with agouti-induced lipogenesis, represents a coordinate control of adipocyte lipid metabolism that may contribute to an agouti-induced obesity syndrome.

Adenosine Triphosphate↗
PubMed1998

Nutritional and endocrine modulation of intracellular calcium: implications in obesity, insulin resistance and hypertension.

Regulation of intracellular Ca2+ ([Ca2+]i) plays a key role in obesity, insulin resistance and hypertension, and [Ca2+]i disorders may represent a fundamental factor linking these three conditions. We have shown insulin to be a direct vasodilator, attenuating voltage-gated Ca2+ influx and stimulating Ca(2+)-ATPase transcription via a glucose-6-phosphate response element. These result in a net decrease in [Ca2+]i and thereby decrease vascular resistance, while these effects are blunted in insulin resistance, leading to increased vascular resistance. Consistent with this concept, pharmacological amplification of peripheral insulin sensitivity results in reduced arterial pressure. While insulin regulates [Ca2+]i, Ca2+ also regulates insulin signaling, as increasing [Ca2+]i impairs insulin signaling in some systems, possibly due to Ca2+ inhibition of insulin-regulated dephosphorylation. Finally, in recent studies of the mouse agouti gene, we have also demonstrated increased [Ca2+]i to play a key role in adipocyte lipogenesis, as follows. We have found dominant agouti mutants to exhibit increased [Ca2+]i in most tissues, leading to increased vascular reactivity and insulin resistance in vascular smooth muscle and skeletal muscle cells, respectively. Further, we have found recombinant agouti protein to directly increase [Ca2+]i in a variety of cells, including murine and human adipocytes, and to stimulate both the expression and activity of adipocyte fatty acid synthase and increase triglyceride accumulation in a Ca(2+)-dependent manner. These effects can be mimicked by stimulation of Ca2+ influx and blocked by Ca2+ channel inhibition, while treatment of mice with a Ca2+ antagonist attenuates agouti-induced obesity. Since humans express agouti in adipose tissue, it may similarly exert paracrine effects on [Ca2+]i and thereby stimulate de novo lipogenesis and promote obesity. Thus, Ca2+ signaling represents a target for therapeutic intervention in obesity as well as hypertension and insulin resistance.

Animals↗
PubMed1997

Combined effects of insulin treatment and adipose tissue-specific agouti expression on the development of obesity.

The agouti gene product is a secreted protein that acts in a paracrine manner to regulate coat color in mammals. Several dominant mutations at the agouti locus in mice cause the ectopic, ubiquitous expression of agouti, resulting in a condition similar to adult-onset obesity and non-insulin-dependent diabetes mellitus. The human agouti protein is 85% homologous to mouse agouti; however, unlike the mouse agouti gene, human agouti is normally expressed in adipose tissue. To address whether expression of agouti in human adipose tissue is physiologically relevant, transgenic mice were generated that express agouti in adipose tissue. Similar to most humans, these mice do not become obese or diabetic. However, we found that daily insulin injections significantly increased weight gain in the transgenic lines expressing agouti in adipose tissue, but not in nontransgenic mice. These results suggest that insulin triggers the onset of obesity and that agouti expression in adipose tissue potentiates this effect. Accordingly, the investigation of agouti's role in obesity and non-insulin-dependent diabetes mellitus in mice holds significant promise for understanding the pathophysiology of human obesity.

Adipose Tissue↗
PubMed1997

Agouti regulation of intracellular calcium: role of melanocortin receptors.

Several dominant mutations at the murine agouti locus cause a syndrome of marked obesity and insulin resistance. We have recently reported that intracellular free Ca2+ concentration ([Ca2+]i) is elevated in viable yellow mice. Because [Ca2+]i has a key role in the pathogenesis of insulin resistance, obesity, and hypertension, the role of the purified agouti gene product in regulating [Ca2+]i was evaluated in a number of cell types. Purified murine agouti induced slow, sustained increases in [Ca2+]i in A7r5 vascular smooth muscle cells and 3T3-L1 adipocytes in a dose-dependent fashion. In L6 skeletal myocytes, agouti stimulated an increase in [Ca2+]i with an apparent concentration eliciting 50% of the maximal response (EC50) of 62 nM. This response was substantially inhibited by Ca2+ entry blockade with nitrendipine. To determine whether melanocortin receptors play a role in agouti regulation of [Ca2+]i, we examined the effect of melanocortin peptides and agouti in cells stably transfected with human melanocortin receptors. Human embryonic kidney cells (HEK-293 cells) transfected with either the human melanocortin 1 receptor (MC1R) or melanocortin 3 receptor responded to human agouti with slow, sustained increases in [Ca2+]i, whereas nontransfected HEK-293 cells with no melanocortin receptors did not respond to agouti. Dose-response curves in the MC1R line showed that agouti had an EC50 of 18 nM, which is comparable to that for agouti antagonism of (125)I-Nle,D-Phe-alpha-melanocyte-stimulating hormone binding in the same cell line. This direct effect of agouti on stimulating increases in [Ca2+]i suggests a potential mechanism for agouti-induced insulin resistance.

Adipocytes↗
PubMed1996

Insulin regulation of vascular smooth muscle glucose transport in insulin-sensitive and resistant rats.

Previous studies from this laboratory demonstrate insulin regulation of vascular smooth muscle intracellular calcium transport and of vascular tone. Accordingly, we measured insulin regulated glucose transport and GLUT4 protein in aortic tissue from insulin-resistant and sensitive rats in order to document classical insulin action in this tissue. We assessed (via in vitro muscle-bath and perfusion) basal and insulin-stimulated [3H]-2-deoxyglucose transport in aorta from lean and obese Zucker rats. In the muscle-bath system, insulin significantly stimulated aortic 2-deoxyglucose transport by 30%, but there was no difference in either baseline or insulin-stimulated glucose transport in lean versus obese rats. However, in the perfusion system, in which only the luminal surface of the vessel was exposed, insulin significantly (p < 0.04) stimulated 2-deoxyglucose transport in the aorta of only the lean animals. Western blot analysis demonstrated that aorta from lean and obese Zucker rats contained substantial concentrations of GLUT4 protein. Surprisingly, in the muscle-bath system 2-deoxyglucose transport was markedly higher in the aorta than in the soleus (p < 0.0002). These data demonstrate that vascular smooth muscle is an insulin-sensitive tissue which may be partially impaired in insulin resistant animals.

Animals↗
PubMed1996

Dietary fat and sex modify heterozygote effects of the rat fatty (fa) allele.

Rats carrying one copy of the obesity gene fa may exhibit intermediate phenotypes between lean (+/+) and homozygous mutants (fa/fa). Previous data suggested to us that fa heterozygotes may be more sensitive than wild-type rats to high fat diets. To test this hypothesis, we generated +/+ and fa/+ rats and fed them diets containing 12% or 48% energy as fat for 7 wk. Energy efficiency was significantly greater in males than in females and in high fat-fed vs. low fat-fed rats. Perirenal fat pad weights were significantly greater in males than in females, in high fat-vs. low fat-fed rats and in fa/+ vs. +/+ rats. Adipose and soleus plasma membrane calcium-ATPase concentrations were significantly lower in rats fed the high fat diet. This protein was also lower in soleus of fa/+ rats compared with +/+ rats. There were significant diet x genotype interactions such that the high fat diet had the greatest effect on fat pads and calcium-ATPase in fa/+ rats. The results of the present study show heterozygote effects of the fa allele and suggest that these effects may be modulated by both sex-related factors and dietary manipulation.

Adipocytes↗
PubMed1996

Upregulation of adipocyte metabolism by agouti protein: possible paracrine actions in yellow mouse obesity.

Mutations leading to ectopic expression of the murine agouti gene (a) result in progressive obesity. To further characterize this model, we analyzed adipose and hepatic mRNA levels for fatty acid synthase (FAS) and stearoyl-CoA desaturase (SCD), two key enzymes in de novo fatty acid synthesis and desaturation, respectively. FAS and SCD mRNA in both tissues of obese (Avy) mice were dramatically increased relative to lean (ala) controls. Excessive expression of these genes in this model could be due to direct effects of the agouti gene product; to test this possibility we treated 3T3-L1 adipocytes in vitro with recombinant agouti protein. Agouti treatment increased FAS and SCD mRNA levels by 1.5- and 4-fold, respectively. In addition, FAS activity and triglyceride content were 3-fold higher in agoutitreated 3T3-L1 cells relative to controls; these effects were attenuated by simultaneous treatment with a calcium channel blocker (nitrendipine). These data demonstrate that the agouti protein can directly increase lipogenesis in adipocytes and suggest that these effects are mediated through an intracellular calcium-dependent mechanism.

Adipocytes↗
PubMed1996

Adipose tissue stearoyl-CoA desaturase mRNA is increased by obesity and decreased by polyunsaturated fatty acids.

Stearoyl-CoA desaturase (SCD) is a key regulatory enzyme in the synthesis of unsaturated fatty acids. Although regulation of hepatic SCD by obesity and polyunsaturated fatty acids (PUFA) has been well investigated, no studies have addressed whether similar regulation occurs in adipose tissue. We addressed these questions by feeding control (12% corn oil) and high-PUFA (48% corn oil) diets to lean and obese Zucker rats and analyzing SCD mRNA levels in adipose tissue and liver. We report that SCD mRNA content was dramatically elevated in adipose tissue of obese vs. lean rats on both diets and was significantly decreased by PUFA in both genotypes. Interestingly, we demonstrate that SCD expression was directly downregulated in a dose dependent manner by PUFA in 3T3-L1 adipocytes. We conclude that 1) obese Zucker rats overexpress the SCD gene in both liver and adipose tissue and 2) PUFA directly suppress SCD expression in adipocytes. Further studies will elucidate the mechanisms responsible for obesity- and PUFA-mediated regulation of SCD in adipose cells.

Adipocytes↗
PubMed1996

The effects of calcium channel blockade on agouti-induced obesity.

We have previously observed that obese viable yellow (Avy/a) mice exhibit increased intracellular Ca2+ ([Ca2+]i) and fatty acid synthase (FAS) gene expression; further, recombinant agouti protein increases in cultured adipocytes and these effects are inhibited by Ca2+ channel blockade. Accordingly, we determined the effect of Ca2+ channel blockade (nifedipine for 4 wk) on FAS and obesity in transgenic mice expressing the agouti gene in a ubiquitous manner. The transgenic mice initially were significantly heavier (30.5+/-0.6 vs. 27.3+/-0.3 g; P<0.001) and exhibited a 0.81 degrees C lower initial core temperature (P<0.0005), an approximately twofold increase in fat pad weights (P=0.002), a sevenfold increase in adipose FAS activity (P=0.009), and a twofold increase in plasma insulin level (P<0.05) compared to control mice. Nifedipine treatment resulted in an 18% decrease in fat pad weights (P<0.007) and a 74% decrease in adipose FAS activity (P=0.03), normalized circulating insulin levels and insulin sensitivity (P<0.05), and transiently elevated core temperature in the transgenic mice, but was without effect in the control mice. These data suggest that agouti regulates FAS, fat storage, and possibly thermogenesis, at least partially, via a [Ca2+]i-dependent mechanism, and that Ca2+ channel blockade may partially attenuate agouti-induced obesity.

Adipose Tissue↗
PubMed1995

Insulin stimulation of intracellular free Ca2+ recovery and Ca(2+)-ATPase gene expression in cultured vascular smooth-muscle cells: role of glucose 6-phosphate.

Wa have previously reported that insulin accelerates recovery of intracellular Ca2+ concentrations ([Ca2+]i) from pressor agonist-induced Ca2+ loads and stimulates both plasmalemmal and sarcoplasmic-reticulum Ca(2+)-ATPase gene expression in cultured and freshly isolated vascular smooth-muscle cells (VSMCs), suggesting that insulin attenuation of vascular tone may result from modulation of [Ca2+]i. Accordingly, we have now evaluated the linkage between this insulin-regulation of VSMC[Ca2+]i and classical actions of insulin (i.e. glucose transport and metabolism). Cultured VSMCs were incubated in the presence or absence of insulin in a medium containing either pyruvate, glucose, 3-O-methylglucose or 2-deoxyglycose. Insulin caused an 87% increase in [Ca2+]i recovery rate after stimulation with arginine-vasopressin (P < 0.01) and caused a marked increase in Ca(2+)-ATPase mRNA and protein levels in the presence of glucose. Comparable increases in both [Ca2+]i recovery and Ca2(+)-ATPase expression were found when glucose was replaced by 2-deoxyglucose. In contrast, no stimulation was found in either the glucose-free or 3-O-methylglucose-containing medium. As both glucose analogues are transported, but only 2-deoxyglucose is phosphorylated, this indicates that glucose transport and metabolism to glucose 6-phosphate is essential for insulin regulation of VSMC [Ca2+]i, possibly via a glucose-6-phosphate-dependent carbohydrate-response element in the Ca2(+)-ATPase gene.

Animals↗
PubMed1995

Agouti regulation of intracellular calcium: role in the insulin resistance of viable yellow mice.

Several dominant mutations at the agouti locus in the mouse cause a syndrome of marked obesity, hyperinsulinemia, and insulin resistance. Although it is known that the agouti gene is expressed in an ectopic manner in these mutants, the precise mechanism by which the agouti gene product mediates these effects is unclear. Since intracellular Ca2+ is believed to play a role in mediating insulin action and dysregulation of Ca2+ flux is observed in diabetic animals and humans, we examined the status of intracellular Ca2+ in mice carrying the dominant agouti allele, viable yellow (Avy). We show here that in mice carrying this mutation, the intracellular free calcium concentration ([Ca2+]i) is elevated in skeletal muscle, and the degree of elevation is closely correlated with the degree to which the mutant traits are expressed in individual animals. Moreover, we demonstrate that the agouti gene product is capable of inducing increased [Ca2+]i in cultured and freshly isolated skeletal muscle myocytes from wild-type mice. Based on these findings, we present a model in which we propose that the agouti polypeptide promotes insulin resistance in mutant animals through its ability to increase [Ca2+]i.

Aging↗
PubMed1995

Variations of blood pressures in lean Zucker rats fed low or high fat diets.

The purpose of this study was to determine whether lean (Fa/?) Zucker rats which exhibit greater metabolic efficiency in response to a high (48%) fat diet also exhibit hypertension.Twenty-nine lean (Fa/?) and eleven obese (fa/fa) Zucker rats were fed low (12% energy) and high (48% energy) fat diets. Food intake and weight change were recorded weekly to obtain an index of energy efficiency. After 6 wk, direct intra-arterial blood pressure was measured in conscious, unrestrained animals. Arterial blood was collected for analyses of plasma insulin and glucose. The high fat diet had no effect on blood pressure in obese rats, although obese animals fed both diets had significantly higher blood pressure than lean siblings. High fat feeding in lean rats unmasked a heterogeneity in blood pressure. Blood pressure in this group was significantly correlated with weight change, energy efficiency ratio and plasma glucose concentration. Lean rats fed the high fat diet with blood pressures above the group median exhibited significantly greater plasma insulin (P < 0.02) and glucose (P < 0.01) concentration and energy efficiency ratios (P < 0.01) than those with lower blood pressures. Thus, it appears that there were two groups of lean animals present, one of which was more phenotypically sensitive to high fat feeding than the other. This suggests a possible gene dosage effect of the fa allele in the Zucker rat model.

Animals↗
PubMed1995

Insulin resistance vs. hyperinsulinemia in hypertension: insulin regulation of Ca2+ transport and Ca(2+)-regulation of insulin sensitivity.

Hypertension in obesity and insulin resistance has been attributed to insulin stimulation of sympathetic neural output and renal sodium retention. However, recent data demonstrates a significant vasodilatory effect of insulin and suggests that vascular smooth muscle resistance to this action may instead be the cause of hypertension in insulin resistance. This concept is supported by the observation that pharmacological amplification of peripheral insulin sensitivity results in reduced arterial pressure. Insulin attenuates vasoconstrictor responses to pressor agonists and accelerates vascular smooth muscle relaxation, while these effects are blunted in obesity and insulin resistance. Insulin regulation of vasoconstriction and vascular relaxation appears to be secondary to regulation of intracellular Ca2+ ([Ca2+]i), as insulin attenuates both voltage- and receptor-mediated Ca2+ influx and stimulates both the transcription and activity of Ca(2+)-ATPase in vascular smooth muscle cells. Further, these effects are also blunted in insulin resistance. Although [Ca2+]i plays a poorly understood role in insulin signalling, increases beyond an optimal range results in impaired insulin sensitivity, possibly by Ca(2+)-inhibition of insulin-induced dephosphorylation of insulin-sensitive substrates. Consistent with this concept, ectopic overexpression of the agouti gene in the viable yellow (Avy) mouse results in increased skeletal myocyte [Ca2+]i. Accordingly, increased [Ca2+]i in primary insulin target tissues appears to result in peripheral insulin resistance which then results in aberrant regulation of vascular smooth muscle [Ca2+]i and increases in arterial pressure.

Animals↗