Skeletal muscle overexpression of nuclear respiratory factor 1 increases glucose transport capacity

Nuclear respiratory factor 1 (NRF-1) is a transcriptional activator of nuclear genes that encode a range of mitochondrial proteins including cytochrome c, various other respiratory chain subunits, and delta-aminolevulinate synthase. Activation of NRF-1 in fibroblasts has been shown to induce increases in cytochrome c expression and mitochondrial respiratory capacity. To further evaluate the role of NRF-1 in the regulation of mitochondrial biogenesis and respiratory capacity, we generated transgenic mice overexpressing NRF-1 in skeletal muscle. Cytochrome c expression was increased approximately twofold and delta-aminolevulinate synthase was increased approximately 50% in NRF-1 transgenic muscle. The levels of some mitochondrial proteins were increased 50-60%, while others were unchanged. Muscle respiratory capacity was not increased in the NRF-1 transgenic mice. A finding that provides new insight regarding the role of NRF-1 was that expression of MEF2A and GLUT4 was increased in NRF-1 transgenic muscle. The increase in GLUT4 was associated with a proportional increase in insulin-stimulated glucose transport. These results show that an isolated increase in NRF-1 is not sufficient to bring about a coordinated increase in expression of all of the proteins necessary for assembly of functional mitochondria. They also provide the new information that NRF-1 overexpression results in increased expression of GLUT4.

Energy metabolism and cytotoxicity

Fatty acids constitute a major source of metabolic fuel for energy production in kidney tissue. During acute renal failure (ARF) injury to the proximal tubule and medullary thick ascending limb leads to structural and functional alterations that result in reduced expression and activity of mitochondrial and peroxisomal fatty acid oxidation (FAO) enzymes. Reduced DNA binding activity of peroxisome proliferator activated receptor-alpha (PPARalpha) to its target genes and decreased expression of its tissue-specific coactivator PPAR-gamma-coactivator-1 (PGC-1) in the mouse proximal tubule and the medullary thick ascending limb, represent 2 potential mechanisms that account for the observed alterations of FAO during ARF. Pretreatment with PPARalpha ligands restores the expression and activity of renal FAO enzymes, and this metabolic alteration leads to amelioration of acute tubular necrosis caused by ischemia/reperfusion or cisplatin-induced ARF. More studies are needed to examine further the cellular mechanisms of substrate inhibition, and to determine if metabolic pathways, in addition to the recovery of FAO, account for the protective effect (s) of PPARalpha ligands during acute renal failure.

Effects of chronic AICAR treatment on fiber composition, enzyme activity, UCP3, and PGC-1 in rat muscles

This study was designed to determine the histological and metabolic effects of the administration of 5'-AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) for 14 successive days. AICAR treatment caused a significant decrease in the percentage of type IIB fibers and the concomitant increase in the percentage of type IIX fibers in extensor digitorum longus (EDL) muscle. The capillary density and the capillary-to-fiber ratio were not altered by AICAR. AICAR treatment increased the glycolytic and oxidative enzyme activities but not the antioxidant enzyme activities. The AICAR treatment increased the uncoupling protein 3 (UCP3) level in EDL and the peroxisome proliferator-activated receptor-gamma coactivator-1alpha protein level in the soleus and EDL muscles, whereas the myogenin level was not altered by AICAR. These results seem to imply that the chronic activation of AMPK alters such muscle histochemical and metabolic characteristics.

Skeletal muscle metabolism in physiology and in cancer disease

Skeletal muscle is a tissue of high demand and it accounts for most of daily energy consumption. The classical concept of energy metabolism in skeletal muscle has been profoundly modified on the basis of studies showing the influence of additional factors (i.e., uncoupling proteins (UCPs) and peroxisome proliferator activated receptors (PPARs)) controlling parameters, such as substrate availability, cellular enzymes, carrier proteins, and proton leak, able to affect glycolysis, nutrient oxidation, and protein degradation. This extremely balanced system is greatly altered by cancer disease that can induce muscle cachexia with significant deleterious consequences and results in muscle wasting and weakness, delaying or preventing ambulation, and rehabilitation in catabolic patients.

Intramyocitic lipid accumulation and SREBP-1c expression are related to insulin resistance and cardiovascular risk in morbid obesity

The aim of this study was to investigate the sterol regulatory element-binding protein 1c (SREBP1c) mRNA muscle expression in morbid obese subjects before and after massive lipid malabsorption due to bariatric surgery (bilio-pancreatic diversion, BPD). We studied 11 obese subjects (BMI 49+/-2 kg/m2) before and 24 months after BPD. Skeletal muscle SREBP1c mRNA expression was determined using RT-competitive PCR. Intramyocytic triglycerides were quantified by HPLC. Insulin sensitivity (M/I) was assessed by euglycemic-hyperinsulinemic clamp. Energy expenditure and respiratory quotient (RQ) were measured over 24 h in a calorimetric chamber. Total cardiovascular risk dropped from 2 before to -2.5 after BPD (P<0.0001). The M/I value was normalized after surgery (0.036+/-0.0148 to 0.095+/-0.0147 micromol kgFFM(-1) min(-1) pmoles(-1) P<0.001). SREBP-1c mRNA levels were decreased (from 4.12+/-2.43 to 2.69+/-1.83% of cyclophilin mRNA, P=0.02) after BPD. In a multiple regression analysis, M/I values (P<0.0001) as well as the intramyocytic triglyceride levels (P=0.039) were the most powerful independent variables for predicting cardiovascular risk. Our results show that the reduction of cardiovascular risk after bariatric massive weight loss is strongly related to the reversion of insulin resistance and to the lowering of intramyocytic triglyceride depots. These two parameters are associated with a significant reduction in SREBP-1c mRNA expression in skeletal muscle, suggesting that this transcription factor might be involved in the accumulation of triglycerides in muscle cells of morbidly obese subjects.

Acylation stimulating protein stimulates insulin secretion

Acylation stimulating protein (ASP) is a hormone produced by adipocytes and is of importance for the storage of energy as fat. We examined whether ASP might also have effects on islet function. In clonal INS-1 cells, ASP dose-dependently augmented glucose-stimulated insulin secretion. The lowest effective dose of ASP at 10 mmol/l glucose was 5 micro mol/l. The effect was glucose-dependent because ASP did not increase insulin secretion at 1 mmol/l glucose but had clear effect at 10 and 20 mmol/l glucose. Similarly, ASP augmented glyceraldehyde-induced insulin secretion but the hormone did not enhance insulin secretion in response to depolarization by 20 mmol/l of KCl. ASP-induced insulin secretion was completely abolished by competitive inhibition of glucose phosphorylation by glucokinase with 5-thio-glucose and was partially inhibited by the calcium channel blocker, nifedipine, and by the protein kinase C inhibitor, GF109203. Furthermore, thapsigargin, an inhibitor of Ca(2+)-ATPase in the endoplasmic reticulum, did not affect ASP-induced insulin secretion. ASP (>5 micro mol/l) also augmented glucose-stimulated insulin secretion from islets isolated from C57BL/6J mice, and intravenous administration of ASP (50 nmol/kg) augmented the acute (1 and 5 min) insulin response to intravenous glucose (1 g/kg) in C57BL/6J mice. This was accompanied by an increased rate of glucose disposal. Minimal model analyses of data derived from the intravenous glucose tolerance test revealed that whereas ASP augmented insulin secretion, the hormone did not affect insulin sensitivity (S(I)) or glucose effectiveness (S(G)). We conclude that ASP augments glucose-stimulated insulin secretion through a direct action on the islet beta cells. The effect is dependent on glucose phosphorylation, calcium uptake and protein kinase C. Stimulation of insulin secretion by ASP in vivo results in augmented glucose disposal.

Cidea-deficient mice have lean phenotype and are resistant to obesity

The thermogenic activity of brown adipose tissue (BAT), important for adaptive thermogenesis and energy expenditure, is mediated by the mitochondrial uncoupling protein1 (Ucp1) that uncouples ATP generation and dissipates the energy as heat. We show here that Cidea, a protein of unknown function sharing sequence similarity with the N-terminal region of DNA fragmentation factors Dffb and Dffa, is expressed at high levels in BAT. Cidea-null mice had higher metabolic rate, lipolysis in BAT and core body temperature when subjected to cold treatment. Notably, Cidea-null mice are lean and resistant to diet-induced obesity and diabetes. Furthermore, we provide evidence that the role of Cidea in regulating thermogenesis, lipolysis and obesity may be mediated in part through its direct suppression of Ucp1 activity. Our data thus indicate a role for Cidea in regulating energy balance and adiposity.

Acquirement of brown fat cell features by human white adipocytes

Obesity, i.e. an excess of white adipose tissue (WAT), predisposes to the development of type 2 diabetes and cardiovascular disease. Brown adipose tissue is present in rodents but not in adult humans. It expresses uncoupling protein 1 (UCP1) that allows dissipation of energy as heat. Peroxisome proliferator-activated receptor gamma (PPAR gamma) and PPAR gamma coactivator 1 alpha (PGC-1 alpha) activate mouse UCP1 gene transcription. We show here that human PGC-1 alpha induced the activation of the human UCP1 promoter by PPAR gamma. Adenovirus-mediated expression of human PGC-1 alpha increased the expression of UCP1, respiratory chain proteins, and fatty acid oxidation enzymes in human subcutaneous white adipocytes. Changes in the expression of other genes were also consistent with brown adipocyte mRNA expression profile. PGC-1 alpha increased the palmitate oxidation rate by fat cells. Human white adipocytes can therefore acquire typical features of brown fat cells. The PPAR gamma agonist rosiglitazone potentiated the effect of PGC-1 alpha on UCP1 expression and fatty acid oxidation. Hence, PGC-1 alpha is able to direct human WAT PPAR gamma toward a transcriptional program linked to energy dissipation. However, the response of typical white adipocyte targets to rosiglitazone treatment was not altered by PGC-1 alpha. UCP1 mRNA induction was shown in vivo by injection of the PGC-1 alpha adenovirus in mouse white fat. Alteration of energy balance through an increased utilization of fat in WAT may be a conceivable strategy for the treatment of obesity.

The human mitochondrial translation initiation factor 2 gene (MTIF2): transcriptional analysis and identification of a pseudogene

Mitochondrial translation initiation factor 2 (MTIF2) is nuclear-encoded and functions in mitochondria to initiate the translation of proteins encoded by the mitochondrial genome. To gain insight into mechanisms that regulate MTIF2 gene expression, the genomic copy and the 5' and 3' flanking regions of MTIF2 were isolated using a combination of genomic library screening and polymerase chain reaction (PCR). MTIF2 is approximately 33.5-kb long and contains 16 exons, confirming data from the Human Genome Project. With RNA ligase-mediated rapid amplification of cDNA ends (RLM-RACE), we mapped the transcription start point in human heart tissue to a cytosine residue 296 bp upstream from the translation initiation site. The region surrounding the transcription start point contains consensus binding sites for transcription factors Sp1, nuclear respiratory factor 2 (NRF-2) and estrogen receptor, while enhancer binding sites were identified upstream. Promoter constructs were prepared in a luciferase reporter vector and transiently transfected into 293T cells. The minimal promoter gave an expression level 3.5x higher than the SV40 control (P=0.001), while the construct containing the minimal promoter plus the enhancer region gave a 3.8x higher level of expression compared to the control (P<0.001). We also discovered a pseudogene of MTIF2 and mapped it to chromosome 1p13-12.

PGC-1beta in the regulation of hepatic glucose and energy metabolism

Peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) is a transcriptional coactivator that regulates multiple aspects of cellular energy metabolism, including mitochondrial biogenesis, hepatic gluconeogenesis, and beta-oxidation of fatty acids. PGC-1alpha mRNA levels are increased in both type-1 and type-2 diabetes and may contribute to elevated hepatic glucose production in diabetic states. We have recently described PGC-1beta, a novel transcriptional coactivator that is a homolog of PGC-1alpha. Although PGC-1beta shares significant sequence similarity and tissue distribution with PGC-1alpha, the biological activities of PGC-1beta in the regulation of cellular metabolism is unknown. In this study, we used an adenoviral-mediated expression system to study the function of PGC-1beta both in cultured hepatocytes and in the liver of rats. PGC-1beta, like PGC-1alpha, potently induces the expression of an array of mitochondrial genes involved in oxidative metabolism. However, in contrast to PGC-1alpha, PGC-1beta poorly activates the expression of gluconeogenic genes in hepatocytes or liver in vivo, illustrating that these two coactivators play distinct roles in hepatic glucose metabolism. The reduced ability of PGC-1beta to induce gluconeogenic genes is due, at least in part, to its inability to physically associate with and coactivate hepatic nuclear receptor 4alpha (HNF4alpha) and forkhead transcription factor O1 (FOXO1), two critical transcription factors that mediate the activation of gluconeogenic gene expression by PGC-1alpha. These data illustrate that PGC-1beta and PGC-1alpha have distinct arrays of activities in hepatic energy metabolism.

Overexpression of peroxisome proliferator-activated receptor gamma coactivator-1alpha down-regulates GLUT4 mRNA in skeletal muscles

Exercise training increases mitochondria and GLUT4 in skeletal muscles. Recent studies indicate that an increased expression of the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) by exercise may promote mitochondrial biogenesis and fatty acid oxidation. To examine whether increased PGC-1alpha expression was also responsible for an increase of GLUT4 expression, transgenic mice that overexpress PGC-1alpha in skeletal muscles driven by a human alpha-skeletal actin promoter were made. PGC-1alpha was overexpresssed in skeletal muscles including type I and II fiber-rich muscles but not in the heart. With an increase of PGC-1alpha mRNA, type II fiber-rich muscles were redder, and genes of mitochondrial oxidative metabolism were up-regulated in skeletal muscles, whereas the expression of GLUT4 mRNA was unexpectedly down-regulated. In parallel with a decrease of GLUT4 mRNA, an impairment of glycemic control after intraperitoneal insulin administration was observed. Thus, an increase of PGC-1alpha plays a role in increasing mitochondrial biogenesis and fatty acid oxidation but not in increasing GLUT4 mRNA in skeletal muscles.

Disruption of leptin signaling contributes to cardiac hypertrophy independently of body weight in mice

BACKGROUND

Whether left ventricular hypertrophy (LVH) in obesity results from increased hemodynamic load or altered neurohormonal signaling remains controversial. Dysregulation of leptin, a neurohormone essential to energy homeostasis, is implicated in the pathogenesis of obesity. Because leptin has cardiovascular bioactivity, we hypothesized that disruption of leptin signaling mediates the development of obesity-associated LVH.

METHODS AND RESULTS

We measured left ventricular (LV) wall thickness and LV mass with echocardiography in mice lacking leptin (ob/ob, n=15) or functional receptor (db/db, n=10) and controls at 2, 4, and 6 months of age. None of the mice had LVH at 2 months. Progressive obesity developed in ob/ob and db/db mice. At 6 months, LVH occurred in ob/ob and db/db compared with controls. We observed corresponding myocyte hypertrophy by light microscopy. To separate the direct contribution of leptin deficiency from mechanical effects of obesity, we induced weight loss in 6- to 8-month-old ob/ob mice either by leptin infusion or caloric restriction. Mice in both groups lost similar weight compared with placebo-treated controls. Leptin infusion completely reversed the increase in wall thickness with partial resolution of myocyte hypertrophy, whereas calorie-restricted mice had no decrease in wall thickness and a lesser change in myocyte size.

CONCLUSIONS

Together these data show that the effect of leptin on LV remodeling is not attributable to weight loss alone, indicating that leptin has antihypertrophic effects on the heart, either directly or through a leptin-regulated neurohumoral pathway. Disruption of leptin signaling may represent a novel mechanism in LVH and related cardiovascular disorders.

Expression of Leptin Receptor Isoforms in vitro : Lack of Effects of Leptin on Endometrial Cytokine and MMP Production

PROBLEM

Leptin has a key role to play in human female reproduction. Its receptor is expressed highly throughout the reproductive tract. Cytokines have an important role in preparing the endometrium for implantation and leptin is known to modulate cytokine production in other tissues. We, therefore, investigated the possible role of leptin in endometrial growth and function.

METHOD OF STUDY

Reverse transcriptase polymerase chain reaction and immunocytochemistry were used to determine the pattern of expression of leptin receptor isoforms in primary human endometrial epithelial and stromal cells in culture. The effect of leptin on cell growth and on the production of cytokines [Leukaemia Inhibitory Factor (LIF), interleukin 6 and tumour necrosis factor-alpha] and matrix metalloproteinases (MMP) (MMP2 and MMP-9) was also investigated.

RESULTS

Expression of the long form of the leptin was restricted to the cultured endometrial, epithelial cells. Both cultured endometrial stromal and epithelial cells expressed the short and variant isoforms of the receptor. Incubation of epithelial and stromal cell cultures with varying concentrations of leptin (0-1000 ng/mL) had no significant effect on cell growth or levels of MMP-2 or MMP-9 production. Leptin also had no significant effect on cytokine production by epithelial cells.

CONCLUSIONS

This study shows for the first time, the presence of leptin receptor isoforms on endometrial, epithelial and stromal cells in culture. Leptin had no effect on cytokine and MMP production by these cells. However, it is possible that leptin affects other factors within the endometrium not investigated here.

In vivo mutagenesis of the insulin receptor

Mice bearing targeted gene mutations that affect insulin receptor (Insr) function have contributed important new information on the pathogenesis of type 2 diabetes. Whereas complete Insr ablation is lethal, conditional mutagenesis in selected tissues has more limited consequences on metabolism. Studies of mice with tissue-specific ablation of Insr have indicated that both canonical (e.g. muscle and adipose tissue) and noncanonical (e.g. liver, pancreatic beta-cells, and brain) insulin target tissues can contribute to insulin resistance, albeit in a pathogenically distinct fashion. Furthermore, experimental crosses of Insr mutants with mice carrying mutations that affect insulin action at more distal steps of the insulin signaling cascade have begun to unravel the genetics of type 2 diabetes. These studies are consistent with an oligogenic inheritance, in which synergistic interactions among few alleles may account for the genetic susceptibility to diabetes. In addition to mutant alleles conferring an increased risk of diabetes, these studies have uncovered mutations that protect against insulin resistance, thus providing proof-of-principle for the notion that certain alleles may confer resistance to diabetes.

Molecular regulation of individual skeletal muscle fibre types

The purpose of this review is to present current understanding of cellular and molecular regulation of fibre type expression in skeletal muscle. Published literature seems to conclusively suggest that muscle fibre type expression is regulated by multiple signalling pathways and transcription factors rather than a single 'master' switch or signalling pathway. While the current nomenclature for fibre types is convenient for communication, based upon the evolution of this nomenclature, the prediction that fibre type classifications may change in the future to incorporate post-genomic information is made. It is predicted that future fibre type classifications could be based upon the contractile-activity-induced changes in a common regulatory factor(s) within a subpopulation of genes whose expressions are altered to modify and maintain the new muscle fibre phenotype.

Recovery of insulin sensitivity in obese patients at short term after biliopancreatic diversion

OBJECTIVE

To gain insight into the specific mechanisms by which biliopancreatic diversion (BPD) can improve insulin action.

MATERIALS AND METHODS

Nondiabetic severely obese patients (n=20) undergoing BPD were included. Waist-to-hip ratio and serum concentration of glucose, insulin, and leptin were determined before, at 4-day, and at 2 months after the operation. Insulin sensitivity was calculated according to the homeostatic model assessment (HOMA IR).

RESULTS

A marked increase of insulin sensitivity was observed by the fourth day after the operation; at the second postoperative month, when body weight was still in the obese range and the food intake was substantially similar to the preoperative one, a further improvement of insulin action towards normality was found. Moreover, before BPD HOMA IR data were independently correlated both to BMI and waist-to-hip ratio values, whereas at 2 months after the operation data were in positive correlation only with the BMI.

DISCUSSION

In obese patients, BPD seems to achieve recovery of insulin sensitivity by specific mechanisms independent of weight loss: the main causes of this sharp improvement might be both the intramyocellular fat depletion and the interruption of enteroinsular axis.

Effect of training on muscle triacylglycerol and structural lipids: a relation to insulin sensitivity?

We studied whether endurance training impacts insulin sensitivity by affecting the structural and storage lipids in humans. Eight male subjects participated (age 25 +/- 1 years, height 178 +/- 3 cm, weight 76 +/- 4 kg [mean +/- SE]). Single-leg training was performed for 30 min/day for 4 weeks at approximately 70% of single-leg maximal oxygen uptake. After 8, 14, and 30 days, a two-step hyperinsulinemic-euglycemic glucose clamp, combined with catheterization of an artery and both femoral veins, was performed. In addition, a muscle biopsy was obtained from vastus lateralis of both legs. Maximal oxygen uptake increased by 7% in the trained leg (T), and training workload increased (P < 0.05) from 79 +/- 12 to 160 +/- 15 W. At day 8, glucose uptake was higher (P < 0.01) in the trained (0.8 +/- 0.2, 6.0 +/- 0.8, 13.4 +/- 1.2 mg x min(-1) x kg(-1) leg wt) than the untrained leg (0.5 +/- 0.2, 3.7 +/- 0.6, 10.5 +/- 1.5 mg x min(-1) x kg(-1) leg wt) at basal and the two succeeding clamp steps, respectively. After day 8, training did not further increase leg glucose uptake. Individual muscle triacylglycerol fatty acid composition and total triacylglycerol content were not significantly affected by training and thus showed no relation to leg glucose uptake. Individual muscle phospholipid fatty acids were not affected by training, but the content of phospholipid polyunsaturated fatty acids was higher (P < 0.06) after 30 than 8 days in T. Furthermore, after 30 days of training, the sum of phospholipid long-chain polyunsaturates was correlated to leg glucose uptake (r = 0.574, P < 0.04). Endurance training did not influence muscle triacylglycerol content or total triacylglycerol fatty acid composition. In contrast, training induced a minor increase in the content of phospholipid fatty acid membrane polyunsaturates, which may indicate that membrane lipids may have a role in the training-induced increase in insulin sensitivity.

Role of the natriuretic peptide system in lipogenesis/lipolysis

AIM

There is recent evidence that the natriuretic peptide (NP) system promotes adipose tissue lipolysis in primates. This effect is mediated by the interaction of NP with its active receptors through guanylyl cyclase activation and cGMP production. This review will briefly focus on the new aspects of NP pathophysiology in man.

DATA SYNTHESIS

NP receptors have been described in rodent adipocytes, and the expression of their mRNA is found in human adipose tissue together with high level of ANP binding sites. In isolated fat cells, atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) were able to stimulate lipolysis as much as isoproterenol, a non-selective beta-adrenergic receptor agonist, whereas C-type natriuretic peptide (CNP) had the lowest lipolytic effect. The potent lipolytic effect of NP has also been confirmed in samples of abdominal adipose tissue from healthy subjects. The potency order of the lipolytic effect (ANP > BNP > CNP) and ANP-induced cGMP production supported the presence of type A natriuretic peptide receptor in human fat cells. The effect of NP on lipid metabolism is confirmed by the fact that intravenous ANP infusion is followed by plasma NEFA and glycerol concentration increase (reflecting lipid mobilisation).

CONCLUSIONS

The NP system seems to play an important role in lipid metabolism, possibly affecting the pathophysiology of obesity and obesity-related disorders, such hypertension. Further studies, however, are needed to completely establish the mechanisms involved in NP-induced lipolysis and the real relevance of this new pathway specific of primates.

The matrix metalloproteinase system: changes, regulation, and impact throughout the ovarian and uterine reproductive cycle

The ovary and uterus undergo extensive tissue remodeling throughout each reproductive cycle. This remodeling of the extracellular environment is dependent upon the cyclic hormonal changes associated with each estrous or menstrual cycle. In the ovary, tissue remodeling is requisite for growth and expansion of the follicle, breakdown of the follicular wall during the ovulatory process, transformation of the postovulatory follicle into the corpus luteum, as well as the structural dissolution of the corpus luteum during luteal regression. In the uterus, there is extraordinary turnover of the endometrial connective tissue matrix during each menstrual cycle. This turnover encompasses the complete breakdown and loss of this layer, followed by its subsequent regrowth. With implantation, extensive remodeling of the uterus occurs to support placentation. These dynamic changes in the ovarian and uterine extracellular architecture are regulated, in part, by the matrix metalloproteinase (MMP) system. The MMP system acts to control connective tissue remodeling processes throughout the body and is comprised of both a proteolytic component, the MMPs, and a regulatory component, the associated tissue inhibitors of metalloproteinases. The current review will highlight the key features of the MMPs and tissue inhibitors of metalloproteinases, focus on the changes and regulation of the MMP system that take place throughout the estrous and menstrual cycles, and address the impact of the dynamic tissue remodeling processes on ovarian and uterine physiology.

Ultrastructural aspects of the follicular cells of the pars tuberalis in bats related to the seasonal cycle

The topography and structure of the follicular cells and the follicular cavity of the hypophyseal pars tuberalis (PT) were studied in adult hibernating bats (Pipistrellus pipistrellus and Rhinolophus ferrumequinum) of both sexes, during the annual seasonal cycle and the reproductive cycle. The follicular cells were found to be organized around a central cavity. They showed a polyhedral shape and apical microvilli protruding into central cavities. During hibernation, the follicular cells showed active cytoplasmic organelles, clusters of glycogen particles, and lipid droplets. In the supranuclear cytoplasm, 9+2 type cilia, some dense bodies, microvesicular vacuoles, and thin actin-like filaments (rather scarce during autumn) were detected. The contents of the follicular cavity showed well-defined ultrastructural seasonal characteristics, with a colloid-like aspect during awakening and a strongly granular aspect during autumn oestrus and mating. Positive staining for PAS and paraldehyde fuchsin, and a marked reaction to lectins PHA-L4, MAM, and RCA 60 suggested the presence of sialo-glycoproteins in the follicular cavities. Both follicular and endocrine PT-specific cells appeared to mark the boundary of follicular cavities. This finding suggests that the follicular cavity contents are comprised of both types of cells, rather than by cell fragmentation or degeneration products.

Bioenergetic analysis of peroxisome proliferator-activated receptor gamma coactivators 1alpha and 1beta (PGC-1alpha and PGC-1beta) in muscle cells

Peroxisome proliferator-activated receptor gamma coactivator (PGC)-1alpha is a coactivator of nuclear receptors and other transcription factors that regulates several components of energy metabolism, particularly certain aspects of adaptive thermogenesis in brown fat and skeletal muscle, hepatic gluconeogenesis, and fiber type switching in skeletal muscle. PGC-1alpha has been shown to induce mitochondrial biogenesis when expressed in muscle cells, and preliminary analysis has suggested that this molecule may specifically increase the fraction of uncoupled versus coupled respiration. In this paper, we have performed detailed bioenergetic analyses of the function of PGC-1alpha and its homolog PGC-1beta in muscle cells by monitoring simultaneously oxygen consumption and membrane potential. Cells expressing PGC-1alpha or PGC-1beta display higher proton leak rates at any given membrane potential than control cells. However, cells expressing PGC-1alpha have a higher proportion of their mitochondrial respiration linked to proton leak than cells expressing PGC-1beta. Although these two proteins cause a similar increase in the expression of many mitochondrial genes, PGC-1beta preferentially induces certain genes involved in the removal of reactive oxygen species, recently recognized as activators of uncoupling proteins. Together, these data indicate that PGC-1alpha and PGC-1beta profoundly alter mitochondrial metabolism and suggest that these proteins are likely to play different physiological functions.

Coordinated reduction of genes of oxidative metabolism in humans with insulin resistance and diabetes: Potential role of PGC1 and NRF1

Type 2 diabetes mellitus (DM) is characterized by insulin resistance and pancreatic beta cell dysfunction. In high-risk subjects, the earliest detectable abnormality is insulin resistance in skeletal muscle. Impaired insulin-mediated signaling, gene expression, glycogen synthesis, and accumulation of intramyocellular triglycerides have all been linked with insulin resistance, but no specific defect responsible for insulin resistance and DM has been identified in humans. To identify genes potentially important in the pathogenesis of DM, we analyzed gene expression in skeletal muscle from healthy metabolically characterized nondiabetic (family history negative and positive for DM) and diabetic Mexican-American subjects. We demonstrate that insulin resistance and DM associate with reduced expression of multiple nuclear respiratory factor-1 (NRF-1)-dependent genes encoding key enzymes in oxidative metabolism and mitochondrial function. Although NRF-1 expression is decreased only in diabetic subjects, expression of both PPAR gamma coactivator 1-alpha and-beta (PGC1-alpha/PPARGC1 and PGC1-beta/PERC), coactivators of NRF-1 and PPAR gamma-dependent transcription, is decreased in both diabetic subjects and family history-positive nondiabetic subjects. Decreased PGC1 expression may be responsible for decreased expression of NRF-dependent genes, leading to the metabolic disturbances characteristic of insulin resistance and DM.

Dietary fat interacts with QTLs controlling induction of Pgc-1 alpha and Ucp1 during conversion of white to brown fat

To identify novel regulatory factors controlling induction of the brown adipocyte-specific mitochondrial uncoupling protein (Ucp1) mRNA in the retroperitoneal white fat depot, we previously mapped quantitative trait loci (QTLs) that control this trait to chromosomes 2, 3, 8, and 19. Since the peroxisome proliferator activator receptor-gamma coactivator-1alpha (PGC-1alpha) regulates Ucp1 and other genes of energy metabolism, we have evaluated whether the QTLs controlling Ucp1 mRNA levels also modulate Pgc-1alpha mRNA levels by analysis of backcross progeny from the A/J and C57BL/6J strains of mice. The results indicate that a locus on chromosome 3 orchestrates expression of Pgc-1alpha and Ucp1 in retroperitoneal fat of mice fed a low-fat diet; however, the effect of this locus on Pgc-1alpha is lost, and a significant correlation between Ucp1 and Pgc-1alpha is severely reduced in mice fed a high-fat diet. An additional QTL located on chromosome 5 has also been identified for the selective regulation of Ucp1 mRNA levels. Similar to the effects of a high-fat diet on the chromosome 3 QTL, linkage of the chromosome 5 QTL is also lost in mice on a high-fat diet. Thus dietary fat has a profound influence on PGC-1alpha-regulated pathways controlling energy metabolism in white fat. The allelic variation observed in the regulation of Ucp1 and Pgc-1alpha expression in brown adipocytes of white fat but not interscapular brown fat suggests that fundamentally different regulatory mechanisms exist to control the thermogenic capacities of these tissues.

Suppression of beta cell energy metabolism and insulin release by PGC-1alpha

beta cell dysfunction is an important component of type 2 diabetes, but the molecular basis for this defect is poorly understood. The transcriptional coactivator PGC-1alpha mRNA and protein levels are significantly elevated in islets from multiple animal models of diabetes; adenovirus-mediated expression of PGC-1alpha to levels similar to those present in diabetic rodents produces a marked inhibition of glucose-stimulated insulin secretion from islets in culture and in live mice. This inhibition coincides with changes in metabolic gene expression associated with impaired beta cell function, including the induction of glucose-6-phosphatase and suppression of GLUT2, glucokinase, and glycerol-3-phosphate dehydrogenase. These changes result in blunting of the glucose-induced rise in cellular ATP levels and membrane electrical activity responsible for Ca(2+) influx and insulin exocytosis. These results strongly suggest that PGC-1alpha plays a key functional role in the beta cell and is involved in the pathogenesis of the diabetic phenotype.

Metabolic partitioning of endogenous fatty acid in adipocytes

OBJECTIVE

To develop an accurate new method to measure the partitioning of adipocyte endogenous fatty acids among different metabolic pathways, a critical step toward understanding the regulatory mechanism by which fat disposition is modulated.

RESEARCH METHODS AND PROCEDURES

Isolated primary rat adipocytes were pre-incubated with isotope-labeled fatty acids. This allows determination of the specific activity of labeled fatty acids in the endogenous lipid pool. After the removal of exogenous fatty acids, the disposition of endogenous fatty acids into the three major metabolic pathways, namely, oxidation, re-esterification, and release into the medium, was measured independently. This was compared with the total lipolytic release of endogenous fatty acids, as measured by glycerol release. Adipocytes from normal fed and fasted animals were used to determine the effects of physiological variations on the metabolic fate of endogenous fatty acids.

RESULTS

In normal fed animals, 0.2% of endogenous fatty acids were oxidized, 50.1% were released, and 49.7% were re-esterified. Fasting doubled the partitioning of fatty acids toward oxidation (p < 0.05) in association with increased lipolysis (1.4-fold increase) (p < 0.05). This effect was completely abolished by the addition of insulin to the cells (61% reduction) (p < 0.05).

DISCUSSION

The endogenous fatty acids in adipocytes are actively oxidized. This process can be regulated by altered physiological conditions or by insulin. Over time, it is possible that a small shift of fatty acids toward oxidation could have a significant impact on body fuel economy. This hypothesis needs to be tested.

Characterization of the human, mouse and rat PGC1 beta (peroxisome-proliferator-activated receptor-gamma co-activator 1 beta) gene in vitro and in vivo

PGC1 alpha is a co-activator involved in adaptive thermogenesis, fatty-acid oxidation and gluconeogenesis. We describe the identification of several isoforms of a new human PGC1 alpha homologue, cloned independently and named PGC1 beta. The human PGC1 beta gene is localized to chromosome 5, has 13 exons and spans more than 78 kb. Two different 5' and 3' ends due to differential splicing were identified by rapid amplification of cDNA ends PCR and screening of human cDNA libraries. We show that PGC1 beta variants in humans, mice and rats are expressed predominantly in heart, brown adipose tissue, brain and skeletal muscle. PGC1 beta expression, unlike PGC1 alpha, is not up-regulated in brown adipose tissue in response to cold or obesity. Fasting experiments showed that PGC1 alpha, but not PGC1 beta, is induced in liver and this suggests that only PGC1 alpha is involved in the hepatic gluconeogenesis. No changes in PGC1 beta gene expression were observed associated with exercise. Human PGC1 beta-1a and -2a isoforms localized to the cell nucleus and, specifically, the isoform PGC1 beta-1a co-activated peroxisome-proliferator-activated receptor-gamma, -alpha and the thyroid hormone receptor beta1. Finally, we show that ectopic expression PGC1 beta leads to increased mitochondrial number and basal oxygen consumption. These results suggest that PGC1 beta may play a role in constitutive adrenergic-independent mitochondrial biogenesis.

Ontogenic expression of TGFbeta 1, 2, and 3 and its receptors in the rat gastric mucosa

The stomach of the rat undergoes extensive changes during the formation and maturation of gastric glands. The presence of transforming growth factor beta (TGFbeta) in rat milk and in the gastrointestinal tract of pups may suggest its role in this process. The current study evaluated the in vivo dynamic expression and distribution of TGFbeta1, beta2, beta3 and their receptors TbetaRI and TbetaRII in the gastric epithelium of 20-day fetal rats and 1-, 14-, 21-, and 30-day-old pups. Immunohistochemistry was used to detect the proteins, and staining was classified according to intensity and cell type. The results showed that the gastric epithelium expresses TGFbeta isoforms and receptors throughout development. We found that immunoreactivity paralleled the appearance of differentiated cells, such that surface mucous cells were the first to be immunostained and chief cells were the last. The intensity of reactions followed this same pattern, showing that the expression of TGFbeta isoforms spread along the gland with growth. Of interest, the highest apparent activity of TGFbeta was observed from 21 days onward, a period that is concomitant with weaning and maturation of most gastric cell types. In addition, surface mucous cells were strongly labeled at the basal cytoplasm at 14 days, suggesting an interaction with the connective tissue. In conclusion, the dynamic expression of TGFbeta1, beta2, beta3, and TbetaRI and TbetaRII through stomach development suggests significant paracrine and autocrine roles for this growth factor. We propose that temporal and spatial differences may be regulated by dietary changes, which in turn control cell proliferation and differentiation in the gastric epithelium.

Parathyroid hormone immunohistochemistry in dogs with primary and secondary hyperparathyroidism: the question of adenoma and primary hyperplasia

In primary hyperparathyroidism, calcium homeostasis is disrupted by excessive synthesis and secretion of parathyroid hormone (PTH), which is usually caused by a solitary adenoma, or less often by nodular hyperplasia or carcinoma of the parathyroid glands. So far, the distinction between these forms of primary hyperparathyroidism has been made by histological examination. In this report clinical and histological findings, including PTH immunohistochemistry, are described in five dogs with primary hyperparathyroidism, three dogs with secondary hyperparathyroidism due to chronic renal failure, and eight control dogs. In the dogs with primary hyperparathyroidism, nodular adenomatous hyperplasia was found in two animals and parathyroid adenoma in three. The dogs with chronic renal failure had diffuse parathyroid gland hyperplasia. The parathyroid glands of the control dogs and the inactive cells surrounding the hyperplastic nodules showed slight to moderate, localized, paranuclear PTH immunolabelling. In the primary nodular and secondary diffuse hyperplasia, all parathyroid cells had a diffuse cytoplasmic PTH labelling pattern, sometimes in combination with localized paranuclear labelling. In parathyroid adenoma, areas with either paranuclear labelling or diffuse cytoplasmic labelling were observed. As both parathyroid adenoma and primary nodular parathyroid gland hyperplasia have characteristics of intrinsic autonomy (i.e., suppression of the remaining endocrine tissue), there would seem to be no functional difference between the two abnormalities. It is argued that primary (multi)nodular hyperplasia is a multiple form of parathyroid adenoma.

Weight loss-induced plasticity of glucose transport and phosphorylation in the insulin resistance of obesity and type 2 diabetes

We tested the hypothesis that weight loss alleviates insulin resistance in skeletal muscle within the proximal steps of glucose metabolism, namely substrate delivery, glucose transport, and glucose phosphorylation. In obese subjects with and without type 2 diabetes, in vivo skeletal muscle assessments were obtained with dynamic positron emission tomography (PET) imaging performed during euglycemic clamps at moderate hyperinsulinemia (40 mU x min(-1) x m(-2)), using [(15)O]H(2)O and [(18)F]fluoro-deoxyglucose ([(18)F]FDG) to quantify tissue perfusion and glucose metabolism. Dynamic [(18)F]FDG PET data were analyzed using both a novel muscle-specific compartmental model and a compartmental model originally developed for the brain and often used for [(18)F]FDG muscle image quantification. Weight loss in obese subjects with (n = 9) and without (n = 9) type 2 diabetes over a 4-month intervention was substantial (14 +/- 2 kg, P < 0.05). Muscle insulin resistance, assessed by insulin-stimulated [(18)F]FDG uptake, decreased threefold in diabetic subjects and twofold in nondiabetic subjects (P < 0.001). Kinetic parameters for [(18)F]FDG transport and phosphorylation improved substantially in both groups, whereas tissue blood flow did not change. In particular, clinically significant weight loss fully corrected insulin resistance in type 2 diabetes at the step of glucose phosphorylation and largely, but incompletely, corrected insulin resistance at the glucose transport step.

Cold-induced metabolism

PURPOSE OF REVIEW

Cold response can be insulative (drop in peripheral temperature) or metabolic (increase in energy expenditure). Nonshivering thermogenesis by sympathetic, norepinephrine-induced mitochondrial heat production in brown adipose tissue is a well known component of this metabolic response in infants and several animal species. In adult humans, however, its role is less clear. Here we explore recent findings on the role and variability of nonshivering thermogenesis in adults.

RECENT FINDINGS

Large individual differences exist in mild cold response with respect to the relative contribution of the insulative response and the metabolic (nonshivering) response. In search for the possible explanations of this variation, recent studies on potential mechanisms of nonshivering thermogenesis in humans are presented. Emphasis is given to the role of uncoupling proteins, mitochondrial ATP-synthase, and calcium cycling. The potential contribution of human skeletal muscle to nonshivering thermogenesis is discussed. The differences in nonshivering thermogenesis can partly be attributed to factors such as age, gender, physical fitness, adaptation, and diet. There are indications that genetic variation affect cold response.

SUMMARY

The implications of the observed large individual variation in cold response is that a low metabolic response to cold can partly explain increased risk to develop obesity. Both the effect of environmental factors and genetic factors on nonshivering thermogenesis require more well controlled studies. With extended knowledge on these factors it can be ascertained if a pharmacological regimen is possible which would mimic the effects of chronic cold or elevated catecholamine levels, without attendant side effects.

Differential effects of cyclic AMP on induction of nitric oxide synthase in 3T3-L1 cells and brown adipocytes

The aim of this study was to determine whether cyclic AMP (cAMP) pathways alter the nitric oxide (NO) production mediated by inducible NO synthase (iNOS) in adipocytes. The treatment of 3T3-L1 cells, a model of white adipocytes, with the combination of lipopolysaccharide (L), tumor necrosis factor-alpha (T), and interferon-gamma (I) synergistically induced iNOS, leading to the production of NO. Enhancers of intracellular cAMP (dibutyryl cAMP, forskolin, and IBMX) inhibited the NO production elicited by LTI, whereas H89, a specific inhibitor of PKA, stimulated the NO production in 3T3-L1 cells. In rat brown adipocyte cell line, the combined treatment with LT synergistically elicited the NO production, and the cAMP analogues further enhanced it. Forskolin inhibited the NO production in 3T3-L1 cells, but enhanced it in brown adipocytes, in a dose-dependent manner. The changes in NO production paralleled the change in iNOS mRNA and protein level in both cell types. The activation of NF-kappaB by LTI/LT was blocked in 3T3-L1 cells, but enhanced in brown adipocytes, by the co-treatment with cAMP analogues. The protein level of 1-kappaBalpha, a NF-kappaB stabilizer, changed reciprocally to that of NF-kappaB activity in each cell type. These results suggest that cAMP regulates iNOS expression in adipocytes through modulating NF-kappaB activity. The differential regulation of iNOS in 3T3-L1 cells from that in the brown adipocytes indicates that intracellular signal pathways activated by cAMP are different between the cell types.

Duodenal leptin stimulates cholecystokinin secretion: evidence of a positive leptin-cholecystokinin feedback loop

Some of the actions of leptin depend on cholecystokinin (CCK). However, it is unknown whether leptin modulates the release of CCK. Here, we demonstrate in vitro that leptin induces the phosphorylation of extracellular signal-related kinase (ERK)-1/2 proteins and increases CCK release (EC(50) = 0.23 nmol/l) in CCK-secreting STC-1 cells. We showed that rat duodenal juice contains leptin that circulates free and bound to macromolecules, suggesting that leptin has a lumenal action on the intestine. In vivo in the rat, duodenal infusion of leptin increased plasma CCK at levels comparable to those induced by feeding. Moreover, meal-induced increases in plasma CCK were markedly reduced in obese fa/fa rats, whereas the mobilization of the gastric leptin pool was similar in lean and obese Zucker rats. The release of CCK by leptin presumably generates a positive feedback loop. Indeed, the blockade of CCK receptors reversed the meal reduction of the stomach leptin pool and the meal-increased plasma insulin, consistent with the previous concept of an entero-insular axis. Collectively, these data support a novel mode of action of leptin where leptin and CCK may potentiate their own effects by cross-stimulating their secretion. The impairment of this leptin-CCK loop may have pathological implications related to obesity and diabetes.

Lean, nondiabetic Asian Indians have decreased insulin sensitivity and insulin clearance, and raised leptin compared to Caucasians and Chinese subjects

OBJECTIVE

To study and compare the insulin sensitivity of healthy, nondiabetic Asian Indians with that of two other ethnic groups (Caucasian and Chinese) living in Singapore.

DESIGN

Study of insulin sensitivity using euglycaemic hyperinsulinaemic glucose clamp.

SUBJECTS

A total of 10 healthy, lean, young male subjects of each ethnic group, matched for age, body mass index (BMI) and physical activity. They all had normal glucose tolerance and had no family history of diabetes.

MEASUREMENTS

Anthropometric parameters (BMI, waist-hip ratio (WHR) and percentage body fat (PBF)), fasting lipid profile and leptin concentration, insulin sensitivity index, and insulin clearance.

RESULTS

Healthy lean (BMI 22.1+/-1.5 kg/m(2) (mean+/-s.d.)) Indians had significantly higher fasting serum leptin (5.1+/-2.5 vs Chinese 1.0+/-0.9 vs Caucasian 2.3+/-1.2 ng/ml; P<0.001), lower insulin sensitivity index (9.9+/-3.3 vs Chinese 14.1+/-3.5 vs Caucasian 18.8+/-9.2 mg/min kg fat-free mass/microU/ml; P<0.002), and lower insulin clearance (461.4+/-54.8 vs Chinese 621.0+/-99.3 vs Caucasian 646.9+/-49.2 ml/min m(2); P<0.001). Indians also had a higher PBF (26.5+/-5.2 vs Chinese 19.5+/-2.2 vs Caucasians 22.9+/-1.4%; P<0.001), diastolic blood pressure (P=0.036), fasting insulin (P<0.006) and fasting triglyceride (P=0.022). Stepwise regression analysis showed that ethnicity was the only significant independent determinant variable for the differences in insulin sensitivity index (P=0.008).

CONCLUSION

Healthy lean nondiabetic Indians were more insulin resistant compared to other ethnic groups despite the similarity in living environment. These findings may warrant preventive health-care strategies for type II diabetes and coronary artery disease to target Indians at an earlier stage compared to other ethnic groups.

PGC-1alpha-responsive genes involved in oxidative phosphorylation are coordinately downregulated in human diabetes

DNA microarrays can be used to identify gene expression changes characteristic of human disease. This is challenging, however, when relevant differences are subtle at the level of individual genes. We introduce an analytical strategy, Gene Set Enrichment Analysis, designed to detect modest but coordinate changes in the expression of groups of functionally related genes. Using this approach, we identify a set of genes involved in oxidative phosphorylation whose expression is coordinately decreased in human diabetic muscle. Expression of these genes is high at sites of insulin-mediated glucose disposal, activated by PGC-1alpha and correlated with total-body aerobic capacity. Our results associate this gene set with clinically important variation in human metabolism and illustrate the value of pathway relationships in the analysis of genomic profiling experiments.

Modeling of Ca2+ flux in pancreatic beta-cells: role of the plasma membrane and intracellular stores

We have developed a detailed mathematical model of ionic flux in beta-cells that includes the most essential channels and pumps in the plasma membrane. This model is coupled to equations describing Ca2+, inositol 1,4,5-trisphosphate (IP3), ATP, and Na+ homeostasis, including the uptake and release of Ca2+ by the endoplasmic reticulum (ER). In our model, metabolically derived ATP activates inward Ca2+ flux by regulation of ATP-sensitive K+ channels and depolarization of the plasma membrane. Results from the simulations support the hypothesis that intracellular Na+ and Ca2+ in the ER can be the main variables driving both fast (2-7 osc/min) and slow intracellular Ca2+ concentration oscillations (0.3-0.9 osc/min) and that the effect of IP3 on Ca2+ leak from the ER contributes to the pattern of slow calcium oscillations. Simulations also show that filling the ER Ca2+ stores leads to faster electrical bursting and Ca2+ oscillations. Specific Ca2+ oscillations in isolated beta-cell lines can also be simulated.

Origins and consequences of mitochondrial variation in vertebrate muscle

This review addresses the mechanisms by which mitochondrial structure and function are regulated, with a focus on vertebrate muscle. We consider the adaptive remodeling that arises during physiological transitions such as differentiation, development, and contractile activity. Parallels are drawn between such phenotypic changes and the pattern of change arising over evolutionary time, as suggested by interspecies comparisons. We address the physiological and evolutionary relationships between ATP production, thermogenesis, and superoxide generation in the context of mitochondrial function. Our discussion of mitochondrial structure focuses on the regulation of membrane composition and maintenance of the three-dimensional reticulum. Current studies of mitochondrial biogenesis strive to integrate muscle functional parameters with signal transduction and molecular genetics, providing insight into the origins of variation arising between physiological states, fiber types, and species.

Mitochondrial uncoupling protein 2 in the central nervous system: neuromodulator and neuroprotector

Uncoupling proteins (UCPs) are localized in the inner membrane of the mitochondria in diverse tissues and decrease mitochondrial membrane potential. The first of these proteins, UCP1, was discovered in brown adipose tissue, where it has a well-described role in thermogenesis. The functional significance of other UCPs, including UCP2, is less well understood. Here we summarize the recent advancements on the role of UCP2 in the brain and portray this uncoupler as an important player in normal neuronal function as well as a key cell death-suppressing device. These previously unknown functions of UCPs offer new avenues not only for the better understanding of these proteins but also for the furthering of our knowledge on the central nervous system in healthy and disease states.

An autoregulatory loop controls peroxisome proliferator-activated receptor gamma coactivator 1alpha expression in muscle

Skeletal muscle adapts to chronic physical activity by inducing mitochondrial biogenesis and switching proportions of muscle fibers from type II to type I. Several major factors involved in this process have been identified, such as the calcium/calmodulin-dependent protein kinase IV (CaMKIV), calcineurin A (CnA), and the transcriptional component peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha). Transgenic expression of PGC-1alpha recently has been shown to dramatically increase the content of type I muscle fibers in skeletal muscle, but the relationship between PGC-1alpha expression and the key components in calcium signaling is not clear. In this report, we show that the PGC-1alpha promoter is regulated by both CaMKIV and CnA activity. CaMKIV activates PGC-1alpha largely through the binding of cAMP response element-binding protein to the PGC-1alpha promoter. Moreover, we show that a positive feedback loop exists between PGC-1alpha and members of the myocyte enhancer factor 2 (MEF2) family of transcription factors. MEF2s bind to the PGC-1alpha promoter and activate it, predominantly when coactivated by PGC-1alpha. MEF2 activity is stimulated further by CnA signaling. These findings imply a unified pathway, integrating key regulators of calcium signaling with the transcriptional switch PGC-1alpha. Furthermore, these data suggest an autofeedback loop whereby the calcium-signaling pathway may result in a stable induction of PGC-1alpha, contributing to the relatively stable nature of muscle fiber-type determination.

Molecular basis of skeletal muscle plasticity--from gene to form and function

Skeletal muscle shows an enormous plasticity to adapt to stimuli such as contractile activity (endurance exercise, electrical stimulation, denervation), loading conditions (resistance training, microgravity), substrate supply (nutritional interventions) or environmental factors (hypoxia). The presented data show that adaptive structural events occur in both muscle fibres (myofibrils, mitochondria) and associated structures (motoneurons and capillaries). Functional adaptations appear to involve alterations in regulatory mechanisms (neuronal, endocrine and intracellular signalling), contractile properties and metabolic capacities. With the appropriate molecular techniques it has been demonstrated over the past 10 years that rapid changes in skeletal muscle mRNA expression occur with exercise in human and rodent species. Recently, gene expression profiling analysis has demonstrated that transcriptional adaptations in skeletal muscle due to changes in loading involve a broad range of genes and that mRNA changes often run parallel for genes in the same functional categories. These changes can be matched to the structural/functional adaptations known to occur with corresponding stimuli. Several signalling pathways involving cytoplasmic protein kinases and nuclear-encoded transcription factors are recognized as potential master regulators that transduce physiological stress into transcriptional adaptations of batteries of metabolic and contractile genes. Nuclear reprogramming is recognized as an important event in muscle plasticity and may be related to the adaptations in the myosin type, protein turnover, and the cytoplasma-to-myonucleus ratio. The accessibility of muscle tissue to biopsies in conjunction with the advent of high-throughput gene expression analysis technology points to skeletal muscle plasticity as a particularly useful paradigm for studying gene regulatory phenomena in humans.

Mitochondrial biogenesis: which part of "NO" do we understand?

A recent paper by Nisoli et al. [1] provides the first evidence that elevated levels of nitric oxide (NO) stimulate mitochondrial biogenesis in a number of cell lines via a soluble guanylate-cyclase-dependent signaling pathway that activates PGC1alpha (peroxisome proliferator-activated receptor gamma coactivator-1alpha), a master regulator of mitochondrial content. These results raise intriguing possibilities for a role of NO in modulating mitochondrial content in response to physiological stimuli such as exercise or cold exposure. However, whether this signaling cascade represents a widespread mechanism by which mammalian tissues regulate mitochondrial content, and how it might integrate with other pathways that control PGC1alpha expression, remain unclear.

IUGR alters postnatal rat skeletal muscle peroxisome proliferator-activated receptor-gamma coactivator-1 gene expression in a fiber specific manner

Uteroplacental insufficiency and subsequent intrauterine growth retardation (IUGR) increase the risk of insulin resistance in humans and rats. Aberrant skeletal muscle lipid metabolism contributes to the pathogenesis of insulin resistance. Peroxisome proliferator-activated receptor-gamma co-activator-1 (PGC-1) is a transcriptional co-activator that affects gene expression of key lipid metabolizing enzymes such as carnitine palmitoyl-transferase I (mCPTI). Because gene expression of lipid metabolizing enzymes is altered in IUGR postnatal skeletal muscle, and we hypothesized that PGC-1 expression would be similarly affected. To prove this hypothesis, bilateral uterine artery ligation and sham surgery were used to produce IUGR and control rats respectively. Western Blotting demonstrated that PGC-1 hind limb skeletal muscle protein levels were increased in perinatal and postnatal IUGR rats. Conventional RT-PCR demonstrated that PGC-1 mRNA levels were similarly increased in perinatal hind limb skeletal muscle and juvenile extensor digitorum longus (EDL), but were decreased in juvenile soleus. Because a gender specific trend was noted in PGC-1 mRNA levels, real time RT-PCR was used for further differentiation. Real time RT-PCR revealed that changes in postnatal skeletal muscle PGC-1 expression were more marked in male IUGR rats versus female IUGR rats. Down stream targets of PGC-1 followed a similar pattern of expression. We conclude that PGC-1 expression is altered in rat IUGR skeletal muscle and speculate that it contributes to the pathogenesis of insulin resistance in the IUGR rat.

Leptin alters the structural and functional characteristics of adipose tissue before birth

This study aimed to determine for the first time whether leptin can act to alter the structural and functional characteristics of adipose tissue before birth. Leptin (0.48 mg/kg/day) or saline was infused intravenously into fetal sheep for 4 days from either 136 or 137 days of gestation (term=147+/-3 days). Circulating leptin concentrations were increased approximately four- to fivefold by leptin infusion. Leptin infusion resulted in a significant increase in the proportion of smaller lipid locules present within fetal perirenal adipose tissue (PAT), and this was associated with a significant increase in the proportion of multilocular tissue and a significant decrease in the proportion and relative mass of unilocular tissue in fetal PAT. The relative abundance of leptin mRNA in fetal PAT was significantly lower in the leptin-infused group, and there was a positive correlation between the relative abundance of leptin mRNA and the proportion of unilocular adipose tissue in fetal PAT. The amount of uncoupling protein 1 tended to be higher (P=0.06) in leptin-infused compared with saline-infused fetuses. This is the first demonstration that leptin can act to regulate the lipid storage characteristics, leptin synthetic capacity, and potential thermogenic functions of fat before birth.

Prospective evaluation of human chorionic gonadotropin in the differentiation of undescended testes from retractile testes

PURPOSE

We prospectively evaluated the efficacy of human chorionic gonadotropin (HCG) in the treatment of undescended testis and sought to determine whether HCG assists in the differentiation of undescended testis from retractile testis.

MATERIALS AND METHODS

Patients with undescended testes were offered HCG. Testis position, laterality and the presence or absence of a hypoplastic scrotum were noted. The same physician (G. W. K.) recorded physical findings prospectively and stated clinical impression of descent.

RESULTS

A total of 67 patients with 90 undescended or retractile testes were treated and evaluated with HCG. Of the 64 undescended testes 13 (20%) descended with HCG therapy, with none requiring subsequent surgery. Of the 26 retractile testes 15 (58%) descended with HCG (p <0.001). Based on physical examination, 100% of retractile testes descended if the testis was in the high scrotal position but only 40% descended if the testis was in the superficial pouch or inguinal area. In the undescended testes group no ectopic or nonpalpable testis descended with HCG. Evaluation of HCG with age demonstrated minimal response (15%) to HCG at less than 24 months, and a peak response between ages 2 and 6 years (75%) with response decreasing thereafter.

CONCLUSIONS

HCG may have a limited role in the evaluation of undescended testis in patients younger than 2 years. HCG can serve as an adjunct in the clinical diagnosis of retractile testis in older children.

PPARgamma coactivator-1alpha expression during thyroid hormone- and contractile activity-induced mitochondrial adaptations

The transcriptional coactivator the peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1alpha) has been identified as an important mediator of mitochondrial biogenesis based on its ability to interact with transcription factors that activate nuclear genes encoding mitochondrial proteins. The induction of PGC-1alpha protein expression under conditions that provoke mitochondrial biogenesis, such as contractile activity or thyroid hormone (T(3)) treatment, is not fully characterized. Thus we related PGC-1alpha protein expression to cytochrome c oxidase (COX) activity in 1) tissues of varying oxidative capacities, 2) tissues from animals treated with T(3), and 3) skeletal muscle subject to contractile activity both in cell culture and in vivo. Our results demonstrate a strong positive correlation (r = 0.74; P < 0.05) between changes in PGC-1alpha and COX activity, used as an index of mitochondrial adaptations. The highest constitutive levels of PGC-1alpha were found in the heart, whereas the lowest were measured in fast-twitch white muscle and liver. T(3) increased PGC-1alpha content similarly in both fast- and slow-twitch muscle, as well as in the liver, but not in heart. T(3) also induced early (6 h) increases in AMP-activated protein kinase (AMPKalpha) activity, as well as later (5 day) increases in p38 MAP kinase activity in slow-twitch, but not in fast-twitch, muscle. Contractile activity provoked early increases in PGC-1alpha, coincident with increases in mitochondrial transcription factor A (Tfam), and nuclear respiratory factor-1 (NRF-1) protein expression, suggesting that PGC-1alpha is physiologically important in coordinating the expression of the nuclear and mitochondrial genomes. Ca(2+) ionophore treatment of muscle cells led to an approximately threefold increase in PGC-1alpha protein, and contractile activity induced rapid and marked increases in both p38 MAP kinase and AMPKalpha activities. 5-Aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR) treatment of muscle cells also led to parallel increases in AMPKalpha activity and PGC-1alpha protein levels. These data are consistent with observations that indicate that increases in PGC-1alpha protein are affected by Ca(2+) signaling mechanisms, AMPKalpha activity, as well as posttranslational phosphorylation events that increase PGC-1alpha protein stability. Our data support a role for PGC-1alpha in the physiological regulation of mitochondrial content in a variety of tissues and suggest that increases in PGC-1alpha expression form part of a unifying pathway that promotes both T(3)- and contractile activity-induced mitochondrial adaptations.

Increased expression of the SNARE accessory protein Munc18c in lipid-mediated insulin resistance

Fatty acids inhibit insulin-mediated glucose metabolism in skeletal muscle, an effect largely attributed to defects in insulin-mediated glucose transport. Insulin-resistant mice transgenic for the overexpression of lipoprotein lipase (LPL) in skeletal muscle were used to examine the molecular mechanism(s) in more detail. Using DNA gene chip array technology, and confirmation by RT-PCR and Western analysis, increases in the yeast Sec1p homolog Munc18c mRNA and protein were found in the gastrocnemius muscle of transgenic mice, but not other tissues. Munc18c has been previously demonstrated to impair insulin-mediated glucose transport in mammalian cells in vitro. Of interest, stably transfected C2C12 cells overexpressing LPL not only demonstrated increases in Munc18c mRNA and protein but also in transcription rates of the Munc18c gene. To confirm the relevance of fatty acid metabolism and insulin resistance to the expression of Munc18c in vivo, a 2-fold increase in Munc18c protein was demonstrated in mice fed a high-fat diet for 4 weeks. Together, these data are the first to implicate in vivo increases in Munc18c as a potential contributing mechanism to fatty acid-induced insulin resistance.

Knockout studies defining different roles for melanocortin receptors in energy homeostasis

Proopiomelanocortin (POMC) is expressed in the arcuate nucleus of the hypothalamus (ARC) and the commissural nucleus of the solitary tract (cNTS). Post-translational processing of POMC produces two melanocortin receptor ligands, alpha- and gamma-melanocyte-stimulating hormone (MSH). Two melanocortin receptors (MC3R, MC4R) are expressed in brain regions receiving projections of POMC fibers, most of which also receive projections from a population of ARC neurons that co-express neuropeptide Y (NPY) and the MC3R/MC4R antagonist agouti-related peptide (AgRP). MC4R haploinsufficient humans and MC4R knockout (MC4RKO) mice exhibit increased adiposity and linear growth. MC4RKO mice exhibit hyperleptinemia and hyperinsulinemia and sometimes, but not always, develop type 2 diabetes (T2D). Individually housed MC4RKO mice fed low-fat diets are not hyperphagic when food intake is corrected for lean mass, whereas hyperphagia is observed after the introduction of diets with increased fat content. POMC knockout (POMCKO) mice are similar in that the severity of hyperphagia increases with the introduction of high-fat diets. By contrast, targeted deletion of the MC3R in the mouse results in increased adiposity despite the absence of hyperphagia. MC3RKO mice also exhibit reduced linear growth and lean mass; while MC3RKO mice are hyperleptinemic and hyperinsulinemic, the development of T2D has not been reported. The MC4R, but not the MC3R, is required for the stimulation of energy expenditure in response to melanocortin agonists and voluntary hyperphagia. Evidence for altered physical activity has also been reported for both knockout models. Analysis of MC4RKO mice indicates that this receptor is involved in rapidly coordinating energy consumption with energy expenditure through diet-induced thermogenesis and activity.