Ramamani Arumugam

PPARγ coactivator-1α contributes to exercise-induced regulation of intramuscular lipid droplet programming in mice and humans

Intramuscular accumulation of triacylglycerol, in the form of lipid droplets (LD), has gained widespread attention as a hallmark of metabolic disease and insulin resistance. Paradoxically, LDs also amass in muscles of highly trained endurance athletes who are exquisitely insulin sensitive. Understanding the molecular mechanisms that mediate the expansion and appropriate metabolic control of LDs in the context of habitual physical activity could lead to new therapeutic opportunities. Herein, we show that acute exercise elicits robust upregulation of a broad program of genes involved in regulating LD assembly, morphology, localization, and mobilization. Prominent among these was perilipin-5, a scaffolding protein that affects the spatial and metabolic interactions between LD and their surrounding mitochondrial reticulum. Studies in transgenic mice and primary human skeletal myocytes established a key role for the exercise-responsive transcriptional coactivator PGC-1α in coordinating intramuscular LD programming with mitochondrial remodeling. Moreover, translational studies comparing physically active versus inactive humans identified a remarkably strong association between expression of intramuscular LD genes and enhanced insulin action in exercise-trained subjects. These results reveal an intimate molecular connection between intramuscular LD biology and mitochondrial metabolism that could prove relevant to the etiology and treatment of insulin resistance and other disorders of lipid imbalance.

Growth hormone and prolactin receptors in adipogenesis : STAT-5 activation, suppressors of cytokine signaling, and regulation of insulin-like growth factor I

Growth hormone (GH) stimulates lipolysis in mature adipocytes and primary preadipocytes but promotes adipogenesis in preadipocyte cell lines. The lactogenic hormones (prolactin [PRL] and placental lactogen) also stimulate adipogenesis in preadipocyte cell lines but have variable lipolytic and lipogenic effects in mature adipose tissue. We hypothesized that differences in expression of GH receptors (GHR) and PRL receptors (PRLR) during adipocyte development might explain some of the differential effects of the somatogens and lactogens on fat metabolism. To that end, we compared: (a) the expression of GHR and PRLR mRNAs in 3T3-L1 preadipocytes during the course of adipocyte differentiation; (b) the induction of STAT-5 activity by GH and PRL during adipogenesis; and (c) the acute effects of GH and PRL on the suppressors of cytokine signaling (SOCS-1–3 and cytokine-inducible SH2-domain-containing protein [CIS]) and IGF-I. In confluent, undifferentiated 3T3-L1 cells, the levels of GHR mRNA were ∼250-fold higher than the levels of PRLR mRNA. Following induction of adipocyte differentiation the levels of PRLR mRNA rose 90-fold but GHR mRNA increased only 0.8-fold. Expression of both full-length (long) and truncated (short) isoforms of the PRLR increased during differentiation but the long isoform predominated at all time points. Mouse GH (mGH) stimulated increases in STAT-5a and 5b activity in undifferentiated as well as differentiating 3T3-L1 cells; mouse PRL (mPRL) had little or no effect on STAT-5 activity in undifferentiated cells but stimulated increases in STAT-5a and 5b activity in differentiating cells. mGH stimulated increases in SOCS-2 and SOCS-3 mRNAs in undifferentiated cells and SOCS-1–3 and CIS mRNAs in differentiating cells; mPRL induced CIS in differentiating cells but had no effect on SOCS-1–3. mPRL and mGH stimulated increases in IGF-I mRNA in differentiating cells but not in undifferentiated cells; the potency of mGH (3–6-fold increase, p < 0.01) exceeded that of mPRL (40–90% increase, p < 0.05). Our findings reveal disparities in the expression of PRLR and GHR during adipocyte development and differential effects of the hormones on STAT-5, the SOCS proteins, CIS, and IGF-I. These observations suggest that somatogens and lactogens regulate adipocyte development and fat metabolism through distinct but overlapping cellular mechanisms.

The Interplay of Prolactin and the Glucocorticoids in the Regulation of β-Cell Gene Expression, Fatty Acid Oxidation, and Glucose-Stimulated Insulin Secretion: Implications for Carbohydrate Metabolism in Pregnancy

Carbohydrate metabolism in pregnancy reflects the balance between counterregulatory hormones, which induce insulin resistance, and lactogenic hormones, which stimulate beta-cell proliferation and insulin production. Here we explored the interactions of prolactin (PRL) and glucocorticoids in the regulation of beta-cell gene expression, fatty acid oxidation, and glucose-stimulated insulin secretion (GSIS). In rat insulinoma cells, rat PRL caused 30-50% (P < 0.001) reductions in Forkhead box O (FoxO)-1, peroxisome proliferator activator receptor (PPAR)-gamma coactivator-1alpha (PGC-1alpha), PPARalpha, and carnitine palmitoyltransferase 1 (CPT-1) mRNAs and increased Glut-2 mRNA and GSIS; conversely, dexamethasone (DEX) up-regulated FoxO1, PGC1alpha, PPARalpha, CPT-1, and uncoupling protein 2 (UCP-2) mRNAs in insulinoma cells and inhibited GSIS. Hydrocortisone had similar effects. The effects of DEX were attenuated by coincubation of cells with PRL. In primary rat islets, PRL reduced FoxO1, PPARalpha, and CPT-1 mRNAs, whereas DEX increased FoxO1, PGC1alpha, and UCP-2 mRNAs. The effects of PRL on gene expression were mimicked by constitutive overexpression of signal transducer and activator of transcription-5b. PRL induced signal transducer and activator of transcription-5 binding to a consensus sequence in the rat FoxO1 promoter, reduced nuclear FoxO1 protein levels, and induced its phosphorylation and cytoplasmic redistribution. DEX increased beta-cell fatty acid oxidation and reduced fatty acid esterification; these effects were attenuated by PRL. Thus, lactogens and glucocorticoids have opposing effects on a number of beta-cell genes including FoxO1, PGC1alpha, PPARalpha, CPT-1, and UCP-2 and differentially regulate beta-cell Glut-2 expression, fatty acid oxidation, and GSIS. These observations suggest new mechanisms by which lactogens may preserve beta-cell mass and function and maternal glucose tolerance despite the doubling of maternal cortisol concentrations in late gestation.

Adenylosuccinate Is an Insulin Secretagogue Derived from Glucose-Induced Purine Metabolism

Pancreatic islet failure, involving loss of glucose-stimulated insulin secretion (GSIS) from islet β cells, heralds the onset of type 2 diabetes (T2D). To search for mediators of GSIS, we performed metabolomics profiling of the insulinoma cell line 832/13 and uncovered significant glucose-induced changes in purine pathway intermediates, including a decrease in inosine monophosphate (IMP) and an increase in adenylosuccinate (S-AMP), suggesting a regulatory role for the enzyme that links the two metabolites, adenylosuccinate synthase (ADSS). Inhibition of ADSS or a more proximal enzyme in the S-AMP biosynthesis pathway, adenylosuccinate lyase, lowers S-AMP levels and impairs GSIS. Addition of S-AMP to the interior of patch-clamped human β cells amplifies exocytosis, an effect dependent upon expression of sentrin/SUMO-specific protease 1 (SENP1). S-AMP also overcomes the defect in glucose-induced exocytosis in β cells from a human donor with T2D. S-AMP is, thus, an insulin secretagogue capable of reversing β cell dysfunction in T2D.

Androgen profiles during pubertal Leydig cell development in mice

Postnatal Leydig cell (LC) development in mice has been assumed empirically to resemble that of rats, which have characteristic hormonal profiles at well-defined maturational stages. To characterize the changes in LC function and gene expression in mice, we examined reproductive hormone expression from birth to 180 days, and quantified in vivo and in vitro production of androgens during sexual maturation. Although the overall plasma androgen and LH profiles from birth through puberty were comparable to that of rats, the timing of developmental changes in androgen production and steroidogenic capacity of isolated LCs differed. In mice, onset of androgen biosynthetic capacity, distinguished by an acute rise in androstenedione and testosterone production and an increased expression of the steroidogenic enzymes, cytochrome P450 cholesterol side-chain cleavage enzyme and 17alpha-hydroxylase, occurred at day 24 (d24) rather than at d21 as reported in rats. Moreover, in contrast to persistently high testosterone production by pubertal and adult rat LCs, testosterone production was maximal at d45 in mice, and then declined in mature LCs. The murine LCs also respond more robustly to LH stimulation, with a greater increment in LH-stimulated testosterone production. Collectively, these data suggest that the mouse LC lineage has a delayed onset, and that it has an accelerated pace of maturation compared with the rat LC lineage. Across comparable maturational stages, LCs exhibit species-specific developmental changes in enzyme expression and capacity for androgen production. Our results demonstrate distinct differences in LC differentiation between mice and rats, and provide informative data for assessing reproductive phenotypes of recombinant mouse models.

Differential and Complementary Effects of Glucose and Prolactin on Islet DNA Synthesis and Gene Expression

The mechanisms by which lactogenic hormones promote β-cell expansion remain poorly understood. Because prolactin (PRL) up-regulates β-cell glucose transporter 2, glucokinase, and pyruvate dehydrogenase activities, we reasoned that glucose availability might mediate or modulate the effects of PRL on β-cell mass. Here, we used male rat islets to show that PRL and glucose have differential but complementary effects on the expression of cell cyclins, cell cycle inhibitors, and various other genes known to regulate β-cell replication, including insulin receptor substrate 2, IGF-II, menin, forkhead box protein M1, tryptophan hydroxylase 1, and the PRL receptor. Differential effects on gene expression are associated with synergistic effects of glucose and PRL on islet DNA synthesis. The effects of PRL on gene expression are mirrored by β-cell overexpression of signal transducer and activator of transcription 5b and are opposed by dexamethasone. An ad-small interfering RNA specific for cyclin D2 attenuates markedly the effects of PRL on islet DNA synthesis. Our studies suggest a new paradigm for the control of β-cell mass and insulin production by hormones and nutrients. PRL up-regulates β-cell glucose uptake and utilization, whereas glucose increases islet PRL receptor expression and potentiates the effects of PRL on cell cycle gene expression and DNA synthesis. These findings suggest novel targets for prevention of neonatal glucose intolerance and gestational diabetes and may provide new insight into the pathogenesis of β-cell hyperplasia in obese subjects with insulin resistance.

Regulation of Islet β-Cell Pyruvate Metabolism: Interactions of Prolactin, Glucose, and Dexamethasone

Prolactin (PRL) induces beta-cell proliferation and glucose-stimulated insulin secretion (GSIS) and counteracts the effects of glucocorticoids on insulin production. The mechanisms by which PRL up-regulates GSIS are unknown. We used rat islets and insulinoma (INS-1) cells to explore the interactions of PRL, glucose, and dexamethasone (DEX) in the regulation of beta-cell pyruvate carboxylase (PC), pyruvate dehydrogenase (PDH), and the pyruvate dehydrogenase kinases (PDKs), which catalyze the phosphorylation and inactivation of PDH. PRL increased GSIS by 37% (P < 0.001) in rat islets. Glucose at supraphysiological concentrations (11 mm) increased PC mRNA in islets; in contrast, PRL suppressed PC mRNA levels in islets and INS-1 cells, whereas DEX was without effect. Neither PRL nor DEX altered PC protein or activity levels. In INS-1 cells, PRL increased PDH activity 1.4- to 2-fold (P < 0.05-0.001) at glucose concentrations ranging from 2.5-11 mm. DEX reduced PDH activity; this effect was reversed by PRL. PDK1, -2, -3, and -4 mRNAs were detected in both islets and insulinoma cells, but the latter expressed trivial amounts of PDK4. PRL reduced PDK2 mRNA and protein levels in rat islets and INS-1 cells and PDK4 mRNA in islets; DEX increased PDK2 mRNA in islets and INS-1 cells; this effect was reversed by PRL. Our findings suggest that PRL induction of GSIS is mediated by increases in beta-cell PDH activity; this is facilitated by suppression of PDKs. PRL counteracts the effects of DEX on PDH and PDK expression, suggesting novel roles for the lactogens in the defense against diabetes.

Pathogenesis of growth failure and partial reversal with gene therapy in murine and canine Glycogen Storage Disease type Ia

Glycogen Storage Disease type Ia (GSD-Ia) in humans frequently causes delayed bone maturation, decrease in final adult height, and decreased growth velocity. This study evaluates the pathogenesis of growth failure and the effect of gene therapy on growth in GSD-Ia affected dogs and mice. Here we found that homozygous G6pase (-/-) mice with GSD-Ia have normal growth hormone (GH) levels in response to hypoglycemia, decreased insulin-like growth factor (IGF) 1 levels, and attenuated weight gain following administration of GH. Expression of hepatic GH receptor and IGF 1 mRNAs and hepatic STAT5 (phospho Y694) protein levels are reduced prior to and after GH administration, indicating GH resistance. However, restoration of G6Pase expression in the liver by treatment with adeno-associated virus 8 pseudotyped vector expressing G6Pase (AAV2/8-G6Pase) corrected body weight, but failed to normalize plasma IGF 1 in G6pase (-/-) mice. Untreated G6pase (-/-) mice also demonstrated severe delay of growth plate ossification at 12 days of age; those treated with AAV2/8-G6Pase at 14 days of age demonstrated skeletal dysplasia and limb shortening when analyzed radiographically at 6 months of age, in spite of apparent metabolic correction. Moreover, gene therapy with AAV2/9-G6Pase only partially corrected growth in GSD-Ia affected dogs as detected by weight and bone measurements and serum IGF 1 concentrations were persistently low in treated dogs. We also found that heterozygous GSD-Ia carrier dogs had decreased serum IGF 1, adult body weights and bone dimensions compared to wild-type littermates. In sum, these findings suggest that growth failure in GSD-Ia results, at least in part, from hepatic GH resistance. In addition, gene therapy improved growth in addition to promoting long-term survival in dogs and mice with GSD-Ia.