Physiologic concentrations of beta-lipotropin stimulate lipolysis in rabbit adipocytes

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

Non-adrenergic control of lipolysis and thermogenesis in adipose tissues

The enormous plasticity of adipose tissues, to rapidly adapt to altered physiological states of energy demand, is under neuronal and endocrine control. In energy balance, lipolysis of triacylglycerols and re-esterification of free fatty acids are opposing processes operating in parallel at identical rates, thus allowing a more dynamic transition from anabolism to catabolism, and vice versa. In response to alterations in the state of energy balance, one of the two processes predominates, enabling the efficient mobilization or storage of energy in a negative or positive energy balance, respectively. The release of noradrenaline from the sympathetic nervous system activates lipolysis in a depot-specific manner by initiating the canonical adrenergic receptor-G_s-protein-adenylyl cyclase-cyclic adenosine monophosphate-protein kinase A pathway, targeting proteins of the lipolytic machinery associated with the interface of the lipid droplets. In brown and brite adipocytes, lipolysis stimulated by this signaling pathway is a prerequisite for the activation of non-shivering thermogenesis. Free fatty acids released by lipolysis are direct activators of uncoupling protein 1-mediated leak respiration. Thus, pro- and anti-lipolytic mediators are bona fide modulators of thermogenesis in brown and brite adipocytes. In this Review, we discuss adrenergic and non-adrenergic mechanisms controlling lipolysis and thermogenesis and provide a comprehensive overview of pro- and anti-lipolytic mediators.

Neuropeptides, Including Neuropeptide Y and Melanocortins, Mediate Lipolysis in Murine Adipocytes

OBJECTIVE

To determine whether key appetite-regulating neuropeptides such as melanin-concentrating hormone (MCH), neuropeptide Y (NPY), and alpha-melanocyte-stimulating hormone (alpha-MSH), which are known to mediate energy balance through centrally mediated pathways, also have direct acute effects on the lipolytic activity of murine adipocytes.

RESEARCH METHODS AND PROCEDURES

Fully differentiated 3T3-L1 adipocytes serum starved overnight in Dulbecco's modified Eagle medium containing 2% bovine serum albumin or freshly isolated mouse adipocytes were incubated for up to 2 hours in the absence and presence of 100 nM each of NPY, MCH, alpha-MSH, the melanocortin receptor agonist MTII, or isoproterenol as a control. Free fatty acids secreted into the incubation medium were measured using a commercially available nonesterified fatty acid C test kit.

RESULTS

Treatment of 3T3-L1 cells with 100 nM NPY decreased basal free fatty acid secretion (basal, 0.006 +/- 0.001 vs. NPY, 0.001 +/- 0.0003 nM at 90 minutes; p < 0.05), whereas both alpha-MSH and MTII stimulated up to a 7-fold increase in free fatty acid release (MTII, 0.238 +/- 0.004 vs. basal, 0.024 +/- 0.002 nM at 2 hours; p < 0.05; and alpha-MSH, 0.22 +/- 0.005 vs. basal, 0.04 +/- 0.003 nM at 2 hours; p < 0.05). Treatment with 100 nM MCH had no effect on basal free fatty acid release or on alpha-MSH-induced lipolysis during concurrent treatment. Conversely, concurrent treatment with 100 nM NPY dramatically inhibited (by approximately 90%) alpha-MSH-induced lipolysis. Similar treatment of freshly isolated mouse adipocytes showed virtually identical results.

DISCUSSION

In addition to their centrally mediated actions, appetite-regulating neuropeptides modulate adipose tissue mass through direct peripheral effects. Systemic administration of pharmacological agents altering the effects of these neuropeptides may form the basis of future obesity therapies. Thus, some of these agents will likely have direct effects on adipocytes that may serve to alter their therapeutic effectiveness.

Several agents and pathways regulate lipolysis in adipocytes

Adipose tissue is the only tissue capable of hydrolyzing its stores of triacylglycerol (TAG) and of mobilizing fatty acids and glycerol in the bloodstream so that they can be used by other tissues. The full hydrolysis of TAG depends on the activity of three enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL) and monoacylglycerol lipase, each of which possesses a distinct regulatory mechanism. Although more is known about HSL than about the other two enzymes, it has recently been shown that HLS and ATGL can be activated simultaneously, such that the mechanism that enables HSL to access the surface of lipid droplets also permits the stimulation of ATGL. The classical pathway of lipolysis activation in adipocytes is cAMP-dependent. The production of cAMP is modulated by G-protein-coupled receptors of the Gs/Gi family and cAMP degradation is regulated by phosphodiesterase. However, other pathways that activate TAG hydrolysis are currently under investigation. Lipolysis can also be started by G-protein-coupled receptors of the Gq family, through molecular mechanisms that involve phospholipase C, calmodulin and protein kinase C. There is also evidence that increased lipolytic activity in adipocytes occurs after stimulation of the mitogen-activated protein kinase pathway or after cGMP accumulation and activation of protein kinase G. Several agents contribute to the control of lipolysis in adipocytes by modulating the activity of HSL and ATGL. In this review, we have summarized the signalling pathways activated by several agents involved in the regulation of TAG hydrolysis in adipocytes.

Peripheral administration of the N-terminal pro-opiomelanocortin fragment 1-28 to Pomc-/- mice reduces food intake and weight but does not affect adrenal growth or corticosterone production

Pro-opiomelanocortin (POMC) is a polypeptide precursor that undergoes extensive processing to yield a range of peptides with biologically diverse functions. POMC-derived ACTH is vital for normal adrenal function and the melanocortin alpha-MSH plays a key role in appetite control and energy homeostasis. However, the roles of peptide fragments derived from the highly conserved N-terminal region of POMC are less well characterized. We have used mice with a null mutation in the Pomc gene (Pomc(-/-)) to determine the in vivo effects of synthetic N-terminal 1-28 POMC, which has been shown previously to possess adrenal mitogenic activity. 1-28 POMC (20 mug) given s.c. for 10 days had no effect on the adrenal cortex of Pomc(-/-) mice, with resultant cortical morphology and plasma corticosterone levels being indistinguishable from sham treatment. Concurrent administration of 1-28 POMC and 1-24 ACTH (30 mug/day) resulted in changes identical to 1-24 ACTH treatment alone, which consisted of upregulation of steroidogenic enzymes, elevation of corticosterone levels, hypertrophy of the zona fasciculate, and regression of the X-zone. However, treatment of corticosterone-depleted Pomc(-/-) mice with 1-28 POMC reduced cumulative food intake and total body weight. These anorexigenic effects were ameliorated when the peptide was administered to Pomc(-/-) mice with circulating corticosterone restored either to a low physiological level by corticosterone-supplemented drinking water (CORT) or to a supraphysiological level by concurrent 1-24 ACTH administration. Further, i.c.v. administration of 1-28 POMC to CORT-treated Pomc(-/-) mice had no effect on food intake or body weight. In wild-type mice, the effects of 1-28 POMC upon food intake and body weight were identical to sham treatment, but 1-28 POMC was able to ameliorate the hyperphagia induced by concurrent 1-24 ACTH treatment. In a mouse model which lacks all endogenous POMC peptides, s.c. treatment with synthetic 1-28 POMC alone can reduce food intake and body weight, but has no impact upon adrenal growth or steroidogenesis.

Proopiomelanocorticotropin (POMC) peptides and lipoprotein lipase activity in vitro

Effects of alpha-melanocyte-stimulating hormone (alpha-MSH), beta-melanocyte-stimulating hormone (beta-MSH), beta-lipotropin (beta-LPH), and beta-endorphin (beta-EPH) at concentrations from 10(-9) M up to 10(-6) M on human adipose tissue lipoprotein lipase (LPL) were studied in a cell-free system. alpha-MSH and beta-MSH did not exert any effect on LPL; no degradation of these peptides in the incubation medium could be detected by HPLC analysis. beta-LPH and beta-EPH failed to alter enzyme activity. However, HPLC analysis revealed an unspecific rapid degradation of the peptides due to the activity of tissue proteases. Therefore, the protease inhibitors amastatin, antipain, APMSF, and TPCK were tested at concentrations of 10(-5), 10(-4), and 10(-3) M for their efficacy to inhibit degradation. None of the inhibitors was able to substantially reduce proteolysis of beta-LPH, as was the case with amastatin, APMSF, and TPCK for beta-EPH. However, antipain at 10(-4) M preserved at least 20% of the initial peptide concentration from proteolysis up to 150 min. Antipain caused a decrease in lipoprotein lipase activity (LPLA), which was dependent on concentration. The adverse effect of antipain at concentrations of 10(-4) M on LPL was completely abolished by beta-EPH at a concentration of 10(-6) M.

Processing of the lipid-mobilizing peptide beta-lipotropin in rabbit adipose tissue

beta-Lipotropin, a pituitary peptide, is a strong stimulator of lipolysis in rabbit adipose tissue. This polypeptide is shown to be degraded by intact fat pads, homogenized adipose tissue and adipocytes of the rabbit dependent on the amount of adipose tissue, time and the pH of the incubation medium. In subcellular fractions of rabbit adipocytes the proteolytic activity could be localized into the cytosol and the microsomal fraction. To obtain information about the processing of beta-lipotropin in its target cell lipolysis and degradation of this polypeptide were investigated in the presence of inhibitors of distinct cellular mechanisms and in different physiological states such as obesity and starvation. Thus, the stronger lipolytic response in adipocytes from obese rabbits respectively animals fed ad libitum was accompanied by a significantly increased degradation in comparison to lean respectively starved rabbits. The six lysosomotropic agents (chloroquine, NH4Cl, propranolol, quinacrine, acridine orange and tetracaine), the proteinase inhibitors alpha 2-macroglobulin and monodansylcadaverine, cellular ATP depletion by 2-deoxy-D-glucose and 2,4-dinitrophenol and the omission of Ca2+ ions from the incubation medium inhibited dose-dependently the lipolytic activity as well as the degradation of beta-lipotropin in intact and homogenized adipose tissue. Inhibitors of the cytoskeleton such as colchicine, cytochalasin B, vinblastine and concanavalin A also reduced lipolysis but only the degradation in intact adipose tissue. It can be concluded that after receptor-mediated uptake the cytoskeleton and lysosomal proteases are involved in the processing of beta-lipotropin.

Effect of carbohydrate refeeding on free fatty acids after a fast in obese diabetic and obese non-diabetic females

The metabolic effects of refeeding with oral or intravenous carbohydrate were studied in obese women after ten or 14 days of fasting. Seven patients were refed with protein-free fruit juice for a total of 250 g of carbohydrate (1,000 kcal) over ten hours. The juice was sipped continuously throughout this time, causing a drop in free fatty acids (FFA) from 1.07 +/- 0.08 to 0.61 +/- 0.05 mmol/L (P less than .01) over the first four hours. Over the next four hours, despite continuous ingestion of the carbohydrate and elevated plasma glucose (132 +/- 9 mg/dL) and insulin (2.81 +/- 0.86 ng/mL) (1 ng/mL = 25 microU/mL), FFA rose to 0.99 mmol/L (P less than .01). Similar results were obtained in five patients refed with similar amounts of oral glucose and four patients who received an equivalent amount of glucose intravenously (IV). Refeeding with carbohydrate of obese diabetic and non-diabetic women after a two-week fast caused an abrupt decrease in FFA that was followed after four hours by an increase in FFA and glycerol, despite continued ingestion of carbohydrate glucose and insulin.

Effects of caloric restriction and exercise on B-endorphin, ACTH and cortisol circulating levels in obesity

B-Endorphin (B-Ep), ACTH and cortisol circulating levels, before and after a two months therapy with a hypocaloric diet and an increase in physical exercise, were measured by RIA in 17 obese female subjects. After therapy, the body weight excess fell from 56.6 +/- 22.2% to 38.6 +/- 22.1% (p less than 0.01). Plasma levels of B-Ep decreased from 18.3 +/- 12.5 fmol/ml to 6.4 +/- 3.5 fmol/ml (p less than 0.01); those of ACTH from 46.8 +/- 22.8 pg/ml to 31.2 +/- 11.6 pg/ml (p less than 0.01); and those of cortisol from 15.9 +/- 4.6 micrograms% to 10.3 +/- 2.5 micrograms% (p less than 0.01). The reduction of the elevated plasma B-Ep levels found in obese subjects is related principally to the diet therapy. Thus, as shown in experimental animals, excessive feeding results in an increased hypothalamic-pituitary secretion of B-Ep.