Effects of theophylline on insulin receptors and insulin action in the adipocyte

Summary The effects of theophylline on insulin receptors and insulin action in isolated rat adipocytes were studied. Theophylline reduced insulin binding by a decrease of receptor affinity. As concentration-response curves revealed, the effect was paralleled by a reduction of the cellular ATP content. Basal as well as insulin-stimulated glucose transport (2-deoxyglucose and 3-O-methylglucose uptake) were inhibited by much smaller theophylline concentrations (0.15–0.6 mM ) than those necessary to reduce insulin binding and to lower ATP levels (1–4.8 mM), or to stimulate lipolysis (0.3-2.4 mM). Insulin fully antagonized the effect of theophylline on lipolysis but failed to reverse the inhibition of glucose transport completely. The results suggest that (a) theophylline impairs insulin action at a post-receptor level and, at higher concentrations, by a decrease of receptor binding, (b) the reduction of insulin receptor affinity probably reflects ATP depletion of the adipocyte, and (c) the xanthine inhibits glucose transport independently from its effects on lipolysis.     

Regulation of adipocyte differentiation and insulin action with rapamycin

Here, we demonstrated that inhibition of mTOR with rapamycin has negative effects on adipocyte differentiation and insulin signaling. Rapamycin significantly reduced expression of most adipocyte marker genes including PPARgamma, adipsin, aP2, ADD1/SREBP1c, and FAS, and decreased intracellular lipid accumulation in 3T3-L1 and 3T3-F442A cells, suggesting that rapamycin would affect both lipogenesis and adipogenesis. Contrary to the previous report that suppressive effect of rapamycin on adipogenesis is limited to the clonal expansion, we revealed that its inhibitory effect persisted throughout the process of adipocyte differentiation. Thus, it is likely that constitutive activation of mTOR might be required for the execution of adipogenic programming. In differentiated 3T3-L1 adipocytes, chronic treatment of rapamycin blunted the phosphorylation of AKT and GSK, which is stimulated by insulin, and reduced insulin-dependent glucose uptake activity. Taken together, these results suggest that rapamycin not only prevents adipocyte differentiation by decrease of adipogenesis and lipogenesis but also downregulates insulin action in adipocytes, implying that mTOR would play important roles in adipogenesis and insulin action.     

Insulin receptor cycling and insulin action in the rat adipocyte

The possibility that the insulin receptor of adipocytes undergoes cycling was examined by a method involving pronase digestion at 12 degrees C, followed by insulin binding studies to determine receptor location and quantity. In the absence of insulin treatment, the amount of internal receptors (i.e. protected from pronase) was 10% of total receptor content. Following a 30-min insulin treatment (0.1 microM) at 37 degrees C, the internal receptor content increased 2-fold (206 +/- 12% of control, 100%). This effect was rapid, and maximum internalization was approached by 5 min of insulin treatment. Warming pronase-digested cells to 37 degrees C allowed the internal receptors to move to the cell surface. This movement was rapid also, and expansion of the internal pool by insulin pretreatment provided a 2.4-fold increase in the reinsertion of cell-surface receptors (238 +/- 28% of nontreated cells, 100%). Insulin-pretreated and nontreated cells had approximately 13 and 6%, respectively, of their original cell-surface receptor content, i.e. their content before pronase digestion. These receptors appeared intact after the cycling process, as judged by affinity labeling and sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the receptor and its binding subunit. The ability of the recycled receptor to respond to insulin was examined by studies of glucose incorporation into lipids and the inhibition of isoproterenol-stimulated lipolysis. Cells pretreated with insulin and allowed to recycle (e.g. 13% of normal receptor content) were 2-3-fold more responsive and 7-fold more sensitive to subsequent insulin stimulation than nontreated cells (e.g. 6% of normal receptor content), indicating that the recycled receptors are biologically active and coupled to cellular effector systems.     

Depot-specific regulation of glucose uptake and insulin sensitivity in HIV-lipodystrophy

Altered fat distribution is associated with insulin resistance in HIV, but little is known about regional glucose metabolism in fat and muscle depots in this patient population. The aim of the present study was to quantify regional fat, muscle, and whole body glucose disposal in HIV-infected men with lipoatrophy. Whole body glucose disposal was determined by hyperinsulinemic clamp technique (80 mU x m(-2) x min(-1)) in 6 HIV-infected men and 5 age/weight-matched healthy volunteers. Regional glucose uptake in muscle and subcutaneous (SAT) and visceral adipose tissue (VAT) was quantified in fasting and insulin-stimulated states using 2-deoxy-[18F]fluoro-D-glucose positron emission tomography. HIV-infected subjects with lipoatrophy had significantly increased glucose uptake into SAT (3.8 +/- 0.4 vs. 2.3 +/- 0.5 micromol x kg tissue(-1) x min(-1), P < 0.05) in the fasted state. Glucose uptake into VAT did not differ between groups. VAT area was inversely related with whole body glucose disposal, insulin sensitivity, and muscle glucose uptake during insulin stimulation. VAT area was highly predictive of whole body glucose disposal (r2 = 0.94, P < 0.0001). This may be mediated by adiponectin, which was significantly associated with VAT area (r = -0.75, P = 0.008), and whole body glucose disposal (r = 0.80, P = 0.003). This is the first study to directly demonstrate increased glucose uptake in subcutaneous fat of lipoatrophic patients, which may partially compensate for loss of SAT. Furthermore, we demonstrate a clear relationship between VAT and glucose metabolism in multiple fat and muscle depots, suggesting the critical importance of this depot in the regulation of glucose and highlighting the significant potential role of adiponectin in this process.     

Physiological consequences of early neonatal growth retardation: effects of alpha-difluoromethylornithine on renal growth and function in the rat

The physiological consequences of early neonatal growth retardation in the kidney were investigated using alpha-difluoromethylornithine (DFMO), a specific irreversible inhibitor of ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines. We administered by s.c. 500 mg/kg/day DFMO, or saline, to Sprague-Dawley rat pups from the day of birth through postnatal day (PD) 6 and evaluated renal function on PD 4, 7, 10, and 13 using tests of basal renal clearance and urinary concentrating ability. Kidney weights and gross pathology were also obtained. On PD 39, serum chemistries and organ weights were determined. In a second experiment, we evaluated concentrating ability on PD 7-10, and basal renal function, concentrating ability, diuretic response, serum chemistries, and organ weights on PD 132-140. DFMO selectively inhibited renal growth but did not inhibit glomerular and tubular functional maturation. In fact, the rates of filtration and reabsorption (per g renal tissue), and concentrating ability were increased in treated pups. These changes were associated with long-term effects on renal function, including uremia, glucosuria, and male-specific concentrating deficits in adulthood. Several hypotheses can be developed concerning the physiological mechanisms underlying these changes (e.g., altered renal urea metabolism), which in turn may reflect either a direct role of ODC in the regulation of maturation or secondary consequences of inhibition of ODC.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of the effects of insulin and insulin-like agents on different aspects of adipocyte metabolism.

The effect of insulin, spermine, cysteine, and diamide on glucose transport, glucose oxidation, and lipolysis were compared using isolated rat adipocytes. Each agent exerted insulin-like effects on glucose transport and glucose oxidation, whereas spermine and cysteine, but not diamide, inhibited lipolysis. Furthermore, the relative potencies of these agents on the various insulin responsive processes were markedly different. Thus, although these agents have much in common with insulin, the differences in relative potencies suggest that at least some of the insulin-like properties of these compounds may not be exerted through the same mechanisms as insulin.     

Regulatory effects of protein S-acylation on insulin secretion and insulin action

Post-translational modifications (PTMs) such as phosphorylation and ubiquitination are well-studied events with a recognized importance in all aspects of cellular function. By contrast, protein S-acylation, although a widespread PTM with important functions in most physiological systems, has received far less attention. Perturbations in S-acylation are linked to various disorders, including intellectual disability, cancer and diabetes, suggesting that this less-studied modification is likely to be of considerable biological importance. As an exemplar, in this review, we focus on the newly emerging links between S-acylation and the hormone insulin. Specifically, we examine how S-acylation regulates key components of the insulin secretion and insulin response pathways. The proteins discussed highlight the diverse array of proteins that are modified by S-acylation, including channels, transporters, receptors and trafficking proteins and also illustrate the diverse effects that S-acylation has on these proteins, from membrane binding and micro-localization to regulation of protein sorting and protein interactions.     

Studies on experimental growth retardation in sheep. The effects of maternal hypoxaemia

Intrauterine growth retardation in fetal sheep was caused by removal of endometrial caruncles prior to conception. Such fetuses are chronically hypoxaemic and to establish their ability to withstand additional episodes of hypoxia, the effects of administration of 9% O2 to the pregnant ewe was investigated. Fetuses were studied at 135-140 days. During maternal hypoxia the small fetuses showed a greater tendency to further hypoxaemia and acidaemia, but the differences compared with controls were not large. Whilst the initial response to hypoxaemia was a fall in heart rate in the small fetuses, unlike the controls, the heart rate returned to normal within 15 min. Metabolite responses to hypoxia in the small fetuses were less than normal and the changes in plasma insulin concentrations were uncommonly small. In contrast the plasma cortisol and ACTH responses to hypoxia were larger than normal in the small fetus. The results are discussed in relation to the altered physiological state of the growth-retarded fetal sheep.     

Tissue-specific regulation of early steps in insulin action in septic rats.

Sepsis is known to induce insulin resistance, but the exact molecular mechanism involved is unknown. In the present study we have examined the levels and phosphorylation state of the insulin receptor and of insulin receptor substrate 1 (IRS-1), as well as the association between IRS-1 and phosphatidylinositol 3-kinase (PI 3-kinase) in the liver and muscle of septic rats by immunoprecipitation and immunoblotting with anti-insulin receptor, anti-IRS-1, anti-PI 3-kinase and anti-phosphotyrosine antibodies. There were no changes in the insulin receptor concentration and phosphorylation levels in the liver and muscle of septic rats. IRS-1 protein levels were decreased by 40+/-3% (p < 0.01) in muscle but not in liver of septic rats. In samples previously immunoprecipitated with anti-IRS-1 antibody and blotted with antiphosphotyrosine antibody, the insulin-stimulated IRS-1 phosphorylation levels in the muscle of septic rats decreased by 38+/-5% (p < 0.01) and insulin-stimulated IRS-1 association with PI 3-kinase decreased by 44+/-7% in muscle (p < 0.01) but no changes were seen in liver. These data suggest that there is a tissue-specific regulation of early steps of insulin signal transduction in septic rats, and the changes observed in muscle may have a role in the insulin resistance of these animals.     

Depot-specific effects of fatty acids on lipid accumulation in children's adipocytes

Circulating concentrations of fatty acids are elevated in obesity, although their effect on regional fat deposition is relatively unexplored. With the increasing prevalence of childhood obesity, we aimed to investigate whether saturated and unsaturated fatty acids lead to differential lipid accumulation (LA) in children's subcutaneous and visceral adipocytes. To examine this, subcutaneous and peri-nephric pre-adipocytes, isolated from fat biopsies from 6 pre-pubertal children, were differentiated in vitro before being exposed to palmitate and/or oleate for 24 h. Lipid accumulation was then quantified by nile red staining. Palmitate significantly increased LA in visceral adipocytes at all doses > or =188 microM (e.g. Palmitate 750 microM: +30.0%[8.2]; p<0.01), whilst only a dose of 375 microM led to a significant, but smaller, increase in LA in subcutaneous adipocytes (Palmitate 375 micro: +13.0%[4.3]; p=0.02). In contrast, oleate significantly increased LA in subcutaneous (Oleate 1000 microM: +36.3%[14.0]; p=0.01), but not visceral (Oleate 1000 microM: +16.2%[9.6]; p=0.25) adipocytes. These data suggest that saturated and unsaturated fatty acids may exert depot-specific effects on lipid accumulation.     

Comparison of the effects of insulin and H2O2 on adipocyte glucose transport

The abilities of insulin and the insulin mimickers spermine and H₂O₂ to stimulate 3-O-methyl glucose transport in isolated rat ft cells were stuided in an attempt to determine possible common mechanisms of action. All three agents caused a seven- to 12-fold stimulation of initial rates of glucose transport with insulin being the most effective agent. Insulin and spermine displayed similar time courses for the onset of their stimulation of transport; an initial lag before any effect was seen and then a gradual rise until the full effect was reached. The time course of H₂O₂ activation of glucose transport was different since stimulation was seen at the earliest time point tested and then gradually rose to the maximal effect. Trypsinization of cells removed insulin receptors and rendered the cells insensitive to insulin but not to spermine or H₂O₂. However, trypsinization did alter the time course of H₂O₂ action, causing an initial lag phase to appear and a general slowing of the activation kinetics. Pretreatment of cells with 2,4-dinitrophenol to lower ATP levels prevented the stimulatory effects of insulin and the mimickers. All three of the agents revealed a similar temperature dependency for stimulated glucose transport, resulting in linear Arrhenius plots with activation energies of 10.2–11.4 kcal/mole. These results show that (1) H₂O₂ does not act directly on the glucose transport system of rat adipocytes and (2) insulin and H₂O most likely act through a common energy-dependent biochemical pathway to stimulate glucose transport, but H₂O₂ enters the stimulus-response sequence distal to the initial steps in insulin action.     

Discriminative insulin antagonism of stimulatory effects of various cAMP analogs on adipocyte lipolysis and hepatocyte glycogenolysis

Although insulin effectively blocked hormone-stimulated glycerol output in adipocytes or phosphorylase activation in hepatocytes, the inhibitory effect of insulin on cAMP analog-stimulated cells depended on the cAMP analog used. Of the 20 analogs tested in adipocytes and 13 tested in hepatocytes, the effects of about half of them were effectively blocked by insulin, whereas the effects of many of them were not inhibited at all. In order to approach the explanation for this discriminative insulin action, the inhibitory effects of insulin on the responses to the analogs in the intact cells were correlated with the in vitro cAMP analog specificity for the hepatocyte cAMP-dependent protein kinase isozymes and the low Km, hormone-sensitive phosphodiesterases from both cell types. No correlation was found between insulin resistance of analog-stimulated hepatocyte phosphorylase and the concentration of analog required in vitro for half-maximal activation of either type I or type II cAMP-dependent protein kinase from hepatocytes. However, a good correlation was found between insulin resistance of cAMP analog-stimulated responses and the analog I50 values for the phosphodiesterase from both cell types. Using a new method capable of measuring hydrolysis at low analog concentrations, several of those analogs which had relatively low, but not high, phosphodiesterase I50 values were shown to be directly hydrolyzed by the low Km adipocyte phosphodiesterase. The insulin inhibition of cell responses when stimulated by hydrolyzable analogs, but not by poorly hydrolyzable analogs, is best explained by insulin stimulation of the low Km phosphodiesterases from both cell types.     

Increased adipocyte S-nitrosylation targets anti-lipolytic action of insulin: relevance to adipose tissue dysfunction in obesity

Protein S-nitrosylation is a reversible protein modification implicated in both physiological and pathophysiological regulation of protein function. In obesity, skeletal muscle insulin resistance is associated with increased S-nitrosylation of insulin-signaling proteins. However, whether adipose tissue is similarly affected in obesity and, if so, what are the causes and functional consequences of increased S-nitrosylation in this tissue are unknown. Total protein S-nitrosylation was increased in intra-abdominal adipose tissue of obese humans and in high fat-fed or leptin-deficient ob/ob mice. Both the insulin receptor β-subunit and Akt were S-nitrosylated, correlating with body weight. Elevated protein and mRNA expression of inducible NO synthase and decreased protein levels of thioredoxin reductase were associated with increased adipose tissue S-nitrosylation. Cultured differentiated pre-adipocyte cell lines exposed to the NO donors S-nitrosoglutathione (GSNO) or S-nitroso-N-acetylpenicillamine exhibited diminished insulin-stimulated phosphorylation of Akt but not of GSK3 nor of insulin-stimulated glucose uptake. Yet the anti-lipolytic action of insulin was markedly impaired in both cultured adipocytes and in mice injected with GSNO prior to administration of insulin. In cells, impaired ability of insulin to diminish phosphorylated PKA substrates in response to isoproterenol suggested impaired insulin-induced activation of PDE3B. Consistently, increased S-nitrosylation of PDE3B was detected in adipose tissue of high fat-fed obese mice. Site-directed mutagenesis revealed that Cys-768 and Cys-1040, two putative sites for S-nitrosylation adjacent to the substrate-binding site of PDE3B, accounted for ～50% of its GSNO-induced S-nitrosylation. Collectively, PDE3B and the anti-lipolytic action of insulin may constitute novel targets for increased S-nitrosylation of adipose tissue in obesity.