Characterization of the chicken muscle insulin receptor

Insulin signaling in chicken liver and muscle

This review addresses the control exerted by insulin through its receptor on the general metabolism and gene expression in chicken liver and muscle. Compared with mammals, chickens have similar concentrations of circulating insulin, but still maintain high plasma glucose levels. This may be a consequence of the low sensitivity of the chicken to exogenous insulin. In order to determine whether this low sensitivity is the result of differences in insulin receptor signaling between mammals and birds, insulin receptors have been characterized in several chicken tissues and two insulin receptor substrates (IRS-1 and Shc) have been described in liver and muscle. Compared with mammals current knowledge of insulin signaling in birds is incomplete. This is particularly evident when considering the number of isoforms of the components involved in the insulin cascade (IRSs, AKT, ERK and others) many of which may have not been characterized in the chicken. Despite these shortfalls in available data, it appears that insulin signaling in chicken liver is similar to that in mammals, but is unlike that in mammals in muscle. In leg muscle, chickens differ from mammals in the early steps of the insulin signaling cascade (IR, IRS-1 and PI3K) where PI3K activity is about 30-fold greater in the chicken than in the rat. This "constitutive" hyperactivity of PI3K in chicken muscle may over-stimulate a feedback inhibitory pathway described in mammals thereby desensitizing chicken muscle to insulin.

Mechanisms involved in the nutritional regulation of mRNA translation: features of the avian model

Abstract:Insulin and amino acids are key factors in regulating protein synthesis. The mechanisms of their action have been widely studied for several years. The insulin signal is mediated by the activation of intracellular kinases such as phosphatidylinositol–3'kinase and the mammalian target of rapamycin (mTOR), affecting the phosphorylation of some major effectors involved in the regulation of translation initiation, i.e. p70 S6 kinase (p70S6K) and the translational repressor eukaryotic initiation factor 4E binding protein (4E-BP1). The amino acid–induced signalling cascade also originates from mTOR and promotes p70S6K and 4E–BP1 activation. However, the mechanisms of regulation are complex and little understood, especiallyin vivo. Elucidating these mechanisms is important for both fundamental physiology and nutritional applications, i.e. better control of the use of nutrients and optimisation of dietary amino acid supplies in various physiological and physiopathological situations. In comparative physiology, the chicken is an interesting model to gain better understanding of the nutritional regulation of mRNA translation because of the very high rates of muscle growth and protein synthesis, and the unusual features compared with mammals. In the present review we provide an overview of the roles of insulin and amino acids as regulators of protein synthesis in both mammals and avian species.

Regulation of protein metabolism by insulin: value of different approaches and animal models

Insulin induces protein accretion by stimulating protein synthesis and inhibiting proteolysis. However, the mechanisms of regulation of protein metabolism by insulin are complex and still not completely understood. The use of approaches combining hyperinsulinemic clamp and isotopic methods, or measurement of the activation of intracellular kinases involved in insulin signaling, in addition to the use of different animal models in a comparative physiology process, provide better understanding of the potential regulation of protein metabolism by insulin. Studies using the clamp technique in lactating goats have shown a clear inhibitory effect of insulin on proteolysis, with an interaction between the effects of insulin and amino acids. Such studies revealed that the insulin-inhibited proteolysis is improved in lactating goats, this adaptative process limiting the mobilization of body protein under the conditions of amino acid deficit which occurs during early lactation. Insulin signaling studies in growing chickens have also provided some interesting features of insulin regulation compared to mammals. Refeeding or insulin injection leads to the activation of the early steps of insulin receptor signaling in the liver but not in the muscle. Muscle p70 S6 kinase, a kinase involved in the insulin activation of protein synthesis, was found to be markedly activated in response to insulin and to refeeding, suggesting that other signaling pathways than those classically described in mammalian muscles may be involved in signal transduction. Finally, although the role of insulin has been doubtful and has long been considered to be minor in ruminants and in avian species, this hormone clearly regulates protein metabolism in both species.

Adipose tissue compensates for defect of phosphatidylinositol 3'-kinase induced in liver and muscle by dietary fish oil in fed rats

The present work aimed to study in rats whether substitution of a low level of fish oil (FO; 2.2% of calories) into a low-fat diet (6.6% of calories from fat as peanut-rape oil or control diet) 1) has a tissue-specific effect on insulin signaling pathway and 2) prevents dexamethasone-induced alteration of insulin signaling in liver, muscle, and adipose tissue. Sixteen rats were used for study of insulin signaling, and sixteen rats received an oral glucose load (3 g/kg). Eight rats/group consumed control diet or diet containing FO over 5 wk. Four rats from each group received a daily intraperitoneal injection of saline or dexamethasone (1 mg.kg(-1).day(-1)) for the last 5 days of feeding. In liver, FO decreased phosphatidylinositol 3'-kinase (PI 3'-kinase) activity by 54% compared with control diet. A similar result was obtained in muscle. In both liver and muscle, FO clearly amplified the effect of dexamethasone. FO did not alter early steps of insulin signaling, and in muscle GLUT4 protein content remained unaltered. In adipose tissue, FO increased PI 3'-kinase activity by 74%, whereas dexamethasone decreased it by 65%; inhibition of PI 3'-kinase activity by dexamethasone was similar in rats fed FO or control diet, and GLUT4 protein content was increased by 61% by FO. Glycemic and insulinemic responses to oral glucose were not modified by FO. In conclusion, FO increased PI 3'-kinase activity in adipose tissue while inhibiting it in liver and muscle. The maintenance of whole body glucose homeostasis suggests an important role of adipose tissue for control of glucose homeostasis.

Early steps of insulin receptor signaling in chicken and rat: apparent refractoriness in chicken muscle

The early steps of insulin receptor (IR) signaling (tyrosine phosphorylation of IR beta-subunit, IRS-1 and Shc and PI 3'-kinase activity) have been characterized in two target tissues in the chicken: liver and muscle. The signaling cascade appeared to depend on nutritional status in the liver, but not in muscle (with a possible exception for a minor tyrosine phosphorylation of the 52 kDa Shc isoform). In this study, we compared the responses of the liver and muscle to exogenous insulin (10 or 1000 mU/kg) in chickens and rats. In the liver, IRS-1 and Shc proteins were present in smaller amounts and the regulatory subunit p85 of PI 3'-kinase was present in larger amounts in chickens than in rats. In the basal state (saline injection), the level of tyrosine phosphorylation of IR was lower, and that of Shc higher, in chickens than in rats. PI 3'-kinase activity in chickens was half that in rats. Insulin activated all components of the cascade in a dose-dependent manner in both species. A different pattern was observed in the muscle. In the basal state, the levels of tyrosine phosphorylation of IR and of PI 3'-kinase activity were much higher in chickens than in rats (by factors of 2 and 30, respectively). Insulin strongly activated all components of the cascade in rats (but with no significant increase in the phosphorylation of Shc). No activation was observed in chickens (with only a slight but significant increase in the tyrosine phosphorylation of Shc). The insulin cascade therefore appears to respond normally in chicken liver but to be refractory in chicken muscle. The large amount of p85 and high levels of PI 3'-kinase activity in muscle may contribute to this situation, making chicken muscle an interesting model of insulin resistance.

Insulin, insulin-like growth factor-I (IGF-I) and glucagon: the evolution of their receptors

Insulin and glucagon, two of the most studied pancreatic hormones bind to specific membrane receptors to exert their biological actions. Insulin-like growth factors IGF-I and IGF-II are structurally related to insulin, although they are expressed ubiquitously. The biological functions of the IGFs are mediated by different transmembrane receptors, which includes the insulin, IGF-I and IGF-II receptors. The interaction of insulin, insulin related peptides and glucagon with the corresponding receptors has been studied extensively in mammals and continues to be so. At the same time, research on ectothermic animals has made enormous progress in the recent years. This paper summarizes current knowledge on insulin, IGF-I and glucagon receptors, from a comparative point of view with special attention to non-mammalian vertebrates. The review covers adult and mostly typical target tissues, and with very few exceptions, developmental aspects are not considered. Binding characteristics, tissue distribution and structure of insulin and IGF-I receptors will be considered first, because both ligands and receptors are structurally related and have overlapping functions. These sections will be followed by similar distribution of information on glucagon receptors. Readers interested in either structure or functions of insulin, IGFs and glucagon in nonmammalian vertebrates are referred to other reviews (Mommsen TP, Plisetskaya EM. Insulin in fishes and agnathans: history, structure and metabolic regulation. Rev Aquat Sci 1991;4:225-259; Mommsen TP, Plisetskaya EM. Metabolic and endocrine functions of glucagon-like peptides: evolutionary and biochemical perspectives. Fish Physiol Biochem 1993;11:429-438; Duguay SJ, Mommsen TP. Molecular aspects of pancreatic peptides. In: Sherwood NM, Hew CL, editors, Fish Physiology. vol 13. 1994:225-271; Plisetskaya EM, Mommsen TP. Glucagon and glucagon-like peptides in fishes. Int Rev Citol 1996;168:187-257.).

Muscle insulin-like growth factor-I (IGF-I) receptors in chickens with high or low body weight: effects of age and muscle fibre type

IGF-I receptors were characterized in leg and breast muscles from 1- and 7-week-old chickens selected for high (HG) or low (LG) growth rate. Following whole muscle solubilization, receptors were partially purified by wheat germ agglutinin (WGA) chromatography. IGF-I and insulin binding did not differ between genotypes, but significantly decreased with age, without any change in receptor affinity. In the older birds, insulin binding was lower in breast than in leg muscle, whereas IGF-I binding was similar. Using the artificial substrate poly(Glu-Tyr) 4:1, the IGF-I-stimulated tyrosine kinase activity of the receptors was measured and compared between genotypes (at 1 week), and in HG chickens between muscle types (1 and 7 weeks) and ages (in leg muscle). It was not modified by any of these factors. We conclude that: (1) IGF-I and insulin receptor number do not differ between genotypes, (2) the number of both receptors decreases with age, and (3) in 7-week-old birds, breast muscle could become less responsive to insulin than leg muscles, following a decrease in receptor number.

Nutritional state regulates insulin receptor and IRS-1 phosphorylation and expression in chicken

After insulin binding, insulin receptors (IR) phosphorylate the insulin receptor substrate 1 (IRS-1) on specific motifs and thereby initiate insulin action. The interaction between IR and IRS-1 and their expression were studied in vivo in two target tissues (muscle and liver) in chickens, a species that is insulin resistant. To induce extreme changes in plasma insulin levels, chickens were subjected to three different nutritional states (ad libitum fed, fasted for 48 h, and refed for 30 min after 48-h fast). Liver membrane IR number was significantly increased in fasted compared with fed chickens. This upregulation of IR number was concomitant with the an enhanced expression of IR mRNA as determined by reverse transcription-polymerase chain reaction. In leg muscle, IR mRNA was not altered by the nutritional state. Using specific antibodies directed toward human IR, anti-phosphotyrosines, or mouse IRS-1, we demonstrated that IR and IRS-1 are associated in vivo in liver and muscles. Tyrosine phosphorylation of liver IR and IRS-1 were significantly decreased by prolonged fasting and restored by 30-min refeeding. These alterations were not observed in muscle. Fasting increased IRS-1 mRNA expression in liver but not in muscle. These results are the first evidence showing that chicken liver and muscle express IRS-1. Therefore, the chicken insulin resistance is not accounted for by the lack of IRS-1. The differences observed for the regulation of IR and IRS-1 messengers and phosphorylation between liver and muscle in response to alterations of the nutritional state remain to be explained.

Effects of obesity and gender on insulin receptor expression in liver of SHHF/Mcc-FAcp rats

In SHHF/Mcc-FAcp rats (formerly SHR/Mcc-cp), obesity and male gender synergistically modulate hyperinsulinemia, insulin resistance and predisposition to diabetes. Our previous studies showed gender and obesity modulate hepatic cell surface insulin binding and insulin clearance additively. Hepatic insulin receptors (IR) bind insulin as a first step in insulin clearance through internalization and degradation. We hypothesize that the synergistic effects of obesity and gender on hepatic insulin binding and clearance result from interaction of these two factors on hepatic IR expression. To address IR expression in SHHF/Mcc-FAcp rats, we quantitated IR protein levels in detergent-solubilized liver homogenates by Western blotting and IR mRNA levels by a solution hybridization/RNase protection assay. Obesity reduced total hepatic IR content in males and females, 50% and 68% respectively. Male gender reduced IR protein content 24% in lean, but had no effect on IR protein content in obese rats. Neither gender nor obesity affected hepatic IR mRNA content. Thus, obesity appears to affect hepatic IR protein content and cell surface binding through post-transcriptional mechanisms; similarly, male gender in lean rats reduces IR protein levels and cell surface binding through mechanisms not involving changes in mRNA levels. In obese rats, the synergistic effects of male gender appears to involve changes in IR trafficking and consequently cell surface insulin binding and processing.

Insulin receptor and insulin sensitivity in a chicken hepatoma cell line

Insulin receptors have been characterized in a cell line recently isolated from a chicken hepatoma (LMH). The binding of 125I-insulin to LMH cells or membranes displayed the expected criteria for insulin receptors: affinity, temperature dependency, curvilinearity of Scatchard plot, rank order of potency for insulin analogs and insulin induced down-regulation. The alpha-subunit of LMH cell insulin receptors exhibited a normal size of 135 kDa. Following autophosphorylation, LMH WGA-purified receptors revealed a 95 kDa beta-subunit and a 72 kDa protein (pp72). Both proteins were phosphorylated in a time-, insulin- (and insulin-like growth factor 1; IGF-1) and manganese-dependent manner, and were precipitated by antiphosphotyrosine and two anti-insulin receptor antibodies. The 72 kDa protein was not present under non-reducing condition PAGE or in normal chicken liver. These results strongly suggest that pp72 is either a truncated form of the insulin receptor beta-subunit specific to LMH cells or a degradation product. Lectin-purified insulin receptors from LMH cells or chicken liver membranes exhibited similar tyrosine kinase activity, using artificial substrate poly(Glu-Tyr) 4:1. Finally, amino acid uptake by LMH cells was insulin stimulatable.

Insulin receptor binding and kinase activity in liver and skeletal muscles of lactating goats

Lactation in goats has been shown to modify in vivo insulin action. [Debras, E., J. Grizard, E. Aina, S. Tesseraud, C. Champredon, and M. Arnal. Am. J. Physiol. 256 (Endocrinol. Metab. 19): E295-E302, 1989]. To further elucidate the mechanism of insulin action, we studied insulin binding and insulin receptor tyrosine kinase activity in solubilized and partially purified receptor preparations from liver and skeletal muscles (longissimus dorsi, tensor fascia lata, diaphragm, and masseter) from lactating and nonlactating goats. Lactation did not alter insulin receptors in the various skeletal muscles and had a minor influence on liver receptors (where only a 20% increase in receptor number was visible, P less than 0.05). Insulin-stimulated autophosphorylation and the kinase activity against polyglutamyltyrosine (4:1) were not significantly modified in skeletal muscle receptor preparations from lactating goats when compared with nonlactating animals. They tended to decrease in liver preparations, but not significantly. Thus the changes in insulin action in vivo during lactation in goats were not related to modifications in insulin kinase activity but were probably localized at a postreceptor level.

Annual variations in the binding of insulin to hepatic membranes of the frog Rana esculenta

Amphibia undergo regular annual cycles of metabolic activity that are influenced by both exogenous factors and hormones. Insulin binding to crude frog hepatic membranes was studied throughout the year. The general character of insulin binding was similar to that in other vertebrates; the maximum specific binding was achieved after 4 hr at 4 degrees, the optimum pH was 7.8, half-maximal displacement of bound insulin was from 9 x 10(-10) to 1 x 10(-9) M, and insulin analogs competed for the insulin receptor in line with their relative biological potencies. A biphasic Scatchard plot and negative cooperativity of the receptor were also observed in frog liver membranes. Affinity constants from Scatchard plots revealed high and low affinity binding sites which were unchanged during the year. The seasonal cycle, however, markedly affected the binding capacity for both sites. Maximum binding occurred in May-June and the minimum in November-December for both classes of receptors. Binding capacities ranged from 1.71 to 11.33 fmol/mg protein for the high affinity sites and from 432 to 3171 fmol/mg protein for the low affinity sites. It is concluded that annual cycles of insulin binding reflect modulation of receptor number rather than receptor affinity.

Insulin-receptor binding in skeletal muscle of trout

Two hundred rainbow trout (Oncorhynchus mykiss) age 0 +, weight range 11.3 - 11.5 g, were distributed randomly in two groups and maintained for five weeks on either 10% dextrin, or 20% dextrin diet. The fish were sampled 3-5 h and 18-20 h after the last feeding and insulin binding to partially purified insulin receptors in white and red skeletal muscles and to liver plasma membranes was assessed. Plasma insulin, plasma glucose, and liver glycogen content were analyzed in the same fish.Fish fed a diet with higher carbohydrate content (HC) had elevated insulin and glucose levels in peripheral blood, but lower liver glycogen contents compared to the fish fed a diet with lower carbohydrate content (LC). No growth retardation was observed in the fish from HC group.Three to five hours after the last feeding, insulin-receptor binding in white skeletal muscles was higher in HC group of fish, mostly because of an increase in number of high affinity binding sites. Eighteen to twenty hours after the last feeding this difference disappeared. In contrast, the specific binding of insulin to the liver plasma membranes appeared to be lower in the HC group of fish. The lower insulin binding to the liver plasma membranes observed 3-5 h after feeding, could be attributed to the lower quantity of binding sites, while the same phenomenon 18 h after feeding was likely a result of affinity changes. We conclude that higher glycemic levels observed in trout fed a HC diet as compared to LC group of fish, are not a consequence of impaired binding of insulin to its receptors in skeletal muscles.

Insulin binding and receptor tyrosine kinase activity in rat liver and skeletal muscle: effect of starvation

Insulin binding and insulin receptor kinase activity were measured in solubilized and partially purified receptor preparations from liver and skeletal muscles of rats that were either fed a standard diet or subjected to a 72-hour fasting period. Insulin binding capacity was increased in both tissues from fasted rats as determined by Scatchard analysis. The affinity of the receptors was not modified by fasting. Affinity labeling of the alpha-subunit of insulin receptors also suggested an increase in the number of insulin receptors in both tissues. The ability of insulin to stimulate the autophosphorylation of the beta-subunit as well as the phosphorylation of the artificial substrate Glu80-Tyr20 was significantly impaired in liver from fasted rats and by contrast unchanged in skeletal muscles. These findings indicate that in rats, fasting produces changes in insulin receptor kinase activity in liver but not in muscle. The physiological significance of this tissue-specific regulation of receptor kinase activity in relation to insulin action during fasting remains to be established.