Keys for microRNA expression profiling of single rat hypothalamic nuclei and multiplex sequencing strategies

Integrative research has taken on the challenge of addressing questions in physiology by using novel knowledge and novel techniques. Recently, small and long non-coding RNAs have emerged as key regulators of gene expression, while next-generation sequencing technologies have revolutionized the characterization of genomes and gene expression. For a decade, it has been known that microRNAs (miRNAs) are RNAs of 18-24 bases that regulate gene expression in mammals. Here, we first describe the nature of miRNAs and the advantages of high-throughput sequencing technologies for establishing miRNA expression profiles. The hypothalamus harbours a dozen specialized areas or nuclei, the sampling of which is required to establish physiologically relevant miRNA expression profiles. MicroRNA expression profiling from single animals is also important for investigating potential genetic or epigenetic differences between individuals. Establishing a large number of miRNA expression profiles of individual hypothalamic nuclei of single rats at a cost compatible with laboratory finance can be achieved by using tagged cDNA libraries constructed from purified small RNAs and a multiplex sequencing strategy. We continue this report by surveying specificities of the different strategies that are used at present for constructing tagged cDNA libraries and provide a comparative analysis of miRNA expression profiles from hypothalamic arcuate nuclei of seven male Wistar rats.

Adiponectin receptors are expressed in hypothalamus and colocalized with proopiomelanocortin and neuropeptide Y in rodent arcuate neurons

Adiponectin is involved in the control of energy homeostasis in peripheral tissues through Adipor1 and Adipor2 receptors. An increasing amount of evidence suggests that this adipocyte-secreted hormone may also act at the hypothalamic level to control energy homeostasis. In the present study, we observed the gene and protein expressions of Adipor1 and Adipor2 in rat hypothalamus using different approaches. By immunohistochemistry, Adipor1 expression was ubiquitous in the rat brain. By contrast, Adipor2 expression was more limited to specific brain areas such as hypothalamus, cortex, and hippocampus. In arcuate and paraventricular hypothalamic nuclei, Adipor1, and Adipor2 were expressed by neurons and astrocytes. Furthermore, using transgenic green fluorescent protein mice, we showed that Adipor1 and Adipor2 were present in pro-opiomelanocortin (POMC) and neuropeptide Y (NPY) neurons in the arcuate nucleus. Finally, adiponectin treatment by intracerebroventricular injection induced AMP-activated protein kinase (AMPK) phosphorylation in the rat hypothalamus. This was confirmed by in vitro studies using hypothalamic membrane fractions. In conclusion, Adipor1 and Adipor2 are both expressed by neurons (including POMC and NPY neurons) and astrocytes in the rat hypothalamic nuclei. Adiponectin is able to increase AMPK phosphorylation in the rat hypothalamus. These data reinforced a potential role of adiponectin and its hypothalamic receptors in the control of energy homeostasis.

A putative physiological role of hypothalamic CNTF in the control of energy homeostasis

Administration of CNTF durably reduces food intake and body weight in obese humans and rodent models. However, the involvement of endogenous CNTF in the central regulation of energy homeostasis needs to be elucidated. Here, we demonstrate that CNTF and its receptor are expressed in the arcuate nucleus, a key hypothalamic region controlling food intake, and that CNTF levels are inversely correlated to body weight in rats fed a high-sucrose diet. Thus endogenous CNTF may act, in some individuals, as a protective factor against weight gain during hypercaloric diet and could account for individual differences in the susceptibility to obesity.

Lack of cross-desensitization between leptin and prolactin signaling pathways despite the induction of suppressor of cytokine signaling 3 and PTP-1B

Hyperprolactinemia and hyperleptinemia occur during gestation and lactation with marked hyperphagia associated with leptin resistance. Prolactin (PRL) induces the expression of orexigenic neuropeptide Y (NPY) through the activation of JAK-2/STAT-3 signaling pathway in hypothalamic paraventricular nucleus (PVN) leading to hyperphagia. PRL may also act through the inhibition of anorexigenic effect of leptin via induction of suppressor of cytokine signaling 3 (SOCS-3). This paper aimed to co-localize PRL (PRL-R) and leptin (ObRb) receptors in the hypothalamus of female rats and investigate the possible cross-desensitization between PRL-R and ObRb. We showed that: 1) PRL-R and ObRb are expressed in the PVN and co-localized in the same neurons; 2) in lactating females leptin failed to activate JAK-2/STAT-3 signaling pathway; 3) in Chinese Hamster Ovary (CHO) stably co-expressing PRL-R and ObRb, overexposure to PRL did not affect leptin signaling but totally abolished PRL-dependent STAT-5 phosphorylation. The overexposure to leptin produces similar results with strong alteration of leptin-dependent STAT-3 phosphorylation, whereas PRL-dependent STAT-5 was not affected; and 4) CHO-ObRb/PRL-R cells overexposure to leptin or PRL induces the expression of negative regulators SOCS-3 and PTP-1B. Thus, we conclude that these negative regulators affect specifically the inducer signaling pathway; for instance, SOCS-3 induced by PRL will affect PRL-R signaling but not ObRb signaling and vice versa. Finally, the lack of cross-desensitization between PURL-R and ObRb suggests that hyperphagia observed during gestation and lactation may be attributed to a direct effect of PRL on NPYexpression, and is most likely exacerbated by the physiological leptin resistance state.

Effects of immunisation against leptin on feed intake, weight gain, fat deposition and laying performance in chickens

1. Three experiments were conducted to study the effects of leptin on weight gain and body composition in laying hens. 2. The effects of immunisation against chicken leptin on feed intake (FI), fat deposition and laying rate were observed in laying Guangdong yellow-feathered hens. Ten hens were inoculated with leptin immunogen on d 3, 31, 63 and 84, together with 10 control hens immunised with bovine serum albumin (BSA). In the 100-d experiment, immunisation against leptin increased blood anti-leptin antibody titres, slightly reduced plasma T3 concentrations, slightly decreased FI and increased live weight; however, laying rate was significantly depressed and abdominal fat mass was increased by the end of the 100-d experiment. 3. Passive immunisation of 50-d-old pullets with yolk extract containing anti-leptin antibody IgY significantly increased FI within 6 h of treatment compared with physiological saline treated controls. 4. In growing 70-d-old pullets, inoculation with 0.5 (group 1) or 1 (group 2) ml leptin immunogen on d 1 and 28 of the experiment slightly increased FI and significantly increased daily gain compared with BSA-immunised control pullets. Abdominal fat mass on d 49 increased from 48+/-4.5 g in controls to 66+/-3.5 and 80+/-3.1 g in groups 1 and 2, respectively. 5. It was suggested that immunisation against leptin mimicked loss of leptin bioactivity and might become a novel technique to stimulate fat growth in certain types of animal production.

Is There Peripheral or Ovarian Insulin Action Alteration in Broiler Breeder Hens Fed ad Libitum?

We investigated whether a change in peripheral glucose homeostasis, a local change in the insulin-related ovarian regulatory system, or both occurred in ad libitum-fed broiler breeder hens compared with feed-restricted counterparts. Feed-restricted (R, from 5 to 16 wk of age) and ad libitum-fed (A) hens from a standard commercial line (S) and an experimental dwarf genotype (E) were studied. Basal and stimulated plasma insulin and glucose concentrations were measured during the prebreeding and laying periods. In the basal state (after 16 h fasting) plasma glucose concentrations were significantly lower in SA chickens (-5% at 17 wk, -7.5% at 32 wk) compared with EA, SR, and ER chickens, with no difference in plasma insulin concentrations (n = 16). In 17-wk-old SA birds, 30 min after oral glucose loading, plasma glucose concentrations increased significantly compared with the basal state and were also significantly lower as compared with SR but did not differ significantly from EA and ER. Plasma insulin concentrations did not differ significantly between genotypes or regimens (n = 16). A potential modification of intracellular mediators involved in the regulation of cell growth and survival in small follicles that were overrecruited in SA compared with SR was also investigated in SA and SR hens at 32 wk. There was no effect of food restriction in phospho-Akt, Akt, phospho-ERK, and phospho-S6 in the small white ovarian follicles (n = 6) in the basal state and after 30 min of refeeding. In conclusion, the present study does not demonstrate any evidence of glucose intolerance during the prebreeding period, specific change in the ovarian small follicle insulin signalling pathway, or both, in laying broiler breeders fed ad libitum compared with feed-restricted hens.

Peripheral leptin effect on food intake in young chickens is influenced by age and strain

The acute effect of leptin on the regulation of food intake was investigated in layer and broiler chickens. In an initial study, we observed that a single intraperitoneal injection of recombinant chicken leptin (1 mg/kg BW) dramatically reduced (38%) food intake in 56-day-old layer chickens, more moderately reduced (15%) food intake in 9-day-old layer chicks, and had no significant effect in 9-day-old broiler chicks. In a subsequent study, body weight and plasma concentrations of leptin were measured weekly in layer and broiler chicks from day 1 to 35 of age and brain leptin receptor and neuropeptide Y (NPY) mRNA expression were analyzed at 1, 9, and 35 days of age. At day 1 of age, peripheral concentrations of leptin were significantly greater in layer than broiler chicks. Subsequently, despite increases in body weight and differences in growth rates between layer and broiler chicks from day 8 to day 35 of age, peripheral concentrations of leptin were constant and similar in both genotypes. Leptin receptor and NPY mRNA were expressed in brain from day 1 in chicks of both genotypes and increased significantly to day 35 of age. These observations provide evidence that the inhibitory effect of leptin on the regulation of food intake in growing chicks is an age dependent process. Furthermore, acquisition of the anorectic effect of leptin is likely to be associated with greater expression of the leptin receptor and NPY mRNAs than to changes in blood levels of leptin. Finally, this study provides evidence that chickens selected for high growth rates may be less sensitive or responsive to peripheral concentrations of leptin than chickens with low growth rates (layers), suggesting that the faster growth of broiler chicks may be related to a lessened responsiveness to anorexigenic factors.

Sex ratios in mule duck embryos at various stages of incubation

Mule duck hatcheries have long reported varying degrees of unbalance in the sex ratio, with a preponderance of male mules at hatching. The aim of the present study was to assess the distributions of sex ratios at various stages of development in embryos originating from intra- and intergeneric crosses between parental lineages (Muscovy male x Muscovy female, Pekin male x Pekin female, Muscovy male x Pekin female or Mule, and Pekin male x Muscovy female or Hinny). In Experiment I, embryo sexing was performed on Days 1 and 5 of incubation (by multiplex PCR) and at hatching (by vent observation). The sex ratio was not significantly modified during the early stages of embryo development whatever the genetic origin (P>0.05, Days 1 and Day 5) but our results in mule and hinny ducklings confirmed the preponderance of males among normally hatched ducklings originating from the intergeneric lineage (58.9 and 55.4% males in mules and hinnies, respectively; P<0.05 in both cases). Sex ratio (vent sexing) in second grade (cull) ducklings revealed that 68% of these ducklings were females (P<0.05). In Experiment II, the distribution of sex ratio was also performed in mule duck eggs from 6 batches (400,000 eggs/batch) first examined for fertility (candling) on Day 18 of incubation. These results indicate that the percentage of males present in the population of normally hatched ducklings increases when fertility decreases. In addition, this experiment also revealed that 83.7-90.5% of viable male mule embryos develop up to hatching, compared to only 43.0-51.0% of female mule embryos. Given that a deviation in sex ratio during the first stages of incubation is unlikely (Experiment I), it is concluded that the skewed sex ratio of mule ducks at hatching is primarily due to increased late mortality in female mule embryos occurring between egg transfer and hatching. This mortality originated, at least in part, from the intergeneric origin of female mules, and was marked to a greater or lesser extent depending on the initial success of fertilization in a given batch, a possible indication that the initial quality of gametes may selectively exert its influence at the later stages of embryo development.

Leptin and insulin downregulate leptin receptor gene expression in chicken-derived leghorn male hepatoma cells

In chickens, leptin is expressed mainly in the liver, where its receptor gene expression has also been reported, and in adipose tissue. In view of the key role played by the liver in lipogenesis in avian species, the hepatic expression of leptin may have physiological significance. In this study, we showed that leptin is constitutively expressed and secreted in a chicken-derived hepatoma cell line (LMH). Although insulin regulates leptin expression in vivo, incubation of LMH cells in the presence of 100 nM insulin for 24 or 48 h had no effect on leptin expression or its secretion in the culture medium. In addition, we developed a specific chicken leptin receptor real-time reverse transcription (RT)-PCR, and downregulation of leptin receptor gene expression by homologous and heterologous signals was demonstrated, as relative leptin receptor mRNA levels were significantly decreased after exposure of LMH cells to recombinant chicken leptin or porcine insulin. In conclusion, our results indicate that leptin is probably able to desensitize its own response in the chicken liver. Finally, the ability of insulin and leptin to regulate chicken leptin receptor gene expression suggests a direct role of leptin in the control of hepatic metabolism.

Leptin fully suppresses acetylcholine-induced insulin secretion and is reversed by tolbutamide in isolated perfused chicken pancreas

So far, there has been no evidence for any direct pancreatic effect of leptin in the chicken. The present study was aimed at detecting chicken leptin receptor (cOb-R) expression in isolated chicken islets of Langerhans and to examine the direct effect of leptin on insulin secretion after stimulation by acetylcholine (1 micro M) + glucose (14 mM) from isolated perfused chicken pancreas. We will show that i) full length cOb-R mRNA was expressed in isolated pancreatic islets of chickens, ii) recombinant chicken leptin (10 nM) or diazoxide (100 micro M) rapidly (within 2 min) and significantly suppressed insulin secretion induced by acetylcholine stimulation without any change in volume outflow rate, iii) tolbutamide (100 micro M) introduced 10 min after leptin and perfused for 10 min fully reversed the suppressive effect of leptin on pre-established acetylcholine-induced insulin release. In conclusion, we found that leptin has a profound inhibitory influence upon insulin secretion in perfused chicken pancreas. The results suggest that leptin inhibits insulin secretion by acting before or at the level of K ATP channels in chicken pancreatic beta-cells. Further studies are warranted to clarify the specific inhibitory mechanism.

Early-age thermal conditioning reduces uncoupling protein messenger RNA expression in pectoral muscle of broiler chicks at seven days of age

Early-age thermal conditioning (TC) by exposing young chicks to 40 C for 24 h reduces body temperature (Tb) and has been showed by others to improve long-term resistance of broilers to heat stress. Uncoupling oxidative phosphorylation in pectoral muscle mitochondria might be related to heat production. Fertile eggs were hatched under video control, and 161 pedigree chicks froml2 sires and 22 dams were immediately allocated to two groups (T, a group composed of 81 chicks exposed to TC at 5 d of age, and N, a control group of 80 nonexposed chicks). Body weights and Tb were measured at 2 and 7 d of age. Five pairs (one N and one T) of full sib chicks from families that exhibited the largest difference of Tb variation from 2 to 7 d of age between the two treatments were chosen for pectoral muscle sampling. Avian uncoupling protein (avUCP) messenger RNA expression was measured by reverse transcript-PCR coupled to southern blot in the pectoral muscle of 7-d-old broiler chicks. At 7 d of age, there were no BW differences between treatments and Tb was significantly reduced by TC (-0.13 C on average). Heritability of Tb variation between 2 and 7 d was 0.38 +/- 0.20 (SE) for T chicks and 0.35 +/- 0.17 for N chicks without a significant genetic correlation between the two environments. Expression of avUCP mRNA was significantly (85%) lower in T chicks than in N chicks. Uncoupling protein mRNA expression in pectoral muscle and Tb are quickly adjusted in broiler chicks 24 h after early thermal conditioning.

Chicken leptin: properties and actions

Chicken leptin cDNA shows a high homology to mammalian homologous, with an expression localized in the liver and adipose tissue. It is noteworthy, that the hepatic expression is most likely associated with the primary role that this organ plays in lipogenic activity in avian species. As in mammals, chicken leptin expression is regulated by hormonal and nutritional status. This regulation is tissue-specific and with a high sensitivity in the liver compared to adipose tissue. The blood leptin levels are regulated by the nutritional state with high levels in the fed state compared to the fasted state. The recombinant chicken leptin markedly inhibits food intake as reported in mammals, suggesting the presence of an hypothalamic leptin receptor. The chicken leptin receptor has been identified and all functional motifs are highly conserved compared to mammalian homologous. Chicken leptin receptor is expressed in the hypothalamus but also in other tissues such as pancreas, where leptin inhibits insulin secretion and thus may have a key role in regulating nutrient utilization in this species.

An uncoupling protein homologue putatively involved in facultative muscle thermogenesis in birds

The cDNA of an uncoupling protein (UCP) homologue was obtained by screening a chicken skeletal-muscle library. The predicted 307-amino-acid sequence of avian UCP (avUCP) is 55, 70, 70 and 46% identical with mammalian UCP1, UCP2 and UCP3 and plant UCP respectively. avUCP mRNA expression is restricted to skeletal muscle and its abundance was increased 1.3-fold in a chicken line showing diet-induced thermogenesis, and 3.6- and 2.6-fold in cold-acclimated and glucagon-treated ducklings developing muscle non-shivering thermogenesis respectively. The present data support the implication of avUCP in avian energy expenditure.

Biological activities of recombinant chicken leptin C4S analog compared with unmodified leptins

The chicken leptin sequence, in contrast to mammalian leptins, contains an unpaired Cys at position 3 of the original cDNA (AF012727). The presence of an extra Cys may confer a different structure and affect the leptin's biological activity. To address this, we studied the effects of wild-type and mutated (C4S) chicken leptins in vitro and in vivo and compared them with mammalian leptin prepared from ovine leptin cDNA. The prokaryotic expression vector pMON, encoding full-size A(-1) chicken leptin (AF012727), was mutated using a mutagenesis kit, yielding the C4S analog. Escherichia coli cells transformed with this vector overexpressed large amounts of chicken leptin C4S upon induction with nalidixic acid. The expressed protein, found in the inclusion bodies, was refolded and purified to homogeneity on a Q-Sepharose column, yielding three electrophoretically pure fractions, eluted from the column by 100, 125, and 150 mM NaCl, respectively. All three fractions showed a single band of the expected molecular mass (16 kDa) and were composed of >95% monomeric protein. Proper refolding was evidenced by comparing the circular dichroism spectrum of the analog with spectra of nonmutated chicken and ovine leptins. The biological activity of the C4S analog was evidenced by its ability to stimulate proliferation of leptin-sensitive BAF/3 cells transfected with a long form of human leptin receptor construct similar to its nonmutated counterpart, indicating that Cys4 plays no role in leptin activity. The in vitro activity of both wild-type and mutated chicken leptins was approximately 10-fold lower than that of ovine leptin. After intravenous or intraperitoneal injections, C4S analog and the nonmutated chicken and ovine leptins all lowered the food intake of starved 9-day-old broiler or 5-wk-old layer male chickens by 11-34%. Monitoring food behavior revealed that the attenuated food intake resulted not from a decreased number of approaches to the feeders but from a decrease in the average time spent eating during each approach.

A chicken leptin-specific radioimmunoassay

Recombinant chicken leptin was used to produce an antiserum in order to develop a specific and sensitive radioimmunoassay (RIA) for chicken leptin in plasma and serum. We have used either murine or chicken leptin as tracer and competition curves were performed using recombinant chicken leptin. Variations in leptin plasma levels in different chicken strains and various nutritional states were correlated with the physiological status. Leptin plasma concentrations were regulated by the nutritional state with higher levels in the fed state as compared to the fasted state (3.36 +/- 0. 13 versus 2.78 +/- 0.11 ng/ml) and being dependent upon the age. Higher leptin levels were found in 22 week-old as compared to 15 week-old layer chickens (2.709 +/- 0.172 versus 1.478 +/- 0.102 ng/ml). We have also shown that the multispecies leptin RIA kit (LINCO Inc.) underestimated leptinemia compared to the chicken leptin- specific RIA reported here. In conclusion the RIA developed in the present study is specific to the chicken and thus may be considered as powerful tool for investigating the physiological significance of leptin in chickens.

Metabolic differences between genetically lean and fat chickens are partly attributed to the alteration of insulin signaling in liver

Insulin signaling [tyrosine phosphorylation of insulin receptor (IR), insulin receptor substrate-1 (IRS-1), Src homology and collagen protein (Shc) and phosphatidyl inositol 3'-kinase activity (PI 3'-kinase)] was studied in the liver and thigh muscles of fat (FL) and lean (LL) chickens. These lines result from a divergent selection on abdominal fat pad size. The divergence is of metabolic origin. Extreme nutritional states were studied (fed, 48-h starved and 30-min refed). Such conditions significantly altered insulin signaling in chicken liver, but surprisingly not in the muscle (except the phosphorylation of Shc in the refed state). No major differences that could account for this divergence were found in muscle. Liver IR number and Shc protein did not differ between genotypes. Liver IRS-1 (protein and messenger) was lower in the fed state and higher in the starved state in FL compared to that in LL chickens. In the fed state, tyrosine phosphorylation of liver IR, IRS-1 and Shc action was higher in FL than in LL chickens that in the absence of insulin resistance rely on higher plasma insulin levels. In the starved state, phosphorylation of liver IR was lower, but the phosphorylation of IR and IRS-1 were higher in LL than in FL chickens, most likely in response to higher plasma glucose and insulin in the lean genotype. In the refed state, the phosphorylation of liver IR and IRS-1 did not differ between genotypes despite significantly lower plasma insulin in FL chickens. Finally, PI 3'-kinase was not affected by the genotype. A significant activation of early steps of insulin signaling in liver of fed FL chickens may at least partly account for their increased liver lipogenesis and ultimately their fattening.

Corticosterone alters insulin signaling in chicken muscle and liver at different steps

Chronic treatment with corticosterone evokes insulin resistance in chickens, a species which is already resistant to insulin compared with mammals. The in vivo effects of corticosterone on insulin signaling were investigated in chicken liver and thigh muscle in two nutritional states: basal (overnight fasted) and stimulated (30 min refeeding). Corticosterone significantly decreased specific insulin binding in liver and the amount of insulin receptor substrate-1 (IRS-1) and p85 (regulatory subunit of phosphatidylinositol (PI) 3'-kinase) in both tissues. Insulin receptor (IR) and IRS-1 mRNAs generally varied accordingly. Src homology and collagen protein (Shc) and messenger were not altered. In liver, in the basal state, the tyrosine phosphorylation of IR, IRS-1 and Shc, and the IR-associated PI 3'-kinase activity were largely decreased by corticosterone. Following refeeding the cascade was activated in control but totally inhibited in treated chickens. In muscle, as previously observed, IR and IRS-1 phosphorylation and PI 3'-kinase were not stimulated by refeeding in controls. Only the phosphorylation of Shc was increased. On this background, corticosterone decreased the basal PI 3'-kinase activity and prevented the phosphorylation of Shc in response to refeeding. In conclusion, corticosterone largely impaired insulin signaling in liver and to some extent in muscle. This should contribute to the large impairment of growth. In addition, the present studies further emphasize the peculiarities of insulin signaling in chicken muscle, which needs further investigation.

Large-scale preparation of biologically active recombinant chicken obese protein (leptin)

Prokaryotic expression vector pMON3401 encoding full size A(-1) chicken leptin (AF012727) was prepared by PCR of previously described cDNA. Escherichia coli cells transformed with this vector overexpressed large amounts of chicken leptin upon induction with nalidixic acid. The expressed protein found in the inclusion bodies was refolded and purified to homogeneity on a Q-Sepharose column, yielding two electrophoretically pure fractions (leptin-1 and leptin-2), eluted from the column by 100 and 125 mM NaCl. Both fractions showed a single band of the expected molecular mass of 16 kDa and were composed of over 95% of monomeric protein. The biological activity of both fractions, resulting from proper renaturation, was further evidenced by their ability to stimulate proliferation of leptin-sensitive BAF/3 cells transfected with a long form of human leptin-receptor construct and by lowering the food intake of starved chicken following intravenous or intraperitoneal injections.

Effect of nutritional state on the formation of a complex involving insulin receptor IRS-1, the 52 kDa Src homology/collagen protein (Shc) isoform and phosphatidylinositol 3'-kinase activity

The Src homology and collagen protein (Shc) is tyrosine phosphorylated in response to insulin; however, evidence for its interaction with insulin receptor (IR) in normal tissues is missing. Interactions between IR, Shc and regulatory subunits of the phosphatidylinositol 3'-kinase (PI 3'-kinase) were characterized in the present study in liver and muscles of chickens submitted to various nutritional states. A chicken liver Shc cDNA fragment encoding a 198 amino acid long fragment, including the phosphotyrosine binding domain was sequenced. It shows 89% homology with the corresponding human homologue. The amounts of the three Shc isoforms (66, 52 and 46 kDa) and Shc messenger were not altered by the nutritional state. Shc tyrosine phosphorylation was decreased by fasting in both liver and muscle. Importantly, Shc was immunoprecipitated by IR antibody (mostly the 52 kDa isoform) or by alphaIRS-1(mostly the 46 kDa isoform). IR-Shc association was decreased by fasting and restored by refeeding. In liver, alphaShc immunoprecipitated the three forms of regulatory subunits of PI 3'-kinase and a PI 3'-kinase activity which was decreased by fasting. In muscle, alphaShc immunoprecipitated only the p85 isoform; the associated PI 3'-kinase activity was not altered by the nutritional state. Conversely, in both tissues anti-p85 antibody precipitated only the 52 kDa Shc isoform. In liver, antibodies to insulin receptor substrate-1 (alphaIRS-1), Shc or IR immunoprecipitated the three regulatory subunits of PI 3'-kinase and an equal PI 3'-kinase activity, without any residual activity left in the supernatants, suggesting the presence of a large complex involving IR, IRS-1, Shc (mainly the 52 kDa isoform) and PI 3'-kinase activity. The presence of another complex containing IRS-1 and the 46 kDa Shc isoform, but no PI 3'-kinase activity, is suggested.

Cloning the chicken leptin gene

Chicken is characterized by a relative insulin resistance and a physiological hyperglycemia (2g/L) and is also subjected to fattening. Fat deposits in chicken, as in mammals, are regulated by environmental and genetic factors. In mammals, leptin, an adipose cell-specific secreted protein has been characterized that is encoded by ob gene. Leptin regulates satiety through hypothalamic specific receptors, energy balance, energy efficiency and contributes to adaptation to starvation. The leptin gene has been characterized in various mammalian species, and the cloning and sequencing of the chicken leptin gene (ob gene) are reported. Using RT-PCR and primers flanking the coding region of the leptin gene selected from known mammalian sequences, we have successfully amplified a 600-bp fragment from chicken liver and adipose tissue total ARNs. The amplified fragment exhibits a similar size to that of the coding region of the mammalian leptin gene. The sequences of the coding region of chicken liver and adipose tissue are identical and presented 97%, 96% and 83% similarity to the mouse, rat and human sequences, respectively. Finally, this is the first report showing that leptin gene expression in chicken is not exclusively localized in adipose tissue but is also expressed in liver. The expression of leptin in liver may be associated with a key role of this organ in avian species in controlling lipogenesis.

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.

Insulin receptor substrate 1 antisense expression in an hepatoma cell line reduces cell proliferation and induces overexpression of the Src homology 2 domain and collagen protein (SHC)

In mammalian cells, the insulin receptor substrate 1 protein (IRS-1) is a specific substrate for insulin and IGF-1 receptor tyrosine kinases which is involved in mediating metabolic and mitogenic actions of insulin and IGFs. In order to determine if IRS-1 is also essential in a chicken derived hepatoma cell line (LMH cells), IRS-1 gene has been invalidated in these cells. For this, we subcloned chicken IRS-1 gene in an antisense orientation into a mammalian expression vector driven by the cytomegalovirus early promoter. LMH cells were stably transfected with this construct or with the empty vector carrying only the neomycin resistance gene and selected for cIRS-1 expression. One subclone, C2, showed a complete repression of cIRS-1 expression at both protein and mRNA levels. Proliferation of C2 cells was dramatically reduced (54%) compared with Neo(r) cells. Furthermore this reduction was accompanied by a decrease in insulin-dependent [3H]thymidine incorporation, indicating a reduction in DNA synthesis. Insulin-dependent [U-14C]glucose incorporation into cellular lipids was also significantly reduced in C2 cell line suggesting an alteration in lipogenesis. In wild type LMH cells, SHC which is involved in Ras pathway, also served as a substrate for insulin receptor tyrosine kinase. In C2 cells, SHC expression, its association with the insulin receptor and its tyrosine phosphorylation were largely increased. Two forms of the regulatory subunit of PI 3-kinase were present: p85 and p55 forms. Furthermore, C2 cells displayed increased basal phosphatidylinositol (PI) 3'-kinase activity. This report demonstrates a role for cIRS-1 in the metabolic and mitogenic actions of insulin in LMH cells. However, the overexpression of cIRS-1 antisense did not completely abolish cell proliferation. This may be explained by the exacerbation of an alternative pathway that only partly compensate for the knocking out of cIRS-1 gene: the overexpression of SHC.

Cloning of the chicken insulin receptor substrate 1 gene

The action of insulin, IGF-1, and IGF-2 is mediated via two receptor tyrosine kinases, the insulin and IGF-1 receptors. Upon ligand binding, these receptors become active kinases, undergoing autophosphorylation and phosphorylating cellular substrates, including insulin receptor substrate-1 (IRS-1). IRS-1 acts as a docking protein and mediates multiple interactions among other proteins, resulting in transduction of the metabolic and mitogenic signals. The IRS-1 gene has been cloned from four species (human, rat, mouse, and frog). In the present study, the chicken IRS-1 gene was cloned. Chicken, as is true of birds in general, have a higher fasting and fed blood glucose than do mammals. Chicken IRS-1 DNA sequence encodes a 1240 amino acid protein. The most conserved regions were the IRS homology-2 (IH-2), the pleckstrin homology, and the shc and IRS-1 NPXY-binding (SAIN) domains. Twelve of the cIRS-1 tyrosine residues are in sequence motifs that, when phosphorylated, could interact with proteins containing SH2 domains. All twelve of these motifs were conserved. IRS-1 mRNA is expressed during embryogenesis in chicken and persists after hatching. In LMH cells, derived from a chicken hepatoma, two bands were tyrosine phosphorylated in an insulin-dependent manner: IRS-1 (approximately 180 kDa) and the insulin receptor beta subunit (approximately 95 kDa). Chicken IRS-1 is structurally and functionally similar to its human homolog, despite the difference in blood glucose levels and the evolutionary distance between birds and mammals.

Biological activity of immunoreactive insulin-like activity extracted from rat submandibular gland

Earlier studies indicate the presence of an insulin-like immunoreactivity (ILI) in rat submandibular salivary glands (SSG). Previous observations also showed that streptozotocin (STZ)-induced diabetes was accompanied by an increase in SSG ILI concentrations. In the present work we studied the effect of SSG ILI from normal and STZ diabetic rats (ILI-N and ILI-D, respectively) on insulin receptor binding and function in LMH cell line. ILI-N and ILI-D inhibited 125I-insulin binding to intact cells and wheat germ agglutinin (WGA)-purified insulin receptors with a high affinity. Furthermore, ILI-N and ILI-D activated, although weakly, the beta-subunit autophosphorylation of solubilized and WGA-purified insulin receptors. An ATP hydrolytic activity was present in ILI-N and, to a greater extent, in ILI-D extracts, which can at least in part explain their low potency for activating autophosphorylation and kinase activity of insulin receptors in vitro. However, after ILI treatment of intact cells and immunoprecipitation of insulin receptors, ILI induced a dose-dependent tyrosine phosphorylation of the insulin receptor beta-subunit. Finally, ILI-N and ILI-D stimulated amino acid uptake and lipogenesis in LMH cells. These findings suggest that SSG ILI is biologically active and can participate in metabolic regulations.

Insulin-induced activation of phosphatidyl inositol 3-kinase. Demonstration that the p85 subunit binds directly to the COOH terminus of the insulin receptor in intact cells

Insulin activates the insulin receptor tyrosine kinase to phosphorylate signaling molecules such as insulin receptor substrate-1 (IRS-1). Phosphorylated IRS-1 binds to SH2 domains in the p85 regulatory subunit of phosphatidyl inositol (PI) 3-kinase, thereby stimulating the catalytic activity of PI 3-kinase. For most growth factor receptor tyrosine kinases (including receptors for epidermal growth factor and platelet-derived growth factor), the p85 regulatory subunit of PI 3-kinase binds directly to phosphorylated YXXM motifs contained in the cytoplasmic domain of the receptor itself. Previous studies in cell-free systems have shown that the phosphorylated YHTM sequence (amino acid residues 1322-1325) in the COOH terminus of the insulin receptor has the ability to bind to the p85 subunit of PI 3-kinase, thereby activating the enzyme. In this investigation, we demonstrate the occurrence of the same direct binding interaction in intact cells. Subsequent to insulin-stimulated phosphorylation of the insulin receptor, a complex is formed that contains the insulin receptor and PI 3-kinase. This complex can be immunoprecipitated by antibodies directed against either the insulin receptor or the p85 subunit of PI 3-kinase. The delta 43 mutant insulin receptor that lacks 43 amino acids at the COOH terminus does not bind p85. In addition, the delta 43 truncation impairs the ability of the receptor to mediate the activation of PI 3-kinase. Thus, by binding directly to p85, the phosphorylated YHTM motif in the COOH terminus of the insulin receptor contributes partially to mediating the effect of insulin to activate PI 3-kinase.

Corticosterone-induced insulin resistance is not associated with alterations of insulin receptor number and kinase activity in chicken kidney

Chicken renal insulin receptors have been recently characterized; their number and kinase activities vary in response to altered nutritional status. In the present study, the effect of chronic corticosterone treatment was examined in 5-week-old chickens. The development of an insulin resistance following corticosterone was suggested after 1 and 2 weeks of treatment by a significant increases in plasma insulin levels (1.63 +/- 0.13 vs 0.56 +/- 0.14 ng insulin/ml in controls) and in renal cytosolic phosphoenolpyruvate carboxykinase activity (17.2 +/- 0.8 vs 13.7 +/- 0.7 nm/mn/mg tissue in controls). No significant changes were present at the level of insulin receptor number and kinase activity. Therefore, in kidney and, as previously observed, in muscles, corticosterone can induce insulin resistance at postreceptor steps in the cascade of events leading to insulin action.

Rescue and activation of a binding-deficient insulin receptor. Evidence for intermolecular transphosphorylation

Binding of insulin to the alpha subunit of the insulin receptor (IR) leads to autophosphorylation of the beta subunit. The reaction proceeds as intramolecular transphosphorylation between alpha beta half-receptors of the heterotetrameric receptor dimer (alpha 2 beta 2). Since IRs are mobile in the plane of the plasma membrane, it is also possible that transphosphorylation may occur between adjacent holoreceptors (alpha 2 beta 2) by an intermolecular reaction. To address this question, we cotransfected NIH-3T3 cells with two IR cDNA constructs: a truncated but functionally normal IR lacking the C-terminal 43 amino acids (delta 43) and a full-length Leu323 mutant receptor that is expressed on the cell surface but that does not bind insulin. A clonal cell line was selected from cells cotransfected with a 1/5 ratio of delta 43 cDNA/Leu323 cDNA. The two homodimers (Leu323 and delta 43) were expressed without detectable formation of hybrid receptors. By using specific antibodies, we demonstrate that in cells coexpressing both homodimers, the Leu323 mutant receptor was phosphorylated in vivo by the delta 43 IR in an insulin-dependent manner. However, when the Leu323 mutant receptor was expressed alone, no phosphorylation was detected. In addition, we demonstrate the association of the phosphorylated Leu323 mutant receptor with insulin receptor substrate-1 and with phosphatidylinositol 3-kinase. These findings indicate that insulin binding is not required for phosphorylation of the Leu323 mutant receptor, that the phosphorylation of the Leu323 mutant receptor occurs by an intermolecular transphosphorylation mechanism, and, finally, that the Leu323 mutant receptor, once phosphorylated, can associate with insulin receptor substrate-1 and phosphatidylinositol 3-kinase.

Structural basis by which a recessive mutation in the alpha-subunit of the insulin receptor affects insulin binding

Recently, a mutation substituting Leu for Ser323 in the alpha-subunit of the human insulin receptor has been identified in an insulin-resistant patient. The Leu323 mutation leads to a severe impairment in insulin binding without significantly altering the processing or cell surface expression of the receptor. In order to study how alpha beta half-receptors interact to form the insulin-binding site, we cotransfected NIH-3T3 cells with two insulin receptor cDNA constructs: a truncated insulin receptor lacking the C-terminal 43 amino acids (delta 43) and the full-length Leu323 mutant receptor. A clonal cell line from cotransfected cells expresses a hybrid receptor consisting of a Leu323 half-receptor and a delta 43 half-receptor. We demonstrate that the Leu323-delta 43 hybrid receptor binds insulin with high affinity. Furthermore, by cross-linking 125I-insulin to immobilized hybrid receptors, we show that only the alpha beta delta half of the hybrid receptor binds insulin. Since the isolated half-insulin receptor has low affinity for insulin, this suggests that the addition of even a non-binding alpha-subunit can result in high affinity binding to the holoreceptor (alpha alpha mut beta delta beta). Both beta and beta delta-subunits of the Leu323-delta 43 hybrid receptor are phosphorylated in vivo and in vitro in an insulin-dependent manner, suggesting an intramolecular transphosphorylation mechanism and that the presence of the Leu323 mutant receptor that lacks an intrinsic high affinity binding site does not prevent the associated beta-subunit from functioning either as a tyrosine kinase or as a phosphate acceptor in the hybrid insulin receptor molecule (alpha alpha mut beta delta beta). Furthermore, we show that the hybrid receptor can phosphorylate insulin receptor substrate-1 (IRS-1) in response to insulin and can be coimmunoprecipitated together with IRS-1 by anti-IRS-1 antibody.

Increased insulin receptor number and insulin responsiveness in a chicken hepatoma cell line

Insulin receptor number and insulin responsiveness were compared in a chicken hepatoma cell line (LMH) and in normal chicken hepatocyte (cHep) cells cultured in the same conditions. LMH cells expressed two- to threefold more insulin receptors than cHep cells, without significant changes in affinity. The tyrosine kinase activity of solubilized and lectin (lentil+wheat germ agglutinin; WGA)-purified LMH receptors was higher than that of cHep receptors. The ATP hydrolytic activity previously observed in WGA-purified receptors from chicken liver membranes was also present in WGA-purified receptors from cultured cHep cells. This unidentified membrane-associated ATPase was absent from LMH membrane-solubilized material and therefore from WGA-purified LMH insulin receptors. Finally, LMH cells incorporated at least tenfold more amino isobutyric acid than cHep cells in the absence of insulin and were more responsive to insulin. The enhanced basal amino acid transport of LMH cells was most probably the consequence of their proliferative activity. The enhanced insulin responsiveness of LMH cells can be accounted for, at least in part, by one or several of the modifications presently demonstrated in LMH cells when compared with normal cultured hepatocytes: increased insulin receptor number and tyrosine kinase activity and possibly the loss of the membrane-associated ATPase.

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.

Corticosterone effect on insulin receptor number and kinase activity in chicken muscle and liver

The effects of chronic corticosterone treatment (6 mg/kg/day) on insulin sensitivity and on liver and muscle insulin receptors were examined in 5-week-old chickens. The hypoglycemic effect of exogenous insulin was completely abolished within 2 weeks of treatment, suggesting a corticosterone-induced insulin resistance. Hepatic insulin receptor numbers were slightly reduced (P < 0.001) after 2 weeks of treatment. After 1 or 2 weeks, corticosterone treatment significantly reduced liver insulin receptor kinase activity toward the artificial substrate poly(Glu4,Tyr1). Muscle insulin receptor kinase activity was also significantly decreased after 1 week of treatment but this effect was accounted for by a decrease in basal activity. Therefore the corticosterone-induced insulin resistance is accounted for, at least in part, by altered hepatic receptor numbers and kinase activity. The impairment of muscle development involves postreceptor defects.

Transcainide: biochemical evidence for state-dependent interaction with the class I antiarrhythmic drug receptor

The mechanism of action of the lidocaine derivative transcainide was examined using [3H]batrachotoxinin 20 alpha-benzoate, which binds specifically to and stabilizes activated states of the sodium channel. Transcainide (IC50 0.3 microM) inhibited equilibrium [3H]batrachotoxinin binding to sodium channels present on freshly isolated rat cardiac myocytes. Scatchard analysis of [3H]batrachotoxinin binding showed that transcainide both reduced maximal binding and altered the KD for [3H]batrachotoxinin binding, indicating noncompetitive, allosteric inhibition. Inhibition by transcainide of [3H]batrachotoxinin binding was reversible within 60 min. We used state-dependent [3H]batrachotoxinin binding assays to examine whether transcainide preferentially binds to activated or nonactivated sodium channels. Transcainide had little effect on the k-1 of [3H]batrachotoxinin even at concentrations 1000-fold greater than its IC50, indicating low affinity of transcainide for activated channels. However, transcainide decreased the k + 1 of [3H]batrachotoxinin at a concentration very close to its IC50 concentration for inhibiting equilibrium [3H]batrachotoxinin binding. The results are discussed in terms of a model in which transcainide inhibits [3H]batrachotoxinin binding by binding specifically to and stabilizing a nonactivated state of the cardiac sodium channel.

Upregulation of the rat cardiac sodium channel by in vivo treatment with a class I antiarrhythmic drug

Class I antiarrhythmic drugs inhibit the sodium channel by binding to a drug receptor associated with the channel. In this report we show that in vivo administration of the class I antiarrhythmic drug mexiletine to rats induces sodium channel upregulation in isolated cardiac myocytes. The number of sodium channels was assessed with a radioligand assay using the sodium channel-specific toxin [3H]batrachotoxinin benzoate ([3H]BTXB). The administration of mexiletine to rats induced a dose-dependent increase in [3H]BTXB total specific binding (Bmax) on isolated cardiac myocytes. Sodium channel numbers were 15 +/- 5, 29 +/- 9, and 54 +/- 4 fmol/10(5) cells after 3 d treatment with 0, 50 mg/kg per d, and 150 mg/kg per d mexiletine (P less than 0.001, analysis of variance). Sodium channel number increased monoexponentially to a steady-state value within 3 d with a half-time of increase of 1.0 d. After cessation of treatment with mexiletine the number of sodium channels returned to normal within 12 d. Finally, treatment with mexiletine altered only sodium channel number; the Kd for [3H]BTXB and the IC50 for mexiletine were not different for myocytes prepared from control and mexiletine-treated rats.

Cyclic AMP-dependent regulation of the number of [3H]batrachotoxinin benzoate binding sites on rat cardiac myocytes

We sought to assess the effect of an increase in cAMP on sodium channels on adult rat cardiac ventricular myocytes. Sodium channels were studied with the use of the radiolabeled sodium channel-specific toxin [3H] batrachotoxinin benzoate ([3H]BTXB). Forskolin, isoproterenol, prostaglandin E1, cholera toxin, and pertussis toxin each increased cAMP levels and decreased the number of [3H]BTXB binding sites without changing the affinity of [3H]BTXB for the sodium channel. The cAMP analog 8-bromo-cyclic AMP (8-Br-cAMP) reduced the number of [3H]BTXB binding sites from 19 fmol/10(5) cells to 11 fmol/10(5) cells. [3H]BTXB binding site down-regulation was reversible, cAMP dose-dependent, and time-dependent. To test the hypothesis that the cAMP effect was mediated by cAMP-dependent phosphorylation, we determined the effect of 8-Br-cAMP on [3H]BTXB binding after preincubation of myocytes with N-(2-(methylamino)ethyl)-5-isoquinolinesulfonamide dihydrochloride (H8), a protein kinase A inhibitor. H8 inhibited 70% of the decrease in the number of [3H]BTXB binding sites induced by 8-Br-cAMP. Thus increases in intracellular cAMP in cardiac myocytes reversibly induced a decrease in the number of [3H]BTXB binding sites via cAMP-dependent protein phosphorylation, possibly of the sodium channel.

Aminoalkyl structural requirements for interaction of lidocaine with the class I antiarrhythmic drug receptor on rat cardiac myocytes

The structural and physicochemical determinants of binding of lidocaine and several of its aminoalkyl homologs to specific sites associated with the sodium channel were assessed using a radioligand assay and freshly isolated rat cardiac myocytes. The two series of closely related lidocaine homologs that were studied were composed, first, of homologs differing in the length of the link between the arylamide and amine domains of the molecule and, second, of homologs differing in the number of carbons attached to the terminal amine. Drug affinity was measured with a radioligand binding assay, using [3H]batrachotoxinin A 20 alpha-benzoate and freshly isolated cardiac myocytes. The affinities of the homologs were then compared with the pKa values, partition coefficients, distribution coefficients, and molecular structure of the homologs, to determine the relationship between the affinity for the receptor and the physicochemical and structural properties of the drug. Optimal binding was obtained with a link between the arylamide and amine domains that was two carbons in length. The affinity of the drug for the receptor was optimal with four or more amino-terminal carbons, and the precise arrangement of the carbons was not important. Each of the amino-terminal carbons independently contributed 0.3 kcal of free energy of binding, suggesting that the carbons dissolve in a hydrophobic pocket. The evolving picture of a drug structure that is optimal for receptor binding is one of a compound with a two-carbon arylamide-amine link and four or more amino-terminal carbons.

Lipolytic action of a new alpha-2 adrenergic antagonist of the piperazinopyrimidine family: RP 55462

Recent investigations have demonstrated the in vitro lipid mobilizing effects induced by alpha-2 adrenergic antagonist administration and have focused attention on the putative therapeutic interest of such compounds in the treatment of obesity as adjuvants in caloric restriction programs. We studied the impact of RP 55462 [6-chloro-4-(isopropylamino)-5-(methyl)-2, piperazinopyrimidine], a new alpha-2 adrenergic antagonist compound of the piperazinopyrimidine family, on fat cell function. The alpha-2-blocking properties of this agent, which had been defined initially on the brain were confirmed on adipocytes. RP 55462 competed with [3H]yohimbine binding sites on human fat cell membranes and inhibited the antilipolytic action of alpha-2-agonist compounds (UK 14304, clonidine and epinephrine) in human and hamster fat cells. It was also noticed that RP 55462 alone was able to activate lipolysis in isolated fat cells from various species (man, rat, hamster and dog). Moreover, the lipolytic response induced by isoproterenol or synacthene was largely amplified in the presence of RP 55462 in rat fat cells which are the least alpha-2 adrenergic responsive tested. RP-55462-dependent stimulation of lipolysis was not affected by the presence of other alpha-2 adrenergic antagonists (idazoxan, yohimbine or phentolamine). Intravenous administration of RP 55462 in alert dogs promoted an increment in plasma nonesterified acid concentrations reflecting its lipid mobilizing action. In summary this study focuses attention on a new alpha-2-antagonist compound which exhibits an in vivo lipid mobilizing action which could be attributable to its alpha-2 adrenergic antagonist properties. Inasmuch as the lipolytic activity of RP 55462, revealed in in vitro studies, seems to be independent from its alpha-2 adrenolytic properties; further studies are required to define the mechanism of such a lipolytic effect as well as its possible involvement in in vivo conditions.

Mechanism of lipolytic action of a new alpha-2 adrenergic antagonist of the piperazinopyrimidine family: RP 55462

The in vivo lipid mobilizing effect of alpha-2 adrenergic antagonist has been demonstrated previously. This has attracted attention to the putative interest of such compounds in a lipid-mobilizing strategy. RP 55462 [6-chloro-4-(isopropylamino)-5-(methyl)-2 piperazinopyrimidine], a piperazinopyrimidine derivative, has already been shown to exert alpha-2 adrenergic antagonist actions on fat cell function in vitro. Moreover, RP 55462 exhibits a direct in vitro lipolytic action which is independent of its alpha-2-blocking potency. When administered i.v. RP 55462 is also able to induce an increment in plasma nonesterified levels in dogs. The mechanism of action of RP 55462 was studied and the nature of its lipomobilizing effect was explored. RP 55642-dependent lipolysis was not affected by beta adrenergic blockers on rat fat cells and RP 55462 had no direct effect on adenylylcyclase activity on fat cell membranes. Moreover, RP 55462 did not compete with [3H]phenyl isopropyl adenosine binding (A1-adenosine receptor agonist) on fat cell membranes. In fact, RP 55462 inhibited, in a dose-dependent manner, the cyclic AMP (cAMP)-dependent phosphodiesterase (PDE) activity in rat adipose tissue. Several derivatives with the piperazinopyrimidine structure also inhibited cAMP-dependent PDE activity and exerted lipolytic effects. A short structure-activity study was performed with various derivatives. In dogs, by contrast with yohimbine, the in vivo lipid mobilizing effect of RP 55462 was not abolished by pretreatment with propranolol, and lasted longer. It is concluded that the in vivo lipolytic activity of RP 55462 is connected with its ability to inhibit cAMP-dependent PDE activity; a property of several piperazinopyrimidine derivatives. The lipid mobilizing effect induced in vivo by RP 55462 results from a combination of its alpha-2 adrenergic antagonist properties and its direct lipolytic action mediated by cAMP-dependent PDE inhibiting effects.

Lipomobilizing effects of procaterol and yohimbine in the conscious dog: comparison of endocrinological, metabolic and cardiovascular effects

1. Lipid mobilization during a hypocaloric diet may be enhanced by a pharmacological approach using beta 2-adrenoceptor agonists or alpha 2-adrenoceptor antagonists. Studies were undertaken in the dog, an animal model presenting fat cell antilipolytic alpha 2- and lipolytic beta-adrenoceptors, in order, firstly, to demonstrate the presence of beta 2 subtype adrenoceptors on adipocytes and, secondly, to compare the effects of procaterol (beta 2-adrenoceptor agonist) and of yohimbine (alpha 2-adrenoceptor antagonist) on metabolic, endocrinological and cardiovascular parameters. 2. Procaterol strongly stimulates lipolysis in dog adipocytes in vitro. The utilisation of selective beta 1- and beta 2-adrenoceptor antagonists (bisoprolol and ICI 118,551) in both lipolysis and binding studies (displacement of [3H]-dihydroalprenolol binding) demonstrated the presence of the two beta-adrenoceptor subtypes in dog fat cells. 3. Infusion of either yohimbine or procaterol (10 and 0.4 nmol min-1 kg-1, respectively for 30 min), provoked an equivalent increase in plasma non-esterified fatty acids (+100%). Procaterol, but not yohimbine, induced hyperglycaemia (+120%). Plasma insulin was weakly enhanced by yohimbine (+120%) as compared to the increase given by procaterol (+500%). 4. Both drugs stimulated sympathetic nervous system activity, as indicated by the increased plasma noradrenaline concentration, but only yohimbine increased the plasma adrenaline level. 5. Cardiovascular measurements indicated that procaterol strongly enhances heart rate and transiently decreases mean blood pressure. Yohimbine exhibits a weaker effect on heart rate and slightly increases mean blood pressure. 6. The present work clearly indicates that lipid mobilization is enhanced during fasting in the dog by selective beta 2-adrenoceptor stimulation or by alpha 2-adrenoceptor blockade. This enhanced lipolytic effect may result either from a direct action of the drugs on the adrenoceptors of fat cells or from an activation of the sympathetic nervous system. Procaterol suffers major limitations since it strongly increases heart rate, immunoreactive insulin and glycaemia. On the other hand, yohimbine induces only minor modifications both in cardiovascular and endocrinological parameters.

Mechanism of the lipid-mobilizing effect of alpha-2 adrenergic antagonists in the dog

Alpha-2 adrenergic antagonists may constitute putative antiobesity agents. These drugs may act: 1) by blockade of the antilipolytic alpha-2 adrenoceptor on fat cell membranes and 2) by activation of the sympathetic adrenergic system (blockade of central and presynaptic alpha-2 adrenoceptors). Studies were undertaken in the dog, a species possessing fat cell alpha-2 and beta-adrenoceptors in order: 1) to define and compare the metabolic and endocrinological impacts of recently discovered alpha-2 antagonists [idazoxan and SK&F 86,466 (6-chloro-N-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine)] and of an older one (yohimbine) and 2) to dissociate the contribution of the noradrenergic activation from a postsynaptic action on fat cell and endocrine pancreas alpha-2 adrenoceptors. Binding studies showed that the three compounds have similar affinities for the alpha-2 adrenoceptor on both cerebrocortical and fat cell membranes. Their ability to suppress the antilipolytic effect of an alpha-2 agonist [UK-14,304 (5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline)] on isolated fat cells was equivalent. Yohimbine perfusion induced a dose-dependent increase in plasma nonesterified fatty acids (NEFA) and immunoreactive insulin concentrations. At equivalent doses, the three alpha-2 adrenergic antagonists induced NEFA mobilization, increase in immunoreactive insulin and norepinephrine plasma levels with an order of potency: yohimbine much greater than idazoxan greater than or equal to SK&F 86,466. The weakest effect on sympathetic activation was with SK&F 86,466. The effects of yohimbine were suppressed completely by pretreatment with propranolol. Clonidine infusion reduced NEFA levels in dogs. Yohimbine but not SK&F 86,466 abolished completely this lowering effect of clonidine on NEFA and glycerol levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2-antagonist compounds and lipid mobilization: evidence for a lipid mobilizing effect of oral yohimbine in healthy male volunteers

Investigations were carried out to analyse the interactions of alpha 2-antagonists (yohimbine, idazoxan, SK & F-86,466) with human fat cell alpha 2-adrenoceptors. All the alpha 2-antagonists enhanced the lipolytic potencies of epinephrine with an order of potency: yohimbine greater than idazoxan greater than SK & F-86,466; the same order was also found in 3H-yohimbine competition studies on human fat cell membranes. The most potent agent, yohimbine, was administered orally in humans to define the conditions of appearance and the time-course of a putative lipid-mobilizing action. Oral yohimbine administration (0.2 mg kg-1) elevated plasma glycerol and non-esterified fatty acids in fasting healthy subjects without significant action on heart rate or blood pressure during the time-course of the experiment. The lipid-mobilizing action of yohimbine was reinforced during physical exercise, completely suppressed after a meal and partially blocked by administration of propranolol (0.5 mg kg-1; 60 min before yohimbine). Plasma norepinephrine concentrations were increased (40-50%) after oral yohimbine administration. The rise in plasma catecholamine concentration elicited by yohimbine was not modified by propranolol treatment. The lipid-mobilizing effect of yohimbine could be attributable to: (i) the increase in synaptic norepinephrine with a resultant increment in lipolysis by beta-adrenergic agonism; (ii) a decrease in alpha 2-adrenoceptor stimulation of human fat cell alpha 2-adrenoceptors; (iii) a blockade of presynaptic alpha 2-adrenoceptors. The use of highly selective alpha 2-antagonists will allow investigations into alpha 2-adrenoceptors, which may represent a novel locus for pharmacological intervention in lipid-mobilization strategies.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute release of catecholamines on circulating blood cell adrenoceptors and metabolic indices in dog

The effects of acute release of endogenous catecholamines on both platelet alpha 2 and leukocyte beta adrenoreceptors and metabolic indices (glucose and free fatty acids) were investigated in dogs by means of a model of neurogenic hypertension following acute sinoaortic denervation (ASAD). Despite the marked increase in catecholamine levels (+4.2-fold for noradrenaline and 16.7-fold for adrenaline, for example, at minute 45 following ASAD) and in glucose plasma levels, and the significant decrease in free fatty acid plasma levels, no change in platelet alpha 2 or leukocyte beta adrenoreceptor binding sites (number as well as affinity) was observed during the whole experiment. It is suggested that the number of platelet alpha 2- and leukocyte beta-adrenoreceptors is not submitted to short-term regulation, at least by endogenous catecholamines in dogs.

In vitro study of alpha 2-adrenoceptor turnover and metabolism using the adenocarcinoma cell line HT29

We have utilized the adenocarcinoma cell line HT29 as an in vitro model to investigate the turnover and the metabolism of the alpha 2-adrenoceptor. The biosynthesis rate of the receptor was studied in postconfluent HT29 cells, when its density expressed as fmol/mg of cell membrane protein is constant, by following the recovery of the receptor binding capacity after blockade with the non-reversible alpha-adrenergic antagonist benextramine. Study of the inhibition of [3H]yohimbine and [3H]UK-14,304 binding showed that benextramine was a more potent antagonist at alpha 2-adrenoceptor than phenoxybenzamine. The incubation of intact HT29 cells for 30 min in the presence of 10(-5) M benextramine irreversibly blocked more than 95% of the alpha 2-adrenoceptors and totally suppressed the inhibitory effect of UK-14,304 on cyclic AMP production. The blockade appeared specific, since benextramine effects were prevented by alpha 2-adrenergic agents. Moreover, neither vasoactive intestinal polypeptide responsiveness nor other tested aspects of the regulation of the adenylate cyclase was altered by the treatment. Study of the time course of receptor recovery after irreversible blockade indicated that alpha 2-adrenoceptors reappeared in the cells with a monoexponential kinetic. The linearization of the repopulation curve obtained with the labeled antagonist [3H]yohimbine allowed the determination of the rate constant for receptor degradation (k = 0.0268 +/- 0.0025 hr-1) and the rate of receptor synthesis (6.91 +/- 0.64 fmol/mg of cell membrane protein/hr) corresponding to the synthesis of about 500 receptors/cell/hr. The alpha 2-adrenoceptor half-life was 26 +/- 3 hr. Measurement of the biological effects associated to the alpha-adrenoceptor stimulation during the course of receptor recovery indicated a relationship between the number of cell receptors and the percentage of inhibition of the cyclic AMP accumulation induced by forskolin. The receptor reappearance was totally inhibited by either actinomycin or cycloheximide or tunicamycin, showing that the recovery corresponded to de novo synthesized receptor and giving indirect evidence for the glycoproteic nature of the alpha 2-adrenoceptor. Deprivation for glucose or glutamine also impeded the recovery process; by contrast, addition of UK-14,304 or clonidine did not interfere, indicating that the expression of the alpha 2-adrenoceptor is not subject to homologous regulation in the HT29 cell.

Characterization of dog fat cell adrenoceptors: variations in alpha-2 and beta adrenergic receptors distribution according to the extent of the fat deposits and the anatomical location

In the search of the most accurate animal model for studies on fat cell adrenoceptor regulation, the characterization of dog fat cell beta and alpha-2 adrenoceptor properties was carried out using selected radioligands and biological measurements of fat cell function. Moreover, the distribution of dog fat cell adrenoceptors and the functional consequences were explored in fat deposits from different anatomical locations (perirenal, omental and s.c. adipose tissue). Beta and alpha-2 adrenoceptors involved in the control of the lipolytic activity of dog fat cells exhibit pharmacological characteristics which are very similar to those defined in human adipocytes. However, in contrast with the results obtained in human adipocytes, there is a preponderance of beta over alpha-2 adrenergic binding sites, whatever the location of the fat deposits, in the dog. The higher values of the ratio beta-sites over alpha-2-sites were found in perirenal adipocytes whereas the lowest values of this ratio were found in s.c. deposits. A close positive relationship exist between alpha-2 adrenoceptor number, alpha-2 adrenergic responsiveness and fat cell volume. To conclude, beta and alpha-2 adrenoceptors exhibit similar pharmacological properties in dog and human fat cells. Variations according to the anatomical location of the fat deposits exist in both species. However, as opposed to human fat cells, it is evident that the balance between beta and alpha-2 adrenergic responsiveness is predominantly under the control of beta adrenoceptors in dog fat cells.

Alpha 2-adrenergic receptor turnover in adipose tissue and kidney: irreversible blockade of alpha 2-adrenergic receptors by benextramine

The recovery of post- and extrasynaptic alpha 2-adrenergic receptor-binding sites was studied in vivo in male golden hamsters after treatment with an irreversible alpha-adrenoceptor antagonist benextramine, a tetramine disulfide that possesses a high affinity for alpha 2-binding sites. The kidney alpha 2-adrenergic receptor number was measured with [3H]yohimbine, whereas [3H]clonidine was used for fat cell and brain membrane alpha 2-binding site identification. Benextramine treatment of fat cell, kidney, and brain membranes reduced or completely suppressed, in an irreversible manner, [3H] clonidine and [3H]yohimbine binding without modifying adenosine (A1-receptor) and beta-adrenergic receptor sites. This irreversible binding was also found 1 and 2 hr after intraperitoneal administration of benextramine to the hamsters. Although it bound irreversibly to peripheral and central alpha 2-adrenergic receptors on isolated membranes, benextramine was unable to cross the blood-brain barrier of the hamster at the concentrations used (10-20 mg/kg). After the irreversible blockade, alpha 2-binding sites reappeared in kidney and adipose tissue following a monoexponential time course. Recovery of binding sites was more rapid in kidney than in adipose tissue; the half-lives of the receptor were 31 and 46 hr, respectively in the tissues. The rates of receptor production were 1.5 and 1.8 fmol/mg of protein/hr in kidney and adipose tissue. Reappearance of alpha 2-binding sites was associated with a rapid recovery of function (antilipolytic potencies of alpha 2-agonists) in fat cells inasmuch as occupancy of 15% of [3H]clonidine-binding sites was sufficient to promote 40% inhibition of lipolysis. Benextramine is a useful tool to estimate turnover of alpha 2-adrenergic receptors under normal and pathological situations using the approach described in the present paper.