Cyclic AMP does not mediate the action of synthetic hypertrehalosemic peptides from the corpus cardiacum of Periplaneta americana

Insect peptide hormones, an overview of the present literature

A comprehensive overview of the recent state of the art of insect peptide hormones with chemical structures is presented. An increased interest in insect neuropeptides and dynamic development of that research area has been influenced by a rapid improvement of instrumentation necessary for isolation and structural characterization. Several research teams have studied the relationships between biological properties of insect and vertebrate peptide hormones. Thus hormones from the AKH family can be considered glucagon counterparts, whereas the myotropic hormones such as proctolin and Lem-PK (LPK) are a substance P equivalent. Insect melanization hormones Bom-MRCH in their structural characteristics and properties resemble those of mammal MSH, and leucosulfakinins Lem-SK-I and -II show some similarities with gastrin II and cholecystokinin. Bombyxin-II (Bom-PTTH-II) reveals a structural homology with human insulin and similar biological properties to adenocorticotropic mammal hormone. Allatostatin (Dip-JHS-I) may be compared to somatostatin as it can be inferred from the observations that this peptide modulates JH secretion in cockroach, Blattella germanica. Determination of the primary structure of eclosion hormones Mas-EH and Bom-EH-II as well as the amino acid sequence of allatotropin and allatostatin is a significant contribution to the understanding of the molecular mechanisms of metamorphosis and insect development.

Mode of action of neuropeptides from the adipokinetic hormone family

Neuropeptides of the adipokinetic hormone (AKH) family regulate inter alia mobilisation of various substrates from stores in the fat body of insects during episodes of flight. How is this achieved? In insects which exclusively oxidise carbohydrates for flight (cockroaches), or which oxidise carbohydrates in conjunction with lipids (locusts) or proline (a number of beetles), the endogenous AKHs bind to a G(q)-protein-coupled receptor, activate a phospholipase C and the resulting inositol trisphosphate releases Ca(2+) from internal stores. In addition, influx of extracellular Ca(2+) is increased and, via a kinase cascade, glycogen phosphorylase is activated, glucose-1-phosphate produced, and transformed to trehalose, which is released into the haemolymph. In locusts, additionally, adenylate cyclase is activated and cyclic AMP is synthesised. In insects which use lipids for sustained flight (locust, tobacco hornworm moth) or proline for flight (certain beetles), adenylate cyclase is activated after the AKHs bind to their respective G(s)-protein-coupled receptor. The resulting cyclic AMP, together with the messengers intra- and extracellular Ca(2+), activate a triacylglycerol lipase, which results in the production of 1,2 diacylglycerols (in locusts, moths) or (hypothetically) free fatty acids (fruit beetle).

The role of Ins(1,4,5)P(3) in signal transduction of the metabolic neuropeptide Mem-CC in the cetoniid beetle, Pachnoda sinuata

We have investigated the role of inositol triphosphate, Ins(1,4,5)P(3), in the transduction of the hypertrehalosaemic and hyperprolinaemic signal of the endogenous neuropeptide Mem-CC in the cetoniid beetle Pachnoda sinuata. Flight and injection of Mem-CC into the haemocoel of the beetle induce an increase of Ins(1,4,5)P(3) levels in the fat body of the beetle. When Mem-CC is co-injected with U 73122, which is an inhibitor of phospholipase C, this effect is abolished. Mem-CC also elevates Ins(1,4,5)P(3) concentration in fat body pieces in vitro. The increase in Ins(1,4,5)P(3) levels is tissue-specific and does not occur in brain and flight muscles. Elevation of the Ins(1,4,5)P(3) levels upon injection of Mem-CC is time- and dose-dependent: the maximum response is reached after 3 min and a dose of 10 pmol is needed. Compounds that mimic the action of cAMP (cpt-cAMP, forskolin) do not influence the concentration of Ins(1,4,5)P(3), while those that stimulate G-proteins (aluminium fluoride and cholera toxin) cause an increase of Ins(1,4,5)P(3) levels. The application (in vivo and in vitro) of F-Ins(1,4,5)P(3), an Ins(1,4,5)P(3) analogue that penetrates the cell membrane, causes a mobilisation of carbohydrate reserves via the activation of glycogen phosphorylase but does not stimulate proline synthesis. In addition, U 73122 abolishes the hypertrehalosaemic but not the hyperprolinaemic effect of Mem-CC. The results suggest that the hypertrehalosaemic signal of Mem-CC is mediated via an increase of Ins(1,4,5)P(3) levels in the fat body of P. sinuata.

Adipokinetic hormones of insect: release, signal transduction, and responses

Flight activity of insects provides an attractive yet relatively simple model system for regulation of processes involved in energy metabolism. This is particularly highlighted during long-distance flight, for which the locust constitutes a well-accepted model insect. Peptide adipokinetic hormones (AKHs) are synthesized and stored by neurosecretory cells of the corpus cardiacum, a neuroendocrine gland connected with the insect brain. The actions of these hormones on their fat body target cells trigger a number of coordinated signal transduction processes which culminate in the mobilization of both carbohydrate (trehalose) and lipid (diacylglycerol). These substrates fulfill differential roles in energy metabolism of the contracting flight muscles. The molecular mechanism of diacylglycerol transport in insect blood involving a reversible conversion of lipoproteins (lipophorins) has revealed a novel concept for lipid transport in the circulatory system. In an integrative approach, recent advances are reviewed on the consecutive topics of biosynthesis, storage, and release of insect AKHs, AKH signal transduction mechanisms and metabolic responses in fat body cells, and the dynamics of reversible lipophorin conversions in the insect blood.

Inositol trisphosphate mediates the action of hypertrehalosemic hormone on fat body of the American cockroach, Periplaneta americana

The rate of synthesis of inositol trisphosphate (InsP(3)) in trophocytes derived from disaggregated cockroach (Periplaneta americana) fat body increases following treatment of the cells with hypertrehalosemic hormone I or II (HTH-I, -II) in vitro. Trophocytes preloaded with [3H]inositol display a significant increase in InsP(3) synthesis as early as 15 s after addition of the hormone. When the trophocytes are pre-incubated with LiCl and subsequently incubated with HTH the [3H] content of the InsP(3) fraction is greater than that found with HTH alone. This is taken as evidence that inositol monophosphate phosphatase is part of the mechanism for clearing InsP(3) from the cytosol. In contrast to HTH, octopamine, which is also capable of exerting a hypertrehalosemic effect in the cockroach, does not increase the synthesis of InsP(3). 1-Octadecyl-2-methyl-rac-glycero-3-phosphocholine (ET-18-OCH(3)), a potent and selective inhibitor of phosphatidylinositol phospholipase C, blocks the activation of phosphorylase by HTH-I as well as the hypertrehalosemic effect induced by the hormone.

Flight substrates and their regulation by a member of the AKH/RPCH family of neuropeptides in Cerambycidae

The pattern of metabolic changes during tethered flight with lift-generation was investigated in two South African species of long-horned beetles (family: Cerambycidae), namely Phryneta spinator and Ceroplesis thunbergi. Energy substrates were measured in haemolymph and flight muscles at rest, after a flight period of 1 min at an ambient temperature of 25-29 degrees C, and 1 h thereafter. Flight diminished the levels of proline and carbohydrates in the haemolymph and proline and glycogen in the flight muscles of both species, and caused an increase in the levels of alanine in both compartments. The concentration of lipids in the haemolymph, however, was not changed upon flight in either species. The resting period of 1 h following a 1 min flight episode, was sufficient to reverse the metabolic situation in haemolymph and flight muscles to pre-flight levels in both species. Purification of an extract of the corpora cardiaca from the two beetle species on RP-HPLC, resulted in the isolation and subsequently in the identification (by mass spectrometry, Edman degradation and RP-HPLC) of an octapeptide of the AKH/RPCH family, denoted Pea-CAH-I (pGlu-Val-Asn-Phe-Ser-Pro-Asn-Trpamide), present in each species. It was demonstrated that low doses of Pea-CAH-I elicited increases in the concentration of proline, as well as carbohydrates, in the haemolymph of both species. The levels of lipids, however, remained unchanged upon injection of this peptide. It is concluded that, upon stimulation by flight, the peptide Pea-CAH-I is released from the corpus cardiacum of a cerambycid beetle and is responsible for the regulation of the major flight substrates, proline and carbohydrates, of these beetles.

Cyclic AMP mediates the elevation of proline by AKH peptides in the cetoniid beetle, Pachnoda sinuata

The role of cyclic nucleotides in the transduction of the hyperprolinaemic and hypertrehalosaemic signal of the endogenous neuropeptide Mem-CC was investigated in the cetoniid beetle Pachnoda sinuata. Flight and injection of Mem-CC into the haemocoel of the beetle induce an increase of cAMP levels in the fat body of the beetle. This increase is tissue-specific and does not occur in brain and flight muscles. An elevation of cAMP levels was also found when in vitro preparations of fat body tissue were subjected to Mem-CC. Elevation of the cAMP concentration after injection of Mem-CC is time- and dose-dependent: the maximum response is measured after 1 min, and a dose of 25 pmol Mem-CC is needed. Injection of cpt-cAMP, a cAMP analogue which penetrates the cell membrane, causes a stimulation of proline synthesis but no mobilisation of carbohydrate reserves. The same is measured when IBMX, an inhibitor of phosphodiesterase, is injected. cGMP seems not to be involved in synthesis of proline nor carbohydrate release, because injection of cpt-cGMP has no influence on the levels of proline, alanine and carbohydrates in the haemolymph. Although glycogen phosphorylase of the fat body is activated by Mem-CC in a time- and dose-dependent manner, it cannot be stimulated by cpt-cAMP. The combined data suggest that cAMP is involved in regulation of proline levels by Mem-CC but not in regulation of carbohydrates. Octopamine has no effect on metabolites in the haemolymph and is not capable of activating glycogen phosphorylase, indicating that it is not involved in the regulation of substrates in this beetle. Furthermore, the requirements of the receptor of Mem-CC are different for eliciting a hypertrehalosaemic and a hyperprolinaemic effect, respectively, suggesting that differentiation in signal transduction begins at the receptor level.

New insights into adipokinetic hormone signaling

Flight activity of insects comprises one of the most intense biochemical processes known in nature, and therefore provides an attractive model system to study the hormonal regulation of metabolism during physical exercise. In long-distance flying insects, such as the migratory locust, both carbohydrate and lipid reserves are utilized as fuels for sustained flight activity. The mobilization of these energy stores in Locusta migratoria is mediated by three structurally related adipokinetic hormones (AKHs), which are all capable of stimulating the release of both carbohydrates and lipids from the fat body. To exert their effects intracellularly, these hormones induce a variety of signal transduction events, involving the activation of AKH receptors, GTP-binding proteins, cyclic AMP, inositol phosphates and Ca2+. In this review, we discuss recent advances in the research into AKH signaling. This not only includes the effects of the three AKHs on each of the signaling molecules, but also crosstalk between signaling cascades and the degradation rates of the hormones in the hemolymph. On the basis of the observed differences between the three AKHs, we have tried to construct a physiological model for their action in locusts, in order to answer a fundamental question in endocrinology: why do several structurally and functionally related peptide hormones co-exist in locusts (and animals in general), when apparently one single hormone would be sufficient to exert the desired effects? We suggest that the success of the migratory locust in performing long-distance flights is in part based on this neuropeptide multiplicity, with AKH-I being the strongest lipid-mobilizing hormone, AKH-II the most powerful carbohydrate mobilizer and AKH-III, a modulatory entity that predominantly serves to provide the animal with energy at rest.

The regulation of trehalose metabolism in insects

Trehalose is a non-reducing disaccharide comprising two glucose molecules. It is present in high concentration as the main haemolymph (blood) sugar in insects. The synthesis of trehalose in the fat body (an organ analogous in function to a combination of liver and adipose tissue in vertebrates) is stimulated by neuropeptides (hypertrehalosaemic hormones), released from the corpora cardiaca, a neurohaemal organ associated with the brain. The peptides cause a decrease in the content of fructose 2,6-biphosphate in fat body cells. Fructose 2,6-biphosphate, acting synergistically with AMP, is a potent activator of the glycolytic enzyme 6-phosphofructokinase-1 and a strong inhibitor of the gluconeogenic enzyme fructose 1,6-biphosphatase. This indicates that fructose 2,6-biphosphate is a key metabolic signal in the regulation of trehalose synthesis in insects. Trehalose is hydrolysed by trehalase (E.C. 3.2.1.28). The activity of this enzyme is regulated in flight muscle, but the mechanism by which this is achieved is unknown. Trehalase from locust flight muscle is a glycoprotein bound to membranes of the microsomal fraction. The enzyme can be activated by detergents in vitro and by short flight intervals in vivo, which indicates that changes in the membrane environment modulate trehalase activity under physiological conditions.

Purification and properties of glycogen phosphorylase from the fat body of larval Manduca sexta

Glycogen phosphorylase b has been purified to homogeneity from the fat body of larval Manduca sexta. The purification procedure involved ammonium sulfate precipitation, and chromatography of DEAE-cellulose, 5'-AMP-Sepharose and Q-Sepharose. The final product, which showed a single band on SDS-PAGE with a M(r) = 92,500, was purified 50-fold from the original homogenate in a yield of about 3%. The molecular mass of the native purified phosphorylase b was estimated to be 186,000 Da from gel filtration, suggesting that the native enzyme is a dimer. The apparent Km values for glycogen, phosphate and 5'-AMP were 1.4 mM, 82 mM and 1.1 mM, respectively. The enzyme had a pH optimum of 7.05, and was inhibited by ATP, ADP and glucose, but not by trehalose, even at high concentration. Conversion of phosphorylase b into the a form was achieved by incubation with rabbit phosphorylase kinase and Mg(2+)-ATP. The molecular mass of phosphorylase a was estimated to be 250,000 Da by gel filtration chromatography. The specific activity of the a form in the presence of 5'-AMP was 1.6-1.7-fold higher than the specific activity of the b form under the same conditions. Thus, 5'-AMP activates the a form by about 20%, whereas ATP has no effect on the phosphorylase a activity.

Structure-activity relationships for Periplaneta americana hypertrehalosemic hormone. I: The importance of side chains and termini

Single amino acid replacement analogues for the native hypertrehalosemic hormone I of the American cockroach, Periplaneta americana (Pea-CAH-I: pGlu-Val-Asn-Phe-Ser-Pro-Asn-Trp-NH2), have been prepared by solid-phase peptide synthesis, and complete dose-response curves have been measured in P. americana monitoring the carbohydrate-mobilizing activity in vivo. All analogues that elicited hypertrehalosemia showed similar time-response courses, indicating that transport and degradation rates were comparable. Comparison of the potency and efficacy parameters of the analogues under study in the dose-response curves revealed four activity groups: 1) analogues that had the aromatic amino acids at positions 4 (phenylalanine) or 8 (tryptophan) replaced by alanine and glycine, respectively, had trace activity; 2) analogues with alanine at positions 1 or 2 had low potencies and an apparent biphasic dose-response relationship without much observable loss of efficacy; 3) analogues with glycine at positions 6 and 7 had potencies and efficacies most similar to Pea-CAH-I; and 4) analogues that had either an alanine instead of asparagine residue at position 3, or had a substitution of the carboxylamide function at the C-terminus by a carboxyl function reached apparent saturation, but only achieved 50-57% of the maximum activity of the native peptide. The potency profile for the analogue set is consistent with the importance of the N-terminal pentapeptide and the C-terminal tryptophan interacting with receptor(s) more closely than the side chains at positions 6 and 7, which are predicted to be the corner residues of a beta-turn. Finally, the biphasic dose-response curves observed for more than one analogue suggest the potential that receptors for Pea-CAH-I exist in more than one form.

Intracellular transduction of trehalose synthesis by hypertrehalosemic hormone in the fat body of the tropical cockroach, Blaberus discoidalis

Fat body of adult male Blaberus discoidalis cockroaches exposed to B. discoidalis hypertrehalosemic hormone (HTH) in vitro showed a decline in tissue glycogen as carbohydrate increased in the medium. In vivo HTH injections increased hemolymph carbohydrate and fat body glycogen phosphorylase activity > 2-fold compared to controls. In vivo trehalose synthesis was unaffected by agents that enhance intracellular levels of cyclic nucleotides including: dibutyrl cAMP, dibutyryl cGMP, forskolin (adenylyl cyclase activator) and isobutyl methylxanthine (IBMX) or theophylline (cAMP phosphodiesterase inhibitors). DbcAMP+IBMX stimulated trehalose biosynthesis of fat body in vitro and had additive effects with a minimally stimulatory HTH concentration. However, adenylyl cyclase activity was unaffected by HTH either with isolated fat body or fat body membrane preparations. We conclude that cAMP is not a second messenger for HTH, but cAMP can stimulate trehalose production independent of HTH through actions on common regulatory events related to trehalose biosynthesis. Dibutyryl cGMP and phorbol esters and diacylglycerol (activators of protein kinase C) also failed to stimulate trehalose biosynthesis in vitro. Extracellular Ca2+ enhanced HTH-dependent phosphorylase activity, glycogenolysis and hypertrehalosemia and maintained basal levels of phosphorylase a at twice those observed in the absence of Ca2+. However, Ca2+ entry by Ca2+ ionophore (A23187) did not mimic HTH effects. Results of these studies demonstrated that extracellular Ca2+ is important for HTH-dependent trehalose biosynthesis but cAMP and cGMP are not involved.

Adipokinetic hormone-induced influx of extracellular calcium into insect fat body cells is mediated through depletion of intracellular calcium stores

Adipokinetic hormone I (AKH I) needs extracellular Ca2+ for its activating action on glycogen phosphorylase in locust fat body in vitro. TMB-8 reduces this AKH effect significantly, indicating that for a major part, hormone action also requires the mobilization of Ca2+ from intracellular stores. Using 45Ca2+, AKH was shown to stimulate both the influx and the efflux of Ca2+. Thapsigargin also enhances the influx of extracellular Ca2+ into the fat body cells, indicating that the stimulating effect of AKH on Ca2+ influx may be mediated through depletion of intracellular Ca2+ stores as well. AKH is known to enhance cAMP levels in locust fat body. We show that elevation of cAMP with forskolin or theophylline leads to activation of glycogen phosphorylase, both in the presence and in the absence of extracellular Ca2+. The present data are discussed in an attempt to elucidate further the mechanism underlying transduction of the hormonal signal in locust fat body.

Effects of intestinal insulin-like peptide on glucose catabolism in mealworm larval fat body in vitro: dependence on extracellular Ca2+ for its stimulatory action

In vitro hormonally induced variations of glucose catabolism in mealworm fat body tissue were examined by a microradiorespirometric method. An insulin-like peptide (ILP) extracted from the midgut of last larval instar mealworm larvae significantly modified glucose catabolism and was dependent on energy metabolism and on the Ca2+ concentration in the culture medium. Using two different labelled substrate molecules, the stimulatory effects of ILP (compared with those of mammalian insulin) on the relative use of the pentose cycle as opposed to the glycolytic-citric acid cycle by the mealworm fat body were measured in vitro. Metabolic variations were evaluated using either [1-14C]glucose or [6-14C]glucose as substrates. Time course and dose-response curves of ILP and the hormonally induced variations in total CO2 and 14CO2 kinetics were determined. Modification in the specific radioactivity kinetics of 14CO2 derived from [1-14C] glucose and [6-14C]glucose molecules under hormonal effects were observed. As demonstrated in in vivo studies, ILP stimulated the relative utilization of the pentose cycle. However, this effect was observed much more rapidly, but for a shorter time, with fat body in vitro. Mammalian insulin produced similar, but not identical effects. Variations in transmembranous Ca2+ cellular exchanges, induced by either EGTA, nifedipine, or calcium ionophore ionomycin included in the culture medium, indicated that the stimulatory effects of ILP depends on this cation.

Regulation of fat body mitochondrial respiration in Periplaneta americana by a novel factor from the corpus cardiacum

Respiration of fat body (Periplaneta americana) mitochondria is increased by pretreatment of the tissue with corpus cardiacum (CC) extract. The magnitude of the increase depends on the type of substrate supplied for oxidation. With 5 mM pyruvate the respiration increased 22%, decreasing to 0 with 1 mM pyruvate. In contrast, 50 microM and 0.2 mM palmitic acid supported an increase in CC-stimulated respiration of 14 and 44%, respectively. Unlike crude CC extract, the synthetic hyperglycemic peptides CCI and CCII failed to alter the respiratory activity of fat body mitochondria. In common with the action of CC extract pretreatment of the fat body in vitro with 10(-5) M cyclic AMP, 10(-5) M 8-bromo-cyclic AMP, or 10(-5) M forskolin increased mitochondrial respiration approximately 30%. Octopamine (10(-4) M) elicited a response similar to that obtained with CC extract. Neither 10(-5) M cyclic AMP nor 10(-5) M 8-bromo-cyclic AMP stimulated respiration when applied directly to the mitochondria. These results suggest that the factor in CC extract manifests its effect intracellularly through the activation of a cyclic AMP-dependent protein kinase. This interpretation is also based on the finding that diamide, an inhibitor of protein kinase, inhibits CC-dependent and cyclic AMP-dependent mitochondrial respiration. The physiological role of the CC factor responsible is not known.

Stimulation of carbohydrate metabolising enzymes by synthetic hypertrehalosemic peptides in thoracic musculature of the American cockroach, Periplaneta americana

The ability of the synthetic hypertrehalosemic peptides, HT-I and HT-II, to influence the activities of glycogen phosphorylase, trehalase and hexokinase via elevation of Ca++ and cAMP levels was examined in thoracic musculature of the American cockroach, Periplaneta americana. The peptides effect dose- and time-dependent activation of phosphorylase, trehalase and hexokinase activities that occur concomitantly with elevated levels of intracellular calcium. In addition, HT-I increases the accumulation of cyclic AMP in muscle cells.

Structure-activity relationships on hyperglycemia by representatives of the adipokinetic/hyperglycemic hormone family in Blaberus discoidalis cockroaches

Several members of the adipokinetic/hyperglycemic neurohormone family from several different invertebrate species have been prepared by solid-phase peptide synthesis and assayed by a modified in vivo hyperglycemic bioassay in Blaberus discoidalis cockroaches. The hypertrehalosemic hormone (HrTH) is the endogenous hypertrehalosemic factor for B. discoidalis and was the most potent peptide in the assay. The more divergent the sequence of a family member from Blaberus HrTH, the less potent was the bioanalog. Manduca adipokinetic hormone is the most divergent peptide of the family and was totally inactive in the bioassay. Locusta adipokinetic hormone I had reduced maximum activity in the assay, which suggests that Ser5 is an important residue for the transduction of the hyperglycemic response. The direct relation between bioanalog similarity to Blaberus HrTH sequence and potency suggests that the hormone and target cell receptor for HrTH have evolved to maintain an "optimal fit".

Simulation of the activation of fat body glycogen phosphorylase and trehalose synthesis by peptide hormones in the American cockroach

A model is described which simulates activation of glycogen phosphorylase and induction of trehalose synthesis in the fat body of the American cockroach, Periplaneta americana, by two hypertrehalosaemic peptides. Parameters for the model were estimated from literature data (Siegert et al., Insect Biochem. 16, 365), with the exception of the half-life of the physiologically active peptides, which was estimated from the model. The model describes satisfactorily the activation of glycogen phosphorylase and the increase of haemolymph carbohydrates, which is dependent on the activation of glycogen phosphorylase in the model. It further explains the observed differences in sensitivity for glycogen phosphorylase activation and increase in haemolymph carbohydrates by these peptides. Best fits were obtained with a physiological half-life of about 12-15 min for the peptides. This value is similar to what can be calculated from the in vivo effects of these peptides on heart beat (Gersch et al., Zool. Jb. Physiol. 86, 17), but it is considerably shorter than the published half-life of 1 h for radioactive peptide (Skinner et al., Insect Biochem. 17, 433). However, both values are compatible if part of the peptides in the haemolymph is not present in freely dissolved form, but bound to a haemolymph component. The model half-life would then represent the half-life of the free, physiologically active peptide, which was estimated from the disappearance of radioactive peptide to be about 12-15 min. This suggests, that the model half-life is realistic and physiologically more important than the half-life of radioactive peptide of 1 h.