Pro-opiocortin processing in the pituitary: a model for neuropeptide biosynthesis

GIP_HUMAN[22-51] is a new proatherogenic peptide identified by native plasma peptidomics

We recently established a new plasma peptidomic technique and comprehensively identified a large number of low-molecular weight and low-abundance native peptides using a single drop of human plasma. To discover a novel polypeptide that potently modulates the cardiovascular system, we performed a bioinformatics analysis of the large-scale identification results, sequentially synthesized the selected peptide sequences, tested their biological activities, and identified a 30-amino-acid proatherogenic peptide, GIP_HUMAN[22-51], as a potent proatherosclerotic peptide hormone. GIP_HUMAN[22-51] has a common precursor with the glucose-dependent insulinotropic polypeptide (GIP) and is located immediately N-terminal to GIP. Chronic infusion of GIP_HUMAN[22-51] into ApoE-/- mice accelerated the development of aortic atherosclerotic lesions, which were inhibited by co-infusions with an anti-GIP_HUMAN[22-51] antibody. GIP_HUMAN[22-51] increased the serum concentrations of many inflammatory and proatherogenic proteins, whereas neutralising antibodies reduced their levels. GIP_HUMAN[22-51] induced IκB-α degradation and nuclear translocation of NF-κB in human vascular endothelial cells and macrophages. Immunoreactive GIP_HUMAN[22-51] was detected in human tissues but there was no colocalization with the GIP. The plasma GIP_HUMAN[22-51] concentration in healthy humans determined using a stable-isotope tagged peptide was approximately 0.6 nM. This study discovered a novel endogenous proatherogenic peptide by using a human plasma native peptidomic resource.

Elasmobranch color change: A short review and novel data on hormone regulation

Skins of Potamotrygon reticulatus are light in color in vitro, exhibiting punctate melanophores. Alpha-Melanocyte stimulating hormone (EC(50) = 4.58 x 10(-9) M) and prolactin (EC(50) = 1.44 x 10(-9) M) darken the skins in a dose-dependent manner. The endothelins ET-1, ET-2 and ET-3, and the purines, ATP, and uracil triphosphate (UTP) were not able to induce either skin lightening or darkening. Forskolin and the calcium ionophore A23187 promoted a dose-dependent darkening response, whereas N(2), 2'-O-dibutyryl guanosine 3'-5'-cyclic monophosphate (db cyclic GMP), phorbol-12-myristate-13-acetate (TPA), and 1-oleoyl-2-acetyl-sn-glycerol (OAG) were ineffective. The maximal response obtained with the calcium ionophore A23187 was only 76% of maximal darkening. These results indicate that the cyclic adenosine 3'-5'-monophosphate (cAMP) pathway is probably involved in the pigment dispersion of P. reticulatus melanophores. Other experiments should be done to further investigate how cytosolic calcium may be physiologically increased, and the existence of a putative cross-talk between calcium and cAMP signals. In conclusion, the only hormones effective on P. reticulatus melanophores were prolactin and alpha-MSH. No aggregating agent has been shown to antagonize these actions. Prolactin effect on elasmobranch melanophores adds a novel physiological role to this ancient hormone. J. Exp. Zool. 284:485-491, 1999.

Stress-induced peptide release from rat intermediate pituitary. An ultrastructural analysis

We tested the hypothesis that acute restraint stress results in ultrastructural evidence for enhanced release of alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin from the intermediate lobe (IL) of the rat pituitary. Measurements of plasma alpha-MSH- and beta-endorphin-immunoreactivity (ir) were used to confirm ultrastructural findings. Plasma alpha-MSH-ir was elevated after 20 and 30 min of restraint while plasma beta-endorphin-ir peaked 10 min after the onset of restraint. Ultrastructural analysis revealed a decrease in the content of secretory granules within IL cells of stressed rats. Analysis of Golgi-related immature secretory granules in IL cells indicated that new peptide synthesis was not enhanced after 30 min of restraint. These results confirm previous studies showing an elevation of plasma beta-endorphin and alpha-MSH-ir during acute restraint. Furthermore, these results indicate that quantitative analysis at the ultrastructural level can be used to assess peptide release from IL secretory cells during stress.

Regulated secretion of pro-opiomelanocortin converting enzyme and an aminopeptidase B-like enzyme from dispersed bovine intermediate lobe pituitary cells

Coordinate secretion of two prohormone/proneuropeptide processing enzymes [pro-opiomelanocortin converting enzyme (PCE) and an aminopeptidase B-like enzyme (APBE)] and alpha-melanotropin (alpha-MSH) from bovine intermediate lobe pituitary cells was studied. Stimulation of secretion with 8-bromo-cyclic AMP produced significant increases in levels of immunoreactive alpha-MSH, PCE, and APBE. Treatment of cells with the dopaminergic agonist 2-bromo-alpha-ergocryptine resulted in significant decreases in secretion of alpha-MSH, PCE, and APBE. In neither case were there significant changes in levels of cytosolic lactic dehydrogenase or lysosomal beta-glucuronidase in the medium. The secreted PCE activity was shown to process frog and mouse pro-opiomelanocortin primarily to 23,000-Mr corticotropin (ACTH), 13,000-Mr ACTH, beta-lipotropin, a beta-endorphin-like peptide, and beta-endorphin, products comparable to those synthesized by the mouse and frog intermediate lobe in situ. The secreted enzymatic activity had a pH optimum between 4.0 and 5.0, was strongly inhibited by pepstatin A, and had an inhibitor profile similar to the purified bovine intermediate lobe PCE. The secreted APBE activity cleaved Argo-[Met]-enkephalin to [Met]-enkephalin and had a pH optimum and inhibitor profile similar to that previously reported for an activity from purified secretory vesicle fractions of bovine intermediate and neural lobes. The coordinate regulated secretion of alpha-MSH and enzyme activities (PCE and APBE) strongly indicates their colocalization in the same secretory vesicle compartment within the cell. The characteristics of the two enzymes secreted in the medium paralleled those seen in the tissue and further support their role in pro-opiomelanocortin processing in vivo.

Ultrastructural Characterization of Adrenocorticotrope Hormone (ACTH) Immunoreactive Fibres in the Mesencephalic Central Grey Substance of the Rat

The fine structural localization of fibres immunoreactive for the adrenocorticotrope hormone (ACTH) was studied in the mesencephalic central grey substance (MCG) of the male Wistar rat. Light microscopically, varicose ACTH-immunoreactive fibres were found throughout the MCG in a dorsal, lateral and ventral, periventricular position. Electron microscopically, the immunoreactivity was most prominent in the direct vicinity of electron-dense secretory granules in axonal varicosities, and, although to a lower degree, around other cytoplasmic organelles such as electron-lucent synaptic vesicles, mitochondria and microtubules. With serial section analysis two types of ACTH-immunoreactive varicosity were discerned. The first type is large, contains many, small electron-lucent synaptic vesicles, that are located in the vicinity of a morphologically well-defined synaptic contact. In this type of varicosity, large dense-core secretory granules are scarce. Immunoreactivity is low or absent, particularly near the active zone. The second type is strongly immunoreactive. It always contains many large, dense-core secretory granules; electron-lucent vesicles are rare. The smaller varicosities of this type never make synaptic contacts, but a few of the larger varicosities have synaptic contacts with dendrites of MCG cells.

Beta-endorphin genetics in the etiology of alcoholism

"Reinforcing" effects are ascribed to endogenous opioids, particularly to the pro-opiomelanocortin (POMC)-derived beta-endorphin 1-31, the most potent opiate-active substance. Alcohol induces variations in the genetic processing of the precursor POMC and of beta-endorphin at different levels. Studies focused on changes in POMC gene expression (mRNA quantitation) and post-translational processing. Chronic alcohol intake significantly reduces POMC mRNA in the lobes of the pituitary. In inbred strains of mice, genotypic differences are seen in post-translational processing of hypothalamic beta-endorphin, thus inducing differences in alcohol sensitivity. Clinical studies show a disproportion of POMC cleavage products in the CSF of chronic alcoholics (reduced beta-endorphin versus increased ACTH contents), together with remarkable indications for baseline differences in beta-endorphin levels. Errors within the genetic sequence of POMC are suggested to underlie alcohol-seeking behavior.

Intragranular processing of pro-opiomelanocortin in the intermediate cells of the rat pituitary glands. A quantitative immunocytochemical approach

Quantitative immunocytochemical studies were done by using the immunogold technique on sections of the intermediate lobe of rat pituitary. Antibodies raised (in rabbits) against the precursor proteins pro-opiomelanocortin (POMC) and ACTH were used. The results clearly indicate that the immature granules are the major site of POMC, as their antigenic density (gold beads/micron2) was almost 3 times as high as that of ACTH. In the mature granules, the antigenic density of ACTH was increased by 2.7-fold compared with the immature granules. Using a computer-assisted method, it was possible to categorize the granules' antigenic density according to their size. Using this approach it was found that the antigenic density of POMC remained constant in all mature granules of varied sizes, whereas the antigenic density of ACTH decreased with increasing granule size. The relationship between granule size, degree of maturation, and antigenic density is discussed.

Beta-endorphin concentrations in the hypothalamus, pituitary and plasma of streptozotocin-diabetic rats with and without insulin substitution therapy

The concentrations of beta-endorphin like immunoreactivity (beta-END) in the hypothalamus, pituitary and plasma were studied in rats of either sex, one month after induction of diabetes by single iv injection of streptozotocin. As controls, both normal and undernourished rats, weight-matched with diabetic rats, were used. Diabetic male and female rats had a marked depletion of beta-END stores in the hypothalamus and neurointermediate lobe (NIL) but not in the anterior pituitary. Depletion of beta-END was reversed to normal by insulin replacement therapy. Severe undernourishment was not as effective as diabetes to reduce beta-END stores in the hypothalamus and NIL. A significant reduction of beta-END was observed only in the NIL of undernourished female rats. Plasma beta-END and beta-lipotropin (beta-LPH) concentrations were not significantly altered in diabetic rats. These results indicate that the lack of insulin may affect beta-END synthesis in the hypothalamus and NIL.

Immunocytochemical evidence for intragranular processing of pro-opiomelanocortin in the melanotropic cells of the rabbit

The immunogold technique, employing antisera with clear-cut specificities, was used to localise different processing stages of pro-opiomelanocortin (POMC) in rabbit melanotropic cells. While the antiserum against gamma 3-MSH labelled all the secretory granules including intrasaccular condensations in the Golgi apparatus, antisera against alpha-MSH only labelled extra-Golgi secretory vesicles (SV). All extra-Golgi SV were likewise labelled with the three antisera against alpha-MSH used, despite their different specificities for the deacetylated, N-acetylated or C-amidated forms of the peptide. The antibody against beta-endorphin also labelled the extra-Golgi SV, while only some SV were labelled with the antibody against gamma-endorphin. These results correlate with biochemical data in favour of mainly--if not exclusively--intragranular processing of POMC. Except for gamma 3-MSH, the cleavage of which could coincide with Golgi packaging of secretory material, other post-translational modifications of the precursor seem to occur when SV are discharged outside the Golgi area. The cleavage of gamma-endorphin appears to be a later step in POMC processing, occurring in some mature SV.

Separation and partial characterization by high-performance liquid chromatography and radioimmunoassay of different forms of melanocyte-stimulating hormone from fish (Cyprinidae) neurointermediate lobes

From neurointermediate lobe (NIL) extracts of two species of Cyprinidae, Carassius auratus and Cyprinus carpio, several peptides were separated by high-performance liquid chromatography (HPLC) on a C18 muBondapak column eluted with a methanol/acetic acid/triethylamine mixture. Monitoring all fractions by radioimmunoassay (RIA) with an antibody against melanocyte-stimulating hormone (MSH) C terminal gave positive reactions for fractions 7, 11-12, 15-16, 23-24, and 25-27. For further characterization, the elution positions of these peaks were compared to those of known synthetic reference substances. Peak 7 elutes in the same position as oxidized alpha MSH, whereas peak 15-16 matches the elution position of des-acetyl alpha MSH and 23-24 that of alpha MSH. The product from peak 26-27 has several characteristics of the diacetylated form of alpha MSH: its immunoreactivity in RIA, its sensitivity to weak bases and to HCl and its mass spectrum which is identical with that of mammalian diacetyl alpha MSH. In both species, the diacetylated form is predominant in the intracellular pool. This study establishes the coexistence of three different forms of alpha MSH, a des-acetylated, monoacetylated, and diacetylated in the cyprinid NIL extracts.

Autonomous beta-endorphin secretion from the pituitary neurointermediate lobe: in vivo studies

The existence of independent control mechanisms of beta-endorphin (beta-EP) secretion from the anterior (AP) and intermediate (NIL) pituitary lobes is now ascertained. The aim of this study was to evaluate the effect of surgical separation from the hypothalamus of the two pituitary lobes on beta-EP secretion. Two experimental models of surgical hypothalamo-pituitary disconnection were used: 1) rats with ablation of the medial basal hypothalamus (MBH); 2) rats bearing two entire ectopic pituitaries or two anterior pituitaries (APs) only, transplanted under the kidney capsule. In rats with MBH-ablation plasma beta-EP levels were significantly higher than in sham-operated controls. Plasma beta-EP levels increased in rats transplanted with entire pituitaries 3 days after surgery and were still elevated after 1 week. In rats transplanted with APs only, no significant beta-EP changes in plasma were evident. In both experimental conditions no significant difference was present in beta-LPH plasma levels. Concentrations of beta-EP in the ectopic NILs decreased gradually after transplantation. In all these results indicate that that NIL but not the AP is capable, when is disconnected from the hypothalamus, or secreting autonomously beta-EP.

Enkephalin-containing polypeptides are potent inhibitors of enkephalin degradation

Enkephalin-containing polypeptides derived from pro-enkephalin A, pro-enkephalin B, or pro-opiomelanocortin were inhibitors of enkephalin degradation by aminoenkephalinases purified from cytosol or membranes. Of the peptides, Argo-Met-enkephalin was the most potent inhibitor for the aminoenkephalinases, with an IC50 of about 0.6 microM, it was more effective than bestatin (IC50 = 0.8-1.0 microM). This inhibition was partly due to substrate competition. Argo-Met-enkephalin was hydrolyzed by aminoenkephalinases to form Arg, Tyr, and Gly-Gly-Phe-Met in a substrate-inhibited manner. The hexapeptide also inhibited the breakdown of Arg- and Tyr-beta-naphthylamide by the membrane aminoenkephalinase. Since Argo-Met-enkephalin did not inhibit leucine aminopeptidase, it was a more selective inhibitor than bestatin of Met-enkephalin breakdown by aminopeptidases. Argo-Met-enkephalin inhibited enkephalin breakdown by synaptosomal plasma membranes but not by brain slices. Our data suggest that in addition to their possible role as opioids, the enkephalin-containing polypeptides may be regulators of enkephalin levels.