Cell-free translation product containing corticotropin and beta-endorphin encoded by messenger RNA from anterior lobe and intermediate lobe of bovine pituitary

60 YEARS OF POMC: From the prohormone theory to pro-opiomelanocortin and to proprotein convertases (PCSK1 to PCSK9)

Pro-opiomelanocortin (POMC), is a polyprotein expressed in the pituitary and the brain where it is proteolytically processed into peptide hormones and neuropeptides with distinct biological activities. It is the prototype of multipotent prohormones. The prohormone theory was first suggested in 1967 when Chrétien and Li discovered γ-lipotropin and observed that (i) it was part of β-lipotropin (β-LPH), a larger polypeptide characterized 2 years earlier and (ii) its C-terminus was β-melanocyte-stimulating hormone (β-MSH). This discovery led them to propose that the lipotropins might be related biosynthetically to the biologically active β-MSH in a precursor to end product relationship. The theory was widely confirmed in subsequent years. As we celebrate the 50th anniversary of the sequencing of β-LPH, we reflect over the lessons learned from the sequencing of those proteins; we explain their extension to the larger POMC precursor; we examine how the theory of precursor endoproteolysis they inspired became relevant for vast fields in biology; and how it led, after a long and arduous search, to the novel proteolytic enzymes called proprotein convertases. This family of nine enzymes plays multifaceted functions in growth, development, metabolism, endocrine, and brain functions. Their genetics has provided many insights into health and disease. Their therapeutic targeting is foreseeable in the near future. Thus, what started five decades ago as a theory based on POMC fragments, has opened up novel and productive avenues of biological and medical research, including, for our own current interest, a highly intriguing hypocholesterolemic Gln152His PCSK9 mutation in French-Canadian families.

60 YEARS OF POMC: The proopiomelanocortin gene: discovery, deletion and disease

The cloning of the bovine proopiomelanocortin (POMC) cDNA in 1978 by Nakanishi and colleagues was the result of a remarkable series of exacting and ingenious experiments. With this work, they instantly confirmed the single precursor hypothesis for adrenocorticotrophic hormone-β-lipotropin, as it was then known, and in so doing revealed the existence of additional, largely unpredicted, N-terminal peptides that together formed the POMC precursor peptide. This work paved the way for a host of additional studies into the physiology of these peptides and their regulation. Furthermore, the cloning of the murine Pomc gene was essential for subsequent studies, in which Pomc was intentionally deleted in the mouse illuminating its substantial role in body weight regulation and adrenal function. Contemporaneously with this work, naturally occurring mutations in human POMC came to light underlining the vital role of this gene in appetite regulation. This article reviews each of these aspects of POMC with the benefit of several decades of hindsight and informed by more recent genomic and transcriptomic data.

Design, synthesis, and biological evaluation of a new class of small molecule peptide mimetics targeting the melanocortin receptors

A new bicyclic template has been developed for the synthesis of peptide mimetics. Straightforward synthetic steps, starting from amino acids, allow the facile construction of a wide range of analogs. This system was designed to target the melanocortin receptors (MCRs), with functional group selection based on a known pharmacophore and guidance from molecular modeling to rationally identify positional and stereochemical isomers likely to be active. The functions of hMCRs are critical to myriad biological activities, including pigmentation, steroidogenesis, energy homeostasis, erectile activity, and inflammation. These G-protein-coupled receptors (GPCRs) are targets for drug discovery in a number of areas, including cancer, pain, and obesity therapeutics. All compounds from this series tested to date are antagonists which bind with high affinity. Importantly, many are highly selective for a particular MCR subtype, including some of the first completely hMC5R-selective antagonists reported.

Beta-endorphin and cortisol levels in plasma and CSF following acute experimental spinal traumas

beta-endorphin and cortisol were measured in cerebrospinal fluid (CSF) and plasma by radioimmunological method (RIA) in two groups of rabbits with spinal cord traumatic injuries at cervical and lumbar levels, respectively with and without concomitant spinal shock and arterial hypotension, and in a group of sham operated animals as controls. The two groups with spinal lesions displayed a significant beta-endorphin increase in CSF, whereas the cortisol level remained unchanged both in the spinal traumatized rabbits and in controls. Both the opioid and the cortisol concentration rose significantly in plasma in all three groups and in particular resulted significantly higher in the cervical traumatized group where spinal trauma was associated with spinal shock and hypotension. However, no significant difference was found when beta-endorphin concentrations in plasma were compared between the sham operated animals and the spinal lumbar traumatized animals without concomitant spinal shock. The results seem to suggest that the beta-endorphin increase in CSF is related to the nervous tissue lesion, while its increase in plasma, like that of cortisol, is due to surgery or other stress factors inherent in the experiment. This independent behaviour of beta-endorphin in plasma and in CSF suggests its different origin in these two compartments.

Characterization of opioid binding sites in the neural and intermediate lobe of the rat pituitary gland by quantitative receptor autoradiography

Previous studies have suggested an involvement of enkephalins in regulation of oxytocin (OXT) and vasopressin (AVP) release, which seems to disagree with the very low affinities of Met- and Leu-enkephalin for the kappa opioid receptor. As opioid receptors in the neural lobe exclusively exist of kappa receptors, we studied the binding characteristics of larger pro-enkephalin derived peptides for opioid binding sites in the neural lobe by means of light microscopic receptor autoradiography. In addition, the pharmacological characteristics of opioid binding sites in the neural lobe were compared with those in other parts of the pituitary. In the neural as well as the intermediate lobe both high and low affinity 3H-bremazocine binding sites were present. Binding to these sites was completely displaceable by both naloxone and nor-binaltorphimine suggesting that these sites represent kappa opioid receptors. Also with regard to selectivity and affinity characteristics to other ligands, opioid binding sites in the neural and intermediate lobe were quite similar. In the anterior lobe a very low level of bremazocine binding was present, which could not be displaced by nor-binaltorphimine. Displacement studies with pro-enkephalin and pro-dynorphin derived peptides showed that both groups of peptides could bind to opioid binding sites in the neural and intermediate lobe. Especially the relatively large pro-dynorphin and pro-enkephalin derived peptides, such as dynorphin 1-17 and BAM22, appeared to be very potent ligands for these opioid binding sites and were much more potent than smaller fragments, such as dynorphin 1-8, and Met- and Leu-enkephalin. These results contradict the existence of a mismatch in the neural (and intermediate) lobe with regard to the local type of opioid peptides and receptors present.

Isoproterenol-stimulated release of beta-endorphin and related peptides from the rat pituitary neurointermediate lobe in vitro: evidence for preferential release of certain molecular forms of beta-endorphin

The intermediate lobe of the pituitary gland synthesizes the multifactorial precursor molecule pro-opiomelanocortin (POMC), from which, through a process of post-translational enzymatic processing, beta-endorphin-(1-31) (beta E) and a variety of N alpha-acetylated and C-terminally shortened forms of this peptide are generated. Using an in vitro superfusion system, the release of these endorphins from intact rat neurointermediate lobes (NILs) was investigated under basal and isoproterenol (ISO) stimulated conditions. Superfusion of NILs with the beta-adrenergic agonist ISO (30 min pulse) resulted in a rapid, sustained and concentration-dependent stimulation of the release of beta E-like immunoreactivity (beta E-IR) over basal as determined with an antiserum directed against the C-terminus of the beta E- (1-31) sequence (10(-6) M: + 145%; 10(-7) M: + 73%; 10(-8) m: + 41%). The release of N(alpha)-acetylated-endorphin-like immunoreactivity (AcE-IR) was stimulated to a similar extent. These effects of ISO were antagonized by the competitive alpha-adrenoceptor antagonist propranolol in a concentration-dependent manner, indicating the involvement of alpha-adrenoceptors. The beta-related peptides released from the NILs under basal and ISO-stimulated conditions were further characterized, based on their retention times in a reversed-phase HPLC system and their reactivity with specific antisera recognizing respectively the midportion of beta E, the N-terminus of acetylated endorphins, the C-terminus of tau-endorphin (beta E-(1-17); tau E), or the C-terminus of alpha-endorphin (beta E-(1-16); alpha E). In HPLC fractionated superfusates 10 peaks were resolved that reacted with the midportion beta E antiserum. In superfusates collected under basal conditions, three major peaks possessed chromatographical and immunological characteristics of Ac beta E-(1-26), Ac beta E- (1-27) Ac beta E-(1-31). In addition, a prominent peak was found eluting around the retention time of beta E-(1-31), that contained both acetylated and non-acetylated material. Six smaller peaks were observed, with the characteristics of beta E-(1-26) and beta E-(1-27) (these peptides were not resolved with the HPLC system used), Ac tau E, tau E, Aa alpha E, and des-tyrosine-alpha E (DT alpha E), respectively. In superfusates collected during superfusion of NILs with ISO (10(-6) M) all peaks were increased. However, those eluting as beta E-(1-31), beta E-(1-26)/beta E-(1-27), Ac beta E-(1-26) and Ac tau E appeared to be preferentially stimulated.(ABSTRACT TRUNCATED AT 400 WORDS)

Activities of ACTH-potentiating substances isolated from rat serum on steroidogenesis in isolated rat adrenal cells

Peptides that potentiate the response of adrenal cells to ACTH1-24 were isolated from rat serum. ACTH-potentiating activity was found in fractions of 9,000-40,000 in molecular weight (APS-Fr) and of smaller molecular weight (SM-Fr) on Sephadex G-100 gel filtration of the serum extract. The peptides were isolated from APS-Fr by preparative acid polyacrylamide gel electrophoresis and named d1, d, d2, e, f and g. Their molecular weights, estimated by Sephadex G-75 gel filtration, were 41,000, 33,000, 24,000, 17,500, 17,500 and 16,000, respectively. All of these peptides were glycopeptides. The isoelectric point of peptide d was 8.45 and some of the others, such as f and g, were more basic. Sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis revealed that these peptides were decomposed into various fragments. ACTH-potentiating activity was highest in peptide d1 and lowest in peptide e. The maximum activity of peptide d was observed at 3 X 10(-8) M when steroidogenesis was induced by 9 X 10(-12) M ACTH1-24. While these peptides did not show any ACTH-like activity at any stage of isolation, the fractions of these peptides eluted from a Sephadex G-75 column showed more or less ACTH-like activity. These peptides therefore seemed to possess latent ACTH-like activity. The molecular weight of SM-Fr was smaller than ACTH1-24. The ACTH-potentiating activity of SM-Fr was low, and SM-Fr did not show any ACTH-like activity. SM-Fr therefore seems to be the smallest structure which has ACTH-potentiating activity. The similarity of these peptides to proopiomelanocortin-related substances was discussed.

ACTH and related peptides: molecular biology, biochemistry and regulation of secretion

The precursor of ACTH and beta-LPH is a glycoprotein with a molecular weight of more than 30 000. Its gene consists of three exons with two intervening sequences and most of the protein coding sequence is in exon 3. The gene is expressed not only in the pituitary gland but also in extrapituitary tissues. The gene expression in the anterior pituitary gland is regulated by CRF and glucocorticoids, but it is regulated differently in other tissues. The processing of the ACTH/beta-LPH precursor yields several peptides, but final products vary in tissues due to differential processing. The processing is abnormal in ACTH-producing tumours, especially in ectopic ACTH-producing tumours. Some abnormalities may also occur at the transcriptional or post-transcriptional level as well. Peptides derived from the same precursor are secreted concomitantly from the pituitary gland. CRF is the major stimulating factor, but vasopressin and some other factors are also involved in stimulating ACTH release. On the other hand, glucocorticoids inhibit ACTH release by acting at the hypothalamic and pituitary levels. In the pituitary ACTH-producing adenomas of Cushing's disease, CRF, vasopressin as well as other non-physiological factors stimulate ACTH secretion. Such abnormal receptor mechanisms are also seen in ectopic ACTH-producing tumours.

Isolation of corticotropin-beta-lipotropin common precursor from ovine pituitary glands

Ovine corticotropin-beta-lipotropin common precursor was purified to homogeneity from commercial frozen ovine pituitary glands. A crude preparation was obtained following a procedure published elsewhere (Lee, T.H. and Lee, M.S. (1977) Biochemistry 16, 2824-2829) and was purified by gel filtration on Sephadex G-200 in the presence of 0.5% SDS and 0.1% 2-mercaptoethanol, and under an atmosphere of nitrogen. The gel filtration was repeated once. The partially purified preparation obtained from the second Sephadex G-200 gel filtration was further fractionated by preparative SDS-acrylamide gel electrophoresis, using immunoprecipitated and electrophoretically purified [125I]corticotropin-beta-lipotropin common precursor as a marker. The preparation was judged homogeneous by the appearance of a single protein band in analytical SDS-acrylamide gel electrophoresis, which exhibited both corticotropin and beta-lipotropin immunoreactivities, and a single symmetrical peak in high-pressure liquid chromatography on a reverse phase C18 column. The isolated ovine corticotropin-beta-lipotropin common precursor possessed specific activities of 116 micrograms of immunoreactive corticotropin and 210 micrograms of immunoreactive beta-lipotropin per mg of protein, equivalent to 89 and 62% of theoretical values, respectively. The amino acid composition of the homogeneous preparation was determined.

Effect of dopaminergic drugs on hypothalamic and pituitary immunoreactive beta-endorphin concentrations in the rat

Beta-endorphin concentrations have been evaluated in the hypothalamus, pituitary lobes and plasma after 1-and 3-week treatment with 2-Br-alpha-ergocriptine or lisuride, two potent dopaminergic drugs. Hypothalamic beta-endorphin concentrations were significantly decreased after the administration of the dopaminergic agents for 1 or 3 weeks. Similarly, beta-endorphin concentrations decreased in the neurointermediate lobe and plasma. After gel chromatography, it appeared that in the anterior pituitary, beta-lipotropin concentrations were unchanged or lightly increased concomitantly with a decrease of beta-endorphin. Our data indicate that, both in the hypothalamus and the neurointermediate pituitary lobe, beta-endorphin is under an inhibitory dopaminergic tone. The latter may also play a role in inhibiting beta-endorphin cleavage from beta-lipotropin in the anterior pituitary.

Analysis of brain and pituitary RNA metabolism: a review of recent methodologies

Recent characterization of brain and pituitary RNA metabolism is reviewed. Relative to other tissues, the brain transcribes more of the unique, single-copy DNA. This transcriptional diversity reflects the inherent heterogeneity in organization and development of the brain. The end product of transcriptional regulation in the brain is a population of functional cytoplasmic mRNAs with multiple components, differing in complexity and abundance. Analysis of nuclear and cytoplasmic RNA provides evidence that both brain-specific synthesis and processing may determine the final mRNA population. Both polyadenylated and non-polyadenylated RNA classes contribute significantly to the total brain polysomal mRNA fraction. Characterizations of individual species of mRNA from both brain and pituitary are described. One possible transcriptional modulator in both the pituitary and brain is the presence of steroid hormone at responsive sites. Functional consequences of steroid accumulation within the brain may be (1) interactions with neurotransmitter, especially catecholamine, metabolism and function, (2) developmental interactions with neuronal systems, and (3) differentiation of glial cell function. The pleiotropic nature of steroid hormone effects (both transcriptional and non-transcriptional) within one brain region is considered by examining the biochemical effects of glucocorticoids in the hippocampus.

Isolation of pancreatic trypsin inhibitor from bovine pituitary glands

A trypsin inhibitor has been isolated from bovine pituitary extracts. From its amino acid composition, NH2-terminal residue, mobility in paper electrophoresis, and behavior in high-performance liquid chromatography and from the tryptic pattern of the performic acid-oxidized material, it appears that the inhibitor is identical to the Kunitz and Northrop pancreatic trypsin inhibitor.

Analysis of pro-opiomelancortin gene structure and function

Pro-opiomelanocortin (POMC) is the protein precursor to several physiologically distinct peptide hormones. The POMC gene is expressed in several distinct cell types and appears to be under differential regulation depending upon the cell location. The structure of the POMC gene in human, bovine, rat, and mouse is very similar, suggesting that this particular structure has functional significance. The gene is broken by intervening sequences into a 5' noncoding region, a signal sequence-coding region, and a peptide hormone-coding region. Middle-repetitive DNA sequences are present in both introns of the genes as well as the 5'-flanking regions in all species studied; this too suggests that this structural element has functional significance. The peptide hormone-coding sequences are well conserved among species, whereas the nonhormonal "spacer" sequences are poorly conserved. Differential regulation of the POMC gene appears to occur by the differential recognition of effector molecules. Although the studies are not complete, all available evidence suggests that there is only a single POMC gene which is express in mammalian tissues.

Concomitant secretion of calcitonin, beta-endorphin and ACTH from medullary thyroid carcinoma in vivo and in vitro

The present study was performed to investigate the possible synthesis of a common precursor molecule for calcitonin (CT), adrenocporticotropin (ACTH) and beta-lipotropin (beta-LPH)/ beta-endorphin (beta-EP) by the human medullary thyroid carcinoma (MTC). In a patient with MTC but without Cushing's syndrome, the response of plasma CT, ACTH and cortisol levels to a calcium infusion, lysine vasopressin (LVP) and dexamethasone were measured. A parallel increase of these hormones in response to calcium and LVP was seen, while there was a paradoxical increase of CT during dexamethasone infusion. Incubation of MTC fragments obtained at surgery showed a significant correlation of the secretion of CT, ACTH and beta-LPH/ beta-EP in response to calcium, LVP and dexamethasone. The concomitant release of these hormones in vivo and in vitro could be compatible with the synthesis of a common precursor molecule for CT, ACTH and beta-LPH/ beta-EP in MTC, although this was not substantiated by gel-chromatography of the tumor extract. Corticotropin releasing factor, a regulator of the normal processing of pro-opiococorticoid precursor molecule in the anterior pituitary gland, is also able to activate ACTH, beta-LPH/ beta-EP and calcitonin secretion from the malignant C-cell of the thyroid.

Gamma-melanotrophin-like immunoreactivities in human pituitaries, ACTH-producing pituitary adenomas, and ectopic ACTH-producing tumours: evidence for an abnormality in glycosylation in ectopic ACTH-producing tumours

Using gel exclusion chromatography on Bio-Gel P-60, gamma-melanotropin-like immunoreactivity (gamma-MSH-LI) in three human pituitary glands, two ACTH-producing pituitary adenomas, and three ectopic ACTH-producing tumours (two medullary thyroid carcinomas and one thymoma) was divided into one or two molecular weight classes. The largest component eluted near the position of mouse 16K fragment and was designated big gamma-MSH-LI. This big gamma-MSH-LI was present in all samples. The second one, designated intermediate gamma-MSH-LI, eluted between the position of mouse 16K fragment and human ACTH, and was demonstrated only in two ectopic ACTH-producing tumours. No gamma-MSH-LI emerged at the elution position of synthetic gamma 3-MSH. Affinity chromatography on concanavalin A-agarose revealed that a significant fraction (52-68%) of gamma-MSH-LI from human pituitary glands, ACTH-producing pituitary adenomas, and one ectopic ACTH-producing tumour bound to the column and was eluted with alpha-methyl-D-mannopyranoside. In two ectopic ACTH-producing tumours which contained big and intermediate gamma-MSH-LI, a relatively small fraction (27-35%) of gamma-MSH-LI bound to the column and was similarly eluted. These observations suggest that human gamma-MSH-LI is glycosylated and that there is an abnormality in the glycosylation of gamma-MSH-LI in some ectopic ACTH-producing tumours.

Evidence for a signal sequence at the N terminus of the common precursor to adrenocorticothrophin and beta-lipotropin in mouse pituitary cells

The precursor to corticotropin and beta-endorphin was synthesized in a reticulocyte cell-free system under the direction of mRNA from mouse AtT-20 pituitary tumor cells in the presence of [3H]proline, [3H]phenylalanine, [3H]leucine, [3H]valine, [3H]isoleucine or [35S]methionine. Automatic Edman degradation of the radioactive cell-free product showed the following N-terminal sequence: Pro-1, Met-2, Leu-11, Leu-12, Leu-13, Leu-15, Leu-16, Leu-17, Ile-21 and Val-23. The corticotropin-endorphin precursor was also labeled in AtT-20 cells with [3H]valine, [3H]leucine, [3H]tryptophan, [3H]serine, [35S]methionine or [35S]cysteine. Automatic Edman degradation of the radioactive intact cell form gave the following N-terminal sequence: Trp-1, Cys-2, Leu-3, Ser-5, Ser-6, Val-7, Cys-8, Leu-11, Leu-17, Leu-18 and tentatively Met-27. The sequence of the intact cell form from AtT-20 cells matches the sequence of the cell-free form of bovine pituitary precursor beginning at Trp-27, as determined by recombinant DNA technology [Nakanishi, S., Inoue, A., Kita, T., Nakamura, M., Chang, A. C. Y., Cohen, S. N., and Numa, S. (1979) Nature (Lond.) 278, 423-427]. The sequence of the mouse pituitary mRNA-directed cell-free translation product also matches the bovine precursor beginning at Pro-2. The results suggest that both the mouse and bovine precursors possess a signal sequence of 26 amino acids which is cleaved in intact cells. CNBr cleavage of [35S]cysteine-labelled intact cell precursor gave rise to an N-terminal fragment of a size compatible with the presence of a methionyl residue at or near position 27.

The proopiocortin (adrenocorticotropin/beta-lipoprotein) gene is located on chromosome 2 in humans

The proopiocortin gene is located on chromosome 2 in humans. A 13-kb DNA fragment containing proopiocortin gene sequences was identified in human cells while proopiocortin-related genes sequences of 9.8 and 6.2 kb were present in mouse cells. In human-mouse cell hybrids which contained reduced numbers of human chromosomes and a complete set of mouse chromosomes, the 9.8- and 6.2-kb fragments were always present while the 13-kb fragment segregated with human chromosome 2 and the chromosome 2 enzyme markers acid phosphatase-1 (ACP1), malate dehydrogenase-1 (MDHI), and isocitrate dehydrogenase-1 (IDH1). Analysis of a single cell hybrid with a broken chromosome 2 indicates that the proopiocortin and ACP1 genes are closely linked and in the distal region of the short arm of chromosome 2.

Isolation and characterization of the bovine corticotropin/beta-lipotropin precursor gene

The entire bovine corticotropin/beta-lipotropin precursor gene has been isolated as a set of overlapping genomic DNA fragments which extend over a length of approximately 17000 base pairs. Restriction mapping of the cloned DNA fragments and nucleotide sequence analysis of the whole mRNA-coding segments and their surrounding regions have established that the corticotropin/beta-lipotropin precursor gene is approximately 7300-base-pairs long and contains two intervening sequences; one with an approximate length of 4000 base pairs is located within the segment encoding the 5'-untranslated region of the mRNA, and the other with an approximate length of 220 base pairs interrupts the protein-coding sequence near the signal peptide region. Sequence analysis of more than 200 base pairs preceding the proximal end of the corticotropin/beta-lipotropin precursor gene has revealed a 'Hogness box' and a variant of the model sequence d(G-G-TC-C-A-A-T-C-T) as well as palindrome structures as observed in other eukaryotic genes. Furthermore, some sequence similarities in the 5'-flanking region are found between the corticotropin/beta-lipotropin precursor gene and the mouse alpha-globin and beta-globin genes, all of which are negatively regulated by glucocorticoids. At least four homologous repetitive sequences are distributed at 3000-5000-base-pair distances in the corticotropin/beta-lipotropin precursor gene region; two such sequences are located in the 5'-flanking region, and one within each intervening sequence. Blot hybridization analysis of bovine pituitary nuclear RNA has indicated that the entire corticotropin/beta-lipotropin precursor gene is transcribed into a primary hnRNA product, which is then spliced to form the mature mRNA.

Immunological identification of a common precursor to arginine vasopressin and neurophysin II synthesized by in vitro translation of bovine hypothalamic mRNA

mRNA from membrane-bound polysomes of bovine hypothalamus was translated in an mRNA-dependent cell-free system from reticulocyte lysate or wheat germ. The translation products were identified by immunoprecipitation with antibodies to either neurophysin II or arginine vasopressin followed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. An immunoreactive polypeptide was obtained with an apparent Mr of 21,000. Sequential immunoprecipitation studies indicated that the Mr 21,000 product is a common precursor to neurophysin II and arginine vasopressin. The specificity of the immunoprecipitation was demonstrated by competition with excess amounts of unlabeled neurophysin II or arginine vasopressin; little or no competition was observed with unlabeled neurophysin II or arginine vasopressin; little or no competition was observed with unlabeled neurophysin I or oxytocin. Processing experiments with microsomal membranes from dog pancreas or tunicamycin-treated ascites tumor cells showed that the Mr 21,000 polypeptide is the prepro form. It was converted to a pro form with Mr 19,000 suggesting a pre sequence of approximately 15 amino acids. The Mr 19,000 polypeptide was coreglycosylated to an apparent Mr of 23,000, indicating that the neurophysin II-arginine vasopressin precursor is a glycopolypeptide.

Chemistry and biosynthesis of pro-opiomelanocortin. ACTH, MSH's, endorphins and their related peptides

Studies of lipotropins, melanotropins and endorphins on one hand, and of adrenocorticotropin on the other, has given rise to the concept of a multipotent precursor molecule recently renamed proopiomelanocortin. The preferential sites of cleavage of the precursor to produce its biologically active components are made of pairs of basic amino acid residues as described for the biosynthesis of beta-MSH and pro-insulin. Such structural feature is also found in other pro-hormone molecules. Pulse chase experiments and secretory studies carried out in both anterior and intermediate lobes of rat pituitary glands revealed the transformation of different forms of the precursor into different end-products, the anterior lobe producing preferentially ACTH and beta-LPH while the intermediate produces mainly the alpha-MSH and beta-endorphin. The multiple forms of precursors seem to differ in their carbohydrate content although at least two different gene products are still possible. The presence of similar peptides in the hypothalamus makes it highly probable that neuropeptides are biosynthesized with similar process. Thus the model of beta-LPH precursor, proposed as early as in 1967, is now applicable to the biosynthesis of all other neuropeptides. Major advances in this field are expected in the 1980s.

5'-terminal nucleotide sequence of the messenger RNA coding for bovine corticotropin/beta-lipotropin precursor

The complete 5'-terminal nucleotide sequence of the MRNA coding for the bovine common precursor of corticotropin and beta-lipotropin has been determined. The 5'-32P-labelled, 21-nucleotides-long, single-stranded DNA fragment complementary to a portion of the 5'-noncoding region of the mRNA was prepared from a cDNA clone and elongated by reverse transcriptase reaction with the mRNA as template. The DNA transcript formed was sequenced by the procedure of Maxam and Gilbert, and the resultant sequence was cross-checked by two-dimensional electrophoretic analysis of the partial alkaline digest of the 5'-32P-labelled mRNA. The 5'-terminal nucleotide residue was determined by two-dimensional thin-layer chromatography of the complete hydrolysis product of the 5'-32P-labelled mRNA. The nucleotide sequence determined, which partially overlaps the known sequence of the cloned cDNA, reveals the complete 5'-terminal sequence of the mRNA. This, in conjunction with our previous data, defines the complete primary structure of the mRNA. The mRNA is composed of 1098 nucleotides, including an unusually long 5'-noncoding sequence of 128 nucleotides. The presence of a 'cap' structure at the 5' terminus of the mRNA is suggested. The 5'-terminal 48 nucleotide residues of the mRNA are extremely purine-rich, having an A + G content of 83%, whereas all pyrimidine-rich segments are located downstream from there. Because the 5'-noncoding region of the mRNA contains three segments of potential secondary structure which partially overlap, it can exist in a number of alternative base-pairing configurations. However, its interaction with the 3'-terminal segment of 18-S rRNA at the site of maximal complementarity would fix the mRNA configuration in such a way as to bring the possible site of ribosome binding near the initiation codon.

Immunoreactive beta-endorphin and adrenocorticotropin in human cerebrospinal fluid

To elucidate the significance of beta-endorphin in human cerebrospinal fluid (CSF), CSF levels of beta-endorphin-like immunoreactivity (beta-EP-LI) in various diseases were determined by a specific radioimmunoassay and compared with simultaneously determined ACTH-like immunoreactivity (ACTH-LI) levels in CSF. CSF beta-EP-LI and ACTH-LI in the control group, consisting of 5 normal subjects and 19 patients with nonendocrine diseases, were 22.2+/-1.3 and 14.6+/-0.4 fmol/ml, respectively. CSF levels of these peptides in patients with schizophrenia (n = 19) and acromegaly (n = 10) were not significantly different from those in the control group. Patients with Cushing's disease (n = 7) had significantly lower CSF beta-EP-LI and ACTH-LI levels than those in the control group. Four of them showed a parallel increase in CSF beta-EP-LI and CSF ACTH-LI levels after the complete removal of pituitary microadenomas (P < 0.05). Gel chromatography of CSF beta-EP-LI from a normal volunteer, a control patient, and one patient each with catatonia, Nelson's syndrome, Cushing's syndrome (adrenal adenoma), and acromegaly gave similar patterns consisting of three peaks with the elution positions comparable to those of authentic beta-endorphin, beta-lipotropin, and possibly their precursor molecule. Gel chromatographic patterns of CSF beta-EP-LI and ACTH-LI were compared in a normal volunteer. The first peaks of beta-EP-LI and ACTH-LI eluted at the same position and the second peak of ACTH-LI coincided with the elution position of authentic ACTH.CSF beta-EP-LI and ACTH-LI levels determined every 5 min over a period of 80 min in three normal volunteers did not show moment-to-moment variability.A significant correlation (r = 0.75, P < 0.001) was seen between CSF beta-EP-LI and ACTH-LI levels in normal subjects and patients studied (n = 73). This suggests that beta-endorphin and ACTH in human CSF share the common regulatory mechanism in normal and pathologic conditions.

The protein-coding sequence of the bovine ACTH-beta-LPH precursor gene is split near the signal peptide region

The pituitary hormones corticotropin (ACTH) and beta-lipotropin (beta-LPH) are formed from a large common precursor. Recently, we have elucidated the whole primary structure of the bovine ACTH-beta-LPH precursor (designated alternatively as preproopiocortin) by determining the nucleotide sequence of cloned DNA complementary to the mRNA coding for the precursor protein. The amino acid sequence assigned has disclosed a characteristic repetitive structure of the ACTH-beta-LPH precursor. The repetitive units of the precursor protein each contain a melanotropin (MSH) sequence (alpha-, beta- or gamma-MSH) as well as other peptide components such as beta-endorphin and corticotropin-like intermediate lobe peptide (CLIP). The repetitive units as well as their peptide components are each bounded by paired basic amino acid residues, which apparently represent the sites of proteolytic processing. Several studies have confirmed the translational initiation site and protein structure assigned (see also ref. 11 and refs therein). In view of the recent knowledge about the organization of eukaryotic genes (see refs 12, 13 for reviews), it would be of particular interest to investigate the relationship between the repetitive structure of the ACTH-beta-LPH precursor containing different functional components and the arrangement of the protein-coding sequence in its gene. We have now isolated and characterized bovine genomic DNA fragments encoding this precursor protein and have demonstrated that the protein sequence is encoded by two non-consecutive DNA segments. An intron (intervening sequence) of approximately 2.2 kilobase pairs separates the smaller exon (mRNA-coding sequence), which contains the gene sequence encoding the signal peptide, from the larger exon, which contains the gene sequence for most of the protein structure, including the known biologically active component peptides.

Ectopic hormone syndromes

Ectopic hormone production is not always associated with endocrine manifestations and if extensive studies of tumour hormones are made it is found that asymptomatic hormone production is often concomitant with neoplasms. The nature of tumour hormones seems essentially indistinguishable from that of native hormones, although there exists in some cases an abundance of precursor or hormone fragments and unbalanced biosynthesis of subunits. Production of multiple hormones by a tumour is not uncommon. These facts may suggest abnormal gene expression as the basic mechanism responsible for ectopic hormone production. During the process of cellular differentiation, most of the genes are inactivated. Neoplastic transformation may activate repressed genes, thus producing hormones that are not produced by differentiated cells (derepression hypothesis). This hypothesis, however, makes it difficult to explain the higher incidence of ACTH-LPH-producing tumours among APUD tumours. Some investigators have postulated that only APUD tumours elaborate ACTH-LPH or other APUD hormones (neuro-endocrine cell hypothesis). However, there have been reported some definite non-APUD tumours which elaborate ACTH-LPH. These facts can be explained by the stepwise, irreversible repression hypothesis of cellular differentiation. In APUD cells, the gene for ACTH-LPH coding may be repressed at the terminal stage of differentiation and may, therefore, be very easily derepressed by neoplastic transformation. On the other hand, the ACTH-LPH gene may be repressed at a relatively early stage in non-APUD cells and be difficult to reactivate even after neoplastic transformation. Further studies on ectopic hormone producing tumours may clarify the mechanism of ectopic hormone production and yield new insights into the fundamental process of malignant change.

'gamma-MSH' fragments from ACTH-beta-LPH precursor have an affinity for opiate receptors

gamma-Melanotropin (gamma-MSH), a putative peptide residing in the cryptic N-terminal portion of ACTH-beta-LPH precursor, shares several amino acid residues with alpha-MSH or beta-MSH. The present study revealed that gamma-MSH and structurally related peptides had as potent an affinity for rat brain opiate receptors as did ACTH1-24 when 3H-naloxone was used as a ligand. Thus, gamma-MSH and structurally related peptides may have physiological significance in the activities of the CNS.