In vitro biosynthesis and chemical characterization of beta-lipotropin, gamma-lipotropin, and beta-endorphin in rat pars intermedia

My road to Damascus: how I converted to the prohormone theory and the proprotein convertases

My desire as a young endocrinologist to improve my clinical skills through a better knowledge of hormone chemistry led me to serendipitous discoveries and unexpected horizons. The first discovery, published in 1967, revealed that peptide hormones are derived from endoproteolytic cleavages of larger precursor polypeptides. It was the foundation of the prohormone theory. Initially thought to apply to a few hormones, the theory rapidly extended to many proteins, including neuropeptides, neurotrophins, growth and transcription factors, receptors, extracellular matrix proteins, bacterial toxins, and viral glycoproteins. Its endoproteolytic activation mechanism has become a fundamental cellular process, affecting many biological functions. It implied the existence of specific endoproteolytic enzymes. These proprotein convertases were discovered in 1990. They have been shown to play a wide range of important roles in health and disease. They have opened up novel therapeutic avenues. Inactivation of PCSK9 to reduce plasma cholesterol is currently the most promising. To make this good thing even better, I recently discovered in a French Canadian family a potent PCSK9 (Gln152His) mutation that significantly lowers plasma cholesterol and should confer cardiovascular longevity. The discovery helped me to complete the loop: "From the bedside to the bench and back to the bedside."

Beta-endorphin-related peptides in the pituitary gland: isolation, identification and distribution

A new procedure is described for isolation of four forms of beta-endorphin from bovine pituitary. The four peptides are: the C-fragment of lipotropin (bovine lipotropin residues 63-93, or beta-endorphin, the alpha, N-acetyl derivative of the C-fragment, the C'-fragment (bovine lipotropin residues 63-89) and the alpha, N-acetyl derivative of the C'-fragment. Of these peptides, beta-endorphin alone possesses potent analgesic activity. The procedure has been applied in studying the distribution of beta-endorphin-related peptides in two regions of the pituitary. The results show that in the anterior pituitary of the pig and the rat, beta-endorphin is produced with a high degree of specificity in its opiate active form. In contrast, in the pars intermedia of both species at least six peptides related to beta-endorphin are elaborated and beta-endorphin represents only a minor component. The principal peptides in the pars intermedia have been identified as acetylated derivatives of lipotropin C'-fragment: in the pig the predominant peptide is alpha,N-acetyl C'-fragment and in the rat the major peptide appears to be an epsilon-acetylated derivative of alpha,N-acetyl C'-fragment. Thus, beta-endorphin is activated in the anterior pituitary and inactivated in the pars intermediate. The results demonstrate selective and specific processing of the 31K ACTH-endorphin prohormone in the different regions of the pituitary. In the anterior pituitary two biologically active peptides, ACTH and beta-endorphin, are generated together; in the pars intermedia alpha-melanotropin (alpha-MSH) is accompanied by forms of beta-endorphin that have been inactivated by acetylation and proteolysis.

Isolation and structure-bioactivity characterization of glycosylated N-pro-opiomelanocortin isoforms

The N-terminal fragment of mouse pro-opiomelanocortin (N-POMC) was isolated from AtT-20 cell-conditioned medium on the basis of immunoreactivity to an anti-POMC1-50 monoclonal antibody by a concentration step, a cation exchange step, reversed phase high-performance liquid chromatography (HPLC) and size exclusion HPLC. Two groups of N-POMC isoforms with a molecular weight (MW) of approximately 11 kDa and 13 kDa, respectively, were identified by mass spectrometry and N-terminal amino acid sequencing. C-terminal sequencing indicated that 11 kDa isoforms correspond to POMC1-74 and 13 kDa isoforms to POMC1-95. Isoforms from both groups enhanced the prolactin mRNA content (measured by means of TaqMan real-time reverse transcription-polymerase chain reaction) in cultured rat pituitary cell aggregates in a dose-dependent manner, but not all of them showed this activity. POMC1-74 compounds were significantly more potent than POMC1-95 isoforms. The observed effects were abolished by coincubation with the monoclonal anti-POMC1-50 antibody, showing the specificity of this biological action. Incorporation of bromodeoxyuridine into DNA of immunostained lactotrophs was enhanced by only a minor part of the isoforms. Some of these had no effect on prolactin mRNA expression. The N-POMC isoforms appeared to be N- and at least in part O-glycosylated. After enzymatic N-deglycosylation of selected N-POMC isoforms, the stimulatory effect on the prolactin mRNA level was depressed (in case of the POMC1-95 isoforms) or totally abolished (in case of the POMC1-74 isoforms). The present findings show that N-POMC is a mixture of differentially glycosylated isoforms, that the isoforms of POMC1-74 are the biologically more effective forms and that different isoforms induce different biological responses in the same cell population. The data also show the essential role of N-glycosylation in the biological response.

PC1 and PC2 are proprotein convertases capable of cleaving proopiomelanocortin at distinct pairs of basic residues

A recombinant vaccinia virus vector was used to coexpress the two candidate mouse prohormone convertases, PC1 and PC2, together with mouse proopiomelanocortin (POMC) in the constitutively secreting cell line BSC-40 and in the endocrine tissue-derived cell lines PC12 and AtT-20, which exhibit regulated secretion. Monitoring of POMC processing demonstrated the distinct cleavage specificities of PC1 and PC2, since in the cell lines analyzed (i) PC1 cleaves POMC into corticotropin and beta-lipotropin, (ii) PC2 cleaves POMC into beta-endorphin, an N-terminally extended corticotropin containing the joining peptide, and either alpha MSH or desacetyl-alpha MSH, and (iii) PC2 cleaves POMC at the five pairs of basic residues analyzed, whereas PC1 cleaves two of them preferentially, suggesting that PC2 has a broader spectrum of activity than PC1. These data are consistent with our hypothesis on the physiological role of PC1 and PC2 as distinct proprotein convertases acting alone or together to produce a set of tissue-specific maturation products in the brain and in peripheral tissues.

Presence of arginine-vasopressin in bovine pituitary intraglandular colloid of intermediate lobe origin

In order to determine whether pituitary intraglandular colloid might contain enough hormonal material to be considered a transport medium for intermediate lobe peptides, arginine-vasopressin was assayed in three pools of colloid collected from nearly 100 steers. Colloid samples taken from the pituitary glands of freshly slaughtered cattle were pooled, freed of extraneous tissue, and lyophilized. Three such pools were further extracted on octadecylsilyl-silica and assayed. The radioimmunoassay showed that arginine-vasopressin was indeed present in the bovine intraglandular colloid at levels ranging from 0.18 to 6.95 ng/mg dry weight. These results indicate that the intraglandular colloid contains a sufficient amount of arginine-vasopressin to be considered as a transport medium for this and other intermediate lobe peptides.

Enzymatic maturation of pro-opiomelanocortin by anterior pituitary granules. Methodological approach leading to definite characterization of cleavage sites by means of high-performance liquid chromatography and microsequencing

The coupling of high-performance liquid chromatography, gel-permeation, and reversed-phase chromatography with microsequencing proved to be advantageous for the unambiguous determination of specific sites in pro-opiomelanocortin, cleaved by secretory granule lysates of pig anterior pituitary. This system allows the unambiguous identification of a major chymotrypsin-like enzyme activity, optimal at pH 8.0, in the granule preparation, with a specificity directed towards some Phe decreases X and Tyr decreases X cleavage sites. The approach used emphasizes the necessity to use methodologies leading to the unambiguous determination of conversion activities.

Enzymatic cleavage of pro-opiomelanocortin by anterior pituitary granules. Evidence for a chymotryptic-like enzyme

This paper presents the results obtained when pig anterior pituitary granule lysates are incubated with rat pro-opiomelanocortin (POMC). The resultant peptides were analyzed by 3 systems of high-performance liquid chromatography. This approach, when coupled with microsequence analysis of the conversion products, allowed the unambiguous identification of a major chymotryptic-like activity (pH optimum around 8) associated with the granule lysates with a specificity directed towards selective Tyr-X and Phe-X bonds within the POMC molecule. Furthermore, these results also demonstrate that although the detected enzyme activity gives rise to products 'resembling' those expected, the characterization of the 'elusive' maturation enzyme responsible for the cleavage at pairs of basic residues remain to be critically evaluated.

Primary sequence of two regions of mouse pro-adrenocorticotropin/endorphin

The amino acid sequences of two previously uncharacterized regions of the mouse anterior pituitary common precursor to adrenocorticotropin (ACTH) and beta-endorphin (pro-ACTH/endorphin) were determined. Portions of the NH2-terminal region of pro-ACTH/endorphin (called the 16K fragment) and the region between ACTH and beta-endorphin (called gamma-lipotropin) were sequenced by Edman degradations of biosynthetically labeled immunoprecipitated proteins and by Edman degradations of purified 16K fragment and beta-lipotropin. With a combination of these two approaches, 29 of the first 34 residues at the NH2-terminal end of the mouse 16K fragment were determined. The NH2-terminal region of the mouse 16K fragment was found to be nearly identical with the homologous porcine and bovine molecules. The complete amino acid sequence of the NH2-terminal region of gamma-lipotropin was determined. In contrast to the highly conserved nature of the 16K fragment, mouse gamma-lipotropin was found to differ substantially from the gamma-lipotropins of other species. Although the NH2-terminal and beta-melanotropin-like regions of the mouse gamma-lipotropin are similar to the corresponding regions of other gamma-lipotropins, the intervening region of mouse gamma-lipotropin is substantially shorter than it is in other gamma-lipotropins. In addition, mouse gamma-lipotropin lacks the pair of basic amino acids that normally mark the proteolytic cleavage site used to produce beta-melanotropin from gamma-lipotropin.

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.

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.

beta-Endorphin: possible involvement in the antihypertensive effect of central alpha-receptor activation

Clonidine and L-alpha-methylnoradrenaline (but not D-alpha-methylnoradrenaline) increase the release of a substance with beta-endorphin immunoreactivity from slices of brainstem of spontaneously hypertensive rats, but not that of normotensive rats. It was reported earlier that opiate antagonists inhibit the hypotensive action of clonidine and alpha-methyldopa in spontaneously hypertensive but not in normotensive rats and that beta-endorphin has hypotensive effects of its own. Together, these findings indicate that release of beta-endorphin by central alpha-receptor agonists may contribute to the antihypertensive action of these drugs.

Rat beta-LPH, gamma-LPH and beta-endorphin biosynthesized by isolated cells of pars intermedia and pars distalis. Further characterization

Rat pars intermedia cells were incubated for 3 h with the following amino acids: (a) 35S-methionine and 3H-phenylalanine, (b) 3H-valine; and (c) 3H-valine and 3H-lysine. Radioactive gamma-lipotropin, beta-lipotropin and beta-endorphin were purified on carboxymethyl-cellulose and characterized by polyacrylamide disc gel electrophoresis at pH 4.5, molecular weight estimation and micro-sequencing. Rat gamma-lipotropin was shown to differ slightly from ovine gamma-lipotropin in its NH2-terminal amino acid sequence, in containing no methionine and having phenylalanine at position 6, valine at positions 13 and 27, and lysine at position 20. The same variations were observed in the sequence of rat beta-lipotropin, while rat beta-endorphin was shown to be identical to the ovine beta-endorphin. Following a 3-h pulse of rat pars distalis, the cells were extracted with care to avoid beta-lipotropin degradation by proteolytic enzymes, A peptide was purified and identified to be rat beta-endorphin, thus demonstrating that beta-endorphin is biosynthesized in pars distalis and is not an extraction artifact.

Reversed-phase high-performance liquid chromatographic purification and characterization of the adrenocorticotropin/lipotropin precursor and its fragments

Methods are presented for the effective purification by reversed-phase high-performance liquid chromatography (HPLC) of rat adrenocorticotropin/lipotropin (ACTH/LPH) precursor and its two glycosylated forms. Purification of its NH2-terminal segment from human and porcine pituitaries is presented together with microsequencing data confirming the identity of the purified peptides. The effective separation of various native fragments related to ACTH and beta-LPH from sheep pituitaries is presented. A new putative gamma-MSH hormone has been synthesized and purified by reversed-phase HPLC and tryptic peptide mapping performed to establish the identity of the purified peptide.

Biosynthesis of precursor corticotropin/endorphin-, corticotropin-, alpha-melanotropin-, beta-lipotropin-, and beta-endorphin-like material by cultured neonatal rat hypothalamic neurons

Enzymatically dispersed hypothalamic cells derived from 15-day-old female rats were maintained in tissue culture for 4 days and then incubated for 2 hr in the presence of [35S]methionine. After such incubation, cell extracts contained multiple forms of 35S-labeled products that were specifically bound by immobilized affinity-purified antisera to corticotropin and beta-endorphin. Sequential use of these immobilized antisera revealed two molecular species (apparent Mr of 33,000 and 36,500) that contained both corticotropin and beta-endorphin antigenic determinants within the same molecule. Substances containing only one of these determinants were also present and co-eluted with corticotropin, alpha-melanotropin, beta-lipotropin, or beta-endorphin upon Sephadex G-50 gel filtration. Extracts of similarly labeled rat anterior and intermediate pituitary lobe cells contained two forms of the common precursor molecule corresponding to the same molecular weights estimated for the hypothalamic material and similar to it with respect to other physicochemical parameters. These data suggest that rat hypothalamus synthesizes corticotropin-related and beta-endorphin-related products via sequential cleavage of a larger common precursor molecule in a manner similar to the processing pathway demonstrated for the intermediate lobe of the pituitary.

Neurons of the rat hypothalamus reactive with antisera against endorphins, ACTH, MSH and beta-LPH

In rat brains intraventricularly injected with colchicine, the same discrete neurons of the arcuate and ventromedial nuclei can be stained with antisera against alpha- and beta-endorphins, (1-24)ACTH, (17-39)ACTH, alpha- and beta-MSH, and beta-LPH, as demonstrated by comparative studies in consecutive serial sections. These neurons are strongly reactive with anti-(17-39)ACTH, anti-beta-endorphin, anti-alpha-MSH and anti-beta-MSH, and more faintly stained with anti-alpha-endorphin, anti-beta-LPH and anti-(1-24)ACTH. Exceptionally, neurons reactive with anti-(17-39)ACTH and anti-beta-endorphin are poorly stained or completely negative with anti-alpha-MSH and anti-beta-MSH. Immunoreactive fibers end in the lateral median eminence and in the arcuate nucleus proper, or form ascending pathways along the third ventricle. Comparative studies with other antisera or with the Falck and Hillarp technique show that these neurons differ from the elements producing LH-RH, somatostatin, neurophysin, oxytocin, vasopressin and dopamine. These results suggest that the same neurons of the rat hypothalamus synthesize several neuropeptides identical with or immunologically related to endorphins, ACTH, alpha-MSH and beta-LPH, probably arising from a common precursor molecule similar to that found in the corticotropic cells of the pituitary. These neuropeptides of a common cellular and molecular origin might be involved in basic processes of the central nervous system as neurotramsmitters or neuromodulators.

Action of cathepsin D on human beta-lipotropin: a possible source of human "beta-melanotropin"

Highly purified calf brain cathepsin D (EC 3.4.23.5) selectively splits the Leu77-Phe78 and Ala36-Ala37 peptide bonds of human beta-lipotropin. It is suggested that the formation of human "beta-melanotropin" from gamma-lipotropin, and that of gamma-endorphin from beta-endorphin, is due to the action of cathepsin D during isolation procedures.

Distribution of active and inactive forms of endorphins in rat pituitary and brain

The recent isolation and identification of alpha-N-acetyl forms of the C-Fragment of lipotropin (beta-endorphin, residues 61-91) and the C'-Fragment (residues 61-87) [Smyth, D.G., Massey, D.E., Zakarian, S. & Finnie, M. (1979) Nature (London) 279, 252-254] has led to a study of their distribution in the pituitary and brain of the rat. Regions were mapped by the method of immunofluorescent staining and the reactive peptides were determined by immunoassay after extraction, gel filtration, and ion exchange chromatography. The major immunoreactive peptides in both lobes of the pituitary were found to be C'-Fragment and N-acetyl C'-Fragment, which are weakly active or inactive as opiates; the C-Fragment and its N-acetyl derivative represented minor components. This indicates that in the rat the circulating "endorphins" released from pituitary would have little morphinomimetic activity. The same four immunoreactive peptides were observed in rat brain. In the hippocampus the C'-Fragment was the principal component in the midbrain there was more C-Fragment but C'-Fragment predominated; in the hypothalamus the C-Fragment was the major peptide, almost to the exclusion of the other peptides. The results demonstrate that the processing of lipotropin is under differential control in anatomically distinct regions of the central nervous system. The processing of lipotropin in the hypothalamus is directed specifically to the production of lipotropin C-Fragment.

Concomitant synthesis of beta-endorphin and alpha-melanotropin from two forms of pro-opiomelanocortin in the rat pars intermedia

In the pars intermedia of rat pituitary glands, two forms of a common precursor for corticotropin (ACTH) and beta-lipotropin with apparent molecular weights of 34,000 and 36,000 were resolved by sodium dodecyl sulfate/acrylamide gradient slab gel electrophoresis. High-performance liquid chromatographic analysis of [35S]methionine-labeled tryptic fragments of the two forms of the precursor revealed that both contained copies of ACTH-(1-8) and beta-lipotropin-(61-69) sequences. When biosynthetic studies were performed in the presence of tunicamycin, the 34,000- and 36,000-dalton forms were replaced by a peptide with an apparent molecular weight of 32,000. It was therefore concluded that the 34,000- and 36,000-dalton forms of the precursor represent two glycoprotein variants of similar polypeptides, differing in the number of asparagine-linked carbohydrate moieties. During pulse-chase incubations with [35S]methionine, the precursor forms were cleaved into two major groups of labeled products: (i) beta-endorphin and (ii) a mixture of ACTH fragments closely related to alpha-melanotropin. No ACTH-(1-39) was found at the end of a 2-hr chase period, suggesting that ACTH is not a significant hormone product of the rat pars intermedia.

Localization of corticotropin- and endorphin-related peptides in the intermediate lobe of the rat pituitary

The question is examined whether alpha-melanocyte stimulating hormone (alpha-MSH), adrenocorticotropic hormone (ACTH), met-enkephalin and beta-endorphin are detectable by enzyme immunocytochemistry in the cells of the intermediate lobe (PI) of the rat pituitary. By applying antibodies against alpha-MSH, ACTH and beta-endorphin on light microscopic sections, intense immunostaining was found in all PI-cells. At the ultrastructural level, after treatment of consecutive serial sections with these three antibodies the immunoreactivity was localized in the same secretory granules. No specific metenkephalin immunoreactivity could be detected in the cells of the intermediate lobe.

Beta-lipotropin and endorphins: implications of current knowledge

Evidence that the endorphins, metabolites of the pituitary hormone beta-lipotropin, profoundly affect mood and behavior, are secreted with ACTH in response to stress, and may act as neurotransmitters in the GI tract, suggests that they play an important role in CNS homeostasis. The fact that these peptides are synthesized in the GI tract as well as the brain casts new light on CNS-endocrine-environmental interrelationships.

Biosynthesis of beta-endorphin from beta-lipotropin and a larger molecular weight precursor in rat pars intermedia

When isolated rat pars intermedia cells were incubated for 10 min with radioactive amino acids, one major labeled protein with a molecular weight of 30,000 +/- 1500 was extracted. This protein was shown to contain in its sequence the antigenic determinants for corticotropin and beta-melanotropin by immunoprecipitation. When the radioactivity incorporated into this large molecular weight protein during the first 10 min was chased by a further incubation in presence of an excess of unlabeled amino acid, the initial protein was degraded into several smaller peptides including beta-endorphin and beta-lipotropin. Another 18,000-dalton peptide was also observed and was tentatively identified as a large molecular form of corticotropin. From the kinetics of the maturation of the initial precursor, it is concluded that the initial cleavage of the 30,000-dalton peptide gives rise to beta-lipotropin and the 18,000-dalton form of corticotropin. beta-Lipotropin is subsequently cleaved to form beta-endorphin.