Biosynthesis of proTRH-derived peptides in prohormone convertase 1 and 2 knockout mice

Prohormone convertases (PCs) 1 and 2 are the primary endoproteases involved in the post-translational processing of proThyrotropin Releasing Hormone (proTRH) to give rise to TRH and other proposed biologically active non-TRH peptides. Previous evidence suggests that PC1 is responsible for most proTRH cleavage events. Here, we used the PC1 and PC2 knockout (KO) mouse models to examine the effects of PC1 or PC2 loss on proTRH processing. The PC1KO mouse presented a decrease in five proTRH-derived peptides, whereas the PC2KO mouse showed only lesser reduction in three TRH (Gln-His-Pro), TRH-Gly (Gln-His-Pro-Gly), and the short forms preproTRH(178-184) (pFQ(7)) and preproTRH(186-199) (pSE(14)) of pFE(22) (preproTRH(178-199)). Also, PC1KO and not PC2KO showed a decrease in pEH(24) indicating that PC1 is more important in generating this peptide in the mouse, which differs from previous studies using rat proTRH. Furthermore, downstream effects on thyroid hormone levels were evident in PC1KO mice, but not PC2KO mice suggesting that PC1 plays the more critical role in producing bioactive hypophysiotropic TRH. Yet loss of PC1 did not abolish TRH entirely indicating a complementary action for both enzymes in the normal processing of proTRH. We also show that PC2 alone is responsible for catalyzing the conversion of pFE(22) to pFQ(7) and pSE(14), all peptides implicated in regulation of suckling-induced prolactin release. Collectively, results characterize the specific roles of PC1 and PC2 in proTRH processing in vivo.     

Specific processing of the thyrotropin-releasing prohormone in rat brain and spinal cord

Thyrotropin-releasing hormone (TRH) and TRH extended peptides were extracted from rat hypothalamus and spinal cord and resolved by gel exclusion chromatography under dissociating conditions. Peptides related to TRH were detected by trypsin digestion and radioimmunoassay with an antibody to TRH or an antibody raised against the pentapeptide Glp-His-Pro-Gly-Lys. In addition to the tripeptide hormone a series of C-terminally extended forms of TRH was shown to occur in both tissues; no N-terminally extended peptides were detected. The structure of the TRH-related peptides was confirmed by chromatographic identification of the N-terminal pentapeptide sequence released by trypsin. The TRH extended peptides, which accounted for 15-20% of the total TRH, were present in three groups of different molecular size corresponding to predicted fragments of the TRH prohormone. One of the peptides in the spinal cord was identified by chromatographic comparison with a synthetic 16-residue peptide representing residues 154-169 of the prohormone. In the spinal cord the TRH extended peptides differed in their relative concentrations from the corresponding peptides in the hypothalamus, possibly reflecting differences in processing. The finding of extended forms of TRH in which the extension occurs only on the C-terminal side of the hormone sequence shows that the prohormone undergoes highly specific processing.     

Identification, characterization and localization of thyrotropin-releasing hormone precursor peptides in perinatal rat pancreas

The sequence of rat hypothalamic prepro TRH, deduced from its complementary DNA, contains five TRH progenitor sequences and six cryptic sequences separated by paired basic amino acid residues. We have utilised antisera against two synthetic peptides corresponding to sequences within proTRH, [Tyr53] preproTRH (53-74), part of the amino terminal leader sequence of proTRH and [Cys 74,83] preproTRH-(75-82), representing a TRH progenitor sequence flanked by cysteine residues (pCC10) in radioimmunoassays (RIA) to identify and chromatographically characterize proTRH derived peptides in extracts of rat perinatal pancreas and to localize these peptides immunohistochemically. Two forms of immunoreactive pYT22 (ipYT22) were observed, similar in size to ipYT22 seen in extracts of adult rat brain. By RIA immunoreactive pCC10 was detectable in neonatal but not fetal pancreas. However, immunohistochemical double staining of both fetal and neonatal rat pancreas colocalized both ipYT22 and ipCC10 with immunoreactive insulin in the B-cell of the developing Islets of Langerhans. These findings indicate that the B-cell of the perinatal pancreas synthesizes TRH from a prohormone encoded by a mRNA similar to that present in adult rat hypothalamus.     

Processing of thyrotropin-releasing hormone prohormone (pro-TRH) generates pro-TRH-connecting peptides. Identification and characterization of prepro-TRH-(160-169) and prepro-TRH-(178-199) in the rat nervous system

Rat thyrotropin-releasing hormone prohormone (pro-TRH) contains five separate copies of the TRH progenitor sequence: Gln-His-Pro-Gly. Each of the five sequences is flanked by pairs of basic residues and linked together by one of several predicted connecting sequences. Two of the pro-TRH-connecting peptides, prepro-TRH-(160-169) and prepro-TRH-(178-199), were detected in extracts of rat neural tissues by radioimmunoassay using antibodies directed against the corresponding synthetic probes. Endogenous prepro-TRH-(160-169) and prepro-TRH-(178-199) were purified by gel exclusion chromatography, reverse-phase high pressure liquid chromatography, and ion-exchange chromatography. Structural identification of each peptide was achieved by chromatographic comparison with synthetic standards, immunological analysis, and tryptic mapping. Equimolar amounts of these connecting fragments were observed in hypothalamus and spinal cord. Quantification of TRH in spinal cord and hypothalamus extracts revealed the presence of 4.9-6.3 mol of TRH/mol of prepro-TRH-(178-199) and 4.4-6 mol of TRH/mol of prepro-TRH-(160-169), respectively. By using the indirect immunofluorescence technique, prepro-TRH-(178-199) immunoreactive cell bodies were found in the paraventricular nucleus of the hypothalamus, and a dense plexus of immunopositive nerve terminals was observed in the external zone of the median eminence, in a distribution similar to that described for TRH. These studies demonstrate that prepro-TRH-(160-169) and prepro-TRH-(178-199) are, together with TRH, predominant storage forms of the TRH precursor in hypothalamus and spinal cord, being present in molar ratios corresponding to those expected for a nearly complete processing of the prohormone molecule. The presence of pro-TRH-connecting peptides in various brain regions, including the median eminence, suggests that these peptides might act as neuromodulators in the central nervous system and/or neuroendocrine signals at the pituitary level. In the olfactory lobes, prepro-TRH is processed differently since a C-terminally extended form of TRH, prepro-TRH-(172-199), is found as a major end product along with lower but significant amounts of prepro-TRH-(178-199) and prepro-TRH-(160-169). The striking difference in pro-TRH processing patterns among the various tissues examined suggests differential regulating mechanisms for TRH and/or TRH-related activities.     

Processing of the thyrotropin releasing hormone (TRH) precursor in Xenopus skin and bovine hypothalamus: evidence for the existence of extended forms of TRH

Acid extracts of Xenopus laevis skin were fractionated by gel filtration on Sephadex G50 ion-exchange chromatography and reverse-phase high performance liquid chromatography (HPLC). Peptides related to thyrotropin releasing hormone (TRH) were identified in the eluted fractions by trypsin digestion and radioimmunoassay (RIA) using antibodies to the TRH tripeptide pGlu-His-Pro amide or to a TRH-related pentapeptide pGlu-His-Pro-Gly-Lys. In addition to the tripeptide hormone, evidence was obtained for the presence of peptides containing 10-20 amino acid residues which were extended on the NH2-terminal or COOH-terminal side of TRH. The peptides extending on the NH2-terminal side predominated and were shown to comprise 5 components present in differing concentrations, indicating that the processing sites in the TRH prohormone vary in their susceptibility to proteolysis. Evidence was also obtained for the presence of small amounts of the TRH-related pentapeptide pGlu-His-Pro-Gly-Lys. Using similar procedures it was demonstrated that TRH extended peptides were present in bovine hypothalamus. In this species the peptides extended at the NH2-terminus of TRH occurred in similar concentrations to the peptides extended at the COOH-terminus. The results show that processing of the TRH prohormone in Xenopus and ox leads to the formation of peptides intermediate in size between the prohormone and the tripeptide amide; the TRH extended peptides occur in significant quantity and in Xenopus are formed with a high degree of specificity.     

Thyrotropin-releasing hormone gene expression in normal thyroid parafollicular cells

The rat TRH gene encodes a 255-amino-acid precursor polypeptide, preproTRH, containing five copies of TRH and seven non-TRH peptides. Expression of this gene is well documented in the central nervous system, particularly in the hypothalamus. Thyroids also contain TRH immunoreactivity, but it is unknown whether this immunoreactivity results from expression of the TRH gene or from other genes encoding TRH-like products. Since the CA77 neoplastic parafollicular cell line expresses the TRH gene, we investigated whether TRH gene expression also occurs in normal thyroid parafollicular cells. Northern analysis of total thyroid RNA with a preproTRH-specific RNA probe identified a single hybridizing band the same size as authentic TRH mRNA found in hypothalamus and CA77 cells. Gel filtration analysis of thyroid extracts identified the same 7-kilodalton and 3-kilodalton species of immunoreactive preproTRH53-74 previously identified in hypothalamus and CA77 cells. Immunoreactive preproTRH115-151, not previously identified, was found in all three tissues. Part of this immunoreactivity comigrated with the synthetic preproTRH115-151 standard on gel filtration and reversed-phase HPLC. PreproTRH53-74 was localized to thyroid parafollicular cells by immunostaining. These findings demonstrate authentic TRH gene expression by normal rat thyroid parafollicular cells and establish the CA77 cell line as the only model system of a normal TRH-producing tissue. In addition to expanding the range of neuroendocrine peptides known to be produced by parafollicular cells, these results also suggest a potential paracrine regulatory role for TRH gene products within the thyroid.     

Tissue distribution and processing of proSAAS by proprotein convertases

The conversion of inactive precursor proteins into bioactive neuropeptides and peptide hormones involves regulated secretory proteins such as prohormone convertases PC1 and PC2. The neuroendocrine protein 7B2 represents a specific binding protein for PC2, and the protein proSAAS, which interacts with PC1, exhibits certain structural and functional homologies with 7B2. With the intention of better understanding the physiological role of proSAAS and its derived peptides, we investigated its tissue localization using a new radioimmunoassay (RIA) to a C-terminal proSAAS-derived peptide. Immunoreactivity corresponding to this SAAS-derived peptide is mostly localized to the brain and gut. Analysis of the brain distribution of the proSAAS-derived peptides indicates that the hypothalamus and pituitary are the two richest areas, consistent with the previously described high expression of PC1 in these two areas. In order to investigate the cleavage of proSAAS by prohormone convertases, we incubated recombinant His-tagged proSAAS with recombinant mouse proPC2 or furin, separated the cleavage products using high-pressure gel permeation chromatography and analyzed the products by RIA. Our results indicate that either PC2 or furin can accomplish in vitro rapid removal and efficient internal processing of the C-terminal peptide, exposing the inhibitory hexapeptide to possible further digestion by carboxypeptidases. Finally, we also studied proSAAS processing in the brains of wild-type and PC2 null mice and found that proSAAS is efficiently processed in vivo. Whereas the C-terminal peptide is mostly internally cleaved in wild-type mouse brain, it is not processed as efficiently in the brain of PC2 null mice, suggesting that PC2 is partially responsible for this cleavage in vivo.     

TRH-extended peptides in the olfactory lobe are formed by incomplete cleavage at pairs of arginine residues in the TRH prohormone

High concentrations of thyrotrophin-releasing hormone (TRH) are known to be present in the olfactory lobe, and the processing of the TRH prohormone in this region of the brain has been examined in this study. TRH-extended peptides have been detected in the rat olfactory lobe: these peptides accounted for approximately 11% of the total TRH immunoreactivity present in the tissue and contained the sequence pGlu-His-Pro-Gly-Arg exclusively at their N-termini. Extended peptides containing pGlu-His-Pro-Gly-Lys at their N-termini were not detected suggesting that incomplete cleavage occurs only at Arg-Arg residues in the TRH-prohormone. In view of the highly specific processing of the prohormone, it is likely that the TRH-extended peptides play important physiological roles.     

Gonadal steroid and chronic morphine treatment do not change the posttranslational processing of beta-endorphin in the rat brain

The present study examines whether two treatments known to induce refractoriness to exogenous morphine produce this desensitization through a change in the posttranslational processing of brain beta-endorphin (beta-End). The first experiment examined whether an ovarian steroid regimen which produces a transient desensitization of brain opiate receptor mechanisms alters beta-End processing in the preoptic area (POA), medial basal hypothalamus (MBH), and brainstem (BS). The second experiment monitored the effects of morphine pellet treatment, known to produce morphine dependency, on immunoreactive beta-End forms in the hypothalamus and periaquaductal gray area of the midbrain (PAG). The individual molecular forms of beta-End were separated using ion exchange chromatography and collection fractions were quantitated for beta-End immunoreactivity by RIA. The results show that regional differences occur in the posttranslational processing of beta-End. In the hypothalamus, MBH and POA, beta-End-(1-31) and its non-acetylated C-terminal cleavage products, beta-End-(1-27) and beta-End-(1-16) were the predominant forms of beta-End. The PAG pools produced a beta-End peptide elution profile similar to the hypothalamus, although small amounts of N-acetyl-beta-End-(1-31) were also identified. The BS exhibited the least posttranslational processing of beta-End; beta-End-(1-31) was the primary product with smaller amounts of beta-End-(1-27) and beta-End-(1-26) observed. However, neither ovarian steroid treatment nor chronic morphine produced any changes in posttranslational processing of beta-End or in total beta-End concentration in any of the brain regions examined in these experiments. These data indicate that the refractoriness or tolerance to exogenous morphine associated with steroid or chronic morphine treatment cannot be explained by alterations in the biological activity of beta-End resulting from the differential regulation of its posttranslational processing products.     

Cellular colocalization and coregulation between hypothalamic pro-TRH and prohormone convertases in hypothyroidism

The prohormone convertases (PCs), PC1/3 and PC2, are involved in the tissue-specific endoproteolytic posttranslational processing of many hormonal precursors within the secretory pathway. One important prohormone, pro-thyrotropin-releasing hormone (TRH), is expressed in both hypophysiotropic (where it regulates the secretion of thyroid-stimulating hormone) and nonhypophysiotropic regions of the brain. Pro-TRH is processed at specific sites in the secretory pathway, primarily by PC1/3 followed by PC2. We hypothesized that thyroid hormone status in specific nuclei of the brain would alter pro-TRH processing by inducing changes in PC1/3 and PC2 expression. Therefore, we examined pro-TRH, PC1/3, and PC2 coexpression and coregulation in the paraventricular nucleus (PVN), lateral hypothalamus (LH), and ventromedial nucleus (VMN) of hypothyroid and euthyroid rats. Our results show that 6-n-propyl-2-thiouracil (PTU) treatment producing hypothyroidism induced a significant increase in the expression of PC1/3, PC2, and pro-TRH in the PVN and LH, but not VMN. When confocal studies were performed, an increase in colocalization of PC1/3 or PC2 in pro-TRH was observed only in PVN, a response that was especially prominent in the ventral and medial areas of the PVN. PTU did not regulate colocalization in the VMH or LH. Regulation of colocalization of processing enzyme and prohormone expression is a novel mechanism to alter hormonal biosynthesis.     

Proteolytic Processing of Chromogranin B and Secretogranin II by Prohormone Convertases

Abstract: Two experimental approaches were used to study the processing of chromogranin B and secretogranin II by prohormone convertases. In GH₃ cells various prohormone convertases were overexpressed together with the substrate chromogranin B by use of a vaccinia virus infection system. PC1 appeared to be by far the most active enzyme and converted chromogranin B to several smaller molecules, including the peptide PE-11. In brain this peptide is cleaved physiologically from chromogranin B. Some processing of chromogranin B and formation of free PE-11 were also observed with PC2 and PACE4. Furin produced larger fragments, whereas PC5-A and PC5-B had negligible effects. As a second model, PC12 cells were stably transfected with PC1 or PC2 to investigate the processing of endogenous chromogranins. Both enzymes effectively cleaved chromogranin B and secretogranin II, liberating the peptides PE-11 and secretoneurin, respectively. However, in transfection experiments the ability to generate the free peptides was more pronounced with PC2 than with PC1. The extent of proprotein processing achieved by prohormone convertases apparently differed depending on the experimental system applied. This suggests that in vivo mechanisms to support and fine-tune the activity of the processing enzymes exist, which might be overlooked by using only one methodological approach.     

Role of a pro-sequence in the secretory pathway of prothyrotropin-releasing hormone

The biogenesis of rat thyrotropin releasing hormone (TRH) involves the processing of its precursor (proTRH) into five biologically active TRH peptides and several non-TRH peptides where two of them had been attributed potential biological functions. This process implicates 1) proper folding of proTRH in the endoplasmic reticulum after its biosynthesis and exit to the Golgi apparatus and beyond, 2) initial processing of proTRH in the trans Golgi network and, 3) sorting of proTRH-derived peptides to the regulated secretory pathway. Previous studies have focused on elucidating the processing and sorting determinants of proTRH. However, the role of protein folding in the sorting of proTRH remains unexplored. Here we have investigated the role in the secretion of proTRH of a sequence comprising 22 amino acid residues, located at the N-terminal region of proTRH, residues 31-52. Complete deletion of these 22 amino acids dramatically compromised the biosynthesis of proTRH, manifested as a severe reduction in the steady state level of proTRH in the endoplasmic reticulum. This effect was largely reproduced by the deletion of only three amino acid residues, 40PGL42, within the proTRH31-52 sequence. The decreased steady state level of the mutant DeltaPGL was due to enhanced endoplasmic reticulum-associated protein degradation. However, the remnant of DeltaPGL that escaped degradation was properly processed and sorted to secretory granules. Thus, these results suggest that the N-terminal domain within the prohormone sequence does not act as "sorting signal" in late secretion; instead, it seems to play a key role determining the proper folding pathway of the precursor and, thus, its stability.     

Detection of TRH extended peptides in rat hypothalamus using an antibody raised to pGluHisProGlyLys

An antibody was raised to the synthetic pentapeptide pGluHisProGlyLys which, in radioimmunoassay (RIA), could detect the pentapeptide at a level of 10 fmole per tube and exhibited <0.5 per cent cross reactivity with a series of related peptides. The RIA was used to demonstrate the presence of C-terminally extended forms of thyrotropin releasing hormone (TRH) in rat hypothalamus. After extraction, the endogenous peptides were resolved by gel exclusion chromatography and TRH-extended peptides were revealed by trypsin digestion to release the pentapeptide. The TRH extended peptides occurred in substantial quantity, approximately 11 pmoles/g, indicating that only partial processing of the gene duplicated prohormone takes place.     

Disruption of disulfide bond formation alters the trafficking of prothyrotropin releasing hormone (proTRH)-derived peptides

Rat prothyrotropin releasing hormone (proTRH) is processed in the regulated secretory pathway (RSP) of neuroendocrine cells yielding five TRH peptides and several non-TRH peptides. It is not understood how these peptides are targeted to the RSP. We show here that a disulfide bond in the carboxy-terminus of proTRH plays an important role in the trafficking of this prohormone. Recombinant proTRH was observed to migrate faster on a native gel when treated with dithiothreitol (DTT) suggesting the presence of a disulfide bond. In vitro disulfide bond formation was prevented either by DTT treatment or by mutating cysteines 213 and 219 to glycines. In both cases the peptides derived from these mutants exhibited increased constitutive release and processing defects when expressed in AtT20 cells, a neuroendocrine cell line used in our prior studies on proTRH processing. Immunocytochemistry revealed that wild-type proTRH and mutant proTRH localized in a punctate pattern typical of proteins sorted to the regulated secretory pathway. These data suggest that the proposed disulfide bond of proTRH is involved in sorting of proTRH-derived peptides and in their retention within maturing secretory granules. This is the first evidence of structural motifs being important for the sorting of proTRH.     

Valproate and copper accelerate TRH-like peptide synthesis in male rat pancreas and reproductive tissues

Treatment with valproate (Valp) facilitates the synthesis of TRH-like peptides (pGlu-X-Pro-NH(2)) in rat brain where "X" can be any amino acid residue. Because high levels of TRH-like peptides occur in the pancreas and pGlu-Glu-Pro-NH(2) (Glu-TRH) has been shown to be a fertilization promoting peptide, we hypothesized that these peptides mediate some of the metabolic and reproductive side effects of Valp. Male WKY rats were treated with Valp acutely (AC), chronically (CHR) or chronically followed by a 2 day withdrawal (WD). AC, CHR and WD treatments significantly altered TRH and/or TRH-like peptide levels in pancreas and reproductive tissues. Glu-TRH was the predominant TRH-like peptide in epididymis, consistent with its fertilization promoting activity. Glu-TRH levels in the epididymis increased 3-fold with AC Valp. Phe-TRH, the most abundant TRH-like peptide in the pancreas, increased 4-fold with AC Valp. Phe-TRH inhibits both basal and TRH-stimulated insulin release. Large dense core vesicles (LDCV's) contain a copper-dependent enzyme responsible for the post-translational processing of precursors of TRH and TRH-like peptides. Copper (500 microM) increased the in vitro C-terminal amidation of TRH-like peptides by 8- and 4-fold during 24 degrees C incubation of homogenates of pancreas and testis, respectively. Valp (7 microM) accelerated 3-fold the processing of TRH and TRH-like peptide precursors in pancreatic LDCV's incubated at 24 degrees C. We conclude that copper, an essential cofactor for TRH and TRH-like peptide biosynthesis that is chelated by Valp, mediates some of the metabolic and reproductive effects of Valp treatment via acceleration of intravesicular synthesis and altered release of these peptides.     

Obliteration of alpha-melanocyte-stimulating hormone derived from POMC in pituitary and brains of PC2-deficient mice

Alpha-melanocyte-stimulating hormone (alpha-MSH) is a neuropeptide expressed in pituitary and brain that is known to regulate energy balance, appetite control, and neuroimmune functions. The biosynthesis of alpha-MSH requires proteolytic processing of the proopiomelanocortin (POMC) precursor. Therefore, this study investigated the in vivo role of the prohormone convertase 2 (PC2) processing enzyme for production of alpha-MSH in PC2-deficient mice. Specific detection of alpha-MSH utilized radioimmunoassay (RIA) that does not crossreact with the POMC precursor, and which does not crossreact with other adrenocorticotropin hormone (ACTH) and beta-endorphin peptide products derived from POMC. alpha-MSH in PC2-deficient mice was essentially obliterated in pituitary, hypothalamus, cortex, and other brain regions (collectively), compared to wild-type controls. These results demonstrate the critical requirement of PC2 for the production of alpha-MSH. The absence of alpha-MSH was accompanied by accumulation of ACTH, ACTH-containing imtermediates, and POMC precursor. ACTH was increased in pituitary and hypothalamus of PC2-deficient mice, evaluated by RIA and reversed-phase high pressure liquid chromatography (RP-HPLC). Accumulation of ACTH demonstrates its role as a PC2 substrate that can be converted for alpha-MSH production. Further analyses of POMC-derived intermediates in pituitary, conducted by denaturing western blot conditions, showed accumulation of ACTH-containing intermediates in pituitaries of PC2-deficient mice, which implicate participation of such intermediates as PC2 substrates. Moreover, accumulation of POMC was observed in PC2-deficient mice by western blots with anti-ACTH and anti-beta-endorphin. In addition, increased beta-endorphin1-31 was observed in pituitary and hypothalamus of PC2-deficient mice, suggesting beta-endorphin1-31 as a substrate for PC2 in these tissues. Overall, these studies demonstrated that the PC2 processing enzyme is critical for the in vivo production of alpha-MSH in pituitary and brain.     

Accumulation of mu-type opiate receptor ligands during the culture of phytohemagglutinin-stimulated lymphocytes

Substances displacing labelled ligands from opiate receptors of the rat brain membrane fractions were found in the extracts of human blood lymphocytes. Pronase treatment has shown that at least part of the opiate receptor ligands present in lymphocyte extracts are of peptide nature. Opiate mu-receptor ligands content was identical in the extracts of the total population, T-lymphocyte and B-cell-enriched population. However, opiate delta-receptor ligands concentration in T-cell population is 15 times lower than in B-cell-enriched population. Opiate receptor ligands content is not changed during cultivation of nonstimulated lymphocytes. However, phytohemagglutinin activation results in a threefold increase of opiate mu-receptor ligands content in lymphocytes. The results obtained suggest that synthesis and processing of opiate mu-receptor ligands precursor occur in stimulated T-lymphocytes.     

Distribution of beta-endorphin-related peptides in rat pituitary and brain.

beta-Endorphin, the most potent known naturally occurring analgesic agent, is found in pituitary and brain in company with a series of structurally and biosynthetically related peptides that are essentially devoid of opiate activity. In studies of beta-endorphin it is important to discriminate between the active and inactive forms of the peptide. This review describes the use of chemical and immunological methods for localizing the peptides in the tissues, extracting and resolving the peptides by chromatography, and determining the concentrations of the peptides by radioimmunoassay. These approaches have allowed the distribution of beta-endorphin and its related peptides to be assigned unequivocally in regions of rat pituitary and brain. It has been found that the multifunctional corticotropin-endorphin prohormone can undergo processing by different mechanisms in different tissues, permitting the intrinsic activities of its fragments to be expressed selectively. The different processing patterns can be attributed to the existence of highly specific enzymes, characteristic of individual cells, which regulate the formation of this potent opiate.     

Proteolytic processing of chromogranin A by the prohormone convertase PC2

The neuroendocrine secretory protein chromogranin A (CgA) is a precursor for various biologically active peptides. Several single and paired basic residues are present within its primary amino acid sequence comprising cleavage sites for prohormone convertases. In this study, SH-SY5Y human neuroblastoma cells were stably transfected with the prohormone convertase PC2 to analyse the proteolytic processing of endogenous chromogranin A and, in particular, the formation of the chromogranin-A-derived peptide GE-25. Our analyses revealed a significant change in the pattern of proteolytic conversion of chromogranin A in cells expressing PC2. Mock-transfected control cells contained mainly the intact chromogranin A molecule and hardly any shorter products were found. On the other hand, PC2-transfected cells showed extensive processing of chromogranin A, resulting in significantly lower amounts of the intact precursor and especially high levels of the free peptide GE-25.