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

Up-regulation of splenic prohormone convertases PC1 and PC2 in diabetic rats

Organisms respond to infection in a complex manner involving bidirectional interactions between the neuroendocrine and immune systems. Many of the bioactive endocrine/immune factors are synthesized in a precursor form and are expected to be activated by prohormone convertases (PCs). Since patients with both type 1 and type 2 diabetes have an increased incidence and severity of infections, we hypothesized that in a condition of hyperglycemia, these processing enzymes would be activated in an immune tissue, the spleen. To test this hypothesis, we treated rats with intraperitoneal streptozotocin (STZ) (50 mg/kg/day) daily for 5 days and measured splenic PC1 and PC2 mRNA by ribonuclease protection assay. We found that PC1 mRNA was increased 6.0+/-0.02-fold (P<0.05) and PC2 mRNA was increased 1.80+/-0.01-fold (P<0.005) in the spleen of rats that received STZ compared to rats that received vehicle. Western blot indicated that the 75-kDa form of PC1 was the only form of PC1 present in the spleen and that this form increased with STZ treatment. Immunohistochemistry revealed that PC1 was found in both the white pulp (T-lymphocytes) and red pulp (monocytes and macrophages) and that its increase in immunoreactivity occurred primarily in the white pulp. PC2 and pro-opiomelanocortin (POMC, a possible splenic substrate for PC1/PC2) immunoreactivity was found predominantly in the red pulp. STZ induced an increase in splenic PC1 and POMC, but not PC2 protein levels. We conclude that in the STZ model of diabetes, splenic PCs are induced, which could lead to an increased activation of many immune-derived hormones. We speculate that this up-regulation of prohormone converting enzymes may be related to the increased infections seen in patients with both type 1 and type 2 diabetes.

Lack of neuronal NOS has consequences for the expression of POMC and POMC-derived peptides in the mouse pituitary

The relevance of NO in neuroendocrine signalling has been investigated by analysis of cellular expression of pro-opiomelanocortin (POMC) and the POMC-derived peptides beta-endorphin, alpha-melanocyte stimulating hormone and adrenocorticotropin. Expression patterns were studied in the pituitary gland of 150-day old wild-type and neuronal-NOS (nNOS) knock-out mice by using immunohistochemistry, in situ hybridization and Northern blot analysis. Remaining NO-generating capacities in the knock-out mice were demonstrated by immunohistochemical localization of inducible, endothelial and neuronal NOS isoforms. Quantitative analysis revealed that cellular expression of POMC mRNA was drastically reduced in the pituitary of knock-out mice in comparison to controls. In situ hybridization studies demonstrated that this reduction was most pronounced in the intermediate lobe, while the anterior lobe was much less affected. Immunostaining for the proteolytic fragments of POMC was significantly reduced in the intermediate lobe cells of knock-out mice. A moderate reduction of immunostaining for these peptides was also observed in adenopituitary cells of nNOS knock-out mice. Our data demonstrate that the lack of nNOS substantially affects cellular levels of pituitary opioid peptides, which may have consequences for the response of these animals to stress and pain.

Dietary sodium modulates mRNA abundance of enzymes involved in pituitary processing of proopiomelanocortin

The messenger RNA abundance of proopiome-lanocortin (POMC) is increased in neurointermediate lobe (NIL) of rat pituitary when ingesting a high sodium diet (8%; HSD), as is the plasma concentration of the natriuretic peptide gamma-melanocyte stimulating hormone (gammay-MSH) derived from it. We examined whether the HSD also increases the mRNA abundance in rat NIL of proconvertases 1 and 2 (PC1, PC2), enzymes involved in the processing of POMC into gamma-MSH. PC1 mRNA increased by 40% after two weeks of the HSD and by 84% after three weeks. PC2 mRNA increased by 40% after two weeks and by more than 3 fold after three weeks. These results for PC2 were confined to NIL as shown by in situ hybridization at one and two weeks, and were accompanied by a significant increase in NIL PC2 protein after three weeks of the HSD as measured by immunoblotting. The increases in PC1 and PC2 mRNA abundance were paralleled by an increase in POMC mRNA level in NIL. Plasma gamma-MSH immunoreactivity averaged 35.1 +/- 3.3 fmol/ml in rats on the LSD, but increased to 70.9 +/- 4.8 fmol/ml after 3 weeks of the HSD (p < 0.002 vs LSD). These results confirm that the HSD increases the plasma concentration of gamma-MSH, consistent with a role for it as a circulating natriuretic peptide. The increased NIL expression of PC1 and PC2 in parallel with POMC in response to the HSD suggests that these changes are part of the coordinated response to states of sodium surfeit.

Human dermal fibroblasts express prohormone convertases 1 and 2 and produce proopiomelanocortin-derived peptides

In the last few years it has become apparent that the skin is a locoregional source for several proopiomelanocortin-derived peptides including alpha-melanocyte-stimulating hormone, adrenocorticotropin, and beta-endorphin. The enzymes that regulate expression of these neuropeptides are the prohormone convertases 1 and 2. In this study we demonstrate, by reverse transcriptase polymerase chain reaction and Western immunoblotting, that cultured human dermal fibroblasts express prohormone convertases 1 and 2 as well as 7B2, which is an essential cofactor for enzymatic activity of prohormone convertase 2. Immunofluorescence studies revealed prohormone convertase 1 to be mainly expressed in the perinuclear region in vesicular structures resembling the trans-Golgi network, whereas prohormone convertase 2 was found in the trans-Golgi network as well as in vesicular structures diffusely distributed in the peripheral cytoplasm. Expression of both enzymes was also confirmed in fibroblasts of normal adult human skin by immunohistochemistry using antibodies against prohormone convertases 1 and 2 and vimentin. To assess the relevance of prohormone convertase 1 and 2 expression in human dermal fibroblasts, we studied the expression of proopiomelanocortin and proopiomelanocortin-derived peptides. Proopiomelanocortin expression was detected by reverse transcriptase polymerase chain reaction and Western immunoblotting. Alpha-melanocyte-stimulating hormone, adrenocorticotropin, and beta-endorphin were mainly located in vesicular structures as demonstrated by immunofluorescence. Production of these peptides was confirmed by radioimmunoassay, immunoradiometric assay, or enzyme immunoassay. Among several stimuli tested, interleukin-1 was found to upregulate production of alpha-melanocyte-stimulating hormone in human dermal fibroblasts. In summary, we have shown that human dermal fibroblasts express the enzymatic machinery for proopiomelanocortin processing and make proopiomelanocortin, alpha-melanocyte-stimulating hormone, adrenocorticotropin, and beta-endorphin. Production of proopiomelanocortin peptides by human dermal fibroblasts may be relevant for fibroblast functions such as collagen degradation and/or regulation of dermal immune responses.

Proteolysis of ProPTHrP(1-141) by "prohormone thiol protease" at multibasic residues generates PTHrP-related peptides: implications for PTHrP peptide production in lung cancer cells

The parathyroid hormone-related protein (PTHrP) precursor requires proteolytic processing to generate PTHrP-related peptide products that possess regulatory functions in the control of PTH-like (parathyroid-like) actions and cell growth, calcium transport, and osteoclast activity. Biologically active peptide domains within the PTHrP precursor are typically flanked at their NH2- and COOH-termini by basic residue cleavage sites consisting of multibasic, dibasic, and monobasic residues. These basic residues are predicted to serve as proteolytic cleavage sites for converting the PTHrP precursor into active peptide products. The coexpression of the prohormone processing enzyme PTP ("prohormone thiol protease") in PTHrP-containing lung cancer cells, and the lack of PTP in cell lines that contain little PTHrP, implicate PTP as a candidate processing enzyme for proPTHrP. Therefore, in this study, PTP cleavage of recombinant proPTHrP(1-141) precursor was evaluated by MALDI mass spectrometry to identify peptide products and cleavage sites. PTP cleaved the PTHrP precursor at the predicted basic residue cleavage sites to generate biologically active PTHrP-related peptides that correspond to the NH2-terminal domain (residues 1-37) that possesses PTH-like and growth regulatory activities, the mid-region domain (residues 38-93) that regulates calcium transport, and the COOH-terminal domain (residues 102-141) that modulates osteoclast activity. Lack of cleavage at other types of amino acids demonstrated the specificity of PTP processing at basic residue cleavage sites. Overall, these results demonstrate the ability of PTP to cleave the PTHrP precursor at multibasic, dibasic, and monobasic residue cleavage sites to generate active PTHrP-related peptides. The presence of PTP immunoreactivity in PTHrP-containing lung cancer cells suggests PTP as a candidate processing enzyme for the PTHrP precursor.

Neuroendocrine secretory protein 7B2: structure, expression and functions

7B2 is an acidic protein residing in the secretory granules of neuroendocrine cells. Its sequence has been elucidated in many phyla and species. It shows high similarity among mammals. A Pro-Pro-Asn-Pro-Cys-Pro polyproline motif is its most conserved feature, being carried by both vertebrate and invertebrate sequences. It is biosynthesized as a precursor protein that is cleaved into an N-terminal fragment and a C-terminal peptide. In neuroendocrine cells, 7B2 functions as a specific chaperone for the proprotein convertase (PC) 2. Through the sequence around its Pro-Pro-Asn-Pro-Cys-Pro motif, it binds to an inactive proPC2 and facilitates its transport from the endoplasmic reticulum to later compartments of the secretory pathway where the zymogen is proteolytically matured and activated. Its C-terminal peptide can inhibit PC2 in vitro and may contribute to keep the enzyme transiently inactive in vivo. The PC2-7B2 model defines a new neuroendocrine paradigm whereby proteolytic activation of prohormones and proneuropeptides in the secretory pathway is spatially and temporally regulated by the dynamics of interactions between converting enzymes and their binding proteins. Interestingly, unlike PC2-null mice, which are viable, 7B2-null mutants die early in life from Cushing's disease due to corticotropin ('ACTH') hypersecretion by the neurointermediate lobe, suggesting a possible involvement of 7B2 in secretory granule formation and in secretion regulation. The mechanism of this regulation is yet to be elucidated. 7B2 has been shown to be a good marker of several neuroendocrine cell dysfunctions in humans. The possibility that anomalies in its structure and expression could be aetiological causes of some of these dysfunctions warrants investigation.

The role of dibasic residues in prohormone sorting to the regulated secretory pathway. A study with proneurotensin

The mechanisms by which prohormone precursors are sorted to the regulated secretory pathway in neuroendocrine cells remain poorly understood. Here, we investigated the presence of sorting signal(s) in proneurotensin/neuromedin N. The precursor sequence starts with a long N-terminal domain followed by a Lys-Arg-(neuromedin N)-Lys-Arg-(neurotensin)-Lys-Arg- sequence and a short C-terminal tail. An additional Arg-Arg dibasic is contained within the neurotensin sequence. Mutated precursors were expressed in endocrine insulinoma cells and analyzed for their regulated secretion. Deletion mutants revealed that the N-terminal domain and the Lys-Arg-(C-terminal tail) sequence were not critical for precursor sorting to secretory granules. In contrast, the Lys-Arg-(neuromedin N)-Lys-Arg-(neurotensin) sequence contained essential sorting information. Point mutation of all three dibasic sites within this sequence abolished regulated secretion. However, keeping intact any one of the three dibasic sequences was sufficient to maintain regulated secretion. Finally, fusing the dibasic-containing C-terminal domain of the precursor to the C terminus of beta-lactamase, a bacterial enzyme that is constitutively secreted when expressed in neuroendocrine cells, resulted in efficient sorting of the fusion protein to secretory granules in insulinoma cells. We conclude that dibasic motifs within the neuropeptide domain of proneurotensin/neuromedin N constitute a necessary and sufficient signal for sorting proteins to the regulated secretory pathway.

ProSAAS processing in mouse brain and pituitary

ProSAAS is a newly discovered protein with a neuroendocrine distribution generally similar to that of prohormone convertase 1 (PC1), a peptide-processing endopeptidase. Several proSAAS-derived peptides were previously identified in the brain and pituitary of the Cpe(fat)/Cpe(fat) mouse based on the accumulation of C-terminally extended peptides due to the absence of enzymatically active carboxypeptidase E, a peptide-processing exopeptidase. In the present study, antisera against different regions of proSAAS were used to develop radioimmunoassays and examine the processing profile of proSAAS in wild type and Cpe(fat)/Cpe(fat) mouse tissues following gel filtration and reverse phase high performance liquid chromatography. In wild type mouse brain and pituitary, the majority of proSAAS is processed into smaller peptides. These proSAAS-derived peptides elute from the reverse-phase column in the same positions as synthetic peptides that correspond to little SAAS, PEN, and big LEN. Mass spectrometry revealed the presence of peptides with the expected molecular masses of little SAAS and big LEN in the fractions containing immunoreactive peptides. The processing of proSAAS is slightly impaired in Cpe(fat)/Cpe(fat) mice, relative to wild-type mice, leading to the accumulation of partially processed peptides. One of these peptides, the C-terminally extended form of PEN, is known to inhibit PC1 activity and this could account for the reduction in enzymatically active PC1 seen in Cpe(fat)/Cpe(fat) mice. The observation that little SAAS and big LEN are the major forms of these peptides produced in mouse brain and pituitary raises the possibility that these peptides function as neurotransmitters or hormones.

Biogenesis of regulated exocytotic carriers in neuroendocrine cells

Ca(2+)-triggered exocytosis is a hallmark of neurosecretory granules, but the cellular pathway leading to the assembly of these regulated exocytotic carriers is poorly understood. Here we used the pituitary AtT-20 cell line to study the biogenesis of regulated exocytotic carriers involved in peptide hormone secretion. We show that immature secretory granules (ISGs) freshly budded from the trans-Golgi network (TGN) exhibit characteristics of unregulated exocytotic carriers. During a subsequent maturation period they undergo an important switch to become regulated exocytotic carriers. We have identified a novel sorting pathway responsible for this transition. The SNARE proteins, VAMP4 and synaptotagmin IV (Syt IV), enter ISGs initially but are sorted away during maturation. Sorting is achieved by vesicle budding from the ISGs, because it can be inhibited by brefeldin A (BFA). Inhibition of this sorting pathway with BFA arrested the maturing granules in a state that responded poorly to stimuli, suggesting that the transition to regulated exocytotic carriers requires the removal of a putative inhibitor. In support of this, we found that overexpression of Syt IV reduced the stimulus-responsiveness of maturing granules. We conclude that secretory granules undergo a switch from unregulated to regulated secretory carriers during biogenesis. The existence of such a switch may provide a mechanism for cells to modulate their secretory activities under different physiological conditions.

PreproTRH(178-199) and two novel peptides (pFQ7 and pSE14) derived from its processing, which are produced in the paraventricular nucleus of the rat hypothalamus, are regulated during suckling

Suckling increases preproTRH messenger RNA in hypothalamic paraventricular neurons (PVN) and also markedly increases TRH release during the first period of lactation. Whether lactation alters preproTRH processing resulting in the generation of novel proTRH-derived peptides that may be involved in the regulation of PRL secretion lactation is not known. Therefore, in the present study we determine whether some other peptides derived from proTRH potentially contribute to lactation-induced PRL secretion. We have recently demonstrated that two members of the family of prohormone convertases PC1 and PC2 play a significant role in proTRH processing. PC1 is the major contributor in proTRH processing, whereas PC2 may have a specific role in cleaving TRH from its extended forms. In this study, we used a recombinant vaccinia virus system to coexpress rat preproTRH complementary DNA with PC1, PC2, and the neuropeptide 7B2 in GH4C1 cells (somatomammothophs, rat). We found that two novel peptides, preproTRH(178-184) (pFQ(7)), and preproTRH(186-199) (pSE(14)), were formed after the cleavage of their precursor preproTRH(178-199) (pFE(22)) by only PC2. Their formation was confirmed by microsequence analysis. Anatomical analyses revealed that these peptides are also found in the rat PVN. In addition, we found that pFE(22), pSE(14) and pFQ(7) produced a dose-dependent release of PRL from primary cultures of pituitary cells compared with one of the well studied secretagogues of PRL, TRH. To establish whether these peptides might play a role in vivo in the regulation of PRL release, we took rat litters on postnatal day 4, separated the pups from their mothers for 6 h, and then reunited the pups and mothers for 45 min. At the end of this period, the mothers were killed, acidic extracts of microdissected PVN were prepared and subjected to SDS-PAGE, followed by slicing and analysis by pFE(22) RIA. Forty-five minutes of suckling induced a marked 6-fold increase in serum levels of PRL. In addition, PVN levels of pFE(22) and pSE(14) increased approximately 5-fold during the same period in the acutely suckling females. Lactating animals that were separated from their litters and never reunited with their pups had low levels of PRL, and pFE(22) and pSE(14). These data provide the first evidence for alterations in proTRH processing in the PVN during lactation and suggest that the products of this altered processing may play a physiological role in the regulation of PRL secretion.

Impaired prohormone convertases in Cpe(fat)/Cpe(fat) mice

A spontaneous point mutation in the coding region of the carboxypeptidase E (CPE) gene results in a loss of CPE activity that correlates with the development of late onset obesity (Nagert, J. K., Fricker, L. D., Varlamov, O., Nishina, P. M., Rouille, Y., Steiner, D. F., Carroll, R. J., Paigen, B. J., and Leiter, E. H. (1995) Nat. Genet. 10, 135-142). Examination of the level of neuropeptides in these mice showed a decrease in mature bioactive peptides as a result of a decrease in both carboxypeptidase and prohormone convertase activities. A defect in CPE is not expected to affect endoproteolytic processing. In this report we have addressed the mechanism of this unexpected finding by directly examining the expression of the major precursor processing endoproteases, prohormone convertases PC1 and PC2 in Cpe(fat) mice. We found that the levels of PC1 and PC2 are differentially altered in a number of brain regions and in the pituitary. Since these enzymes have been implicated in the generation of neuroendocrine peptides (dynorphin A-17, beta-endorphin, and alpha- melanocyte-stimulating hormone) involved in the control of feeding behavior and body weight, we compared the levels of these peptides in Cpe(fat) and wild type animals. We found a marked increase in the level of dynorphin A-17, a decrease in the level of alpha-melanocyte-stimulating hormone, and an alteration in the level of C-terminally processed beta-endorphin. These results suggest that the impairment in the level of these and other peptides involved in body weight regulation is mainly due to an alteration in carboxypeptidase and prohormone convertase activities and that this may lead to the development of obesity in these animals.

Evidence that furin is an authentic transforming growth factor-beta1-converting enzyme

Transforming growth factor (TGF)-beta1 plays an essential role in cell growth and differentiation. It is also considered as a gatekeeper of immune homeostasis with gene disruption leading to autoimmune and inflammatory diseases. TGF-beta1 is produced as an inactive precursor polypeptide that can be efficiently secreted but correct proteolytic cleavage is an essential step for its activation. Assessment of the cleavage site has revealed a unique R-H-R-R sequence reminiscent of proprotein convertase (PC) recognition motifs and has previously demonstrated that this PC-like cleavage site is correctly cleaved by furin, a member of the PC family. Here we report that among PC members, furin more closely satisfies the requirements needed to fulfill the role of a genuine TGF-beta1 convertase. Even though six members of the PC family have the ability to cleave TGF-beta1, ectopic expression of alpha(1)-antitrypsin Portland (alpha(1)-AT-PDX), a potent furin inhibitor, blocked 80% of TGF-beta1 processing mediated by endogenous enzymes as demonstrated in an in vitro digestion assay. Genetic complementation of a furin-deficient LoVo cell line with the wild-type gene restores the production of mature and bioactivable TGF-beta1. Moreover, both furin and TGF-beta are coordinately expressed and regulated in vitro and in vivo in the hematopoietic and immune system, an important tissue target. These results demonstrate for the first time that furin is an authentic and adaptive TGF-beta1-converting enzyme whereas other members of the PC family might substitute or supplement furin activity. Our study advances our comprehension of the complexity of the TGF-beta system and should facilitate the development of therapeutically useful TGF-beta inhibitors.

Defective prodynorphin processing in mice lacking prohormone convertase PC2

Prodynorphin, a multifunctional precursor of several important opioid peptides, is expressed widely in the CNS. It is processed at specific single and paired basic sites to generate various biologically active products. Among the prohormone convertases (PCs), PC1 and PC2 are expressed widely in neuroendocrine tissues and have been proposed to be the major convertases involved in the biosynthesis of hormonal and neural peptides. In this study we have examined the physiological involvement of PC2 in the generation of dynorphin (Dyn) peptides in mice lacking active PC2 as a result of gene disruption. Enzymological and immunological assays were used to confirm the absence of active PC2 in these mice. The processing profiles of Dyn peptides extracted from brains of these mice reveal a complete lack of Dyn A-8 and a substantial reduction in the levels of Dyn A-17 and Dyn B-13. Thus, PC2 appears to be involved in monobasic processing, leading to the generation of Dyn A-8, Dyn A-17, and Dyn B-13 from prodynorphin under physiological conditions. Brains of heterozygous mice exhibit only half the PC2 activity of wild-type mice; however, the levels of Dyn peptides in these mice are similar to those of wild-type mice, suggesting that a 50% reduction in PC2 activity is not sufficient to significantly reduce prodynorphin processing. The disruption of the PC2 gene does not lead to compensatory up-regulation in the levels of other convertases with similar substrate specificity because we find no significant changes in the levels of PC1, PC5/PC6, or furin in these mice as compared with wild-type mice. Taken together, these results support a critical role for PC2 in the generation of Dyn peptides.

Biogenesis of regulated exocytotic carriers in neuroendocrine cells

Ca(2+)-triggered exocytosis is a hallmark of neurosecretory granules, but the cellular pathway leading to the assembly of these regulated exocytotic carriers is poorly understood. Here we used the pituitary AtT-20 cell line to study the biogenesis of regulated exocytotic carriers involved in peptide hormone secretion. We show that immature secretory granules (ISGs) freshly budded from the trans-Golgi network (TGN) exhibit characteristics of unregulated exocytotic carriers. During a subsequent maturation period they undergo an important switch to become regulated exocytotic carriers. We have identified a novel sorting pathway responsible for this transition. The SNARE proteins, VAMP4 and synaptotagmin IV (Syt IV), enter ISGs initially but are sorted away during maturation. Sorting is achieved by vesicle budding from the ISGs, because it can be inhibited by brefeldin A (BFA). Inhibition of this sorting pathway with BFA arrested the maturing granules in a state that responded poorly to stimuli, suggesting that the transition to regulated exocytotic carriers requires the removal of a putative inhibitor. In support of this, we found that overexpression of Syt IV reduced the stimulus-responsiveness of maturing granules. We conclude that secretory granules undergo a switch from unregulated to regulated secretory carriers during biogenesis. The existence of such a switch may provide a mechanism for cells to modulate their secretory activities under different physiological conditions.

Cathepsin-B fusion proteins misroute secretory protein partners such as the proprotein convertase PC2-7B2 complex toward the lysosomal degradation pathways

A general strategy is presented for the dominant negative reduction in the levels of heterodimeric soluble proteins within the secretory pathway through fusion of one of its partners C-terminal to the lysosomal enzyme cathepsin B (CB). Stable transfectants of CB-7B2 chimeras in AT20 cells result in a drastic reduction of the endogenous levels of its partner, the proprotein convertase PC2. This dominant negative suppressive effect requires active CB. It was partially reversed by NH(4)Cl, the cell-permeable CB inhibitor CA-074Me, but not by the proteasome inhibitor Lactacystin, suggesting the potential participation of the lysosomal/endosomal degradative pathway in this process.

Neprilysin, a novel target for ultraviolet B regulation of melanogenesis via melanocortins

Compelling evidence suggest a role for melanocortins in the regulation of melanogenesis by ultraviolet radiation. Within the epidermis, melanocytes and keratinocytes produce alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone. The persistence and the strength of the biologic signal delivered by these peptides depend on their local concentration, which is controlled by the rate of peptide production and by the rate of its degradation. In this study, we investigated the mechanism of melanocortin degradation by melanocytes and the effect of ultraviolet on this process. We have focused our attention on a neutral endopeptidase, neprilysin, which has been implicated in the ending of numerous peptidergic signals. We have shown that this enzyme is expressed at the surface of human melanocytes. Interestingly, its activity and its expression are dramatically downregulated by ultraviolet B treatment. Moreover, in the presence of phosphoramidon, a stable inhibitor of neprilysin, we observed an increased efficiency of alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone to stimulate both tyrosinase activity and microphthalmia expression. Taken together, these data indicate that neprilysin expressed by melanocytes has a physiologic role in the regulation of melanogenesis by proopiomelanocortin peptide. Further, its downregulation by ultraviolet B irradiation shed light on a new and appealing mechanism of ultraviolet B induced melanogenesis via the control of melanocortins degradation.

Immunohistochemical evidence for the involvement of protein convertases 5A and 2 in the processing of pro-neurotensin in rat brain

The neuropeptides/neurotransmitters neurotensin (NT) and neuromedin (NN) are synthesized by endoproteolytic cleavage of a common inactive precursor, pro-NT/NN. In vitro studies have suggested that the prohormone convertases PC5A and PC2 might both be involved in this process. In the present study, we used dual immunohistochemical techniques to determine whether either one or both of these two convertases were co-localized with pro-NT/NN maturation products and could therefore be involved in the physiological processing of this propeptide in rat brain. PC2-immunoreactive neurons were present in all regions immunopositive for NT. All but three regions expressing NT were also immunopositive for PC5A. Dual localization of NT with either convertase revealed that NT was extensively co-localized with both PC5A and PC2, albeit with regional differences. These results strongly suggest that PC5A and PC2 may play a key role in the maturation of pro-NT/NN in mammalian brain. The regional variability in NT/PC co-localization patterns may account for the region-specific maturation profiles previously reported for pro-NT/NN. The high degree of overlap between PC5A and PC2 in most NT-rich areas further suggests that these two convertases may act jointly to process pro-NT/NN. At the subcellular level, PC5A was largely co-localized with the mid-cisternae Golgi marker MG-160. By contrast, PC2 was almost completely excluded from MG-160-immunoreactive compartments. These results suggest that PC5A, which is particularly efficient at cleaving the two C-terminal-most dibasics of pro-NT/NN, may be acting as early as in the Golgi apparatus to release NT, whereas PC2, which is considerably more active than PC5A in cleaving the third C-terminal doublet, may be predominantly involved further distally along the secretory pathway to release NN.

The C-terminal region of proSAAS is a potent inhibitor of prohormone convertase 1

ProSAAS is a recently discovered 26-kDa neuroendocrine protein that was previously found to inhibit prohormone convertase (PC) 1 and not PC2. In the present study, the specificity of proSAAS toward other members of the prohormone convertase family was determined. Two microm proSAAS selectively inhibits PC1 but not furin, PACE4, PC5A, or PC7. The PC1 inhibitory region of proSAAS was mapped to an 8-12-residue region near the C terminus that includes a critical Lys-Arg sequence. Synthetic peptides corresponding to this region are competitive inhibitors of PC1 with apparent K(i) values of 14-40 nm. The inhibition becomes more effective with incubation time, indicating that the inhibitor is slow binding. A fusion protein containing the inhibitory region of proSAAS linked to the C terminus of glutathione S-transferase binds the 71-kDa form but not the 85-kDa form of PC1. This binding, which occurs at pH 5.5 and not at pH 7.4, is stable to incubation at room temperature for 1 h in the presence or absence of 0.5% Triton X-100 and/or 0.5 m NaCl. The removal of Ca(2+) with chelating agents partially releases the bound PC1. High concentrations of the inhibitory peptide quantitatively release the bound PC1. Taken together, these data support the proposal that proSAAS functions as an endogenous inhibitor of PC1.

Spatiotemporal expression, distribution, and processing of POMC and POMC-derived peptides in murine skin

In murine skin, after depilation-induced anagen, there was a differential spatial and temporal expression of pro-opiomelanocortin (POMC) mRNA, of the POMC-derived peptides beta-endorphin, ACTH, beta-MSH, and alpha-MSH, and of the prohormone convertases PC1 and PC2 in epidermal and hair follicle keratinocytes and in the cells of sebaceous units. Using a combination of in situ hybridization histochemistry and immunohistochemistry, we found cell-specific variations in the expression of POMC mRNA that were consistent with immunoreactivities for POMC-derived peptides. Cells that contained POMC peptide immunoreactivity (IR) also expressed POMC mRNA, and where the IR increased there was a parallel increase in mRNA. The levels of PC1-IR and PC2-IR also showed cell-specific variations and were present in the same cells that contained the POMC peptides. Based on the cleavage specificities of these convertases and on the spatial and temporal expression of the convertases and of ACTH, beta-endorphin, beta-MSH, and alpha-MSH, we can infer that the activities of PC1 and PC2 are responsible for the cell-specific differential processing of POMC in murine skin.

Molecular cloning demonstrates structural features of homologous bovine prohormone convertases 1 and 2

PC1 and PC2 (prohormone convertase) represent neuroendocrine members of the mammalian subtilisin-like family of proprotein convertases. The goal of this study was to compare the primary sequence motifs of bovine PC1 and PC2 with those of homologs from other mammalian species to establish the structural basis for PC1 and PC2 activities in bovine that resemble other mammalian homologs. Molecular cloning from bovine adrenal medulla resulted in the isolation of cDNAs for bovine PC1 and PC2 with highly conserved primary sequences with respect to signal sequence, prosegment, catalytic domain, and P domain. Bovine PC1 and PC2 contained the catalytic triad residues Asp, His, Ser, which are identical to the triads in PC1 and PC2 from other mammalian species. Bovine PCl contained Asn as the oxyanion hole residue; in contrast, bovine PC2 contained Asp as the oxyanion hole residue, which is identical to PC2 in other mammalian species. Bovine PC1 and PC2 possessed the P domain that contains the functional RRGDL motif. The cloned cDNAs detected expression of PC1 and PC2 mRNAs in bovine adrenal medulla. These results establish the defined structural domains of bovine PC1 and PC2 that are known to be essential for the activities of these enzymes in various species.

PC2 and 7B2 null mice demonstrate that PC2 is essential for normal pro-CCK processing

Analysis of CCK content in extracts of whole forebrain from PC2 and 7B2 null mouse brain showed a significant decrease relative to wild-type brains. More detailed analysis revealed that CCK 8 amide levels in cerebral cortex and forebrain regions were more decreased than in hypothalamus. CCK 8 content in PC2 null mouse intestines was identical to control. Null mutant brains contained less CCK 8 than wild type and no other forms were seen when analyzed by gel filtration chromatography. No brain area examined was completely devoid of CCK, suggesting that other enzymes can partially compensate for the loss of PC2. This is the first demonstration that any endoprotease is important for CCK processing but also suggest the presence of a redundant system to ensure production of active CCK in the brain.

Interaction of Drosophila melanogaster prohormone convertase 2 and 7B2. Insect cell-specific processing and secretion

The prohormone convertases (PCs) are an evolutionarily ancient group of proteases required for the maturation of neuropeptide and peptide hormone precursors. In Drosophila melanogaster, the homolog of prohormone convertase 2, dPC2 (amontillado), is required for normal hatching behavior, and immunoblotting data indicate that flies express 80- and 75-kDa forms of this protein. Because mouse PC2 (mPC2) requires 7B2, a helper protein for productive maturation, we searched the fly data base for the 7B2 signature motif PPNPCP and identified an expressed sequence tag clone encoding the entire open reading frame for this protein. dPC2 and d7B2 cDNAs were subcloned into expression vectors for transfection into HEK-293 cells; mPC2 and rat 7B2 were used as controls. Although active mPC2 was detected in medium in the presence of either d7B2 or r7B2, dPC2 showed no proteolytic activity upon coexpression of either d7B2 or r7B2. Labeling experiments showed that dPC2 was synthesized but not secreted from HEK-293 cells. However, when dPC2 and either d7B2 or r7B2 were coexpressed in Drosophila S2 cells, abundant immunoreactive dPC2 was secreted into the medium, coincident with the appearance of PC2 activity. Expression and secretion of dPC2 enzyme activity thus appears to require insect cell-specific posttranslational processing events. The significant differences in the cell biology of the insect and mammalian enzymes, with 7B2 absolutely required for secretion of dPC2 and zymogen conversion occurring intracellularly in the case of dPC2 but not mPC2, support the idea that the Drosophila enzyme has specific requirements for maturation and secretion that can be met only in insect cells.

Alpha-melanocyte-stimulating hormone reduces impact of proinflammatory cytokine and peroxide-generated oxidative stress on keratinocyte and melanoma cell lines

We have previously shown that alpha-melanocyte-stimulating hormone (alpha-MSH) can oppose tumor necrosis factor alpha activation of NF-kappaB (1-2 h) and intercellular adhesion molecule 1 up-regulation (mRNA by 3 h and protein by 24 h) in melanocytes and melanoma cells. The present study reports on the ability of four MSH peptides to control intracellular peroxide levels and glutathione peroxidase (GPx) activity in pigmentary and nonpigmentary cells. In human HBL melanoma and HaCaT keratinocytes tumor necrosis factor alpha and H(2)O(2) both activated GPx in a time- and concentration-dependent manner (by 30-45 min). alpha-MSH peptides were found to inhibit the stimulated GPx activity and had biphasic dose-response curves. MSH 1-13 and MSH [Nle(4)-d-Phe(7)] achieved maximum inhibition at 10(-10) and 10(-12) m, respectively. Higher concentrations (10-100 fold) of MSH 4-10 and MSH 11-13 were required to produce equivalent levels of inhibition. alpha-MSH was also capable of reducing peroxide accumulation within 15 min, and again this inhibition was biphasic. The data support a role of alpha-MSH in acute protection of cells to oxidative/cytokine action that precedes NF-kappaB and GPx activation. The rapidity and potency of the response to alpha-MSH in pigmentary and nonpigmentary cells suggest this to be a central role of this peptide in cutaneous cells.

Mutations in the catalytic domain of prohormone convertase 2 result in decreased binding to 7B2 and loss of inhibition with 7B2 C-terminal peptide

Prohormone convertases 1 (PC1) and 2 (PC2) are members of a family of subtilisin-like proprotein convertases responsible for proteolytic maturation of a number of different prohormones and proneuropeptides. Although sharing more than 50% homology in their catalytic domains, PC1 and PC2 exhibit differences in substrate specificity and susceptibility to inhibitors. In addition to these differences, PC2, unlike PC1 and other members of the family, specifically binds the neuroendocrine protein 7B2. In order to identify determinants responsible for the specific properties of the PC2 catalytic domain, we compared its primary sequence with that of other PCs. This allowed us to distinguish a PC2-specific sequence at positions 242-248. We constructed two PC2 mutants in which residues 242 and 243 and residues 242-248 were replaced with the corresponding residues of PC1. Studies of in vivo cleavage of proenkephalin, in vivo production of alpha-MSH from proopiomelanocortin, and in vitro cleavage of a PC2-specific artificial substrate by mutant PC2s did not reveal profound alterations. On the other hand, both mutant pro-PC2s exhibited a considerably reduced ability to bind to 21-kDa 7B2. In addition, inhibition of mutant PC2-(242-248) by the potent natural inhibitor 7B2 CT peptide was almost completely abolished. Taken together, our results show that residues 242-248 do not play a significant role in defining the substrate specificity of PC2 but do contribute greatly to binding 7B2 and are critical for inhibition with the 7B2 CT peptide.

The radioresponse of the central nervous system: a dynamic process

Radiation continues to be a major treatment modality for tumors located within and close to the central nervous system (CNS). Consequently, alleviating or protecting against radiation-induced CNS injury would be of benefit in cancer treatment. However, the rational development of such interventional strategies will depend on a more complete understand-ing of the mechanisms responsible for the development of this form of normal tissue injury. Whereas the vasculature and the oligodendrocyte lineage have traditionally been considered the primary radiation targets in the CNS, in this review we suggest that other phenotypes as well as critical cellular interactions may also be involved in determining the radio-response of the CNS. Furthermore, based on the assumption that the CNS has a limited repertoire of responses to injury, the reaction of the CNS to other types of insults is used as a framework for modeling the pathogenesis of radiation-induced damage. Evidence is then provided suggesting that, in addition to acute cell death, radiation induces an intrinsic recovery/repair response in the form of specific cytokines and may

Endoproteolytic processing of integrin pro-alpha subunits involves the redundant function of furin and proprotein convertase (PC) 5A, but not paired basic amino acid converting enzyme (PACE) 4, PC5B or PC7

Several integrin alpha subunits undergo post-translational endoproteolytic processing at pairs of basic amino acids that is mediated by the proprotein convertase furin. Here we ask whether other convertase family members can participate in these processing events. We therefore examined the endoproteolysis rate of the integrin subunits pro-alpha5, alpha6 and alphav by recombinant furin, proprotein convertase (PC)5A, paired basic amino acid converting enzyme (PACE)4, PC1, PC2 and PC7 in vitro and/or ex vivo after overexpression in LoVo cells that were deficient in furin activity. We found that 60-fold more PC1 than furin was needed to produce 50% cleavage of pro-alpha subunit substrates in vitro; the defective pro-alpha chain endoproteolysis in LoVo cells was not rescued by overexpression of PC1 or PC2. No endoproteolysis occurred with PC7 either in vitro or ex vivo, although similar primary sequences of the cleavage site are found in integrins and in proteins efficiently processed by PC7, which suggests that a particular conformation of the cleavage site is required for optimal convertase-substrate interactions. In vitro, 50% cleavage of pro-alpha subunits was obtained with one-third of the amount of PC5A and PACE4 than of furin. In LoVo cells, PC5A remained more active than furin, PACE4 activity was quite low, and PC5B, which differs from PC5A by a C-terminal extension containing a transmembrane domain, was very inefficient in processing integrin alpha-subunit precursors. In conclusion, these results indicate that integrin alpha-subunit endoproteolytic processing involves the redundant function of furin and PC5A and to a smaller extent PACE4, but not of PC1, PC2, PC5B or PC7.

Molecular forms of alpha-melanocyte-stimulating hormone in pheochromocytoma tissue

The molecular forms of alpha-melanocyte-stimulating hormone (MSH) in pheochromocytoma tissues have been characterized using reversed-phase HPLC and radioimmunoassay. Six alpha-MSH-related peptides were detected. Three of the six peaks had elution times identical to those of synthetic desacetyl-alpha-MSH, alpha-MSH and diacetyl-alpha-MSH. The remaining three forms of the alpha-MSH-like immunoreactivity suggest a different processing of pro-opiomelanocortin in this tissue than in the intermediate lobe of the pituitary gland. Current results confirm that alpha-MSH are present in human pheochromocytoma and suggest that alpha-MSH has role in the pathophysiology of this tissue.

Regulation of prohormone convertase 1 (PC1) by gp130-related cytokines

The processing of pro-opiomelanocortin (POMC) to generate bioactive ACTH in the anterior pituitary is mediated by prohormone convertase 1 (PC1). Leukemia inhibitory factor (LIF) and interleukin 6 (IL-6), two cytokines sharing the common gp130 receptor subunit and functioning through activation of the intracellular JAK/STAT pathway, induce POMC synthesis and ACTH release. We investigated the effects of LIF and IL-6 on PC1 expression and its subsequent processing of POMC. A significant time-dependent up-regulation of both PC1 protein and mRNA by LIF and IL-6 was seen in mouse corticotroph AtT-20 cells. IL-6 or LIF increased the synthesis of ACTH-related products with a concomitant increase in bioactive 5 and 13 kDa ACTH indicating coordinated regulation of substrate and processing enzyme. AtT-20 cells transiently transfected with a human PC1-promoter-luciferase reporter construct and treated with LIF or IL-6 showed significantly increased luciferase activity. Additionally, lipopolysaccharide (LPS) administration to rats resulted in an increase in both pituitary PC1 and POMC mRNA. These findings suggest that the ACTH increase induced by LIF and IL-6 is due to both increased POMC synthesis as well as increased POMC processing by up-regulation of PC1. These two coordinately regulated processing events probably exert central roles in the pathophysiological response to some stresses, such as inflammatory stress.

Proprotein and prohormone convertases: a family of subtilases generating diverse bioactive polypeptides

Proproteins and prohormones are the fundamental units from which bioactive proteins and peptides as well as neuropeptides are derived by limited proteolysis within the secretory pathway. Precursors are usually cleaved at the general motif (K/R)--(X)n--(K/R)down arrow, where n=0, 2, 4 or 6 and X is any amino acid and usually is not a Cys. Seven mammalian precursor convertases (PCs) have been identified: PC1, PC2, furin, PC4, PC5, PACE4 and PC7. Each of these enzymes, either alone or in combination with others, is responsible for the tissue-specific processing of multiple polypeptide precursors both in the brain and in periphery. This combinatorial mechanism generates a large diversity of bioactive molecules in an exquisitively regulated manner. The production of null mice allowed the assessment of the critical role of convertases in vivo. Thus, male PC4 (-/-) mice are infertile, furin (-/-) and PC1(-/-) mice are embryonic lethal, and PC2 (-/-) mice are mildly diabetic and runted. Interestingly, animals deficient in 7B2, a PC2-specific binding protein, exhibit a Cushing-like syndrome and die soon after birth. Recently, the first member of a new class of subtilisin--kexin-like convertases, called SKI-1, was identified. Its structure is closer to pyrolysin than to mammalian PCs and it exhibits a specificity for cleavage at the motif (R/K)--X--X--(L,T) down arrow as deduced from its ability to process sterol regulatory element binding proteins and pro-brain derived neurotrophic factor. Thus, while PCs are responsible for the processing of neuropeptides, adhesion molecules, receptors, growth factors, cell surface glycoprotein and enzymes, SKI-1 cleaves proproteins that are critical for the control of cholesterol and fatty acid metabolism and for neuronal protection and growth.

The subtilisin/kexin family of precursor convertases. Emphasis on PC1, PC2/7B2, POMC and the novel enzyme SKI-1

Proopiomelanocortin (POMC) is a precursor to various, bioactive peptides including ACTH, beta LPH, alpha MSH, and beta endorphin (beta END). Processing of POMC at dibasic residues is tissue-specific and is performed by either PC1 alone (resulting in ACTH and beta LPH, anterior pituitary corticotrophes) or by a combination of PC1 and PC2 (yielding alpha MSH and beta END, pituitary neurointermediate lobe and hypothalamus). The PC2-specific binding protein 7B2 is intimately involved in the zymogen activation of proPC2 into PC2. Structure-function studies of these enzymes demonstrated the presence of N- and C-terminal domains, as well as specific amino acids within the catalytic segment that influence the degree of activity of each enzyme and the interaction of PC2 with 7B2. The tissue distribution, plasticity of expression, and the multiple precursors that are differentially cleaved by PC1 and/or PC2, predict a wide array of combinatorial activities of these convertases within the endocrine and neuroendocrine system. The phenotypic consequences of the absence of genetic expression of either PC1 or PC2 are now explored using knockout mice and in human patients suffering from obesity and diabetes.

Characterization of the enzymatic properties of the first and second domains of metallocarboxypeptidase D

Carboxypeptidase D (CPD) contains three domains with homology to other metallocarboxypeptidases. To further characterize the various domains, we constructed a series of point mutants with a critical active site Glu of duck CPD converted to Gln. The proteins were expressed in the baculovirus system, purified to homogeneity, and characterized. Point mutations within both the first and second domains eliminated enzyme activity, indicating that the third domain is inactive toward dansyl-Phe-Ala-Arg. CPD removed only the C-terminal Lys or Arg from peptides, with the first domain more efficient toward Arg and the second domain more efficient toward Lys. Peptides containing Pro in the penultimate position were poorly cleaved by either domain. Cleavage of a peptide with Ala in the penultimate position was most efficient, with the relative order Ala >/= Met > Ser, Phe > Tyr > Trp > Thr >/= Gln, Asp, Leu, Gly >> Pro for CPD with both domains active. There were only minor differences between the first and the second domains regarding the influence of the penultimate amino acid. The first domain was optimally active at pH 6.3-7.5, whereas the second domain was optimally active at pH 5. 0-6.5. Thus, the first and second carboxypeptidase domains have complementary enzyme activities. Furthermore, the finding that CPD with both domains active shows a broad activity to a wide range of substrates is consistent with a role for this enzyme in the processing of many proteins that transit the secretory pathway.

The cell biology of the prohormone convertases PC1 and PC2

Mature peptide hormones and neuropeptides are typically synthesized from much larger precursors and require several posttranslational processing steps--including proteolytic cleavage--for the formation of the bioactive species. The subtilisin-related proteolytic enzymes that accomplish neuroendocrine-specific cleavages are known as prohormone convertases 1 and 2 (PC1 and PC2). The cell biology of these proteases within the regulated secretory pathway of neuroendocrine cells is complex, and they are themselves initially synthesized as inactive precursor molecules. ProPC1 propeptide cleavage occurs rapidly in the endoplasmic reticulum, yet its major site of action on prohormones takes place later in the secretory pathway. PC1 undergoes an interesting carboxyl terminal processing event whose function appears to be to activate the enzyme. ProPC2, on the other hand, exhibits comparatively long initial folding times and exits the endoplasmic reticulum without propeptide cleavage, in association with the neuroendocrine-specific protein 7B2. Once the proPC2/7B2 complex arrives at the trans-Golgi network, 7B2 is internally cleaved into two domains, the 21-kDa fragment and a carboxy-terminal 31 residue peptide. PC2 propeptide removal occurs in the maturing secretory granule, most likely through autocatalysis, and 7B2 association does not appear to be directly required for this cleavage event. However, if proPC2 has not encountered 7B2 intracellularly, it cannot generate a catalytically active mature species. The molecular mechanism behind the intriguing intracellular association of 7B2 and proPC2 is still unknown, but may involve conformational rearrangement or stabilization of a proPC2 conformer mediated by a 36-residue internal segment of 21-kDa 7B2.

The inducible cyclic adenosine monophosphate early repressor (ICER) in the pituitary intermediate lobe: role in the stress response

The cAMP signalling pathway plays a key role in the regulation of the hypothalamic-pituitary-adrenal axis. Transcription factor CREM (cAMP response element modulator) is implicated in the modulation of a number of neuroendocrine functions. By virtue of an alternative, intronic promoter CREM generates the powerful transcriptional repressor ICER (inducible cAMP early repressor), which displays a pronounced neuroendocrine-specific expression. Here we document a remarkable induction of ICER in response to acute stress in the intermediate lobe (IL) of the pituitary gland. The induction is transient and is preceded by CREB phosphorylation. Adrenergic stimulation directs ICER induction in the IL through the activation of both beta2-adrenergic and corticotrophin-releasing hormone receptors. These receptors are positively coupled to the adenylate cyclase signalling pathway, which regulates hormone release from the IL, implicating ICER in the modulation of peptide secretion. We show that targeted ablation of the CREM gene in the mouse causes a chronic increase of beta-endorphin levels. Altered hormonal production occurs both in basal conditions and after stress. Thus, early ICER induction in the IL may be involved in the modulation of gene expression in response to stress.

Developmental expression of proprotein convertase 1/3 in the rat

We have isolated a clone that has 3' end sequence identity with prohormone convertase 1/3 (PC1/3) from a rat islet cDNA library. Northern blot analysis and immunocytochemical studies have confirmed its presence in the endocrine pancreas. Analysis of poly A mRNA from various adult tissues demonstrated that it was relatively abundant in whole brain, lung and spleen, but not detectable in kidney, testis and heart. Using probes consisting of either the coding region or the 3' end sequences, the mRNA transcripts identified were 5.0, 3.0 and 8.5 kb. The 8.5 kb transcript detected has not been described previously. RT-PCR of RNA isolated from rat embryonic tissues using a primer set corresponding to the 3' end of the PC1/3 sequence showed a steady increase of expression in fetal pancreas and intestine during the course of development. In contrast, comparatively high and constant levels of PC1/3 expression were detected in fetal lung, whereas low and constant expression was detected in fetal liver. Double immuno-staining showed that PC1/3 was co-localised with insulin throughout development, and at mid-gestation, PC1/3 immunoreactivity could also be detected within glucagon-producing cells in the developing pancreas. Thus, we have identified a novel PC1/3 mRNA transcript in the rat by using sequence-specific probes and have demonstrated that the developmental expression of prohormone convertase PC1/3 is confined primarily to pancreas and intestine, suggesting that it may play a possible role in regulating growth and differentiation of these tissues.

A role for amontillado, the Drosophila homolog of the neuropeptide precursor processing protease PC2, in triggering hatching behavior

Accurate proteolytic processing of neuropeptide and peptide hormone precursors by members of the kexin/furin family of proteases is key to determining both the identities and activities of signaling peptides. Here we identify amontillado (amon), the Drosophila melanogaster homolog of the mammalian neuropeptide processing protease PC2, and show that in contrast to vertebrate PC2, amontillado expression undergoes extensive regulation in the nervous system during development. In situ hybridization reveals that expression of amontillado is restricted to the final stages of embryogenesis when it is found in anterior sensory structures and in only 168 cells in the brain and ventral nerve cord. After larvae hatch from their egg shells, the sensory structures and most cells in the CNS turn off or substantially reduce amontillado expression, suggesting that amontillado plays a specific role late in embryogenesis. Larvae lacking the chromosomal region containing amontillado show no gross anatomical defects and respond to touch. However, such larvae show a greatly reduced frequency of a hatching behavior of wild-type Drosophila in which larvae swing their heads, scraping through the eggshell with their mouth hooks. Ubiquitous expression of amontillado can restore near wild-type levels of this behavior, whereas expression of amontillado with an alanine substitution for the catalytic histidine cannot. These results suggest that amontillado expression is regulated as part of a programmed modulation of neural signaling that controls hatching behavior by producing specific neuropeptides in particular neurons at an appropriate developmental time.

A role for amontillado, the Drosophila homolog of the neuropeptide precursor processing protease PC2, in triggering hatching behavior

Accurate proteolytic processing of neuropeptide and peptide hormone precursors by members of the kexin/furin family of proteases is key to determining both the identities and activities of signaling peptides. Here we identify amontillado (amon), the Drosophila melanogaster homolog of the mammalian neuropeptide processing protease PC2, and show that in contrast to vertebrate PC2, amontillado expression undergoes extensive regulation in the nervous system during development. In situ hybridization reveals that expression of amontillado is restricted to the final stages of embryogenesis when it is found in anterior sensory structures and in only 168 cells in the brain and ventral nerve cord. After larvae hatch from their egg shells, the sensory structures and most cells in the CNS turn off or substantially reduce amontillado expression, suggesting that amontillado plays a specific role late in embryogenesis. Larvae lacking the chromosomal region containing amontillado show no gross anatomical defects and respond to touch. However, such larvae show a greatly reduced frequency of a hatching behavior of wild-type Drosophila in which larvae swing their heads, scraping through the eggshell with their mouth hooks. Ubiquitous expression of amontillado can restore near wild-type levels of this behavior, whereas expression of amontillado with an alanine substitution for the catalytic histidine cannot. These results suggest that amontillado expression is regulated as part of a programmed modulation of neural signaling that controls hatching behavior by producing specific neuropeptides in particular neurons at an appropriate developmental time.

Proparathyroid hormone processing by the proprotein convertase-7: comparison with furin and assessment of modulation of parathyroid convertase messenger ribonucleic acid levels by calcium and 1,25-dihydroxyvitamin D3

We previously showed that the processing of proparathyroid hormone (proPTH) to PTH was accomplished most efficiently by furin (17). Colocalization studies demonstrated that furin is expressed in the parathyroid, whereas proprotein convertase (PC)1 and PC2 are not. Since that time, another member of the PC family, called PC7, has been identified. Here we show, using coinfection studies, that PC7, as well as furin, can appropriately cleave PTH from proPTH. ProPTH and PTH were purified from cell extracts by reversed-phase HPLC and were identified by Western blot analysis and delayed extraction matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Colocalization studies, using Northern blot and reverse transcriptase-PCR analyses, showed that PC7 messenger RNA (mRNA) is expressed in the parathyroid gland. Therefore, PC7, like furin, has the potential to be involved in the physiological processing of proPTH to PTH. The two major regulators of parathyroid cell synthetic and secretory activity are the extracellular fluid calcium and 1,25-dihydroxyvitamin D [1,25(OH)2D] levels. We investigated whether either of these agents might modulate processing of proPTH to PTH by altering parathyroid convertase gene expression. In both in vitro and in vivo systems in which regulation of PTH mRNA levels were clearly apparent, there was no effect of either calcium or 1,25(OH)2D3 on parathyroid furin or PC7 mRNA levels. This is in contrast to the processing of proinsulin to insulin in the pancreatic beta-cell, which is up-regulated by glucose stimulation of PC1 and PC2 synthesis.

Neuroregulation of ProTRH biosynthesis and processing

This review presents an overview of the current knowledge on proTRH biosynthesis, its processing, its tissue distribution, and the role of known processing enzymes in proTRH maturation. The neuroendocrine regulation of TRH biosynthesis, the biological actions of its products, and the signal transduction and catabolic pathways used by those products are also reviewed. The widespread expression of proTRH, PC1, and PC2 rnRNAs in hypophysiotropic and extrahypophysiotropic areas of the brain, with their overlapping distribution in many areas, indicates the striking versatility provided by tissue-specific processing in generating quantitative and qualitative differences in nonTRH peptide products as well as TRH. Evidence is presented suggesting that differential processing for proTRH at the intracellular level is physiologically relevant. It is clear that control over the diverse range of proTRH-derived peptides within a specific cell is accomplished most from the regulation at the posttranslational level rather than the translational or transcriptional levels. Several examples supporting this hypothesis are presented in this review. A better understanding of proTRH-derived peptides role represents an exciting new frontier in proTRH research. These connecting sequences in between TRH molecules to form the precursor protein may function as structural or targeting elements that guide the folding and sorting of proTRH and its larger intermediates so that subsequent processing and secretion are properly regulated. The particular anatomical distribution of the proTRH end products, as well as regulation of their levels by neuroendocrine or pharmacological manipulations, supports a unique potential biologic role for these peptides.

Cloning, expression, and characterization of human metalloprotease 1: a novel member of the pitrilysin family of metalloendoproteases

A novel cDNA, designated human metalloendoprotease 1 (hMP1), was identified on the basis of homology to known metalloendoproteases of the pitrilysin family. The full-length MP1 codes for a protein with an open reading frame of 1038 amino acids. The N-terminal region contains the HXXEH(X)76E catalytic domain that is conserved in the members of pitrilysin family, namely insulin-degrading enzyme and NRD convertase. The hMP1 mRNA is expressed in a number of cell lines and tissues as a single species of about 3.4 kb. The expression of hMP1 mRNA is higher in muscle and heart than in brain, pancreas, liver, lung, and placenta. The full-length hMP1 was expressed in the baculovirus system and purified to homogeneity using isoelectrofocusing and ion-exchange chromatography. The enzyme exhibited a neutral pH optimum and high sensitivity to thiol reagents. HMP1 was inactivated by 1,10-phenanthroline, a specific inhibitor of Zn(+2)-dependent metalloproteases. The enzyme was not inhibited by agents that inhibit neutral metalloendoproteases of the thermolysin family such as thimet endo-oligopeptidase, enkephalinase, or angiotensin-converting enzyme. HMP1 cleaved a prodynorphin-derived peptide, leumorphin, N-terminal to Arg in the monobasic processing site, as evidenced by MALDI-TOF mass spectrometry. However, the enzyme did not exhibit strict monobasic cleavage specificity, as peptide substrates with amino acid substitutions around the monobasic site was cleaved efficiently by hMP1. Taken together, these results suggest that hMP1 is a novel member of the metalloendoprotease superfamily with ubiquitous distribution that could play a broad role in general cellular regulation.

Expression of human prohormone convertase PC2 in a baculovirus-insect cell system

PC2 is a member of the eukaryotic family of subtilisin-related proprotein convertases which are thought to be involved in the intracellular proteolytic processing of prohormones and proneuropeptides. The presence of only small amounts of PC2 in the secretory granules of certain mammalian neuroendocrine cell types has made the characterization and further study of this enzyme difficult. We report here the expression of proteolytically active human PC2 protein in the insect cell-baculovirus system. Human PC2 expressed in insect cells is a calcium-dependent intracellular protein active at neutral pH. In insect cells, human PC2 was found intracellularly as 75-kDa and 71-kDa proteins. Both 73-kDa and 68-kDa forms were found in the conditioned medium, but no PC2 proteolytic activity was detected. We demonstrated the presence of a soluble inhibitor in infected-cell medium which may block PC2 activity.

The RGD motif and the C-terminal segment of proprotein convertase 1 are critical for its cellular trafficking but not for its intracellular binding to integrin alpha5beta1

Cellular trafficking of subtilisin/kexin-like precursor convertases (PCs) may be regulated by a number of motifs, some of which are present within the P-domain and in the C-terminal sequence. Six of the seven known PCs contain a conserved RGD sequence within the P domain. In order to investigate the functional importance of this motif, we generated mutants of PC1 that contain a Myc tag epitope inserted between the prosegment and the catalytic subunit. Cellular expression of vaccinia virus recombinants revealed that this tag did not seem to influence the autocatalytic conversion of proPC1 into PC1 or its bioactivity. The two PC1 variants produced possess either the wild type RGD sequence or its RGE mutant. Stable transfectants of these variants in AtT20 cells revealed that similar to the wild type enzyme, PC1-RGD-Myc is sorted to secretory granules. In contrast, PC1-RGE-Myc exits the cell via the constitutive secretory pathway. In vitro, a 14-mer peptide spanning the RGD sequence of PC1, but not its RGE mutant, binds to cell surface vitronectin-binding integrins of Chinese hamster ovary cells. However, within the endoplasmic reticulum and in an RGD-independent fashion, integrin alpha5beta1 associates primarily with the zymogens proPC1, proPC1-DeltaC (missing the C-terminal 137 residues), as well as proPC2. Thus, the observed discrimination between the secretion routes of PC1-RGD and PC1-RGE does not implicate integrins such as alpha5beta1.

Miniglucagon (glucagon 19-29), a potent and efficient inhibitor of secretagogue-induced insulin release through a Ca2+ pathway

Using the MIN6 B-cell line, we investigated the hypothesis that miniglucagon, the C-terminal () fragment processed from glucagon and present in pancreatic A cells, modulates insulin release, and we analyzed its cellular mode of action. We show that, at concentrations ranging from 0.01 to 1000 pM, miniglucagon dose-dependently (ID50 = 1 pM) inhibited by 80-100% the insulin release triggered by glucose, glucagon, glucagon-like peptide-1-(7-36) amide (tGLP-1), or glibenclamide, but not that induced by carbachol. Miniglucagon had no significant effects on cellular cAMP levels. The increase in 45Ca2+ uptake induced by depolarizing agents (glucose or extracellular K+), by glucagon, or by the Ca2+channel agonist Bay K-8644 was blocked by miniglucagon at the doses active on insulin release. Electrophysiological experiments indicated that miniglucagon induces membrane hyperpolarization, probably by opening potassium channels, which terminated glucose-induced electrical activity. Pretreatment with pertussis toxin abolished the effects of miniglucagon on insulin release. It is concluded that miniglucagon is a highly potent and efficient inhibitor of insulin release by closing, via hyperpolarization, voltage-dependent Ca2+ channels linked to a pathway involving a pertussis toxin-sensitive G protein.

Protein 7B2 is essential for the targeting and activation of PC2 into the regulated secretory pathway of rMTC 6-23 cells

Among the prohormone convertases, PC2 is unique in that it specifically binds to the neuroendocrine-specific protein 7B2 in the endoplasmic reticulum (ER) and is activated late in the regulated secretory pathway of neuroendocrine cells. Several roles, sometimes contradictory, have been suggested for 7B2 with regard to PC2 cellular fate. Thus, 7B2 was proposed to act as a PC2 chaperone in the ER, or to facilitate 7B2 transport from the ER to the trans-Golgi network and to be necessary for proPC2 activation, or to inhibit PC2 enzymatic activity until the latter reaches the secretory granules. To gain insight into the function of 7B2, we sought to block its expression in PC2-expressing endocrine cells using antisense strategies. We have previously shown that the endocrine rMTC 6-23 cell line expresses PC2 and that the enzyme is responsible for the processing of pro-neurotensin/neuromedin N (proNT/NN). Here, we show that rMTC 6-23 cells express 7B2 and that the protein was coordinately induced with PC2 and proNT/NN by dexamethasone. Stable transfection of rMTC 6-23 cells with 7B2 antisense cDNA led to a marked reduction (>90%) in 7B2 levels. ProPC2 was expressed to normal levels and cleaved to yield a PC2 form that was constitutively released, was not stored within secretory granules and was unable to process proNT/NN. We conclude that 7B2 is essential for the sorting and activation of PC2 into the regulated secretory pathway of endocrine cells.

Confocal microscopy reveals thimet oligopeptidase (EC 3.4.24.15) and neurolysin (EC 3.4.24.16) in the classical secretory pathway

Thimet oligopeptidase (EC 3.4.24.15; EP24.15) and neurolysin (EC 3.4.24.16; EP24.16) are closely related enzymes involved in the metabolic inactivation of bioactive peptides. Both of these enzymes were previously shown to be secreted from a variety of cell types, although their primary sequence lacks a signal peptide. To investigate the mechanisms responsible for this secretion, we examined by confocal microscopy the subcellular localization of these two enzymes in the neuroendocrine cell line AtT20. Both EP24.15 and EP24.16 were found by immunohistochemistry to be abundantly expressed in AtT20 cells. Western blotting experiments confirmed that the immunoreactivity detected in the soma of these cells corresponded to previously cloned isoforms of the enzymes. At the subcellular level, both enzymes colocalized extensively with the integral trans-Golgi network protein, syntaxin-6, in the juxtanuclear region. In addition, both EP24.15 and EP24.16 were found within small vesicular organelles distributed throughout the cell body. Some, but not all, of these organelles also stained positively for ACTH. These results demonstrate that both EP24.15 and EP24.16 are present within the classical secretory pathway. Their colocalization with ACTH further suggests that they may be targeted to the regulated secretory pathway, even in the absence of a signal peptide.

The neuroendocrine protein 7B2 is required for peptide hormone processing in vivo and provides a novel mechanism for pituitary Cushing's disease

The neuroendocrine protein 7B2 has been implicated in activation of prohormone convertase 2 (PC2), an important neuroendocrine precursor processing endoprotease. To test this hypothesis, we created a null mutation in 7B2 employing a novel transposon-facilitated technique and compared the phenotypes of 7B2 and PC2 nulls. 7B2 null mice have no demonstrable PC2 activity, are deficient in processing islet hormones, and display hypoglycemia, hyperproinsulinemia, and hypoglucagonemia. In contrast to the PC2 null phenotype, these mice show markedly elevated circulating ACTH and corticosterone levels, with adrenocortical expansion. They die before 9 weeks of severe Cushing's syndrome arising from pituitary intermediate lobe ACTH hypersecretion. We conclude that 7B2 is indeed required for activation of PC2 in vivo but has additional important functions in regulating pituitary hormone secretion.