PC12 cells can be induced to produce, but do not process, the neurotensin/neuromedin N precursor

Modulation of prohormone convertase 1/3 properties using site-directed mutagenesis

Prohormone convertase (PC)1/3 and PC2 cleave active peptide hormones and neuropeptides from precursor proteins. Compared with PC2, recombinant PC1/3 exhibits a very low specific activity against both small fluorogenic peptides and recombinant precursors, even though the catalytic domains in mouse PC1/3 and PC2 share 56% amino acid sequence identity. In this report, we have designed PC2-specific mutations into the catalytic domain of PC1/3 in order to investigate the molecular contributions of these sequences to PC1/3-specific properties. The exchange of residues RQG(314) with the SY sequence present in the same location within PC2 paradoxically shifted the pH optimum of PC1/3 upward into the neutral range; other mutations in the catalytic domain had no effect. Although none of the full-length PC1/3 mutants examined exhibited increased specific activity, the 66-kDa form of the RQG(314)SY mutant was two to four times more active than the 66-kDa form of wild-type PC1/3. However, stable transfection of RQG(314)SY into PC12 cells did not result in greater activity against the endogenous substrate proneurotensin, implying unknown cellular controls of PC1/3 activity. Mutation of GIVTDA(243-248) to QPFMTDI, a molecular determinant of 7B2 binding, resulted in increased zymogen expression but no propeptide cleavage or secretion, suggesting that this mutant is trapped in the endoplasmic reticulum due to an inability to cleave its own propeptide. We conclude that many convertase-specific properties are attributable less to convertase-specific catalytic cleft residues than to convertase-specific domain interactions.

Neurotensin and neuromedin N are differentially processed from a common precursor by prohormone convertases in tissues and cell lines

Neurotensin (NT) is synthesized as part of a larger precursor that also contains neuromedin N (NN), a six amino acid NT-like peptide. NT and NN are located in the C-terminal region of the precursor (pro-NT/NN) where they are flanked and separated by three Lys-Arg sequences. A fourth dibasic sequence is present in the middle of the precursor. Dibasics are the consensus sites recognized and cleaved by specialized endoproteases that belong to the family of proprotein convertases (PCs). In tissues that express pro-NT/NN, the three C-terminal Lys-Arg sites are differentially processed, whereas the middle dibasic is poorly cleaved. Processing gives rise mainly to NT and NN in the brain, NT and a large peptide with a C-terminal NN moiety (large NN) in the gut, and NT, large NN, and a large peptide with a C-terminal NT moiety (large NT) in the adrenals. Recent evidence indicates that PC1, PC2, and PC5-A are the prohormone convertases responsible for the processing patterns observed in the gut, brain, and adrenals, respectively. As NT, NN, large NT, and large NN are all endowed with biological activity, the evidence reviewed here supports the idea that posttranslational processing of pro-NT/NN in tissues may generate biological diversity of pathophysiological relevance.

Differential processing of pro-neurotensin/neuromedin N and relationship to pro-hormone convertases

Neurotensin (NT) is synthesized as part of a larger precursor that also contains neuromedin N (NN), a six amino acid neurotensin-like peptide. NT and NN are located in the C-terminal region of the precursor (pro-NT/NN) where they are flanked and separated by three Lys-Arg sequences. A fourth dibasic sequence is present in the middle of the precursor. Dibasics are the consensus sites recognized and cleaved by endoproteases that belong to the recently identified family of pro-protein convertases (PCs). In tissues that express pro-NT/NN, the three C-terminal Lys-Arg sites are differentially processed, whereas the middle dibasic is poorly cleaved. Pro-NT/NN processing gives rise mainly to NT and NN in the brain, to NT and a large peptide ending with the NN sequence at its C-terminus (large NN) in the gut and to NT, large NN and a large peptide ending with the NT sequence (large NT) in the adrenals. Recent evidence indicates that PC1, PC2 and PC5-A are the pro-hormone convertases responsible for the processing patterns observed in the gut, brain and adrenals, respectively. As NT, NN, large NT and large NN are all endowed with biological activity, the evidence reviewed here supports the idea that post-translational processing of pro-NT/NN in tissues may generate biological diversity.

Involvement of neurotensin in cancer growth: evidence, mechanisms and development of diagnostic tools

Focusing on the literature of the past 15 years, we evaluate the evidence that neurotensin and neurotensin receptors participate in cancer growth and we describe possible mechanisms. In addition, we review the progress achieved in the use of neurotensin analogs to image tumors in animals and humans. These exciting advances encourage us to pursue further research and stimulate us to consider novel ideas regarding the multiple inputs to cancer growth that neurotensin might influence.

Prohormone convertases differentially process pro-neurotensin/neuromedin N in tissues and cell lines

Neurotensin (NT) is synthesized as part of a larger precursor that also contains neuromedin N (NN), a six-amino acid neurotensin-like peptide. NT and NN are located in the C-terminal region of the precursor (pro-NT/NN) where they are flanked and separated by three Lys-Arg sequences. A fourth dibasic sequence is present in the middle of the precursor. Dibasics are the consensus sites recognized and cleaved by specialized endoproteases that belong to the family of proprotein convertases (PCs). In tissues that express pro-NT/NN, the three C-terminal Lys-Arg sites are differentially processed, whereas the middle dibasic is poorly cleaved. Processing gives rise mainly to NT and NN in the brain, to NT and a large peptide with a C-terminal NN moiety (large NN) in the gut, and to NT, large NN, and a large peptide with a C-terminal NT moiety (large NT) in the adrenals. Recent evidence indicates that PC1, PC2, and PC5-A are the prohormone convertases responsible for the processing patterns observed in the gut, brain, and adrenals, respectively. As NT, NN, large NT, and large NN are all endowed with biological activity, the evidence reviewed in this paper supports the idea that posttranslational processing of pro-NT/NN in tissues may generate biological diversity of pathophysiological relevance.

Differential regulation of gene expression of neurotensin and prohormone convertases PC1 and PC2 in the bovine ocular ciliary epithelium: possible implications on neurotensin processing

Prohormone convertases PC1 and PC2 are enzymes involved in the intracellular processing of pro-neurotensin/neuromedin N (pro-NT/NN) through the regulated secretory pathway. In this study, we present evidence of the differential gene expression of pro-NT/NN, pro-PC1 and pro-PC2 in two cell lines established from the neuroendocrine ocular ciliary epithelium. Dexamethasone and forskolin were found to synergistically up-regulate NT/NN mRNA expression in both cell types. The pigmented cells released NT, and this release was enhanced by agents that induced its biosynthesis. In contrast, nonpigmented cells exhibited a significantly reduced neurotensin secretion in response to inducers, leading to an accumulation of the peptide. PC1 and PC2 mRNA expression was induced in a cell-specific manner by the same agents that enhanced pro-NT/NN biosynthesis. These results demonstrate cell-specific processing of pro-NT/NN by the ciliary epithelium.

Production of recombinant large proneurotensin/neuromedin N-derived peptides and characterization of their binding and biological activity

Proneurotensin/neuromedin N (pro-NT/NN) is the common precursor of two biologically active related peptides, neuromedin N (NN) and neurotensin (NT). It undergoes a tissue-specific processing leading to the formation in some tissues and cancer cell lines of large peptides ending with the NT (large NT) or NN (large NN) sequence. In this study, we prepared and purified high amounts of recombinant large NT and large NN using the Drosophila S2 cell expression system. The binding and pharmacological properties of recombinant large peptides were characterized and compared to those of NT and NN using either COS cells transfected with the human subtype-1 NT receptor (hNTS1) or the human colon adenocarcinoma HT29 cell line that endogenously expresses hNTS1. Furthermore, the metabolic stability of the large peptides, when exposed to HT29 cells, was compared to that of NT and NN. Both large NT and large NN were able to bind to and activate hNTS1 with potencies that were approximately 10 times lower than that of their small counterpart. In addition, the large forms proved to be far less sensitive to degradation than the small peptides. Taken together, these data suggest that the large forms might represent endogenous, long-lasting activators of hNTS1 in a number of physiopathological situations.

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.

PC5-A-mediated processing of pro-neurotensin in early compartments of the regulated secretory pathway of PC5-transfected PC12 cells

Among the members of the proprotein convertase (PC) family, PC1 and PC2 have well established roles as prohormone convertases. Another good candidate for this role is PC5-A that has been shown to be present in the regulated secretory pathway of certain neuroendocrine tissues, but evidence that it can process prohormones is lacking. To determine whether PC5-A could function as a prohormone convertase and to compare its cleavage specificity with that of PC1 and PC2, we stably transfected the rat pheochromocytoma PC12 cell line with PC5-A and analyzed the biosynthesis and subcellular localization of the enzyme, as well as its ability to process pro-neurotensin/neuromedin N (pro-NT/NN) into active peptides. Our data showed that in transfected PC12 cells, PC5-A was converted from its 126-kDa precursor form into a 117-kDa mature form and, to a lesser extent, into a C-terminally truncated 65-kDa form of the 117-kDa product. Metabolic and immunochemical studies showed that PC5-A was sorted to early compartments of the regulated secretory pathway where it colocalized with immunoreactive NT. Furthermore, pro-NT/NN was processed in these compartments according to a pattern that differed from that previously described in PC1- and PC2-transfected PC12 cells. This pattern resembled that previously reported for pro-NT/NN processing in the adrenal medulla, a tissue known to express high levels of PC5-A. Altogether, these data demonstrate for the first time the ability of PC5-A to function as a prohormone convertase in the regulated secretory pathway and suggest a role for this enzyme in the physiological processing of pro-NT/NN.

Pro-neurotensin/neuromedin N expression and processing in human colon cancer cell lines

The regulatory peptide neurotensin NT has been proposed to exert an autocrine trophic effect on human colon cancers. In the present study, pro-neurotensin/neuromedin N (proNT/NN) expression and processing were investigated in 13 human colon cancer cell lines using a combination of radioimmunoassay and HPLC techniques. All 13 cell lines displayed low to moderate levels of proNT/NN ranging from 10 to 250 fmol/mg protein. However, only 6 (HCT8, LoVo, HT29, C119A, LS174T, and coloDM320) processed the precursor. Three of the latter (HCT8, LS174T, and coloDM320) were analysed in detail with regard to proNT/NN processing pattern and were found to produce NT and large precursor fragments ending with the NT or NN sequence. They had no detectable level of NN. Such a processing pattern resembles that generated by the prohormone convertase PC5. Northern and Western blot analysis of prohormone convertase expression in the 3 cell lines revealed that they were devoid of PC1 and PC2, whereas they all expressed PC5. These data indicate that proNT/NN is a good marker of human colon cancer cell lines while NT is found in only about half of the cell lines. They also suggest that, in addition to NT, several proNT/NN-derived products, possibly generated by PC5, might exert an autocrine positive effect on human colon cancer growth.

Lithium increases melanin-concentrating hormone mRNA stability and inhibits tyrosine hydroxylase gene expression in PC12 cells

Melanin-concentrating hormone (MCH) is a cyclic peptide involved in the regulation of food-intake behaviour and stress response in mammals. Expression of the MCH gene predominates in hypothalamic neurons. Mechanisms governing the regulation of expression of MCH gene in established cell lines were not explored yet. Here, we analysed the actions of nerve growth factor (NGF), dexamethasone, forskolin and lithium on MCH mRNA levels in the PC12 pheochromocytoma cell line. We compared them with those observed on tyrosine hydroxylase (TH) mRNA, constitutively expressed in PC12 cells, and neurotensin (NT) mRNA, taken as a control. In untreated cells, MCH RNA species of high molecular weight were found. Exposure of cells at a combination of NGF and lithium resulted in decreased expression of these MCH RNAs and in the transient production of mature MCH mRNA. Strikingly, after short exposure of PC12 cells to NGF, lithium per se elicited a marked increase in MCH mRNA levels whilst it exerted a potent inhibitory action on TH mRNA expression. Detailed investigations revealed that lithium enhanced MCH mRNA expression through post-transcriptional mechanisms whereas it regulated TH gene expression mainly at the level of transcription. These results demonstrate that lithium, an agent widely used for treatment of manic depressive illness, can exert an opposite effect on MCH and TH mRNA production in PC12 cells. The MCH gene system in NGF-treated PC12 cells provides a good opportunity for studying the effect of lithium on gene expression at post-transcriptional levels in a neuron-like cellular model.

Evidence that PC2 is the endogenous pro-neurotensin convertase in rMTC 6-23 cells and that PC1- and PC2-transfected PC12 cells differentially process pro-neurotensin

The neuropeptide precursor proneurotensin/neuromedin N (pro-NT/NN) is mainly expressed and differentially processed in the brain and in the small intestine. We showed previously that rMTC 6-23 cells process pro-NT/NN with a pattern similar to brain tissue and increase pro-NT/NN expression in response to dexamethasone, and that PC12 cells also produce pro-NT/NN but are virtually unable to process it. In addition, PC12 cells were reported to be devoid of the prohormone convertases PC1 and PC2. The present study was designed to identify the proprotein convertase(s) (PC) involved in pro-NT/NN processing in rMTC 6-23 cells and to compare PC1- and PC2-transfected PC12 cells for their ability to process pro-NT/NN. rMTC 6-23 cells were devoid of PC1, PC4, and PC5 but expressed furin and PC2. Stable expression of antisense PC2 RNA in rMTC 6-23 cells led to a 90% decrease in PC2 protein levels that correlated with a > 80% reduction of pro-NT/NN processing. PC2 expression was stimulated by dexamethasone in a time- and concentration-dependent manner. Stable PC12/PC2 transfectants processed pro-NT/NN with a pattern similar to that observed in the brain and in rMTC 6-23 cells. In contrast, stable PC12/PC1 transfectants reproduced the pro-NT/NN processing pattern seen in the gut. We conclude that (i) PC2 is the major pro-NT/NN convertase in rMTC 6-23 cells; (ii) its expression is coregulated with that of pro-NT/NN in this cell line; and (iii) PC2 and PC1 differentially process pro-NT/NN with brain and intestinal phenotype, respectively.

Mutational analysis of PC1 (SPC3) in PC12 cells. 66-kDa PC1 is fully functional

The proteinase mPC1, a neuroendocrine member of the mammalian family of subtilisin-like enzymes, has previously been shown to be converted to a carboxyl-terminally truncated 66-kDa form during transport through the secretory pathway. The cleavage site and the function of this carboxyl-terminal truncation event are unknown. We have performed site-directed mutagenesis of two paried basic sites in the mPC1 carboxyl-terminal tail and expressed these constructs in PC12 cells, a rat pheochromocytoma known to lack endogenous PC1. We found that the most likely site for the truncation event was at Arg590-Arg591 since mutation of this site to Lys-His prevented processing of 87-kDa PC1. A PC1 mutant carboxyl-terminally truncated at this site and expressed in PC12 cells was efficiently routed to the secretory pathway and stored in secretory granules, indicating that the carboxyl-terminal extension is not required for sorting of this enzyme. The function of the various PC1 constructs was assessed by analyzing proneurotensin cleavage to various forms. The carboxyl-terminally truncated PC1 mutant was found to perform most of the cleavages of this precursor as well as wild-type PC1; however, the blockade mutant processed proneurotensin much less efficiently. Differences between the site preferences of the various enzymes were noted. Our results support the notion that carboxyl-terminal processing of PC1 serves to regulate PC1 activity.

BON cells display the intestinal pattern of neurotensin/neuromedin N precursor processing

Antisera towards the bioactive peptides, neurotensin (NT, 13 residues) and neuromedin N (NMN, 6 residues), as well as towards three regions of their 147-residue canine precursor were used to identify and to quantitate precursor-derived peptides in extracts of human BON cells. This cell-line, which was obtained from a human pancreatic carcinoid tumor, constitutively expresses NT/NMN mRNA and secretes NT. Quantitation of seven precursor-derived peptides led us to conclude that BON cells display the intestinal pattern of NT/NMN precursor processing, which is primarily characterized by the production of a large molecular (125 amino acid) form of NMN. Four large molecular components, identified by immunochemical analyses and Western blotting, displayed physico-chemical properties which, for the most part, were consistent with the structures predicted from the partially-known human mRNA sequence. However, as shown previously for these peptides in canine gut, the empirically determined M(r) and pI values were slightly higher than those predicted solely from the amino acid content, perhaps due to the presence of additional substituents. These results suggest that BON cells may provide a good in vitro model in which to study the regulation of intestinal NT/NMN precursor processing and the nature of the enzyme(s) involved.