Prosomatostatin II processing is initiated in the trans-Golgi network of anglerfish pancreatic cells

Processing and intracellular targeting of prosomatostatin-derived peptides: the role of mammalian endoproteases

Prosomatostatin is cleaved at dibasic and monobasic sites to produce somatostatin-14 and somatostatin-28 respectively. The mammalian pro-protein convertases comprising furin, PACE4 and PC1-6 have recently been identified and are believed to mediate endoproteolysis of prohormone precursors such as prosomatostatin. Furin is membrane bound, localized to the Golgi and mediates constitutive processing. PC1 and PC2 are soluble and are expressed solely in endocrine and neuroendocrine tissues suggesting a key role in prohormone processing. We have investigated the endogenous and heterologous synthesis and processing of rat prosomatostatin in 1027B2 rat islet somatostatinoma cells and in constitutive (COS-7, PC-12) and regulated (AtT-20, GH3/GH4C1) secretory cells. We have correlated processing efficiency with: secretion through the constitutive or regulated pathways; endogenous expression of furin, PC1 and PC2; and expression or overexpression of furin, PC1 and PC2. Pulse-chase studies showed that prosomatostatin is rapidly and independently processed to somatostatin-14 and somatostatin-28. Furin is capable of monobasic processing of prosomatostatin and is a candidate somatostatin-28 convertase. PC1 and PC2 both effect dibasic processing of prosomatostatin and qualify as putative somatostatin-14 convertases. PC1 is active in constitutive and regulated secretory cells, has a broader specificity and is overall more potent than PC2. Efficient processing of prosomatostatin begins in a Golgi or pre Golgi compartment. It requires the milieu of the secretory cell but not the secretory granule.

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.

Predominant basolateral proteolytic processing of prosomatostatin into somatostatin-28 in polarized LLC-PK1 cells

Polarized epithelial cells secrete specific proteins through their apical or basolateral membrane. In the present study, we have expressed the human prosomatostatin cDNA in the pig kidney epithelial cell line (LLC-PK1) and monitored the processing and release of the somatostatin-related peptides. Analysis by high-performance liquid chromatography and radioimmunoassay of the somatostatin-related peptides synthesized by the transfected cells showed that the LLC-PK1 cells released prosomatostatin and somatostatin-28 (S-28) in the culture medium. Furthermore, when the cells were polarized, we observed release of prosomatostatin from both membrane domains (apical and basolateral), while liberation of S-28 was mostly from the basolateral side. This observation suggests that, in these cells, the proprotein convertase(s) responsible for prosomatostatin processing is(are) associated with the basolateral secretory pathway.

Proteolytic processing of sulfated secretogranin II in the trans-Golgi network of GH3B6 prolactin cells

Secretogranin II (SgII) is a protein specific to the matrix of the secretory granules in neurons and neuroendocrine cells. We have already demonstrated the precursor-product relationship between sulfated SgII and four N-terminal derived peptides in GH3B6 prolactin cells. In this study, we have investigated the subcellular compartment in which the cleavage of SgII is initiated by taking advantage of its tyrosine sulfation in the trans-Golgi network (TGN). In order to prevent export of radiosulfated SgII from the TGN, we used brefeldin A (BFA) as well as incubation at 20 degrees C. BFA completely inhibited the cleavage of SgII when added immediately post-pulse. BFA added a few minutes post-pulse or after a 20 degrees C incubation, however, permitted the cleavage of SgII in the presence of the drug. These SgII-derived peptides generated in the presence of BFA could not be released upon stimulation of the cells by either thyroliberin, a physiological secretagogue, or KCl. These results demonstrate that SgII can be cleaved in the TGN. They also evidence that the cleavage occurs in a distal compartment of the TGN different from the sulfation site. The transfer of SgII from the sulfation site to this distal compartment of the TGN involves BFA-sensitive membrane dynamics.

Differential effects of temperature blockade on the proteolytic processing of three secretory granule-associated proteins

Vesicular transport within the secretory pathway can be arrested by incubating cells at 15 degrees C or 20 degrees C to block exit from the endoplasmic reticulum or trans-Golgi network, respectively. Using this powerful tool we have compared the intracellular sites of endoproteolytic processing of proopiomelanocortin and two prohormone processing enzymes in AtT-20 mouse pituitary corticotrope tumor cells. For comparison, proopiomelanocortin processing was also evaluated in primary neurointermediate pituitary cultures. AtT-20 cells synthesize and store endogenous proopiomelanocortin and prohormone convertase 1; AtT-20 cells expressing high levels of integral membrane or soluble peptidylglycine alpha-amidating monooxygenase were generated by stable transfection. Cells were incubated with [35S]methionine and chased at 4 degrees C, 15 degrees C, 20 degrees C or 37 degrees C. The endoproteolytic processing of peptidylglycine alpha-amidating mono-oxygenase, prohormone convertase 1, and proopiomelanocortin was compared following immunoprecipitation. Endoproteolytic processing of integral membrane and soluble peptidylglycine alpha-amidating monooxygenase proteins was completely blocked by incubation of cells at 20 degrees C. In contrast, prohormone convertase 1 processing from the 87 kDa precursor to the 81 kDa intermediate proceeded to completion at both 15 degrees C and 20 degrees C, while cleavage to generate the 63 kDa prohormone convertase 1 protein was completely blocked at 20 degrees C. In AtT-20 cells and neurointermediate pituitary cultures, generation of beta-lipotropin from proopiomelanocortin continued at a slow but significant rate at 20 degrees C, while processing of beta-lipotropin to beta-endorphin was blocked. Thus prohormone convertase 1 processing begins in the endoplasmic reticulum and is not completed until after the trans-Golgi network, while peptidylglycine alpha-amidating monooxygenase processing begins after the trans-Golgi network. Selected proopiomelanocortin cleavages begin before entry into immature granules.

Regulation of carboxypeptidase E. Effect of pH, temperature and Co2+ on kinetic parameters of substrate hydrolysis

Carboxypeptidase E is a member of the carboxypeptidase A and B gene family, with many of the putative active-site and substrate-binding residues conserved between these enzymes. However, the pH optimum of carboxypeptidase E is substantially lower than that of carboxypeptidases A and B. To evaluate whether the difference in the pH optima of these carboxypeptidases reflects fundamental differences in the ionization behaviour of active-site residues, the influence of pH on carboxypeptidase E activity was examined. The V(max) for hydrolysis of dansyl-Phe-Ala-Arg is pH-independent between 5 and 7, but decreases at pH values below 5. The pKa for the group the protonation of which leads to the loss of activity is approximately 4.8, and the slope of the V(max.)/pH profile suggests that only a single ionizable group is involved. In contrast, Km and V(max.)/Km are dramatically influenced by pH over the range 5-7, with multiple ionizable groups detected in this pH range. The pKa of the group the protonation of which decreases the V(max.) of substrate hydrolysis is lower (4.5) for carboxypeptidase E which had been reconstituted with Co2+. The enthalpy of ionization of the group observed in the V(max.) profile for carboxypeptidase E is approx. 28.9 kJ/mol. These results are compatible with the active-site model of the homologous carboxypeptidase A: in this model the ionization of a metal-bound water molecule is responsible for the observed decrease in V(max.).

Ontogeny of thyrotropin-releasing hormone biosynthesis and release in hypothalamic neurons

Thyrotropin-releasing hormone (TRH) is expressed at early postmitotic stages of hypothalamic neuron development, in the mouse and rat, as revealed by the presence of the mature peptide, of pro-TRH mRNAs, and of large precursor forms. This indicates a coordinate expression of several genes encoding, respectively, pro-TRH, its processing enzymes, and the cell machinery for intracellular transport, sorting, and release of TRH. During development, an acceleration of pro-TRH processing is revealed by an increased proportion of the mature peptide. This is correlated with changes in the respective distribution of pro-TRH and TRH along neurites and the ontogenesis of neurosecretory granules.

Investigation of a possible role of the amino-terminal pro-region of proopiomelanocortin in its processing and targeting to secretory granules

Proopiomelanocortin (POMC) is a polyprotein which is targeted to the regulated secretory pathway of neuroendocrine cells where it undergoes tissue-specific proteolysis to yield peptides such as adrenocorticotropic hormone, beta-lipotropin and beta-endorphin. The pro-region of POMC is 49 amino acid long with two disulfide bonds between cysteine residues 2 and 24 and 8 and 20. These cysteine residues are conserved across the species. The pro-region contains no known hormonal sequence. Sorting to the regulated secretory pathway is thought to involve targeting signals encoded in the structure of secretory proteins. In the present study, we have examined the possibility that the disulfide bridges located in the NH2-terminal portion of the pro-region of POMC are essential for maintaining a determinant involved in the sorting of POMC to the regulated secretory pathway. Using site-directed and deletion mutagenesis of the porcine POMC cDNA, we created mutants in which one or both disulfide bridges were disrupted or in which the first 26 amino acid residues of the pro-region were deleted. Recombinant retroviruses carrying the mutated POMC cDNAs were used to infect Neuro2A cells. Immunofluorescence and immunoelectron microscopy studies performed on infected cells revealed that the unmutated and mutated POMC-immunoreactive peptides were localized in dense-core vesicles at the tips of cellular extensions. Analysis of the POMC-immunoreactive peptides extracted from the infected Neuro2A cells indicated that the mutated precursors in which one disulfide bridge was disrupted (POMC-S2 or POMC-S8) were stored and processed as efficiently as the unmutated POMC. By contrast, the mutated precursor in which both disulfide bridges were disrupted (POMC-S2,8) did not accumulate in intracellular compartments to the same extent as unmutated POMC. Moreover, this mutant was very inefficiently processed and no release could be observed upon stimulation of the cells with K+/Ca2+. These results suggest that POMC-S2,8 entered the regulated secretory pathway less efficiently than the unmutated precursor. However, when both disulfide bridges were removed from the precursor from the precursor by deletion of the first 26 amino acid residues of POMC, the truncated precursor (POMC delta 1-26) behaved as the unmutated POMC. Taken together our results indicate that the NH2-terminal portion of the pro-region including both disulfide bridges can be deleted without affecting the targeting of the molecule to secretory granules. However, when the entire POMC sequence is expressed in Neuro2A cells, the proper folding of the NH2-terminal region might be important for efficient processing and targeting.

Differential processing of hormone precursor. Independent production of somatostatins 14 and 28 in transfected neuroblastoma 2A cells

Neuro 2A cells infected with a retroviral vector carrying human prosomatostatin cDNA expressed and processed correctly the precursor into somatostatins-14 and -28 [(1989) EMBO J. 8, 2911-2916]. In order to study the mechanisms by which the active hormone sequences arise, site directed mutagenesis was performed on either the dibasic (ArgLys) or monobasic (Arg) cleavage sites involved in the production of somatostatins-14 and -28, respectively. Radioimmunochemical analysis of the somatostatin-related products indicated that replacement of either Arg-2-Lys-1 by Asn-2-Asn-1 or of Arg-15 by Asn-15 resulted in the exclusive production of either somatostatin-28 or -14, respectively. Moreover only prosomatostatin[1-76] was detected and no somatostatin-28[1-12] could be measured in cell extracts. Selective suppression of either somatostatin-14 or somatostatin-28 release by mutation did not affect the level of production of the other hormone but resulted in a correlative increase of unprocessed prosomatostatin. It is concluded that in this cell type (i) somatostatin-14 is exclusively generated by dibasic cleavage at the Arg-2-Lys-1 site of the intact precursor with concomitant production of prosomatostatin[1-76], and (ii) no direct interactions between the monobasic and dibasic processing domains occur.