PubMed Health⌕ Search

Biomedical subjects

Y C Patel

Publications and source records attributed to Y C Patel.

At least 37 records · Page 2Linked to original sources
PubMed1998

Polymorphism in the 5' flanking region of the human somatostatin receptor subtype 5.

The human somatostatin receptor subtype 5 (hSSTR5) gene has previously been cloned and localized to chromosome 16 p13.3. This region is evolutionarily conserved in all vertebrate genomes from the puffer fish (Fugu rubripes) to human, and also contains loci for genes associated with two common multisystemic disorders, adult polycystic kidney disease (PKD1) and tuberous sclerosis (TSC2). Analysis of the 5' flanking region of the hSSTR5 gene has revealed consensus sequences for a number of transcription factors as well as Alu-like repeat elements. In the present study, genomic DNA from 53 unrelated individuals was analysed by PCR and Southern blots probed with radiolabeled fragments generated from different segments of the hSSTR5 gene. We have identified two restriction fragment length polymorphisms (RFLP) with high heterozygosity values at the 5' flanking region of the hSSTR5 gene. These RFLP markers will be useful for determining the allelic loss of genetic material from this region. The observed polymorphism in the promoter region may affect the function of the hSSTR5 gene.

Adult↗
PubMed1998

Expression of somatostatin receptor subtypes in human brain tumors.

Expression of mRNA for the 5 somatostatin receptors (sst1-5) was characterized by Northern blot and RT-PCR analysis in 20 meningioma and 9 glioma samples. sst1 mRNA was detectable by Northern blots of poly-A+ RNA in meningiomas but not gliomas. In contrast, sst2 mRNA was readily detected by Northern blots of total RNA as a major 2.3 kb transcript and 2 minor 4.3 kb and 8 kb transcripts in all meningiomas and 6 out of 9 gliomas. Quantitation of the 2.3 kb sst2 mRNA showed that 15 out of 20 tumors expressed 1.3- to 33-fold higher levels than control normal human brain. Mean sst2 mRNA for the 20 meningioma samples was 978% that of normal brain. Three gliomas showed 7- to 14-fold higher sst2 mRNA than normal brain whereas the remaining samples displayed very low or undetectable levels. Immunocytochemistry of meningioma and glioma samples, with a sst2-specific antibody revealed immunoreactivity in tumor cells and peritumoral tissue, with prominent expression in blood vessels. mRNA for sst3,4,5 could not be detected by Northern blots in any of the tumors. RT-PCR analysis of meningiomas and gliomas revealed the following percent of tumors positive for a given sst mRNA: sst1 (86%), sst2 (100%), sst3 (60%), sst4 (58%), and sst5 (67%); 85% of tumors expressed 3 of the 5 subtypes. No correlation was found between the pattern of expression of sst mRNA and tumor type, location, and histology for either the meningiomas or gliomas. Our results show that meningiomas and gliomas are all positive for at least one sst subtype, the majority expressing multiple subtypes. sst2 is the most abundant isoform with a rich expression in both tumor and peritumoral tissue especially blood vessels.

Adult↗
PubMed1998

Cellular cholesterol storage in the Niemann-Pick disease type C mouse is associated with increased expression and defective processing of apolipoprotein D.

Apolipoprotein D (apoD), a member of the lipocalin superfamily of ligand transporters, has been implicated in the transport of several small hydrophobic molecules including sterols and steroid hormones. We have previously established that apoD is a secreted protein from cultured mouse astrocytes and that treatment with the oxysterol 25-hydroxycholesterol markedly stimulates apoD release. Here, we have investigated expression and cellular processing of apoD in the Niemann-Pick type C (NPC) mouse, an animal model of human NPC, which is a genetic disorder affecting cellular cholesterol transport. NPC is phenotypically characterized by symptoms of chronic progressive neurodegeneration. ApoD gene expression was up-regulated in cultured NPC astrocytes and in NPC brain. ApoD protein levels were also increased in NPC brain with up to 30-fold higher apoD content in the NPC cerebellum compared with control mice. Subcellular fractionation of NPC brain homogenates revealed that most of the apoD was associated with the myelin fraction. ApoD was found to be a secreted protein from cultured normal astrocytes and treatment with the oxysterol, 25-hydroxycholesterol, markedly stimulated apoD release (by five- to 10-fold). By contrast, secretion of apoD from NPC astrocytes was markedly reduced and could not be stimulated by oxysterol treatment. Secretion of apoE, another apolipoprotein normally produced by astrocytes, was similar in NPC and control cells. Furthermore, apoE secretion was not potentiated by oxysterol treatment in either cell type. Plasma levels of apoD were sixfold higher in NPC, whereas hepatic levels were substantially reduced compared with controls, possibly reflecting reduced hepatic clearance of the circulating protein. These results reveal hitherto unrecognized defects in apoD metabolism in NPC that appear to be linked to the known defects in cholesterol homeostasis in this disorder.

Animals↗
PubMed1997

Somatostatin-14, somatostatin-28, and prosomatostatin[1-10] are independently and efficiently processed from prosomatostatin in the constitutive secretory pathway in islet somatostatin tumor cells (1027B2).

We have characterized the biosynthetic origin of somatostatin-14 (SS-14), SS-28, and pro-SS[1-10] from pro-SS (PSS) in 1027B2 rat islet tumor cells. Because these cells lack regulated secretion and show unresponsiveness of the SS gene to cAMP, we have additionally carried out morphological and functional studies to elucidate the molecular defect in cAMP signalling and to localize the sites of PSS maturation along the secretory pathway. Cell extracts and secretion media were analysed by high performance liquid chromatography and specific C- and N-terminal radioimmunoassays. Electron microscopic sampling of 1027B2 cell cultures showed that most cells had very few dense core secretory granules for heterogeneous sizes. The cells expressed the endoproteases furin, PC1, and PC2 and contained large quantities of fully processed SS-14 and SS-28 with very little unprocessed PSS (ratio SS-14:SS-28:PSS = 39:51:10%). They secreted high concentrations of SS-14, SS-28, and PSS[1-10] constitutively along with PC1 and PC2. Pulse-chase studies demonstrated that PSS is rapidly (within 15 min), and efficiently processed to SS-14, SS-28, and PSS[1-10] via separate biosynthetic pathways: PSS --> SS-14 + 8 kDa; PSS --> SS-28 + 7 kDa; PSS --> PSS[1-10]. Monensin reduced intracellular SS-like immunoreactivity without altering processing efficiency. Transfection with the catalytic subunit of protein kinase A (PKA-C) activated SS promoter-CAT activating indicating that the defect in cAMP-dependent signaling in 1027B2 cells lies at the level of PKA-C. PKA-C overexpression failed to alter the ratio of processed SS-14 and SS-28. These results demonstrate that SS-14, SS-28, and PSS[1-10] are independently synthesized from PSS and that efficient precursor processing can occur within the constitutive secretory pathway in the relative absence of dense core secretory vesicles.

Adenoma, Islet Cell↗
PubMed1997

Expression of NMDA receptor-1 (NR1) and huntingtin in striatal neurons which colocalize somatostatin, neuropeptide Y, and NADPH diaphorase: a double-label histochemical and immunohistochemical study.

The subset of striatal neurons which colocalize SS/NPY/NADPH-d are selectively resistant to neurodegeneration in Huntington's Disease (HD) and to excitotoxic cell death induced experimentally with NMDA receptor (NMDAR) agonists. Here we have analyzed the expression of immunoreactive NMDAR-1 (NR1) subunit (as an index of NMDAR protein) and of huntingtin (the normal product of the HD gene) in primary cultures of rat striatum to see if differential expression of the two antigens in the subset of SS/NPY/NADPH-d and other striatal neurons can explain their selective resistance or vulnerability. Double-label histochemical and immunocytochemical studies were carried out using conventional and confocal laser scanning microscopy to characterize the cellular and subcellular expression of NR1 and SS, or NPY or bNOS, together with NADPH-d histochemistry. The percentages of cultured striatal neurons that were positive for NADPH-d, SS, NPY, bNOS, and NRI were, respectively, 3.8, 8.4, 10.2, 5.1, and 80%. The majority of striatal NADPH-d neurons coexpressed SS and NPY; 17% of SS-producing neurons were strongly positive for NR1; the remaining cells (approximately 80%) exhibited only weak NR1 expression. Comparable data were obtained for NPY-positive neurons, 15% of which colocalized NR1 strongly and 70-80% weakly. By double-label immunofluorescence, huntingtin was nonselectively expressed in virtually all striatal neurons including SS/NPY/NADPH-d neurons. These results show that the majority of striatal SS/NPY/NADPH-d neurons express NR1. The relative abundance of NR1 in SS/NPY/NADPH-d neurons, however, varies between a small subset of neurons that are receptor rich and the remainder that express low levels only and may determine susceptibility to NMDAR-mediated neurotoxicity. Huntingtin is nonselectively expressed in virtually all striatal neurons and does not appear to be a determinant of the selective resistance of normal striatal SS/NPY/NADPH-d neurons to NMDA toxicity.

Animals↗
PubMed1997

Probing the structure of the ligand binding cavity of lipocalins by fluorescence spectroscopy.

The lipocalin superfamily constitutes a phylogenetically conserved group of more than 40 proteins that function in the binding and transport of a variety of physiologically important ligands. Members of this family subserve diverse functions as carriers of retinoids (retinol binding protein), odorants (odorant binding proteins), chromophores (insecticyanin, INS), pheromones (aphrodisin) and sterols (apolipoprotein D, apoD). Despite the pivotal importance of the ligand binding function of these proteins, a suitable approach for characterizing the molecular determinants of such binding has not been available. In studies using three homogeneously purified lipocalins INS, beta-lactoglobulin (BLG) and human apoD, we find that the fluorescence reporter BIS (1,1'-bi(4-anilino) naphthalene-5,5'-disulfonic acid) is an ideal candidate for use in rapid kinetic experiments and in fluorescence resonance energy transfer (FRET). These methods require only small amounts of reagents and yield molecular coordinates of the ligand binding cavity of lipocalins in solution that are in remarkably close agreement to those obtained from crystallographic work with solids. Extremely fast ligand binding dynamics is indicated.

Animals↗
PubMed1997

Ligand binding pocket of the human somatostatin receptor 5: mutational analysis of the extracellular domains.

The ligand binding domain of G protein-coupled receptors for peptide ligands consists of a pocket formed by extracellular and transmembrane domain (TM) residues. In the case of somatostatin (SRIF), however, previous studies have suggested that the binding cavity of the octapeptide analog SMS201-995 (SMS) is lined by residues in TMs III-VII. The additional involvement of the extracellular domains for binding SMS or the natural SRIF ligands (SRIF-14, SRIF-28) has not been clarified. Using a cassette construct cDNA for the human somatostatin 5 receptor (sst5R), we systematically examined the role of exofacial structures in ligand binding by creating a series of mutants in which the extracellular portions have been altered by conservative segment exchange (CSE) mutagenesis for the extracellular loops (ECLs) and by deletion (for the NH2-terminal segment) or truncation analysis (ECL3). CHO-K1 cells were stably transfected with wild type or mutant human sst5R constructs, and agonist binding was assessed using membrane binding assays with 125I-LTT SRIF-28 ligand. Deletion of the NH2 terminus or CSE mutagenesis of ECL1 and ECL3 produced minor 2-8-fold decreases in affinity for SRIF-14, SRIF-28, and SMS ligands. Truncation of ECL3 to mimic the size of this loop in sst1R and sst4R (the two subtypes that do not bind SMS) did not interfere with the binding of SMS, SRIF-14, or SRIF-28. In contrast, both ECL2 mutants failed to bind 125I-LTT SRIF-28. Immunocytochemical analysis of nonpermeabilized cells with a human sst5R antibody revealed that the mutant receptors were targeted to the plasma membrane. Labeled SMS (125I-Tyr3 SMS) also failed to bind to the mutant ECL2 receptors. These results suggest a potential contribution of ECL2 (in addition to the previously identified residues in TMs III-VII) to the SRIF ligand binding pocket.

Amino Acid Sequence↗
PubMed1997

Expression of the five somatostatin receptor (SSTR1-5) subtypes in rat pituitary somatotrophes: quantitative analysis by double-layer immunofluorescence confocal microscopy.

Using quantitative double-label fluorescence immunocytochemistry and confocal microscopy, we have analysed the pattern of expression of SSTR1-5 in normal rat pituitary somatotrophes. Antipeptide rabbit polyclonal antibodies were produced against the extracellular domains of SSTR1-5. SSTR antigens were colocalized in GH positive cells using rhodamine conjugated secondary antibody for SSTRs and FITC-conjugated secondary antibody for GH. SSTR5 was the predominant subtype which was expressed in 86 +/- 9.7% of GH cells followed by SSTR2 in 42 +/- 6.4% of GH positive cells. SSTR4 and SSTR3 were modestly expressed in 23 +/- 4.7% and 18 +/- 3.2% of somatotrophes respectively whereas SSTR1 was the least expressed subtype occurring in only 5 +/- 1.2% of somatotrophes. These results demonstrate variable expression of the 5 SSTRs in somatotrophes. The preponderance of the SST-28 preferring SSTR5 subtype correlates with the reported higher potency of SST-28 than SST-14 for inhibiting GH secretion.

Animals↗
PubMed1996

Molecular biology of somatostatin receptor subtypes.

Somatostatin (SRIF) receptors (ssts) comprise a family of heptahelical membrane proteins encoded by five related genes that map to separate chromosomes and which, with the exception of sst1, are intronless. The ssts1-4 display weak selectivity for SRIF-14 binding, whereas sst5 is SRIF-28-selective. Based on structural similarity and reactivity for octapeptide and hexapeptide sst analogs, ssts2,3 and sst5 belong to a similar sst subclass; ssts1-4 react poorly with these analogs and belong to a separate subclass. All five ssts are functionally coupled to inhibition of adenylyl cyclase via pertussis toxin-sensitive guanosine triphosphate (GTP)-binding proteins. mRNA for ssts1-5 is widely expressed in brain and peripheral organs and displays an overlapping but characteristic pattern that is subtype-selective and tissue- and species-specific. All pituitary cell subsets express sst2 and sst5, with sst5 being more abundant. Individual pituitary cells coexpress multiple sst subtypes. The binding pocket for SRIF-14 ligand lies deep within the membrane in transmembrane domains (TMDs) 3 to 7. Except for extracellular loop 2, it does not involve the other exofacial structures. Human (h)sst2A and hsst5 undergo agonist-mediated desensitization, associated with receptor internalization. The C-tail segment of hsst5 displays positive molecular internalization signals. The ssts inhibit the growth of tumor cells directly, through blockade of mitogenic signaling leading to growth arrest and through induction of apoptosis. This process is associated with translocation of phosphotyrosine phosphatase (PTP) 1C from the cytosol to the membrane.

Amino Acid Sequence↗
PubMed1996

Agonist-dependent regulation of cloned human somatostatin receptor types 1-5 (hSSTR1-5): subtype selective internalization or upregulation.

Agonist regulation of somatostatin receptors (SSTRs) was investigated in stable CHO-K1 cells individually expressing the 5 human (h) SSTR subtypes. hSSTR 2,3,4, and 5 displayed rapid agonist-dependent internalization of [125I] LTT SST-28 ligand in a time- and temperature-dependent manner over 60 min. Maximum internalization of radioligand occurred with hSSTR3 (78%) followed by hSSTR5 (66%), hSSTR4 (29%) and hSSTR2 (20%). In contrast, hSSTR1 displayed virtually no internalization. Prolonged agonist treatment led to differential upregulation of some of the SSTRs. After 22 h, hSSTR1 was upregulated at the membrane by 110%, hSSTR2 and hSSTR4 by 26% and 22% respectively, whereas hSSTR3 and hSSTR5 showed little change. Agonist-induced recruitment of hSSTR1 to the membrane was confirmed by immunocytochemistry with hSSTR1 antibodies. These results show that SST regulates all 5 hSSTRs by differential subtype selective internalization or upregulation. Subtype selectivity for internalization and upregulation is inversely related.

Animals↗
PubMed1996

Subtype-selective induction of wild-type p53 and apoptosis, but not cell cycle arrest, by human somatostatin receptor 3.

Somatostatin (SST) exerts direct antiproliferative effects in tumor cells, triggering either growth arrest or apoptosis. The cellular actions of SST are transduced through a family of five distinct somatostatin receptor subtypes (SSTR1-5). Whereas growth inhibition has been reported to follow stimulation of protein tyrosine phosphatase via SSTR2 or inhibition of Ca2+ channels via SSTR5 in heterologous expression systems, the subtype selectivity for signaling apoptosis has not been investigated. The tumor suppressor protein p53 and the protooncogene product c-Myc regulate cell cycle progression (growth factors present) or apoptosis (growth factors absent). The p53-induced G1 arrest requires induction of p21, an inhibitor of cyclin-dependent kinases, whereas apoptosis requires induction of Bax. c-Myc is capable of abrogating p53-induced G1 arrest by interfering with the inhibitory action of p21 on cyclin-dependent kinases. We have, therefore, investigated the regulation of p53, p21, c-Myc, and Bax and cellular apoptosis in relation to cell cycle progression in CHO-K1 cells stably expressing individual human SSTR1-5. We demonstrate that apoptosis is signaled uniquely through human SSTR3 and is associated with dephosphorylation-dependent conformational change in wild-type (wt) p53 as well as induction of Bax. The induction of wt p53 occurs rapidly and precedes the onset of apoptosis. We show that the increase in wt p53 is not associated with the induction of p21 or c-Myc when octreotide-induced apoptosis becomes evident, suggesting that such apoptosis does not require G1 arrest and is not c-Myc dependent. These findings provide the first evidence for hormonal induction of wt p53-associated apoptosis via G protein-coupled receptor in a subtype-selective manner.

Animals↗
PubMed1995

Heterologous processing of rat prosomatostatin to somatostatin-14 by PC2: requirement for secretory cell but not the secretion granule.

The role of PC2 in prosomatostatin (PSS) processing was investigated in GH3/GH4C1 pituitary cells. These cells are sparsely granulated, express different amounts of PC2 and no PC1. We described heterologous processing of rat PSS (rPSS) co-expressed with PC2 in stably transfected cells, correlate PC2 protein levels under different conditions of transfection with efficiency of PSS processing to somatostatin-14 (SS-14), determine the effect of modulating cell granularity on enzyme expression and PSS processing, and compare the relative potency of PC2 with that of PC1, PSS and cleavage products were monitored by HPLC and radioimmunoassay of SS-like immunoreactivity (SSLI). Radioimmunoassay analysis of N-terminal PC2-like immunoreactivity (PC2 LI) in GH4C1:rPSS, GH4C1:rPSS + PC2 and GH3:rPSS transfectants showed a gradient of PC2 protein of 1:2.6:3.4 in cell extracts and 1:4.7:9 in secretion media from these cells respectively. The concentration of PC2 protein correlated with SS-14 conversion efficiency was 36 +/- 3% in GH4C1:rPSS cells, 56 +/- 7% in GH4C1:rPSS-PC2 cells and 100% in GH3:rPSS cells. Treatment of GH4C1:rPSS + PC2 cells with epidermal growth factor, insulin, and beta-estradiol to induce granules, significantly increased basal and forskolin-stimulated co-release of SS LI and PC2 LI, but had no influence on SS-14 processing efficiency. Hormone treatment led to a small increase in the ratio of mature PC2 (68 kDa) to proPC2 (75 kDa) forms. PC1 stably transfected in GH4C1 cells produced significantly greater SS-14 conversion (62% in cells, 66% in media) compared with PC2 transfectants (53% in cells, 47% in media) These results provide the first proof that PC2 can effect dibasic processing of mammalian PSS, and, along with PC1, qualifies as an authentic SS-14 convertase. The activity of PC2 requires the milieu of the secretory cell but not the secretory granule.

Animals↗
PubMed1995

Cloning of the gene encoding human somatostatin receptor 2: sequence analysis of the 5'-flanking promoter region.

Using a cDNA probe, genomic clones were obtained for the 5' flanking promoter sequence of the human somatostatin receptor 2-encoding gene (SSTR2). A 3.8-kb sequence directly upstream from the start codon was analyzed. This sequence shares a number of characteristics with the promoters of other G-protein-coupled receptor (GPCR)-encoding genes including a number of G+C-rich regions, binding sites for several transcriptional factors, and the absence of coupled TATAA and CAAT sequences.

Base Sequence↗
PubMed1995

Direct role of furin in mammalian prosomatostatin processing.

We have previously reported that rat prosomatostatin (rPSS) undergoes conversion at Arg decreases and Lys decreases monobasic sites to SS-28 and PSS-(1-10) respectively in COS-7 cells, and have proposed furin or a related enzyme of the constitutive secretory pathway as the endoproteinase responsible. Here we have tested directly the ability of furin to cleave rPSS at the two monobasic sites as well as at the RXRK dibasic site of SS-14 conversion (a furin motif, except for Lys substituting for Arg at P1). Recombinant vaccinia virus (VV) vectors were used to co-express rPSS with graded doses of furin in COS-7 cells and LoVo colon carcinoma cells deficient in furin. PSS and cleavage products in cell extracts and media were characterized by HPLC analysis and C-terminal [SS-14-like immunoreactivity (SS-14 LI)] and N-terminal [PSS-(1-10) LI] directed radioimmunoassays. There was a dose-dependent increase in SS-28 production from rPSS by furin in COS-7 cells from 29% (control) to 58% (high-dose furin) associated with a progressive decrease in unprocessed PSS from > 60% to approximately 20% of total SS-14 LI. Significant SS-14 production occurred only at high levels of furin infection. Control LoVo cells infected with VV:rPSS exhibited production of approximately 21% SS-28, approximately 15% PSS-(1-10) and 3.5% SS-14. Infection of LoVo cells with VV:hfurin (hfurin = human furin) enhanced SS-28 production to 30-34%. SS-14 synthesis also increased to 25-40%, probably by conversion from SS-28. Overexpression of furin in COS-7 or LoVo cells failed to increase PSS-(1-10) production. These results show that furin is a candidate SS-28 convertase. Arginine is the preferred residue at the P1 site of furin cleavage. Furin does not process rPSS to PSS-(1-10), suggesting the existence of another monobasic convertase with a preference for Lys rather than Arg at P1. Such an enzyme could also explain the presence of endogenous SS-28-, PSS-(1-10)- and SS-14-producing activities in LoVo cells.

Animals↗
PubMed1995

Comparative proteolytic processing of rat prosomatostatin by the convertases PC1, PC2, furin, PACE4 and PC5 in constitutive and regulated secretory pathways.

Recombinant vaccinia virus vectors were used to coexpress each of the candidate prohormone convertases PC1, PC2, furin, PACE4 and PC5 with rat prosomatostatin (rProSOM) in the constitutive secreting cell line LoVo and in the endocrine corticotroph cell line AtT-20, which exhibits regulated secretion. Mammalian ProSOM is cleaved at a dibasic Arg-Lys decreases site to produce somatostatin-14 (S-14) and at a monobasic Gln-Arg decreases site to yield somatostatin-28 (S-28). The analysis of processed products by gel-permeation high performance liquid chromatography shows that in LoVo cells PC1, furin and PACE4 generate S-14, S-28 and a mixture of S-14 and S-28, respectively, while PC2 is unable to process ProSOM in these constitutive cells. In contrast, PC2 can generate S-14 in AtT-20 cells. The convertase PC5 is unable to process ProSOM in either cell line. These data suggest that PC2, PC1 and PACE4 are candidate S-14 convertases, while PACE4 and furin are candidate S-28 convertases.

Animals↗
PubMed1995

Astrocytes synthesize and secrete the lipophilic ligand carrier apolipoprotein D.

Expression of the lipophilic ligand transporter, apolipoprotein D (apoD) by primary astrocyte cultures derived from neonatal mouse brain was investigated. Western blot analysis of cell lysates and media showed that apoD is constitutively secreted by astrocytes with little intracellular storage. The secreted apoD floated primarily at density 1.063-1.21 g ml-1 upon sequential ultracentrifugation indicating its association with lipids. Treatment of astrocytes with the carboxylic ionophore, monensin, resulted in intracellular retention and decreased secretion of apoD that was of slightly reduced M(r). Progesterone, a steroid hormone that binds to apoD with high affinity (10(-6) mol l-1) and the oxysterol, 25-hydroxycholesterol which is a potent regulator of cellular cholesterol homeostasis in mammalian cells, differentially stimulated apoD, but not apoE secretion. These results show that astrocytes synthesize and constitutively secrete apoD and suggest a physiologic role for this lipocalin in cholesterol metabolism in the nervous system.

Animals↗
PubMed1995

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.

Animals↗