(Hypertension. 1996;28:840-846.)
© 1996 American Heart Association, Inc.
Articles |
the Laboratory of Molecular Biochemistry of Hypertension (C.M., I.J., T.L.R.) and the JA DeSeve Laboratory of Biochemical Neuroendocrinology (R.D., N.G.S.), Clinical Research Institute of Montreal (Quebec, Canada).
| Abstract |
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Key Words: renin-angiotensin system renin adrenal glands
| Introduction |
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In an effort to identify novel PPEs, we recently determined the distribution of processing enzymes in an established renin-expressing tissue culture cell line derived from an oncogene-induced mouse tumor (As4.1 cells13 ). We found one such enzyme, the mouse prohormone convertase PC5, to be capable of partially activating human prorenin (I.J. and T.L.R., unpublished data, 1996). In the current study, we describe the isolation and characterization of a human homologue of this enzyme, hPC5. We demonstrate that hPC5 proteolytically activates human prorenin with the expected site and organelle specificity and that it is coexpressed with prorenin in the zona glomerulosa of the adrenal cortex.
| Methods |
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RT-PCR
One microgram of polyA+ RNA from total human adrenal (Clontech Laboratories) was subjected to RT-PCR with the use of a published procedure16 and the following oligonucleotides: The forward oligonucleotide was derived from a region corresponding to the signal peptide of mouse PC5.15 An artificial HindIII restriction enzyme cleavage site added to the 5' end of the amplified fragment for the purpose of cloning is underlined: 5'-CCAAGCTTGGCTGCTGTGCGTGCTGGC-3'. The reverse oligonucleotide was derived from the 5' end of the phage hPC5B. An internal BglII restriction enzyme site is underlined: 5'-CTGCCTCAGATCTGTAGTG-3'.
The entire RT-PCR reaction was repeated four times, and four independently derived clones of the amplified fragment were sequenced and the sequences compared. The sequence submitted to GenBank (accession No. U49114) represents the consensus sequence, defined as any nucleotide appearing in three of four clones.
Northern Blot Analysis
Tissue distribution of PC5 mRNA was determined by hybridization of commercially purchased nitrocellulose filters containing aliquots (2 µg) of polyadenylate RNA from various human tissues (Clontech Laboratories). The probe used was a complementary RNA derived from the full-length hPC5 cDNA. Probe labeling and hybridization were carried out as previously described.17
Expression Vector Construction
A cDNA fragment from the Kpn I site (Fig 1
) to just past the stop codon was excised from the phage hPC5B and combined with a Kpn I to HindIII (see above) fragment derived from portions of two independent RT-PCR clones (so as to eliminate errors arising from the Taq polymerase). A region corresponding to the first 12 amino acids of the signal peptide derived from mouse PC5 was attached to the 5' end by overlap-extension PCR.18 Thus, the entire cDNA, encoding amino acids 1 through 16 derived from the mouse PC5 signal peptide and the remainder from hPC5, was subcloned into the expression vector RSV globin,19 which places the cDNA under the control of the Rous sarcoma virus promoter and provides a 3' intron and polyadenylation signal from the rabbit ß-globin gene. The entire subcloned fragment was subsequently verified by DNA sequencing.
Cell Culture and Transfection
GH4C1 cells were plated in six-well culture dishes at a density of 5x105 cells per well. Twenty-four hours later, the medium was changed, and the cells were transfected by the (diethylamino)ethyl-dextran method with the use of a commercial kit (CellPhect Transfection kit, Pharmacia Biotech) according to the manufacturer's instructions. Each well received 0.18 µg of either the hPC5 expression vector or a neutral plasmid vector (pUC18) and 0.18 µg of an expression vector for human prorenin (pRHR1100) or its equivalents in which amino acids 42 or 43 of the prorenin prosegment were mutated to alanine (K/A-2 and R/A-1, respectively20 ). Supernatants were collected 30 hours after transfection and assayed for prorenin and renin content as previously described.20 To verify that conversion of the prorenin occurred in the secretory granules, we stimulated GH4C1 cells transfected with the human prorenin and hPC5 expression vectors to release secretory granules by depolarization using a previously published technique.21 Forty hours after cotransfection, the culture medium in parallel wells of transfected cells was replaced with prewarmed medium supplemented to a final concentration of 50 mmol/L with either NaCl (control) or KCl (secretagogue). The media were collected after 20 minutes and assayed for renin/prorenin. A potassium-dependent increase in the percentage of active renin contained in cell supernatants was taken as an indication of active renin release from the secretory granules of the transfected cells. Results shown in Fig 5
represent the mean of three independent transfection experiments.
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Immunolocalization of hPC5 in Human Tissues
Human tissue was obtained postmortem (kidney and adrenal gland) or postpartum (placental cotyledon), fixed in Bouin's solution, and embedded in paraffin. For immunolocalization, 5-µm sections were mounted on gelatin-coated slides, deparaffinized, and incubated with a 1:50 dilution of a polyclonal rabbit antiserum raised against a peptide corresponding to the N-terminal 16 amino acids of rat PC5 (PC5.MAP antibody) or a 1:200 dilution of a polyclonal rabbit antiserum against recombinant human prorenin. For kidney and placental specimens, immune complexes were revealed by incubation with protein Acolloidal gold (15-nm particles) synthesized from tetrachloroauric acid (BDH) according to the method of Ghitescu and Bendayan.22 Gold particles were enhanced for viewing in the light microscope by incubation with silver (IntenSE M Silver Enhancement kit, Amersham Life Science), and sections were counterstained with hematoxylin and methyl green. Immune complexes on human adrenal sections were detected with a 1:200 dilution of biotin-labeled donkey anti-rabbit IgG and a 1:300 dilution of streptavidinhorseradish peroxidase complex (Amersham Life Science) and were incubated with diaminobenzidine and hydrogen peroxide (Sigma Chemical Co) as chromogen. All positive staining patterns were subsequently verified for specificity by omission of the first antibody.
| Results |
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Northern analysis of mRNA from a variety of human tissues revealed a major band of approximately 6.6 kb and a minor band of approximately 3.8 kb (Fig 3
). PC5 RNA was detected in the brain, heart, placenta, lung, thyroid gland, and testes and at lower levels in the skeletal muscle, kidney, pancreas, small intestine, and stomach. In the adrenal gland, PC5 was particularly enriched in the cortex (Fig 3
).
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Because PC5 RNA appears to be expressed in a number of tissues previously reported to contain active renin, we tested the ability of hPC5 to cleave human prorenin in a cell cotransfection assay (Fig 4A
). As has been previously reported,10 when cultured rat somatotrophic GH4C1 cells were cotransfected with an expression vector encoding human prorenin and a neutral plasmid vector, only unprocessed prorenin was secreted into the culture supernatant. In contrast, if the human prorenin expression vector was cotransfected with an expression vector encoding hPC5, a portion of the expressed prorenin was secreted as active renin. Coexpression of hPC5 with prorenin mutated at either of the basic residues forming the native cleavage site (lysine 42 or arginine 43) prevented activation. These results suggest that hPC5 activates human prorenin by proteolytic cleavage at the site previously reported for activation of renin in humans.5 Although hPC5 cleaved human prorenin in GH4C1 cells, there was no apparent increase in active renin secretion when cotransfections were carried out in Chinese hamster ovary (CHO) cells (Fig 4B
). One obvious difference in the CHO cell line compared with GH4C1 cells is their lack of secretory granules, suggesting that either hPC5, human prorenin, or both require the secretory granule environment for this proteolytic step. This conclusion is supported by the acute increase in active renin detected in the supernatants of cotransfected GH4C1 cells treated for 20 minutes with potassium chloride (Fig 5
), a depolarizing agent that causes the release of secretory granules.21
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Using a polyclonal antibody raised against a peptide derived from mouse PC5, we studied the distribution of hPC5 in several human tissues (Fig 6
). To date, we have been unable to detect staining for PC5 in the human kidney, although our sections stained positively for renin. In the placental cotyledon, PC5 was located in the syncytiotrophoblast layer of the chorionic villi, and antibody against renin stained primarily the chorionic mesoderm. In the adrenal gland, the antibodies against both renin and PC5 showed a preferential staining of zona glomerulosa cells in the adrenal cortex, with very little staining of the capsule and zona fasciculata. No staining was evident with omission of the first antibody (data not shown). Thus, our immunohistochemical studies would suggest that of the three tissues studied, it is likely that prorenin and PC5 are clearly colocalized only in the zona glomerulosa of the human adrenal cortex.
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| Discussion |
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The principal source of circulating active renin in humans is the juxtaglomerular cells of the kidney. Although low levels of hPC5 RNA were detected by Northern blot analysis in a sample of total kidney mRNA (Fig 3
), we were unable to localize PC5 immunostaining in kidney sections (Fig 6
), raising the possibility that PC5 is expressed at low levels in diffuse cell types in the kidney. Thus, although these results do not formally rule out PC5 as a PPE in the kidney, our inability to detect it in juxtaglomerular cells makes it unlikely that it plays a major role in the production of renal renin. In contrast, relatively abundant amounts of PC5 mRNA and protein were detected in the placenta although evidence suggests that placental cells in culture33 and in vivo34 secrete only prorenin. However, immunostaining revealed that the cells producing PC5 and prorenin in the human placenta are distinct. It is also unlikely that PC5 would activate prorenin once the two proteins are secreted because of the apparent requirement of a granular environment for the cleavage of prorenin by hPC5 in transfected cells. Thus, in contrast to the case in the adrenal gland, it is unlikely that PC5 expressed in the human placenta would activate placental prorenin. In the mouse, two forms of PC5 have been predicted on the basis of cloned cDNAs. The first would be analogous to the hPC5 cDNA described in the present study and to that cloned from rat tissues,15 23 and the second, called PC6B, would be extended at its 3' end because of a differential RNA splicing event.35 Although the hPC5 cDNA we have cloned is only roughly 3 kb in length, the major RNA band seen in human tissues is approximately 6.6 kb long. The identity of the longer band hybridizing to the hPC5 probe is currently unknown. It should be noted that neither of the cDNA clones isolated from a screening of 1.2 million phages from the adrenal library was extended at its 3' end (Fig 1
), although the probes used in their isolation cover the region of homology with the mouse PC6B variant.35 In mouse tissues, expression of the PC6B variant is restricted to few tissues,35 whereas the abundance of the 6.6-kb variant detected with the hPC5 probe is directly proportional to the abundance of the 3.8-kb band. Hybridization of RNA blots from rodent tissues with a PC5 probe also has revealed RNA bands of 3.8, 6.5, and 7.5 kb,15 35 and the use of a PC5-specific probe has revealed a band of 6.5 kb. Thus, it is possible that additional PC5 RNA species exist in mammals that are extended at their 5' ends. Alternatively, human tissues may be particularly enriched in a homologue to PC6B that was not picked up in our screenings. Recent data suggest that the alternate C-terminal tail present on PC6B may serve to retain the enzyme in the Golgi network, whereas the "short" form of mouse PC5 is targeted to dense core secretory granules (N.G.S., unpublished observations, 1996). These data and the results of our cotransfection assays (Fig 4
) would suggest that the "short" form of hPC5 described here is the form that would be active in renin processing in secretory granules. The PC5 enzymes isolated from humans and mice show a remarkably high degree of conservation at the nucleotide and protein sequence levels. This degree of similarity is higher than that seen for the other mammalian prohormone convertase enzymes that seem to diverge in the C-terminal half of the enzyme.36 37 This high degree of sequence conservation may reflect an essential function of PC5 (and the C-terminus of PC5) in mammals.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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Received February 23, 1996; first decision April 10, 1996; accepted June 25, 1996.
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