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(Hypertension. 1996;27:514-517.)
© 1996 American Heart Association, Inc.

Articles

Cathepsin B Is a Prorenin Processing Enzyme

Francisco A.R. Neves; Keith G. Duncan; John D. Baxter

From the Metabolic Research Unit, University of California, San Francisco.

Correspondence to Francisco A.R. Neves, University of California, San Francisco, Metabolic Research Unit, HSW 1141 PO Box 0540, San Francisco, CA 94143-0540.

	Abstract

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Abstract Conversion of prorenin to renin results from proteolytic cleavage of a 43-amino-acid prorenin prosegment in renal juxtaglomerular cells. The enzyme that performs this processing is not known. Of several enzymes proposed, cathepsin B is a candidate because it colocalizes with renin in juxtaglomerular cell secretory granules and accurately cleaves the prosegment of human prorenin in vitro. It is not known whether cathepsin B can perform this function in the cell. We examined this using secretory granule–containing rat GH4C1 cells transfected with a human preprorenin expression vector. When treated with secretagogue (KCl 50 mmol/L+forskolin 10 µmol/L), these cells secrete 95% prorenin and 5% active renin into the medium, indicating little prorenin processing activity. In contrast, when the cells are cotransfected with a vector that expresses human preprocathepsin B or mouse prohormone convertase 1, secretagogue-induced secretion of active renin increased to 12% and 16.5%, respectively. With antisera that recognize the prosegment and renin, prorenin and renin were identified as proteins of 47 and 43 kD, respectively, and an antibody specific to the prosegment precipitated only the 47-kD species. These results do not address whether cathepsin B is the authentic renal prorenin processing enzyme. However, the results do demonstrate that cathepsin B can localize to the appropriate subcellular compartment and process prorenin to renin in GH4C1 cells and are consistent with a role for this enzyme in prorenin processing.

Key Words: renin-angiotensin system • enzymes • prohormones

	Introduction

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Renin participates in the renin-angiotensin cascade by catalyzing the formation of angiotensin I from angiotensinogen.¹ The major source of circulating active renin is renal JG cells,^{1 2} in which it is produced from prorenin, a larger inactive precursor.³ Prorenin is sorted to both the regulated and constitutive secretory pathways in JG cells. In the regulated pathway, inactive prorenin is converted to active renin in the dense secretory granules by cleavage after a dibasic amino acid (Lys-Arg); this removes the 43 N-terminal amino acids of the prorenin.^{3 4} Active renin is stored in the granules and released into the blood stream in response to cellular stimuli. In contrast, intact prorenin is released in the constitutive pathway.^{1 3}

The enzyme that performs the proteolytic removal of the prosegment in the JG cell secretory granules is not known. Several enzymes have been proposed to be capable of activating prorenin. These include cathepsin B,⁵ cathepsin D,⁶ cathepsin G,⁷ tissue kallikrein,⁸ PC1,^{9 10} trypsin,¹¹ and mouse submandibular gland prorenin converting enzyme.¹² However, none of these enzymes meet all the criteria necessary for an authentic renal prorenin convertase. There is no evidence that kallikrein, cathepsin G, or trypsin colocalizes with renin in the secretory granules of JG cells; the optimum pHs for kallikrein⁸ and cathepsin G⁷ are higher than the pH of secretory granules¹³ ; trypsin degrades renin¹¹ ; and prorenin converting enzyme, despite processing mouse Ren-2 to prorenin, is unable to activate human prorenin.¹²

Cathepsin B, however, remains a strong candidate because it cleaves prorenin at the correct site in vitro⁵ without degrading renin further, and it colocalizes with prorenin in the human⁵ and rat kidney JG cell¹⁴ and human prolactin-producing pituitary cell¹⁵ secretory granules. The optimum pH for prorenin processing by cathepsin B is 6, consistent with the pH inside the secretory granule of 5.6.¹³ Moreover, a thiolprotease purified in the human kidney can accurately cleave the 43-amino-acid prosegment of human recombinant prorenin without degrading renin.¹⁶ This enzyme is identical to cathepsin B in its molecular weight and in a partial amino-terminal amino acid sequence.⁵

Nevertheless, these excellent correlations do not demonstrate that cathepsin B is the renal prorenin–processing enzyme. Of the remaining unanswered questions, it is not known whether in secretory cells containing the regulated secretory pathway, cathepsin B localizes to the appropriate subcellular compartment to allow conversion of prorenin to renin and whether it can function in this capacity in this setting.

Cultured rat pituitary GH4C1 cells serve as a good model to address the latter issue, since they contain a regulated secretory pathway and are known not to process prorenin expressed from a transfected vector.¹⁷ In the present studies, we addressed the issue of whether cathepsin B localizes to the appropriate subcellular compartment and processes prorenin by coexpressing prorenin with cathepsin B in GH4C1 cells. The data demonstrate that cathepsin B can activate prorenin into renin in vivo, consistent with a role for this enzyme as a prorenin processing species in the cell.

	Methods

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GH4C1 cells, DME H21, FCS, insulin, glutamine, and penicillin-streptomycin were obtained from the cell culture facility, University of California, San Francisco.

Plasmid Constructions
The human preprocathepsin B expression vector was constructed by inserting the Pst I/BamHI fragment of plasmid phCB79-2¹⁸ (kindly provided by D.F. Steiner), which encodes human preprocathepsin B downstream of the RSV long-terminal repeat in plasmid pUC 9. The human prorenin expression vector pRhR1100 (driven by an RSV promoter),¹⁹ human procathepsin D (driven by a CMV promoter)²⁰ (kindly provided by J.M. Chirgwin), and mouse PC1 (driven by a CMV promoter)⁹ (kindly provided by N.G. Seidah) have been described previously.

Cell Culture and DNA Transfection
Rat GH4C1 cells were maintained in monolayer culture in DME H21 supplemented with 10% FCS, 2 mmol/L glutamine, 50 U/mL penicillin, and 50 µg/mL streptomycin. Cells were grown at 37°C in 6% CO₂. To increase the number of secretory granules, the cells were treated for 7 to 9 days before transfection with human epidermal growth factor (10 nmol/L) (Collaborative Biomedical Products), estrogen (1 nmol/L), and insulin (300 nmol/L).²¹ Cells were transfected with 10 µg pRhR1100 and either 40 µg RSV vector (neutral plasmid), 40 µg human cathepsin B, 40 µg human cathepsin D, or 40 µg mouse PC1. The cells were electroporated (Bio-Rad Gene Pulser) at 960 µF and 0.3 kV at a density of approximately 5x10⁶ cells in 0.4 mL PBS containing 0.1% glucose and 10 µg/mL BioBrene (Applied Biosystems). Each electroporation was split into two 2.5-cm wells. Twenty-four to 36 hours after transfection, the cells were washed twice with PBS and incubated with 1 mL fresh medium either with or without secretagogues. The secretagogues were 50 mmol/L KCl and 10 µmol/L forskolin in ethanol.^{22 23} Control cells without secretagogue were treated with 50 mmol/L NaCl plus ethanol. After 16 hours, the culture medium was collected for assay of prorenin and renin.

Radiolabeling and Immunoprecipitation
At 24 to 36 hours after the transfection, the cells were washed twice with PBS and grown for 16 hours in originally methionine-free DME H21 containing 0.15 mCi/mL [³⁵S]methionine (Du Pont/New England Nuclear) and 5% dialyzed FCS with or without secretagogue. Cell culture media were collected and immunoprecipitated with an antibody that recognizes both renin and prorenin or with an antibody that recognizes only prorenin (a prosegment-specific antibody) with protein A–Sepharose (Sigma) as previously described.¹⁹ The precipitates were analyzed on 12% polyacrylamide gels containing 0.1% SDS.

Renin and Prorenin Assay
Prorenin and renin levels were determined by enzymatic assays of angiotensin I generation (Incstar Corp) as previously described.¹⁹ Briefly, active renin is determined by the ability of the sample to generate angiotensin I from nephrectomized sheep serum substrate (kindly provided by R.A.S. Gomes, Federal University of Uberaba, Minas Gerais, Brazil). Renin levels were determined before and total renin (renin plus prorenin) was determined after activation with 50 µg/mL trypsin at 21°C for 1 hour. Prorenin levels were calculated as the difference between these two values. The percentage of active renin represents the active renin divided by total renin times 100.

The percentage of active renin is expressed as mean±SEM; the data were analyzed by the Kruskall-Wallis test, and multiple comparisons were made with Dunn's test. Values of P<.05 were considered statistically significant.

	Results

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To assay their ability to process prorenin to renin, GH4C1 cells were transfected with the preprorenin expression vector. After 40 to 48 hours, culture media were assayed for prorenin and renin levels (see "Methods"). The results are summarized in Fig 1. In the absence of secretagogues, 95% of the secreted enzyme was inactive prorenin and only 5% was active renin. Secretagogues stimulate release of prolactin from secretory granules stored in GH4C1 cells.^{22 23} Secretory granule release was stimulated with KCl plus forskolin. The relative amount of renin secreted into the medium after treatment of the cells with secretagogues remained at 5% of total renin (renin plus prorenin).

View larger version (14K): [in this window] [in a new window]   Figure 1. Bar graph showing relative amount of renin (active renin/total reninx100, expressed as %) secreted into the cell culture medium. GH4C1 cells expressing either human prorenin (RSV VECTOR), human prorenin and cathepsin D, human prorenin and cathepsin B, or human prorenin and mouse PC1 (PC1) were grown for 16 hours in media either with NaCl and ethanol (open bars; without secretagogue) or with KCl and forskolin (solid bars; with secretagogue). Aliquots of cell culture media were assayed for renin activity as described in "Methods" (n=8). *P<.05 vs RSV vector with secretagogue and **P<.05 vs RSV vector without secretagogue.

To test the ability of cathepsin B to process prorenin, GH4C1 cells were cotransfected with preprorenin expression vector and a plasmid that expresses human preprocathepsin B. As shown in Fig 1, when the cells expressed cathepsin B, the relative amount of renin secreted into the culture medium in the absence of secretagogues was 8.6% of total renin, and when the cells were treated with secretagogues, the relative amount of renin secreted into the medium increased to 12% of total renin. This result suggests that cathepsin B processed prorenin into renin in the secretory granules.

Mouse PC1 has been shown to process prorenin to renin in GH4C1 cells.^{9 10} GH4C1 cells were cotransfected with vectors that express prorenin and mouse PC1. As summarized in Fig 1, when the cells expressed PC1, the relative amount of renin secreted into the cell culture medium by GH4C1 cells in the absence of secretagogues was 12% of total renin; this value increased to 16.5% of total renin with secretagogue stimulation. By contrast, cotransfection of human preprorenin and human preprocathepsin D expression vector did not increase the basal or secretagogue-stimulated release of renin, which was 6% of total renin in both cases.

Prorenin processing by GH4C1 cells was characterized further by PAGE. Cells that had been transfected with the prorenin expression vector or cotransfected with the human prorenin and human procathepsin B or with the human prorenin and mouse PC1 expression vectors were labeled with [³⁵S]methionine. Culture media from cells grown in the presence or absence of secretagogues were immunoprecipitated with an antibody that recognizes both prorenin and renin, and the products were analyzed by SDS-PAGE (Fig 2). Prorenin, identified as a protein of 47 kD, is present in all lanes. Renin, identified as a protein of 43 kD, is observed only in the media of cells that expressed either cathepsin B or mouse PC1 (lanes 3 and 4 and 7 and 8, respectively). In the presence of secretagogues, the quantity of the renin protein band is increased, confirming the renin assay results.

View larger version (31K): [in this window] [in a new window]   Figure 2. SDS-PAGE analysis of immunoprecipitated prorenin and renin. GH4C1 cells expressing either human prorenin (CONT), human prorenin and human cathepsin B (CATB), human prorenin and human cathepsin D (CATD), or human prorenin and mouse PC1 (PC1) were labeled for 16 hours with [35S]methionine as described in the "Methods." Duplicate wells were grown with (+) or without (-) KCl and forskolin (FORSK). The cell culture media were precipitated with an antibody that recognizes renin and prorenin, and precipitated proteins were analyzed by SDS-PAGE. The 47-kD prorenin and 43-kD renin bands are indicated.

To confirm that the 43-kD species is renin produced by cleavage of the prosegment and is not the result of a carboxy-terminal cleavage, GH4C1 cells were cotransfected with the prorenin and either the procathepsin B or PC1 expression vectors and labeled with [³⁵S]methionine. The cells were treated with secretagogue, and cell culture media were precipitated with either an antibody that recognizes only the prosegment of prorenin or an antibody that recognizes both prorenin and renin (see "Methods"). The immunoprecipitated products were analyzed by SDS-PAGE (Fig 3). The antibody that recognizes renin and prorenin precipitated both the 47- and 43-kD species (lanes 1 and 3). In contrast, the prosegment-specific antibody precipitated only the 47-kD species (lanes 2 and 4). Taken together, these results suggest that the 43-kD band is authentic renin produced by the removal of the amino-terminal prosegment.

View larger version (34K): [in this window] [in a new window]   Figure 3. SDS-PAGE analysis of immunoprecipitated prorenin and renin. GH4C1 cells expressing either human prorenin and human cathepsin B (CATB) or human prorenin and mouse PC1 (PC1) were labeled for 16 hours with [35S]methionine as described in the "Methods" in media with KCl and forskolin. The cell culture media were precipitated either with an antibody that recognizes renin and prorenin (REN) or with prosegment-specific antibodies (PRO). Precipitated proteins were analyzed by SDS-PAGE. The 47-kD prorenin and 43-kD renin bands are indicated.

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
In the present study, we have investigated the role of cathepsin B in prorenin processing in rat pituitary GH4C1 cells. The results demonstrate that in vivo, cathepsin B can localize to the appropriate subcellular compartment and process prorenin to renin. GH4C1 cells, which expressed both transfected prorenin and cathepsin B, secreted 12% active renin in the media, twice the amount of renin activity observed in the absence of cathepsin B. These results were confirmed by SDS-PAGE of [³⁵S]methionine-labeled proteins, by which renin was identified as a protein of 43 kD only in medium from cells that expressed cathepsin B. This is the first functional evidence that cathepsin B can be a prorenin-processing enzyme in intact cells.

Our results also confirm previous results that mouse PC1 can process prorenin into renin in GH4C1 cells.^{9 10} Although PC1 can process prorenin in GH4C1 cells and may be the enzyme responsible for prorenin processing in mouse pituitary AtT20 cells,²⁴ this enzyme is expressed mainly in neuroendocrine cells^{25 26} and has not been found in JG cells.¹⁰ Thus, PC1 is an unlikely candidate to be the renal prorenin processing enzyme. Recently, another prohormone convertase, mouse PC5, has been proposed to process prorenin into renin in GH4C1 cells, and its RNA has been found in As4.1 JG cells derived from a transgenic mouse kidney tumor.²⁷ However, further studies are necessary to investigate the role of PC5 in the renal processing of prorenin.

The percentages of active renin secreted by GH4C1 cells expressing prorenin and cathepsin B, 12%, or expressing prorenin and PC1, 17%, were not very high. Nevertheless, these differences did represent more than twice the amount of active renin secreted from cells expressing only prorenin, and they were reproducible. The value with PC1 is less than the 25% observed by Benjannet et al.⁹ This variation may be due to different transfection efficiencies that can result with the calcium phosphate precipitation method. Additionally, we did not measure the fraction of procathepsin B that was correctly targeted to the regulated secretory pathway and converted into active cathepsin B. It is likely that some fraction of the cathepsin B was targeted to lysosomes. Although it has been suggested that procathepsin B is converted into mature cathepsin B by autocatalysis,²⁸ it is possible that inefficient targeting and activation of human procathepsin B in rat pituitary cells could contribute to the low prorenin conversion given by cathepsin B in our experiments.

Unlike the case with cathepsin B and PC1, we were unable to observe increased processing of prorenin with a vector that expressed cathepsin D. The latter colocalizes with renin in secretory granules of JG cells,²⁹ and it has been shown that cathepsin D can activate prorenin in human amniotic fluid.⁶ However, the optimum pH for this activation (4.8) is less than the range expected of an authentic renal prorenin convertase because the pH inside the secretory granules is 5.6.¹³ Furthermore, cathepsin D was relatively ineffective in processing human recombinant prorenin at pH 4.6 and did not cleave a synthetic peptide spanning the human prorenin processing site, suggesting that the activation of prorenin by cathepsin D does not occur at the authentic processing site.³⁰ Nevertheless, the data do not exclude the possibility that cathepsin D can function as a prorenin-processing enzyme. We did not prove that cathepsin D was expressed in GH4C1 cells or that it was targeted to the secretory granules with prorenin. Unlike the other members of the aspartic proteinase family, which are mostly secretory proteins, procathepsin D is sorted primarily to the lysosomes,³¹ and intracellular localization of cathepsin D may differ from one cell to another. Thus, further studies would be needed to define the potential role of cathepsin D in prorenin processing.

The evidence obtained in the present studies that cathepsin B can process prorenin to renin in the cell is complementary to the in vitro evidence cited in the introduction showing that cathepsin B cleaves the prorenin prosegment, does not further degrade renin, has an optimum pH close to the pH of secretory granules, and colocalizes with renin in JG cell secretory granules. This collective evidence is consistent with a role for cathepsin B as a renal prorenin–processing enzyme. Nevertheless, these studies do not demonstrate whether this enzyme is the actual prorenin-processing enzyme in the kidney, and further studies will be needed to address this issue with respect to cathepsin B, PC5, or possibly other enzymes.

	Selected Abbreviations and Acronyms

 

CMV = cytomegalovirus DMEM H21 = Dulbecco's modified Eagle's medium H21 FCS = fetal calf serum JG = juxtaglomerular PBS = phosphate-buffered saline PC1 = prohormone convertase 1 RSV = Rous sarcoma virus SDS-PAGE = sodium dodecyl sulfate–polyacrylamide gel electrophoresis

	Acknowledgments

 
This work was supported by National Institutes of Health grant HL-35706. Dr Neves was supported by a postdoctoral fellowship from the Brazilian Research Council. We would like to thank Dr Ralff Ribeiro and Fran DeNoto for their suggestions and helpful discussion.

	References

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