(Received for publication, December 11, 1996, and in revised form, December 31, 1996)
From the Howard Hughes Medical Institute and
§ Department of Biochemistry and Molecular Biology,
University of Chicago, Chicago, Illinois 60637, the
Department
of Medical Oncology, Imperial Cancer Research Fund, St.
Bartholomew's Hospital, London EC1M 6BQ, United Kingdom, and the
** Institute of Biological Sciences, University of Tsukuba,
Tsukuba, Ibaraki 305, Japan
Insulin-like growth factor I (IGF-I) is required for normal embryonic development and postnatal growth. Like most hormones and growth factors, IGF-I is synthesized as a proprotein that is converted to the mature form by endoproteolysis. Processing of pro-IGF-I to mature IGF-I occurs by cleavage within the unique pentabasic processing motif Lys-X-X-Lys-X-X-Arg71-X-X-Arg-X-X-Arg77. We have previously shown that human embryonic kidney 293 cells process pro-IGF-IA at Arg71 to generate IGF-I-(1-70) and at Arg77 to produce IGF-I-(1-76). Cleavage at each of these sites requires upstream basic residues, indicating that subtilisin-related proprotein convertases (SPCs) may be involved. In order to investigate the identity of the endogenous enzymes involved in maturation of pro-IGF-IA, we have expressed wild-type and mutant pro-IGF-IA in 293 cells and in the furin-deficient Chinese hamster ovary cell line, RPE.40. We have also co-expressed these constructs with SPCs that are thought to play a role in processing precursor proteins in the constitutive pathway: furin, PACE4, PC6A, PC6B, and LPC. The results show that furin is most active at cleaving wild-type and mutant pro-IGF-IA and can cleave these precursors at multiple sites within the pentabasic motif. PC6A and LPC are less active than furin but cleave only at Arg71. PACE4 and PC6B have very little activity on pro-IGF-IA precursors. Wild-type pro-IGF-IA was correctly processed to mature IGF-I in 10 of 10 cell lines that were tested. Since furin, PC6A, and LPC are known to have a broad pattern of tissue distribution and we have demonstrated expression of LPC in RPE.40 cells, our results suggest that these SPCs may be responsible for the endogenous pro-IGF-IA processing activity observed in a wide variety of cell lines.
Insulin-like growth factor I (IGF-I)1 is expressed ubiquitously throughout embryonic development and postnatal growth of mammalian organisms (1). IGF-I is an important regulator of growth; mice that are homozygous for a disrupted IGF-I gene display a number of developmental abnormalities and most die shortly after birth (2, 3). Overproduction of IGF-I has been associated with many cancers, in which IGF-I may act as an autocrine or paracrine growth factor (4) or an inhibitor of apoptosis (5). Regulation of the biological activity of IGF-I is therefore an important physiological process.
Most hormones and growth factors are initially synthesized as inactive propeptide precursors that are post-translationally processed to the biologically active mature peptide. The subtilisin-like proprotein convertases (SPCs) are a family of enzymes that have been shown to play an important role in the maturation of prohormones and growth factors as well as a number of other protein precursors. PC2 and PC3 (also known as PC1) are restricted in expression to endocrine and neuroendocrine cells and process substrates such as proinsulin and proopiomelanocortin. PC4 is expressed in the testis, and substrates for this processing enzyme have not yet been identified. In contrast, furin, PACE4, PC6A, PC6B, and LPC (also known as PC7, SPC7, and PC8) are expressed in many tissues and may have an important function in proteolytic processing of precursors in the constitutive secretory pathway (6-12).
The mature, 70-amino acid IGF-I molecule can be generated from two
different prohormone precursors, pro-IGF-IA or pro-IGF-IB. Pro-IGF-IA
and pro-IGF-IB are derived from a single gene by alternative splicing
of 3-exons. The prohormone sequences are identical through the B, C,
A, and D domains of mature IGF-I as well as the first 16 amino acids of
the E domain or pro-region. Pro-IGF-IA and pro-IGF-IB, therefore,
contain the same prohormone processing site at the junction of the D
and E domains but diverge in sequence at the carboxyl terminus of the
pro-region (1).
In our previous study on pro-IGF-I processing, we found that expression
of pro-IGF-IA in human embryonic kidney 293 cells resulted in secretion
of N-glycosylated pro-IGF-IA, pro-IGF-IA, IGF-I-(1-76), and
IGF-I-(1-70). Using site-directed mutagenesis, we demonstrated that
IGF-I-(1-76) is produced by cleavage at Arg77 and that
IGF-I-(1-70) is generated by cleavage at Arg71. Cleavage
at both of these positions required a P4 basic residue (see Fig. 1).
Furthermore, cleavage at Arg71 occurred in the
furin-deficient LoVo and RPE.40 cell lines, indicating that another
processing enzyme must be involved in maturation of pro-IGF-IA (13). In
an attempt to identify the enzyme(s) involved in maturation of
pro-IGF-IA, we have expressed wild-type and mutant pro-IGF-IA in 293 and RPE.40 cells. We have also co-expressed these constructs with
candidate pro-IGF-I-converting enzymes, furin, PACE4, PC6A, PC6B, and
LPC.
The expression vector pCMVigf1-FLAG
has been described previously (13). The human furin cDNA was
excised from pBluescript with EcoRI and SalI and
cloned into EcoRI/SalI-digested pCMV6c to
generate pCMV-furin. The human PACE4 cDNA was removed from pBluescript by digestion with XhoI and SalI. The
purified PACE4 fragment was end-filled and blunt-end ligated to pCMV6c
that had been digested with SmaI to generate pCMV-PACE4.
Human pBSK-LPC was digested with SacI and 3-overhangs were
removed with T4 DNA polymerase under conditions favoring exonuclease
activity. The purified fragment was blunt-end ligated to pCMV6c to
create pCMV-LPC. pCMV-furin, pCMV-PACE4 and pCMV-LPC were analyzed by
restriction site digestion and DNA sequencing. cDNAs coding for
mouse PC6A (14) and PC6B (15) were subcloned into pRcCMV
(Invitrogen).
The following pCMVigf1-FLAG
mutants have been described previously (13): R71A, R74A/R77A, and
R77X. Additional mutants of pCMVigf1-FLAG were made using
the restriction site elimination method (16) with the following
mutagenic oligonucleotides: K65R, 5-TTGCGCACCCCTCCGGCCTGCCAAGTCAG;
K68R, 5
-TCAAGCCTGCCCGGTCAGCTCGCTCTG; K65R/K68R,
5
-TTGCGCACCCCTCCGGCCTGCCCGGTCAGCTCGCTCTG; K68R/
A70K,5
-CCCCTCAAGCCTGCCCGGTCAAAGCGCTCTGTCCGTGCC.
Human embryonic kidney 293 cells were grown in Dulbecco's modified Eagle's medium (Life Technologies, Inc) with 10% fetal calf serum and 100 units/ml penicillin and 100 µg/ml streptomycin. 293 cells were transfected by the calcium phosphate method (17). RPE.40 cells were grown in Ham's F12 medium with 5% fetal calf serum, 100 units/ml penicillin, and 100 µg/ml streptomycin. RPE.40 cells were transfected using LipofectAMINETM following the manufacturer's protocol (Life Technologies, Inc). For co-transfection experiments, 5 µg of prohormone cDNA was co-precipitated with 5 µg of SPC cDNA.
Cell Labeling, Immunoprecipitation, and Gel Electrophoresis24 h after transfection, cells were washed two times in PBS and incubated for 2 h in cysteine-free Dulbecco's modified Eagle's medium with 1 mg/ml bovine serum albumin, 100 units/ml penicillin, and 100 µg/ml streptomycin. The medium was then replaced with cysteine-free Dulbecco's modified Eagle's medium containing 100 µCi/ml [35S]cysteine (Amersham Corp.; 1000 Ci/mmol), 1 mg/ml bovine serum albumin, 100 units/ml penicillin, and 100 µg/ml streptomycin. After labeling for 24 h, conditioned medium was collected and centrifuged to remove cellular debris. Conditioned medium was concentrated on a Sep-Pak (Waters), eluted and lyophilized, as described previously (13). Lyophilized proteins were reconstituted in 500 µl of immunoprecipitation buffer (25 mM Tris-Cl, pH 7.4, 300 mM NaCl, 1 mM CaCl2, 10 µg/ml leupeptin, 1 mM phenylmethylsulfonyl fluoride, 1% Triton X-100). Immunoprecipitation was performed with 3 µg of M1 anti-FLAG monoclonal antibody (Kodak/IBI) and protein G-Sepharose (Pharmacia Biotech Inc.). The immunoprecipitates were washed two times in buffer containing 25 mM Tris-Cl, pH 7.4, 300 mM NaCl, 1 mM CaCl2 and then once in buffer consisting of 25 mM Tris-Cl, pH 7.4, 140 mM NaCl, 1 mM CaCl2.
Immunoprecipitates were solubilized in SDS-sample buffer containing 2-mercaptoethanol and denatured by heating at 95 °C for 5 min. Samples were electrophoresed on 16% Tricine-buffered polyacrylamide gels with a 4% stack at 4 °C (18). Gels were fixed in 10% acetic acid, 25% isopropyl alcohol for 30 min and then soaked in a fluorographic solution (Amplify, Amersham) for 15 min. Dried gels were exposed to x-ray film (Kodak) with an intensifying screen for 4 h to 2 days unless stated otherwise. Quantitation of individual bands was done using a PhosphorImager (Molecular Dynamics).
Cloning and Expression of Hamster LPCTotal RNA was
prepared from RPE.40 cells and transcribed to cDNA with Superscript
(Life Technologies, Inc.). Hamster LPC was amplified using degenerate
PCR primers LPC-3 and LPC-4 that were designed to anneal to unique
sequences coding for the P domain. The LPC-3 primer
(5-TT(T/C)TG(T/C)CCNAG(T/C)GGNATGATG) corresponds to the amino acid
sequence 547FCPSGMM553, and LPC-4
(5
-AT(G/C)GT(A/G)TANCC(A/G)TC(T/C)TC(T/C)TC) encodes 654EEDGYTI660. The 342-bp PCR product was
end-filled, blunt end-ligated to pBluescript (Stratagene), and
sequenced.
For Northern blot analysis, 4 µg of poly(A)+ RNA from RPE.40 cells was electrophoresed through 1% agarose, blotted to a HyBond membrane (Amersham), and probed with a 32P-labeled 342-base pair PCR product encoding the P domain of hamster LPC. Standard molecular biology protocols were followed (17).
Our previous study on pro-IGF-IA processing utilized the
pCMVigf1-FLAG expression vector. This vector directs expression of human prepro-IGF-IA containing the eight-amino acid FLAG epitope between Ala1 of the signal peptide and Gly+1
of the B domain. We have shown that the FLAG peptide does not affect
co-translational or post-translational processing of prepro-IGF-IA. Human embryonic kidney 293 cells transfected with pCMVigf1-FLAG secrete
N-glycosylated pro-IGF-IA, pro-IGF-IA, IGF-I-(1-76), and IGF-I-(1-70) (13). In the present work, we sought to determine 1) if
any of the known SPCs could process pro-IGF-IA and 2) if Arg for Lys
substitutions at the P4 and/or the P7 positions would affect processing
by endogenous or exogenous enzymes. The primary sequences of the
cleavage sites of wild-type and mutant pro-IGF-IA-FLAG constructs used
in this study are indicated in Fig. 1. The results of
expression of these constructs in 293 cells and their processing by
endogenous enzymes or SPCs are shown in Fig. 2 and Table
I.
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When wild-type pro-IGF-IA and pCMV6c (as a negative control) are co-transfected in 293 cells, two major bands corresponding to IGF-I-(1-70) and IGF-I-(1-76) are detected in conditioned medium (Fig. 2A). The diffuse band that migrates at approximately 20 kDa corresponds to N-glycosylated pro-IGF-IA (see marker on Fig. 2E). Cells are continuously labeled for approximately 24 h after transfection, and under these conditions approximately 45% of the IGF recovered from conditioned medium is IGF-I-(1-70) and 45% is IGF-I-(1-76). When wild-type pro-IGF-IA is co-transfected with pCMV-furin, 89% of the precursor is processed to IGF-I-(1-70). An additional band is visible that migrates slightly faster than IGF-I-(1-70) and has a molecular mass of approximately 7 kDa. The 7-kDa band is not present in the absence of pCMV-furin. Furin has been reported to cleave at lysine residues (19), and this band may result from processing at Lys65-X-X-Lys68 to generate IGF-I-(1-67). The pattern observed for processing of the K65R mutant by enzymes endogenous to 293 cells is qualitatively and quantitatively identical to processing of wild-type pro-IGF-IA. In the presence of furin the main processing product is again IGF-I-(1-70). A 7-kDa band is also present and is more intense than the the minor cleavage product produced by co-expression of wild-type pro-IGF-IA with furin. This band may result from cleavage at Arg65 or Lys68. The K68R mutant shifts the Lys68-X-X-Arg71 processing motif of pro-IGF-IA upstream by three amino acids to Lys65-X-X-Arg68 and creates a new minimal furin site at Arg68-X-X-Arg71 (see Fig. 1). When the K68R mutant is co-transfected with pCMV6c in 293 cells, IGF-I-(1-70) is the major product, and IGF-I-(1-76) is not detected (Fig. 2A). A faint band is visible below IGF-I-(1-70) in Fig. 2A. However, this product is not produced consistently from K68R by the endogenous processing enzymes (see Fig. 2, B, C, D, and E). In the presence of furin, 54% of the K68R precursor is processed to IGF-I-(1-70), and 34% is processed to a 7-kDa product that likely represents cleavage at the Lys65-X-X-Arg68 site. The K65R/K68R mutant contains arginine residues at positions 65, 68, 71, 74, and 77. Processing enzymes endogenous to 293 cells generate two products from this precursor (Fig. 2A). The upper band represents 65% of the precursor and corresponds to IGF-I-(1-70). The 7-kDa band represents 26% of the precursor and may result from cleavage at Arg65 or Arg68. Furin processes 89% of the K65R/K68R precursor to the 7-kDa form, and IGF-I-(1-70) is not detected.
When PACE4 is co-expressed with wild-type pro-IGF-IA or the K65R mutant, IGF-I-(1-70) and IGF-I-(1-76) are the only cleavage products, lower molecular weight forms are not produced (Fig. 2B). PhosphorImager analysis indicates a slight increase in the production of IGF-I-(1-70) from the wild type and the K65R precursor in the presence of pCMV-PACE4. The K68R mutant is processed only to IGF-I-(1-70), and the amount of product produced is not affected by PACE4. As seen previously, the K65R/K68R mutant is processed primarily to IGF-I-(1-70) by endogenous enzymes, with a smaller peptide composing 27% of the final product. Processing of this mutant is unaffected by PACE4.
Co-expression of PC6A with either the wild-type pro-IGF-IA or the K65R mutant enhances production of IGF-I-(1-70), and lower molecular weight forms are not generated (Fig. 2C). PC6A processes 59% of the wild-type precursor and 71% of the K65R mutant to IGF-I-(1-70). PC6A does not qualitatively or quantitatively alter the processing of the K68R or K65R/K68R mutants.
Fifty-four percent of the wild-type precursor and 59% of the K65R mutant are processed to IGF-I-(1-70) in the presence of PC6B, compared with 45 and 48% IGF-I-(1-70) product by endogenous enzymes (Fig. 2D). PC6B does not qualitatively or quantitatively alter the processing of the K68R or K65R/K68R mutants.
In the presence of LPC, 73% of wild-type pro-IGF-IA was converted to IGF-I-(1-70), as compared with 45% conversion by endogenous enzymes (Fig. 2E). LPC was less active on the K65R, K68R, and K65R/K68R mutants. LPC did not generate peptides smaller than IGF-I-(1-70) from any of the precursors.
Processing of Wild-type and Mutant Pro-IGF-IA by SPCs in RPE.40 CellsThe co-expression experiments in 293 cells indicated that
furin will efficiently process wild-type and mutant pro-IGF-IA to IGF-I-(1-70) and will also cleave the mutant precursors at other sites. Processing enzymes endogenous to 293 cells process all precursors to IGF-I-(1-70) and cleave only the K65R/K68R mutant at an
alternate site. By co-expressing pro-IGF-IA constructs with SPCs in the
furin-deficient RPE.40 cell line (20), we sought to determine 1) if
furin was responsible for alternative cleavage of K65R/K68R and 2) the
activity of PACE4, PC6A, PC6B and LPC for pro-IGF-IA in the absence of
furin. The results of these experiments are shown in Fig.
3 and Table II.
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As we have shown previously (13), when pro-IGF-IA is expressed in RPE.40 cells, IGF-I-(1-70), but not IGF-I-(1-76), is generated. In RPE.40 cells, approximately 80% of the precursor is processed to IGF-I-(1-70), while 20% is secreted as N-glycosylated pro-IGF-IA (Fig. 3A). When co-expressed with furin, nearly 100% of the pro-IGF-IA is converted to IGF-I-(1-70). Approximately 80% of the K65R and K68R precursors appeared as IGF-I-(1-70). In the presence of furin IGF-I-(1-70) is still the major product, but a smaller peptide is also generated from both mutants. The processing enzymes endogenous to RPE.40 cells generate only IGF-I-(1-70) from the K65R/K68R mutant. However, when co-expressed with furin, nearly 100% of the precursor is processed at an alternative site to generate the smaller 7-kDa form.
When wild-type or mutant pro-IGF-IA is co-expressed with PACE4 (Fig. 3B), PC6A (Fig. 3C), or PC6B (Fig. 3D) in RPE.40 cells, IGF-I-(1-70) is the only cleavage product. Approximately 80% of each precursor is converted to mature IGF-I, and this figure is not affected by the presence of exogenous enzymes.
In the background of RPE.40 cells, LPC is nearly as efficient as furin at processing pro-IGF-IA. Eighty-five to 93% of each precursor is processed to IGF-I(1-70) when co-expressed with LPC, and smaller peptides are not produced (Fig. 3E).
Cleavage of Pro-IGF-IA at Arg71 or Arg77 by SPCsIn order to determine if SPCs discriminate between the
Lys68-X-X-Arg71 and
Arg74-X-X-Arg77 sites of
pro-IGF-IA we co-expressed these enzymes with the R74A/R77A and R71A
mutants. Each of these mutants contains only one of the two potential
processing sites (Fig. 1). When the R74A/R77A mutant is expressed in
293 cells, 48% of the precursor is processed at Arg71
(Fig. 4A). In the presence of furin, nearly
100% of the precursor is cleaved at Arg71. PACE4, PC6A,
PC6B, and LPC cleave at Arg71 to generate 59, 66, 60, and
58% IGF-I-(1-70), respectively. Seventy-seven percent of the R71A
precursor is cleaved by endogenous enzymes in 293 cells (Fig.
4B). Processing at Arg77 increased to 96, 81, 91, 88, and 70% of the precursor in the presence of furin, PACE4,
PC6A, PC6B, and LPC, respectively.
When expressed in RPE.40 cells, 47% of the R74A/R77A precursor is processed at Arg71. Furin processed this precursor very efficiently, but PACE4, PC6A, and PC6B had no apparent affect. In contrast, LPC had a moderate affect, processing 57% of the precursor (Fig. 4C). Sixty-eight percent of the R71A mutant is cleaved at Arg77 in RPE.40 cells. In the presence of furin, cleavage at this site increases to 93% of precursor. PACE4, PC6A, PC6B, and LPC do not affect processing of this mutant in the RPE.40 cells (Fig. 4D). We often observe some additional processing or degradation of the R71A precursor in RPE.40 cells, resulting in faint bands below the IGF-I-(1-76) product.
Processing of IGF-I-(1-76) to IGF-I-(1-70)The previous
experiments show that PACE4, PC6A, and PC6B can cleave wild-type and
mutant pro-IGF-IA when co-expressed in 293 cells, but these enzymes do
not appear to process pro-IGF precursors in RPE.40 cells. One possible
explanation is that furin first cleaves pro-IGF-IA at Arg77
to generate IGF-I-(1-76), which then becomes a substrate for these
other SPCs. To test this hypothesis we co-expressed the R77X
truncation mutant with the SPCs. In 293 cells, 61% of the IGF-I-(1-76) is processed to IGF-I-(1-70). Furin and PC6A convert 90% of this precursor to IGF-I-(1-70). PACE4, PC6B, and LPC have no
effect on processing efficiency (Fig. 5A). In
RPE.40 cells, 56% of the IGF-I-(1-76) is processed to IGF-I-(1-70).
Furin very efficiently processes the precursor to IGF-I-(1-70), but
PACE4, PC6A, PC6B, and LPC have no effect (Fig. 5B).
Processing of the K68R/A70K Mutant by SPCs in RPE.40 Cells
In
the previous experiments we have not observed processing of IGF-IA
precursors by PACE4, PC6A, or PC6B in RPE.40 cells. In order to
determine if these enzymes are active in RPE.40 cells, we co-expressed
them with the K68R/A70K mutant. This mutant contains an
Arg-X-Lys-Arg71 cleavage site (Fig. 1), which is
also found in other precursors that are processed by PACE4, PC6A, and
PC6B (21-24). The K68R/A70K mutant was processed very efficiently by
the endogenous RPE.40 processing enzymes, and it was therefore
necessary to overexpose the gel to detect pro-IGF-IA. Overexposure of
the gel revealed that processing is enhanced by furin, PACE4, PC6A, and
PC6B (Fig. 6). This effect was observed in three
independent experiments.
Cloning and Expression of Hamster LPC
The data on
co-expression of pro-IGF-IA with SPCs in RPE.40 cells indicates that
furin and LPC are most active in this cell line. The fact that both
alleles of furin are known to be inactive in RPE.40 cells due to
mutation (20) suggests that LPC may be responsible for the observed
pro-IGF-IA processing. In order to determine if RPE.40 cells express
LPC, we obtained a partial clone by PCR of RPE.40 cDNA. The
342-base pair cDNA encodes a 113-amino acid segment of the P domain
that shares 92% sequence identity to human LPC and 95% sequence
identity to rat PC7 (data not shown). When this cDNA was used to
probe a Northern blot of RPE.40 poly(A)+ RNA, a single
transcript of 4.4 kilobase pairs was detected (Fig. 7),
indicating that LPC is expressed in RPE.40 cells.
Processing of Human Pro-IGF-IA in Various Cell Lines
Since IGF-I is synthesized by all tissues of the body, we were interested in determining if pro-IGF-IA-processing enzymes are also ubiquitous. We expressed pro-IGF-IA-FLAG in cell lines derived from several mammalian sources as well as one invertebrate cell line. Secreted IGF-I peptides were immunoprecipitated from conditioned medium and analyzed by gel electrophoresis, as described under "Experimental Procedures." The results have been summarized in Table III. All cell lines examined processed pro-IGF-IA to IGF-I-(1-70). IGF-I-(1-76) was also generated by all cells except the furin-deficient LoVo and RPE.40 cells and the Drosophila cell line. Pro-IGF-IA was also processed very efficiently. Endogenous enzymes were able to convert at least 50% of overexpressed precursor to final product in all cell lines (data not shown).
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We have expressed wild-type and mutant pro-IGF-IA with furin, PACE4, PC6A, PC6B, and LPC in order to determine which of these enzymes might play a role in pro-IGF processing. In 293 cells, endogenous enzymes process wild-type pro-IGF-IA and the K65R mutant at Lys68-X-X-Arg71 to yield IGF-I-(1-70) and at Arg74-X-X-Arg77 to produce IGF-I-(1-76) (Fig. 2). The K68R mutant is processed at Arg68-X-X-Arg71 to generate IGF-I-(1-70), and IGF-I-(1-76) is apparently not produced from this precursor by 293 enzymes. The lack of IGF-I-(1-76) could be due to a preference for Arg68-X-X-Arg71 over Arg74-X-X-Arg77. It is also possible that pro-IGF-IA is processed sequentially from IGF-I-(1-76) to IGF-I-(1-70) and that the Arg for Lys68 substitution creates a more efficient cleavage site or a site favorable to additional processing enzymes, resulting in complete processing to the final product. Pulse-chase analysis will be necessary to determine if the K68R precursor is sequentially cleaved. Endogenous processing enzymes in 293 cells produce two products from the K65R/K68R precursor (Fig. 2). The main product corresponds to IGF-I-(1-70) generated by cleavage at Arg68-X-X-Arg71. The smaller peptide likely corresponds to IGF-I-(1-67) resulting from cleavage at the newly created furin site, Arg65-X-X-Arg68. This suggestion is supported by the fact that the smaller peptide is not generated from K65R/K68R in the furin-deficient RPE.40 cell line (Fig. 3).
When furin is co-expressed with wild-type pro-IGF-IA in 293 cells, IGF-I-(1-70), but not IGF-I-(1-76), is produced (Fig. 2A). A faint band migrating slightly faster than IGF-I-(1-70) is also produced and could represent cleavage at an upstream lysine. The fact that furin generates IGF-I-(1-70), but not IGF-I-(1-76), from pro-IGF-IA could be due to a preference for Lys68-X-X-Arg71 over Arg74-X-X-Arg77 or the result of sequential processing, with IGF-I-(1-70) being the major final product. Since each site is cleaved with high efficiency when the R74A/R77A and R71A mutants are co-expressed with furin in 293 or RPE.40 cells (Fig. 4, A and B), we suggest that the latter hypothesis is more likely. Furin processes the K65R and K68R mutants to IGF-I-(1-70) and a smaller band that may result from cleavage at one of the two upstream basic residues (Fig. 2A). Only one product, which migrates slightly ahead of IGF-I-(1-70), is produced from the K65R/K68R mutant. This 7-kDa band likely represents IGF-I-(1-67) produced by cleavage at the Arg65-X-X-Arg68 site. Again, cleavage may be sequential or reflect processing site specificity.
In contrast to furin, co-expression of PACE4, PC6A, PC6B, or LPC with
pro-IGF precursors in 293 cells does not qualitatively change the
pattern of processing products generated by the endogenous enzymes
(Fig. 2, B, C, D, and E).
However, some quantitative changes are apparent. PC6A and LPC are most
efficient at generating IGF-I-(1-70) from pro-IGF-IA, followed by PC6B
and then PACE4 (Table I). These enzymes appear to have a more strict
substrate specificity than furin, since they produce only IGF-I-(1-70)
from pro-IGF precursors. However, PACE4, PC6A, and PC6B are capable of
cleaving at
Lys68-X-X-Arg71 or
Arg74-X-X-Arg77 (Fig. 4,
A and B). Other studies comparing specificity and
activity of SPCs have also found that furin has the widest substrate
specificity and is more active than PACE4, PC6A, and PC6B. Furin will
efficiently cleave both Arg-X-(Lys/Arg)-Arg and
Arg-X-X-Arg motifs (25, 26), while PACE4 and PC6A
prefer substrates with a basic residue in the P2 position (14, 19, 27,
28). Neurotropin precursors and human immunodeficiency virus gp160
contain Arg-X-(Lys/Arg)-Arg motifs that are cleaved more efficiently by
furin than PACE4 or PC6B, and not at all by PC6A (21, 22, 24). However,
PC6A is more active than PACE4 at cleaving the
Arg-X-(Lys/Arg)-Arg site of receptor protein-tyrosine
phosphatase µ (23), indicating that structural features are also
important for processing activity. Although we have not extensively
examined the substrate specificity of LPC, our data suggest that this
convertase may prefer the
Lys-X-X-Arg71 site to the
Arg-X-X-Arg77 site (Fig. 4).
Processing of pro-IGF by endogenous and transfected enzymes in 293 cells is summarized in Fig. 8A.
From our experiments in 293 cells it is clear that the endogenous processing enzymes cleave pro-IGF-IA very efficiently and that furin is more active on pro-IGF precursors than PACE4, PC6A, PC6B, or LPC. We therefore thought it would be informative to examine processing of pro-IGF-IA in a furin-deficient cell line. We have used RPE.40 cells for this purpose. These cells were derived from CHO-K1 cells by exposure to the mutagen ethyl methane sulfonate and selection for resistance to Pseudomonas exotoxin A (29). RPE.40 cells fail to process several proprotein precursors due to mutations in both furin alleles (20). When expressed in RPE.40 cells, wild-type pro-IGF-IA is processed exclusively to IGF-I-(1-70) by endogenous enzymes (Fig. 3). Similar results were obtained previously in LoVo cells (13). It therefore appears that furin is required to generate IGF-I-(1-76), although co-expression of wild-type pro-IGF-IA and furin in 293 or RPE.40 cells results in processing to IGF-I-(1-70), not IGF-I-(1-76) (Figs. 2A and 3A). This may be due to sequential processing of pro-IGF-IA by furin, as discussed above.
An unexpected result from the experiments with RPE.40 cells was that PACE4, PC6A, and PC6B do not process pro-IGF-IA precursors in this cell line (Fig. 3 and Table II). This could be due to a requirement for furin to activate PACE4, PC6A, and PC6B. However, others have shown that these enzymes cleave prosomatostatin and neurotropin precursors in LoVo cells (21, 22, 30). Furthermore, we have found that PACE4, PC6A, and PC6B will cleave the K68R/A70K mutant in RPE.40 cells. This mutant contains a basic residue in the P2 position and may therefore be a better substrate for these enzymes (Fig. 6). It is also possible that PACE4, PC6A, and PC6B act primarily on IGF-I-(1-76) that would be generated by furin but is not produced in RPE.40 cells. This possibility was eliminated by showing that PACE4, PC6A, and PC6B process IGF-I-(1-76) to IGF-I-(1-70) in 293 cells but not in RPE.40 cells (Fig. 5). A third possibility is that expression levels of exogenous enzymes may vary in 293 and RPE.40 cells. Using ribonuclease protection assays, we determined that mRNA levels for PACE4, PC6A, and PC6B were 1.5-2.3-fold higher in 293 cells than RPE.40 cells (data not shown), suggesting that lower expression may account for their lack of activity in RPE.40 cells. Expression levels did not strictly correlate with processing activity, however. For example, PACE4 had the highest mRNA levels but the lowest activity in both cell lines. Processing of pro-IGF by endogenous and transfected enzymes in RPE.40 cells is summarized in Fig. 8B.
Our previous work clearly identified the processing site for final maturation of pro-IGF-IA as Lys68-X-X-Arg71 (13). This dibasic motif has been shown to be cleaved by enzymes of the SPC family (8). The data presented here indicate that furin, PC6A, and LPC are capable of efficiently processing pro-IGF-IA to mature IGF-I. We have also found that processing activity can be influenced by enzyme expression levels and that all cells examined contain endogenous enzymes that efficiently and correctly process pro-IGF-IA. Furin, PC6A, and LPC are expressed in many tissues (6-12), suggesting that these and perhaps other members of the SPC family that have not yet been described may account for our observation that pro-IGF-I processing is a function common to all cells.
We thank Dr. Graeme Bell for providing the pCMV6c expression vector and Dr. Thomas Moehring for the RPE.40 cell line. Furin and PACE4 cDNAs were gifts from Dr. Steve Smeekens and Terry Calarco, respectively. We thank Jeff Stein for oligonucleotide synthesis. We are grateful to Dr. Shu Chan for advice, to Dr. An Zhou for comments on this manuscript, and to Florence Rozenfeld for secretarial support.