From the Department of Molecular and Cellular
Biochemistry, Loyola University of Chicago, Stritch School of Medicine,
Maywood, Illinois 60153 and the ** Department of Pharmacology and
Molecular Biology, Chicago Medical School,
North Chicago, Illinois 60064
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ABSTRACT |
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Cathepsin B is a lysosomal cysteine protease
whose increased expression is believed to be linked to the malignant
progression of tumors. Alternative splicing and the use of alternative
transcription initiation sites in humans produce cathepsin B mRNAs
that differ in their 5'- and 3'-untranslated ends. Some human tumors
also contain cathepsin B-related transcripts that lack exon 3 which encodes the N-terminal signal peptide and 34 of the 62-amino acid inhibitory propeptide. In this study we show that one such transcript, CB(2,3), which is missing exons 2 and 3, is likely to be a functional message in tumors. Thus, CB(
2,3) was found to be otherwise complete, containing the remainder of the cathepsin B coding sequence and the
part of the 3'-untranslated region that is common to all previously characterized cathepsin B mRNAs in humans. Its in vitro
translation product can be folded to produce enzymatic activity against
the cathepsin B-specific substrate,
N
-benzyloxycarbonyl-L-Arg-L-Arg-4-methylcoumaryl-7-amide.
Endogenous CB(
2,3) from the metastatic human melanoma cell line,
A375M, co-sediments with polysomes, indicating that it engages the
eukaryotic translation machinery in these cells. Epitope-tagged forms
of the truncated cathepsin B from CB(
2,3) are produced in amounts comparable to the normal protein after transient transfection into COS
cells. Immunofluorescence microscopy and subcellular fractionation show
this novel tumor form of cathepsin B to be associated with nuclei and
other membranous organelles, where it is likely to be bound to the
cytoplasmic face of the membranes. This subcellular distribution was
different from the lysosomal pattern shown by the epitope-tagged,
full-length cathepsin B in COS cells. These results indicate that the
message missing exons 2 and 3 is likely to be translated into a
catalytically active enzyme, and that alternative splicing (exon
skipping) could contribute to the aberrant intracellular trafficking of
cathepsin B that is observed in some human cancers.
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INTRODUCTION |
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Cathepsin B is a lysosomal cysteine protease whose expression and trafficking are frequently altered in transformed and malignant cells (1). Both its association with the plasma membrane and its secretion have been linked to the increased capacity of human and rodent tumors to invade and metastasize (2-5). The association of cathepsin B with cell fractions containing plasma membrane-derived vesicles and endosomes was found to increase after transfection of a human breast epithelial cell line with the c-Ha-ras oncogene (6). Cathepsin B has been detected in nuclei from human ectocervical tumors and in basal and columnar cells of the normal and hyperplastic prostate (7, 8). In human lung tumor cell lines cathepsin B activity was also localized to the endoplasmic reticulum, nuclear membrane, and plasma membrane (9). Forms of cathepsin B greater in size and stability than the mature, lysosomal form have been observed in tumors from humans and animals or in media conditioned by them (10-13). At present, many of these findings are unexplained.
Normal human tissues and human tumors contain different cathepsin B
mRNAs due to alternative post-transcriptional processing of their
5'- and 3'-untranslated ends and to the probable existence of more than
one promoter in the cathepsin B gene (14-17). Some human tumors also
contain cathepsin B-related transcripts that differ in their 5'-coding
region (14, 17). These appear to arise from the skipping of exons 2 and
3 during splicing of the pre-mRNA (14), and from transcription
initiation in exon 4 (17). Both events result in the loss of the
sequences for the signal peptide and a part of the inhibitory
propeptide. Reticulocyte lysates were able to translate a synthetic
message missing exons 2 and 3 with high efficiency, producing a
truncated form of cathepsin B that lacked the first 51 amino acid
residues (51CB) (14). In this paper we show that the
transcript missing exons 2 and 3 is likely to be a functional message
and that its translation product retains the capacity to be folded into
an active enzyme. When tagged with the C (connecting)-peptide of proinsulin and expressed in COS cells, the truncated protein,
51CB-CP, associates with the cytosolic surface of nuclei
and other membranous organelles. Therefore, alternative splicing could
explain some of the diversity of size and location reported for
cathepsin B in some human tumors, and suggests that cathepsin B could
enhance their malignant behaviors by novel and previously unsuspected mechanisms.
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EXPERIMENTAL PROCEDURES |
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Cell Culture-- Human melanomas A375P (parental, poorly metastatic) and A375M (metastatic) were obtained from I. J. Fidler, M.D. Anderson Cancer Ctr., Houston, TX. Human prostate carcinomas PC3P (parental) and PC3M (metastatic) were obtained from J. Kozlowski, Dept. of Urology, Northwestern Medical School, Chicago, IL. These cells were cultured as described previously (14). COS 7 cells were obtained from L. Gissmann, Loyola University Medical Center, Maywood, IL. These cells were cultured in Dulbecco's modified Eagle's medium high glucose medium (Life Technologies, Inc., Grand Island, NY) supplemented with penicillin (100 units/ml), streptomycin (100 µg/ml), and 10% fetal bovine serum.
Measurement of Variant Cathepsin B mRNAs--
Total RNA was
prepared from cultured tumor cells according to the method of
Chomczynski and Sacchi (18). Complementary DNA was synthesized from 5 µg of RNA using Superscript Reverse Transcriptase (Life Technologies,
Inc.) and oligo(dT) primers (Pharmacia Biotech Inc., Piscataway, NJ).
PCR1 was performed in a
Temp·Tronic thermal cycler (Barnstead/Thermolyne Corp., Dubuque, IA)
programmed for 30 s at 95 °C, 1 min at 72 °C, and 2 min at
72 °C for 30 cycles. The primers used in the PCR step to
detect the truncated cathepsin B mRNA, CB(2,3), were 5'-CAGCGCTGGGCCGGGCAC (sense, exon 1-exon 4 junction) and
5'-CCAGGACTGGCACGACAGGC (antisense, exon 11). Other cathepsin B
mRNAs, CB and CB(
2), were detected using the corresponding exon
junction specific primers, 5'-CAGCGCTGGGCTGGTGTG (exon 1-exon 2)
and 5'-CAGCGCTGGGTGGATCTA (exon 1-exon 3) (14). Variant cathepsin B
messages were also quantified by RNase protection using an RPA II kit
(Ambion, Austin, TX) according to the manufacturer's instructions.
Construction of Tagged cDNAs--
Three-way PCR (19) was
used to fuse the connecting (C-) peptide of human pre-proinsulin
(residues 57 to 87 plus a stop codon) to the carboxyl terminus of
cathepsin B. The 5' primer (5'-GGTCTAGAGGCAACCGCTCCGGCAACGCC) corres- ponded to the start of exon 1 (20). The 3' primer
(5'-GGTCCGGACTACTGCAGGGACCCCTC) was complementary to 15 nucleotides at
the 3' end of the C-peptide coding region (21). The bridging primer
(5'-GCCCCACCTGCAGGTCCTCTGCCTCGATCTTTTCCCAGTACTGATCGGTG) spanned
25 nucleotides of the 5' end of the C-peptide and the 3' end of
cathepsin B just prior to the stop codon. The plasmid pGEM4Z-phins,
which contains the full-length cDNA for pre-proinsulin, was a kind
gift from S. J. Chan and D. F. Steiner at the Howard Hughes
Medical Institute, the University of Chicago. The cathepsin B template,
phCB79-2(2,3), was described previously (14). Standard cloning
procedures were used to replace the 5' end of the PCR product with
those from phCB79-2 and phCB79-2(
2) (14). For expression in COS
cells, the three cathepsin B-C peptide cDNA fusions were
introduced into the XbaI and SacI cloning sites
of the vector, pEUKC1 (CLONTECH Laboratories, Inc.,
Palo Alto, CA), adjacent to the SV40 late promoter.
In Vitro Transcription and Translation--
Plasmid
phCB79-2(2,3) was used as a template for the preparation of
m7G(5')ppp(5')G capped RNA as described previously (14).
Purified, synthetic capped mRNA (0.6 µg) was translated with a
rabbit reticulocyte lysate (Promega, Madison, WI), according to the
instructions of the supplier. To promote protein folding, the
translation mixture was dialyzed in the cold against buffer (0.05 M Tris-HCl, 20% glycerol, 0.1 mg/ml bovine serum albumin,
0.15 M NaCl, 1 mM Na2EDTA) to which
had been added 6.0 M guanidinium chloride. At intervals of
10 or 30 min, the dialysis buffer was diluted with an equal volume of
buffer to reduce the guanidinium chloride concentration by half, and
the dialysis continued for two additional cycles of dilution. The final
cycle was followed by dialysis against buffer only (23, 24). The
samples were clarified by centrifugation at 14,000 × g
for 15 min and quick frozen for later analysis. In an alternative
procedure, some samples were treated with 0.1 volume of a 10 × "oxido-shuffling" redox buffer (1 × contains 0.15 M Tris-HCl, pH 8.0, 1 mM Na2 EDTA,
0.3 mM glutathione disulfide, and 3.0 mM
reduced glutathione) and incubated at room temperature for 24 h
(25). Cathepsin B activity was measured fluorometrically with
Z-Arg-Arg-NMec as described previously (26). Some samples were
pretreated for 30 min at 37 °C with pepsin (0.83 mg/ml) to remove
the remainder of the inhibitory propeptide (26, 27).
Polysome Isolation-- Polysomes were analyzed by a modification of earlier procedures (31, 32). Exponentially growing A375M cells (five 175-cm2 flasks, ~20 × 106 cells/flask) were fed 10-12 h before harvesting. PBS-washed cells were collected by scraping into 5 ml of buffer containing 30 mM HEPES, pH 7.4, 150 mM NaCl, and 11 mM glucose. Cells were pelleted at 1,000 rpm for 5 min at 4 °C and resuspended in 10 ml of lysis buffer (10 mM HEPES, pH 7.4, 100 mM KCl, 5 mM MgCl2, 0.5% Nonidet P-40, 1 mM dithiothreitol, 50 units/ml RNasin, 100 µg/ml cycloheximide). After a 10-min incubation on ice, cells were treated with cold 0.5% (w/v) sodium deoxycholate for an additional 10 min with occasional vortexing. Cell debris was removed by centrifugation at 3,000 rpm for 30 min at 4 °C and 1.0-ml aliquots were applied to continuous sucrose gradients (15-45% in 13 ml of buffer which also contained 50 mM HEPES, pH 7.0, 100 mM KCl, 10 mM MgCl2, 0.5 mg/ml heparin, and 150 µg/ml cycloheximide). The tubes were centrifuged at 87,000 × g in a Beckman SW40 rotor for 3.5 h, and polysome profiles were determined from absorbance measurements at 254 nm on isolated fractions. Total RNA was isolated from each fraction and the presence of variant cathepsin B mRNAs was determined by reverse transcription linked PCR using exon junction-specific primers.
In an independent experiment, A375M cells were suspended in 20 ml of lysis buffer, or in lysis buffer containing 30 mM EDTA in place of the cycloheximide and MgCl2. The cells were incubated on ice for 10 min and lysed with sodium deoxycholate as above. After centrifugation, the supernatant was loaded onto a 1.0 and 2.0 M sucrose step gradient in 18 ml of lysis buffer containing either MgCl2 and cycloheximide, or EDTA (33). The samples were centrifuged at 20,000 rpm for 22 h at 4 °C in a Beckman SW28 rotor. Total RNA was extracted from the pellets and the supernatants, and variant cathepsin B mRNAs were quantified by RNase protection (14).Transfection of COS Cells with Tagged Forms of Cathepsin B-- COS 7 cells, grown to 70-80% confluency in 35-mm tissue culture dishes, were transfected by either of two methods, with the LipofectAMINE reagent (Life Technologies, Inc., Grand Island, NY) according to the instructions of the supplier, or by the DEAE-dextran method (34). Twenty-four h after transfection the complete medium was replaced with serum-free medium and the incubation continued for an additional 24 h. Media samples were concentrated by centrifugation through Centricon 10 filters (Amicon, Inc., Beverly, MA), and diluted with an equal volume of 2 × sample buffer for SDS-PAGE (30). Cells were washed with PBS and scraped into 2 × sample buffer. The samples were subjected to electrophoresis on a 12% polyacrylamide gel in the presence of SDS and transferred to nitrocellulose with a miniblotter (Bio-Rad). To detect the C-peptide tagged proteins, the membrane was incubated overnight at 4 °C in 1 × TTBS buffer (35) containing 5% nonfat powdered milk (Carnation Co., Los Angeles, CA) and a 1:500 dilution of rabbit anti-proinsulin C-peptide antiserum (S. J. Chan, the Howard Hughes Medical Institute, the University of Chicago). The membrane was washed with 1 × TTBS buffer for 30 min and incubated for 2 h in 1 × TTBS buffer containing 5% nonfat milk and a 1:5000 dilution of goat anti-rabbit IgG conjugated to peroxidase. The blot was developed using the LumiGLO substrate kit (KPL, Gaithersburg, MD) for chemiluminescence detection according to the supplier's instructions. Monoclonal mouse anti-Flag antibody, M2 (Eastman Scientific Imaging Systems), was used to detect the Flag epitope according to the supplier's instructions. In some experiments 2 µg/ml tunicamycin was added to the cell culture medium 12 h prior to harvesting cells and media for analysis.
Immunofluorescence Microscopy-- COS cells were seeded on 12-mm coverslips at a density of 104 cells/coverslip in 100-mm tissue culture dishes. Forty-eight h after transfection, expressed proteins were visualized by a prior procedure, which was modified for secondary immunofluorescence microscopy (36). Briefly, cells were fixed for 30 min in 4% paraformaldehyde, and rinsed three times, 10 min each, with PBS, pH 7.2. The cells were blocked in normal goat serum (NGS) blocking solution (PBS, pH 7.2, 10% NGS, 1% bovine serum albumin, 0.3% Triton X-100) for 30 min. Primary antibodies were added to the NGS carrier solution (PBS, pH 7.2, 1% NGS, 1% bovine serum albumin, 0.3% Triton X-100) at a dilution of 1:100 for both the rabbit anti-human C-peptide (Dr. S. J. Chan, the University of Chicago, Chicago, IL) and mouse anti-PDI monoclonal antibody (StressGen Biotechnologies Corp., British Columbia, Canada). These were incubated with fixed cells for 1 h, after which the cells were rinsed twice, 10 min each, in PBS, pH 7.2. The cells were then incubated for 1 h at room temperature with either fluorescein isothiocyanate-labeled goat anti-rabbit or anti-mouse antibody at a dilution of 1:250 in NGS carrier solution. The cells were rinsed three times, 10 min each, with PBS, rinsed once with distilled water, and mounted in glycerol on a glass slide. The slides were viewed in the dark with a Zeiss microscope equipped for fluorescence, and suitable fields were photographed (36).
Subcellular Fractionation-- COS cells were washed with PBS and collected by scraping into homogenization buffer (0.25 M sucrose, 1 mM EDTA, pH 7.4). The cells were pelleted at 1,000 rpm for 10 min at 4 °C, resuspended in 1.5 ml of homogenization buffer, and lysed by repeated passage (40 times) through a 271/2-gauge needle (37). The homogenates were fractionated by differential centrifugation (38). Nuclei were collected at 800 × g for 10 min in a Sorvall SS34 rotor and the pellet resuspended in 100 µl of homogenization buffer. The post-nuclear supernatant was centrifuged at 4 °C in a Beckman TLA 103.3 rotor at 25,000 × g for 20 min to collect mitochondria and lysosomes, and then at 100,000 × g for 30 min to obtain the microsomal fraction. The pellets were resuspended in 100 µl of homogenization buffer. The post-microsomal supernatant were taken to represent the soluble cytosolic proteins. The concentration of protein in each fraction was determined using the Bio-Rad colorimetric protein assay according to the suppliers instructions. After SDS-PAGE, individual proteins were identified by Western blotting. To determine whether the full-length or truncated cathepsin B was delivered to the lumen of a membrane bound organelle, some samples (10 µg of total protein) were pretreated with 0.1 mg/ml of proteinase K as described above, with and without the addition of Triton X-100 to a final concentration of 0.15%.
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RESULTS |
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Transcripts Missing Exons 2 and 3 from A375 Human Melanoma Cells
Appear to be Functional Messages--
In humans, the occasional loss
of exon 2 during the splicing of cathepsin B pre-mRNA results in
messages that differ by 88 nucleotides in the length of their
5'-untranslated regions. The ratio of these messages vary among
different tissues. The shorter message, which was more active in an
in vitro translation assay, is the predominant form in many
human tumors (14). Remarkably, some human tumors also contain
transcripts that are additionally missing exon 3 which encodes the
signal peptide and part of the inhibitory propeptide (14, 17). In Fig.
1, reverse transcription and PCR
amplification were used to confirm that messages missing exons 2 and 3 are otherwise complete. For the reverse transcriptase reaction
oligo(dT) was used to prime the synthesis of cDNA from transcripts
containing poly(A) tracts. For the PCR, the 5' primer contained 9 nucleotides on each side of the exon 1-exon 4 junction. The 3' primer
was complementary to a region of exon 11 that is found in the
3'-untranslated regions of both the 2.3- and 4.0-kilobase cathepsin B
mRNAs from various human tissues and tumors (14-16). The plasmid
phCB79-2, and the cDNA reverse transcribed from PC3M human prostate
carcinoma cellular RNA, were included to verify the specificity of the
5' primer for the exon 1-exon 4 junction. RNase protection assays had
previously shown that PC3M contains messages with and without exon 2, CB and CB(2), but lacks the message CB(
2,3) in which both exons 2 and 3 are skipped (14). The plasmid phCB79-2(
2,3) was a positive
control. Its 934-base pair product is indistinguishable from those
obtained with the two human melanoma variants, A375M and A375P,
indicating that the mRNAs missing exons 2 and 3 in these cells are
otherwise complete. Also note the higher levels of this message in the
more metastatic melanoma variant. The identity of this product was
confirmed by hybridization with a human cathepsin B cDNA (Fig.
1B).
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The Truncated Form of Cathepsin B Can be Expressed in Vivo--
To
further assess the translational potentials of variant messages for
cathepsin B from human tumors, COS cells were transiently transfected
with cDNAs encoding carboxyl-terminal-tagged forms of the enzyme.
The truncated procathepsin B, 51CB, labeled with the
insulin C-peptide (Fig. 4, CB-CP(
2.3))
was readily detected in lysates of COS cells, producing a band with the
predicted molecular mass of 35 kDa.
51CB-CP was not
present in the medium, as expected for a protein that lacks a signal
peptide. In contrast, the tagged products of the vectors encoding the
full-length pre-proenzyme (CB-CP and CB-CP(
2) in Fig. 4) were
detected in both cell lysates and media from COS cells. Their
appearance in the medium agreed with prior observations that
overexpression of lysosomal pre-proenzymes can result in their
secretion (40, 41). The presence of additional bands in lysates from
cells transfected with CB-CP and CB-CP(
2) also suggests that the
tagged proenzymes can undergo post-translational transport and
processing in COS cells (42). Very similar results were also obtained
for the flag-tagged full-length and truncated forms of cathepsin B
(data not shown).
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The Truncated Form of Cathepsin B Can be Folded in Vitro--
The
propeptide in lysosomal enzymes is thought to guide their folding after
synthesis (41, 43, 44). The expected protein product of the cathepsin B
mRNA missing exons 2 and 3 retains 28 of the 62-amino acid
propeptide. Therefore, it was of interest to determine whether
51CB had lost information essential to its correct
folding. To initiate folding, the product of in vitro
translation was treated in one of two ways: with an "oxido
shuffling" buffer to facilitate disulfide bond formation, and with
decreasing concentrations of guanidinium chloride, as described under
"Experimental Procedures." The ability to acquire enzymatic
activity is a highly stringent test for the folding of a protein into a
native conformation. The results in Fig.
5 demonstrate that the product of the
message missing exons 2 and 3 has the potential to acquire biological
activity. Also noteworthy is the finding that the guanidinium
chloride-treated product was active (33%) prior to treatment with
pepsin to remove the remainder of the inhibitory propeptide. This
indicates that removal of the entire propeptide is not needed for
activation. In contrast, in vitro translation reactions
which lacked the synthetic mRNA templates gave no activity after
any of these treatments (data not shown).
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Localization of the Truncated Form of Cathepsin B in COS
Cells--
Indirect immunofluorescence microscopy was used to locate
C-peptide-tagged forms of cathepsin B when expressed in COS cells from
the SV40 late promoter in pEUKC1. The full-length pre-proenzyme produced a punctate pattern of fluorescence, showing localization of
the protein in peripheral lysosomes (Fig.
6, panels A and B). Some of the tagged protein was also concentrated in the juxtanuclear region of the cell. Similar patterns have been seen previously for
lysosomal enzymes in COS cells (45). In contrast, the tagged, truncated
cathepsin B from CB-CP(2,3) acquired a predominantly juxtanuclear,
vesicular distribution, unlike that expected for a cytosolic protein
that lacks a signal peptide (Fig. 6, panels C and
D). A fluorescent halo that surrounds the nucleus was also evident at higher magnification, suggesting that some of the tag may
have accumulated in the perinuclear space or been bound to the surface
of the nuclear envelope. Non-transfected cells and cells transfected
with vector alone produced little discernible fluorescence under these
same conditions (not shown). As the pattern obtained with
51CB-CP appeared ER-like, its localization was compared
with that of the ER-resident protein, PDI (Fig.
7). The tubular and vesicular pattern of
labeling observed for the truncated enzyme (panel A) clearly
overlapped that of PDI (panel B), suggesting that the two
were associated with the same membranous structures.
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The Truncated Cathepsin B Is Not Resident in the Lumen of the
ER--
The overlap in the staining pattern for the truncated
cathepsin B with that of PDI suggested that it might be associated with the endoplasmic reticulum. To determine if the truncated enzyme contained a cryptic signal peptide-like sequence, we examined the
translation of tagged and untagged messages in the presence of canine
microsomes. This reconstitutes an in vitro system to study
signal recognition particledependent transport into the lumen of the ER (28). Extraluminal and luminal proteins were distinguished by their sensitivity to digestion with proteinase K. As
shown in Fig. 9, the full-length,
C-peptide-tagged message produced two bands in the presence of
microsomes. The lower 40-kDa band, which was also seen in the absence
of microsomes, was sensitive to proteinase K digestion and corresponded
to the extraluminal, tagged, pre-procathepsin B. The upper, 44-kDa
band, which appeared only in the presence of microsomes, was largely
insensitive to relatively high concentrations of proteinase K, and is
likely due to a glycosylated proenzyme in the lumen of the microsome (42). In contrast, the message for CB-CP(2,3) produced only a single
band of about 35 kDa that was proteinase K sensitive, irrespective of
the presence of microsomes. Identical results were obtained for the
untagged forms of cathepsin B (not shown), indicating that the
C-peptide did not affect the translocation of the variant forms of
cathepsin B into microsomes.
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Cathepsin B Is Associated with the Extraluminal Surfaces of Isolated Membrane Organelles-- Immunofluorescence microscopy and subcellular fractionation suggested that the truncated cathepsin B was associated with membrane-bound organelles in COS cells. However, as documented above, it is not likely to be present in the lumen of the endoplasmic reticulum or any other compartment connected to the ER by vesicular transport. Therefore, the overlap in the distribution of the C-peptide-tagged, truncated cathepsin B and PDI in COS cells suggests that the former may be associated with the extraluminal surface of the ER. This possibility is supported by the results in Fig. 11, which shows the truncated enzyme in isolated membrane fractions was sensitive to proteinase K digestion. In contrast, the full-length enzyme was resistant to digestion by proteinase K unless the membranes were first solubilized with Triton X-100.
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DISCUSSION |
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Cathepsin B is a lysosomal cysteine proteinase which may not be exclusively localized to lysosomes. Plasma membrane and secreted forms, possibly resulting from missorting in the trans-Golgi network (26), are thought to promote tumor invasion and metastasis. Cathepsin B activity and immunoreactivity have also been found in the nucleus (7) and endoplasmic reticulum (9) in some cancer cells. This finding suggests that cathepsin B could have other important and as yet unsuspected functions in cancer. Molecular mechanisms that might explain the abnormal intracellular locations of cathepsin B are largely unknown. We and others have examined transcripts of the cathepsin B gene in human tumors for evidence of an altered protein which lacks the capacity to be targeted to lysosomes. Most of the messages detected differed in their 5'- and 3'-untranslated ends due to differences in mRNA splicing (14-17) or the selection of alternative transcription start sites (17). These might contribute to the regulated expression of cathepsin B. In addition, two transcripts were found which lacked sequences encoding the signal peptide and part of the inhibitory propeptide (14, 17). Despite their different presumptive origins, the coding regions in both begin with the same initiation codon in exon 4 and predict a product that lacks the signal peptide and 34 of the 62 amino acids which comprise the inhibitory propeptide. However, the methods employed to detect these putative messages, RNase protection, reverse transcriptase-PCR, and 5'-rapid amplification of cDNA ends, cannot distinguish between incomplete or nonfunctional transcripts and functional, full-length mRNAs. To better characterize the transcript missing exons 2 and 3, we have used oligo(dT) primers to reverse transcribe messages containing poly(A) tracts from two human melanoma cell lines, and PCR to amplify the specific messages that contain the exon 1-exon 4 junction. A single product was obtained which encompassed all the remainder of the coding region and the 3'-untranslated tail characteristic of normal cathepsin B mRNAs. In addition, the majority of this message was found to associate with polysomes in a manner sensitive to EDTA treatment. Collectively, these results support a claim that transcripts missing exons 2 and 3 are functional messages that can recruit ribosomes to initiate translation in the A375 melanoma cells.
An amino-terminal propeptide is often required for correct folding of the corresponding proprotein (41, 43, 44). However, some exceptions have been described (54, 55). In yeast carboxypeptidase Y, the entire propeptide is not absolutely required for folding in vivo, provided the vacuolar signal and C-terminal region of the propeptide are retained (55). The role of the propeptide in cathepsin B folding is unknown. Our results show that the retention of 28 amino acids from the C-terminal end of the 62-amino acid propeptide is sufficient for recovery of a native-like conformation in an in vitro folding assay, as deduced by activity measurements. Furthermore, the resulting truncated proprotein has about 1/3 the Z-Arg-Arg-AMC hydrolase activity of the mature enzyme generated by removal of the entire propeptide. These results suggest that the truncated cathepsin B, when expressed in tumor cells, could have biological activity in the absence of further proteolytic processing.
The ability to detect the product of messages missing exons 2 and 3 in tumor cells required distinguishing the truncated enzyme from other forms of cathepsin B which can arise from the proteolytic processing, glycosylation, and glycolytic modification of the endogenous enzyme (42). We have used two different epitope tags to follow the synthesis and localization of the full-length and truncated cathepsin B in COS cells. The crystal structure of cathepsin B shows its C terminus to be exposed to solvent and disordered (56), suggesting that a C-terminal peptide extension might not interfere with protein folding. This was confirmed by the finding that when overexpressed in COS cells, the secreted flag- and C-peptide-tagged procathepsin B acquired enzymatic activity after pepsin cleavage of the inhibitory propeptide.2 In addition, the flag peptide and C-peptide are not likely to directly affect protein targeting. The flag peptide is highly polar and resembles no known protein targeting signal. The C-peptide is poorly conserved among species, it does not cause retention of proinsulin when expressed in a non-insulin producing cell, and recombinant insulins that lack the C-peptide are efficiently delivered to secretory granules in insulin-producing cells (57, 58). It was recently reported that the isolated human C-peptide can produce biological effects in the diabetic rat in a manner which suggests that it could be membrane active (59). However, its polarity, and the kinetics with which it is metabilized, would seem to argue against an association with cell membranes (60). The Flag- and C-peptide-tagged full-length and truncated cathepsin B accumulated to similar levels in COS cells, confirming that the message missing exons 2 and 3 is translationally active in vivo, and that its product is sufficiently stable in COS cells to be readily detected. The presence of the tagged, full-length proenzyme in the media results from partial missorting of overexpressed lysosomal enzymes to the secretory limb of the exocytic-lysosomal biosynthetic transport pathway (40, 41). The truncated cathepsin B was not secreted, as would be predicted of a protein that is missing the signal peptide.
To begin to identify the potential function of the truncated cathepsin B in cancer, it was necessary to determine its cellular location. The truncated cathepsin B lacks the N-terminal signal peptide and was expected to be directed to the cytyosol of COS cells where it should have produced a diffuse cellular fluorescence. Unexpectedly, indirect immunofluorescence microscopy showed most of the label to be concentrated in regions of the cell proximal to the nucleus and to appear to be associated with tubules and interconnected vesicles. Staining of the nuclear envelope was also evident. This distribution of label resembled that of the ER resident protein, PDI. In agreement with the results of immunofluorescence microscopy, differential centrifugation of COS cell lysates demonstrated that the majority of the tagged, truncated cathepsin B co-sedimented with nuclei and other membranous organelles.
An analysis of CB(2,3) with
PSORT3 failed to detect any
sequence that could function as a cryptic signal peptide for its
transport into the lumen of the endoplasmic reticulum. In agreement,
the truncated enzyme was not taken up by canine microsomes in an
in vitro translation reaction. In addition, it is also
unlikely that the truncated enzyme was targeted to the lumen of the ER
by a putative signal recognition particle and signal peptide
independent mechanism (50, 51). First, when the synthesis of
CB-CP(
2,3) was examined in COS cells in the presence and absence of
tunicamycin, no evidence could be obtained for glycosylation at
Asn-113, as would have been expected if the truncated enzyme had been
directed to the lumen of the ER. Second, treatment of membrane
fractions from COS cells with proteinase K indicated that CB-CP(
2,3)
was accessible to proteolytic digestion and unlikely to be in the interior of the isolated organelles. Therefore, it appears that the
association of the truncated form of cathepsin B with membranous fractions from COS cells may involve interactions with the cytosolic surfaces of these organelles.
Observations of a novel form of cathepsin B, which is associated with intracellular membranes in human tumors, suggests the possibility that this enzyme may have other important functions in cancer beyond its likely participation in cell invasion and metastasis. Thus, a number of important regulatory proteins are also associated with the cytosolic surface of the ER and other organelles. These include the anti-apoptotic protein Bcl-2 (61, 62), the pro-apoptotic protein, Presenilin 2 (63, 64), and members of the Ras superfamily of small GTP-binding proteins. The later are involved in the regulation of diverse biological functions which include the assembly of actin stress fibers and focal adhesions (65), the expression of genes involved in cellular proliferation and transformation (66, 67), the maintenance of membranous organelles, and the transport of vesicles between them (65, 68). It may be relevant that limited proteolysis of Presenilin 2 produces C-terminal fragments (69) which can act in a dominant negative manner to inhibit apoptosis (64). It is also interesting to note that v-Ha-Ras and c-Ha-Ras bind with high affinity to cathepsin B (KI values of 48 and 165 nM, respectively) (48, 70).
In conclusion, we have shown that mRNAs missing exons 2 and 3, which encode a truncated form of cathepsin B, are likely to be
functional messages. They contain the remainder of the propeptide and
mature enzyme, and are found to associate with polysomes in the A375
human melanoma. The product of in vitro translation retains the capacity to fold into a native-like protein which is partially active in the absence of pepsin pretreatment to remove the remainder of
the propeptide. Thus, alternative splicing may partly account for
observations of forms of cathepsin B in human tumors that have
unexplained locations, and may be greater in size and/or stability than
the normal, mature enzyme. Immunofluorescence microscopy revealed a
distribution for the truncated enzyme which appeared to overlap that of
the ER resident protein, PDI. The absence of glycosylation and the
sensitivity of CB-CP(2,3) in isolated membrane fractions to
proteinase K digestion suggested that it was located on the cytosolic
surface of intracellular membranes. In these locations the truncated
protein could be expected to interact with important regulatory
proteins such as Bcl-2 and Ras-related proteins. Further work is
required to determine it the truncated tumor form of cathepsin B could
act to alter the apoptotic program in tumor cells or to modify other
important regulatory pathways known to be dysfunctional in cancer.
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ACKNOWLEDGEMENTS |
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We acknowledge the help of Shih-Yen Tsai and Dr. John McNulty, Dept. of Cell Biology, Neurobiology & Anatomy, Loyola University Medical School, in obtaining the immunofluorescence microscopy data presented in this paper. We also thank Dr. Reid Gilmore, Dept. of Biochemistry and Molecular Biology, University of Massachusetts Medical School, for the kind gift of nuclease-treated canine microsomes.
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FOOTNOTES |
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* This work was supported in part by Grant CB103 from the American Cancer Society and in part by an award from the estate of Earl M. Bane.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
§ Portions of this work were submitted in partial fulfillment of the requirements for a Ph.D. degree in the Dept. of Molecular and Cellular Biochemistry, Loyola University of Chicago.
¶ Present address: Cancer Center, Massachusetts General Hosp., Charlestown, MA 02129.
Present address: Dept. of Pediatrics, University of Wisconsin,
Madison, WI 53706.
To whom correspondence should be addressed: Dept. of Molecular
and Cellular Biochemistry, Loyola University Medical Center, 2160 S. First Ave., Maywood, IL 60153. Tel.: 708-216-8109; Fax: 708-216-8523;
E-mail: afrankf{at}luc.edu.
1 The abbreviations used are: PCR, polymerase chain reaction; NGS, normal goat serum; -NMec, N-methylcoumarinylamide; PBS, phosphate-buffered saline; PAGE, polyacrylamide gel electrophoresis; RNasin, human placental ribonuclease inhibitor; Z-, benzyloxycarbonyl; ER, endoplasmic reticulum; PDI, protein disulfide isomerase.
2 S. Mehtani and A. Frankfater, unpublished results.
3 Available on the World Wide Web at www.expasy.ch. Last revision on Oct. 20, 1994 by K. Nakai, Institute for Molecular and Cellular Biology, Osaka University, Osaka, Japan.
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REFERENCES |
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