In VivoExpression of an Alternatively Spliced Human Tumor Message That Encodes a Truncated Form of Cathepsin B

Abstract

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.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 ...

Diversity in Tissue Expression, Substrate Binding, and SCF Complex Formation for a Lectin Family of Ubiquitin Ligases

“…A similar discrepancy in expression patterns has been reported for other ubiquitin ligases (52), and tissue-specific distributions have been reported for different isoforms of lysosomal enzymes (63)(64)(65). These differences in expression could reflect tissue-specific auto-ubiquitination of ubiquitinprotein isopeptide ligases (50,51) or the expression of splice variants.…”

Human Cathepsin F

“…Interestingly, the staining pattern of cathepsin F is similar to that observed for a truncated form of cathepsin B, which also is missing its signal sequence and a part of its prodomain (33). Mehtani et al (33) could demonstrate that truncated cathepsin B is associated with the cytosolic surface of membrane organelles. Whether cathepsin F is located inside the observed vesicles or facing the cytosolic side of vesicular membranes is presently under investigation.…”

Deletion of the Collagen-specific Molecular Chaperone Hsp47 Causes Endoplasmic Reticulum Stress-mediated Apoptosis of Hepatic Stellate Cells

“…The localization of procollagen within cells was next examined by immunostaining HSCs after they had been permeabilized using Triton X-100. Type I procollagen localized primarily in the Golgi apparatus in control HSCs, as shown by its colocalization with GM130 (cis-Golgi marker) (41) but not with protein-disulfide isomerase (ER marker) (42). However, in Hsp47-KO HSCs, the level of type I procollagen in the ER was markedly increased, and this procollagen was also clearly observed in the Golgi apparatus (Fig.…”