
<ArticleTitle>Alternative messenger RNA splicing and enzyme forms of <prot>cathepsin B</prot> in human osteoarthritic cartilage and cultured chondrocytes.</ArticleTitle>
<AbstractText>OBJECTIVE: In previous studies, we suggested that <prot>cathepsin B</prot>, which is present at sites of cartilage remodeling in osteoarthritis (OA), may act as an antagonist of cartilage repair, an enhancer of the action of metalloproteinases, and a mediator of cartilage neovascularization and mineralization. Alternative splicing of <prot>cathepsin B</prot> pre-messenger RNA (pre-mRNA) and/or mRNA overexpression is a plausible regulatory mechanism. In the present study, we investigated the abundance of <prot>cathepsin B</prot> transcripts and the properties of <prot>cathepsin B</prot> protein in normal and OA cartilage, osteophytes, and cultured chondrocytes. METHODS: <prot>Cathepsin B</prot> mRNA splice variants containing the full-length transcript (<prot>CB</prot>) and the variants lacking either exon 2 (<prot>CB</prot>[-2]) or lacking exons 2 and 3 (<prot>CB</prot>[-2,3]) were measured by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and Northern blot assays and were localized by in situ RT-PCR. Cathepsin B protein was analyzed by electrophoretic, Western blot, and chromatographic methods. RESULTS: The relative content of <prot>CB</prot>, <prot>CB</prot>(-2), and <prot>CB</prot>(-2,3) varied considerably in OA cartilage and osteophytes, with less variation in normal cartilage. The mean cathepsin B mRNA level was significantly higher in OA cartilage and osteophytes than in normal cartilage. Normal cultured chondrocytes attained <prot>cathepsin B</prot> mRNA levels similar to those in OA cartilage. Enzyme overexpression resulted in the secretion of <prot>procathepsin B</prot>, followed by activation to the proteolytically active form. CONCLUSION: The high levels of <prot>CB</prot> and <prot>CB</prot>(-2) are consistent with an overproduction of secreted <prot>procathepsin B</prot> in OA. Up-regulation of chondrocyte <prot>cathepsin B</prot>, which takes place at both the transcriptional and the translational level, suggests a leading role of the enzyme in the progression of OA.</AbstractText>
