The expression of cathepsin B and other lysosomal proteinases in normal tissues and in tumors

The mRNA for the lysosomal proteinases cathepsins B, D, H, L, and S are broadly distributed in normal rodent tissues. Although total cathepsin mRNA levels generally parallel the protein catabolic activity of the tissues, the expressions of the individual enzymes do not appear to be linked. Thus, the relative proportions of the individual messages are found to vary from tissue to tissue. Further evidence for the independent regulation of lysosomal proteinase expression is derived from observations of selective increases in mRNA levels for individual proteinases in rodent tumors. Only cathepsin B mRNA is elevated in a highly metastatic murine B16a melanoma and in a Walker-256 rat carcinosarcoma, while Moloney murine sarcoma virus-transformed fibroblasts express increased mRNA for cathepsins B, D, and L and normal levels for H and S. To address the regulation of cathepsin B expression, the mouse cathepsin B gene and its 5'-upstream region were cloned. The gene has 10 exons and 9 introns spanning about 20 kilobases. The 5'-upstream region and exon 1 are GC-rich with several potential Sp1 binding sites. TATA and CAAT motifs adjacent to the transcription start site are not evident. These properties are characteristic of mammalian "housekeeping" genes. B16 melanoma cells contain three cathepsin B transcripts of 2.2, 4.0 and 5.0 kilobases. The two larger messages, which were not found in normal tissues, contain unusually long 3'-untranslated regions resulting from the alternative cleavage and polyadenylation of the 3' end of the cathepsin B pre-mRNA in B16 melanomas. As all three messages encoded normal preprocathepsin B, cathepsin B secretion by melanoma cells is probably due to posttranslational mechanisms and not to alternative splicing or gene mutation.