CATHEPSIN L2, A NOVEL HUMAN CYSTEINE PROTEINASE PRODUCED BY BREAST AND COLORECTAL CARCINOMAS

We have identified and cloned a new member of the papain family of cys teine proteinases from a human brain cDNA library. The isolated cDNA c odes for a polypeptide of 334 amino acids that exhibits all of the str uctural features characteristic of cysteine proteinases, including the active site cysteine residue essential for peptide hydrolysis. Pairwi se comparisons of this amino acid sequence with the remaining human cy steine proteinases identified to date showed a high percentage of iden tity (78%) with cathepsin L; the percentage of identity with all other members of the family was much lower (<40%), On the basis of these st ructural characteristics, se have tentatively called this novel protei n cathepsin L2, The cDNA encoding the mature cathepsin L2 was expresse d in Escherichia coli, and after purification, the recombinant protein was able to degrade the synthetic peptide yloxycarbonyl-L-phenylalany l-L-arginine-7-amido-4- methylcoumarin, which is commonly used as a su bstrate for cysteine proteinases, Cathepsin L2 proteolytic activity on this substrate was abolished by s-epoxysuccinyl-L-leucylamido-(4-guan idino)butane, an inhibitor of cysteine proteinases, thus providing add itional evidence that the isolated cDNA encodes a functional cysteine proteinase. Northern blot analysis of polyadenylated RNAs isolated fro m a variety of human tissues demonstrated that cathepsin L2 is predomi nantly expressed in the thymus and testis, This finding is in marked c ontrast with the wide tissue distribution of most cysteine proteinases characterized to date, including cathepsin L, and suggests that cathe psin L2 may play a specialized role in the thymus and testis, Expressi on analysis of cathepsin L2 in human tumors revealed a widespread expr ession in colorectal and breast carcinomas but not in normal colon or mammary gland or in peritumoral tissues. Cathepsin L2 was also express ed by colorectal and breast cancer cell lines as well as by some tumor s of diverse origin, including ovarian and renal carcinomas. These res ults open the possibility that this novel enzyme may be involved in tu mor processes, as already reported for other cysteine proteinases, inc luding cathepsin L.