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An Opposing View on WWOX Protein Function as a Tumor Suppressor

¹ Genome Science Division and
² Laboratory of Systems Biology and Medicine, Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo;
³ Perseus Proteomics, Inc., Tokyo; and
⁴ First Department of Surgery, Osaka City University Medical School, Osaka, Japan

WW domain-containing oxidoreductase (WWOX) is a candidate tumor suppressor gene. Because mutation or deletion in the coding region of WWOX is rarely found, it is speculated that the appearance of aberrant transcripts affects progression of various cancers. However, little is known about the role in these cancers of the WWOX protein. To characterize endogenous WWOX proteins, we analyzed WWOX expression using newly generated monoclonal antibodies. In immunoblot analysis of 49 cancer cell lines, only the normal form of the protein was detectable, although some of cell lines exhibited aberrant WWOX RNA transcripts. Accumulation of truncated proteins was observed by inhibiting proteasomal degradation with MG-132, whereas expression level of normal protein did not change, suggesting truncated proteins may be subjected to rapid degradation through proteasomal machinery. Immunohistochemistry for cancer cells demonstrated that WWOX protein levels are not decreased but rather elevated in gastric and breast carcinoma, challenging the notion of WWOX as a classical tumor suppressor. In noncancerous cells, WWOX was observed only in epithelial cells, including hormone-regulated cells such as Leydig cells, follicular cells, prostate epithelium, and mammary glands. Interestingly, restricted staining in nuclei was observed in some mammary gland cells while other epithelial cells exhibited localization of WWOX in cytoplasm. Nuclear localization of WWOX was also confirmed in confluent human fibroblast KMS-6, whereas WWOX was associated mainly with mitochondria before reaching confluence, indicating that WWOX shuttles between cytoplasm and nuclei. These findings provide novel insights into aspects of human WWOX function in both normal and malignant cells.

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