LIV-1 is a type III transmembrane protein with zinc uptake activity whose expression has previously been correlated with estrogen receptor positive breast cancers. Using genomic profiling, we have found that in addition to the previously described association of LIV-1 expression in breast cancers, LIV-1 mRNA is also overexpressed in prostate cancers, compared to normal human tissues. We generated monoclonal antibodies to LIV-1 and found that LIV-1 protein is expressed in a significant percentage of both breast and prostate tumors, as detected by immunohistochemistry, as well as on variety of cultured human tumor cell lines, as detected by flow cytometry. Confirming the mRNA data, very little protein was detected in normal human tissues using these methods. To further characterize LIV-1 cellular functions, we used RNAi to modulate LIV-1 protein levels. Several different siRNAs to LIV-1 reduced cell-surface LIV-1 protein levels, as determined by flow cytometry. The LIV-1 siRNAs, in turn, caused a reduction in cellular proliferation in a variety of human tumor cell lines in long-term cultures (7-14 days). In addition, we investigated the potential of LIV-1 for membrane type matrix metalloproteinase (MT-MMP) activity, as its protein sequence contains a potential metalloproteinase motif (Taylor, et al. Biochem J. 375:51). Using a qualitative in situ gelatin degradation assay as a measure of MT-MMP activity, we found that most cancer cell lines tested had significant MT-MMP activity. However, reducing LIV-1 protein levels via RNAi significantly reduced gelatin degradation by the colon carcinoma cell line, HCT-116. This result suggests that LIV-1 can contribute to cellular MT-MMP levels, but given that the LIV-1 sequence contains the potential metalloproteinase motif, it is possible that LIV-1 itself has MT-MMP activity. These findings, together with the previously described correlation of LIV-1 expression and lymph node positive breast cancers (Manning, et al. Eur. J. Cancer 30A:675), suggest that LIV-1 can contribute to the metastatic potential of tumor cells by increasing the invasive potential as well as the viability of tumor cells and therefore plays an important functional role for LIV-1 in tumorigenesis.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]