FXYD3 protein involved in tumor cell proliferation is overproduced in human breast cancer tissues.

FXYD3, also known as Mat-8 (Mammary tumor 8 kDa), is one of mRNAs highly expressed in mouse and human breast cancers. Here, we newly found that FXYD3 protein was also overexpressed in human breast cancer specimens; invasive ductal carcinomas and intra-ductal carcinomas, whereas its expression was low in benign lesion specimens; mastopathy, fibroadenoma and phyllodes tumors. Although human FXYD3 has two isoforms (shorter FXYD3a and longer FXYD3b), there have been no reports in which these two mRNAs were separately quantified. Here, we found that FXYD3a mRNA is a major transcript product expressed in human normal tissues as well as in breast, colon, stomach and pancreas cancer cell lines. In addition, expression level of FXYD3b was much lower than that of FXYD3a in these cancer cell lines. Cell proliferation rate of MCF-7 breast cancer cell line was drastically decreased when FXYD3a and 3b mRNAs were suppressed by the small interfering RNA. These results suggest that FXYD3a protein is highly expressed in breast cancers, and responsible for cancer cell proliferation.