Leupaxin is expressed in mammary carcinoma and acts as a transcriptional activator of the estrogen receptor alpha

Leupaxin belongs to the group of paxillin proteins and was reported to play a major role in the invasion and migration of prostate cancer cells. In the present study we were able to show by using a cDNA cancer profiling array that leupaxin is upregulated in breast and endometrial cancer, whereas downregulation of leupaxin was observed in lung cancer. In addition, immunohistochemical studies using a leupaxin-specific antibody on human breast cancer specimens (n=127) revealed that leupaxin is expressed mainly in invasive ductal carcinomas and ductal carcinoma in situ (40 and 49% respectively), and only in a minority of lobular mammary carcinomas. To further investigate the role of leupaxin in the progression of breast cancer the expression of leupaxin was analysed in six breast cancer cell lines. The estrogen receptor a (ER alpha)-positive HCC70 and the ER alpha-negative MDA-MB-231 cells showed leupaxin expression on the RNA and protein level. Leupaxin localizes in these mammary carcinoma cells at focal adhesion sites and shuttles between membrane and nucleus via its LD4 motif as major nuclear export signal. Interaction partners of leupaxin in the nucleus represent the estrogen receptors ER alpha and ER beta. Both ERa and ER beta bind to the LIM domains of leupaxin via their AF-1/DNA binding domains. Furthermore, leupaxin is able to induce transcriptional activity of ERa independent of the presence of estradiol. The specific downregulation of leupaxin expression using siRNAs in mammary carcinoma cells resulted in reduced migratory capability and diminished invasiveness whereas no effect on proliferation was observed. Collectively, these results show that leupaxin has particular influence on the progression and invasion of breast cancer cells and may therefore represent an interesting candidate protein for diagnosis and therapeutic interventions.