Estrogenically regulated LRP16 interacts with estrogen receptor alpha and enhances the receptor's transcriptional activity

Previous studies have shown that leukemia related protein 16 (LRP16) is estrogenically regulated and that it can stimulate the proliferation of MCF-7 breast Cancer cells, but there are no data on the mechanism of this pathway. Here, we demonstrate that the LRP16 expression is estrogen dependent in several epithelium-derived tumor cells. In addition, the suppression of the endogenous LRP16 in Estrogen Receptor alpha (ERalpha)-positive MCF-7 cells not only inhibits cells growth, but also significantly attenuates the cell line's estrogen-responsive proliferation ability. However, ectopic expression of LRP16 in ERalpha-negative MDA-MB-231 cells has no effect on proliferation. These data suggest the involvement of LRP16 in estrogen signaling. We also provide novel evidence by both ectopic expression and small interfering RNA knockdown approaches that LRP16 enhances ERalpha-mediated transcription activity. In stably LRP16-inhibitory MCF-7 cells, the estrogen-induced upregulation of several well-known ERalpha target genes including cyclin D1 and c-Myc is obviously impaired. Results from Glutathione S-transferase pull-down and coimmunoprecipitation assays revealed that LRP16 physically interacts with ERalpha in a manner that is estrogen independent but is enhanced by estrogen. Furthermore, a mammalian two-hybrid assay indicated that the binding region of LRP16 localizes to the A/B activation function 1 domain of ERalpha. Taken together, these results present new data supporting a role for estrogenically regulated LRP16 as an ERalpha coactivator, providing a positive feedback regulatory loop for ERalpha signal transduction.