Regulation of Estrogen Receptor Signaling in Breast and Endometrial Cancer by the Src Kinase Pathway, the Micronutrient Selenium, and by Novel Tamoxifen-regulated Biomarkers

Tamoxifen is the most widely prescribed selective estrogen receptor modulator (SERM) for breast cancer. Despite the benefits of tamoxifen therapy, almost all tamoxifen-responsive breast cancer patients develop resistance to therapy and tamoxifen exhibits ER agonist action in the uterus that is associated with an increased incidence of endometrial cancer. These problems with tamoxifen therapy indicate a need to identify new biomarkers for tamoxifen therapy that are predictive of tamoxifen resistance and/or the agonist effects associated in the endometrium. Protein expression profiles associated with a graded estrogen response were generated in an ER-positive and tamoxifen resistant Ishikawa endometrial adenocarcinoma cell-line using two-dimensional gel electrophoresis and mass spectrometry sequencing. Among the identified proteins were chaperones proteins, RNA and RNA polymerase binding proteins, cytoskeletal proteins, an ubiquitin ligase, and a multi-functional kinase. The proteins identified may serve as useful biomarkers for development of tamoxifen resistance or serve as novel tumor targets. In related projects designed to identify novel mechanisms to increase tamoxifen efficacy, it was found that src kinase inhibitors and the micronutrient selenium modulated ERa signaling and potentiated the antagonist action of tamoxifen in sensitive and resistant breast and endometrial cell lines through discrete mechanisms. Src kinase induced ERa phosphorylation at serine 167 via the PI3K/AKT pathway that was shown to be critical for ERa promoter interaction. In addition, src specifically enhanced the transcriptional activity of steroid receptor coactivator-1 (SRC-1), which was sufficient to enhance its coactivation of ERa. Inhibition of src kinase blocked these mechanisms and prevented tamoxifen agonist action in tamoxifen resistant cell lines. Methylseleninic acid (MSA) is a monomethylated selenium compound that inhibits growth of cancer cells in vitro. MSA decreased ERa mRNA and protein levels in tamoxifen-sensitive and -resistant breast and endometrial cancer cell lines in vitro that resulted in subsequent antagonism of estradiol-dependent ERE2e1b-luciferase and endogenous c-myc and pS2 gene expression. MSA also reversed tamoxifen activation of these genes in endometrial Ishikawa and HEC-1A cells. In addition, MSA potentiated the growth inhibitory affects of tamoxifen in tamoxifen-sensitive and -resistant breast and endometrial cancer cells.