IGF-II regulates phosphorylation of estrogen receptors In breast cancer cellsfrom African American patients

A76 Recognition that racial differences may account for a disparity in survival among African American (AA) women with breast cancer has stimulated nationwide interest in research to elucidate the contributing factors. The increased mortality rate for AA women is paradoxical in view of their lower risk of breast cancer (BC) incidence. Thus, a comprehensive approach to understand the biological factors associated with poorer outcomes among AA patients is urgently needed. We are addressing this need by investigating how IGF-II contributes to the survival disparity observed among AA breast cancer patients. IGF-II is a fetal growth factor regulated by estrogen to promote proliferation and prevent apoptosis. Expression of this growth factor in human breast cancer cells stimulated rapid tumor growth in a nude mice model of human BC developed in our laboratory. Of great significance, the expression of IGF-II abrogated the requirement of estrogen for tumor growth and metastasis in this animal model. These observations led us to propose that IGF-II plays a key role in BC by promoting estrogen-independent growth, a hallmark of aggressive BC characteristic of tumors observed among AA patients. How does IGF-II promotes estrogen independent growth? What is the interaction between estrogen signaling and IGF signaling? Estrogens play a key role in the progression of human breast cancer. The effects of estrogen are largely mediated by two different, but related, estrogen receptor (ER) isoforms named estrogen receptor alpha (ER- α) and estrogen receptor beta (ER- β). While both, ER- αand ER- β, share a number of structural and functional similarities, they differ in term of subcellular localization and transactivation of certain estrogen responsive genes. On the other hand, IGF-II binds the IGF-1 receptor, a membrane associated tyrosine kinase that initiates a downstream signaling cascade to stimulate growth. In spite of distinct signaling cascades, recent published work revealed that the estrogen-IGF signaling pathways interact in a“cross-talk” where the activation of the IGF-IR can phosphorylate and activate the ER in cell membranes to stimulate a rapid non-genomic response. This activation of the estrogen receptor does not require estrogen. These observations have prompted us to investigate whether IGF-II alters the phosphorylation and sub-cellular localization of these ERs. Breast cancer cells from Caucasian and AA breast cancer patients were obtained from ATCC. Cells were sonicated and fractionated to separate the distinct compartments (cytosol, organelles, nucleus and extracellular matrix). Phosphorylated and non-phosphorylated ER- α and ER- β were analyzed by Western blot. Densitometric analysis was used to quantify the differences detected in the bands revealed by chemiluminescence in x-ray films. Our study revealed that phosphorylated ER- α and ER- β were detected in membranes, but not nucleus, of cells classified as ER-α. Cells from AA patients expressed higher amounts of ER- α in the plasma membrane while ER- β was mainly detected in the mitochondria. We propose that IGF-II mediates estrogen actions in these cells by phosphorylating/activating both receptors to promote estrogen independent growth and control mitochondrial function to provide energy and chemoresistance, thus contributing to the survival disparity observed among AA breast cancer patients.