Abstract B37: The role of IGF-II in estrogen independent tumors in African American breast cancer cells

Introduction: Breast cancer is the second leading cause of cancer deaths in women. In 2008, an estimated 180,000 people will be diagnosed with breast cancer while approximately 40,000 will die from this disease. Breast cancer has been found to affect different ethnic groups disproportionately. Estrogen receptor negative (ER-) tumors are more common in AA women, when compared to Caucasian women. These aggressive tumors are more common at every age and stage of breast cancer, which causes increased mortality rate. 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 the role of insulin-like growth factor-II (IGF-II) in the survival disparity gap observed in AA women. ProIGF-II is a fetal growth factor regulated by estrogen to promote proliferation, prevent apoptosis and regulate energy production by signaling through the IGF-I (IGF-IR) and Insulin Receptors (IR).

Methods: In our studies we used obtained four breast cancer cell lines from ATCC: AA ER−/PR− and HER2/Neu− (CRL-2335), AA ER+/PR− and Her2/Neu− (CRL-2315), Caucasian ER−/PR− and Her2/Neu+ (HS578t) and Caucasian ER+/PR+ and Her2/Neu+ (MCF-7). We evaluated the phosphorylation and subcellular localization of ER-α and ER-β, with and without proIGF-II treatment, using Real-time Polymerase Chain Reaction (RT-PCR) and western blot analysis.

Results and Discussion: We have demonstrated that not only do these cell lines express ER-α and ER-β, but also the ERs become phosphorylated/activated and translocate to other cellular compartments when treated with precursor Insulin-like growth factor II (proIGF-II). Our study demonstrates that proIGF-II binding to the IGF-IR and 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. We report here, for the first time, that ER-α negative breast cancer cells express significant levels of ER-α and ER-β in the cytosol, membranes and mitochondria. When treated, with proIGF-II, ER-α and ER-β shifted from its predominant localization in the cytosol to the membrane/organelles and nucleus. Thus, without the requirement of estrogen binding, IGF-II can stimulate estrogen-like responses.

Conclusion: We propose that proIGF-II mediates estrogen actions in these cells by phosphorylating/activating both ERs to promote estrogen independent growth and control mitochondrial and nuclear function to provide energy and cell survival proteins. Thus, IGF-II contributes to the survival disparity observed among AA breast cancer patients, because it promotes higher activation/phosphorylation of the IGF-1R and ERs. This independent activation of ERs promotes estrogen independent breast cancer cell survival and chemoresistance characteristic of triple-negative (ER−-/PR− and HER2/neu-) tumors.