Abstract 203: PKR is a doxorubicin sensitivity biomarker for breast cancer

Abstract

Breast cancer is the second most frequently diagnosed cancer in U. S. women with >200,000 new cases estimated for 2010. Resistance to standard chemotherapy remains a significant clinical challenge. Thus, the identification of novel molecular biomarkers that predict response to treatment may improve therapeutic decision making and outcomes. Recently it has been reported that the level and activity of the dsRNA-dependent protein kinase, PKR, is increased in mammary carcinoma cell lines and more aggressive primary tumor specimens. Previously, our laboratory has determined that PKR is a key regulator of cell proliferation, apoptosis and transformation. PKR is activated by diverse cellular stresses including growth factor withdrawal from hematopoietic cells, inflammatory cytokine treatment or viral infection to initiate apoptosis presumably as a result eIF2α phosphorylation and inhibition of protein synthesis. However, activated PKR may also promote apoptosis by inhibiting Bcl2 phosphorylation and regulate cell proliferation by mechanisms dependent on p53, STAT1 or NF-κB. In order to gain insight into how PKR signaling may affect breast cancer development and sensitivity to chemotherapy, we measured the level of PKR in primary breast cancer (and paired, normal adjacent tissue) by immunohistochemical staining of a breast tumor tissue microarray containing 50 cases. Increased PKR staining was observed in all tumor types tested (infiltrating ductal carcinoma, mucinous adenocarcinoma, medullary carcinoma, Paget's disease, carcinosarcoma, cystosarcoma phyllodes and benign fibroadenoma) compared to normal adjacent tissue. Furthermore, elevated PKR expression directly correlates with increased tumor grade indicating a more aggressive phenotype. To determine whether PKR may be required for tumor response to doxorubicin, we employed a siRNA strategy to knock-down PKR by >75% in several breast cell lines. Interestingly, basal PKR activity is dramatically higher in those cell lines derived from human mammary carcinoma (i.e. MCF7 and T-47D) compared to a “nontransformed” mammary epithelial cell line (MCF10A). Furthermore, timecourse and dose response studies with doxorubicin, a frequently used and effective chemotherapy, revealed that cells with reduced PKR are significantly less sensitive to the drug compared to control siRNA cells with higher PKR. Interestingly, phosphorylation of eIF2α is unaffected by reduced PKR expression in MCF7 cells even with the highest dose of doxorubicin. This indicates that the mechanism by which PKR promotes doxorubicin sensitivity may be independent of eIF2α phosphorylation and protein synthesis inhibition. Together, these results suggest that increased PKR expression may be a useful biomarker for more aggressive breast tumors and for predicting response to doxorubicin treatment. Based on this model, aggressive tumors that do not display elevated PKR are predicted to be resistant to doxorubicin. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 203. doi:10.1158/1538-7445.AM2011-203