Abstract P4-05-08: Cellular localization dictates the role for c-Abl in breast cancer

c-Abl is a ubiquitously expressed non-receptor tyrosine kinase that regulates numerous aspects of normal mammary epithelial cell (MEC) physiology. However, the exact role of c-Abl during mammary tumorigenesis remains controversial. Clinical trials utilizing the c-Abl antagonist, imatinib mesylate, for the treatment of metastatic breast cancer (BC) patients failed due to disease progression. Additionally, our previous studies overexpressing a constitutively active c-Abl mutant in aggressive murine triple negative breast cancer (TNBC) cells abrogated tumorigenesis when injected into syngeneic mice. Through data mining we found c-Abl expression to be down-regulated across BC subtypes and this correlated with a significantly worse relapse free survival rate for patients of triple negative and luminal A BC subtypes. Furthermore, through in silico analyses we determined that BC patients with high c-Abl expression responded to docetaxel treatment, whereas their low c-Abl expressing counterparts did not. Taken together these findings support a tumor suppressive role for c-Abl in BC and that its targeted activation could inhibit BC tumorigenesis as well as sensitize BC patients to docetaxel treatment. We show here that shRNA mediated depletion of c-Abl elicited docetaxel resistance in malignant MECs through three dimensional (3-D) culture and clonogenic assays on plastic. Furthermore, we show through data mining that c-Abl expression is down-regulated in response to docetaxel treatment in a cohort of BC patients. This finding was recapitulated in malignant MECs through which docetaxel treatment selected for cells that retained lower c-Abl expression highlighting the importance of c-Abl in this cell death pathway. Targeted activation of c-Abl utilizing a novel small molecule activator, DPH, presents a novel therapeutic strategy for sensitizing BC patients to docetaxel treatment. However, we show DPH-mediated c-Abl activation to inhibit the proliferation of luminal A MCF7 BC cells, but enhance proliferation of the TNBC MDA-MB-231-L2 cell line in vitro. The cellular localization of c-Abl is crucial for its tumor suppressive functions. In Philadelphia chromosome positive chronic myelogenous leukemia (CML) the constitutively active BCR-Abl gene fusion has a strict cytoplasmic localization, however when nuclear expression of BCR-Abl is enforced CML cells undergo apoptosis. Cellular fractionation of normal murine MECs cells indicate that c-Abl expression is detected both in cytoplasmic and nuclear compartments before and with greater amounts detected in the nucleus upon activation. A similar trend is seen in the luminal A MCF7 BC cell line, however c-Abl is predominantly expressed in the cytoplasm basally in the TNBC MDA-MB-231-L2 cell line and retains a strict cytoplasmic localization upon activation with DPH. These findings suggest a similar mechanism as CML of cytoplasmic isolation as a means of circumventing the tumor suppressive role of c-Abl across different BC subtypes. Data from ongoing immunohistochemical analyses of a cohort of BC patient tissue microarrays will inform whether nuclear versus cytoplasmic c-Abl will correlate with available patient outcome data to determine if cellular localization dictates a tumor suppressive or tumor promoting role for c-Abl. Citation Format: Chevaun D Morrison, Hannah Gilmore, William P Schiemann. Cellular localization dictates the role for c-Abl in breast cancer [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P4-05-08.