The Role of Signal Transducer and Activator of Transcription 1 (STAT1) and 3 (STAT3) in Primary and Metastatic Breast Cancer

Breast cancer is the most frequently diagnosed malignancy and the second most lethal cancer in women. Metastasis in breast cancer is invariably responsible for patient death and is comprised of many steps, of which proliferation in vital organs is responsible for morbidity and mortality due to vital organ failure. Patients with the metastatic disease are limited to chemotherapy, which non-specifically targets proliferating cells. Despite it being initially effective, chemotherapy is associated with high toxicity and many patients develop resistance. Thus, there is an urgent need to characterize the biology of metastatic breast cancer to develop targeted therapies for the late-stage disease.

EGFR is a member of the ErbB family of receptor tyrosine kinases, which have particular relevance in breast tumorigenesis. Clinical studies show that high expression levels of EGFR in the primary mammary tumors correlate with poor prognosis and decreased survival of breast cancer patients due to metastasis. Patient data is supported by experimental and pre-clinical studies, which describe various signaling pathways that mediate the oncogenic effects of EGFR, such as the MAPK, STAT3, and PI3K pathways. Despite these well-documented roles of EGFR, clinical trials evaluating EGFR inhibitors (EGFRi) in metastatic breast cancer have been unanimously unsuccessful in improving patient prognosis, and the mechanisms that contribute to this intrinsic resistance are unknown.

To characterize the signaling events that govern EGFR behavior in metastatic breast cancer resistant to EGFRi, we utilized multiple pre-clinical breast cancer progression series and patient-derived cells that display the intrinsic resistance phenomenon. In these models, EGFR functions as a pro-apoptotic molecule whose ligand-mediated activation results in growth inhibition and/or apoptosis of metastatic breast cancer cells. Here we show that in the later stages of metastasis, increased nuclear translocation of EGFR leads to increased physical access to STAT1 and STAT3 molecules residing in the nucleus. Indeed, an EGFR mutant that is defective in endocytosis is unable to elicit STAT1/3 phosphorylation. Additionally, specific inhibition of nuclear EGFR function using the EGFR kinase inhibitor gefitinib linked to a nuclear localization signal (NLS-gefitinib) prevents EGF-induced STAT1/3 phosphorylation. Altogether, these findings implicate nuclear localization of EGFR in downstream STAT1/3 signaling in metastatic breast cancer.

Subsequently, we examined the involvement of nuclearly-activated STAT1/3 signaling in the apoptotic function of EGFR. NLS-gefitinib treatment or genetic/pharmacologic inhibition of STAT1/3 efficiently blocks EGF-induced apoptosis in metastatic breast cancer cells resistant to EGFRi. These findings were utilized therapeutically by activating EGFR with EGF treatment while simultaneously blocking the downstream proliferative MAPK:ERK1/2 pathway using the MEK1/2 inhibitor trametinib. EGF + trametinib combination preserved STAT1 signaling while effectively blocking the MAPK pathway, thus potentiating EGF-mediated apoptosis in metastatic breast cancer cells. Importantly, combined administration of trametinib and EGF resulted in STAT1-mediated apoptosis of primary mammary tumor cells, which respond to EGF in a proliferative fashion. These data provide a novel approach of targeting metastatic breast cancer by biasing EGFR signaling towards nuclear activation of STAT1/3 signaling resulting in apoptosis.

Our studies herein also examined the role of STAT3 in primary mammary tumor cells overexpressing EGFR. Depletion of STAT3 expression normalized the transformed phenotype of these cells in vitro and resulted in smaller mammary tumors in vivo. These results implicate STAT3 in EGFR-driven breast tumorigenesis localized to the mammary tissues. Further, systemic dissemination of breast cancer is associated with activation of the JAK1/2:STAT3 signaling axis. Despite the involvement of STAT3 in EGFR-mediated oncogenesis in the primary tumor setting, targeting JAK1/2:STAT signaling with the JAK1/2 inhibitor ruxolitinib proved ineffective in inhibiting the growth and invasion of metastatic cells derived from these primary tumors. These results are in agreement with the role of STAT1/3 in driving the pro-apoptotic function of EGFR in metastatic breast cancer cells. Altogether, these investigations provide a plausible explanation for the inability of JAK1/2 inhibitors to effectively target metastatic breast cancer in clinical and experimental investigations. Further, these findings indicate that the development of therapeutics or molecular tools that efficiently activate STAT1/3 signaling in metastatic breast cancer may represent an important concept for eradicating tumors resistant to targeted therapies.