Abstract 3345: Inhibition of STAT3 reduces radiation induced changes in cellular plasticity and sensitizes breast cancer cells to radiation

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Triple negative breast cancer (TNBC) is categorized as a lack of expression of the hormone receptors, estrogen and progesterone, and HER2, and is a highly aggressive form of breast cancer. Due to the receptor status, TNBC is unresponsive to conventional targeted treatment resulting in a poor prognosis and increased risk of recurrence and metastasis. We have observed that TNBC patients have higher amounts of infiltrating leukocytes compared to estrogen receptor positive breast cancer patients. Therefore, inflammation may be a contributing factor in TNBC and inhibition of key inflammatory pathways may aid in sensitizing TNBC cells to treatment. Growing evidence suggests that radioresistant breast cancer stem cells are responsible for the tumor growth, recurrence, and metastasis following treatment. Previous studies have suggested that radiation alters non-stem cell populations to become more stem-like, indicating that radiation may induce changes in cellular plasticity. Cytokine activation of the signal transducer and activator of transcription 3 (STAT3) has been shown to mediate tumor-promoting inflammation and expansion of stem cell populations. Thus, we wanted to determine how inhibition of the STAT3 pathway alters the response of TNBC to radiation. To determine the effect of STAT3 inhibition on breast cancer stem populations, TNBC cells were treated with two different small molecule inhibitors to STAT3, C188-9 or Stattic, to block STAT3 signaling and the enzymatic activity of the cancer stem cell marker, aldehyde dehydrogenase (ALDH), was measured by Aldefluor assay. Treatment with 10 μM C188-9 decreased the ALDH positive (ALDH+) population from 18.3% to 5.2% and treatment with 1 μM Stattic decreased the ALDH+ from 12.2% to 4.9% indicating that inhibition of STAT3 decreases the stem cell population of TNBC. In order to determine the effect of STAT3 inhibition on radiation induced plasticity of breast cancer cells, TNBC cells were isolated into ALDH+ and ALDH negative (ALDH-) populations using flow cytometry and irradiated with 8 Gy of radiation. Following five days of treatment, both populations exhibited an increase in the percentage of ALDH+ cells indicating an increase in stem-like cells following radiation. Treatment with either 10 μM C188-9 or 1 μM Stattic significantly blocked the radiation induced increase in breast cancer stem cells. The combined effect of radiation and STAT3 inhibition on colony formation was also assessed. Exposure to 8 Gy of radiation decreased colony formation; however, inhibition of STAT3 prior to radiation exposure significantly reduced colony formation and thus sensitized the breast cancer cells to radiation treatment. These data suggest that targeting STAT3 could potentially be used to prevent alterations in cellular plasticity induced by radiation, reduce populations of resistant stem-like cells, and improve treatment outcomes. Citation Format: Kimberly M. Arnold, Lynn M. Opdenaker, Daniel Flynn, Jennifer Sims-Mourtada. Inhibition of STAT3 reduces radiation induced changes in cellular plasticity and sensitizes breast cancer cells to radiation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3345. doi:10.1158/1538-7445.AM2015-3345