Triple-negative breast cancer is a highly aggressive and invasive tumor type which accounts for 15% of all breast cancers and disproportionately affects pre-menopausal women as well as African-American and Hispanic populations. Triple negative breast tumors show an increased risk for metastasis, due to over-activation of tumor-promoting signaling pathways, including the mitogen-activated protein kinase (MAPK) and PI3 kinase (PI3K)-Akt pathways. The limited available targeted therapies for triple negative breast cancer due to the lack of estrogen and progesterone receptor expression and unamplified human epidermal growth factor receptor 2 (HER2) levels indicate a clear need for novel targeted therapies for patients. Angiotensin-(1-7) [Ang-(1-7)] is an endogenous, seven amino acid peptide hormone of the renin-angiotensin system that activates the G protein-coupled receptor mas. Ang-(1-7) inhibited the growth of MDA-MB-231 human triple negative breast tumors growing in the mammary fat pads of athymic mice. The mean tumor volumes of saline- or Ang-(1-7)-treated mice were similar prior to the initiation of treatment (saline, 113.3±7.0 mm3; Ang-(1-7), 107.6±6.8 mm3). The tumor volume of mice treated with saline increased 4.4-fold over the 28 day treatment period; in contrast, the volume of tumors from mice administered Ang-(1-7) did not increase over the 28-day treatment period compared to tumor volume at the initiation of treatment. Cells in tumors from mice treated with saline showed abundant Ki67 immunoreactivity, while Ki67 immunoreactivity was reduced 51% in tumor cells from mice injected with Ang-(1-7). Ang-(1-7) significantly reduced the phosphorylation of ERK1 and ERK2 in triple negative breast tumors as compared to the phospho-modification of the enzymes in tumors from saline-treated mice, by 95.2% and 81.3%, respectively; no change in non-phosphorylated ERK1 and ERK2 was observed. The dual-specificity phosphatase DUSP1, or MKP-1, is a key phosphatase that regulates the activity of ERK1/ERK2. Ang-(1-7) significantly increased DUSP1 protein and mRNA in tumors of mice injected with Ang-(1-7) as compared to tumors from saline-treated mice. Incubation of human MDA-MB-231 triple negative breast cancer cells with an siRNA to DUSP1 prevented the reduction in ERK activation, demonstrating that the heptapeptide hormone up-regulates DUSP1 to reduce MAP kinase activities. Ang-(1-7) also attenuated PI3 kinase-Akt signaling in human triple negative breast tumors by up-regulating the Akt phosphatase PP2A (protein phosphatase 2A) 5-fold (n=4-5, p<0.05). Incubation of MDA-MB-231 human triple negative breast cancer cells with Ang-(1-7) increased PP2A subunits--both the scaffolding A subunit (a 4-fold increase, n=3-4, p<0.05) and the catalytic C subunit (a 2-fold increase, n=3-4, p<0.05). The heptapeptide hormone also significantly inhibited the growth of 4T1 murine triple negative breast cancer cells (n=3, p<0.05). The reduction in 4T1 cell growth was similarly associated with a 5-fold increase in PP2A-A and a 2-fold increase in PP2A-C (n=3-4, p<0.05 and p<0.01, respectively), with a concomitant 80 to 90% reduction in phospho-Akt at both the serine and threonine residues (n=3-4, p<0.001 and p<0.0001, respectively). Taken together, these results suggest that Ang-(1-7) may serve as a novel, targeted therapy for triple negative breast cancer by regulating protein phosphatases to reduce activation of protein kinase-mediated proliferative signaling cascades.
Citation Format: Patricia E. Gallagher, Alison L. Arter, David R. Soto-Pantoja, E. Ann Tallant. Angiotensin-(1-7) attenuates triple-negative breast cancer growth and progression through regulation of protein phosphatases. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr C66.