Abstract 2102: Phenformin down-regulates mammary aromatase expression via the AMPK pathway in humanized aromatase expressing ERBB2 mice

Local mammary estrogen production may be vital for breast cancer development because about 70% of breast cancers occur in postmenopausal women. Aromatase is a key enzyme essential for estrogen biosynthesis, and aromatase inhibitors (AIs) are the most effective endocrine treatment for estrogen-responsive breast cancer in postmenopausal women. However, they are also accompanied a number of severe side-effects on bone and joints. Many researchers are currently focusing on developing the novel mammary-specific AIs to prevent or treat breast cancer to minimize the side effects of treatment. Phenformin, an antidiabetic drug, lowers serum glucose levels via activating the AMP-activated protein kinase (AMPK) pathway and reduces tumor burden and prolongs survival in non-small cell lung cancers. In vitro studies show that activated AMPK inhibits human mammary adipose tissue aromatase expression via suppression of binding of cAMP-responsive element binding protein (CREB) to aromatase promoters I.3/II, leading to decreased estrogen formation. We hypothesized that phenformin will be an effective adipose-specific AI for prevention and treatment of breast cancer. However, one of the major obstacles to define in vivo effects of phenformin on mammary aromatase expression is the lack of suitable mouse models because aromatase expression, present in the human breast, is absent in the mouse mammary gland. We developed a unique humanized aromatase mouse model (Arom hum ) to mimic the human aromatase expression pattern and physiology in the mouse. We observed that tumorigenesis was accelerated in the mice overexpressing ERBB2 that were crossed with Arom hum mice (AE mice). Phenformin treatment (i.p., 70 mg/kg /day for 14 days) significantly decreased blood glucose levels by 37% in AE mice as compared to vehicle treatment. Phosphorylation of CREB, a downstream effector of the AMPK pathway, which mediates aromatase expression, was significantly decreased. Furthermore, after phenformin administration, mammary aromatase mRNA expression in AE mice was significantly decreased by 58% and estrogen response genes (SUSD3 and cyclin D1) were also reduced by about 65%. Real-time PCR showed that mammary AMPKα1 mRNA expression is 10-fold higher compared with AMPKα2 mRNA. Moreover, phenformin treatment significantly decreased AMPKα1 mRNA levels by 53% but not AMPKα2 mRNA expression in AE mice. Interestingly, daily oral phenformin treatment for 7 days significantly reduced mammary tumor volume of AE mice by 76%. In conclusion, phenformin decreased mammary aromatase expression and possible estrogen production, and reduced tumorigenesis via upregulation of the AMPK pathway in breast tissue of AE mice. This study provides justification for future testing of the adipose-specific preventive and therapeutic potential of the metabolic drug phenformin in postmenopausal breast cancer with minimal side-effects. Citation Format: Hong Zhao, Robert T. Chatterton, Timothy Prajka, Lin Li, Serdar E. Bulun. Phenformin down-regulates mammary aromatase expression via the AMPK pathway in humanized aromatase expressing ERBB2 mice. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2102. doi:10.1158/1538-7445.AM2014-2102