Abstract P6-17-01: MicroRNA-34a suppresses breast cancer bone metastasis by inhibiting osteoclastogenesis and targeting tgif2

Introduction: Approximately 70% of people living with metastatic breast cancer have metastases to their bones. Osteolytic bone metastasis is a common, debilitating and essentially incurable skeletal complication of breast cancer, in which tumor cells migrate to and destroy bones, causing extreme pain, fractures, life-threatening hypercalcemia, limited mobility and eventually motality. The bone resorbing osteoclasts significantly contribute to this process. MicroRNAs (miRNAs) play important roles in physiology and disease, and present tremendous therapeutic potential. Nonetheless, how miRNAs regulate skeletal biology is underexplored. We hypothesize that miRNAs that can suppress osteoclast function may ameliorate breast cancer bone metastasis. Methods: A strategy of ex vivo osteoclast differentiation from mouse bone marrow cells was used to examine the levels of several cancer-related miRNAs during a time course of osteoclastogenesis, and test the effects of miR-34a mimic or inhibitor. Micro-Computed Tomography was conducted to quantify bone mass. ELISA analyses were performed to measure serum bone resorption and bone formation markers. Ovariectomy was employed as a model for postmenopausal osteoporosis. As a model for bone metastases, luciferase labelled bone-metastasis prone MDA-MB-231 human breast cancer cell subline were injected into the left cardiac ventrical of nu/nu mice. Bone metastases were detected and quantified weekly post injection by bioluminescence imaging. Results: MiR-34a is down-regulated during osteoclast differentiation. Osteoclastic miR-34a over-expressing transgenic mice exhibit lower bone resorption and higher bone mass. Conversely, miR-34a knockout and heterozygous mice exhibit elevated bone resorption and reduced bone mass. Consequently, ovariectomy-induced osteoporosis, as well as bone metastasis of breast cancer are diminished in osteoclastic miR-34a transgenic mice. Pharmacologically, systemic delivery of miR-34a mimics via a chitosan nanoparticle vehicle (miR-34a-CH) can target multiple tissues. Our bio-distribution analysis shows that miR-34a-CH delivery to the bone marrow is among the highest compared to other tissues. Administration of miR-34a nanoparticles not only diminished ovariectomy induced bone loss but also attenuated breast cancer bone metastasis. Systemic miR-34a-CH delivery affected neither tumor growth when cancer cells were injected subcutaneously nor tumor metastasis to other organs such as lung when cancer cells were injected intravenously. These results further support the notion that the suppression of bone metastases by miR-34a-CH was mediated by its inhibition of the bone metastatic niche rather than the cancer cells. Mechanistically, we identify the homeodomain transcription factor Tgif2 as an essential direct miR-34a target that is pro-osteoclastogenic. Tgif2 deletion reduces bone resorption and abolishes miR-34a regulation. Conclusion: Using mouse genetic, pharmacological and disease models, we have identified mir-34a as a novel and critical suppressor of osteoclastogenesis, bone resorption and the bone metastatic niche, revealing miR-34a as a potential new therapeutic strategy to confer skeletal protection and ameliorate bone metastasis of breast cancer. Citation Format: Jing Y Krzeszinski, Wei Wei, HoangDinh Huynh, Zixue Jin, Xunde Wang, Tsung-Cheng Chang, Xian-jin Xie, Lin He, Lingegowda S Mangala, Gabriel Lopez-Berestein, Anil K Sood, Joshua T Mendell, Yihong Wan. MicroRNA-34a suppresses breast cancer bone metastasis by inhibiting osteoclastogenesis and targeting tgif2 [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 P6-17-01.