Role of miRNA-155 on tumorigenesis in a transgenic mouse model of breast cancer

Deregulated and aberrant expression of microRNAs (miRNAs) have been linked to a variety of disorders such as cancers, autoimmune diseases, and chronic inflammation. Many miRNAs are important regulators of cellular processes involving development, differentiation, and signaling. We investigated whether miR155, a multifunctional miRNA induced by inflammatory cytokines and involved in hematopoiesis, inflammatory disorders, and macrophage activation, has any promise in reducing tumor burden in a transgenic breast cancer model. To study the effects of miR155 in breast cancer, our lab developed the first MMTV-PyMT mice deficient for miR155 on a C57BL/6 background. Four-week-old female PyMT (n=12) and PyMT/miR155-/- (n=11) mice were euthanized at 19 weeks of age. miR155 deficient mice exhibited reduced tumor number and volume palpations (p<0.05) at approximately 16 weeks of age. Reduced tumor burden displayed in miR155 deficient mice was also confirmed by histopathological analysis. The majority (60%) of miR155 deficient mice exhibited a lower degree of dysplasia characterized as adenoma/mammary intraepithelial neoplasia (MIN), whereas all PyMT mice progressed into the early and late invasive carcinoma grade. Upon euthanizing, mammary glands were excised from tumors for RT-PCR, and tumors were counted, measured, and weighed. PyMT mice deficient in miR155 exhibited reduced tumor volume (p<0.05), number (p<0.05), and weight (p<0.05). To investigate the anti-tumoral effects associated with miR155 deficiency, mammary glands excised from tumors were analyzed to examine inflammatory mediators and macrophage markers within the mammary tumor environment. We demonstrate that miR155 deficiency upregulates suppressor of cytokine signaling 1 (SOCS1) resulting in reduced overall cytokines, and reduced macrophages as indicated by CD68 and Integrin Subunit Alpha X (IGTAX). Mammary glands of miR155-/- mice had reduced pro-inflammatory M1 macrophage cytokines as indicated by reduced interleukin-1-beta (IL-1b), IL-6, and tumor necrosis factor-alpha (TNF-a). Mammary glands excised from tumors of mice deficient in miR155 exhibited even greater inhibition of M2 macrophages, important in reducing inflammation and contributing to tumor growth and immunosuppressive function, as indicated by reduced IL-4, IL-10, IL-13, mannose receptor 1 (Mrc1), and TNF-beta. Given that macrophages are known to promote tumorigenesis, we suggest that the anti-tumor effects of miR155 inhibition are due to its upregulation of SOCS1 and resulting decrease in pro-tumor M2 macrophages. © FASEB.