Chronic inflammation is known to play a role in cancer initiation, promotion,and metastasis. However, the mechanism by which inflammation promotes metastasis is still unclear. We evaluated if chronic inflammation induced by autoimmune arthritis may contribute to increased breast cancer-associated metastasis. We report a three-fold increase in lung metastasis and a significant increase in the incidence of bone metastasis in the pro-arthritic mice compared to control mice. The metastatic breast tumors in turn augment the severity of arthritis resulting in a vicious cycle that increases both bone destruction and metastasis. Enhanced neutrophilic and granulocytic infiltration in lungs and bone of the pro-arthritic mice and subsequent increase in circulating levels of proinflammatory cytokines, such as IL-17, IL-6, VEGF, and TNF-α were the underlying factors contributing to the increased metastasis. The data clearly has important clinical implications for patients diagnosed with metastatic breast cancer
