Abstract
Obesity is a growing health concern worldwide and increases the incidence of multiple types of cancer, including breast cancer. Obese breast cancer patients often develop more aggressive tumors than lean patients and have increased risk for metastasis, tumor recurrence and mortality. Here, we sought to address how obesity alters the biology of breast cancer to promote aggressive tumors. To induce obesity, we fed mice either a control diet (CD) or high fat diet (HFD) for 16 weeks, then transplanted Met-1 tumor cells into mammary fat pads and monitored tumor growth. At end stage, tumors from HFD-fed mice were significantly larger than tumors from CD-fed mice, suggesting obesity promotes tumor growth. To investigate how obesity promotes tumor aggression, we dissociated the tumors from CD- and HFD-fed mice and plated isolated tumor cells in tumorsphere and invasion assays to test for cells with cancer stem-like cell (CSC) properties. Tumor cells from HFD-fed mice demonstrated increased tumorsphere formation and increased capacity for invasion compared to tumor cells from CD-fed mice, suggesting that obesity selects for tumor cells with CSC properties. Next, to address how obesity impacts the tumor microenvironment, we evaluated tumor necrosis and blood vessel formation through CD31 staining. Tumors from HFD-fed mice had significantly less necrosis and greater CD31 staining than those from CD-fed mice, suggesting that obesity promotes tumor angiogenesis. Since obesity promotes chronic, macrophage-driven inflammation within adipose tissue of the mammary gland, we stained tumors for the macrophage marker, F4/80. As with obese mammary glands, tumors from HFD-fed mice had significantly greater macrophage recruitment than tumors from CD-fed mice, together suggesting that obesity alters the tumor microenvironment. To determine how obesity stimulates tumor angiogenesis, we performed an in vitro assay by culturing dissociated tumor cells from HFD or CD-fed mice alone or with macrophages. Conditioned media (CM) isolated from tumor cells from HFD-fed mice cultured with macrophages enhanced the ability of endothelial cells to form networks in vitro. In contrast, CM from HFD tumor cells alone, macrophages alone, or those from CD-fed mice did not promote network formation. Together, these results suggest that cooperation between macrophages and tumor cells from HFD-fed mice promotes angiogenesis. Next, to investigate how macrophages and tumor cells interacting in obesity, we depleted macrophages using anti-F4/80 antibodies in CD-fed and HFD-fed tumor-bearing mice. In HFD-fed mice, macrophage depletion significantly reduced tumor volume and CD31 staining while increasing tumor necrosis compared to controls. Obesity promotes interactions between tumor cells and macrophages to enhance tumor angiogenesis and progression.