Human breast carcinoma is the most commonly diagnosed cancer in women in the United States with patient mortality primarily resulting from metastasis of breast cancer cells to distant organs. Immune cells, including macrophages, are involved in cancer progression and their presence in primary breast tumors is a poor prognostic factor for patients. Therefore, in the present study we investigated the effects of macrophages in mammary tumor progression to elucidate critical intercellular interactions between these two cell types. Our results indicate that (1) co-implantation of 4T1 mammary tumor cells and RAW macrophages significantly decreased primary tumor mass but tended to increase metastases to the bone marrow. Also, (2) CD11b+ cell infiltration was higher in primary tumors derived from co-implantation of 4T1 and RAW cells. (3) Interestingly, apoptosis but not hypoxia was markedly increased in primary tumors generated from injection of 4T1 cells alone. (4) Consistent with the in vivo observations, in normoxic conditions co-culturing of 4T1 and RAW cells led to a reduction in the growth of 4T1 cells compared to 4T1 cells cultured alone. However, in similar co-cultures under hypoxic conditions, 4T1 cell growth was unaffected by the addition of RAW macrophages. (5) Additionally, regardless of oxygen conditions, in contrast with co-cultures using RAW macrophages, the addition of BMDMs to 4T1 cells did not alter the growth of the 4T1 cells. (6) Differential neuropilin-1 expression was observed between RAW macrophages and BMDMs. Taken together our data highlight the key role of oxygen conditions and macrophage phenotype in breast tumor progression.
