Inhibition of Tumor Growth and Alteration of Associated Macrophage Cell Type by an HO-1 Inhibitor in Breast Carcinoma-Bearing Mice

Heme oxygenase-1 [HO-1, also called heat shot protein 32 (HSP32)] can specifically metabolize heme to carbon monoxide, biliverdin, and ferrous iron and plays an important role in the processes of anti-inflammation, tissue protection, and antioxidative stress reaction. It has been reported that HO-1 can promote tumorigenesis and metastasis of many tumors. However, the detailed mechanisms of how HO-1 affects tumor progress are not clear. Here, we used ZnPPIX (a specific inhibitor of HO-1) to evaluate its potential effects on mouse breast cancer and tumor-associated macrophages (TAMs). We found out that mouse 4T1 breast cancer growth can be effectively suppressed through inhibition of HO-1 in vitro and in vivo. Moreover, in the 4T1 mouse model, when HO-1 was suppressed in TAMs, alternatively activated macrophages (M2 type) switched to classically activated macrophages (M1 type). In conclusion, 4T1 breast cancer growth was modulated by HO-1 expression. Furthermore, inhibition of HO-1 may induce tumor-associated immune response by activating TAMs' alternative proliferation. These data suggest that HO-1 may be an important target of breast cancer treatment.