Abstract
Tumor-associated macrophages (TAMs), especially the M2-like phenotype, promote tumor progression, making them candidate targets for anti-tumor therapy. We previously discovered a cyclic sulfur compound, Onionin A (ONA), which suppresses tumor progression by inhibiting the M2-polarization of TAMs. In the present study, we sought to find new candidate compounds possessing a stronger effect compared to ONA by exploring compounds with structures similar to those of ONA among several cyclic sulfur compounds. A total of 81 cyclic sulfur compounds were screened, and their effects on macrophage polarization toward an M2-like phenotype were tested using human monocyte-derived macrophages (HMDMs). The anti-tumor effects of the identified candidate compounds were examined in a tumor-bearing mouse model. Three candidate compounds inhibited both IL-10- and tumor culture supernatant (TCS)-induced M2-polarization of HMDMs. These compounds also suppressed STAT3 activation in HMDMs stimulated by IL-10 and TCS, whereas these compounds had no effect on STAT3 activation in tumor cells. Furthermore, these compounds inhibited tumor cell proliferation under co-culture conditions with HMDMs, indicating that the three candidate compounds suppress tumor proliferation by regulating cell-cell interactions between tumor cells and macrophages. In addition, two of these candidate compounds had inhibitory effects on tumor growth and lung metastasis in the LM8 tumor-bearing mouse model. Our study identified new candidate cyclic sulfur compounds for anti-tumor therapy targeting the M2-polarization of TAMs.