Background The transcription factor signal transducer and activator of transcription 3 (STAT3) is implicated in cancer drug resistance, metastasis, and immunosuppression and has been identified as a promising therapeutic target for new anticancer drugs. Myeloid‐derived suppressor cells (MDSCs) play a major role in the suppression of antitumor immunity and STAT3 is involved in the accumulation, generation, and function of MDSCs. Thus, targeting STAT3 holds the potential of reversing immunosuppression in cancer. This study aims to investigate the effect of the small molecule STAT3 inhibitor galiellalactone on prostate cancer cell– induced generation of MDSCs from monocytes and the effect on immunosuppressive factors and inflammatory cytokines. Methods Primary human monocytes were cocultured with prostate cancer cells (DU145, PC3, and LNCaP‐IL6) or with conditioned medium (CM) from prostate cancer cells in the presence or absence of the STAT3 inhibitor galiellalactone. Monocytes were analyzed by flow cytometry for an MDSC‐like phenotype (CD14+ HLA‐DR−/lo). The secretion and gene expression of immunosuppressive factors and inflammatory cytokines from prostate cancer cells and monocytes were investigated. Results Galiellalactone blocked the prostate cancer cell–induced generation of MDSC‐like monocytes with an immunosuppressive phenotype ex vivo. Monocytes cultured with CM from prostate cancer cells showed increased expression of phosphorylated STAT3. Prostate cancer cells increased the expression of interleukin1β (IL1β), IL10, and IL6 in monocytes which was inhibited by galiellalactone. In addition, galiellalactone decreased indoleamine 2,3‐dioxygenase gene expression in monocytes. Galiellalactone reduced the levels of IL8 and granulocyte macrophage‐colony stimulating factor in prostate cancer cells per se. Conclusion The STAT3 inhibitor galiellalactone may prevent the prostate cancer cell–induced generation of MDSCs and reverse the immunosuppressive mechanisms caused by the interplay between prostate cancer cells and MDSCs. This is a potential new immunotherapeutic approach for the treatment of prostate cancer.

The molecular mechanisms of prostate inflammation are unclear. We hypothesized that heme oxygenase 1 (HMOX1; HO-1), an enzyme responsible for degradation of heme to carbon monoxide, bilirubin, and iron, is an important regulator of inflammation and epithelial responses in the prostate. Injection of non-uropathogenic Escherichia coli (MG1655 strain) or phosphate-buffered saline into the urethra of mice led to increased numbers of CD45 þ leukocytes and mitotic markers (phosphorylated histone H 3 and phosphorylated ERK1/2) in the prostate glands. Leukocyte infiltration was elevated in the prostates harvested from mice lacking HO-1 in myeloid compartment. Conversely, exogenous carbon monoxide (250 ppm) increased IL-1b levels and suppressed cell proliferation in the prostates. Carbon monoxide did not affect the number of infiltrating CD45 þ cells in the prostates of E. colie or phosphate-buffered salineetreated mice. Interestingly, immunomodulatory effects of HO-1 and/or carbon monoxide correlated with early induction of the long-chain acyl-CoA synthetase 1 (ACSL1). ACSL1 levels were elevated in response to E. coli treatment, and macrophage-expressed ACSL1 was in part required for controlling of IL-1b expression and prostate cancer cell colony growth in soft agar. These results suggest that HO-1 and/or carbon monoxide might play a distinctive role in modulating prostate inflammation, cell proliferation, and IL-1b levels in part via an ACSL1-mediated pathway.