PURPOSE: Myoblasts are capable of forming muscle fibers after transplantation and are therefore envisioned as a treatment for urinary incontinence after radical prostatectomy. However, the safety of this treatment and the interaction of myoblasts with remaining neighboring cancerare unknown. We investigated the interactions between myoblasts and prostate carcinoma cellsin vitro and in vivo. MATERIALS AND METHODS: Myoblasts isolated fromrectus abdominis were used in a series of co-culture experiments with prostate cancer cells and subcutaneously co-injected in vivo. Cell proliferation, cell cycle arrest and apoptosis of cancers in co-culture with myoblasts were assessed. Tumor volume and metastasis formation were evaluated in a mouse model. Tissue specific markers were assessed by immunohistochemistry, FACS analyses, Western blot and RT-qPCR. RESULTS: In this study we have demonstrated that myoblasts, in proximity to tumor, provide paracrine TNFα to their microenvironment, decreasing tumor growth of all prostate cancer cell lines examined. Co-culture experiments showed induction of cell cycle arrest, tumor death by apoptosis and increased differentiation of myoblasts. This effect was largely blocked by TNFα inhibition. The same outcome was demonstrated in a mouse model, where co-injected human myoblasts also inhibited tumor growth and metastasis formation of all prostate cancer cell lines evaluated. CONCLUSIONS: Myoblasts restrict prostate cancer growth and limit metastasis formation by paracrine TNFα secretion in vitro and in vivo.