In a previous study, it was reported that secondary untreated melanoma tumors implanted several weeks after and at distance from primary chloroethylnitrosourea (CENU)-treated tumors underwent differentiation and growth inhibition. To see whether the primary treated tumor released soluble factors that mediated the secondary tumor response, serum transfer experiments were performed in vivo. Administration of serum from CENU-treated tumor-bearing donors arrested tumor proliferation, decreased vessel formation and induced tumor metabolite alterations encompassing glutathione decrease and polyunsaturated fatty acid and phosphoethanolamine increase. These changes mimicked secondary tumor phenotype. To reproduce the model in vitro, cell culture supernatant transfer experiments were performed. CENU-treated cell cultures showed polyploidy and reactive oxygen species (ROS) production. Cell cultures challenged by a conditioned medium of CENU-treated cells underwent growth inhibition, cytoskeleton disorders, cytokinesis retardation, metabolite alterations, glutathione decrease and phosphoethanolamine increase, without ROS elicitation. Proteomics of CENU-treated cell conditioned media revealed altered protein secretion activity by CENU-treated cells. Among de novo secreted proteins, the most expressed were phosphatidylethanolamine-binding protein (PEBP), cardiovascular heat shock protein (cHsp), Rho-associated coiled-coil forming kinase 2 (ROCK) and actin fragments. These proteins testified of cytoskeleton disorders, growth inhibition and metabolite alterations. This article demonstrates the release by CENU-treated tumors of growth inhibitory differentiation-inducing soluble factors. These factors mediate remote bystander effects and attest persistent biological activity of residual tumors after chemotherapy. ' 2006 Wiley-Liss, Inc.Key words: chloroethylnitrosoureas; distant double tumor models; growth inhibition; bystander effects; proteomics There is increasing experimental evidence that therapy-induced tumor cell modification (cell-based gene and ionizing radiation therapies) may promote the immune system or induce the expression of cytokines or other factors that may be inhibitory for local or distant unmodified tumor cells. [1][2][3][4][5][6][7][8] Cell-based gene therapy after cytokine or suicide gene transfer has been investigated for autologous and allogeneic vaccination in cancer therapy. Tumor cells engineered to produce cytokines such as interleukin-12 (IL-12) induced antitumor immunity capable of inhibiting a challenge with unmodified parental tumor cells.1 Recently, in a model of allogeneic tumor cells transduced with IL-12 mixed with autologous lymphoma cells, IL-12 induced microenvironmental changes that, through the induction of an immune response, yielded eradication of distant preestablished lymphoma.2 Cells transfected with a suicide gene such as that of thymidine kinase from herpes simplex virus 3-6 or of cytosine desaminase, 7-8 enzymes that activate a drug or prodrug, inhibited distant tumors and provok...