ABSTRACT
Melanoma is notoriously resistant to chemotherapy, but variable responses to biotherapies, including the IFNs and IL-2, provide intriguing avenues for further study. Systemic IL-2 treatment has provided significant clinical benefit in a minority of patients with metastatic melanoma, leading to long-term survival in a few cases. We hypothesize that one previously unidentified mechanism of effective IL-2 therapy is through direct upregulation of the tumor suppressor IL-24 in melanoma tumor cells resulting in growth suppression. In this study, five melanoma cell lines were treated with high dose recombinant human IL-2. Three (A375, WM1341, WM793) showed statistically significant increases in IL-24 protein; two (WM35, MeWo) remained negative for IL-24 message and protein. This increase was abolished by preincubating with anti-IL-2 antibody or blocking with antibodies against the IL-2 receptor chains. These IL-2 responsive melanoma cell lines expressed IL-2Rß and IL-2Rγ mRNA. The IL-2Rßγ complex was functional, as measured by IL-2-induced signal transducers and activators of transcription activation as well as IL-15 signaling through its shared receptor complex. IL-24 upregulation was observed in response to either IL-2 or IL-15. Cell growth was significantly decreased by treatment of IL-24-positive cells with IL-2 or IL-15, whereas no effect was seen in negative cells. Incubating the IL-24 inducible-cells with anti-IL-24 antibody as well as transfecting with IL-24 small interfering RNA effectively reversed the growth suppression seen with IL-2. Thus, we have shown that one mechanism of clinically effective IL-2 therapy may be the direct action of IL-2 on a biologically distinct subset of melanoma cells leading to upregulation of the tumor suppressor IL-24.
