771. Human mda-7/Interleukin 24 (IL-24) Protein Kills Breast Cancer Cells Via the IL-20 Receptor and is Antagonized by IL-10

The melanoma differentiation-associated gene-7 (mda-7/IL-24) is a unique member of the interleukin 10 (IL-10) family of cytokines, with ubiquitous tumor cell pro-apoptotic activity. Recent data have shown that IL-24 is secreted as a glycosylated protein and functions as a pro-Th1 cytokine and as a potent anti-angiogenic molecule. In this study, we analyzed the activity of Ad-mda7 and its protein product, secreted human IL-24, against human breast cancer cells. We show that Ad-mda7 transduction of breast cancer cells results in G2/M phase cell-cycle arrest and killing, which correlates with secretion of IL-24 protein. Neutralizing antibody against IL-24 significantly inhibited Ad-mda7 killing. IL-24 and IL-10 both engage their cognate receptors on breast cancer cells resulting in phosphorylation and activation of STAT3, however, IL-10 receptor binding failed to induce cell killing, indicating that tumor cell killing by IL-24 is independent of STAT3 phosphorylation. Exogenous L-24 killed breast cancer cells by induction of apoptosis and this effect was abolished by addition of anti-IL-24 antibody or anti-IL-20R1, indicating that bystander cell killing is mediated via IL-24 binding to the IL-20R1/IL-20R2 heterodimeric receptor complex. Furthermore, IL-10 treatment inhibits killing mediated by IL-24. In summary, we have defined a tumor-selective cytotoxic bystander role for secreted IL-24 protein and identified a novel receptor- mediated death pathway in breast cancer cells, wherein the related cytokines IL-24 and IL-10 exhibit antagonistic activity.