Increased Interleukin-10 mRNA Stability in Melanoma Cells Is Associated with Decreased Levels of A + U-Rich Element Binding Factor AUF1

Abstract

Abnormal production of interleukin-10 (IL-10) is observed in some pathologic conditions. For example, compared with normal melanocytes, IL-10 expression is elevated in melanoma cells. IL-10 overexpression could inhibit both immune surveillance and tumor rejection. We investigated a potential posttranscriptional mechanism for IL-10 overexpression in melanoma cells. In normal melanocytes, the half-life of IL-10 mRNA is 7 min, whereas in the melanoma cell line MNT1, the half-life is 75 min. This 10-fold difference could account, at least in part, for IL-10 overexpression in MNT1 cells. Examination of the 3'-untranslated region (3'-UTR) of IL-10 mRNA revealed a suspected A + U-rich element (ARE) that might target the mRNA for rapid degradation. Transfection experiments confirmed that these sequences promote rapid degradation when inserted into a normally stable mRNA, indicating ARE functionality. As AREs act via their interactions with ARE-binding proteins, we examined cytoplasmic proteins from normal melanocytes and MNT1 cells for IL-10 ARE-binding activity. Compared with cytoplasmic extracts of normal melanocytes, cytoplasmic extracts of MNT1 cells possess substantially less ARE-binding activity, consistent with the extended half-life of IL-10 mRNA in MNT1 cells. Finally, we find that the ARE-binding protein AUF1 comprises the major ARE-binding activity in cytoplasmic extracts of normal melanocytes. By contrast, AUF1 is not detectable in cytoplasmic extracts of MNT1 cells but appears restricted to the nuclear fraction. Together, these data suggest a mechanism whereby reduced cytoplasmic levels of AUF1 in MNT1 melanoma cells may lead to IL-10 overexpression, with deleterious consequences for tumor surveillance and rejection.