Characterization of RNA binding proteins associated with CD40 ligand (CD154) mRNA turnover in human T lymphocytes

CD153 (CD40 ligand (CD40L)) has been demonstrated to play an essential role in the development of humoral and cellular immunity through its interactio n with CD40, While earlier studies have examined the regulation of CD154 ex pression by transcriptional and posttranslational pathways, scant data exis t on its regulation at a posttranscriptional level. In this report we demon strate that CD154 mRNA is rapidly turned over in primary culture of activat ed human T lymphocytes, Moreover, we demonstrate that CD154 mRNA is unstabl e, but can be stabilized by treatment with either phorbol esters or calcium ionophores, To address this lability of CD154 mRNA, we examined the abilit y of cytoplasmic proteins to bind to its 3' untranslated region (3'UTR), Tw o major proteins (p25 and p50) capable of binding the 3'UTR of CD154 were i dentified. The p25 binding activity was associated with polysomes and appea red to correlate with CD154 mRNA instability, Intriguingly, these proteins did not appear to bind to the AU-rich elements present in the 3'UTR of CD15 4, Rather, their binding was localized to unique sites between nt 471-811 o f the 3'UTR, which lack any classical AU-rich elements. These data suggest that these proteins interact with distinct cis-acting elements that are imp ortant in the posttranscriptional regulation of CD154 expression. As such, identifying these proteins will help us understand the signals that are nec essary for CD154 expression by activated T cells.