Post-transcriptional mechanisms play a major role in regulating T cell activation and lymphokine production. Cytokine, proto-oncogene, and transcription factor genes encode mRNAs frequently distinguished by cytoplasmic lability (half-life<30 minutes). Many of these unstable mRNAs contain reiterations of a specific sequence (AUUUA) in their 3'UTR. Reiterations of this AUUUA sequence (ARE) have been shown capable of conferring instability on mRNA as well as modulating its ability to be translated. Based on the studies of IRE-BP/aconitase in cellular iron metabolism, we hypothesize that posttranscriptional regulation of lymphokine gene expression is highly dependent on trans- acting factors that bind to reiterated AUUUA sequences. In this proposal, we plan to identify, functionally characterize, and clone the trans-acting proteins (AU-rich sequence binding proteins; AUBP) that bind to AUUUA multimers and modulate lymphokine mRNA turnover and translation. With identification of four cytoplasmic AUBPs present in T lymphocytes as hnRNP A1, hnRNP C, UP-1, and glyceraldehyde 3- phosphate dehydrogenase, considerable progress has already been made, that now permits study of their posttranslational regulation. Given the importance of post-transcriptional mechanisms in the regulation of lymphokine gene expression, these studies have immediate relevance to our understanding of T cell activation and differentiation. Defining the AUBP(s) relevant to lymphokine mRNA stability and turnover will not only enable insight into the mechanism(s) regulating their expression, but also elucidate the normal physiologic mechanism(s) through which deactivation of T cells occurs. Thus, characterizing AUBP function may be important in diseases in which excessive immunoreactivity (autoimmune disease, allergy) is present. Insights derived from these studies will have widespread application beyond immunobiology. Characterization of AUBP, their regulation, and their function will have particular relevance to understanding the pathways that regulate cell growth and differentiation of all eukaryotic cells. Considerable data exists to indicate that an important component of tumorigenesis is disordered post- transcriptional regulation, resulting in enhanced expression of a proto-oncogene or growth factor. Given our identification of hnRNP A1 as an AUBP, it was very exciting to learn that transformation by Friend murine erythroleukemia virus is frequently associated with retroviral integration and silencing of the hnRNP A1. These data provide correlative evidence that AUBP, particularly hnRNP A1, may be important in the regulation of cell growth and hence a target in neoplastic transformation. Thus, understanding the role of these trans-acting factors in lymphokine gene expression will have direct relevance to immunobiology and growth regulation, as well as the clinical disorders manifest by their dysregulation.
