Tristetraprolin (TTP) is the only trans-acting factor shown to be capable of regulating AU-rich element-dependent mRNA turnover at the level of the intact animal; however, the mechanism by which TTP mediated RNA instability is unknown. Using an established model system, we performed structure/function analysis with TTP as well as examined the current hypothesis that TTP function is regulated by p38-MAPKAP kinase 2 (MK2) activation. Deletion of either the N- or C-terminal domains inhibited TTP function. Extensive mutagenesis, up to 16%, of serines and threonines, some of which were predicted to mediate proteasomal targeting, did not alter human TTP function. Mutation of the conserved MK2 phosphorylation sites enhanced human TTP function in both resting and p38-stress-activated protein kinase-MK2-activated cells. However, p38-stress-activated protein kinase-MK2 activation did not alter the activity of either wild-type or mutant TTP. TTP localized to the stress granules, with arsenite treatment reducing this localization. In contrast, arsenite treatment enhanced stress granule localization of the MK2 mutant, consistent with the involvement of additional pathways regulating this event. Finally, we determined that, in response to LPS stimulation, human TTP moves onto the polysomes, and this movement occurs in the absence of 14-3-3. Taken together, these data indicate that, although p38 activation alters TTP entry into the stress granule, it does not alter TTP function. Moreover, the interaction of TTP with 14-3-3, which may limit entry into the stress granule, is not involved in the downstream message stabilization events.

The macrophage is critical to the innate immune response and contributes to human diseases, including inflammatory arthritis and plaque formation in atherosclerosis. Vascular endothelial growth factor (VEGF) is an angiogenic cytokine that is produced by macrophages. To study the regulation of VEGF production in macrophages we show that stimulation of monocyte‐macrophage‐like RAW‐264.7 cells by lipopolysaccharide (LPS) increases expression of VEGF mRNA and protein. Three alternative splicing VEGF mRNA isoforms are produced, and the stability of VEGF mRNA increases following cellular activation. To study post‐transcriptional regulation of the VEGF gene the 3′‐untranslated region (3′ UTR) was introduced into the 3′ UTR of the luciferase gene in a reporter construct. In both RAW‐264.7 cells and thioglycollate‐elicited macrophages, the 3′ UTR sequence dramatically reduces reporter expression. Treatment with activators of macrophages, including LPS, lipoteichoic acid, and VEGF protein, stimulates expression of 3′ UTR reporters. Finally, mapping studies of the 3′ UTR of VEGF mRNA show that deletion of the heterogeneous nuclear ribonucleoprotein l binding site affects basal reporter expression in RAW‐264.7 cells, but does not affect reporter activation with LPS. Together these results demonstrate that a post‐transcriptional mechanism contributes to VEGF gene expression in activated macrophage cells.

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