Key Points This study has identified a novel mechanism by which TF expression is posttranscriptionally regulated in macrophages. The mechanism involves the control of mRNA stability by a cooperation between PARP-14 and TTP.

IgG may accelerate atherosclerosis via ligation of proinflammatory Fcγ receptors; however, IgM is unable to ligate FcγR and is often considered vasculoprotective. IgM aggravates ischemia-reperfusion injury, and solid-phase deposits of pure IgM, as seen with IgM-secreting neoplasms, are well known clinically to provoke vascular inflammation. We therefore examined the molecular mechanisms by which immunoglobulins can aggravate vascular inflammation, such as in atherosclerosis. We compared the ability of fluid- and solid-phase immunoglobulins to activate macrophages. Solid-phase immunoglobulins initiated prothrombotic and proinflammatory functions in human macrophages, including NF-κB p65 activation, H(2)O(2) secretion, macrophage-induced apoptosis, and tissue factor expression. Responses to solid-phase IgG (but not to IgM) were blocked by neutralizing antibodies to CD16 (FcγRIII), consistent with its known role. Macrophages from mice deficient in macrophage scavenger receptor A (SR-A; CD204) had absent IgM binding and no activation by solid-phase IgM. RNA interference-mediated knockdown of SR-A in human macrophages suppressed activation by solid-phase IgM. IgM binding to SR-A was demonstrated by both co-immunoprecipitation studies and the binding of fluorescently labeled IgM to SR-A-transfected cells. Immunoglobulins on solid-phase particles around macrophages were found in human plaques, increased in ruptured plaques compared with stable ones. These observations indicate that solid-phase IgM and IgG can activate macrophages and destabilize vulnerable plaques. Solid-phase IgM activates macrophages via a novel SR-A pathway.

No abstract

Background Although monocyte-derived tissue factor (TF) plays critical roles in atherothrombosis, little is known about its posttranscriptional regulation. Tristetraprolin (TTP) binds the 3′UTR of target mRNAs and promotes their degradation, with its function being negatively regulated by p38 MAPK. Whether TTP posttranscriptionally regulates TF is unknown. Methods We used human monocytes and bone marrow derived macrophages from TTP+/+ and TTP-/-mice. Procoagulant activity was determined using a clot turbimetric assay. mRNA decay was determined following transcriptional arrest using actinomycin D. TTP knockdown was achieved using siRNA transfection. RNA and protein interaction was determined using ribonucleoprotein (RNP) immunoprecipitation and RNA biotin pulldown assays. Results p38 inhibition with SB203580 and SB202190 (1µMM) reduced procoagulant activity, TF mRNA and protein expression in human macrophages (p<0.05). p38 inhibition reduced TF mRNA stability in both human and murine macrophages (p<0.05). TTP knockdown increased TF expression in human macrophages (p<0.05). Both TF mRNA and protein expression were significantly increased in TTP-/-versus TTP-/-macrophages (p<0.05). Moreover TF mRNA decay was reduced in TTP-/-macrophages and p38 inhibition had no effect on this (p<0.01). RNP immunoprecipitation demonstrated TTP and TF mRNA interaction. Furthermore, a more specific interaction with TTP and TF 3′UTR was confirmed using RNA biotin pulldown techniques. Conclusions These data provide evidence, for the first time, that p38 and TTP post-transcriptionally regulate TF expression in macrophages. A better understanding of the post-transcriptional regulation of TF expression will provide novel insights into the interface between inflammation and thrombosis in vascular biology, and holds therapeutic potential.