Antagonistic roles of four SR proteins in the biosynthesis of alternatively spliced tissue factor transcripts in monocytic cells

Following recruitment to solid tissues, peripheral blood monocytes express a number of proinflammatory molecules including TF, a trigger of coagulation that also promotes cell-cell interactions and tissue remodeling. Monocytes express two forms of TF: flTF, a highly coagulant transmembrane form, and asTF, a highly proangiogenic, soluble TF form. Biosynthesis of the two TF forms occurs via alternative processing of exon 5 during pre-mRNA splicing. Its inclusion results in flTF mRNA and its exclusion, asTF mRNA. We developed a splicing reporter system recently and determined that two spliceosomal constituents, SR proteins ASF/SF2 and SRp55, play a pivotal role in exon 5 inclusion. In this report, we show for the first time that two other SR proteins expressed in human monocytes, SRp40 and SC35, antagonize ASF/SF2 and SRp55 by competing for binding to certain sites in exon 5, thereby promoting TF exon 5 exclusion, an event unique to asTF biosynthesis. We also show that the intron preceding TF exon 5 possesses characteristics rarely found in U2 introns. Our findings indicate that modulation of TF pre-mRNA splicing can be accomplished via modification of SR proteins' activity, facilitating development of novel therapeutic strategies to modulate the "TF profile" of monocytes/macrophages.