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Role of mRNA transcript stability in modulation of expression of the gene encoding thrombin activable fibrinolysis inhibitor

Summary.  Regulation of mRNA stability has emerged as a major control point in eukaryotic gene expression. The abundance of a particular mRNA can be rapidly regulated in response to a stimulus by altering the stability of existing translatable transcripts rather than by altering the rate of transcription initiation. Alternative polyadenylation of transcripts during mRNA processing can be important in determining transcript abundance if the different forms of mRNA possess different stabilities or translatability. The mRNA transcript encoding thrombin activable fibrinolysis inhibitor (TAFI) is an attractive candidate for regulation of mRNA stability because of the relatively long length of its 3′‐untranslated region and because the transcript can be polyadenylated at three different sites. As well, we have previously reported that treatment of HepG2 cells with interleukins (IL) − 1β and − 6 destabilizes the endogenous TAFI mRNA expressed in this cell line. In the current study, we report that the TAFI 3′‐untranslated region contains cis‐acting instability element(s) and that these elements in fact determine the intrinsic stability of the TAFI transcript. Moreover, we found that the three different polyadenylated mRNA forms have different intrinsic stabilities, with the mRNA half‐life increasing from the longest to the shortest transcript. Interestingly, treatment with IL‐1β plus IL‐6 not only resulted in a 2‐fold decrease in stability of the transcript produced using the 3′‐most polyadenylation site but also resulted in profound shifts in the relative abundances of the respective polyadenylated forms through changes in the frequency of utilization of the three polyadenylation sites. As such, in the presence of IL‐1β and IL‐6, the longest transcript is over a thousand times more abundant than the two shorter transcripts whereas in the absence of the stimulus it comprises only 1% of the total TAFI transcripts.