Abstract 3197: RNase-L modulates transcriptional and post-transcriptional responses to mitogenic stimuli through serum response factor and tristetraprolin.
Authors: Brennan-Laun Sarah E. , Li X-L , Ezelle Heather J. , Wilson Gerald M. , Hassel Bret A. 
Published: August 2014

Abstract
RNase-L is an endoribonuclease that functions in a range of important physiologic activities including the innate immune response and control of cell proliferation and is dysregulated in pathologic conditions including cancer. RNase-L is tightly regulated and requires the binding of 2’,5’-linked oligoadenylates (2-5A: px5′A(2′p5′A)n; x=1-2;n&gt;2) to induce oligomerization and enzymatic activity. Activated RNase-L cleaves single-stranded regions of cellular and pathogen RNAs, and distinct profiles of RNA are targeted by RNase-L to mediate its biologic functions in different cellular contexts. Therefore an understanding of the mechanism(s) by which specific RNAs are targeted for RNase-L cleavage is essential to modulate its substrate selection and biologic activity for therapeutic applications. Towards this goal, we identified the RNA binding protein tristetraprolin (TTP) as a novel RNase-L interacting partner. TTP binds to A-U-rich elements (AREs) in the 3’-untranslated region (3’UTR) of target mRNAs, including its own mRNA, to promote their degradation. We hypothesize that TTP functions to direct RNase-L cleavage of a subset of ARE mRNAs. Consistent with this prediction, RNase-L interacted with TTP mRNA, and decreased steady-state levels and increased half-life of TTP mRNA in RNase-L-/- fibroblasts. In addition to this direct effect on basal TTP mRNA stability, RNase-L inhibited the transcriptional induction of TTP primary transcripts following serum stimulation. In this context, we determined that RNase-L targeted the mRNA encoding serum response factor (SRF), a transcription factor that induces the expression of proliferation and tumorigenesis-associated genes. The TTP promoter contains SRF binding sites suggesting that the RNase-L-mediated downregulation of SRF indirectly reduced TTP transcription; experimental validation of this regulation is in progress. Together these data support a model in which following mitogen stimulation, RNase-L constrains the transcriptional induction of proliferation-associated genes via its regulation of SRF and interacts with TTP to post-transcriptionally downregulate an overlapping set of mRNAs. The RNase-L-dependent regulation of TTP, which itself functions to limit the mitogenic response, suggests that RNase-L contributes to proliferative homeostasis through the regulation of both positive and negative effectors. These findings have implications for the therapeutic targeting of RNase-L-mediated cleavage in regenerative repair and tumorigenesis.
Citation Format: Sarah E. Brennan-Laun, X-L Li, Heather J. Ezelle, Gerald M. Wilson, Bret A. Hassel. RNase-L modulates transcriptional and post-transcriptional responses to mitogenic stimuli through serum response factor and tristetraprolin. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3197. doi:10.1158/1538-7445.AM2013-3197