Abstract 1009: Critical role of Pin1 for the regulation of TRF1 stability and ubiquitination

Pin1 is a peptidyl-prolyl cis-trans isomerase (PPIase) that isomerizes only phosphorylated Ser/Thr-Pro peptide bonds. These Pin1-catalyzed conformational changes after phosphorylation can have profound effects on many key proteins in diverse cellular processes. Importantly, Pin1 is tightly regulated normally by multiple mechanisms in response to upstream signals and it often acts on multiple targets to help drive certain cellular processes in one direction under a given condition synergistically. Moreover, Pin1 is highly overexpressed in many human cancers including breast cancer and is important for the activation of multiple oncogenic pathways, indicating that Pin1 plays a key role in the pathogenesis of cancer. TRF1 is a major telomeric DNA-binding protein that negatively controls telomere length. Moreover, TRF1 levels increase at entry into mitosis, and then decreased as cells exit from mitosis, and that TRF1 is important for mitotic regulation. Furthermore, TRF1 is tightly regulated by the Fbx4-mediated ubiquitination pathway and its stability can have a dramatic impact on telomere length and cell growth. In addition, TRF1 is often reduced and correlated with poor clinical outcome in many tumors. However, it is not known whether the Fbx4-mediated proteolytic pathway is regulated. Here, we report that TRF1 stability is regulated by Pin1-mediated prolyl isomerization. Pin1 interacts with the conserved Thr149-Pro motif in TRF1 in a phosphorylation-dependent manner. Inhibition of endogenous Pin1 function via multiple approaches renders TRF1 fully resistant to protein ubiquitination and degradation by Fbx4. We have also examined stability and ubiquitination of the TRF1 point mutant (TRF1 T149A) that fails to bind Pin1. This TRF1 mutant is completely stable not only in Pin1+/+ cells, but also in Pin1-/- cells. Moreover, no ubiquitinated TRF1 mutant has been detected in the presence or absence of Pin1, indicating that the Pin1-binding site in TRF1 is essential for Pin1 to promote ubiquitin-mediated degradation of TRF1. In addition, amounts of TRF1 protein are increased in tissues examined from Pin1-deficient mice, in comparison with those from wild-type mice, indicating that TRF1 level is upregulated in Pin1 knockout mice. Furthermore, TRF1 expression is decreased and correlated with Pin1 overexpression in human breast cancer tissues, suggesting that a close relationship between Pin1 and TRF1 levels under both physiological and pathological conditions. Thus, Pin1 is a novel regulator of TRF1 and its overexpression might contribute to the downregulation of TRF1 in tumors such as breast cancer. Given that the Pin1 targeting pThr149-Pro motif is located in the middle of the TRF1 homology domain that also interacts with Fbx4, Pin1-catalyzed conformational changes might regulate the interaction between TRF1 and Fbx4 to promote Fbx4-mediated degradation of TRF1. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1009.