Abstract 919: Death associated protein kinase 1 phosphoylates Pin1 and inhibits its cellular function

Protein phosphorylation on certain serine or threonine residues preceding proline is a central signaling mechanism in cell growth and proliferation. Pin1 is a peptidyl-prolyl cis-trans isomerase that isomerizes only phosphorylated Ser/Thr-Pro peptide bonds. The WW domain of Pin1 binds only to specific pSer/Thr-Pro-motifs, targeting the Pin1 catalytic domain close to its substrates, where the PPIase domain isomerizes specific pSer/Thr-Pro motifs. These Pin1-catalyzed conformational changes after phosphorylation can have profound effects on many key proteins in diverse cellular processes. Moreover, Pin1 is highly overexpressed in many human cancers and is important for the activation of multiple oncogenic pathways, indicating that Pin1 plays a key role in the pathogenesis of cancer. However, since prolyl isomerases are often believed to be constitutively active, nothing is known whether and how Pin1 catalytic activity is regulated. Death associated protein kinase 1 (DAPK1) is a calcium/calmodulin-regulated Ser/Thr kinase and function as a positive mediator of apoptosis induced by various stimuli. DAPK1 is a well-known tumor suppressor based on the following lines of evidence. First, loss of DAPK1 expression is frequently observed in tumor tissues of different origins. Second, loss of DAPK1 expression has been shown to correlate strongly with recurrence and metastasis incidence. Finally, DAPK1 has been shown to be capable of suppressing c-Myc- and E2F-induced oncogenic transformation. Together, these findings indicate that DAPK1 suppresses tumorigenesis. However, only a handful of direct DAPK1 substrates have been identified, and the molecular mechanisms of its tumor suppression are poorly understood. Here we report that Pin1 catalytic activity is regulated in vitro and in vivo. Pin1 is phosphorylated in the catalytic domain and this phosphorylation fully inactivates Pin1 phosphorylation-specific PPIase activity likely by blocking the access of phosphorylated substrate to the catalytic active site. Furthermore, we identify DAPK1 as a kinase responsible for such inhibitory phosphorylation. In contrast to all known Pin1-binding proteins, DAPK1 specifically interacts only to the PPIase domain, but not the WW domain of Pin1 and more importantly, DAPK1 phosphorylates Pin1 in vitro and in vivo. Moreover, inducible DAPK1 expression fully inhibits the ability of Pin1 to activate transcription factors, stabilize proteins and induce centrosome amplification, chromosome instability and cell transformation. Finally, Pin1 phosphorylation levels correlated positively with DAPK1 levels, but negatively with centrosome amplification in breast cancer tissues. Thus, phosphorylation of Pin1 by DAPK1 is a novel mechanism inhibiting Pin1 catalytic activity and function, documenting an essential role of the catalytic activity for Pin1 function in vivo. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 919. doi:10.1158/1538-7445.AM2011-919