Objectives: Tumor necrosis factor ≤ (TNF) is an inflammatory cytokine, which is released upon different stimuli, including irradiation. Recently it has been shown, that the Death-associated protein kinase (DAPK) mediates TNF-induced apoptosis in colon cancer cells [1]. Here, we aimed to identify new DAPK binding partners and to characterize the functional role of novel protein interaction complexes during TNF-induced apoptosis in colon cancer cells. Methods/Results: HCT116 colorectal cancer cells were cultured for 6 to 48 hours in either normal or TNF-conditioned medium. For phosphopeptide microarray (PPM) whole cell lysates were incubated on peptide platforms with radioactive-labeled P33. Apoptosis was detected by Annexin V staining and caspase 3 cleavage in Western Blotting. PPM analysis revealed heat shock transcription factor 1 (HSF1) as a new potential substrate of DAPK phosphorylation under TNF-stimulation. DAPK Co-IP, co-immunofluorescence (Co-IF), and mass spectrometry showed that DAPK interacts with HSF1 after TNF-treatment. Phosphorylation on serine residues of HSF1 is necessary for HSF1 nuclear translocation and the initiation of transcription of its target genes. Among them, Ser 230 is part of a consensus phosphorylation motif for DAPK. Maximal level of pHSF1Ser230 was observed at the time point where massive apoptosis was induced. Co-IF microscopy confirmed enrichment of pHSF1Ser230 in the nucleus already after 24 hours of TNF-stimulation. EMSA and Chromatin-IP revealed that pHSF1Ser230 binds to the heat shock response element in the DAPK promoter region and enhances its transcriptional activity. Exogenous over-expression of HSF1 protein led to a significant increase in mRNA DAPK levels and consequently to an enforcement of apoptosis. As expected, DAPK knockdown cells did not show any variation in pHSF1Ser230 level, supporting again that DAPK is an important mediator of the TNF-driven signaling pathway in colorectal cancer cells. The significance of the DAPK/pHSF1Ser230 interaction for response prediction was evaluated by immunohistochemical staining on tissue microarrays of colorectal cancer before and after radiotherapy. Conclusion: Our data show a novel functional interaction between HSF1 and DAPK under TNF-stress and highlight a positive feedback mechanism in DAPK-regulation. These results help to understand cell death pathways in response to radiotherapy. [1].Bajbouj K.et al., Am J Pathol. (2009)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4673A. doi:1538-7445.AM2012-4673A