Background and Aims: The death-associated protein kinase (DAP-kinase) is a cytoskeleton-associated protein which is crucially involved in the induction of early apoptotic pathways. Aberrant hypermethylation of the DAP-kinase promotor plays a major role in tumorigenesis. We aimed to investigate the inactivation of DAP-kinase and its association with apoptotic cell death in 94 colorectal carcinomas. Methods: DAP-kinase promotor hypermethylation and mRNA expression were investigated using methylation-specific PCR and real-time RT-PCR, respectively. The immunohistochemical expression of DAP-kinase, Fas and Fas-ligand (FasL) proteins and TUNEL-positive tumor cells were studied by single, double and triple immunofluorescence using laser-scan confocal microscopy. Histopathological findings were confirmed functionally in a co-culture model of U937 monocytic cells and HCT-116 colorectal tumor cells. Results: We found an aberrant promotor hypermethylation in more than 50% of carcinomas resulting in DAP-kinase inactivation at the mRNA and protein level and decreased apoptosis in tumor cells. DAP-kinase protein was localized in tumor cells and specifically in invading tumor-associated macrophages. High DAP-kinase expression co-localized with FasL overexpression in tumor-associated macrophages. Co-culture experiments revealed that macrophageal DAP-kinase expression and Fas-L secretion were indeed required for apoptosis in human colorectal carcinoma cells. Conclusions: Our study provides the first evidence for a synergistic interaction between macrophages and colorectal tumor cells during apoptosis in vivo and in vitro. Moreover, DAP-kinase expression in tumor-associated macrophages is an important mechanism of FasL-mediated tumor cell apoptosis. Thus, targeting DAP-kinase might represent an interesting new therapeutic option in order to enhance anti-tumor defense by the innate immune system.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]