Recently the proapoptotic caspase family has been described to be involved also in proliferative processes. Initially, in this work, the relevance of the FADD/Caspase signaling pathway for cell growth was investigated using murine fibroblasts. Expression of a dominant-negative FADD mutant caused a reduced growth rate of these cells compared to the wildtype. Subsequently the molecular contribution of caspases during mitogen-induced proliferation of human T lymphocytes was characterized. Experiments using human T cells showed an impaired proliferative response of mitogen-activated T cells, when caspase activity was blocked by pharmacological inhibitors. Moreover, caspase inhibition led to a G1-cell cycle arrest of cells and caused inappropriate regulation of cell cycle-associated proteins. Proliferation assays implicated a positive contribution of caspases to the autocrine IL-2/IL-2R system, since exogeneous IL-2 could partially revert the proliferative defects mediated by caspase inhibitors. Along those lines, caspase inhibition caused a strong reduction in the activation-induced IL-2 secretion of primary T cells. Jurkat T cells lacking functional caspase-8 also exhibited a disability to produce physiological amounts of IL-2. This inhibition occurred at the transcriptional level as shown by quantitative RT-PCR. Luciferase-reporter assays using the human T cell line Jurkat revealed an impaired transcriptional activity of NFkB and the composite RE/AP element within the IL-2 promoter. Finally, caspases might be involved in activation-induced cytokine production of human monocytes, since caspase inhibition led to a reduced TNF secretion after stimulation with bacterial antigens
