Defective TNF-alpha-induced apoptosis in STAT1-null cells due to low constitutive levels of caspases.

Signal transducers and activators of transcription (STATs) enhance transcription of specific genes in response to cytokines and growth factors. STAT1 is also required for efficient constitutive expression of the caspases Ice, Cpp32, and Ich-1 in human fibroblasts. As a consequence, STAT1-null cells are resistant to apoptosis by tumor necrosis factor alpha (TNF-alpha). Reintroduction of STAT1alpha restored both TNF-alpha-induced apoptosis and the expression of Ice, Cpp32, and Ich-1. Variant STAT1 proteins carrying point mutations that inactivate domains required for STAT dimer formation nevertheless restored protease expression and sensitivity to apoptosis, indicating that the functions of STAT1 required for these activities are different from those that mediate induced gene expression.