C/EBP beta phosphorylation by RSK creates a functional XEXD caspase inhibitory box critical for cell survival

Upon activation by liver injury, hepatic stellate cells produce excessive f ibrous tissue leading to cirrhosis. The hepatotoxin CCl4 induced activation of FISK, phosphorylation of C/EBP beta on Thr(217), and proliferation of s tellate cells in normal mice, but caused apoptosis of these cells in C/EBP beta (-/-) or C/EBP beta -Ala(217) (a dominant-negative nonphosphorylatable mutant) transgenic mice. Both C/EBP beta -PThr(217) and the phosphorylatio n mimic C/EBP beta -Glu(217), but not C/EBP beta -Ala(217), were associated with procaspases 1 and 8 in vivo and in vitro and inhibited their activati on. Our data suggest that C/EBP beta phosphorylation on Thr(217) creates a functional XEXD caspase substrate/inhibitor box (K-Phospho-(TVD)-V-217) tha t is mimicked by C/EBP beta -Glu(217) ((KEVD)-V-217). C/EBP beta (-/-) and C/EBP beta -Ala(217) stellate cells were rescued from apoptosis by the cell permeant (KEVD)-V-217 tetrapeptide or C/EBP beta -Glu(217).