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

Upon activation by liver injury, hepatic stellate cells produce excessive fibrous tissue leading to cirrhosis. The hepatotoxin CCl4 induced activation of FISK, phosphorylation of C/EBP beta on Thr(217), and proliferation of stellate 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 phosphorylation 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 activation. Our data suggest that C/EBP beta phosphorylation on Thr(217) creates a functional XEXD caspase substrate/inhibitor box (K-Phospho-(TVD)-V-217) that 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).