Stress-inducible transcription factor CHOP/gadd153 induces apoptosis in mammalian cells via p38 kinase-dependent and -independent mechanisms.
CHOP/gadd153 is a transcription factor induced by cellular stresses such as UV light, genotoxic agents, and protein misfolding in the endoplasmic reticulum. The fact that these stresses induce CHOP expression, and at the same time cause cellular apoptosis, suggests that CHOP may be directly involved in apoptosis. However, evidence has been circumstantial. Here, we show that CHOP can directly induce apoptosis. A GFP-tagged CHOP vector, ectopically overexpressed in several cell types (3T3 fibroblasts, keratinocytes, and HeLa cells), caused apoptosis as defined by morphology, DNA fragmentation, and FACS analysis. Apoptosis was quantified using a rapid fluorescence assay that measures the signal from cells collected in culture supernatants. The apoptosis-modulating effects of p38 kinase, previously shown to phosphorylate CHOP, were also examined. Simultaneous overexpression of CHOP and p38 significantly augmented apoptosis. However, although p38 kinase clearly modulated the activity of full-length CHOP, it was not absolutely required. Deletion mapping experiments showed that the bZIP region of CHOP stimulates apoptosis to nearly the same extent as wild-type CHOP. Thus, while the amino-terminal region of CHOP serves an important modulatory role (i.e., regulation by p38), the underlying apoptosis-inducing activity of CHOP resides within the bZIP region of the molecule.