Enhanced expression of C/EBP homologous protein (CHOP) precedes degeneration of fibrocytes in the lateral wall after acute cochlear mitochondrial dysfunction induced by 3-nitropropionic acid.

We previously reported that treatment of the rat cochlea with a mitochondrial toxin, 3-nitropropionic acid (3-NP), causes temporary to permanent hearing loss depending on the amount of the drug. Furthermore, apoptosis of cochlear lateral wall fibrocytes, which are important for maintaining the endolymph, is a predominant pathological feature in this animal model. 3-NP is known to induce oxidative stress as well as neuronal apoptosis. C/EBP homologous protein gene (chop) is one of the marker genes induced during endoplasmic reticulum (ER) stress, and is also considered to be involved in apoptosis. To elucidate the molecular mechanism of cochlear fibrocyte apoptosis induced by 3-NP, we studied spatiotemporal expression of C/EBP homologous protein (CHOP) and other signaling molecules related to ER stress as well as the appearance of apoptotic cells in the cochlear lateral wall after 3-NP treatment. Quantitative real-time PCR revealed that chop and activating transcription factor 4 gene (atf-4) showed marked increase within 6h, whereas expression of other ER stress-responsive genes such as grp78 and grp94 did not change. Immunohistochemistry showed that 3-NP treatment caused up-regulation of CHOP, especially in type II and type IV fibrocytes, followed by the appearance of terminal deoxynucleotidyl transferase mediated dUTP nick end-labeling (TUNEL)-positive apoptotic cells in the same confined area. Thus, apoptosis of lateral wall fibrocytes induced by 3-NP is likely to be mediated by induction of CHOP. These results contribute clarification of pathological mechanism of cochlear fibrocytes and may lead to development of novel therapeutic strategy for hearing loss.