Contribution of persistent C-Jun N-terminal kinase activity to the survival of human vestibular schwannoma cells by suppression of accumulation of mitochondrial superoxides

Abstract

Vestibular schwannomas (VSs) result from inactivating mutations in the merlin tumor suppressor gene. The merlin protein suppresses a variety of progrowth kinase-signaling cascades, including extracellular regulated kinase/mitogen-activated protein kinase (ERK/MAPK), c-Jun N-terminal kinase (JNK), and phosphatidyl-inositol 3-kinase (PI3-K)/Akt. Recent studies indicate that ERKs and Akt are active in human VSs, and here we show that JNKs are also persistently active in human VS cells. With use of cultures of human VSs, we investigated the contribution of each of these signals to the proliferative and survival response of VS cells. Inhibition of ERK or Akt signaling reduced VS cell proliferation but did not increase apoptosis, whereas inhibition of JNK with SP600125, I-JIP, or siRNA knock-down reduced VS cell proliferation and survival by inducing apoptosis. By contrast, JNK activity promotes apoptosis in normal Schwann cells. Inhibition of JNK increased the fluorescence intensity of VS cells loaded with 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate (H(2)DCFDA), a fluorescent probe for reactive oxygen species (ROS). Furthermore, ebselen, a ROS scavenger, rescued VS cells with suppressed JNK from apoptosis, suggesting that JNK activity protects VS cells from apoptosis by limiting accumulation of ROS. VS cultures treated with JNK inhibitors demonstrated significantly higher levels of MitoSOX Red fluorescence, implying that persistent JNK activity specifically suppresses superoxide production in the mitochondria. Overexpression of superoxide dismutase 2 (MnSOD; mitochondrial SOD) prevented apoptosis in VS cells with suppressed JNK signaling. Taken together, these results indicate that persistent JNK activity enhances VS cell survival, at least in part, by suppressing accumulation of mitochondrial superoxides.