Abstract 1036: ProNGF/p75NTR signaling promotes survival of merlin-deficient vestibular schwannoma cells by activating NF-kB

Abstract

Abstract Vestibular schwannomas (VSs) represent benign Schwann cell (SC) neoplasms arising from the eighth cranial nerve and result from loss of function of the tumor suppressor gene, merlin. The mechanisms by which loss of merlin function promotes tumorigenesis in SCs is not well-defined. Denervation by axotomy leads to increased merlin inactivation by phosphorylation, increased p75NTR expression, JNK activation, and eventual death of normal SCs. In contrast to SCs, VS cells survive in the absence of axons. We found that p75NTR expression is elevated in sciatic nerves of mice harboring a SC specific dominant negative merlin mutation (P0SchΔ36-121) compared to nerves from wild-type mice suggesting that merlin status regulates p75NTR expression. Consistent with this we also find that p75NTR is over-expressed in primary human VS tissue compared with normal nerves. Treatment of normal rat SCs with proNGF, a high affinity p75NTR ligand, induces apoptosis by activating JNK. Primary cultured human VS cells, by contrast, display persistent JNK activity which promotes cell survival. Treatment of human primary VS cultures with the JNK inhibitors (I-JIP and SP600125) or JNK siRNA knock down results in decreased proliferation and increased apoptosis; treatment with proNGF protects cells from apoptosis due to JNK inhibition suggesting a p75NTR-mediated pro-survival signal in primary cultured VS cells. We explored the possibility that the pro-survival effect of p75NTR signaling results from activation of NF-κB. Indeed, proNGF treatment activated NF-κB in primary cultured VS cells while inhibition of JNK with SP600125 or siRNA-mediated JNK1/2 knockdown reduced NF-κB activity. Significantly, proNGF also activated NF-κB in primary cultured VS cells treated with JNK inhibitors. Thus, persistent JNK activity appears to be required for basal levels of NF-κB activity, but not for proNGF-induced NF-κB activity. To confirm that the increase in NF-κB activity by proNGF contributes to the prosurvival effect, we infected primary human VS cultures with Ad.IκB-SermutS32/36A virus which inhibits NF-κB activation. Compared to control virus, Ad.IκB-SermutS32/36A significantly increased VS cell apoptosis in VS cultures including those treated with proNGF. These results suggest that, in contrast to normal SCs which undergo apoptosis in response to p75NTR and JNK signaling, proNGF/p75NTR signaling provides a prosurvival response by activating NF-κB independent of JNK in merlin deficient VS cells. Such differences may account for the ability of human VS cells to proliferate and survive in the absence of axons. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1036. doi:10.1158/1538-7445.AM2011-1036