Transglutaminase 2 and NF-κB interplay during NGF-induced differentiation of neuroblastoma cells

NGF treatment of neuroblastoma cells stimulates outgrowth of neurite processes associated with the expression of TrkA receptor and several differentiation markers. In this study, a 6 DIV exposure to NGF (50 ng/ml) increased immunostaining for α-tubulin, and expression of both α-tubulin and protein kinase C in the neuroblastoma cell line Neuro2a. Further, up-regulation of transglutaminase 1 and transglutaminase 2 expression, and reduction of transglutaminase 3 levels, were also observed in NGF-treated cells in comparison to untreated cells. Moreover, when Neuro2a cells were treated with the specific NF-κB inhibitor SN-50, the strong reduction of NF-κB activation was concomitant with a significant decrease of transglutaminase 2 expression, suggesting that NGF-evoked transglutaminase 2 induction could be related to NF-κB activation. To characterize the possible transglutaminase 2/NF-κB interplay, NGF treatment was carried out in Neuro2a cells which already over-expressed transglutaminase 2 after retinoic acid treatment. An additive effect of NGF was observed on the retinoic acid-induced transglutaminase 2 expression and enzyme activity, and NF-κB activation. However, a cystamine-mediated significant inhibition of transglutaminase activity (70%) was accompanied by a drastically reduced NF-κB activation only in cells exposed to NGF following retinoic acid treatment. We hypothesize that NF-κB activation was dependent on the transamidating activity related to high levels of TG2, and NGF enhanced NF-κB activation by a different, synergistically acting, pathway. These data suggest that the combined use of NGF and retinoic acid, or mimicking drugs, may provide the basics for the development of novel strategies in the therapeutic management of neuroblastomas.