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
alpha-tubulin, and expression of both
alpha-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-kappaB inhibitor SN-50, the strong reduction of
NF-kappaB activation was concomitant with a significant decrease of
transglutaminase 2 expression, suggesting that
NGF-evoked
transglutaminase 2 induction could be related to
NF-kappaB activation. To characterize the possible
transglutaminase 2/
NF-kappaB 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-kappaB activation. However, a
cystamine-mediated significant inhibition of
transglutaminase activity (70%) was accompanied by a drastically reduced
NF-kappaB activation only in cells exposed to
NGF following
retinoic acid treatment. We hypothesize that
NF-kappaB activation was dependent on the transamidating activity related to high levels of TG2, and
NGF enhanced
NF-kappaB 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.