Elucidating signaling pathways that mediate cell survival or apoptosis will facilitate the development of targeted
therapies in
cancer. In
neuroblastoma tumors,
brain-derived neurotrophic factor (
BDNF) and its
receptor TrkB are associated with poor prognosis. Our previous studies have shown that
BDNF activation of TrkB induces resistance to
chemotherapy via activation of phosphoinositide-3-kinase (PI3K)/Akt pathway. To study targets of PI3K/Akt that mediate protection from
chemotherapy, we focused on
glycogen synthase kinase-3beta (GSK-3beta), which is a known modulator of apoptosis. We used pharmacologic and genetic methods to study the role of
GSK-3beta in the
BDNF/TrkB/PI3K/Akt protection of
neuroblastoma from
chemotherapy.
BDNF activation of TrkB induced the Akt-dependent phosphorylation of
GSK-3beta, resulting in its inactivation. Treatment of
neuroblastoma cells with inhibitors of
GSK-3beta, LiCl,
GSK-3beta inhibitor VII,
kenpaullone, or a GSK-3beta-targeted
small interfering RNA (
siRNA) resulted in a 15% to 40% increase in
neuroblastoma cell survival after cytotoxic treatment. Transfection of
neuroblastoma cells with a constitutively active
GSK-3beta S9A9 caused
a 10% to 15% decrease in cell survival. Using real-time, dynamic measurements of cell survival, we found that 6 to 8 h after
etoposide treatment was the period during which critical events regulating the induction of cell death or
BDNF/TrkB-induced protection occurred. During this period,
etoposide treatment was associated with the dephosphorylation and activation of
GSK-3beta in the mitochondria that was blocked by
BDNF activation of TrkB. These data indicate that the inactivation of
GSK-3beta contributes to the
BDNF/TrkB/PI3K/Akt protection of
neuroblastoma cells from
chemotherapy.