The β-
carboline alkaloids are plant substances that exhibit a wide spectrum of neuropharmacological, psychopharma-cological and antitumor effects. In the present study, we found that
harmol, a β-
carboline alkaloid, induced autophagy and suppression of
survivin expression, and subsequently induced apoptotic cell death in U251MG human
glioma cells. Autophagy was induced within 12 h by treatment with
harmol. When treated for over 36 h, however, apoptotic cell death was induced.
Harmol treatment also reduced
survivin protein expression.
Small interfering RNA (
siRNA)-mediated knockdown of
survivin enhanced the
harmol-induced apoptosis. Knockdown of
survivin by
siRNA also induced autophagy. Therefore,
harmol-induced apoptosis is a result of the reduction in
survivin protein expression. Treatment with
3-methyladenine (3-MA) in the presence of
harmol did not affect the expression of
survivin and diminished
harmol-induced cell death. Treatment with
chloroquine in the presence of
harmol did not suppress the reduction of
survivin expression and increased
harmol-induced cell death. From these results,
harmol-induced reduction of
survivin expression was closely related to autophagy. It is assumed that when isolation membrane formation is inhibited by treatment with 3-MA, reduction of
survivin protein expression and apoptotic cell death were not induced. However, when isolation membrane formation is started and an autophagosome is formed,
survivin expression is suppressed and apoptosis is executed.
Harmol treatment reduced phosphorylation of Akt,
mammalian target of rapamycin (mTOR) and its downstream targets p70-ribosomal
protein S6 kinase and 4E-binding
protein 1, resulting in induction of autophagy. Conversely, activation of the Akt/mTOR pathway inhibited
harmol-induced autophagy and cell death. These findings indicate that
harmol-induced autophagy involves the Akt/mTOR pathway. Taken together, autophagy induced by
harmol represented a pro-apoptotic mechanism, and
harmol suppressed the expression of
survivin and subsequently induced apoptosis.