AMPK‐mediated autophagy inhibits apoptosis in cisplatin‐treated tumour cells

Abstract

The role of autophagy in cisplatin anticancer action was investigated using human U251 glioma, rat C6 glioma and mouse L929 fibrosarcoma cell lines. A dose- and time-dependent induction of autophagy was observed in tumour cells following cisplatin treatment, as demonstrated by up-regulation of autophagy-inducing protein beclin-1 and subsequent appearance of acridine orange-stained acidic autophagic vesicles. The presence of autophagosomes in cisplatin-treated cells was also confirmed by electron microscopy. Inhibition of autophagy with lysosomal inhibitors bafilomycin A1 and chloroquine, or a PI3 kinase inhibitor wortmannin, markedly augmented cisplatin-triggered oxidative stress and caspase activation, leading to an increase in DNA fragmentation and apoptotic cell death. The mechanisms underlying the protective effect of autophagy apparently involved the interference with cisplatin-induced modulation of Bcl-2 family proteins, as inhibition of autophagy potentiated cisplatin-mediated up-regulation of proapoptotic Bax and down-regulation of anti-apoptotic Bcl-2. Autophagy induction in cisplatin-treated cells was preceded by activation of adenosine monophosphate-activated protein kinase (AMPK) and concomitant down-regulation of mammalian target of rapamycin (mTOR)-mediated phosphorylation of p70S6 kinase. The ability of cisplatin to trigger autophagy was reduced by small interfering RNA (siRNA)-mediated AMPK silencing, while transfection with mTOR siRNA was sufficient to trigger autophagy in tumour cells. Finally, siRNA-mediated AMPK down-regulation and AMPK inhibitor compound C increased cisplatin-induced tumour cell death, while mTOR siRNA and AMPK activator metformin protected tumour cells from cisplatin. Taken together, these data suggest that cisplatin-triggered activation of AMPK and subsequent suppression of mTOR activity can induce an autophagic response that protects tumour cells from cisplatin-mediated apoptotic death.

Antiproliferative action of metformin in human lung cancer cell lines

“…However, they also found augmented cisplatin sensitivity by metformin in a mouse melanoma cell line. Harhaji-Trajkovic et al (19) reported the antagonistic action of metformin on cisplatin. These findings again suggest that metformin affects different types of cancer, differently.…”

“…Despite these findings, the precise mechanisms of the metformin-induced effects are not fully understood. In particular, controversy remains about whether metformin is apoptotic (6,10) or just cytostatic (5,9) and whether it kills cancer cells synergistically with cytotoxic agents including cisplatin (11,13), paclitaxel (8), and doxorubicin (17), or if it is antagonistic to cisplatin (18,19).…”

Redox implications of AMPK-mediated signal transduction beyond energetic clues

“…cisplatin), and to naturally occurring molecules (e.g. polyphenols), which have been considered to be promising anticancer drugs owing to their ability to induce metabolic oxidative stress (Harhaji-Trajkovic et al, 2009;Filomeni et al, 2010b). In light of these results, it is reasonable to consider that although targeting AMPK remains a feasible approach to selectively kill tumor cells, the possible activation of AMPKmediated pathways mediating tumor resistance, such as autophagy, should be taken into account.…”

MTOR inhibition attenuates DNA damage and apoptosis through autophagy-mediated suppression of CREB1

“…32,33 Recently, it has been reported that induction of autophagy is frequently observed in many human cancer cell lines treated with chemotherapeutics. [34][35][36] However, whether autophagy induced by chemotherapy acts as a protective mechanism that allows treated cancer cells to survive or rather as an alternative cell death-inducing process has been an issue of great controversy. Although MTOR plays an inhibitory role in regulation of autophagy, little is known about the role of autophagy in the chemotherapy of cancers driven by aberrant activation of MTOR signaling.…”