ERK and Akt have been shown to regulate cell sensitivity to death-inducing stress by phosphorylating GSK-3β, a major modulator of the threshold for mitochondrial permeability transition. Here we examined intra-mitochondrial localization of the pro-survival kinases and their regulation by phosphatases. Stepwise trypsin digestion of mitochondria isolated from HEK293 or H9c2 cells was performed, and immunoblotting revealed that GSK-3β and ERK localized dominantly in the outer membrane (OM), while Akt resided at comparable levels in OM, the inner membrane (IM) and the matrix. Treatment with IGF-1 increased the protein level of Akt in the matrix, while ERK and GSK-3β protein levels were increased in OM. Simultaneously, IGF-1 treatment elevated the level of Thr202/Tyr204-phospho-ERK in IM and matrix and levels of Ser473-phospho-Akt and Ser9-phospho-GSK-3β in OM, IM and matrix. Exposing cells to reactive oxygen species (ROS) by using antimycin A increased the levels of DUSP5 and PHLPP-1 mainly in OM and induced dephosphorylation of Akt, ERK and GSK-3β. The mitochondrial localization of DUSP5 was confirmed by experiments with mitochondria purified by Percoll gradient centrifugation and by transfection of cells with GFP-tagged DUSP5. Knockdown of either DUSP5 or PHLPP-1 increased the levels of both Thr202/Tyr204-phospho-ERK and Ser473-phospho-Akt in mitochondria. Cell death induced by antimycin A was suppressed by siRNA-mediated knockdown of DUSP5. The results suggest that Akt and ERK in mitochondria show distinct intra-mitochondrial localization and crosstalk in GSK-3β regulation and that recruitment of DUSP5 as well as PHLPP-1 to mitochondria contributes to ROS-induced termination of the protective signaling.

Excessive reactive oxygen species (ROS) induce apoptosis and are associated with various diseases and with aging. SIRT1 (sirtuin-1), an NAD+-dependent protein deacetylase, decreases ROS levels and participates in cell survival under oxidative stress conditions. SIRT1 modulates the transcription factors p53, a tumor suppressor and inducer of apoptosis, and the forkhead O (FOXO) family, both of which play roles for cell survival and cell death. In this study, we aimed to know which is working greatly among p53 and FOXOs transcription factors in SIRT1's cell protective functions under oxidative stress conditions. The antimycin A-induced increase in ROS levels and apoptosis was enhanced by SIRT1 inhibitors nicotinamide and splitomicin, whereas it was suppressed by a SIRT1 activator, resveratrol, and a SIRT1 cofactor, NAD+. SIRT1-siRNA abolished the effects of splitomicin and resveratrol. p53-knockdown experiment in C2C12 cells and experiment using p53-deficient HCT116 cells showed that splitomicin and resveratrol modulated apoptosis by p53-dependent and p53-independent pathways. In p53-independent cell protective pathway, we found that FOXO1, FOXO3a, and FOXO4 were involved in SOD2's upregulation by resveratrol. The knockdown of these three FOXOs by siRNAs completely abolished the SOD2 induction, ROS reduction, and anti-apoptotic function of resveratrol. Our results indicate that FOXO1, FOXO3a and FOXO4, are indispensable for SIRT1-dependent cell survival against oxidative stress, although deacetylation of p53 has also some role for cell protective function of SIRT1.

Loss of mitochondrial membrane potential (ΔΨm) is known to be closely linked to cell death by various insults. However, whether acceleration of the ΔΨm recovery process prevents cell necrosis remains unclear. Here we examined the hypothesis that facilitated recovery of ΔΨm contributes to cytoprotection afforded by activation of the mitochondrial ATP-sensitive K+ (mKATP) channel or inactivation of glycogen synthase kinase-3β (GSK-3β). ΔΨm of H9c2 cells was determined by tetramethylrhodamine ethyl ester (TMRE) before or after 1-h exposure to antimycin A (AA), an inducer of reactive oxygen species (ROS) production at complex III. Opening of the mitochondrial permeability transition pore (mPTP) was determined by mitochondrial loading of calcein. AA reduced ΔΨm to 15 ± 1% of the baseline and induced calcein leak from mitochondria. ΔΨm was recovered to 51 ± 3% of the baseline and calcein-loadable mitochondria was 6 ± 1% of the control at 1 h after washout of AA. mKATP channel openers improved the ΔΨm recovery and mitochondrial calcein to 73 ± 2% and 30 ± 7%, respectively, without change in ΔΨm during AA treatment. Activation of the mKATP channel induced inhibitory phosphorylation of GSK-3β and suppressed ROS production, LDH release and apoptosis after AA washout. Knockdown of GSK-3β and pharmacological inhibition of GSK-3β mimicked the effects of mKATP channel activation. ROS scavengers administered at the time of AA removal also improved recovery of ΔΨm. These results indicate that inactivation of GSK-3β directly or indirectly by mKATP channel activation facilitates recovery of ΔΨm by suppressing ROS production and mPTP opening, leading to cytoprotection from oxidant stress-induced cell death.