Valproic acid‐induced hepatotoxicity in alpers syndrome is associated with mitochondrial permeability transition pore opening‐dependent apoptotic sensitivity in an induced pluripotent stem cell model

Abstract

Valproic acid (VPA) is widely used to treat epilepsy, migraine, chronic headache, bipolar disorder, and as adjuvant chemotherapy, but potentially causes idiosyncratic liver injury. Alpers-Huttenlocher syndrome (AHS), a neurometabolic disorder caused by mutations in mitochondrial DNA polymerase gamma (POLG), is associated with an increased risk of developing fatal VPA hepatotoxicity. However, the mechanistic link of this clinical mystery remains unknown. Here, fibroblasts from 2 AHS patients were reprogrammed to induced pluripotent stem cells (iPSCs) and then differentiated to hepatocyte-like cells (AHS iPSCs-Hep). Both AHS iPSCs-Hep are more sensitive to VPA-induced mitochondrial-dependent apoptosis than controls, showing more activated caspase-9 and cytochrome c release. Strikingly, levels of both soluble and oligomeric optic atrophy 1, which together keep cristae junctions tight, are reduced in AHS iPSCs-Hep. Furthermore, POLG mutation cells show reduced POLG expression, mitochondrial DNA (mtDNA) amount, mitochondrial adenosine triphosphate production, as well as abnormal mitochondrial ultrastructure after differentiation to hepatocyte-like cells. Superoxide flashes, spontaneous bursts of superoxide generation, caused by opening of the mitochondrial permeability transition pore (mPTP), occur more frequently in AHS iPSCs-Hep. Moreover, the mPTP inhibitor, cyclosporine A, rescues VPA-induced apoptotic sensitivity in AHS iPSCs-Hep. This result suggests that targeting mPTP opening could be an effective method to prevent hepatotoxicity by VPA in AHS patients. In addition, carnitine or N-acetylcysteine, which has been used in the treatment of VPAinduced hepatotoxicity, is able to rescue VPA-induced apoptotic sensitivity in AHS iPSCs-Hep. Conclusion: AHS iPSCs-Hep are more sensitive to the VPA-induced mitochondrial-dependent apoptotic pathway, and this effect is mediated by mPTP opening. Toxicity models in genetic diseases using iPSCs enable the evaluation of drugs for therapeutic targets. (HEPATOLOGY 2015;61:1730-1739

Mitochondrial dysfunction as a mechanism of drug-induced hepatotoxicity: current understanding and future perspectives

“…Investigation of the effect of VPA on mitochondrial activity in the human hepatoma cell line, HepG2, showed that VPA treatment reduced oxygen consumption and ATP levels [167]. VPA treatment also reduced expression of DNA polymerase gamma, which plays a major role in mitochondrial DNA (mtDNA) replication and repair [168], as well as mtDNA levels in HepG2 cells [168]. VPA has been shown to significantly diminish mitochondrial membrane potential in HepG2 cells [167].…”

“…Treatment with VPA induced hepatic microvesicular steatosis accompanied by decreased respiratory control ratio, increased mitochondrial swelling and altered calcium flux in hepatic mitochondria [169]. VPA also increased the level of ROS and inhibited the antioxidant function of superoxide dismutase (SOD) in a murine hepatic microsomal system and in HepG2 cells [167, 168, 170]. In hepatocyte like cells differentiated from fibroblasts obtained from patients with an intractable epilepsy condition, VPA induced a mitochondrial dependent apoptosis [168].…”

“…VPA also increased the level of ROS and inhibited the antioxidant function of superoxide dismutase (SOD) in a murine hepatic microsomal system and in HepG2 cells [167, 168, 170]. In hepatocyte like cells differentiated from fibroblasts obtained from patients with an intractable epilepsy condition, VPA induced a mitochondrial dependent apoptosis [168]. This finding was also supported by an in vitro study in HepG2 cells [167], where VPA led to morphological abnormalities in mitochondria, and in addition, the levels of optic atrophy 1, which keep cristae junctions tight, were reduced.…”

“…This is likely due to VPA mediated opening of the MPT pore. This intermittent ROS discharge leads to mitochondrial cytochrome c release and caspase 9 activation, which triggers mitochondria mediated apoptosis [168, 171]. Thus, VPA-induced hepatotoxicity is characterized by excessive mitochondrial dependent apoptosis[168].…”

“…This intermittent ROS discharge leads to mitochondrial cytochrome c release and caspase 9 activation, which triggers mitochondria mediated apoptosis [168, 171]. Thus, VPA-induced hepatotoxicity is characterized by excessive mitochondrial dependent apoptosis[168]. During VPA-induced toxicity, the mitochondrial oxidant stress depletes GSH and can also induce lipid peroxidation [172].…”