Antioxidants
Given the critical role of PDH in cerebral energy metabolism and the notion that its reduced activity contributes to ischemic brain injury, interventions that prevent PDH inhibition or that compensate for its impairment have been explored as neuroprotective strategies. Based on data implicating oxidative stress as a cause of PDH inactivation, it is reasonable that antioxidants should provide a beneficial effect. Alpha-LA has been identified as a potent metabolic antioxidant that may serve as an ideal treatment for ischemic injury involving free radical processes.
[35],
[36],
[37],
[38] The influence of R-(+)-alpha-LA, the naturally occurring enantiomer of LA, on pyruvate metabolism has been documented in primary cultured hepatocytes isolated from 24 h fasted rats. The results showed enhanced pyruvate oxidation and decreased gluconeogenesis. Of note, these changes were associated with significant increases in the activation state of PDH, which may reflect a return of normal metabolic function conferred by antioxidant therapy.
[39]
While exogenous antioxidants may improve mitochondrial resistance to oxidative stress, another promising approach utilizes pharmacologic stimulation of endogenous gene expression to protect against metabolic dysfunction.
[40] The transcriptional activating factor Nrf2 regulates expression of many genes encoding mitochondrial antioxidant enzymes, as well as targets of oxidative stress.
[41],
[42] Of note, Nrf2 has been found to exert control over key mediators of cellular energy metabolism, including pyruvate dehydrogenase lipoamide β and PDK. Stimulation of the Nrf2 pathway by sulforaphane, a molecule with known antioxidative effects that is obtained from cruciferous vegetables, has proven effective in reducing brain infarct volume and increasing expression of the stress-response protein, heme oxygenase-1, in a rat model of focal ischemic stroke.
[43] These results, coupled with additional findings of reduced flux through the PDH pathway in Nrf2 knockdown cells, suggest that PDH or its regulators may be of those proteins under Nrf2 influence.
[44] The critical role that PDH plays in energy metabolism and its vulnerability to oxidative stress may explain the protective effect that genetic manipulation by Nrf2 pathway activation has upon cerebral ischemic injury.
Combination therapy with ethanol and normobaric oxygen
Dose-dependent neuroprotection by ethanol (EtOH) has been observed in rat models of middle cerebral artery occlusion.
[45],
[46] EtOH has been found to raise expression levels of PDH and PDP and decrease those of PDK. Other signs of improved oxidative metabolism, including reduced ROS levels, lower ADP/ATP ratios and fewer neurological deficits, accompanied these changes. When these same parameters were assessed in rats treated with EtOH + normobaric oxygen (NBO), it was found that combination therapy conferred a greater therapeutic effect than each agent alone.
[17] EtOH's ability to reduce energy demands and to inhibit glucose metabolism more likely accounts for the limited ROS generation detected in EtOH treatment groups.
[47],
[48] Removal of PDH from ROS-mediated inhibition promotes oxidative metabolism and is, therefore, one of the mechanisms that have been proposed in EtOH-induced neuroprotection. Conversely, NBO has been utilized to counteract ischemia-induced hypoxic conditions. Although NBO has been reported to confer neuroprotective effects during ischemic events when administered in clinical settings, its limited time window for efficacy and minor therapeutic effect limit its potential for clinical application. However, when administered concomitantly, NBO enhances the effects of EtOH, evidenced by a greater attenuation of impaired PDH activity and protein expression, which may reflect further facilitation of aerobic metabolism.
[49],
[50],
[51],
[52],
[53],
[54] While further studies are needed to characterize PDH modulation by EtOH and NBO at the molecular level, their role in stabilizing cerebral energy metabolism makes these agents promising neuroprotectants in ischemic stroke injury.
Dichloroacetate
Dichloroacetate (DCA), a pharmacologic agent that activates PDH by inhibiting PDK, has also revealed significant neuroprotective potential.
[55],
[56],
[57] Administration of DCA has been shown to enhance regional lactate removal and limit the lactic acidosis associated with brain hypoperfusion and metabolic dysregulation.
[58],
[59],
[60],
[61] In addition, a proton magnetic resonance spectroscopy study revealed that DCA delivered in high dose or within 2 days of ischemic stroke produced similar reductions in lactate levels.
[62] Treatment with DCA appears to be most effective during reperfusion by enhancing the postischemic reactivation of PDH.
[63],
[64],
[65] This effect on PDH activity has been demonstrated in rat and gerbil models of cerebral ischemia, which exhibited a reduction in lactate levels in addition to a restoration of ATP and phosphocreatine levels later on in reperfusion, but displayed no demonstrable effect during the ischemic phase.
[57],
[66] These findings of reduced lactate production and increased oxidative energy metabolism by DCA administration further implicate PDH impairment in the delayed cerebral energy failure that occurs after ischemic insult. By enhancing activity of the rate-limiting enzyme that links pyruvate production with pyruvate oxidation, DCA promotes oxidative metabolic recovery. Despite early recognition of DCA's selectivity and ease of delivery, clinical studies have raised concern regarding its potential toxicity.
[67],
[68],
[69] This includes a randomized, controlled clinical trial evaluating the efficacy of 25 mg/kg/day DCA in patients with mitochondrial encephalopathy with lactic acidosis and stroke-like episodes that resulted in early termination due to associated peripheral nerve toxicity.
[70]
Acetyl-L-carnitine
Acetyl-L-carnitine (ALCAR) is an endogenous metabolic intermediate which has been shown to be neuroprotective in cerebral ischemia models when administered at supraphysiologic doses.
[71],
[72],
[73],
[74] Human and animal studies suggest that ALCAR's neuroprotective effect is derived from its restoration of oxidative energy metabolism. Delivery of ALCAR acetyl groups to the tricarboxylic acid cycle is understood to improve aerobic energy metabolism by providing a fuel supply alternative to pyruvate, allowing for circumvention of the PDH pathway.
[75] This hypothesis of metabolically mediated neuroprotection by ALCAR is supported by rat models of global cerebral ischemia. These models exhibit reductions in lactate and inorganic phosphate levels, along with elevations in levels of ATP and creatine-phosphate,
[71] consistent with the metabolism of ALCAR acetyl units, and indicative of augmented oxidative cerebral energy production, and diminished anaerobic glycolysis and lactic acidosis. Another proposed mechanism of ALCAR-mediated neuroprotection is by relief of oxidative tissue injury.
[76] This effect has been demonstrated in a canine cardiac arrest model by the substance's ability to limit protein carbonyl formation, a marker of oxidative stress, in brain tissue during reperfusion.
[77] PDH's critical function in oxidative energy metabolism and its known susceptibility to inactivation by ROS support the role ALCAR may play in attenuating the mitochondrial dysfunction observed during ischemic stroke injury by either preventing PDH inhibition or compensating for its impairment.
