Hypoxia-induced deactivation of NGF-mediated ERK1/2 signaling in hippocampal cells: Neuroprotection by acetyl-L-carnitine

Cellular and molecular pathways underlying hypoxic neurotoxicity and cell death are multifaceted and complex. Although many potentially neuroprotective agents have been investigated, the protection conferred is often inadequate, resulting in their insufficient clinical utility. In light of the above, we investigated the therapeutic potential and mechanism of action of acetyl-L-carnitine (ALCAR) in protecting hippocampal neurons from hypoxia-induced neurotoxicity and cellular death. Results showed decreased viability of hippocampal cells when exposed to hypoxia (3% O-2) for 48 hr along with concomitant membrane depolarization, adenosine triphosphate depletion, DNA fragmentation, accentuated free radical production, and lactate dehydrogenase activity. Pretreatment with ALCAR significantly attenuated hypoxia-induced cytotoxicity in a dose-dependent manner and improved cellular glutathione levels and cytochrome c oxidase activity compared with normoxic controls. Supplementation of ALCAR also prevented apoptosis by down-regulating caspase-3 levels, cytochrome c release, and p-Bcl-2 expression. A decrease in nerve growth factor (NGF) was observed in hypoxic stress despite increased phosphorylation of ERK1/2 (extracellular signal-related kinase) and its downstream effector, Elk-1. Supplementation of ALCAR, on the other hand, up-regulated NGF and tyrosine kinase A expression along with concomitant increase in ERK1/2 phosphorylation, thus enhancing cell survival. ALCAR therefore provides neuroprotection by stabilizing mitochondrial membrane, restoring the cholinergic transmission, and more importantly, it stimulates NGF receptors, thus triggering cell survival pathway via ERK phosphorylation. Therefore, ALCAR may be useful as an effective therapeutic agent for hypoxic stress and associated neurodegenerative diseases. (C) 2008 Wiley-Liss, inc.