Potent homocysteine-induced ERK phosphorylation in cultured neurons depends on self-sensitization via system Xc−

Homocysteine is increased during pathological conditions, endangering vascular and cognitive functions, and elevated homocysteine during pregnancy may be correlated with an increased incidence of schizophrenia in the offspring. This study showed that millimolar homocysteine concentrations in saline medium cause phosphorylation of extracellular-signal regulated kinases 1 and 2 (ERK1/2) in cerebellar granule neurons, inhibitable by metabotropic but not ionotropic glutamate receptor antagonists. These findings are analogous to observations by Zieminska et al. (2003), that similar concentrations cause neuronal death. However, these concentrations are much higher than those occurring clinically during hyperhomocysteinemia. It is therefore important that a ∼ 10-fold increase in potency occurred in the presence of the glutamate precursor glutamine, when ERK1/2 phosphorylation became inhibitable by NMDA or non-NMDA antagonists and dependent upon epidermal growth factor (EGF) receptor transactivation. However, glutamate release to the medium was reduced, suggesting that reversal of the cystine/glutamate antiporter, system Xc− could be involved in potentiation of the response by causing a localized release of initially accumulated homocysteine. In agreement with this hypothesis further enhancement of ERK1/2 phosphorylation occurred in the additional presence of cystine. Pharmacological inhibition of system Xc− prevented the effect of micromolar homocysteine concentrations, and U0126-mediated inhibition of ERK1/2 phosphorylation enhanced homocysteine-induced death. In conclusion, homocysteine interacts with system Xc− like quisqualate (Venkatraman et al. 1994), by “self-sensitization” with initial accumulation and subsequent release in exchange with cystine and/or glutamate, establishing high local homocysteine concentrations, which activate adjacent ionotropic glutamate receptors and cause neurotoxicity.