Identification of Oct-2 as a mediator of lead neurotoxicity

Lead exposure is highly neurotoxic, particularly to the developing brain. A potential mechanism through which lead could precipitate cerebral damage is by perturbations in the regulation of genes mediated by transcription factors involved in neural development. Macroarray screening analysis was conducted in vivo in an attempt to identify novel transcription factors that are involved in the response to lead exposure in the hippocampus of developing rats. Oct-2 was identified as a novel transcription factor prominently induced by lead. Lead-induced elevation of Oct-2 was confirmed by RT-PCR analysis and the functional properties of Oct-2 were examined using EMSA. We found that Oct-2 mRNA expression and DNA-binding follow a similar pattern after lead exposure. Furthermore, we found that Oct-2 DNA-binding and Oct-2 protein levels mirrored each other suggesting that the elevations in Oct-2 DNA-binding are a product of de novo synthesis. We further examined whether lead-induced perturbations of Oct-2 impact the expression of its target genes such as NOS and TOH. The basal mRNA expression and protein levels of both NOS and TOH increased with age and were primarily down-regulated by exposure to lead. These data suggest that exposure to lead induces alterations in the expression of Oct-2, consequently changing the expression and levels of its target genes. To explore the signaling pathways that mediate lead-induced disturbances in Oct-2, we used mouse cortical neuronal primary cultures and examined the role of signaling pathways such as PKC and MAPK in mediating the effects of lead on Oct-2 DNA-binding. Consistent with in vivo studies, exposure to lead in vitro increased Oct-2 DNA-binding as well as Oct-2 protein levels. Addition of the PKC and MAPK inhibitors reduced the lead-induced Oct-2 DNA-binding, but failed to influence basal Oct-2 DNA-binding. These findings suggest that lead increases Oct-2 DNA-binding via a pathway that involves both PKC and MAPK. This further suggests that lead-induced elevations in Oct-2 activity require that MAPK activate an intermediate transcription factor, which promotes Oct-2 gene expression. These findings indicate that Oct-2 is a molecular target for lead and may play a role in lead-induced disturbances in gene expression during brain development.