Lead exposure in pheochromocytoma (PC12) cells alters neural differentiation and Sp1 DNA-binding

Previous studies have revealed that lead modulates the DNA-binding profile of the transcription factor Sp1 both in vivo and in vitro (Der Brain Res 1998;107:291). Sp1 is a zinc finger protein, that is selectively up-regulated in certain developing cell types and plays a regulatory role during development and differentiation (Mol Cell Biol 1991;11:2189). In NGF-stimulated PC12 cells, Sp1 DNA-binding activity was induced within 48 h of exposure of NGF naive cells. Exposure of undifferentiated PC12 cells to lead alone (0.1 μM) also produced a similar increase in Sp1 DNA-binding. Since lead altered the DNA-binding profile of Sp1 in newly differentiating cells, neurite outgrowth was assessed as a morphological marker of differentiation to determine whether or not the effects of lead on differentiation were restricted to the initiation phase (unprimed) or the elaboration phase of this process (NGF-primed). NGF-primed and unprimed PC12 cells were prepared for bioassay following exposure to various concentrations of NGF and/or lead. Neurite outgrowth was measured at 48 and 72 h during early stages of NGF-induced differentiation and at 14 h in NGF primed/ replated cells. In the absence of NGF, exposure to lead alone (0.025, 0.05, 0.1 μM) promoted measurable neurite outgrowth in unprimed PC12 cells at 48 and 72 h. A similar phenomenon was also observed in primed/replated PC12 cells at 14 h. However, this effect was two to five times greater than unprimed control cells. In the presence of NGF, a similar trend was apparent at lower concentrations, although the magnitude and temporal nature was different from lead alone. In most cases, the administration of higher lead concentrations (1 and 10 μM), in both the absence or presence of NGF, was less effective than the lower concentrations in potentiating neurite outgrowth. These results suggest that lead alone at low doses may initiate premature stimulation of morphological differentiation that may be related to lead-induced alterations in Sp1 binding to DNA.