Enhancement of TNF-α expression does not trigger apoptosis upon exposure of glial cells to lead and lipopolysaccharide

Lead (Pb) and lipopolysaccharide (LPS) affect not only neurons but also glial cells, and cause neurological impairment in the brain. We have previously shown an enhancement of tumor necrosis factor-α (TNF-α) in U-373MG glial cells after exposure of Pb and LPS. To determine whether Pb- or LPS-induced TNF-α triggers glial cell death, the apoptotic response was investigated in glial cells both in vitro and in vivo. In vitro, Pb ranging from 0.1 to 10 μM increased the expression of TNF-α in a dose-dependent manner in U-373MG cells. Similarly, LPS (500 ng/ml) stimulated TNF-α expression in the same cells. A combination of LPS and Pb did not cause higher mRNA TNF-α expression than LPS or Pb alone. However, exposure to Pb and LPS did not affect apoptosis in U-373MG cells detected by flow cytometric analysis with merocyanine 540 staining and propidium idodine staining. In B6 mice, glial cells in the brain were immunostained positive for TNF-α after an intraperitoncal administration of Pb (12.5 mg/kg) or LPS (10 mg/kg). Consistent with the findings in the cell culture system, few apoptotic cells were detected in glial and neurons by terminal deoxynucleotidyl transferase (TdT) -mediated dUTP nick end-labeling (TUNEL) in the brain of B6 mice. We conclude that induction of TNF-α alone by Pb and LPS does not cause cell apoptosis in cells of glial origin.