Protective effects of noradrenaline on benzo[a]pyrene-induced oxidative stress responses in brain tumor cell lines.

Benzo[a]pyrene (B[a]P) is an ubiquitous environmental pollutant that is generated during combustion of fossil fuels. We examine the effect of noradrenaline (NA) on B[a]P-induced neurotoxicity in brain tumor cell lines like neuroblastoma (Neuro2a) and glioma (C6). We pre-treated tumor cells with NA for 6 h, followed by addition of B[a]P for additional 24 h. Cell viability was measured using trypan blue dye-exclusion assay and comet assay was performed to measure DNA damage. Cell cycle status was analyzed using flow cytometry and oxidative DNA damage (8-oxodG) production was examined by immunostaining. The intracellular Ca²⁺ concentration was analyzed using Fura-2AM. Our results showed viability of Neuro2a and C6 cells declined (24% and 20%) in B[a]P-treated groups. However, pre-treating with NA increased viability of cells by reducing percentage of cell death in both. Furthermore, B[a]P-induced deregulation of cell cycle (G2/M and S phase cell arrest) was significantly restored by pre-treatment with NA in Neuro2a cells as compared to C6 cells. We further observed increased 8-oxodG production in B[a]P-treated cells; however, NA pre-treatment significantly (p < 0.05) reduced the 8-oxodG production in Neuro2a, while C6 cells were less affected possibly due to better protective machinery. B[a]P-induced intracellular Ca²⁺ influx was significantly reduced in both the cell lines due to co-treatment of NA possibly by reducing Ca²⁺ influx. NA protects brain tumor cells against B[a]P-induced neurotoxicity may be by decreasing percentage of G2 cell arrest, oxidative DNA damage, and reducing intracellular Ca²⁺ influx. These findings suggested that NA may be considered as a natural potential protective agent against B[a]P-induced neurotoxicity. Graphical abstract Graphical abstract showing differential protective mechanism of NA against B[a]P-induced toxicity through antioxidant mechanism maintaining homeostasis for oxidative stress in Neuro2a and C6 cell lines. The schematic graph showed the biological significance of the NA that regulates the induction of metabolic processes of cell cycle after exposure to the environmental pollutants. B[a]P increases the intracellular levels of Ca²⁺, but also induces damage to cellular molecules including DNA causing cell cycle arrest. The B[a]P-induced DNA damage due to base lesions generated in the genome, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) is one of the most abundant because of guanine's lowest redox potential among DNA bases through intracellular calcium homoeostasis.