Activation of DNA biosynthesis in human hepatoblastoma HEPG2 cells by the nitric oxide donor, sodium nitroprusside.

The role of nitric oxide (NO) in carcinogenesis is controversial as it has been shown to both stimulate and inhibit tumour growth. Also, there are contradictory opinions regarding the effects of NO on the proliferation of normal and tumour cells. The aim of our study was to use an in vitro model to determine the influence of exogenous NO donors on DNA biosynthesis by measuring [3H] thymidine incorporation in human hepatoblastoma cells (HepG2). The studies were conducted with the following NO precursors: sodium nitroprusside (SNP), S-nitrosoglutathione, and nitroglycerine (NTG). Out of all three NO donors, SNP increased NO levels and strongly stimulated DNA biosynthesis. A SNP concentration of 150 microM induced optimal NO levels necessary for the activation of DNA biosynthesis. Lower levels of DNA biosynthesis (118% increase over the control) were observed in the presence of NTG, whereas S-nitrosoglutathione had no effect. Antioxidants such as thiol-containing drugs, N-acetylcysteine and tocopherol, proved to be the most efficient co-activators of SNP-induced DNA synthesis. On the other hand, supplementing the SNP-containing medium with compounds that induce oxidative stress and lower the level of -SH groups such as hydrogen peroxide, doxorubicin, and N-ethylmaleimide, led to the inhibition of DNA synthesis. Therefore, our results firmly confirm the hypothesis that biological effects of exogenous NO donors depends on the redox status of the cell.