L-arginine increases UVA cytotoxicity in irradiated human keratinocyte cell line: potential role of nitric oxide.

Human fibroblasts and keratinocytes possess nitric oxide synthases (NOS), which metabolize L-arginine (L-Arg) for producing nitric oxide (NO*). This report delineates the relations between NO* and UVA in the human keratinocyte cell line HaCaT. NOS activity was stimulated by exposure of cells to L-Arg just after irradiation. L-Arg (5 mM) supply led to an increase in UVA (25.3 J/cm(2)) cytotoxicity (% of viability 18 +/- 3%) whereas neither L-Arg itself nor UVA irradiation induced cell death at the doses used in this study. Cells were also treated either with L-thiocitrulline (L-Thio), an irreversible inhibitor of NOS, or with exogenous superoxide dismutase (SOD) and catalase. L-Thio and SOD prevented L-Arg-mediated deleterious effects in irradiated cells, whereas catalase was ineffective. Intracellular antioxidant enzyme activities were also determined. UVA/L-Arg stress altered catalase (66% decrease) and glutathione peroxidase (83% decrease). DNA damage was evaluated using the 'comet assay' and quantified using the 'tail moment'. UVA alone was genotoxic (mean tail moment: 25.43 +/- 1.23, P<0.001 compared control cells). The addition of L-Arg potentiated DNA damage (mean tail moment: 41.05+/-3.9) whereas L-Thio prevented them (mean tail moment 9.86 +/- 0.98). We attempted to assess the effect of poly(ADP-ribose) polymerase (PARP) inhibition on cell death. Using the PARP inhibitor 3-aminobenzamide, we established that PARP determines both cell lysis and DNA damage induced by UVA and/or L-Arg. Our findings demonstrated that L-Arg was able to increase UVA-mediated deleterious effects in keratinocytes (both DNA damage and cytotoxicity) and that the ratio NO*/O2*- plays a key role in these processes.