Differential roles of nitric oxide synthases in regulation of ultraviolet B light-induced apoptosis

Ultraviolet B light (UVB) activates nitric oxide synthase(s) (NOSs) and nitric oxide (NO) production, which plays a role in regulation of apoptosis. However, the role of NO in UVB-induced apoptosis remains controversial. In this study, we analyzed expression and activation of constitutive NOSs (cNOSs) and their roles in UV-induced apoptosis of HaCaT keratinocytes. Our data showed that the expression of neuronal NOS (nNOS) was increased while endothelial NOS (eNOS) was uncoupled in the early phase (0–6 h) post-UVB. The expression of both cNOSs peaked at 12 h post-UVB and NO was transiently elevated with 30 min and then steadily rose from 6 to 18 h post-UVB. The expression of iNOS was detected at 6 h post-UVB and then sturdily increased. Inhibition of cNOSs with l-NAME reduced the inducibility of NO in the early and late phases of irradiation. Along with the eNOS uncoupling, an increased level of peroxynitrite (ONOO−) was detected in the early phase, but not in the late phase post-UVB. Inhibition of cNOSs reduced the production of ONOO− in the early time, but led to an increase of ONOO− in the late time after UVB-irradiation. The results indicate that cNOSs regulate NO/ONOO− imbalance after UVB-irradiation. Our data suggested that the activation of cNOSs in the early phase post-UVB leads to NO/ONOO− imbalance and promotes apoptosis via a caspase 3-independent pathway. The elevation of NO in the late phase of UVB-irradiation is mainly produced by inducible NOS (iNOS). However, cNOSs also contribute to the NO production and to maintain a higher NO/ONOO− ratio, which reduces caspase 3 activity and protects cells from UVB-induced apoptosis.