In Vitro Studies On The Protective Mechanisms Of Silibinin In Ultraviolet B (UVB)-irradiated Epidermoid Carcinoma A431 Cells And Dermal Fibroblast L929 Cells

Ultraviolet B (UVB) from sunlight is a major cause of cutaneous lesion, which accompanies complex alterations in irradiated skin cells, including DNA lesions, oxidative stress, inflammation and caspase activation. The protection against UVB damage requires multiple interruptions such as repair of the DNA lesions, scavenging of the reactive oxygen species (ROS), repression of the inflammation and others. Silibinin, a traditional hepatic protectant, elicits protective effects against UVB-induced cellular damage, but the underlying mechanisms have not been fully elucidated.Our previous in vivo study revealed that the natural flavonoid silibinin had marked anti-inflammatory effect on UVB-irradiated murine skin. UVB-exposure caused reduced autophagy in epidermis while it promoted autophagy in dermis. Nevertheless, silibinin inhibited the inflammatory flux in the skin epidermis as well as dermis through the modulation of autophagy. In order to elucidate the underlying protective mechanisms of silibinin for UVB damage on skin, the separate studies on epidermis and dermis are helpful.In human epidermoid carcinoma A431 cells, the activation of insulin-like growth factor-1 receptor (IGF-1R) played a key role in UVB damage. UVB-induced activation of the IGF-1R signalling pathways contributed to caspase-8-mediated cell apoptosis instead of rescuing the cells. Up-regulated IGF-1R stimulated downstream mitogen-activated protein kinases (MAPKs), such as c-Jun N-terminal kinases (JNK) and extracellular signal-regulated protein kinases 1/2 (ERK1/2), to induce cell apoptosis. PI3K-Akt pathway was also activated by IGF-1R, which resulted in the suppression of protective autophagy and thereby augmented apoptosis. Silibinin down-regulated the ERK/JNK cascades and restored autophagy through the suppression of the IGF-1R signalling pathway, therefore protected A431 cells from UVB-induced epidermal apoptosis.Derived from the normal tissue of the mouse, L929 cells were capable to represent some characteristics of dermal cells. UVB irradiation caused L929 cell apoptosis in a time- and dose-dependent manner. But the mechanisms involved were different from those in A431 cells. Ataxia telangiectasia mutated (ATM) protein and p53 were activated in UVB-irradiated L929 cells to cause apoptosis, accompanying up-regulation of the autophagic flux. The pharmacological inhibition of ATM, p53 and autophagy, or the transfection with autophagy-associated protein-targeted siRNAs showed that UVB-activated ATM-p53 axis and autophagy formed a positive feedback loop, which synergistically promoted the cell apoptosis. Silibinin treatment simultaneously repressed the activation of ATM-p53 and autophagy and thereby protected UVB-irradiated L929 cells from apoptotic death.Considering all the findings on A431 cells and L929 cells, one can conclude that the influence of UVB irradiation on different types of cells is not identical, involving multiple interferences such as the alterations in IGF-1R pathway, ATM, p53 and autophagy. Autophagy affected in these processes shows different effects on cells’survival which antagonises apoptosis in epidermal cells but promotes it in dermal fibroblasts. Silibinin has the potential that balances stresses-induced multiple alterations in cells and thus protects both epidermal and dermal cells against UVB damage.