Raffinose increases autophagy and reduces cell death in UVB-irradiated keratinocytes

Autophagy is an important process for maintaining intracellular homeostasis. Our previous study demonstrated that autophagy was down-regulated in ultraviolet B (UVB)-irradiated keratinocytes. Raffinose is a natural oligosaccharide that serves as a novel activator of autophagy and as a balancing agent to regulate the diversity of environmental stress. However, whether raffinose balances ultraviolet stress through the autophagy activation pathway has yet to be established. In this study, we found that raffinose treatment inhibited the LDH release and trypan blue staining in UVB-challenged human keratinocytes cell line HaCaT but did not affect the cleavage of apoptotic markers Caspase-3 and PARP, as well as translocation into nucleus of other cell death markers Endonuclease G and AIF. Moreover, we confirmed that raffinose treatment enhanced autophagy flux in an MTOR-independent manner in HaCaT cells. Importantly, decrease of LC3-II turnover in UVB-irradiated keratinocytes could be rescued by raffinose treatment, indicating that raffinose treatment increased autophagy in UVB-irradiated HaCaT cells. Furthermore, the effect on cell death by raffinose was inhibited when autophagy was suppressed with either a small interfering RNA targeting ATG5 (siATG5) or autophagic inhibitor wortmannin. In conclusion, we demonstrated that raffinose increases MTOR-independent autophagy and reduces cell death in UVB-irradiated keratinocytes. Our study indicated that the natural agent raffinose presents the potential value in opposing photodamage.