Protective effects of epigallocatechin-3-gallate against ultraviolet radiation caused keratinocyte injury: targeting the NLRP3 inflammasome priming and activation

Abstract

Abstract Inflammatory response occurring after ultraviolet (UV) exposure is regarded as one of the major causes of skin photodamage. As a crucial mediator of inflammatory cascades, the NOD-like receptor protein 3 (NLRP3) inflammasome has been reported to be responsible for UV radiation (UVR) evoked inflammation in keratinocytes, emerging as a promising target for therapeutic intervention. Epigallocatechin-3-gallate (EGCG), the most abundant polyphenol in green tea, has been found to afford protection against UVR-caused keratinocyte injury, but mechanism clarifying its precise target still awaits further exploration. The aim of this study was to examine whether EGCG had the ability to inhibit UVR-triggered NLRP3 inflammasome activation in keratinocytes which accounted for the protection against cell damage. The results showed EGCG prevented HaCaT cells from UVR-induced DNA insults by counteracting oxidative stress and inflammatory cytokines production. NLRP3 inflammasome activation in response to UVR was markedly suppressed by EGCG. Moreover, EGCG inhibited the induction of new mtDNA synthesis by downregulating the expression of IRF-1, CMPK2 and phosphorylated STAT1. Meanwhile, EGCG reduced the excessive production of mitochondrial reactive oxygen species (mtROS) and ox-mtDNA in UV-exposed cells so that both deubiquitination of NLRP3 and the binding of ox-mtDNA to NLRP3 were suppressed, resulting in impaired NLRP3 inflammasome priming and activation. Our results have identified the potent activity of EGCG to ameliorate NLRP3 inflammasome-driven photodamage in keratinocytes caused by UVR. Apart from the mtROS-scavenging activity which enabled EGCG to antagonize mtROS-triggered NLRP3 inflammasome activation, a novel antioxidation-independent mechanism of EGCG in NLRP3 inhibition was revealed.