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Toll-like receptor 3 (TLR3), a member of pattern recognition receptors, is reported to initiate skin inflammation by recognizing double-strand RNA (dsRNA) released from UVB-irradiated cells. Recently, we have discovered the NF-κB pathway activated by TLR3 is involved in apoptosis of UVB-Poly I:C-treated HaCaT cells. The real culprit for apoptosis has not been precisely identified since the system of NF-κB pathway is complex. In this study, we silenced main transcriptional factors in NF-κB family, RelA, RelB and c-Rel, but to our surprise the results show that none of them participate in apoptosis induction in UVB-Poly I:C-treated HaCaT cells. Therefore, we moved to investigate the apoptosis-associated molecules in the upstream of NF-κB pathway. We firstly checked the expression of IκBα, an NF-κB inhibitor. UVB (4.8 mJ/cm 2 ) and Poly I:C (0.3 μg/mL) co-treatment decreased IκBα expression level in a time-dependent manner. Silencing IκBα with siRNA further enhanced UVB-Poly I:C-induced cell death. We then investigated IκB kinase (IKK) complex that contributes to the degradation of IκBα. IKK is composed of IKKα, IKKβ and NEMO. Treatment with IKK-16, an IKKα/β inhibitor, significantly diminished UVB-Poly I:C-induced IκBα degradation and thus apoptosis. Silencing either IKKα or NEMO but not IKKβ with corresponding siRNA inhibited apoptosis. Tumor repressor p73, a homologue of p53, is reported to mediate IKKα-induced apoptosis in DNA damage response. Silencing p73 reduced cell apoptosis in UVB-Poly I:C-treated HaCaT cells. In summary, UVB and Poly I:C co-treatment activates IKKα and NEMO, which diminishes anti-apoptotic IκBα, resulting in enhancement of apoptosis through p73. The findings partially clarify the possible molecular mechanism of pro-apoptotic NF-κB pathway activated by TLR3 in the fate of UVB-irradiated epidermis., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Ultraviolet B (UVB) irradiation causes skin inflammation and apoptosis. Mitochondria are highly dynamic and undergo constant fusion and fission that are essential for maintaining physiological functions of cells. Although dysfunction of mitochondria has been implicated in skin damages, little is known about the roles of mitochondrial dynamics in these processes. UVB irradiation increases abnormal mitochondrial content but decreases mitochondrial volume in immortalized human keratinocyte HaCaT cells. UVB irradiation resulted in marked upregulation of mitochondrial fission protein dynamin-related protein 1 (DRP1) and downregulation of mitochondrial outer membrane fusion proteins 1 and 2 (MFN1 and MFN2) in HaCaT cells. Mitochondrial dynamics was discovered to be crucial for NLRP3 inflammasome and cGAS-STING pathway activation, as well as the induction of apoptosis. Inhibition of mitochondrial fission by treatments with a DRP1 inhibitor, mdivi-1, or with DRP1-targeted siRNA, efficiently prevented UVB-induced NLRP3/cGAS-STING mediated pro-inflammatory pathways or apoptosis in the HaCaT cells, whereas inhibition of mitochondrial fusion with MFN1and 2 siRNA increased these pro-inflammatory pathways or apoptosis. The enhanced mitochondrial fission and reduced fusion caused the up-regulation of reactive oxygen species (ROS). Application of an antioxidant, N -acetyl- l -cysteine (NAC), which scavenges excessive ROS, attenuated inflammatory responses through suppressing NLRP3 inflammasome and cGAS-STING pathway activation, and rescued cells from apoptosis caused by UVB-irradiation. Together, our findings revealed the regulation of NLRP3/cGAS-STING inflammatory pathways and apoptosis by mitochondrial fission/fusion dynamics in UVB-irradiated HaCaT cells, providing a new strategy for the therapy of UVB skin injury. [Display omitted] • UVB perturbs mitochondrial fission/fusion dynamics in HaCaT cells. • Disturbed mitochondrial dynamics control the amount of mitochondria. • Disturbed mitochondrial dynamics regulate NLRP3/cGAS-STING mediated pro-inflammatory pathways and apoptosis. • ROS are responsible for pro-inflammatory pathways and apoptosis initiated by the dysregulated mitochondrial dynamics. [ABSTRACT FROM AUTHOR]

Ultraviolet B (UVB) irradiation causes skin damages. In this study, we focus on the involvement of mitochondrial disorders in UVB injury. Surprisingly, UVB irradiation increases the amounts of mitochondria in human immortalized keratinocytes HaCaT. However, further analysis shows that ATP levels decreased by UVB treatment in accordance with the collapse of mitochondrial membrane potential (MMP), suggesting an accumulation of dysfunctional mitochondria in UVB-irradiated HaCaT cells. Mitophagy, mainly mediated by PINK1 and parkin, is critical for the elimination of damaged mitochondria. Western blot results show that the levels of both PINK1 and parkin are decreased in UVB-irradiated cells, indicating the impairment of mitophagy. Silencing the expression of PINK1 or parkin by transfection of siRNA shows essentially the same damage to the cells as UVB irradiation does, including increased mitochondrial amount, decreased MMP and ATP production, and enhanced apoptosis, evidencing that repression of PINK1/parkin-mediated mitophagy plays a primary cause of UVB-caused cells damages. We previously found that HaCaT cells exposed to UVB showed activation of the cGAS-STING pathway and apoptosis. Here, silencing PINK1 or parkin also increases the protein levels of cGAS and STING, facilitates nuclear accumulation of NF-κB, and promotes the transcription of IFNβ, suggesting for the activation of STING pathway. Mitophagy impairment either by UVB-irradiation or by PINK1/parkin silencing initiates caspase-3-mediated apoptosis, as shown by the activation of caspase-3 and cleavage of PARP, as well as the increase of Hoechst-positive stained cells and Annexin V-positive cells. Further studies find that Bax-mediated permeabilization of mitochondrial membrane is critical for cell apoptosis, as well as the cytosolic leakage of mtDNA in UVB-treated cells, which results in cGAS-STING activation, and these processes are negatively-regulated by PINK1/parkin-mediated mitophagy. This study reveals the involvement of dysfunctional mitochondria due to impaired mitophagy in the damaging effect of UVB irradiation on HaCaT cells. Restoring the mitophagy has the potential to be developed as a new strategy to protect skin from UVB damages. [Display omitted] • UVB decreases mitophagy and accumulates dysfunctional mitochondria in HaCaT cells. • UVB reduces PINK1/parkin expression that is involved in mitophagy of damaged mitochondria. • Mitophagy arrest caused by UVB leads to the activation of cGAS-STING/apoptosis pathway. • Mitophagy arrest caused by UVB increases mitochondrial membrane permeability. • Mitophagy arrest caused by UVB results in the mtDNA leakage into cytosol. [ABSTRACT FROM AUTHOR]

Collagen I is a major component of extracellular matrix in human skin, and is also widely used in a variety of skin-care products. In this study, we investigated the modulatory roles of collagen I on human immortalized keratinocytes HaCaT, especially when cells were irradiated with UVB. Interestingly, the cells grown on plates coated by molecular collagen I, but not fibrillar collagen I, acquired certain resistance against UVB damages, as shown by increased survival and reduced apoptosis. The accumulation of dysfunctional mitochondria in UVB-treated cells was attenuated by molecular collagen I-coating. Interestingly, molecular collagen I rescued the loss of mitochondrial biogenesis in cells treated with UVB. Loss of PINK1/parkin-mediated mitophagy was dominant for the accumulation of dysfunctional mitochondria after UVB irradiation. Of note, cells cultured on molecular collagen I-precoated plates exhibited reserved mitophagy after UVB irradiation, as reflected by the enhanced protein level of PINK1/parkin, increased mitochondrial ubiquitin and the co-localization of lysosomes and mitochondria. Moreover, in UVB-treated cells, inhibiting mitophagy by Cyclosporin A, or by silencing PINK1 or parkin, disturbed the resolution of mitochondrial stress and reduced the protective effect of molecular collagen I, indicating that mitophagy is pivotal for the protection of collagen I against UVB damage in keratinocytes HaCaT. Collectively, this study reveals an unexpected protective role of collagen I, which facilitates mitophagy to rescue cells under UVB irradiation, providing a new direction for clinical application of collagen products. [Display omitted] • Precoating culture plates with collagen I alleviates UVB-induced HaCaT cell apoptosis. • Collagen I inhibits the accumulation of damaged mitochondria in UVB-treated cells. • Arrest of mitophagy accelerates UVB damage by accumulating damaged mitochondria. • UVB protection caused by collagen I depends on rescuing mitophagy. [ABSTRACT FROM AUTHOR]