Binding of collagen I to integrins alleviates UVB-caused mitochondrial disorders in human keratinocytes HaCaT through enhancement of F-actin polymerization

Collagen I is one of the major components of the extracellular matrix in human skin, and is frequently used in skin cares and medications. Previously, we revealed that human keratinocytes HaCaT cells grown on collagen I (Col)-coated dishes gain resistance against UVB damages owing to the restored mitophagy. In this study, we further investigate the mechanisms by which collagen I modulates mitophagy. UVB irradiation causes loss of integrin β1 and collapse of F-actin cytoskeleton. Considering the requirement of actin skeleton in various cellular processes, we are curious about the participation of F-actin collapse in UVB damage. Integrin β1, whose activation enhances F-actin assembly, is a potential target for Col in UVB-treated cells. Notably, inhibiting integrin by adding an inhibitor RGDS or siRNA attenuates the effect of Col against UVB damages, confirming the participation of integrin in cell protection. The collapse of F-actin is rescued by Col, accompanying increases in the mRNA of F-actin polymerization-associated proteins and decreases in the mRNA of depolymerization-associated proteins. Inhibiting actin polymerization by using cytochalasin D represses the protective effect of Col, confirming the cytoprotective role of F-actin in UVB-treated cells. Remarkably, mitophagy in UVB-treated cells restored by Col-coating is inhibited by adding cytochalasin D or RGDS, as shown by the decreases of lysosomes, mitochondrial ubiquitin proteins, and co-localization of autophagosomes and mitochondria, resulting in accumulation of damaged mitochondria, which stresses the importance of F-actin and integrin in mitophagy. In summary, integrins and F-actin are required for mitophagy in UVB-irradiated HaCaT cells, and their enhancements by Col-coating facilitate timely elimination of damaged mitochondria caused by UVB, finally contributing to cell survival.