Abstract
Excessive exposure of the skin to UV radiaton (UVR) accelerates the aging process and leads to a photoaging state which involves similar pathological alterations to those occurring in chronological aging. UVR exposure, containing both UVA and UVB radiation, triggers cellular senescence and a chronic inflammatory state in skin. UVR promotes oxidative stress and a leakage of double-stranded DNA (dsDNA) from nuclei and mitochondria into the cytoplasm of keratinocytes and fibroblasts. It is recognized that cytosolic dsDNA is a specific danger signal which stimulates cytoplasmic DNA sensors. The activation of the signaling through the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) is a major defence and survival mechanism combatting against tissue injuries. There is abundant evidence that UVR exposure of skin stimulates cGAS-STING signaling which promotes cellular senescence and remodels both the local and systemic immune network. cGAS-STING signaling activates the IRF3 and NF-κB signaling pathways which trigger both pro-inflammatory and immunosuppressive responses. Moreover, cGAS-STING signaling stimulates inflammatory responses by activating the NLRP3 inflammasomes. Senescent fibroblasts secrete not only cytokines but also chemokines and colony-stimulating factors which induce myeloid differentiation and recruitment of immune cells into inflamed skin. Photoaging is associated with an immunosuppressive state in skin which is attributed to an expansion of immunosuppressive cells, such as Tregs. UVR-induced cGAS-STING signaling also stimulates the expression of PD-L1, a ligand for inhibitory immune checkpoint receptor, which evokes an exhaustion of effector immune cells. There is clear evidence that cGAS-STING signaling can also accelerate chronological aging by remodeling the immune network.