Elimination of damaged mitochondria during UVB‐induced senescence is orchestrated by NIX‐dependent mitophagy

Skin aging is the result of two types of aging, “intrinsic aging” an inevitable consequence of physiologic and genetically determined changes and “extrinsic aging,” which is dependent on external factors such as exposure to sunlight, smoking, and dietary habits. UVB causes skin injury through the generation of free radicals and other oxidative byproducts, also contributing to DNA damage. Appearance and accumulation of senescent cells in the skin are considered one of the hallmarks of aging in this tissue. Mitochondria play an important role for the development of cellular senescence, in particular stress‐induced senescence of human cells. However, many aspects of mitochondrial physiology relevant to cellular senescence and extrinsic skin aging remain to be unraveled. Here, we demonstrate that mitochondria damaged by UVB irradiation of human dermal fibroblasts (HDF) are eliminated by NIX‐dependent mitophagy and that this process is important for cell survival under these conditions. Additionally, UVB‐irradiation of human dermal fibroblasts (HDF) induces the shedding of extracellular vesicles (EVs), and this process is significantly enhanced in UVB‐irradiated NIX‐depleted cells. Our findings establish NIX as the main mitophagy receptor in the process of UVB‐induced senescence and suggest the release of EVs as an alternative mechanism of mitochondrial quality control in HDF. Upon sporadic UVB exposure (1), NIX is translocated from the nucleus to damaged mitochondria, inducing NIX‐dependent mitophagy (NDM). As a result, cells can recycle their mitochondria and remain healthy. Under chronic UVB exposure (2), NDM is not sufficient to fully restore mitochondrial function, resulting in cellular senescence. Upon inhibition of NDM (3), cells release EVs containing mitochondria. Under these conditions, a shift from senescence to cell death is observed.