Sublethal UV irradiation induces squamous differentiation via a p53-independent, DNA damage-mitosis checkpoint

Abstract

The epidermis is a self-renewal epithelium continuously exposed to the genotoxic effects of ultraviolet (UV) light, the main cause of skin cancer. Therefore, it needs robust self-protective mechanisms facing genomic damage. p53 has been shown to mediate apoptosis in sunburn cells of the epidermis. However, epidermal cells daily receive sublethal mutagenic doses of UV and massive apoptosis would be deleterious. We have recently unravelled an anti-oncogenic keratinocyte DNA damage-differentiation response to cell cycle stress. We now have studied this response to high or moderate single doses of UV irradiation. Whereas, as expected, high levels of UV induced p53-dependent apoptosis, moderate levels triggered squamous differentiation. UV-induced differentiation was not mediated by endogenous p53. Overexpression of the mitosis global regulator FOXM1 alleviated the proliferative loss caused by UV. Conversely, knocking-down the mitotic checkpoint protein Wee1 drove UV-induced differentiation into apoptosis. Therefore, the results indicate that mitosis checkpoints determine the response to UV irradiation. The differentiation response was also found in cells of head and neck epithelia thus uncovering a common regulation in squamous tissues upon chronic exposure to mutagens, with implications into homeostasis and disease.

Squamous differentiation requires G2/mitosis slippage to avoid apoptosis

“…8a), at levels comparable with acute UVB irradiation (Fig. 7a) [7]. There was also loss of the G2/M (4C) and polyploid (>4C) fractions ( Fig.…”

“…Therefore, in keratinocytes inactivation of Cdk1 during G2 or mitosis arrest might be required to allow G2/M slippage and to drive irreversible terminal differentiation. Within these lines, it is interesting that knocking-down Cdk1 inhibitor Wee1 drives UV-induced keratinocyte differentiation into apoptosis [7]. Consistently, CDC20 strongly accumulates in mitotic cells in differentiating layers of the epidermis.…”

“…We have found a cell-autonomous mechanism that triggers differentiation in response to cell cycle stress in human primary keratinocytes in vitro [4][5][6]. DNA damage caused by genotoxic agents (chemicals, UV irradiation) [6][7][8] or oncogenic replication stress, induces squamous differentiation. This differentiation response is triggered via mitotic checkpoints and involves cytokinesis failure and endoreplication.…”

“…Apoptosis was measured by CF647-Annexin V (FCCH100108, Millipore Bedford, MA, USA) binding assay in live cells following manufacturer instructions. As apoptotic positive control, mouse keratinocytes were irradiated with a 150 mJ/cm 2 dose of UVB (312 nm; UPV CL-1000 Series UV crosslinker) as previously described [7].…”

The Role of Geomagnetic Field Intensity in Late Quaternary Evolution of Humans and Large Mammals

“…which implies an evolutionary selective trend in the increased number of copies of TP53 in the genomes of these related megafauna (Sulak et al, 2016). TP53 is considered the guardian of the genome due to its role in mitigating DNA damage, and is itself a target of UV-induced mutations (Aylon & Oren, 2011;de Pedro et al, 2018). Huang et al (2017) analyzed the body weight of species in two orders of large ungulate herbivores (Artiodactyla and Perissodactyla) from the Neogene fossil record in Europe and North America.…”