ATR Kinase Activity Limits Mutagenesis and Promotes the Clonogenic Survival of Quiescent Human Keratinocytes Exposed to UVB Radiation

The Endoplasmic Reticulum Stress Sensor IRE1α Regulates the UV DNA Repair Response through the Control of Intracellular Calcium Homeostasis

The unfolded protein response is activated by UVB irradiation, but the role of a key mediator, IRE1α, is not clear. In this study, we show that mice with an epidermal IRE1α deletion are sensitized to UV with increased apoptosis, rapid loss of UV-induced cyclopyrimidine dimer‒positive keratinocytes, and sloughing of the epidermis. In vitro, Ire1α-deficient keratinocytes have increased UVB sensitivity, reduced cyclopyrimidine dimer repair, and reduced accumulation of γH2AX and phosphorylated ATR, suggesting defective activation of nucleotide excision repair. Knockdown of XBP1 or pharmacologic inhibition of the IRE1α ribonuclease did not phenocopy Ire1α deficiency. The altered UV response was linked to elevated intracellular calcium levels and ROS, and this was due to dysregulation of the endoplasmic reticulum calcium channel InsP3R. Pharmacologic, genetic, and biochemical studies linked the regulation of the Ins3PR, intracellular calcium, and normal UV DNA damage response to CIB1 and the IRE1α‒TRAF2‒ASK1 complex. These results suggest a model where IRE1α activation state drives CIB1 binding either to the InsP3R or ASK1 to regulate endoplasmic reticulum calcium efflux, ROS, and DNA repair responses after UV irradiation.

Insulin-like Growth Factor-1 Impacts p53 Target Gene Induction in UVB-irradiated Keratinocytes and Human Skin

The tumor suppressor protein p53 limits mutagenesis in response to ultraviolet-B (UVB) light exposure by activating the transcription of genes that mitigate the damaging effects of UVB radiation on DNA. Because most nonmelanoma skin cancers (NMSCs) occur in older individuals, it is important to understand the process of mutagenesis in the geriatric skin microenvironment. Based on previous studies demonstrating that geriatric skin expresses lower levels of the growth factor insulin-like growth factor-1 (IGF-1) than young adult skin, a role for IGF-1 in the regulation of p53 target genes was investigated in both human keratinocytes in vitro and human skin explants ex vivo. The products of the p53 target genes p21 and DNA polymerase eta (pol η) were found to be increased by UVB exposure in both experimental systems, and this induction was observed to be partially abrogated by depriving keratinocytes of IGF-1 in vitro or by the treatment of keratinocytes in vitro and human skin explants with an IGF-1 receptor antagonist. Because p21 and pol η function to limit mutagenic DNA replication following UVB exposure, these results suggest that NMSC risk in geriatric populations may be due to age-dependent decreases in IGF-1 signaling that disrupt p53 function in the skin.

Spironolactone and XPB: An Old Drug with a New Molecular Target

Spironolactone (SP) is commonly used for the treatment of heart failure, hypertension, and complications of cirrhosis by antagonizing the mineralocorticoid receptor. However, SP also antagonizes the androgen receptor, and thus SP has also been shown to be effective in the treatment of acne, hair loss, and hirsutism in women. Interestingly, recent drug repurposing screens have identified new and diverse functions for SP as a simulator of tumor immunosurveillance and as an inhibitor of DNA repair and viral infection. These novel pharmacological effects of SP have all been linked to the ability of SP to induce the rapid proteolytic degradation of the xeroderma pigmentosum group B (XPB) protein. XPB is a critical enzymatic component of the multi-subunit complex known as transcription factor II-H (TFIIH), which plays essential roles in both DNA repair and the initiation of transcription. Given the critical functions for XPB and TFIIH in these processes, the loss of XPB by SP could lead to mutagenesis. However, the ability of SP to promote cancer stem cell death and facilitate immune recognition may counteract the negative consequences of SP to mitigate carcinogenic risk. Thus, SP appears to have new and interesting pharmacological effects that may extend its potential uses.

Regulation of UV damage repair in quiescent yeast cells

Non-growing quiescent cells face special challenges when repairing lesions produced by exogenous DNA damaging agents. These challenges include the global repression of transcription and translation and a compacted chromatin structure. We investigated how quiescent yeast cells regulated the repair of DNA lesions produced by UV irradiation. We found that UV lesions were excised and repaired in quiescent cells before their re-entry into S phase, and that lesion repair was correlated with high levels of Rad7, a recognition factor in the global genome repair sub-pathway of nucleotide excision repair (GGR-NER). UV exposure led to an increased frequency of mutations that included C->T transitions and T > A transversions. Mutagenesis was dependent on the error-prone translesion synthesis (TLS) DNA polymerase, Pol zeta, which was the only DNA polymerase present in detectable levels in quiescent cells. Across the genome of quiescent cells, UV-induced mutations showed an association with exons that contained H3K36 or H3K79 trimethylation but not with those bound by RNA polymerase II. Together, the data suggest that the distinct physiological state and chromatin structure of quiescent cells contribute to its regulation of UV damage repair.