Downregulation of SMAD4 protects HaCaT cells against UVB-induced damage and oxidative stress through the activation of EMT

As a member of the SMAD family, SMAD4 plays a crucial role in several cellular biological processes. However, its function in UVB radiation-induced keratinocyte damage is not yet clarified. Our study aims to provide mechanistic insight for the development of future UVB protective therapies and therapeutics involving SMAD4. HaCaT cells were treated with UVB, and the dose dependence and time dependence of UVB were measured. The cell function of UVB-treated HaCaT cells and the activity of epithelial-mesenchymal transition (EMT) after overexpression or silencing of SMAD4 was observed by flow cytometry, quantitative reverse transcription PCR (qRT-PCR) and Western Blots (WB). We found that a significant decrease in SMAD4 was observed in HaCaT cells induced by UVB. Our data confirm SMAD4 as a direct downstream target of miR-664. The down-regulation of SMAD4 preserved the viability of the UVB-treated HaCaT cells by inhibiting autophagy or apoptosis. Furthermore, the silencing of SMAD4 activated the EMT process in UVB-treated HaCaT cells. Down-regulation of SMAD4 plays a protective role in UVB-treated HaCaT cells via the activation of EMT.

Cyclobutane Pyrimidine Dimers Produced with Narrowband UVB Are on Average More Mutagenic than Those with Broadband UVB in Mouse Skin

Although narrowband UVB (NB-UVB) has replaced broadband UVB (BB-UVB) because of its greater effectiveness in dermatological phototherapy, it is twice as carcinogenic as BB-UVB at an equivalent inflammatory dose. To clarify the basis of the different genotoxicities, we comparatively evaluated the mutagenicities in mouse skin of the two UVB types along with their efficiencies in the formation of cyclobutane pyrimidine dimer (CPD), which is a major mutagenic DNA photolesion specifically produced by UVR. We found that the mutagenicity averaged per single molecule of CPD was 2.5- and 1.8-fold higher in NB-UVB-exposed epidermis and dermis, respectively, which indicates that NB-UVB is more mutagenic for the skin than BB-UVB at doses producing an equimolar amount of CPD. Analysis of effective wavelengths for UV photolesion formation with each UVB source revealed a remarkable difference in the peak effective wavelengths for CPD formation: 15 nm longer for NB-UVB in the epidermis. Although the analysis of mutation profiles showed largely similar UV-specific signatures between the two UVB types, a relatively stronger preference for UVA-specific mutations was observed for NB-UVB. These results suggest that the difference in the effective wavelengths for CPD formation leads to the different mutagenicities and carcinogenicities between the UVB sources.

Wavelength- and Tissue-dependent Variations in the Mutagenicity of Cyclobutane Pyrimidine Dimers in Mouse Skin

The cyclobutane pyrimidine dimer (CPD) is a main mutagenic photolesion in DNA produced by UVR. We previously studied the wavelength-dependent kinetics of mutation induction efficiency using monochromatic UVR sources and transgenic mice developed for mutation assay and established the action spectra of UVR mutagenicity in the mouse epidermis and dermis. Here, we further established the action spectra of CPD and pyrimidine(6-4)pyrimidone photoproduct formation in the same tissues and in naked DNA using the same sources and mouse strain. Quantitative ELISA helped us estimate the photolesion formation efficiencies on a molecule-per-nucleotide basis. Using these action spectra, we confirmed that the UVR mutation mostly depends on CPD formation. Moreover, the mutagenicity of a CPD molecule (CPD mutagenicity) was found to vary by wavelength, peaking at approximately 313 nm in both the epidermis and dermis with similar wavelength-dependent patterns. Thus, the CPD formation efficiency is a main determinant of UVR mutagenicity in mouse skin, whereas a wavelength-dependent variation in the qualitative characteristics of CPD molecules also affects the mutagenic consequences of UVR insults. In addition, the CPD mutagenicity was always higher in the epidermis than in the dermis, suggesting different cellular responses to UVR between the two tissues irrespective of the wavelength.

Mechanistic considerations on the wavelength-dependent variations of UVR genotoxicity and mutagenesis in skin: the discrimination of UVA-signature from UV-signature mutation

Ultraviolet radiation (UVR) predominantly induces UV-signature mutations, C → T and CC → TT base substitutions at dipyrimidine sites, in the cellular and skin genome. I observed in our in vivo mutation studies of mouse skin that these UVR-specific mutations show a wavelength-dependent variation in their sequence-context preference. The C → T mutation occurs most frequently in the 5'-TCG-3' sequence regardless of the UVR wavelength, but is recovered more preferentially there as the wavelength increases, resulting in prominent occurrences exclusively in the TCG sequence in the UVA wavelength range, which I will designate as a "UVA signature" in this review. The preference of the UVB-induced C → T mutation for the sequence contexts shows a mixed pattern of UVC- and UVA-induced mutations, and a similar pattern is also observed for natural sunlight, in which UVB is the most genotoxic component. In addition, the CC → TT mutation hardly occurs at UVA1 wavelengths, although it is detected rarely but constantly in the UVC and UVB ranges. This wavelength-dependent variation in the sequence-context preference of the UVR-specific mutations could be explained by two different photochemical mechanisms of cyclobutane pyrimidine dimer (CPD) formation. The UV-signature mutations observed in the UVC and UVB ranges are known to be caused mainly by CPDs produced through the conventional singlet/triplet excitation of pyrimidine bases after the direct absorption of the UVC/UVB photon energy in those bases. On the other hand, a novel photochemical mechanism through the direct absorption of the UVR energy to double-stranded DNA, which is called "collective excitation", has been proposed for the UVA-induced CPD formation. The UVA photons directly absorbed by DNA produce CPDs with a sequence context preference different from that observed for CPDs caused by the UVC/UVB-mediated singlet/triplet excitation, causing CPD formation preferentially at thymine-containing dipyrimidine sites and probably also preferably at methyl CpG-associated dipyrimidine sites, which include the TCG sequence. In this review, I present a mechanistic consideration on the wavelength-dependent variation of the sequence context preference of the UVR-specific mutations and rationalize the proposition of the UVA-signature mutation, in addition to the UV-signature mutation.

Quantitative analysis of UV photolesions suggests that cyclobutane pyrimidine dimers produced in mouse skin by UVB are more mutagenic than those produced by UVC

The amount of photolesions produced in DNA after exposure to physiological doses of ultraviolet radiation (UVR) can be estimated with high sensitivity and at low cost through an immunological assay, ELISA, which, however, provides only a relative estimate that cannot be used for comparisons between different photolesions such as cyclobutane pyrimidine dimer (CPD) and pyrimidine(6-4)pyrimidone photoproduct (64PP) or for analysis of the genotoxicity of photolesions on a molecular basis. To solve this drawback of ELISA, we introduced a set of UVR-exposed, calibration DNA whose photolesion amounts were predetermined and estimated the absolute molecular amounts of CPDs and 64PPs produced in mouse skin exposed to UVC and UVB. We confirmed previously reported observations that UVC induced more photolesions in the skin than UVB at the same dose, and that both types of UVR produced more CPDs than 64PPs. The UVR protection abilities of the cornified and epidermal layers for the lower tissues were also evaluated quantitatively. We noticed that the values of absorbance obtained in ELISA were not always proportional to the molecular amounts of the lesion, especially for CPD, cautioning against the direct use of ELISA absorbance data for estimation of the photolesion amounts. We further estimated the mutagenicity of a CPD produced by UVC and UVB in the epidermis and dermis using the mutation data from our previous studies with mouse skin and found that CPDs produced in the epidermis by UVB were more than two-fold mutagenic than those by UVC, which suggests that the properties of CPDs produced by UVC and UVB might be different. The difference may originate from the wavelength-dependent methyl CpG preference of CPD formation. In addition, the mutagenicity of CPDs in the dermis was lower than that in the epidermis irrespective of the UVR source, suggesting a higher efficiency in the dermis to reduce the genotoxicity of CPDs produced within it. We also estimated the minimum amount of photolesions required to induce the mutation induction suppression (MIS) response in the epidermis to be around 15 64PPs or 100 CPDs per million bases in DNA as the mean estimate from UVC and UVB-induced MIS.

Role of p53 in silibinin-mediated inhibition of ultraviolet B radiation-induced DNA damage, inflammation and skin carcinogenesis

Non-melanoma skin cancers (NMSC) are a growing problem given that solar ultraviolet B (UVB) radiation exposure is increasing most likely due to depletion of the atmospheric ozone layer and lack of adequate sun protection. Better preventive methods are urgently required to reduce UV-caused photodamage and NMSC incidence. Earlier, we have reported that silibinin treatment activates p53 and reduces photodamage and NMSC, both in vitro and in vivo; but whether silibinin exerts its protective effects primarily through p53 remains unknown. To address this question, we generated p53 heterozygous (p53^+/-) and p53 knockout (p53^-/-) mice on SKH-1 hairless mouse background, and assessed silibinin efficacy in both short- and long-term UVB exposure experiments. In the chronic UVB-exposed skin tumorigenesis study, compared to p53^+/+ mice, p53^+/- mice developed skin tumors earlier and had higher tumor number, multiplicity and volume. Silibinin topical treatment significantly reduced the tumor number, multiplicity and volume in p53^+/+ mice but silibinin' protective efficacy was significantly compromised in p53^+/- mice. Additionally, silibinin treatment failed to inhibit precursor skin cancer lesions in p53^-/- mice but improved the survival of the mice. In short-term studies, silibinin application accelerated the removal of UVB-induced DNA damage in p53^+/+ mice while its efficacy was partially compromised in p53^-/- mice. Interestingly, silibinin treatment also inhibited the UVB-induced inflammatory markers in skin tissue. These results further confirmed that absence of the p53 allele predisposes mice to photodamage and photocarcinogenesis, and established that silibinin mediates its protection against UVB-induced photodamage, inflammation and photocarcinogenesis partly through p53 activation.

In Vivo Spectrum of UVC-induced Mutation in Mouse Skin Epidermis May Reflect the Cytosine Deamination Propensity of Cyclobutane Pyrimidine Dimers

Although ultraviolet radiation (UVR) has a genotoxicity for inducing skin cancers, the skin may tolerate UVC component because the epidermal layer prevents this short wavelength range from passing through. Here, UVC genotoxicity for mouse skin was evaluated in terms of DNA damage formation and mutagenicity. UVC induced UVR photolesions and mutations remarkably in the epidermis but poorly in the dermis, confirming the barrier ability of the epidermis against shorter UVR wavelengths. Moreover, the epidermis itself responded to UVC mutagenicity with mutation induction suppression, which suppressed the mutant frequencies to a remarkably low, constant level regardless of UVC dose. The mutation spectrum observed in UVC-exposed epidermis showed a predominance of UV-signature mutation, which occurred frequently in 5'-TCG-3', 5'-TCA-3' and 5'-CCA-3' contexts. Especially, for the former two contexts, the mutations recurred at several sites with more remarkable recurrences at the 5'-TCG-3' sites. Comparison of the UVC mutation spectrum with those observed in longer UVR wavelength ranges led us to a mechanism that explains why the sequence context preference of UV-signature mutation changes according to the wavelength, which is based on the difference in the mCpG preference of cyclobutane pyrimidine dimer (CPD) formation among UVR ranges and the sequence context-dependent cytosine deamination propensity of CPD.

p53 requires the stress sensor USF1 to direct appropriate cell fate decision

Genomic instability is a major hallmark of cancer. To maintain genomic integrity, cells are equipped with dedicated sensors to monitor DNA repair or to force damaged cells into death programs. The tumor suppressor p53 is central in this process. Here, we report that the ubiquitous transcription factor Upstream Stimulatory factor 1 (USF1) coordinates p53 function in making proper cell fate decisions. USF1 stabilizes the p53 protein and promotes a transient cell cycle arrest, in the presence of DNA damage. Thus, cell proliferation is maintained inappropriately in Usf1 KO mice and in USF1-deficient melanoma cells challenged by genotoxic stress. We further demonstrate that the loss of USF1 compromises p53 stability by enhancing p53-MDM2 complex formation and MDM2-mediated degradation of p53. In USF1-deficient cells, the level of p53 can be restored by the re-expression of full-length USF1 protein similarly to what is observed using Nutlin-3, a specific inhibitor that prevents p53-MDM2 interaction. Consistent with a new function for USF1, a USF1 truncated protein lacking its DNA-binding and transactivation domains can also restore the induction and activity of p53. These findings establish that p53 function requires the ubiquitous stress sensor USF1 for appropriate cell fate decisions in response to DNA-damage. They underscore the new role of USF1 and give new clues of how p53 loss of function can occur in any cell type. Finally, these findings are of clinical relevance because they provide new therapeutic prospects in stabilizing and reactivating the p53 pathway.

Cancer is a complex disease that is characterized by the sequential accumulation of genetic mutations. Exposure to environmental agents, such as solar ultraviolet, induces such alterations and thus contributes to the development of genomic instability. The tumor suppressor p53 has a central role in orchestrating cellular responses to genotoxic stress. In response to DNA-damage, p53 is stabilized and activated to direct cell fate decisions. Cells in which p53 stabilization is compromised become more vulnerable to mutagenic agents and hence the mutation rate increases, which promotes tumor development. Stabilization of p53 is thus a critical step towards cancer prevention. Using a genetic approach, we demonstrate that the ubiquitous transcription factor Upstream Stimulatory factor 1 (USF1) is required for immediate p53 stabilization and appropriate cell fate decisions following genotoxic stress. Furthermore, we show that this involves a novel function of USF1 that underscores its critical role as a stress sensor. The loss of USF1 expression should thus be considered as a potential initiator of tumorigenesis in the context of environmental insults.

Whales use distinct strategies to counteract solar ultraviolet radiation

A current threat to the marine ecosystem is the high level of solar ultraviolet radiation (UV). Large whales have recently been shown to suffer sun-induced skin damage from continuous UV exposure. Genotoxic consequences of such exposure remain unknown for these long-lived marine species, as does their capacity to counteract UV-induced insults. We show that UV exposure induces mitochondrial DNA damage in the skin of seasonally sympatric fin, sperm, and blue whales and that this damage accumulates with age. However, counteractive molecular mechanisms are markedly different between species. For example, sperm whales, a species that remains for long periods at the sea surface, activate genotoxic stress pathways in response to UV exposure whereas the paler blue whale relies on increased pigmentation as the season progresses. Our study also shows that whales can modulate their responses to fluctuating levels of UV, and that different evolutionary constraints may have shaped their response strategies.

Absence of a p53 allele delays nitrogen mustard-induced early apoptosis and inflammation of murine skin

Bifunctional alkylating agent sulfur mustard (SM) and its analog nitrogen mustard (NM) cause DNA damage leading to cell death, and potentially activating inflammation. Transcription factor p53 plays a critical role in DNA damage by regulating cell cycle progression and apoptosis. Earlier studies by our laboratory demonstrated phosphorylation of p53 at Ser15 and an increase in total p53 in epidermal cells both in vitro and in vivo following NM exposure. To elucidate the role of p53 in NM-induced skin toxicity, we employed SKH-1 hairless mice harboring wild type (WT) or heterozygous p53 (p53+/-). Exposure to NM (3.2mg) caused a more profound increase in epidermal thickness and apoptotic cell death in WT relative to p53+/- mice at 24h. However, by 72h after exposure, there was a comparable increase in NM-induced epidermal cell death in both WT and p53+/- mice. Myeloperoxidase activity data showed that neutrophil infiltration was strongly enhanced in NM-exposed WT mice at 24h persisting through 72h of exposure. Conversely, robust NM-induced neutrophil infiltration (comparable to WT mice) was seen only at 72h after exposure in p53+/- mice. Similarly, NM-exposure strongly induced macrophage and mast cell infiltration in WT, but not p53+/- mice. Together, these data indicate that early apoptosis and inflammation induced by NM in mouse skin are p53-dependent. Thus, targeting this pathway could be a novel strategy for developing countermeasures against vesicants-induced skin injury.

Action spectrum analysis of UVR genotoxicity for skin: the border wavelengths between UVA and UVB can bring serious mutation loads to skin

UVR causes erythema, which has been used as a standardized index to evaluate the risk of UVR for human skin. However, the genotoxic significance of erythema has not been elucidated clearly. Here, we characterized the wavelength dependence of the genotoxic and erythematic effects of UVR for the skin by analyzing the induction kinetics of mutation and inflammation in mouse skin using lacZ-transgenic mice and monochromatic UVR sources. We determined their action spectra and found a close correlation between erythema and an epidermis-specific antigenotoxic response, mutation induction suppression (MIS), which suppressed the mutant frequencies (MFs) to a constant plateau level only 2-3-fold higher than the background MF at the cost of apoptotic cell death, suggesting that erythema may represent the threshold beyond which the antigenotoxic but tissue-destructive MIS response commences. However, we unexpectedly found that MIS attenuates remarkably at the border wavelengths between UVA and UVB around 315 nm, elevating the MF plateaus up to levels ∼40-fold higher than the background level. Thus, these border wavelengths can bring heavier mutation loads to the skin than the otherwise more mutagenic and erythematic shorter wavelengths, suggesting that erythema-based UVR risk evaluation should be reconsidered.

Photo-oxidative stress by ultraviolet-B radiation and antioxidative defense of eckstolonol in human keratinocytes

Ultraviolet-B (UV-B) irradiation has been known to generate oxidative stress by increasing reactive oxygen species (ROS) in skin cells. Several naturally occurring antioxidant compounds isolated from marine algae are believed to protect against ROS. In this study, we assessed the antioxidative effect of eckstolonol isolated from Ecklonia cava against UV-B-induced ROS in human keratinocytes (HaCaTs). We investigated the effects of photo-oxidative stress by UV-B (50 mJ/cm(2)) and the antioxidative effects of eckstolonol using fluorometry, flow cytometry, microscopy, and cell viability and comet assays. UV-B irradiation decreased cell viability, which was restored in a dose-dependent manner with eckstolonol treatment (0, 5, 50, 100, and 200 μM). Moreover, eckstolonol reduced UV-B-induced ROS, lipid peroxidation, damaged DNA levels, and cell death. These antioxidative effects seem to be due to the enzymatic activities of catalase (CAT) and superoxide dismutase (SOD). Collectively, these results indicate that eckstolonol is capable of protecting keratinocytes from photo-oxidative stress.

Geldanamycin analog 17-DMAG limits apoptosis in human peripheral blood cells by inhibition of p53 activation and its interaction with heat-shock protein 90 kDa after exposure to ionizing radiation

Exposure to ionizing radiation induces p53, and its inhibition improves mouse survival. We tested the effect of 17-dimethylamino-ethylamino-17-demethoxygeldanamycin (17-DMAG) on p53 expression and function after radiation exposure. 17-DMAG, a heat-shock protein 90 (Hsp90) inhibitor, protects human T cells from ionizing radiation-induced apoptosis by inhibiting inducible nitric oxide synthase (iNOS) and subsequent caspase-3 activation. Using ex vivo human peripheral blood mononuclear cells, we found that ionizing radiation increased p53 accumulation, acute p53 phosphorylation, Bax expression and caspase-3/7 activation in a radiation dose- and time postirradiation-dependent manner. 17-DMAG inhibited these increases in a concentration-dependent manner (IC(50)  =  0.93 ± 0.01 µM). Using in vitro models, we determined that inhibition of p53 by genetic knockout resulted in lower levels of caspase-3/7 activity 1 day after irradiation and enhanced survival at 10 days. Analysis of p53-Hsp90 interaction in ex vivo cell lysates indicated that the binding between the two molecules occurred after irradiation but 17-DMAG prevented the binding. Taken together, these results suggest the presence of p53 phosphorylation and Hsp90-dependent p53 stabilization after acute irradiation. Hsp90 inhibitors such as 17-DMAG may prove useful with radiation-based cancer therapy as well as for general radioprotection.

The mechanisms of UV mutagenesis

Ultraviolet (UV) light induces specific mutations in the cellular and skin genome such as UV-signature and triplet mutations, the mechanism of which has been thought to involve translesion DNA synthesis (TLS) over UV-induced DNA base damage. Two models have been proposed: "error-free" bypass of deaminated cytosine-containing cyclobutane pyrimidine dimers (CPDs) by DNA polymerase η, and error-prone bypass of CPDs and other UV-induced photolesions by combinations of TLS and replicative DNA polymerases--the latter model has also been known as the two-step model, in which the cooperation of two (or more) DNA polymerases as misinserters and (mis)extenders is assumed. Daylight UV induces a characteristic UV-specific mutation, a UV-signature mutation occurring preferentially at methyl-CpG sites, which is also observed frequently after exposure to either UVB or UVA, but not to UVC. The wavelengths relevant to the mutation are so consistent with the composition of daylight UV that the mutation is called solar-UV signature, highlighting the importance of this type of mutation for creatures with the cytosine-methylated genome that are exposed to the sun in the natural environment. UVA has also been suggested to induce oxidative types of mutation, which would be caused by oxidative DNA damage produced through the oxidative stress after the irradiation. Indeed, UVA produces oxidative DNA damage not only in cells but also in skin, which, however, does not seem sufficient to induce mutations in the normal skin genome. In contrast, it has been demonstrated that UVA exclusively induces the solar-UV signature mutations in vivo through CPD formation.