Irradiation of cells with ultraviolet-A (320-400 nm) in the presence of cell culture medium elicits biological effects due to extracellular generation of hydrogen peroxide

Endogenous Photosensitizers in Human Skin

Endogenous photosensitizers play a critical role in both beneficial and harmful light-induced transformations in biological systems. Understanding their mode of action is essential for advancing fields such as photomedicine, photoredox catalysis, environmental science, and the development of sun care products. This review offers a comprehensive analysis of endogenous photosensitizers in human skin, investigating the connections between their electronic excitation and the subsequent activation or damage of organic biomolecules. We gather the physicochemical and photochemical properties of key endogenous photosensitizers and examine the relationships between their chemical reactivity, location within the skin, and the primary biochemical events following solar radiation exposure, along with their influence on skin physiology and pathology. An important take-home message of this review is that photosensitization allows visible light and UV-A radiation to have large effects on skin. The analysis presented here unveils potential causes for the continuous increase in global skin cancer cases and emphasizes the limitations of current sun protection approaches.

MAB21L1 promotes survival of lens epithelial cells through control of αB-crystallin and ATR/CHK1/p53 pathway

The male abnormal gene family 21 (mab21), was initially identified in C. elegans. Since its identification, studies from different groups have shown that it regulates development of ocular tissues, brain, heart and liver. However, its functional mechanism remains largely unknown. Here, we demonstrate that Mab21L1 promotes survival of lens epithelial cells. Mechanistically, Mab21L1 upregulates expression of αB-crystallin. Moreover, our results show that αB-crystallin prevents stress-induced phosphorylation of p53 at S-20 and S-37 through abrogating the activation of the upstream kinases, ATR and CHK1. As a result of suppressing p53 activity by αB-crystallin, Mab21L1 downregulates expression of Bak but upregulates Mcl-1 during stress insult. Taken together, our results demonstrate that Mab21L1 promotes survival of lens epithelial cells through upregulation of αB-crystallin to suppress ATR/CHK1/p53 pathway.

The Damaging Effects of Long UVA (UVA1) Rays: A Major Challenge to Preserve Skin Health and Integrity

Within solar ultraviolet (UV) light, the longest UVA1 wavelengths, with significant and relatively constant levels all year round and large penetration properties, produce effects in all cutaneous layers. Their effects, mediated by numerous endogenous chromophores, primarily involve the generation of reactive oxygen species (ROS). The resulting oxidative stress is the major mode of action of UVA1, responsible for lipid peroxidation, protein carbonylation, DNA lesions and subsequent intracellular signaling cascades. These molecular changes lead to mutations, apoptosis, dermis remodeling, inflammatory reactions and abnormal immune responses. The altered biological functions contribute to clinical consequences such as hyperpigmentation, inflammation, photoimmunosuppression, sun allergies, photoaging and photocancers. Such harmful impacts have also been reported after the use of UVA1 phototherapy or tanning beds. Furthermore, other external aggressors, such as pollutants and visible light (Vis), were shown to induce independent, cumulative and synergistic effects with UVA1 rays. In this review, we synthetize the biological and clinical effects of UVA1 and the complementary effects of UVA1 with pollutants or Vis. The identified deleterious biological impact of UVA1 contributing to clinical consequences, combined with the predominance of UVA1 rays in solar UV radiation, constitute a solid rational for the need for a broad photoprotection, including UVA1 up to 400 nm.

Selective blue-filtering spectacle lens protected primary porcine RPE cells against light emitting diode-induced cell damage

This study aimed to investigate whether use of a selective-blue-filtering (S-BF) lens can protect cultured primary porcine RPE cells against photo-irradiation. Transmittance of S-BF and UV-filtering (UVF) lenses was characterised spectrophotometrically. RPE cells were exposed to 1700 lux of white (peak λ at 443 and 533 nm; 0.44 mW/cm²) or blue (peak λ at 448 and 523 nm; 0.85 mW/cm²) LED light for 16 h to evaluate the influence of light source on the culture. The effect of the S-BF and UVF ophthalmic lenses on RPE cell cultures under blue light irradiation was then investigated. Cell viability was compared using trypan blue and MTT assays. Intracellular ROS production was detected by a fluorescein probe CM-H2DCFDA. Expression levels of catalase and Prdx3 were analysed by western blot. Trypan blue staining showed blue light caused more cell death than no light (p = 0.001) or white light (p = 0.005). MTT assay supported the hypothesis that exposure to blue light damaged RPE cells more severely than no light (p = 0.002) or white light (p = 0.014). Under blue light, use of the S-BF lens, which blocked 17% more blue light than the UVF lens, resulted in higher cellular viability (S-BF: 93.4±1.4% vs UVF: 90.6±1.4%; p = 0.022; MTT: 1.2-fold; p = 0.029). Blue and white light both significantly increased ROS production. The S-BF lens protected cells, resulting in lower levels of ROS and higher expression of catalase and Prdx3. To conclude, blue LED light exposure resulted in significant cytotoxicity to RPE cells. Partial blockage of blue light by an S-BF lens led to protective effects against retinal phototoxicity, which were mediated by reduction of ROS and increased levels of antioxidant enzymes.

Riboflavin Plays a Pivotal Role in the UVA-Induced Cytotoxicity of Fibroblasts as a Key Molecule in the Production of H2O2 by UVA Radiation in Collaboration with Amino Acids and Vitamins

To investigate environmental factors that contribute to ultraviolet A (UVA)-induced oxidative stress, which accelerates the senescence and toxicity of skin cells, we irradiated human fibroblasts cultured in commonly used essential media with UVA and evaluated their viability and production of reactive oxygen species. The viability of fibroblasts exposed to a single dose of 3.6 J/cm² UVA was not reduced when cultured in Hanks balanced salt solution, but it was significantly decreased when cultured in Dulbecco’s modified Eagle’s medium (DMEM), which contains various amino acids and vitamins. Furthermore, cell viability was not reduced when fibroblasts were cultured in DMEM and treated with a hydrogen peroxide (H₂O₂) scavenger such as glutathione or catalase added after UVA irradiation. In addition, we confirmed that the production of H₂O₂ was dramatically increased by UVA photosensitization when riboflavin (R) coexisted with amino acids such as tryptophan (T), and found that R with folic acid (F) produced high levels of H₂O₂ after UVA irradiation. Furthermore, we noticed that R and F or R and T have different photosensitization mechanisms since NaN₃, which is a singlet oxygen quencher, suppressed only R and T photosensitization. Lastly, we examined the effects of antioxidants (L-ascorbic acid, trolox, L-cysteine, and L-histidine), which are singlet oxygen or superoxide or H₂O₂ scavengers, on R and F or on R and T photosensitization, and found that 1 mM ascorbic acid, Trolox, and L-histidine were strongly photosensitized with R, and produced significant levels of H₂O₂ during UVA exposure. However, 1 mM L-cysteine dramatically suppressed H₂O₂ production by UVA photosensitization. These data suggest that a low concentration of R-derived photosensitization is elicited by different mechanisms depending on the coexisting vitamins and amino acids, and regulates cellular oxidative stress by producing H₂O₂ during UVA exposure.

Blue and Long-Wave Ultraviolet Light Induce in vitro Neutrophil Extracellular Trap (NET) Formation

Neutrophil Extracellular Traps (NETs) are produced by neutrophilic granulocytes and consist of decondensed chromatin decorated with antimicrobial peptides. They defend the organism against intruders and are released upon various stimuli including pathogens, mediators of inflammation, or chemical triggers. NET formation is also involved in inflammatory, cardiovascular, malignant diseases, and autoimmune disorders like rheumatoid arthritis, psoriasis, or systemic lupus erythematosus (SLE). In many autoimmune diseases like SLE or dermatomyositis, light of the ultraviolet-visible (UV-VIS) spectrum is well-known to trigger and aggravate disease severity. However, the underlying connection between NET formation, light exposure, and disease exacerbation remains elusive. We studied the effect of UVA (375 nm), blue (470 nm) and green (565 nm) light on NETosis in human neutrophils ex vivo. Our results show a dose- and wavelength-dependent induction of NETosis. Light-induced NETosis depended on the generation of extracellular reactive oxygen species (ROS) induced by riboflavin excitation and its subsequent reaction with tryptophan. The light-induced NETosis required both neutrophil elastase (NE) as well as myeloperoxidase (MPO) activation and induced histone citrullination. These findings suggest that NET formation as a response to light could be the hitherto missing link between elevated susceptibility to NET formation in autoimmune patients and photosensitivity for example in SLE and dermatomyositis patients. This novel connection could provide a clue for a deeper understanding of light-sensitive diseases in general and for the development of new pharmacological strategies to avoid disease exacerbation upon light exposure.

Effects of light-emitting diode radiations on human retinal pigment epithelial cells in vitro

Human visual system is exposed to high levels of natural and artificial lights of different spectra and intensities along lifetime. Light-emitting diodes (LEDs) are the basic lighting components in screens of PCs, phones and TV sets; hence it is so important to know the implications of LED radiations on the human visual system. The aim of this study was to investigate the effect of LEDs radiations on human retinal pigment epithelial cells (HRPEpiC). They were exposed to three light-darkness (12 h/12 h) cycles, using blue-468 nm, green-525 nm, red-616 nm and white light. Cellular viability of HRPEpiC was evaluated by labeling all nuclei with DAPI; Production of reactive oxygen species (ROS) was determined by H2DCFDA staining; mitochondrial membrane potential was quantified by TMRM staining; DNA damage was determined by H2AX histone activation, and apoptosis was evaluated by caspases-3,-7 activation. It is shown that LED radiations decrease 75-99% cellular viability, and increase 66-89% cellular apoptosis. They also increase ROS production and DNA damage. Fluorescence intensity of apoptosis was 3.7% in nonirradiated cells and 88.8%, 86.1%, 83.9% and 65.5% in cells exposed to white, blue, green or red light, respectively. This study indicates three light-darkness (12 h/12 h) cycles of exposure to LED lighting affect in vitro HRPEpiC.

Medium-mediated effects increase cell killing in a human keratinocyte cell line exposed to solar-simulated radiation

PURPOSE

The objective of this study was to investigate whether cell culture medium is a biologically relevant exposure medium that can be employed in non-ionising photobiological investigations.

METHODS

The effect of solar-simulated irradiation on cell culture medium and its ability to elicit cell death was studied. The role of reactive oxygen species (ROS), cell secreted factors, and the contribution of individual components of the medium were investigated.

RESULTS

Cell death was found to be primarily mediated through the formation of ROS via riboflavin photosensitisation and degradation in the cell culture medium. Phenol red was found to significantly reduce the cell killing ability of riboflavin. Exposures in riboflavin-free medium resulted in significantly increased cell survival compared to identical exposures in riboflavin containing medium.

CONCLUSIONS

This study has shown that solar radiation toxicity is augmented by cell culture medium due to the presence of riboflavin. Results suggest that exposures performed in phenol red-free medium may serve to increase phototoxic effects if riboflavin is present. Riboflavin-free media is recommended for solar radiation investigations to eliminate concerns regarding riboflavin photosensitisation and nutrient deprivation.

Role of HuR and p38MAPK in ultraviolet B-induced post-transcriptional regulation of COX-2 expression in the human keratinocyte cell line HaCaT

COX-2 (cyclooxygenase-2) is a pivotal player in inflammatory processes, and ultraviolet radiation is a known stimulus for COX-2 expression in skin cells. Here, an induction of COX-2 expression in HaCaT human keratinocytes was observed only upon exposure of cells to UVB (280-320 nm) but not to UVA radiation (320-400 nm), as demonstrated by reverse transcription-PCR and Western blotting. Prostaglandin E(2) levels were elevated in cell culture supernatants of HaCaT cells exposed to UVB. COX-2 mRNA stability was dramatically increased by UVB irradiation. Both the stabilization of COX-2 mRNA and the enhancement of COX-2 steady-state mRNA and protein levels caused by UVB were prevented both by inhibition and small interfering RNA-induced depletion of p38(MAPK), a kinase strongly activated upon exposure to UVB, suggesting p38(MAPK)-dependent mRNA stabilization as a mechanism of UVB-induced COX-2 expression. A dramatic decrease in COX-2 expression induced by UVB was elicited by small interfering RNA-based depletion of a stress-responsive mRNA stabilizing protein regulated by p38(MAPK), i.e. HuR; UVB-induced elevation of COX-2 mRNA and protein levels coincided with an accumulation of HuR in the cytoplasm and was attenuated in cells depleted of HuR. Moreover, UVB-induced generation of prostaglandin E(2) by HaCaT cells was blunted by HuR depletion, suggesting that stress kinases (such as p38(MAPK)) as well as HuR are excellent targets for approaches aiming at interfering with induction of COX-2 expression by UVB.

Ultraviolet-A irradiation but not ultraviolet-B or infrared-A irradiation leads to a disturbed zinc homeostasis in cells

Changes of the redox balance in cells alter the availability of intracellular free Zn(2+). Here, cells were exposed to ultraviolet (UV)-A, UV-B, or infrared (IR)-A light irradiation, and the intracellular free zinc pool was monitored. Under sublethal conditions only UV-A irradiation resulted in a transient cytoplasmic and nuclear increase of intracellular free Zn(2+). Likewise, tert-butyl hydroperoxide and singlet oxygen, but not H(2)O(2) or intracellular generation of O(2)(*-) by redox cyclers, mimicked the effects of UV-A irradiation, while disulfide stress by diamide only led to a transient cytoplasmic zinc release. These results show that only certain types of subtoxic cellular stress massively disturb the zinc homeostasis in cells.

Total anti-oxidant capacity of cell culture media

1. The composition of synthetic cell culture media is important for the behaviour of cultured cells in vitro and may affect the results of many in vitro experiments. The total anti-oxidant capacity (TAC) of an extracellular medium may be an important factor in cell redox homeostasis. 2. In the present study, the TAC of cell culture media used for the cultivation of mammalian, yeast and bacterial cells (RPMI1640, Iscove's modified Dulbecco's medium, Dulbecco's modified Eagle's medium, minimum essential medium Eagle's 1959 with Earle's salts, Parker medium 199 with Hanks salts, bacterial Luria-Bertani medium, yeast extract-peptone-glucose and yeast nitrogen base media) was estimated using the 2,2'-azinobis(3-ethylbenzthiazoline-6-sulphonic acid) (ABTS(.+)) decolourization assay and the ferric ion reducing anti-oxidant power assay. 3. We found that components of the media such as cysteine, tyrosine, tryptophan and Phenol Red are important contributors to the TAC of cell culture media.

5-Methyltetrahydrofolate inhibits photosensitization reactions and strand breaks in DNA

The known functions of folate are to support one-carbon metabolism and to serve as photoreceptors for cryptochromes and photolyases. We demonstrate that 5-methyltetrahydrofolate (5-MTHF, the predominant folate in plasma) is also a potent, near diffusion limited, scavenger of singlet oxygen and quencher of excited photosensitizers. Both pathways result in decomposition of 5-MTHF, although ascorbate can protect against this loss. In the absence of photosensitizers, 5-MTHF is directly decomposed only very slowly by UVA or UVB. Although synthetic folic acid can promote DNA damage by UVA, submicromolar 5-MTHF inhibits photosensitization-induced strand breaks. These observations suggest a new role for reduced folate in protection from ultraviolet damage and have bearing on the hypothesis that folate photodegradation influenced the evolution of human skin color.

Singlet oxygen inactivates protein tyrosine phosphatase-1B by oxidation of the active site cysteine

Singlet oxygen ((1)O(2)), an electronically excited form of molecular oxygen, is a mediator of biological effects of ultraviolet A radiation, stimulating signaling cascades in human cells. We demonstrate here that (1)O(2) generated by photosensitization or by thermodecomposition of 3,3'-(1,4-naphthylidene)dipropionate-1,4-endoperoxide inactivates isolated protein tyrosine phosphatases (PTPases). PTPase activities of PTP1B or CD45 were abolished by low concentrations of (1)O(2), but were largely restored by post-treatment with dithiothreitol. Electrospray ionization mass spectrometry analysis of tryptic digests of PTP1B exposed to (1)O(2) revealed oxidation of active-site Cys215 as the only cysteine residue oxidized. In summary, (1)O(2) may activate signaling cascades by interfering with phosphotyrosine dephosphorylation.

Extracellular generation of hydrogen peroxide is responsible for activation of EGF receptor by ultraviolet A radiation

Receptor tyrosine kinases such as the epidermal growth factor receptor (EGFR) have been proposed to be activated in cells exposed to ultraviolet A (UVA) radiation (320-400 nm) and to be involved in photocarcinogenesis. Singlet oxygen and hydrogen peroxide are being discussed as mediators of the activation of signal transduction pathways by UVA. It is demonstrated here that EGFR is not activated in cells exposed to UVA in the absence of extracellular photosensitizers. Rather, UVA was capable of activating the EGFR and the related ErbB2 receptor tyrosine kinase in HeLa cells and human keratinocytes only under conditions that allowed for the extracellular photochemical generation of H(2)O(2), such as when cells were covered with cell culture medium during exposure to UVA. Pretreatment of cells with vanadate was required for UVA-induced EGFR activation, pointing to the involvement of protein tyrosine phosphatases. Unlike H(2)O(2), photochemically generated singlet oxygen did not activate EGFR but instead impaired the activation of EGFR by its ligand, EGF. In summary, extracellularly generated H(2)O(2) mediates UVA-induced activation of the EGFR and of ErbB2, whereas intracellular generation of reactive oxygen species upon exposure of cells to UVA is not sufficient for activation of the receptor.

Endogenous UVA-photosensitizers: mediators of skin photodamage and novel targets for skin photoprotection

Endogenous chromophores in human skin serve as photosensitizers involved in skin photocarcinogenesis and photoaging. Absorption of solar photons, particularly in the UVA region, induces the formation of photoexcited states of skin photosensitizers with subsequent generation of reactive oxygen species (ROS), organic free radicals and other toxic photoproducts that mediate skin photooxidative stress. The complexity of endogenous skin photosensitizers with regard to molecular structure, pathways of formation, mechanisms of action, and the diversity of relevant skin targets has hampered progress in this area of photobiology and most likely contributed to an underestimation of the importance of endogenous sensitizers in skin photodamage. Recently, UVA-fluorophores in extracellular matrix proteins formed posttranslationally as a consequence of enzymatic maturation or spontaneous chemical damage during chronological and actinic aging have been identified as an abundant source of light-driven ROS formation in skin upstream of photooxidative cellular stress. Importantly, sensitized skin cell photodamage by this bystander mechanism occurs after photoexcitation of sensitizers contained in skin structural proteins without direct cellular photon absorption thereby enhancing the potency and range of phototoxic UVA action in deeper layers of skin. The causative role of photoexcited states in skin photodamage suggests that direct molecular antagonism of photosensitization reactions using physical quenchers of photoexcited states offers a novel chemopreventive opportunity for skin photoprotection.

Ultraviolet Light (UVB and UVA) Induces the Damage-Responsive Transcription Factor CHOP/gadd153 in Murine and Human Epidermis: Evidence for a Mechanism Specific to Intact Skin

C/EBP-homologous protein (CHOP)/gadd153 (or CHOP) is a transcription factor induced by endoplasmic reticulum (ER) stress. Forcible overexpression of CHOP causes apoptosis in keratinocytes in culture. Here, we asked whether CHOP might be increased in the skin after UVB (280-320 nm) exposure, thus implicating CHOP in sunburn cell (SBC) formation. SKH-1 hairless mice were exposed to a ultraviolet (UV) source (80 mJ per cm2; approximately 74% UVB, approximately 16% UVA), and skin biopsies examined by immunohistology and immunoprecipitation. Compared with non-irradiated epidermis, CHOP expression was significantly increased at 30 min, and reached maximal levels by 24 h. Similar increases in CHOP following UVB exposure were observed in human buttock skin. The time course of CHOP expression preceded SBC formation and another marker of apoptosis, caspase-3 cleavage. Intracellular CHOP accumulated mainly in cytoplasmic and perinuclear locations, with little remaining in the nucleus. To examine mechanisms, cultured keratinocytes were irradiated in vitro and examined by western blotting. Under conditions that eliminated ER stress because of cell handling, CHOP did not accumulate (and was in fact decreased) in the cells. Thus, induction of CHOP in keratinocytes requires factors present only in the native skin. Overall, the data suggest that CHOP participates in adaptive responses of the epidermis following UVB/UVA exposure in vivo.

Quinone-induced Cdc25A inhibition causes ERK-dependent connexin phosphorylation

Gap junctional intercellular communication (GJC) varies during progression of the cell cycle. We propose here that Cdc25A, a dual specificity phosphatase crucial for cell cycle progression, is linked to connexin (Cx) phosphorylation and the modulation of GJC. Inhibition of Cdc25 phosphatases in rat liver epithelial cells employing a 1,4-naphthoquinone-based inhibitor, NSC95397, induced cell cycle arrest, tyrosine phosphorylation of the epidermal growth factor receptor (EGFR), and activation of extracellular signal-regulated kinases ERK-1 and -2. ERK activation was blocked by specific inhibitors of MAPK/ERK kinases 1/2 or of the EGFR tyrosine kinase. An EGFR-dephosphorylation assay suggested that Cdc25A interacts with the EGFR, with inhibition by NSC95397 resulting in activation of the receptor. As a consequence of ERK activation, Cx43 was phosphorylated, resulting in a downregulation of GJC. Loss of GJC was prevented by inhibition of ERK activation. In summary, cell cycle and GJC are connected via Cdc25A and the EGFR-ERK pathway.

Long-term lens organ culture system to determine age-related effects of UV irradiation on the eye lens

Aging of the eye lens represents the life-long accumulation of damage. Factors responsible for age-related cataract are unknown because medical evaluations of aged populations demonstrate a wide range of systemic diseases and medical disorders. There are some main suspected factors, which may contribute to accumulated age-related damage in the eye lens. (1) Diseases, such as diabetes, substantially increase the probability of cataract formation in the age group from 40 to 49, and double or triple this probability for ages 50 to 69. (2) Drugs, including systemic medications such as steroids. (3) Environmental factors, such as UV radiation, heat and electromagnetic radiation. Our study represents an effort to determine the effects of suspected cataractogenic factors on the eye lens. The experiments are performed using a unique long-term lens organ culture system of bovine lenses. In our system it is possible to give controlled amounts of insult and monitor changes in lens optical quality throughout the culture period of 8-15 days. The optical properties, monitored in association with biochemical analysis of lens epithelium, cortex and nuclear samples, help in determining the mechanisms of cataract formation. The present study investigates mechanisms by which UV-A radiation at 365 nm causes damage to the lens. It is believed that solar radiation is one of the major environmental factors involved in lens cataractogenesis. Bovine lenses were placed in our special culture cells for pre-incubation of 24 hr followed by irradiation of 29 or 33 J cm(-2). The lenses were maintained in the cells during irradiation. After irradiation, lens optical quality was monitored throughout the culture period and lens epithelium was taken for enzyme analysis. Using the culture system we learned that: (a) young lenses (less than one-year-old) are less sensitive to UV radiation than 3-year-old lenses; (b) the lenses have the ability to recover in organ culture conditions; (c) applying the insult in one step results in less damage than dividing the same insult in 4 steps with 24 hr interval between each one; and (d) the damage from UV is greater if the intervals between each irradiation stage are insufficient to permit full recovery.

Human αA- and αB-crystallins prevent UVA-induced apoptosis through regulation of PKCα, RAF/MEK/ERK and AKT signaling pathways

AlphaA- and alphaB-crystallins are distinct antiapoptotic regulators. Regarding the antiapoptotic mechanisms, we have previously demonstrated that under staurosporine treatment, HalphaA- and HalphaB-crystallins can interact with Bax and Bcl-XS, proapoptotic members of the Bcl-2 family, to sequester their translocation into mitochondria, and thus prevent the staurosporine-induced apoptosis. In the present study, we further compared the anti-apoptotic mechanisms of HalphaA- and HalphaB-crystallin in preventing human lens epithelial cells from UVA-induced apoptosis. UVA-irradiation of human lens epithelial cells turned on the apoptotic death program. Moreover, associated with the activation of the death program, UVA also activated the RAF/MEK/ERK signaling pathway. In contrast, p38 kinase and JNK1/2 signaling pathways were not activated. Inhibition of the RAF/MEK/ERK pathway by a dominant negative mutant RAF1 greatly attenuated UVA-induced apoptosis. Expression of the exogenous human alphaB-crystallin prevented UVA-induced activation of RAF/MEK/ERK pathway and thus substantially abrogated UVA-induced apoptosis. In contrast, expression of the exogenous human alphaA-crystallin did not prevent UVA-induced activation of RAF/MEK/ERK pathway. Instead, it activated AKT kinase pathway to promote survival and thus counteracted the UVA-induced apoptosis. Together, our results for the first time reveal that by regulating multiple signaling pathways the two alpha-crystallins can prevent stress-induced apoptosis through different mechanisms.

Epidermal growth factor receptor is a common mediator of quinone-induced signaling leading to phosphorylation of connexin-43: role of glutathione and tyrosine phosphatases

Rat liver epithelial cells were exposed to three quinones with different properties: menadione (2-methyl-1,4-naphthoquinone, vitamin K3), an alkylating as well as redox-cycling quinone, the strongly alkylating p-benzoquinone (BQ), and the non-arylating redox-cycler, 2,3-dimethoxy-1,4-naphthoquinone (DMNQ). All three quinones induced the activation of extracellular signal-regulated kinase (ERK) 1 and ERK 2 via the activation of epidermal growth factor receptor (EGFR) and MAPK/ERK kinases (MEK) 1/2. ERK activation resulted in phosphorylation at Ser-279 and Ser-282 of the gap junctional protein, connexin-43, known to result in the loss of gap junctional intercellular communication. Another EGFR-dependent pathway was stimulated, leading to the activation of the antiapoptotic kinase Akt via phosphoinositide 3-kinase. The activation of EGFR-dependent signaling by these quinones was by different mechanisms: (i) menadione, but not BQ or DMNQ, inhibited a protein-tyrosine phosphatase regulating the EGFR, as concluded from an EGFR dephosphorylation assay; (ii) although menadione-induced activation of ERK was unimpaired by pretreatment of cells with N-acetyl cysteine, activation by BQ and DMNQ was prevented; (iii) cellular glutathione (GSH) levels were strongly depleted by BQ. The mere depletion of GSH by application of diethyl maleate EGFR-dependently activated ERK and Akt, thus mimicking BQ effects. GSH levels were only moderately decreased by menadione and not affected by DMNQ. In summary, EGFR-dependent signaling was mediated by protein-tyrosine phosphatase inactivation (menadione), GSH depletion (BQ), and redox-cycling (DMNQ), funneling into the same signaling pathway.