Study of the interaction between triplet riboflavin and the alpha-, betaH- and betaL-crystallins of the eye lens

Structural and functional alteration of human αA-crystallin after exposure to full spectrum solar radiation and preventive role of lens antioxidants

The chronically exposure of eye lenses to ultra violet and visible light of the solar radiation is an important risk factor for development of the senile cataract diseases. Various photosensitizer molecules including riboflavin (RF) play a significant role in photo-oxidative damages of lens proteins underlying development of opacity in the lenticular tissues. In the current study, RF-mediated photo-oxidation of human αA-crystallin (αA-Cry) was assessed using SDS-PAGE analysis, dynamic light scattering and other spectroscopic assessments. The RF-photosensitized reactions led to non-disulfide covalent cross-linking, oligomerization and significant structural changes in αA-Cry. The photo-damaging of αA-Cry under solar radiation was also accompanied by the reduction in both Trp and Tyr fluorescence intensities which followed by the formation of new photosensitizer chromophores. The solvent exposed hydrophobic patches, secondary structures and chaperone-like activity of αA-Cry were significantly altered after exposure to the solar radiation in the presence of RF. Although glutathione and ascorbate were capable to partially protect the photo-induced structural damages of human αA-Cry, they also disrupted its chaperone function when co-exposed with this protein to the solar radiation. Also, the most promising data were obtained with cysteine which its availability in the lenticular tissues is a rate limiting factor for the biosynthesis of glutathione. Overall our results suggest that glutathione and ascorbate, as the major anti-oxidant compounds within lenticular tissues, demonstrate controversial effect on structure and chaperone-like activity of human αA-Cry. Elucidation of this effect may demand further experiments.

An endogenous tryptophan photo-product, FICZ, is potentially involved in photo-aging by reducing TGF-β-regulated collagen homeostasis

BACKGROUND

Persistent ultraviolet (UV) radiation in the form of sunlight causes photo-aging of the skin by reducing the production of type I collagen, the major constituent of the extracellular matrix of the dermis. Transforming growth factor (TGF)-β transforms dermal fibroblasts into α2-smooth muscle actin (ACTA2)-expressing myofibroblasts. Myofibroblasts produce a precursor form of type I collagen, type I procollagen (collagen I), consisting of pro-alpha1 (produced by the COL1A1 gene) and pro-alpha2 chains (produced by the COL1A2 gene). Smad2/3 is a key downstream molecule of TGF-β signaling. The mechanisms through which UV inhibits collagen I synthesis are not fully understood. 6-Formylindolo[3,2-b]carbazole (FICZ) is an endogenous tryptophan photo-metabolite generated by UV irradiation. FICZ is well known as a high-affinity ligand for aryl hydrocarbon receptor (AHR). However, the physiological roles of FICZ in photo-aging have yet to be addressed.

OBJECTIVE

To evaluate the effects of FICZ on the TGF-β-mediated ACTA2 and collagen I expression in normal human dermal fibroblasts (NHDFs).

METHODS

Quantitative real-time polymerase chain reaction and western blot analysis were performed to determine the expression of ACTA2, COL1A1, and COL1A2 in NHDFs with or without FICZ and TGF-β. The phosphorylated Smad2/3 (pSmad2/3) protein levels in cytoplasmic or nuclear portions were investigated by western blot analysis. Immunofluorescence staining was conducted to evaluate pSmad2/3 localization, and F-actin staining with phalloidin was performed to visualize actin polymerization in myofibroblasts. The actions of FICZ on the TGF-β-mediated collagen I expression and nuclear translocation of pSmad2/3 were analyzed in the presence of selective AHR antagonists or in AHR-knockdown NHDFs.

RESULTS

We found that FICZ significantly inhibited the TGF-β-induced upregulation of mRNA and protein levels of ACTA2 and collagen I and actin polymerization in myofibroblasts. FICZ did not disturb the phosphorylation of Smad2/3. Notably, FICZ reduced the expression of pSmad2/3 in the nucleus, while it increased that in the cytoplasm, suggesting that it inhibits the nuclear translocation of pSmad2/3 induced by TGF-β. The inhibitory actions of FICZ on the TGF-β-mediated collagen I expression and nuclear translocation of pSmad2/3 were independent of AHR signaling. Another endogenous AHR agonist, kynurenine, also inhibited the TGF-β-mediated ACTA2 and collagen I upregulation in NHDFs in an AHR-independent manner; however, its effects were insignificant in comparison with those of FICZ.

CONCLUSIONS

These findings suggest that the endogenous photo-product FICZ may be a key chromophore that involves in photo-aging. Downregulation of FICZ signaling is thus a potential strategy to protect against photo-aging.

Aggregation of α-crystallins in kynurenic acid-sensitized UVA photolysis under anaerobic conditions

Extensive protein aggregation is the major outcome of kynurenic acid-sensitized photolysis of α-crystallin under anaerobic conditions. The main lens antioxidants ascorbate and glutathione effectively inhibit the protein aggregation.

Middle infrared radiation induces G2/M cell cycle arrest in A549 lung cancer cells

There were studies investigating the effects of broadband infrared radiation (IR) on cancer cell, while the influences of middle-infrared radiation (MIR) are still unknown. In this study, a MIR emitter with emission wavelength band in the 3-5 µm region was developed to irradiate A549 lung adenocarcinoma cells. It was found that MIR exposure inhibited cell proliferation and induced morphological changes by altering the cellular distribution of cytoskeletal components. Using quantitative PCR, we found that MIR promoted the expression levels of ATM (ataxia telangiectasia mutated), ATR (ataxia-telangiectasia and Rad3-related and Rad3-related), TP53 (tumor protein p53), p21 (CDKN1A, cyclin-dependent kinase inhibitor 1A) and GADD45 (growth arrest and DNA-damage inducible), but decreased the expression levels of cyclin B coding genes, CCNB1 and CCNB2, as well as CDK1 (Cyclin-dependent kinase 1). The reduction of protein expression levels of CDC25C, cyclin B1 and the phosphorylation of CDK1 at Thr-161 altogether suggest G(2)/M arrest occurred in A549 cells by MIR. DNA repair foci formation of DNA double-strand breaks (DSB) marker γ-H2AX and sensor 53BP1 was induced by MIR treatment, it implies the MIR induced G(2)/M cell cycle arrest resulted from DSB. This study illustrates a potential role for the use of MIR in lung cancer therapy by initiating DSB and blocking cell cycle progression.

Formation of 3-nitrotyrosine by riboflavin photosensitized oxidation of tyrosine in the presence of nitrite

The results of the present investigation show the susceptibility of tyrosine to undergo visible light-induced photomodification to 3-nitrotyrosine in the presence of nitrite and riboflavin, as sensitizer. By changing H2O by D2O, it could be established that singlet oxygen has a minor role in the reaction. The finding that nitration of tyrosine still occurred to a large extent under anaerobic conditions indicates that the process proceeds mainly through a type I mechanism, which involves the direct interaction of the excited state of riboflavin with tyrosine and nitrite to give tyrosyl radical and nitrogen dioxide radical, respectively. The tyrosyl radicals can either dimerize to yield 3,3'-dityrosine or combine with nitrogen dioxide radical to form 3-nitrotyrosine. The formation of 3-nitrotyrosine was found to increase with the concentration of nitrite added and was accompanied by a decrease in the recovery of 3,3'-dityrosine, suggesting that tyrosine nitration competes with dimerization reaction. The riboflavin photosensitizing reaction in the presence of nitrite was also able to induce nitration of tyrosine residues in proteins as revealed by the spectral changes at 430 nm, a characteristic absorbance of 3-nitrotyrosine, and by immunoreactivity using 3-nitrotyrosine antibodies. Since riboflavin and nitrite are both present endogenously in living organism, it is suggested that this pathway of tyrosine nitration may potentially occur in tissues and organs exposed to sunlight such as skin and eye.

The protective function of hydrogen sulfide for lysozyme against riboflavin-sensitized photo-oxidation

Hydrogen sulfide is the third endogenous signaling gasotransmitter, following nitric oxide and carbon monoxide. Recent studies showed that hydrogen sulfide could alleviate many diseases which were related to the oxidative damage of tissues. It reminded us that hydrogen sulfide might serve as an antioxidant to reduce oxidative pressure. This study showed that hydrogen sulfide protected lysozyme from photo-oxidation induced by riboflavin (RF). Laser flash photolysis was used to explore the mechanisms of antioxidant activity of hydrogen sulfide. The scavenging effects of hydrogen sulfide on the triplet state of riboflavin (³RF(*)) and radicals of tryptophan and tyrosine (TyrO· and TrpN·) were attributed to the protection of lysozyme from photo-oxidation. The results suggested that hydrogen sulfide could serve as an antioxidant in alleviation of oxidative pressure.

UVA-induced calcium oscillations in rat mast cells

UVA is a major bio-active component in solar irradiation, and is shown to have immunomodulatory and anti-inflammatory effects. The detailed molecular mechanism of UVA action in regard to calcium signaling in mast cells, however, is not fully understood. In this study, it was found that UVA induced ROS formation and cytosolic calcium oscillations in individual rat mast cells. Exogenously added H2O2 and hypoxanthine/xanthine oxidase (HX/XOD) mimicked UVA effects on cytosolic calcium increases. Regular calcium oscillation induced by UVA irradiation was inhibited completely by the phosphatidylinositol-specific phospholipase C inhibitor U73122, but U73343 was without effect. Tetrandrine, a calcium entry blocker, or calcium-free buffer abolished UVA-induced calcium oscillations. L-type calcium channel blocker nifedipine and stores-operated calcium channel blocker SK&F96365 had no such inhibitory effect. ROS induction by UVA was abolished after pre-incubation with anti-oxidant NAC or with NAD(P)H oxidase inhibitor DPI; such treatment also made UVA-induced calcium oscillation to disappear. UVA irradiation did not increase mast cell diameter, but it made mast cell structure more granular. Spectral confocal imaging revealed that the emission spectrum of the endogenous fluorophore in single mast cell contained a sizable peak which corresponded to that of NAD(P)H. Taken together, these data suggest that UVA in rat mast cells could activate NAD(P)H oxidase, to produce ROS, which in turn activates phospholipase C signaling, to trigger regular cytosolic calcium oscillation.

Photoreduction of oxoisoaporphine dyes by amines: transient-absorption and semiempirical quantum-chemical studies

Photoreduction by amines of oxoisoaporphine dyes occurs via a stepwise mechanism of electron-proton-electron transfer that leads to the metastable N-hydrogen oxoisoaporphine anion. During photoreduction that occurs from the triplet manifold of the oxoisoaporphine, a radical ion A(-)(*), a neutral-hydrogenated radical A-NH(*), and the metastable ion A-NH(-) of the oxoisoaporphine are formed. We present time-resolved spectroscopic data and quantum mechanical semiempirical PM3 and ZINDO/S results for the transient species formed during the flash photolysis of oxoisoaporphines in the presence of amines. These calculations reproduce adequately the experimental spectra of the triplet-triplet absorption near 450 nm, and that of neutral hydrogenated radical of the studied oxoisoaporphines centered at 390 nm. A transient absorption observed near 490 nm, for all of the studied systems, was explained by considering the formation of radical ion pair between the radical anion of the oxoisoaporphine, A(-)(*), and the radical cation of the amine, whose ZINDO/S calculated spectra generate the strongest transition near the experimentally observed absorption maximum at 490 nm, supporting the formation of a radical ion pair complex as the first step of the photoreduction.

Direct Detection of Singlet Oxygen Generated by UVA Irradiation in Human Cells and Skin

UVA light produces deleterious biological effects in which singlet oxygen plays a major role. These effects comprise a significant risk of carcinogenesis in the skin and cataract formation of the eye lens. Singlet oxygen is generated by UVA light absorption in endogenous molecules present in the cells. To elucidate the primary processes and sources of singlet oxygen in tissue, it is a major goal to uncover the hidden process of singlet oxygen generation, in particular in living tissue. When exposing keratinocytes or human skin in vivo to UVA laser light (355 nm) at 6 J/cm2, we measured the luminescence of singlet oxygen at 1,270 nm. This is a positive and direct proof of singlet oxygen generation in cells and skin by UVA light. Moreover, a clear signal of singlet oxygen luminescence was detected in phosphatidylcholine suspensions (water or ethanol) irradiated by UVA. Oxidized products of phosphatidylcholine are the likely chromophores because phosphatidylcholine itself does not absorb at 355 nm. The signal intensity was reduced by mannitol or super oxide dismutase. Additionally, the monochromatic UVA irradiation at 355 nm leads to upregulation of the key cytokine IL-12. This affects the balance of UV radiation on the immune system, which is comparable to effects of broadband UVA irradiation.

Double roles of hydroxycinnamic acid derivatives in protection against lysozyme oxidation

Oxidative damage to protein has been implicated in a number of diseases. Much interest has been focused on preventing oxidative damage to protein. Here we showed that hydroxycinnamic acid derivatives (HCA) were able to inhibit the cross-linking of protein induced by riboflavin-mediated photooxidation. HCA were also found to strongly protect lysozyme from gamma rays irradiation. The antioxidative properties of HCA were further studied by laser flash photolysis. Mechanism of antioxidant activities of HCA on lysozyme oxidation was discussed. HCA were found to protect protein against oxidation by scavenging oxidizing species and repairing the damaged protein.

Effects, quenching mechanisms, and kinetics of water soluble compounds in riboflavin photosensitized oxidation of milk

To protect the nutrient and flavor stability of milk under light, the effects of 0, 0.01, 0.03, and 0.05 M 1,4-diazabicyclo[2,2,2]octane (DABCO) and 2,5-dimethylfuran (DMF) on the riboflavin photosensitized oxidation of milk were studied. The oxidation of milk was studied by measuring the headspace oxygen in sample bottles after 3 h of light exposure at 3000 lux. As the concentration of DABCO and DMF, which are water soluble compounds, increased in the sample from 0, 0.01, and 0.03 to 0.05 M, the depleted headspace oxygen content significantly decreased (P < 0.05). Steady state kinetic studies of singlet oxygen oxidation showed that the antioxidant activity of DABCO and DMF was due to singlet oxygen quenching. The reaction rate constant of singlet oxygen with milk fat was 8.1 x 10(5) M(-1) s(-1). Total singlet oxygen quenching rates of DABCO and DMF were 1.5 x 10(7) and 2.6 x 10(7) M(-1) s(-1), respectively. DABCO and DMF could be used to slow the reaction between singlet oxygen and milk components to protect nutrients, especially riboflavin, and to improve the oxidative stability of milk fat during storage or processing under light.

Singlet oxygen generation by UVA light exposure of endogenous photosensitizers

UVA light (320-400 nm) has been shown to produce deleterious biological effects in tissue due to the generation of singlet oxygen by substances like flavins or urocanic acid. Riboflavin, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), beta-nicotinamide adenine dinucleotide (NAD), and beta-nicotinamide adenine dinucleotide phosphate (NADP), urocanic acid, or cholesterol in solution were excited at 355 nm. Singlet oxygen was directly detected by time-resolved measurement of its luminescence at 1270 nm. NAD, NADP, and cholesterol showed no luminescence signal possibly due to the very low absorption coefficient at 355 nm. Singlet oxygen luminescence of urocanic acid was clearly detected but the signal was too weak to quantify a quantum yield. The quantum yield of singlet oxygen was precisely determined for riboflavin (PhiDelta = 0.54 +/- 0.07), FMN (PhiDelta = 0.51 +/- 0.07), and FAD (PhiDelta = 0.07 +/- 0.02). In aerated solution, riboflavin and FMN generate more singlet oxygen than exogenous photosensitizers such as Photofrin, which are applied in photodynamic therapy to kill cancer cells. With decreasing oxygen concentration, the quantum yield of singlet oxygen generation decreased, which must be considered when assessing the role of singlet oxygen at low oxygen concentrations (inside tissue).

Photosensitizing effect of riboflavin, lumiflavin, and lumichrome on the generation of volatiles in soy milk

Lumichrome and lumiflavin were formed from riboflavin under light. pH had a significant influence on the formation of lumichrome and lumiflavin from riboflavin. Lumichrome was the only major product from riboflavin under neutral or acidic pH values. Lumiflavin was also formed from riboflavin in basic pH. The maximum concentration of lumiflavin from 100 microM riboflavin at pH 8.5 was 30.9 microM, and it was reached after 2 h of exposure at 1500 lux. The maximum concentration of lumichrome formed from 100 microM riboflavin at pH 4.5, 6.5, or 8.5 was 79.9, 58.7, and 73.1 microM, respectively, after 8, 6, or 2 h of light exposure. The formation of lumichrome and lumiflavin from riboflavin was due to the type I mechanism of the riboflavin photosensitized reaction. Singlet oxygen was also involved in the photosensitized degradation of lumiflavin and lumichrome. The reaction rates of riboflavin, lumiflavin, and lumichrome with singlet oxygen were 9.66 x 10(8), 8.58 x 10(8), and 8.21 x 10(8) M(-1) s(-1), respectively. The headspace oxygen depletion and headspace volatile formation were significant in soy milk containing lumichrome or lumiflavin under light (p < 0.05) and were insignificant (p > 0.05) in the dark. Ascorbic acid could inhibit the total volatile changes of soy milk under light. Soy milk should be protected from light to prevent the photodegradation of riboflavin and the oxidation of soy milk.

Photolysis of riboflavin in aqueous solution: a kinetic study

The kinetics of photolysis of aqueous riboflavin solutions on UV and visible irradiation has been studied in the pH range 1-12 using a specific multicomponent spectrophotometric method for the simultaneous determination of riboflavin and its major photoproducts (formylmethylflavin, lumichrome and lumiflavin). The apparent first-order rate constants for the photodegradation reactions in the pH range have been determined. The log k-pH profiles indicate that riboflavin has maximum photostability around pH 5-6, at which the rate of oxidation-reduction of the molecule is lowest. The cationic and anionic forms of riboflavin are non-fluorescent and less susceptible to photolysis than the non-ionised molecule as indicated by the relatively slow rates below pH 3.0 and above pH 10.0. The rate of photolysis is increased up to 80-fold at pH 10.0, compared to that at pH 5.0, due to increase in redox potentials with an increase in pH and consequently the ease with which the molecule is oxidised. The increase in rate at pH 3.0, compared to that at pH 5.0, appears to be due to the involvement of the excited singlet state as well as the triplet state in riboflavin degradation. The apparent first-order rate constants for the photolysis of riboflavin at pH 5.0-10.0 with UV and visible radiation are 0.185 x 10(-2) to 13.182 x 10(-2)min(-1) and 0.098 x 10(-2) to 7.762 x 10(-2)min(-1), respectively.

Lens α-Crystallin and Hypericin: A Photophysical Mechanism Explains Observed Lens Damage†¶

Determining whether alpha-crystallin (the major lens protein) affects the photophysics of hypericin, a photosensitizing agent found in various plants, such as St. John's Wort, is important. Hypericin shows promise in cancer and human immunodeficiency virus therapy but may harm individuals taking St. John's Wort extracts (for mild to moderate depression). Hypericin causes hypericism, which is characterized by cellular damage in light-exposed areas. Ocular tissues are at risk for photosensitized damage; thus, we investigated the effects on hypericin photophysics by alpha-crystallin. We measured the transient absorption spectra and the 1270 nm luminescence of singlet (1Deltag) oxygen produced from hypericin in the presence of alpha-crystallin. alpha-Crystallin complexes hypericin, extending the lifetime of its triplet excited state; the Stern-Volmer slope is negative, but not linear, after a saturation curve. Damage to the lens protein by hypericin is known to occur via singlet oxygen, which oxidizes methionine, tryptophan and histidine residues. Binding to alpha-crystallin does not inhibit singlet oxygen formation by hypericin. alpha-Crystallin reacts with singlet oxygen with a rate constant of 1.3 x 10(8) M(-1) s(-1). Thus, we anticipate that hypericin will be an effective photosensitizer in the lens.