Hydrogen peroxide in the eye lens: radioisotopic determination

Oxidative damage to βL-crystallin in vitro by iron compounds formed in physiological buffers

β_L-crystallin aggregation due to oxidative damage in the presence of H₂O₂ and ferric chloride was studied in-vitro under conditions close to physiological. It was shown that the protein aggregation characterized by the nucleation time and the aggregation rate significantly depended on the composition of the isoosmotic buffers used, and decreased in the series HEPES buffer > Tris buffer > PBS. Ferric chloride at neutral pH was converted into water-insoluble iron hydroxide III (≡Fe^IIIOH). According to the data of scanning electron microscopy the ≡Fe^IIIOH particles formed in HEPES buffer, Tris buffer, and PBS practically did not differ in structure. However, the sizes of ≡Fe^IIIOH floating particles measured by dynamic light scattering differed significantly and were 44 ± 28 nm, 93 ± 66 nm, 433 ± 316 nm (Z_aver ± SD) for HEPES buffer, Tris buffer, and PBS, respectively. It was found by the spin trap method that the ability of ≡Fe^IIIOH to decompose H₂O₂ with the formation of a ^•OH decreases in the series HEPES buffer, Tris buffer, and PBS. The authors suggest that the ability to generate ^•OH during the decomposition of H₂O₂ is determined by the total surface area of ≡Fe^IIIOH particles, which significantly depends on the composition of the buffer in which these particles are formed.

Role of the ubiquitin-proteasome in protein quality control and signaling: implication in the pathogenesis of eye diseases

The ubiquitin-proteasome pathway (UPP) plays important roles in many cellular functions, such as protein quality control, cell cycle control, and signal transduction. The selective degradation of aberrant proteins by the UPP is essential for the timely removal of potential cytotoxic damaged or otherwise abnormal proteins. Conversely, accumulation of the cytotoxic abnormal proteins in eye tissues is etiologically associated with many age-related eye diseases such as retina degeneration, cataract, and certain types of glaucoma. Age- or stress-induced impairment or overburdening of the UPP appears to contribute to the accumulation of abnormal proteins in eye tissues. Cell cycle and signal transduction are regulated by the conditional UPP-dependent degradation of the regulators of these processes. Impairment or overburdening of the UPP could also result in dysregulation of cell cycle control and signal transduction. The consequences of the improper cell cycle and signal transduction include defects in ocular development, wound healing, angiogenesis, or inflammatory responses. Methods that enhance or preserve UPP function or reduce its burden may be useful strategies for preventing age-related eye diseases.

Correlation between hyper-sensitivity to hydrogen peroxide and low defense against Ca(2+) influx in cataractogenic lens of Ihara cataract rats

Our previous studies have demonstrated that lipid peroxidation in the lenses of hereditary cataract model rats (Ihara cataract rat (ICR)/f rats) caused a dysfunction in Ca(2+) regulation. In the present study, we investigated the effect of in vitro hydrogen peroxide (H(2)O(2)) stimulation on lipid peroxide (LPO) and the activities of sarco-/endoplasmic reticulum and plasma membrane Ca(2+)-ATPase (SERCA and PMCA) in the ICR/f rat lenses. An increase in LPO level and decreases in the SERCA and PMCA activities were observed with increasing H(2)O(2) concentration, and pretreatment with diethyldithiocarbamate, a potent radical scavenger, attenuated these changes in normal and ICR/f rat lenses. The glutathione levels, glutathione peroxidase and glutathione reductase activities are significantly lower in ICR/f rat lenses than in normal rat lenses. Furthermore, we presented as two kinetic parameters such as DP (defense point) and K(s) (reactive constant) analyzed from above various biological responses vs. H(2)O(2) concentration-profile curves using a one-exponential equation. The DPs for LPO, SERCA and PMCA in ICR/f rat lenses is lower than in normal rat lenses. In contrast to the results in DP, the K(s) for LPO, SERCA and PMCA in ICR/f rat lenses is higher than in normal rat lenses. In addition, the closed relationship of was observed between DP and K(s) for LPO, SERCA and PMCA. These results show that the resistance to H(2)O(2) in the ICR/f rat lenses is lower than that of normal rats. The DP and K(s) values can provide an useful information for resistances to various stimuli in cells and tissues.

Sustained oxidative stress inhibits NF-kappaB activation partially via inactivating the proteasome

NF-kappaB is a family of important transcription factors involved in many cellular functions, such as cell survival, proliferation, and stress responses. Many studies indicate that NF-kappaB is a stress-sensitive transcription factor and its activation is regulated by reactive oxygen species. In previous studies, we and others demonstrated that this transcription factor can be activated by transient oxidative stress. However, the effects of sustained oxidative stress on NF-kappaB activation are not clear. The objective of this study was to determine the effects of sustained oxidative stress on NF-kappaB activation and to elucidate the signaling events affected by sustained oxidative stress. Human lens epithelial cells (HLEC) that were subjected to 4 h of continuous influx of hydrogen peroxide were used to investigate the effects of sustained oxidative stress on NF-kappaB activation. The data showed that, unlike transient oxidative stress, sustained exposure of HLEC to physiologically relevant levels of H(2)O(2) (50-100 microM for 4 h) did not induce the degradation of I-kappaB and activation of NF-kappaB, but attenuated TNFalpha-induced degradation of I-kappaB and activation of NF-kappaB. Sustained exposure of HLEC to these levels of H(2)O(2) also inactivated proteasome activity by 50-80%. Consistent with the role of the proteasome in degradation of I-kappaB and activation of NF-kappaB, treatment of HLEC with proteasome inhibitors also attenuated TNFalpha-induced I-kappaB degradation and NF-kappaB activation. The data also indicate that activation of NF-kappaB is essential for the cells to recover from oxidative stress. Inhibiting NF-kappaB activation during recovery from transient oxidative stress significantly reduced the cell viability. Together, these data indicate that sustained oxidative stress may inactivate the proteasome and subsequently inhibit NF-kappaB activation by impeding the degradation of I-kappaB. The oxidative inactivation of the proteasome and subsequent impairment of NF-kappaB activation may contribute to the death of lens epithelial cells, a common feature associated with cataract.

The effect of hydrogen peroxide on sarco/endoplasmic and plasma membrane calcium ATPase gene expression in cultured human lens epithelial cells

The loss of calcium homeostasis in the lens of the eye appears to be a factor contributing to lens opacity. In the human lens, calcium homeostasis depends on the Ca²⁺-ATPase pumps found only in the epithelium. A plasma membrane calcium pump, PMCA2 is upregulated in human cataractous lenses. To determine if oxidation caused the plasma membrane Ca²⁺-ATPases (PMCA) or sarcoplasmic/endoplasmic Ca²⁺-ATPases (SERCA) to become upregulated, we cultured a human lens epithelial cell line, in the presence of hydrogen peroxide. We observed an increase in PMCA1, PMCA2 SERCA2b and SERCA3 mRNA levels and protein expression with increasing hydrogen peroxide concentrations and treatment times. Hydrogen peroxide caused a rise in the intracellular calcium which could be an initiating factor in the concerted upregulation of PMCA1 and SERCA3. Our data support the idea that oxidative stress could contribute to a selective rise in PMCA/SERCA expression in human cataractous lenses.

Function of the ubiquitin proteolytic pathway in the eye

The ubiquitin pathway (UP) is involved in regulation of many essential cellular processes usually by the degradation of regulators of these processes. For example the UP is involved in regulation of cell cycle, proliferation, differentiation, organogenesis, development, and signal transduction in the lens and retina. A functional UP has also been documented in the cornea. Upon aging or exposure to stress there is an accumulation of damaged proteins, including ubiquitinated proteins, in the lens and retina. Some of these proteins may be cytotoxic. Thus, an active UP may be required to avoid such age and disease-related accumulation of damaged proteins. In this review we will explain the biochemistry of the UP and we will document the most important studies regarding UP function in the lens, retina and cornea.

Removal of oxidatively damaged proteins from lens cells by the ubiquitin-proteasome pathway

Understanding how oxidized proteins are removed is important since accumulation of such damaged proteins is causally related to cellular and organismic dysfunction, disease and aging. Previous work showed that activity of the ubiquitin-proteasome pathway (UPP) in lens cells increased during recovery from oxidative stress ( Shang et al., 1997b : J. Biol. Chem. 272, 23086-93). In this study we sought to determine if the up-regulation of the UPP during recovery from oxidative stress has a role in selective removal of oxidized proteins from the cells. In cells which were not exposed to peroxide, inhibition of the proteasome with MG132 or clasto-lactacystin beta-lactone had little effect on protein carbonyl levels. However, inhibition of the proteasome in the 20 microM peroxide-treated cells caused an approximate 60% increase in levels of protein carbonyl and an approximate 100% increase in levels of ubiquitin conjugates. The carbonyl-containing proteins that accumulated in the presence of the proteasome inhibitor co-localized with high molecular mass ubiquitin-protein conjugates. Furthermore, isolated carbonyl-containing proteins from H2O2-treated cells were ubiquitinated, and ubiquitin-conjugates were enriched with carbonyl-containing proteins. The diminished effect of proteasome inhibitors on protein carbonyl levels, together with the robust increase in ubiquitin-protein conjugates and accompanied increases in oxidized proteins, upon exposure to 60 microM H2O2 indicate that the proteasomal step of the UPP is more susceptible to oxidative inactivation than the ubiquitination step. In fact, oxidative stress is associated with a hyperactivation of the ubiquitin-activating enzyme. These data indicate that the UPP plays a role in removal of oxidatively damaged proteins from cells and that attenuation of the UPP activity may result in cytotoxic accumulation of damaged proteins, possibly including the ubiquitinated forms.

Activity of ubiquitin-dependent pathway in response to oxidative stress. Ubiquitin-activating enzyme is transiently up-regulated

Relations between the ubiquitin pathway and cellular stress have been noted, but data regarding responses of the ubiquitin pathway to oxidative stress are scanty. This paper documents the response of this pathway to oxidative stress in lens cells. A brief exposure of lens epithelial cells to physiologically relevant levels of H2O2 induces a transient increase in activity of the ubiquitin-dependent pathway. Ubiquitin conjugation activity was maximal and increased 3. 5-9.2-fold over the activity noted in untreated cells by 4 h after removal of H2O2. By 24 h after removal of H2O2, ubiquitin conjugation activity returned to the level noted in untreated cells. In parallel to the changes in ubiquitin conjugation activity, the activity of ubiquitin-activating enzyme (E1), as determined by thiol ester formation, increased 2-6.7-fold during recovery from oxidation. Addition of exogenous E1 resulted in an increase in ubiquitin conjugation activity and in the levels of ubiquitin carrier protein (E2)-ubiquitin thiol esters in both the untreated cells and the H2O2-treated cells. These data suggest that E1 is the rate-limiting enzyme in the ubiquitin conjugation process and that the increases in ubiquitin conjugation activity which are induced upon recovery from oxidation are primarily due to increased E1 activity. The oxidation- and recovery-induced up-regulation of E1 activity is primarily due to post-synthetic events. Substrate availability and up-regulation of E2 activities also appear to be related to the enhancement in ubiquitinylation upon recovery from oxidative stress. The oxidation-induced increases in ubiquitin conjugation activity were associated with an increase in intracellular proteolysis, suggesting that the transient increase in ubiquitinylation noted upon recovery from oxidative stress may play a role in removal of damaged proteins from the cells.

Prevention of cataracts by nutritional and metabolic antioxidants

Among aging disabilities, the one associated with the progressive decline of vision is functionally most disadvantageous. Cataracts are one of the more common causes of such visual disability. Several predisposing factors have been identified in the genesis of this disease. While it is perhaps a multifactorial process, significant developments have taken place in recent years suggesting that oxygen radicals are involved in the development of this aging manifestation. Antioxidant enzymes, such as catalase and superoxide dismutase, have been demonstrated to protect the lens cell membrane from oxidative stress as reflected by the prevention of the Na(+)-K(+)-ATPase-dependent pump deterioration due to oxyradical-dependent oxidation of its proteins and lipids. From the nutritional point of view, antioxidants such as ascorbate and vitamin E also offer significant protection to the lens against damage due to oxidative stress. Evidence regarding the protective effect of these nutrients has been based on lens organ culture studies in the presence of active oxygen, generated photochemically as well as enzymatically. The experiment involving photochemical environs simulate the status of the eye during the photopic vision. In vivo, the effectiveness of ascorbate against cataracts has been tested in rat pups developing cataracts under the oxidative influence of sodium selenite. Certain antioxidants produced metabolically also may be useful in protecting against cataracts. Pyruvate produced in glucose metabolism seems to be an important antioxidant. The efficacy of this compound has been tested within in vitro organ culture as well as in vivo, the latter experiments being done with selenite-treated rats. There is a hope that these and other nutritional and metabolic antioxidants may one day be useful in delaying or even preventing cataract formation in human beings.