Role of glutathione and other endogenous thiols in radiation protection

Pharmacology of drugs used in autoimmune dermatopathies in cats and dogs: A narrative review

Immunosuppressive drugs are the mainstay of treatment for many feline and canine autoimmune skin diseases, either as monotherapy or in combination with other drugs. Treatment with these drugs is often lifelong and may have long-term consequences on the affected animal's overall quality-of-life. Clinicians need to understand the pharmacology of immunosuppressants in planning and executing the treatment regimen for the best possible clinical outcome, as well as reducing the risk of adverse effects. This review paper will focus on the mechanism of action, pharmacokinetics and pharmacodynamics, clinical uses and adverse effects of immunosuppressive drugs used to treat autoimmune dermatoses in cats and dogs. These include glucocorticoids, ciclosporin A, azathioprine, chlorambucil, mycophenolate mofetil, oclacitinib and Bruton's tyrosine kinase inhibitors.

In Vivo Electron Paramagnetic Resonance Molecular Profiling of Tumor Microenvironment upon Tumor Progression to Malignancy in an Animal Model of Breast Cancer

PURPOSE

Hypoxia and acidosis are recognized tumor microenvironment (TME) biomarkers of cancer progression. Alterations in cancer redox status and metabolism are also associated with elevated levels of intracellular glutathione (GSH) and interstitial inorganic phosphate (Pi). This study aims to evaluate the capability of these biomarkers to discriminate between stages and inform on a switch to malignancy.

PROCEDURES

These studies were performed using MMTV-PyMT( +) female transgenic mice that spontaneously develop breast cancer and emulate human tumor staging. In vivo assessment of oxygen concentration (pO2), extracellular acidity (pHe), Pi, and GSH was performed using L-band electron paramagnetic resonance spectroscopy and multifunctional trityl and GSH-sensitive nitroxide probes.

RESULTS

Profiling of the TME showed significant deviation of measured biomarkers upon tumor progression from pre-malignancy (pre-S4) to the malignant stage (S4). For the combined marker, HOP: (pHe × pO2)/Pi, a value > 186 indicated that the tumors were pre-malignant in 85% of the mammary glands analyzed, and when < 186, they were malignant 42% of the time. For GSH, a value < 3 mM indicated that the tumors were pre-malignant 74% of the time, and when > 3 mM, they were malignant 80% of the time. The only marker that markedly deviated as early as stage 1 (S1) from its value in pre-S1 was elevated Pi, followed by a decrease of pHe and pO2 and increase in GSH at later stages.

CONCLUSION

Molecular TME profiling informs on alteration of tumor redox and metabolism during tumor staging. Early elevation of interstitial Pi at S1 may reflect tumor metabolic alterations that demand elevated phosphorus supply in accordance with the high rate growth hypothesis. These metabolic changes are supported by the following decrease of pHe due to a high tumor reliance on glycolysis and increase of intracellular GSH, a major intracellular redox buffer. The appreciable decrease in TME pO2 was observed only at malignant S4, apparently as a consequence of tumor mass growth and corresponding decrease in perfusion efficacy and increase in oxygen consumption as the tumor cells proliferate.

Sulfur Amino Acid Supplementation Abrogates Protective Effects of Caloric Restriction for Enhancing Bone Marrow Regrowth Following Ionizing Radiation

Radiation therapy damages and depletes total bone marrow (BM) cellularity, compromising safety and limiting effective dosing. Aging also strains total BM and BM hematopoietic stem and progenitor cell (HSPC) renewal and function, resulting in multi-system defects. Interventions that preserve BM and BM HSPC homeostasis thus have potential clinical significance. Here, we report that 50% calorie restriction (CR) for 7-days or fasting for 3-days prior to irradiation improved mouse BM regrowth in the days and weeks post irradiation. Specifically, one week of 50% CR ameliorated loss of total BM cellularity post irradiation compared to ad libitum-fed controls. CR-mediated BM protection was abrogated by dietary sulfur amino acid (i.e., cysteine, methionine) supplementation or pharmacological inhibition of sulfur amino acid metabolizing and hydrogen sulfide (H₂S) producing enzymes. Up to 2-fold increased proliferative capacity of ex vivo-irradiated BM isolated from food restricted mice relative to control mice indicates cell autonomy of the protective effect. Pretreatment with H₂S in vitro was sufficient to preserve proliferative capacity by over 50% compared to non-treated cells in ex vivo-irradiated BM and BM HSPCs. The exogenous addition of H₂S inhibited Ten eleven translocation 2 (TET2) activity in vitro, thus providing a potential mechanism of action. Short-term CR or fasting therefore offers BM radioprotection and promotes regrowth in part via altered sulfur amino acid metabolism and H₂S generation, with translational implications for radiation treatment and aging.

Overview on the Effects of N-Acetylcysteine in Neurodegenerative Diseases

N-acetylcysteine (NAC), which is an acetylated cysteine compound, has aroused scientific interest for decades due to its important medical applications. It also represents a nutritional supplement in the human diet. NAC is a glutathione precursor and shows antioxidant and anti-inflammatory activities. In addition to the uses quoted in the literature, NAC may be considered helpful in therapies to counteract neurodegenerative and mental health diseases. Furthermore, this compound has been evaluated for its neuroprotective potential in the prevention of cognitive aging dementia. NAC is inexpensive, commercially available and no relevant side effects were observed after its administration. The purpose of this paper is to give an overview on the effects and applications of NAC in Parkinson's and Alzheimer's disorders and in neuropathic pain and stroke.

Peroxiredoxins in Colorectal Cancer: Predictive Biomarkers of Radiation Response and Therapeutic Targets to Increase Radiation Sensitivity?

Colorectal cancer (CRC) is the third most common cancer in the Western world, with one-third of cases located in the rectum. Preoperative radiotherapy is the standard of care for many patients with rectal cancer but has a highly variable response rate. The ability to predict response would be of great clinical utility. The response of cells to ionizing radiation is known to involve immediate damage to biomolecules and more sustained disruption of redox homeostasis leading to cell death. The peroxiredoxins are an important group of thiol-dependent antioxidants involved in protecting cells from oxidative stress and regulating signaling pathways involved in cellular responses to oxidative stress. All six human peroxiredoxins have shown increased expression in CRC and may be associated with clinicopathological features and tumor response to ionizing radiation. Peroxiredoxins can act as markers of oxidative stress in various biological systems but they have not been investigated in this capacity in CRC. As such, there is currently insufficient evidence to support the role of peroxiredoxins as clinical biomarkers, but it is an area worthy of investigation. Future research should focus on the in vivo response of rectal cancer to radiotherapy and the redox status of peroxiredoxins in rectal cancer cells, in order to predict response to radiotherapy. The peroxiredoxin system is also a potential therapeutic target for CRC.

N-Acetyl-L-cysteine protects thyroid cells against DNA damage induced by external and internal irradiation

We evaluated the effect of the antioxidant N-acetyl-L-cysteine (NAC) on the levels of reactive oxygen species (ROS), DNA double strand breaks (DSB) and micronuclei (MN) induced by internal and external irradiation using a rat thyroid cell line PCCL3. In internal irradiation experiments, ROS and DSB levels increased immediately after ¹³¹I addition and then gradually declined, resulting in very high levels of MN at 24 and 48 h. NAC administration both pre- and also post-¹³¹I addition suppressed ROS, DSB and MN. In external irradiation experiments with a low dose (0.5 Gy), ROS and DSB increased shortly and could be prevented by NAC administration pre-, but not post-irradiation. In contrast, external irradiation with a high dose (5 Gy) increased ROS and DSB in a bimodal way: ROS and DSB levels increased immediately after irradiation, quickly returned to the basal levels and gradually rose again after >24 h. The second phase was in parallel with an increase in 4-hydroxy-2-nonenal. The number of MN induced by the second wave of ROS/DSB elevations was much higher than that by the first peak. In this situation, NAC administered pre- and post-irradiation comparably suppressed MN induced by a delayed ROS elevation. In conclusion, a prolonged ROS increase during internal irradiation and a delayed ROS increase after external irradiation with a high dose caused serious DNA damage, which were efficiently prevented by NAC. Thus, NAC administration even both after internal or external irradiation prevents ROS increase and eventual DNA damage.

Janus-faced tumor microenvironment and redox

SIGNIFICANCE

Tumor microenvironment (TME) is a complex term that includes extracellular matrix, blood vessels, endothelial, stromal, and inflammatory cells, and other supporting structures of the particular organ; and physiological components such as oxygen, pH, nutrients, waste products, signaling molecules, reducing/oxidizing species, growth factors, protumorigenic factors, etc. TME is now widely recognized as a major contributor to cancer aggression and treatment resistance and as a potential target for therapeutic intervention.

RECENT ADVANCES

Among important physiological parameters of the TME, tissue hypoxia is considered to be a consequence of imbalanced angiogenesis and is associated with changes in metabolic pathways, including a higher dependence on glycolysis resulting in tissue acidosis. Both hypoxia and acidosis affect the tissue redox status and its key intracellular component, glutathione (GSH). Numerous publications support that these local TME conditions select for outgrowth of cells with appropriate phenotypes, which can reflect underlying genetics.

CRITICAL ISSUES

Here, we hypothesize that specific patterns of local TME, namely, tumor oxygenation, extracellular pH, redox, and GSH homeostasis, acting in orchestrated mechanism, can promote cancer cell survival, while at the same time being highly toxic and mutagenic for normal cells, thus contributing to the growth of cancers at the expense of the normal tissues they are invading. This review summarizes the experimental observations that support the hypothesized Janus-faced character of the redox axis.

FUTURE DIRECTIONS

Normalizing the TME redox parameters may decrease the selection pressure for malignant phenotypes, therefore providing a tool for TME-targeted anticancer therapy.

The chemistry and biological activities of N-acetylcysteine

BACKGROUND

N-acetylcysteine (NAC) has been in clinical practice for several decades. It has been used as a mucolytic agent and for the treatment of numerous disorders including paracetamol intoxication, doxorubicin cardiotoxicity, ischemia-reperfusion cardiac injury, acute respiratory distress syndrome, bronchitis, chemotherapy-induced toxicity, HIV/AIDS, heavy metal toxicity and psychiatric disorders.

SCOPE OF REVIEW

The mechanisms underlying the therapeutic and clinical applications of NAC are complex and still unclear. The present review is focused on the chemistry of NAC and its interactions and functions at the organ, tissue and cellular levels in an attempt to bridge the gap between its recognized biological activities and chemistry.

MAJOR CONCLUSIONS

The antioxidative activity of NAC as of other thiols can be attributed to its fast reactions with OH, NO2, CO3(-) and thiyl radicals as well as to restitution of impaired targets in vital cellular components. NAC reacts relatively slowly with superoxide, hydrogen-peroxide and peroxynitrite, which cast some doubt on the importance of these reactions under physiological conditions. The uniqueness of NAC is most probably due to efficient reduction of disulfide bonds in proteins thus altering their structures and disrupting their ligand bonding, competition with larger reducing molecules in sterically less accessible spaces, and serving as a precursor of cysteine for GSH synthesis.

GENERAL SIGNIFICANCE

The outlined reactions only partially explain the diverse biological effects of NAC, and further studies are required for determining its ability to cross the cell membrane and the blood-brain barrier as well as elucidating its reactions with components of cell signaling pathways.

Molecular markers of radiation-related normal tissue toxicity

Over the past five decades, those interested in markers of radiation effect have focused primarily on tumor response. More recently, however, the view has broadened to include irradiated normal tissues-markers that predict unusual risk of side-effects, prognosticate during the prodromal and therapeutic phases, diagnose a particular toxicity as radiation-related, and, in the case of bioterror, allow for tissue-specific biodosimetry. Currently, there are few clinically useful radiation-related biomarkers. Notably, levels of some hormones such as thyroid-stimulating hormone (TSH) have been used successfully as markers of dysfunction, indicative of the need for replacement therapy, and for prevention of cancers. The most promising macromolecular markers are cytokines: TGFbeta, IL-1, IL-6, and TNFalpha being lead molecules in this class as both markers and targets for therapy. Genomics and proteomics are still in nascent stages and are actively being studied and developed.

Combined use of sodium borocaptate and buthionine sulfoximine in boron neutron capture therapy enhanced tissue boron uptake and delayed tumor growth in a rat subcutaneous tumor model

We have previously reported that buthionine sulfoximine (BSO) enhances sodium borocaptate (BSH) uptake by down regulating glutathione (GSH) synthesis in cultured cells. This study investigated the influence of BSO on tissue BSH uptake in vivo and the efficacy of BSH-BSO-mediated boron neutron capture therapy (BNCT) on tumor growth using a Fisher-344 rat subcutaneous tumor model. With BSO supplementation, boron uptake in subcutaneous tumor, blood, skin, muscle, liver, and kidney was significantly enhanced and maintained for 12h. Tumor growth was significantly delayed by using BSO. With further improvement in experimental conditions, radiation exposure time, together with radiation damage to normal tissues, could be reduced.

Enhanced levels of glutathione and protein glutathiolation in rat tongue epithelium during 4-NQO-induced carcinogenesis

High glutathione (GSH) levels are commonly found in oral tumors and are thought to play an important role in tumorigenesis. While posttranslational binding of GSH to cellular proteins (protein glutathiolation) has recently been recognized as an important redox-sensitive regulatory mechanism, no data currently exist on this process during carcinogenesis. Our goal was to determine the effects of 4-nitroquinoline-N-oxide (4-NQO)-induced carcinogenesis on tongue levels of protein-bound and free GSH and related thiols in the rat. Male F-344 rats (6 weeks of age) were administered either 4-NQO (20 ppm) in drinking water or tap water alone (controls) for 8 weeks. Twenty-four weeks after cessation of 4-NQO, squamous cell carcinomas of the tongue were observed in all rats. The levels of both free and bound GSH in tumors, as well as in adjacent tissues, were 2- to 3-fold greater than in tongue epithelium from control rats (p < 0.05). Prior to tumor formation, at 8 weeks after cessation of 4-NQO, hyperplasia, dysplasia and carcinoma in situ were observed in 100%, 25% and 12.5% of 4-NQO-treated rats, respectively. At this early stage of carcinogenesis, levels of free and bound GSH were increased 50% compared with tongue tissues from control rats (p<0.05). Glutathione disulfide (GSSG) levels were also 2-fold greater in tongue tissues from 4-NQO treated vs. control rats (p<0.05). Altogether, these results suggest that protein glutathiolation, together with GSH and GSSG levels, are induced during oral carcinogenesis in the rat possibly as a result of enhanced levels of oxidative stress.

Relations between non–protein sulfydryl levels in the nucleus and cytoplasm, tumor oxygenation, and clinical outcome of patients with uterine cervical carcinoma

PURPOSE

The non-protein sulfydryls (NPSH) glutathione and cysteine are able to effect chemical repair of radiation-induced DNA damage, particularly under hypoxic conditions, and are implicated in radioresistance. The levels of NPSH in the nucleus are predicted to be more important than those in cytoplasm. We, therefore, investigated the relation between nuclear NPSH and the clinical outcome of radical radiotherapy (RT).

METHODS AND MATERIALS

A fluorescence image analysis technique to measure nuclear NPSH was developed, based on the SH-reactive probe 1(4-chloromercuryphenyl-azo-2-naphthol) and the DNA dye 4,6-diamidino-2-phenylindole. This was used to measure nuclear and tissue NPSH levels in biopsies obtained from 58 patients with locally advanced uterine cervical carcinoma, treated with radical RT.

RESULTS

An approximately twofold range in the nuclear and tissue NPSH levels between individual tumors was found, although the intratumoral heterogeneity was much smaller. Nuclear and tissue NPSH values correlated closely in all cases. No statistically significant associations were noted between NPSH levels and tumor size, stage, or tumor hypoxia as determined at the time of biopsy using an Eppendorf po(2) probe. The response to RT and patient survival did not correlate with tumor NPSH.

CONCLUSION

These results did not support the existence of an independently regulated nuclear pool of NPSH and showed that tissue and nuclear NPSH are not predictive of the outcome of patients with locally advanced cervical carcinomas treated with RT.

Enhancement of sodium borocaptate (BSH) uptake by tumor cells induced by glutathione depletion and its radiobiological effect

Sodium borocaptate (BSH) is widely used for boron neutron capture therapy (BNCT) of brain tumors. However, the mechanism of uptake by the tumor remains unclear. We investigated the sulfhydryl moiety of this compound. Down regulation of glutathione (GSH) by buthionine sulfoximine in cultured cells resulted in increase of BSH uptake (7.9-36.5%) compared to the control group and consequently the cytocidal effect of neutron irradiation also increased. On the other hand, the radiation caused damage by gamma-ray irradiation was suppressed when BSH uptake increased. These findings suggested that modulation of GSH enhanced the effect of B (n, alpha) reaction and the protective effect of secondary gamma-ray in BNCT.

Protection against ionizing radiation by antioxidant nutrients and phytochemicals

The potential of antioxidants to reduce the cellular damage induced by ionizing radiation has been studied in animal models for more than 50 years. The application of antioxidant radioprotectors to various human exposure situations has not been extensive although it is generally accepted that endogenous antioxidants, such as cellular non-protein thiols and antioxidant enzymes, provide some degree of protection. This review focuses on the radioprotective efficacy of naturally occurring antioxidants, specifically antioxidant nutrients and phytochemicals, and how they might influence various endpoints of radiation damage. Results from animal experiments indicate that antioxidant nutrients, such as vitamin E and selenium compounds, are protective against lethality and other radiation effects but to a lesser degree than most synthetic protectors. Some antioxidant nutrients and phytochemicals have the advantage of low toxicity although they are generally protective when administered at pharmacological doses. Naturally occurring antioxidants also may provide an extended window of protection against low-dose, low-dose-rate irradiation, including therapeutic potential when administered after irradiation. A number of phytochemicals, including caffeine, genistein, and melatonin, have multiple physiological effects, as well as antioxidant activity, which result in radioprotection in vivo. Many antioxidant nutrients and phytochemicals have antimutagenic properties, and their modulation of long-term radiation effects, such as cancer, needs further examination. In addition, further studies are required to determine the potential value of specific antioxidant nutrients and phytochemicals during radiotherapy for cancer.

Inhibition of LPS-induced nitric oxide production in RAW cells by radioprotective thiols

Nitric oxide (NO) is involved in producing damage after exposure to radiation and also in the toxicity associated with bacterial endotoxin (lipopolysaccharide, LPS). We have evaluated different radioprotective thiols for their effects on LPS-stimulated NO production in mouse macrophage cells, RAW 264.7. Our results indicate that although thiols inhibited NO production in general, the degree of inhibition depended upon the thiol compound. Long-chain aminothiols like WR-1065 [N-(2-mercaptoethyl)-1,3-diaminopropane] exerted a strong inhibition; but its parent drug, amifostine, which protects mice against radiation lethality, was not as effective as WR-1065. Diethyl dithiocarbamate, which is less effective than amifostine as a radioprotector, strongly inhibited NO production from macrophages. These results indicate that the radioprotective potential of sulfhydryl compounds is not related to its ability to inhibit NO production by macrophages and suggest that some of the thiol radioprotectors may effectively ameliorate the fatal symptoms of hypotensive shock, associated with endotoxin (LPS)-induced NO production.

G6PD deficient cells and the bioreduction of disulfides: effects of DHEA, GSH depletion and phenylarsine oxide

We used Glucose 6 phosphate dehydrogenase (G6PD) minus cells (89 cells) and G6PD containing cells (K1) to understand the mechanisms of bioreduction of disulfide and the redox regulation of protein and non protein thiols in mammalian cells. The 89 cells reduce hydroxyethyldisulfide (HEDS) to mercaptoethanol (ME) at a slower rate than K1 cells. HEDS reduction results in loss of nonprotein thiols (NPSH) and a decrease in protein thiols (PSH) in 89 cells. The effects are less dramatic with K1 cells. However, the loss of NPSH and PSH in K1 cells are increased in the absence of glucose. Glutathione-depletion with L-BSO partially blocks HEDS reduction in K1 and 89 cells. Treatment with the vicinal thiol reagent phenyl arsenic oxide (PAO) blocks reduction of HEDS in both cells. Surprisingly, dehydroepiandrosterone (DHEA), a known inhibitor of G6PD, inhibits the growth and blocks the reduction of HEDS both in 89 and K1 cells suggesting that its mechanism for inhibition of growth is not G6PD related.

A method for measuring disulfide reduction by cultured mammalian cells: relative contributions of glutathione-dependent and glutathione-independent mechanisms

A method is described for measuring bioreduction of hydroxyethyl disulfide (HEDS) or alpha-lipoate by human A549 lung, MCF7 mammary, and DU145 prostate carcinomas as well as rodent tumor cells in vitro. Reduction of HEDS or alpha-lipoate was measured by removing aliquots of the glucose-containing media and measuring the reduced thiol with DTNB (Ellman's reagent). Addition of DTNB to cells followed by disulfide addition directly measures the formation of newly reduced thiol. A549 cells exhibit the highest capacity to reduce alpha-lipoate, while Q7 rat hepatoma cells show the highest rate of HEDS reduction. Millimolar quantities of reduced thiol are produced for both substrates. Oxidized dithiothreitol and cystamine were reduced to a lesser degree. DTNB, glutathione disulfide, and cystine were only marginally reduced by the cell cultures. Glucose-6-phosphate deficient CHO cells (E89) do not reduce alpha-lipoate and reduce HEDS at a much slower rate compared to wild-type CHO-K1 cells. Depletion of glutathione prevents the reduction of HEDS. The depletion of glutathione inhibited reduction of alpha-lipoate by 25% and HEDS by 50% in A549 cells, while GSH depletion did not inhibit alpha-lipoate reduction in Q7 cells but completely blocked HEDS reduction. These data suggest that the relative participation of the thioltransferase (glutaredoxin) and thioredoxin systems in overall cellular disulfide reduction is cell line specific. The effects of various inhibitors of the thiol-disulfide oxidoreductase enzymes (1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), arsenite, and phenylarsine oxide) support this conclusion.

Effects of the Bowman-Birk inhibitor on clonogenic survival and cisplatin- or radiation-induced cytotoxicity in human breast, cervical, and head and neck cancer cells

Bowman-Birk inhibitor (BBI) is a soybean-derived anticarcinogenic protease inhibitor previously shown to potentiate cisplatin-induced cytoxicity in human lung and ovarian cancer cells. To further assess the potential of BBI as a sensitizing agent for cancer radiotherapy and chemotherapy, we evaluated the effects of BBI and a soybean concentrate enriched in BBI known as BBI concentrate (BBIC) on clonogenic survival and radiation- or cisplatin-induced cell killing in MCF7 human breast carcinoma cells, SCC61 and SQ20B human head and neck carcinoma cells, HeLa, HeLa-R1, and HeLa-R3 human cervical carcinoma cells, MCF10 nontumorigenic human epithelial cells, HTori-3 nontumorigenic human thyroid epithelial cells, and C3H10T1/2 mouse fibroblast cells. BBI and BBIC significantly suppressed the clonogenic survival of MCF7 and SCC61 cells. BBIC also suppressed the survival of SQ20B cells and enhanced radiation-induced cell killing in SCC61 and SQ20B cells and cisplatin-induced cell killing in HeLa, HeLa-R1, and HeLa-R3 cells. In contrast, BBI and/or BBIC did not enhance radiation-induced cell killing in MCF10 cells or cisplatin-induced cell killing in C3H10T1/2 cells. BBI did not significantly affect the survival of SQ20B cells or enhance radiation-induced cell killing in SCC61 and SQ20B cells. The clonogenic survivals of MCF10 and C3H10T1/2 cells were not adversely affected by treatment with BBI or BBIC. The clonogenic survival of HTori-3 cells was only moderately suppressed by treatment with BBIC at > or = 80 micrograms/ml. These results suggest that BBIC could be a useful agent for the potentiation of radiation- and cisplatin-mediated cancer treatment without significant adverse effects on surrounding normal tissues.

Metallothionein in radiation exposure: its induction and protective role

Since its discovery about 40 years ago, there has been a wide interdisciplinary research interest in metallothionein (MT) on its physiological and toxicological aspects. Functionally, MT is involved not only in metal detoxification and homeostasis, but also in scavenging free radicals during oxidative damage. Among over 4500 publications which can be retrieved by Medline search, only about 50 reports have been published on the relationship of MT with ionizing and UV radiation. In this review, we have evaluated critically the published data on the induced synthesis of MT by radiation, and the potential functions of MT in radiation induced cell damage. MT mRNA expression or MT synthesis was found to be induced by exposure of cells in vitro or tissues in vivo to ionizing or UV radiation. In most of the studies in animals and tissue cultures, high doses of ionizing radiation were used to induce MT, and, therefore, it is difficult to extrapolate these results to low level of repeated exposures to radiation in humans. Induced synthesis of MT is considered as one of the mechanisms involved in the adaptive response to low dose radiation exposure. The presence of MT in normal cells may provide protective effects from radiation-induced genotoxicity and cytotoxicity. However, in tumor cells, the presence of MT can result in drug and radiation resistance as well. These effects are modulated by other cellular factors, besides MT, such as antioxidants, and by the cell cycle stages in cell proliferation and differentiation.

Possible role of metallothionein in the cellular defense mechanism against UVB irradiation in neonatal human skin fibroblasts

The role of metallothionein (MT) in protecting skin cells against UVB irradiation was investigated. Fibroblast strains from normal adult (HS-K) and neonatal (NB1RGB) human skins as well as keratinocyte strains from human skin (SV40-HSK) and newborn Balb/c mouse skin (Pam 212) were exposed to UVB irradiation. The sensitivity of HS-K and NB1RGB cells to UVB irradiation was similar; those of SV40-HSK and Pam 212 cells were two- and six-fold as sensitive to UVB irradiation as HS-K cells, respectively. The HS-K cells contained the greatest cellular reduced form of glutathione (GSH) levels compared to the three other skin cells: the levels were 13-, 7- and 6-fold of those in NB1RGB, SV40-HSK and Pam 212 cells, respectively. These results indicated that the sensitivity of skin cells to UVB irradiation was not always associated with their endogenous GSH levels. In particular, despite the fact that NB1RGB cells contained a relatively small amount of GSH, they were less sensitive to UVB irradiation. NB1RGB cells contained 4-30 times more MT than those in other skin cells examined. The sulfhydryl residues of MT molecules in the NB1RGB cells were estimated to be mostly unoccupied by metals, suggesting they act in a similar way to those of GSH. Moreover, NB1RGB cells in which the MT content was elevated by dexamethasone (1 microM) or Zn2+ (7 micrograms/mL) treatment were more resistant to UVB irradiation than nontreated ones. These results suggest that, at least in neonatal human skin fibroblasts, MT may play a role in protection against UVB irradiation.

Mechanisms of protection of hematopoietic stem cells from irradiation

Current therapies for the treatment of malignancies are associated with significant limitations to the hematopoietic system since chemotherapy and radiation therapy do not discriminate between normal and malignant cells. Since bone marrow depression occurs at low to midlethal doses of irradiation, approaches to improving the therapeutic index of treatment must include measures to enhance the sensitivity of the tumor relative to normal hematopoietic tissue or, by reducing toxicity to normal hematopoietic tissues leaving tumor resistance unchanged. Radioprotective agents have been proposed to unravel the fundamental processes by which radiation itself damages hematopoietic tissue. In radiotherapy, the importance of these agents is derived from their potential use as selective protectors against radiation damage to normal hematopoietic tissue such that higher doses of radiation can be delivered to tumors to achieve a therapeutic advantage. A variety of agents have been and are being evaluated as possible protectants. These include aminothiols, synthetic polysaccharides, vitamins and cytokines. This review attempts to summarize the role both chemical and biological response modifiers play as hematopoietic radioprotectors. In addition, possible mechanisms of protection of hematopoietic stem cells from irradiation are discussed.

Protective effect of RibCys following high-dose irradiation of the rectosigmoid

Ribose-cysteine (RibCys) is a prodrug of L-cysteine that stimulates glutathione biosynthesis. Increased glutathione levels have been shown to have a protective effect against radiation-induced injury and oxidative stress. Surface oximetry has previously been used successfully to predict anastomotic leakage.

PURPOSE

The following study was done to evaluate the protective effect of RibCys and the predictive value of PtO2 determinations in a swine model.

METHODS

Domestic swine were divided into three groups: Group A served as a nonradiated control; Group B received 6,000 to 6,500 rad to the rectosigmoid; and Group C received RibCys (1 g/kg) prior to receiving 6,000 to 6,500 rad. Radiated animals and controls underwent rectosigmoid resection after a three-week rest period. Intraoperative anastomotic PtO2 was checked with a modified Clark electrode. Anastomoses were evaluated radiographically at three and seven days; animals were sacrificed, and bursting strength was recorded at 10 days.

RESULTS

Mean bursting pressures were 243.8 +/- 59.4, 199.5 +/- 37.8, and 209.5 +/- 54.9 mmHg (NS) for Groups A, B, and C, respectively. Anastomotic PtO2 ranged from 19 to 98 mmHg and could not be correlated with anastomotic leaks or bursting pressure. There were 11/15 radiation-related deaths and leaks (eight deaths and three leaks) in the radiated group and 4/12 radiation-related deaths and leaks (three deaths and one leak) in the group receiving radiation and RibCys (P < 0.04).

CONCLUSIONS

1) RibCys protected animals against radiation-related deaths and anastomotic leaks following high doses of pelvic irradiation; 2) anastomotic PtO2 levels did not correlate with anastomotic healing in this model.

The effects of bucillamine on glutathione and glutathione-related enzymes in the mouse

The effect of bucillamine (BA) on glutathione (GSH) and GSH-related enzymes was investigated in C57 mouse. Administration of high doses of BA (150-400 mg/kg) produced a dose-dependent depletion (20-44%) of hepatic GSH, which was similar in magnitude to that produced by equimolar doses of other sulphydryl drugs studied previously. GSH depletion after acute BA administration correlated well with the elevation of serum glutamic-pyruvic transaminase (SGPT) (6-9-fold increase above control). The increase in SGPT after chronic administration (7 days), although significantly higher than the controls, was however much less than after acute administration. The hepatic GSH concentrations of mice given 7 days of BA were similar to the controls, again correlating well with SGPT activity. Administration of BA (150-400 mg/kg) caused also a significant dose-dependent increase in the oxidized glutathione (GSSG) in blood by 2-7-fold, as well as a dose-dependent increase in blood glutathione S-transferase (GST) activity (2-13-fold). In an in vitro experiment, hepatic GST activity was activated by various concentrations of BA (1 microM-1mM). There was little or no effect on GSSG reductase and on glutathione peroxidase (GSH-Px) after acute administration of BA. Chronic administration of BA had no effect on hepatic GSSG reductase and GSH-Px, but GSSG reductase activity in blood was increased significantly by 4-fold. It is possible that BA may affect the redox status through auto-oxidation and oxidation with endogenous thiols such as glutathione, affecting GSH concentrations and the GSH/GSSG ratio in tissues and, thus, having both metabolic and toxicological consequences. Whether or not the induction of GST activity in vivo in blood and in vitro in liver enzyme preparations shared the same underlying mechanism(s) requires further investigation.

Relationship between glutathione levels and drug or radiation sensitivities in human gastric cancer cell lines in vitro

Permanent cell lines and clones established from an untreated patient (AGS cells) with gastric carcinoma, and from a similar patient who had been treated with Adriamycin, 5FU and cytoxan (SII cells) were used in a study that compared their drug and radiation survival sensitivities to their glutathidine (GSH) values. The SII parental cell line was more resistant than the AGS cells in vitro to chlorambucil, ACT D, Adria, Bleo, and X-rays. This greater resistance was positively correlated with GSH values that were 1.77 times higher than in the AGS parental cell line. By contrast the SII parental cells were more sensitive than the AGS cells to MeCCNU and Melphalan. The drug and radiation sensitivities expressed among the clones of the two cell lines were heterogeneous and did not correlate with their GSH values.

Glutathione deficiency produced by inhibition of its synthesis, and its reversal; applications in research and therapy

Glutathione, which is synthesized within cells, is a component of a pathway that uses NADPH to provide cells with their reducing milieu. This is essential for (a) maintenance of the thiols of proteins (and other compounds) and of antioxidants (e.g. ascorbate, alpha-tocopherol), (b) reduction of ribonucleotides to form the deoxyribonucleotide precursors of DNA, and (c) protection against oxidative damage, free radical damage, and other types of toxicity. Glutathione interacts with a wide variety of drugs. Despite its many and varied cellular functions, it is possible to achieve therapeutically useful modulations of glutathione metabolism. This article emphasizes an approach in which the synthesis of glutathione is selectively inhibited in vivo leading to glutathione deficiency. This is achieved through use of transition-state inactivators of gamma-glutamylcysteine synthetase, the enzyme that catalyzes the first and rate-limiting step of glutathione synthesis. The effects of marked glutathione deficiency, thus produced in the absence of applied stress, include cellular damage associated with severe mitochondrial degeneration in a number of tissues. Such glutathione deficiency is not prevented or reversed by giving glutathione. The cellular utilization of GSH involves its extracellular degradation, uptake of products, and intracellular synthesis of GSH. This is a normal pathway by which cysteine moieties are taken up by cells. Glutathione deficiency induced by inhibition of its synthesis may be prevented or reversed by administration of glutathione esters which, in contrast to glutathione, are readily transported into cells and hydrolyzed to form glutathione intracellularly. Research derived from this model has led to several potentially useful therapeutic approaches, one of which is currently in clinical trial. Thus, certain tumors, including those that exhibit resistance to several drugs and to radiation, are sensitized to these modalities by selective inhibition of glutathione synthesis. An alternative interpretation is suggested which is based on the concept that some resistant tumors have high capacity for glutathione synthesis and that such increased capacity may be as significant or more significant in promoting the resistance of some tumors than the cellular levels of glutathione. Therapeutic approaches are proposed in which normal cells may be selectively protected against toxic antitumor agents and radiation by cysteine- and glutathione-delivery compounds. Current studies suggest that research on other modulations of glutathione metabolism and transport would be of interest.

Isolation of mammalian cell variants with enhanced endogenous thiol content at low survival levels following irradiation

Approximately half of a group of Chinese hamster V79 cell clones isolated from radiation survivors at low surviving fractions had significantly higher endogenous levels of non-protein and protein thiols than unirradiated cells. A similar group of cell lines cloned from unirradiated cells had thiol levels in the same range as the original unirradiated population. In some cases, clones isolated following irradiation are also more resistant to misonidazole toxicity and to radiation. This phenotype can persist through many cell generations for weeks or months of continuous growth; however, in many clones with altered phenotypes isolated following irradiation, reversion of the population to the same phenotype as that of unirradiated populations has been observed. Induction of elevated thiol levels in tumours by radiotherapy could reduce both the efficacy of the radiation itself and of radiation-modifying or chemotherapeutic drugs given in combination with the radiation.