Toxicity of t-butylhydroperoxide in hepatocyte monolayers exposed to hypoxia and reoxygenation

Hypoxia and oxygen dependence of cytotoxicity in renal proximal tubular and distal tubular cells

Ischemia and hypoxia are major causes of renal failure and altered oxygen supply may affect renal responses to toxic chemicals. In vitro experiments were designed to evaluate the susceptibility of isolated proximal tubular (PT) and distal tubular (DT) cells from rat kidney to brief periods of oxygen deprivation and to assess how variations in oxygen supply affect chemical-induced cytotoxicity. Isolated cells were incubated for 1 hr in either oxygen (95% O2/5% CO2), air (21% O2), or nitrogen (95% N2/5% CO2) atmosphere. PT cells exhibited no injury due to brief oxygen deprivation whereas DT cells exhibited moderate, but significant injury, indicating that DT cells are more susceptible than PT cells to hypoxic injury. The cytotoxicity of chemicals that alter cellular redox status [i.e. tert-butyl hydroperoxide (tBH), menadione, methyl vinyl ketone] and the cytotoxicity of "chemical hypoxia" [i.e. KCN + iodoacetic acid] were greatest in air, intermediate in oxygen, and lowest in nitrogen. In contrast, the cytotoxicity of the alkylating agent N-dimethylnitrosamine was independent of oxygen concentration and the cytotoxicity of p-aminophenol was related directly to oxygen concentration. The mechanism of the oxygen dependence of chemical injury was investigated further, employing tBH as a model toxicant. tBH metabolism was oxygen independent in both PT and DT cells. Depletion of cellular protein sulfhydryl groups by tBH increased with increasing oxygen concentration and lipid peroxidation due to tBH was inhibited in nitrogen but was not different in air as compared with oxygen. Although these processes may contribute to the much lower toxicity in nitrogen as compared with oxygen, it does not explain the higher toxicity in air as compared with that in oxygen. Other processes that predominate at lower oxygen concentrations but that only produce injury if enough oxygen is present are likely to be responsible for the enhanced susceptibility of both PT and DT cells to oxidants in air as compared with oxygen.

Oxygen dependence of oxidative stress. Rate of NADPH supply for maintaining the GSH pool during hypoxia

NADPH supply for oxidized glutathione (GSSG) reduction was studied in hepatocytes under different steady-state O2 concentrations with controlled infusions of diamide, a thiol oxidant. When bis-chloro-nitrosourea (BCNU) was used to inhibit GSSG reductase, the rate of GSH depletion approximated the rate of diamide infusion, showing that diamide reacted preferentially with GSH under these experimental conditions. Under aerobic conditions without BCNU treatment, the GSH and NADPH pools were largely unaffected and little diamide accumulation or protein thiol oxidation occurred with diamide infusion rates up to 5.3 nmol/10(6) cells per min. However, at greater infusion rates, GSH and NADPH decreased, diamide and GSSG concentrations increased, and protein thiols were oxidized. This critical infusion rate was easily discernible and provided a convenient means to assess the capacity of cells to reduce GSSG as a function of O2 concentration. As the O2 concentration was decreased below 15 microM, the critical infusion rate decreased from the aerobic value of 5.3 to less than 2 nmol/10(6) cells per min in anoxic cells; half-maximal change occurred at 5 microM O2. Although cells could not maintain normal thiol and NADPH pools at infusion rates above the critical value, analysis of the rates of thiol depletion showed that the maximal NADPH supply rate for GSSG reduction under aerobic conditions was 7-8 nmol/10(6) cells per min and was affected by hypoxia to the same degree as the critical value. Thus, hypoxia and anoxia impair the capability of cells to supply NADPH for the reduction of thiol oxidants. This could be an important factor in the sensitivity of hypoxic and ischemic tissues to oxidative injury.

Atrial natriuretic peptide protects hepatocytes against damage induced by hypoxia and reactive oxygen. Possible role of intracellular free ionized calcium

Elevated concentrations of atrial natriuretic peptide reportedly mitigate acute renal failure in vivo and in the isolated perfused kidney (M. Nakamoto, J.I. Shapiro, P.F. Shanley, L. Chan & R.W. Shrier (1987) J. Clin. Invest. 80, 698-705; S.G. Shaw, J. Weidmann, J. Hodler, A. Zimmermann & A. Paternostro (1987) J. Clin. Invest. 80, 1232-1237). Since atrial natriuretic peptide has been shown to be a potent vasodilator, this beneficial effect may be due entirely to improved haemodynamics. To determine whether atrial natriuretic peptide also has a protective effect at the cellular level, rat hepatocyte cell cultures were treated with atrial natriuretic peptide prior to or after induction of cell damage by hypoxia (0.5% O2 for 4 h) or reactive oxygen (hypochlorous acid). Bleb formation, degradation of radiolabeled trichloroacetic acid-precipitable peptides, release of lactate dehydrogenase and trypan blue exclusion were used as indicators of cell damage. Atrial natriuretic peptide treatment distinctly protected the cell cultures against damage in both cases. This beneficial effect of atrial natriuretic peptide was partly mimicked by sodium nitroprusside, which, like atrial natriuretic peptide, largely increased the cellular cGMP content. 6-Anilino-5,8-quinolinedione (Ly 83583), an inhibitor of particulate guanylate cyclase, blocked the protective effect of atrial natriuretic peptide. Therefore a cGMP-mediated mechanism seems to be involved in the cytoprotective action of atrial natriuretic peptide. Fluorometric measurements using the Ca2+-sensitive dye Quin-2 showed that the elevation of intracellular Ca2+ after cellular insult by hypochlorous acid is prevented by atrial natriuretic peptide. These results suggest that atrial natriuretic peptide may attenuate hypoxic and toxic cell damage by increasing cGMP and reducing intracellular Ca2+.

Drug metabolism and toxicity during hypoxia

Oxygen concentration affects the metabolism and toxicity of various drugs. A considerable amount of information is now available on the effects of hypoxia on the major pathways of drug metabolism, including oxidation (i.e., by cytochromes P-450, NAD+-dependent dehydrogenases, and monoamine oxidase), glucuronidation, sulfation, glutathione conjugation, glycine conjugation, and acetylation. Some pathways are essentially independent of O2 concentration while others are highly dependent upon O2. Certain drugs are activated to reactive and toxic metabolites by O2-dependent pathways. This aspect of drug toxicity serves as a basis for treatment of slow-growing solid tumors which have hypoxic regions that are resistant to chemo- and radiation therapies. Recent studies have also established that hypoxic cells have increased susceptibility to oxidative injury, and this can predispose cells to other pathological processes. However, in spite of the available knowledge concerning the O2 dependence of metabolism and toxicity of drugs, relatively little is known about the effects of chronic hypoxia on the expression of drug-metabolizing enzymes or upon the absorption, elimination, or toxicity of drugs. Thus, in addition to the information presently reviewed, major gaps exist in the knowledge needed to provide optimal drug therapy in the large population of patients who experience O2 deficiency. Comments and Perspectives. Specific basic research areas which need to be studied include the effects of hypoxia on drug absorption and elimination, the changes of neahypoxia that lead to enhanced susceptibility to drug toxicity, and the effects of chronic hypoxia on the metabolic systems involved in absorption, metabolism, and elimination of drugs. At an applied level, the available data on the O2 dependences of drug metabolism pathways need to be extended to examine in detail the O2 dependence to metabolism and toxicity of relevant, currently used therapeutic agents. Such efforts can be expected to continue to improve drug therapies and reduce toxicities in hypoxic patients.

Toxicity of 1,2-dibromoethane in primary hepatocyte monolayer cultures: lack of dependence on oxygen concentration

Hepatic 1,2-dibromoethane (DBE) metabolism proceeds via two pathways: oxidation by cytochrome P-450 and direct conjugation with the ubiquitous tripeptide glutathione (GSH) via the GSH S-transferases. The toxicity of DBE in monolayers of hepatocytes was assessed to establish whether the toxicity of this compound is increased under conditions of reductive metabolism at low oxygen concentrations. Our previous studies with t-butyl hydroperoxide and the calcium ionophore A23187 suggested that hypoxia would exacerbate toxicity that was mediated through lipid peroxidation or loss of calcium homeostasis. Monolayers of hepatocytes were exposed for 2 hr to 0, 14, 140, 1400, or 14,000 ppm of DBE in an atmosphere of either 1, 2, or 20% oxygen. Toxicity was measured by leakage of aspartate aminotransferase (AST) and trypan blue exclusion. The time course of the development of cytotoxicity was examined by assaying cell death both immediately following a 2-hr exposure and 24 hr later. The LC50 of DBE vapor was found to be approximately 14,000 ppm when assayed immediately after exposure but only 140 ppm when assayed 24 hr after exposure. The similarity of the percentages of DBE-induced cell death after incubations at 1, 2, and 20% oxygen demonstrates that the toxicity of DBE is oxygen-independent. We conclude that while DBE is highly toxic to rat hepatocytes, hypoxia does not appear to contribute to the toxicity of DBE, even under conditions of low oxygen concentrations. This result is in direct contrast to a previous report where we showed that the toxicity of halothane is potentiated under hypoxic conditions.