Experimental Mathematical Validation of a Novel Concept of Extended 2-Butoxyethanol Autoprotection

Prior administration of a low dose of thioacetamide protects type 1 diabetic rats from subsequent administration of lethal dose of thioacetamide

Previously, we reported that an ordinarily non-lethal dose of thioacetamide (TA, 300 mg/kg) causes 90% mortality in type 1 diabetic rats due to inhibited liver tissue repair, whereas 30 mg TA/kg allows 100% survival due to stimulated although delayed tissue repair. Objective of this investigation was to test whether prior administration of a low dose of TA (30 mg/kg) would lead to sustainable stimulation of liver tissue repair in type 1 diabetic rats sufficient to protect from a subsequently administered lethal dose of TA. Therefore, in the present study, the hypothesis that preplacement of tissue repair by a low dose of TA (30 mg TA/kg, ip) can reverse the hepatotoxicant sensitivity (autoprotection) in type 1 diabetic rats was tested. Preliminary studies revealed that a single intraperitoneal (ip) administration of TA causes 90% mortality in diabetic rats with as low as 75 mg/kg. To establish an autoprotection model in diabetic condition, diabetic rats were treated with 30 mg TA/kg (priming dose). Administration of priming dose stimulated tissue repair that peaked at 72h, at which time these rats were treated with a single ip dose of 75 mg TA/kg. Our results show that tissue repair stimulated by the priming dose enabled diabetic rats to overexpress, calpastatin, endogenous inhibitor of calpain, to inhibit calpain-mediated progression of liver injury induced by the subsequent administration of lethal dose, resulting in 100% survival. Further investigation revealed that protection observed in these rats is not due to decreased bioactivation. These studies underscore the importance of stimulation of tissue repair in the final outcome of liver injury (survival/death) after hepatotoxicant challenge. Furthermore, these results also suggest that it is possible to stimulate tissue repair in diabetics to overcome the enhanced sensitivity of hepatotoxicants.

Molecular mechanisms of enhanced renal cell division in protection againstS-1,2-dichlorovinyl-l-cysteine-induced acute renal failure and death

Sustained activation of ERK 1/2 by a low dose (15 mg/kg ip) of S-1,2-dichlorovinyl-l-cysteine (DCVC) 72 h before administration of a lethal dose of DCVC (75 mg/kg ip) enhances renal cell division and protects mice against acute renal failure (ARF) and death (autoprotection). The objective of this study was to determine correlation among extent of S-phase DNA synthesis, activation of transcription factors, expression of G1/S cyclins, cyclin-dependent kinases (CDKs), and CDK inhibitors downstream of ERK 1/2 following DCVC-induced ARF in autoprotection. Administration of the lethal dose alone caused a general downregulation or an unsustainable increase, in transcriptional and posttranscriptional events thereby preventing G1-S transition of renal cell cycle. Phosphorylation of IκBα was inhibited resulting in limited nuclear translocation of NF-κB. However, cyclin D1 expression was high probably due to transcriptional cooperation of AP-1. Cyclin D1/cyclin-dependent kinase 4 (cdk4)-cdk6 system-mediated phosphorylation of retinoblastoma protein was downregulated due to overexpression of p16 at 24 h after exposure to the lethal dose alone. Inhibition of S-phase stimulation was confirmed by proliferating cell nuclear antigen assay (PCNA). This inhibitory response was prevented if the lethal dose was administered 72 h after the low priming dose of DCVC due to promitogenic effect of the low dose. NF-κB-DNA binding is not limited if mice were pretreated with the priming dose. Cyclin D1/cdk4-cdk6 expression stimulated by the priming dose of DCVC was unaltered even after the lethal dose in the autoprotected group, explaining higher phosphorylated-pRB and S-phase stimulation found in this group. These results were corroborated with PCNA immunohistochemistry. These findings suggest that the priming dose relieves the block on compensatory tissue repair by upregulation of promitogenic mechanisms, normally blocked by the high dose when administered without the prior priming dose.

Tissue repair: an important determinant of final outcome of toxicant-induced injury

Tissue repair is a dynamic compensatory cell proliferation and tissue regeneration response stimulated in order to overcome acute toxicity and recover organ/tissue structure and function. Extensive evidence in rodent models using structurally and mechanistically diverse hepatotoxicants such as acetaminophen (APAP), carbon tetrachloride (CCl4), chloroform (CHCl3), thioacetamide (TA), trichloroethylene (TCE), and allyl alcohol (AA) have demonstrated that tissue repair plays a critical role in determining the final outcome of toxicity, i.e., recovery from injury and survival or progression of injury leading to liver failure and death. Tissue repair is a complex process governed by intricate cellular signaling involving a number of chemokines, cytokines, growth factors, and nuclear receptors leading to promitogenic gene expression and cell division. Tissue repair also encompasses regeneration of hepatic extracellular matrix and angiogenesis, the processes necessary to completely restore the structure and function of the liver tissue lost to toxicant-induced initiation followed by progression of injury. New insights have emerged over the last quarter century indicating that tissue repair follows a dose response. Tissue repair increases with dose until a threshold dose, beyond which it is delayed and impaired due to inhibition of cellular signaling resulting in runaway secondary events causing tissue destruction, organ failure, and death. Prompt and adequately stimulated tissue repair response to toxic injury is critical for recovery from toxic injury. Tissue repair is modulated by a variety of factors including species, strain, age, nutrition, and disease condition causing marked changes in susceptibility and toxic outcome. This review focuses on the properties of tissue repair, different factors affecting tissue repair, and the mechanisms that govern tissue repair and progression of injury. It also highlights the significance of tissue repair as a target for drug development strategies and an important consideration in the assessment of risk from exposure to toxicants.

Butoxyacetic acid-induced hemolysis of rat red blood cells: effect of external osmolarity and cations

Hemolysis is the principal toxicity of acute exposure to ethylene glycol monobutyl ether (EGBE) in rats. EGBE itself is not an active hemolytic agent, but its metabolite, butoxyacetic acid (BAA) formed as a result of dehydrogenase activity is a potent hemolysin. Here we address the role of osmolarity and cation composition of the suspending buffers in the mechanism of BAA-induced hemolysis of rat red blood cells in vitro. Rat erythrocytes were protected from BAA-induced cell swelling and hemolysis by the addition of sucrose to the suspending media. Hemolysis and cell swelling were also reduced by replacing external sodium with potassium. When calcium was not present in the suspending medium or when chelated by EGTA, hemolysis was increased after 2 h incubation with 1 mM or 2 mM BAA. Addition of as little as 0.05 mM CaCl(2) reduced hemolysis significantly while the addition of MgCl(2) had no effect. The dose-response relationship between BAA concentration and hemolysis determined in the presence or absence of calcium showed an increased effect of BAA in the absence of calcium. BAA-induced spherocytosis and cell fragmentation were more pronounced in the absence of calcium. The time course of BAA-induced hemolysis in the presence and absence of calcium demonstrated that the effect of calcium is to delay the onset of hemolysis. Increased intracellular calcium as a result of exposure to BAA was verified by atomic absorption spectroscopy. Charybdotoxin, an inhibitor of the calcium activated potassium channel, blocked the protective effect of calcium suggesting that the delay of onset of hemolysis in the presence of calcium is due to potassium loss caused by this channel. We conclude that the mode of action of BAA is to cause a colloid osmotic lysis of the rat red blood cell. Hemolysis requires external sodium and is associated with calcium uptake. Calcium appears to delay the onset of hemolysis. We speculate that: (1) BAA causes sodium and calcium to enter the cell; (2) calcium initially has a protective effect via the calcium activated potassium channel which facilitates the loss of potassium thereby, compensating for the osmotic effect of increased cell sodium; (3) calcium subsequently may have other deleterious effects through activation of proteases and externalization of phosphatidylserine in the exterior leaflet of the membrane.

Liver regeneration: a critical toxicodynamic response in predictive toxicology

The objective of the present review is to discuss the importance tissue repair in the mixture risk assessment. Studies have revealed the existence of two stages of toxicity: an inflictive stage (stage I) and progressive or regressive stage (stage II). While much is known about mechanisms by which injury is inflicted (stage I), very little is known about the mechanisms that lead to progression or regression of injury. A wide variety of additional experimental evidence suggests that tissue repair impacts decisively on the final toxic outcome and any modulation in this response has profound impact in the final outcome of toxicity. We designed the present research to investigate the importance of tissue repair in the final acute hepatotoxic outcome upon exposures to mixture of toxicants comprising thioacetamide (TA), allyl alcohol (AA), chloroform (CHCl(3)) and trichloroethylene (TCE). Dose response studies with individual compounds, binary mixtures (BM), ternary (TM) and quaternary mixtures (QM) have been conducted. Results of CHCl(3) + AA BM [Anand, S.S., Murthy, S.N., Vishal, V.S., Mumtaz, M.M., Mehendale, H.M., 2003. Tissue repair plays pivotal role in final outcome of supra-additive liver injury after chloroform and allyl alcohol binary mixture. Food Chem. Toxicol. 41, 1123] and CHCl(3) + AA + TA +TCE QM [Soni, M.G., Ramaiah, S.K., Mumtaz, M.M., Clewell, H., Mehendale, H.M., 1999. Toxicant-inflicted injury and stimulated tissue repair are opposing toxicodynamic forces in predictive toxicology. Regul. Phramcol. Toxicol. 19, 165], and two representative individual compounds (TA and AA) [Mangipudy, R.S., Chanda, S., Mehendale, H.M., 1995a. Tissue repair response as a function of dose in thioacetamide hepatotoxicity. Environ. Health Perspect. 103, 260; Soni, M.G., Ramaiah, S.K., Mumtaz, M.M., Clewell, H., Mehendale, H.M., 1999. Toxicant-inflicted injury and stimulated tissue repair are opposing toxicodynamic forces in predictive toxicology. Regul. Phramcol. Toxicol. 19, 165] are described in this review. In addition, modulation of tissue repair in the outcome of hepatotoxicity and its implications in the risk assessment have been discussed. Male Sprague-Dawley (S-D) rats (250-300g) received a single i.p. injection of individual toxicants as well as mixtures. Liver injury was assessed by plasma alanine amino transferase (ALT) and histopathology. Tissue regeneration response was measured by [(3)H]-thymidine ((3)H-T) incorporation into hepatocellular nuclear DNA and PCNA. Only ALT and (3)H-T data have been presented in this review for the sake of simplicity. Studies with individual hepatotoxicants showed a dose-related increase in injury as well as tissue repair up to a threshold dose. Beyond this threshold, tissue repair was inhibited, and liver injury progressed leading to mortality. Since the highest dose of individual compounds resulted in mortality, this dose was not employed for mixture studies. While CHCl(3) + AA BM caused supra-additive liver injury, QM caused additive liver injury. Due to the prompt and robust compensatory tissue repair, all the rats exposed to BM survived. With QM, the rats receiving the highest dose combination experienced some mortality consequent to the progression of liver injury attendant to suppressed tissue repair. These findings suggest that liver tissue repair, the opposing biological response that restores tissue lost to injury, may play a critical and determining role in the outcome of liver injury regardless of the number of toxicants in the mixture or the mechanism of initiation of injury. These data suggest that inclusion of this response in risk assessment might help in fine-tuning the prediction of toxic outcomes.