Mechanisms of Toxicant Mediated Cellular Injury

Transport and biotransformation

Toxicants in general

Toxicant whose primary mechanism of action is ATP depletion

J

th at e d

Oncosis

/OC

o

Apoptosis

Toxicant concentration

Toxicant concentration

FIGURE 15-13

The general relationship between oncosis and apoptosis after nephrotoxicant exposure. For many toxicants, low concentrations cause primarily apoptosis and oncosis occurs principally at higher concentrations. When the primary mechanism of action of the nephrotoxicant is ATP depletion, oncosis may be the predominant cause of cell death with limited apoptosis occurring.

GSH-Hg-GSH -CYS-Hg-CYSs

Lumen

Proximal tubular cell

■ GSH-Hg-GSH VGLY-CYS-Hg-CYS-GLY CYS-Hg-CYS CYS-Hg-CYS Na+

Dipeptidase y-gt'

Dipeptidase

S-"-"5 Neutral amino acid transporter

GSH-Hg-GSH

Urine Acivicin

CYS-Hg-CYS

CYS-Hg-CYS GSH-Hg-GSH

Na+ a-Ketoglutarate—a-Ketoglutarate

Dicarboxylate transporter

Blood

Dicarboxylic acids a-Ketoglutarate v a-Ketoglutarate

Organic anion transporter

Organic anion transporter a-Ketoglutarate v

Organic anions à^(PAH or T probenecid) -R-Hg-R-CYS-Hg-CYS GSH-Hg-GSH

FIGURE 15-14

The importance of cellular transport in mediating toxicity. Proximal tubular uptake of inorganic mercury is thought to be the result of the transport of mercuric conjugates (eg, diglutathione mercury conjugate [GSH-Hg-GSH], dicysteine mercuric conjugate [CYS-Hg-CYS]). At the luminal membrane, GSH-Hg-GSH appears to be metabolized by (-glutamyl transferase ((-GT) and a dipepti-dase to form CYS-Hg-CYS. The CYS-Hg-CYS may be taken up by an amino acid transporter. At the basolateral membrane, mercuric conjugates appear to be transported by the organic anion transporter. (-Ketoglutarate and the dicarboxylate transporter seem to play important roles in basolateral membrane uptake of mercuric conjugates. Uptake of mercuric-protein conjugates by endocytosis may play a minor role in the uptake of inorganic mercury transport. PAH—para-aminohippurate. (Courtesy of Dr. R. K. Zalups.)

High-affinity binding to macromolecules

Altered activity of critical macromolecules

Toxicant

Biotransformation

Reactive intermediate lz

Redox cycling

Covalent binding to macromolecules n

Increased reactive oxygen species

Damage to critical macromolecules

Oxidative damage to critical macromolecules

Cell injury

Cell repair

Cell death

FIGURE 15-15

Covalent and noncovalent binding versus oxidative stress mechanisms of cell injury. Nephrotoxicants are generally thought to produce cell injury and death through one of two mechanisms, either alone or in combination. In some cases the toxicant may have a high affinity for a specific macromolecule or class of macromole-cules that results in altered activity (increase or decrease) of these molecules, resulting in cell injury. Alternatively, the parent nephro-toxicant may not be toxic until it is biotransformed into a reactive intermediate that binds covalently to macromolecules and in turn alters their activity, resulting in cell injury. Finally, the toxicant may increase reactive oxygen species in the cells directly, after being bio-transformed into a reactive intermediate or through redox cycling. The resulting increase in reactive oxygen species results in oxida-tive damage and cell injury.

Plasma RSG

Plasma R-Cys "

Plasma R-NAC

Plasma RSG

Glomerular filtration a r-sg©

Covalent binding Cell injury R-NAC

Basolateral membrane

Brush border membrane

FIGURE 15-16

This figure illustrates the renal proximal tubular uptake, biotransformation, and toxicity of glutathione and cysteine conjugates and mer-capturic acids of haloalkanes and haloalkenes (R). 1) Formation of a glutathione conjugate within the renal cell (R-SG). 2) Secretion of the R-SG into the lumen. 3) Removal of the ^-glutamyl residue (7-Glu) by 7-glutamyl transferase. 4) Removal of the glycinyl residue (Gly) by a dipeptidase. 5) Luminal uptake of the cysteine conjugate (R-Cys). Basolateral membrane uptake of R-SG (6), R-Cys (7), and a mercap-turic acid (N-acetyl cysteine conjugate; R-NAC)(8). 9) Secretion of R-NAC into the lumen. 10) Acetylation of R-Cys to form R-NAC. 11) Deacetylation of R-NAC to form R-Cys. 12) Biotransformation of the penultimate nephrotoxic species (R-Cys) by cysteine conjugate P-lyase to a reactive intermediate (R-SH), ammonia, and pyruvate. 13) Binding of the reactive thiol to cellular macromolecules (eg, lipids, proteins) and initiation of cell injury. (Adapted from Monks and Lau [5]; with permission.)

Representative starting Submitochondrial fractions material A. Untreated B. TFEC (30 mg/kg)

Representative starting Submitochondrial fractions material A. Untreated B. TFEC (30 mg/kg)

Covalent binding of a nephrotoxicant metabolite in vivo to rat kidney tissue, localization of binding to the mitochondria, and identification of three proteins that bind to the nephrotoxicant. A, Binding of tetrafluo-roethyl-L-cysteine (TFEC) metabolites in vivo to rat kidney tissue detected immunohisto-chemically. Staining was localized to the S3 segments of the proximal tubule, the segment that undergoes necrosis. B, Immunoreactivity in untreated rat kidneys. C, Isolation and fractionation of renal cortical mitochondria from untreated and TFEC treated rats and immunoblot analysis revealed numerous proteins that bind to the nephrotoxicant (innerinner membrane, matrix-soluble matrix, outer-outer membrane, inter-intermembrane space). The identity of three of the proteins that bound to the nephrotoxicant: P84, mortalin (HSP70-like); P66, HSP 60; and P42, aspartate aminotransferase. Mr—relative molecular weight. (From Hayden et al. [6], and Bruschi et al. [7]; with permission.)

Lipid peroxidation and mitochondrial dysfunction

O * O Malondialdehyde

O-OH

O-OH

Lipid peroxyl radical

Hydrogen abstraction L'

Lipid h2o

Hydrogen abstraction R

Lipid radical

Diene conjugation

Lipid radical, conjugated diene Oxygen addition R

Lipid peroxyl radical

Hydrogen abstraction L'

Lipid hydroperoxide Fenton reaction HO^

H7i Ethane

Lipid hydroperoxide Fenton reaction HO^

Lipid alkoxyl radical Fragmentation

Lipid aldehyde

H7i Ethane

Lipid aldehyde

FIGURE 15-18

A simplified scheme of lipid peroxidation. The first step, hydrogen abstraction from the lipid by a radical (eg, hydroxyl), results in the formation of a lipid radical. Rearrangement of the lipid radical results in conjugated diene formation. The addition of oxygen results in a lipid peroxyl radical. Additional hydrogen abstraction results in the formation of a lipid hydroperoxide. The Fenton reaction produces a lipid alkoxyl radical and lipid fragmentation, resulting in lipid aldehydes and ethane. Alternatively, the lipid peroxyl radical can undergo a series of reactions that result in the formation of malondialdehyde.

50 4030-

50 4030-

—O— Control

TBHP (0.5 mmol)

-■- TBHP + DEF (1 mM) -r

-A- TBHP + DPPD (2 |M)

O-O------

Time, h

Control

TBHP

10080 60

Control DCVC

Control DCVC

10080 60

Time, h

Control

DCVC

Control

DCVC

FIGURE 15-19

A-D, Similarities and differences between oxidant-induced and halocarbon-cysteine conjugate-induced renal proximal tubular lipid peroxidation and cell death. The model oxidant t-butylhy-droperoxide (TBHP) and the halocarbon-cysteine conjugate dichlorovinyl-L-cysteine (DCVC) caused extensive lipid peroxidation after 1 hour of exposure and cell death (lactate dehydrogenase (LDH) release) over 6-hours' exposure. The iron chelator deferoxamine (DEF) and the antioxidant N,N'-diphenyl-1, 4-phenylenediamine (DPPD) completely blocked both the lipid peroxidation and cell death caused by TBHP. In contrast, while DEF and DPPD completely blocked the lipid peroxidation caused by DCVC, cell death was only delayed. These results suggest that the iron-mediated oxidative stress caused by TBHP is responsible for the observed toxicity, whereas the iron-mediated oxidative stress caused by DCVC accelerates cell death. One reason that cells die in the absence of iron-mediated oxidative stress is that DCVC causes marked mitochondrial dysfunction. (Data from Groves et al. [8], and Schellmann [9].)

Was this article helpful?

0 0

Post a comment