Reduction of trans-4,5-dihydroxy-1,2-dithiane by cellular oxidoreductases activates gadd153/chop and grp78 transcription and induces cellular tolerance in kidney epithelial cells

trans-4,5-Dihydroxy-1,2-dithiane, the intramolecular disulfide form of dithiothreitol (DTTox) transcriptionally activates the stress-responsive genes gadd153(chop) and grp78. Herein, we used a renal epithelial cell line, LLC-PK1, to investigate the mechanism(s) whereby DTTox activates a molecular stress response. DTTox activated both grp78 and gadd153 transcriptionally, but gadd153 mRNA stability also increased suggesting that both transcriptional and posttranscriptional mechanisms are involved. DTTox did not activate hsp70 transcription indicating that a heat shock response was not induced. Structure-activity studies showed that DTTox analogues lacking the intramolecular disulfide were inactive. Furthermore, the ring-open intermolecular disulfide form of DTTox, 2-hydroxyethyl disulfide, was only a weak inducer of grp78 and gadd153 but was a strong inducer of hsp70 mRNA and a potent oxidant that lowered the NADPH/NADP+ ratio and depleted reduced glutathione (GSH). DTTox had little effect on the overall GSH and NADPH levels; thus cells were not undergoing oxidative stress; however, the NADPH/NADP+ ratio decreased slightly indicating that reducing equivalents were consumed. LLC-PK1 cells reduced DTTox to DTT, and the kinetics as well as the concentration dependence for reduction correlated with induction of both grp78 and gadd153 mRNA. Prior treatment with DTTox rendered cells tolerant to the potent nephrotoxicant S-(1,1,2, 2-tetrafluoroethyl)-L-cysteine. Bacitracin, an inhibitor of plasma membrane oxidoreductases, blocked DTTox reduction and gene activation as well as DTTox-induced tolerance. Thus, activation of stress genes and induction of cellular tolerance by DTTox is mediated by a novel mechanism involving cellular oxidoreductases.

The molecular response to reductive stress in LLC-PK1 renal epithelial cells: coordinate transcriptional regulation of gadd153 and grp78 genes by thiols

Organic thiols are toxic to eukaryotic cells. Treatment of cells with thiols activates expression of grp78, but it is not known if, like other forms of stress, there is a battery of stress response genes that are induced by thiols. In LLC-PK1 renal epithelial cells, mRNAs for both grp78 and gadd153 were induced by thiols with similar time, concentration and structure-activity dependence. Dithiothreitol (DTT) was the most potent reductant and inducer of gene expression among the thiols tested. Nuclear run-on assays demonstrated that DTT activated both grp78 and gadd153 genes transcriptionally. A hamster gadd153 promoter construct which contains enhancer elements necessary for gadd153 activation was stably integrated into the LLC-PK1 cell genome and was activated by DTT. Although auto-oxidation of thiols can generate active oxygen species, transcriptional activation of the gadd153 promoter was not due to formation of hydrogen peroxide or superoxide since neither catalase nor superoxide dismutase prevented activation of the gadd153 promoter by DTT. The concentration dependence for activation of the gadd153 promoter correlated with inhibition of dome formation and protein synthesis, two toxic effects of DTT in LLC-PK1 cells. Thus, both grp78 and gadd153 are members of a gene battery which is responsive to reductive stress. There appears to be considerable, but not complete, overlap between the upstream signaling pathways for activation of both genes.

Reversible and irreversible oxidant injury to PC12 cells by hydrogen peroxide

A simple and reproducible model to identify biochemical changes associated with the transition from reversible to irreversible oxidant injury and cell death was established using rat pheochromocytoma PC12 cells. Cells were subjected to a transient oxidative stress induced by exposure to hydrogen peroxide (H2O2). Reversible loss of high-energy phosphates, induced by exposing cells to 0.2 mM H2O2, was preceded by transient increases in cytosolic calcium with no loss of plasma membrane integrity, as indexed by release of cytosolic enzymes. In contrast, permanent loss of high-energy phosphates, induced by treating cells with 0.5 mH H2O2, was associated with sustained rises in cytosolic-free calcium and increased oxidation of pyruvate and palmitate, two mitochondrial substrates. Initial production of pyruvate and lactate was inhibited by exposure to 0.5 mM H2O2 but returned to values comparable to control values at one hour after treatment with H2O2. Compromise of the plasma membrane was a late event, occurring between 1 and 2 hours after exposure to 0.5 mM H2O2. Collectively, these data indicate that irreversible loss of high-energy phosphates and cell death caused by oxidative stress is more closely associated with altered mitochondrial function than with impaired glycolysis.

Axonal degeneration and axonal caliber alterations following combined beta,beta'-iminodipropionitrile (IDPN) and acrylamide administration

A new model of neurofilamentous axonal abnormality is described which employs combined administration of beta,beta'-iminodipropionitrile (IDPN) and acrylamide (AC). The model was developed to test the hypothesis that IDPN-induced swelling increases the vulnerability of the distal axon to a second neurotoxic chemical insult. Rats were given a single intraperitoneal (IP) injection of IDPN (1.5 g/kg) one week before receiving a single injection of AC (75 mg/kg, IP). Axonal degeneration was observed at multiple levels along the sciatic nerve at two weeks (with reference to IDPN administration), and was not progressive up to five weeks. Quantitation of degenerating fibers demonstrated that the extent of degeneration increased distally along the sciatic nerve. Single administration of either IDPN or AC did not produce degeneration. Thus, IDPN-induced neurofilamentous swellings alter the susceptibility of the axon to AC neurotoxicity. Two variations of this model were also studied. First, rats given five daily injections of AC (30 mg/kg, IP) beginning one week following IDPN administration developed accumulations of fast axonally transported materials in IDPN-induced microtubule channels. Second, rats given chronic injections of AC (30 mg/kg, IP, five days/week, for four weeks), to reduce the delivery of neurofilaments to the proximal axon, developed less prominent axonal enlargements when challenged with IDPN. Thus, axonal atrophy can mask the development of neurofilamentous axonal swellings.