We have previously reported that among the other death proteins, hepatic secretory phospholipase A? (sPLA?) is a leading mediator of progression of liver injury initiated by CCl? in rats. The aim of our present study was to test the hypothesis that increased hepatic sPLA? released after acetaminophen (APAP) challenge mediates progression of liver injury in wild type (WT) and COX-2 knockout (KO) mice. COX-2 WT and KO mice were administered a normally non lethal dose (400 mg/kg) of acetaminophen. The COX-2 KO mice suffered 60% mortality compared to 100% survival of the WT mice, suggesting higher susceptibility of COX-2 KO mice to sPLA?-mediated progression of acetaminophen hepatotoxicity. Liver injury was significantly higher at later time points in the KO mice compared to the WT mice indicating that the abatement of progression of injury requires the presence of COX-2. This difference in hepatotoxicity was not due to increased bioactivation of acetaminophen as indicated by unchanged cyp2E1 protein and covalently bound ¹?C-APAP in the livers of KO mice. Hepatic sPLA? activity and plasma TNF-? were significantly higher after APAP administration in the KO mice. This was accompanied by a corresponding fall in hepatic PGE? and lower compensatory liver regeneration and repair (³H-thymidine incorporation) in the KO mice. These results suggest that hindered compensatory tissue repair and poor resolution of inflammation for want of beneficial prostaglandins render the liver very vulnerable to sPLA?-mediated progression of liver injury. These findings are consistent with the destructive role of sPLA? in the progression and expansion of tissue injury as a result of continued hydrolytic breakdown of plasma membrane phospholipids of perinecrotic hepatocytes unless mitigated by sufficient co-induction of COX-2.
Drug resistance is the outcome of multiple-gene interactions in cancer cells under stress of anticancer agents. MDR1 overexpression is most commonly detected in drug-resistant cancers and accompanied with other gene alterations including enhanced glucosylceramide synthase (GCS). MDR1 encodes for P-glycoprotein that extrudes anticancer drugs. Polymorphisms of MDR1 disrupt the effects of P-glycoprotein antagonists and limit the success of drug resistance reversal in clinical trials. GCS converts ceramide to glucosylceramide, reducing the impact of ceramide-induced apoptosis and increasing glycosphingolipid (GSL) synthesis. Understanding the molecular mechanisms underlying MDR1 overexpression and how it interacts with GCS may find effective approaches to reverse drug resistance.
Once initiated, how tissue injury expands after high toxicant doses, even after their complete elimination, is not understood. Free-radical generation was initially proposed to mediate progression of injury. However, mechanisms proposed thus far have remained unsubstantiated. Necrotic injury is characterized by loss of osmoregulation, cell swelling, blebbing, and cell rupture. This exposes cytosolic enzymes, including proteases, phospholipases, and lysosomal Ca(2+)-dependent enzymes, to high extracellular calcium (Ca(2+)). Activated hydrolytic enzymes, termed death proteins, hydrolyze their substrates in the plasma membrane of neighboring cells, commencing self-perpetuated injury progression. Likewise, ischemia-reperfusion injury exposes the hydrolytic enzymes to high Ca(2+), fuelling the progression of tissue injury. This mechanism is independent of the offending toxicant that initiates the injury. I present here a case for therapeutic intervention with inhibitors directed against death proteins as a means to avert organ failure and death well after the poisoning event.
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