Actin-containing sera from patients with adult respiratory distress syndrome are toxic to sheep pulmonary endothelial cells.

Actin released from damaged cells after a variety of tissue injuries appears to be involved in multiple organ dysfunction syndrome. Under experimental conditions, when the quantity of actin present in plasma is made to exceed the protective capacity of the actin-scavenging mechanism, microembolism and pulmonary vascular angiopathy have been noted in rats. It remains to be determined whether this injury is a result of a direct toxic effect or occurs indirectly via platelet activation or fibrin interactions. We examined the effect of sera from patients with adult respiratory distress syndrome (ARDS), as well as G-actin added to normal serum, on the viability, morphology, and function of cultured sheep pulmonary artery endothelial cells (SPAEC). Both patient sera and normal sera to which actin was added were toxic in the cell culture model; this toxicity could be abrogated, at least partially, by preincubation with gelsolin, which is known to complex with actin. A significant portion of the toxicity of sera from patients with ARDS was sensitive to heat (56 degrees C), suggesting an important role of complement. Sera from patients with ARDS were shown to contain filaments of F-actin by immunoblot and rhodamine phalloidin staining after ultracentrifugation. Thus, saturation of the actin-scavenging system by addition of exogenous G-actin to plasma produces direct pulmonary endothelial cell injury. Furthermore, plasma from patients with ARDS secondary to bacterial pneumonia is toxic to SPAEC, and a small but significant contributory role of actin is apparent in these studies.

Bifunctional anti/prooxidant potential of metallothionenin: redox signaling of copper binding and release.

Metallothioneins (MTs) are cysteine-rich metal-binding proteins that exert cytoprotection during metal exposure and oxidative stress. The roles of MT in copper (Cu) binding and release and modulation of redox cycling are unresolved. We hypothesized that Cu-binding to MT renders Cu redox inactive, but that oxidation of free thiols critical for metal binding can reduce MT/Cu interactions and potentiate Cu redox cycling. Overexpression of MT in cells by cadmium pretreatment or ectopic overexpression by gene transfer confers protection from Cu-dependent lipid oxidation and cytotoxicity. Using a chemically defined model system (Cu/ascorbate/H2O2) to study Cu/MT interactions, we observed that MT inhibited Cu-dependent oxidation of luminol. In the absence of H2O2, MT blocked Cu-dependent ascorbyl radical production with a stoichiometry corresponding to Cu/MT ratios < or = 12. In the presence of H2O2, Cu-dependent hydroxyl radical formation was inhibited only up to Cu/MT ratios < or = 6. Using low-temperature EPR of free Cu2+ to assess Cu/MT physical interactions, we observed that the maximal amount of Cu1+ bound to MT corresponded to 12 molar equivalents of Cu/MT with Cu and ascorbate alone and was reduced in the presence of H2O2. 2,2'-Dithiodipyridine titration of MT SH-groups revealed a 50% decrease after H2O2, which could be regenerated by dihydrolipoic acid (DHLA). DHLA regeneration of thiols in MT was accompanied by restoration of MT's ability to inhibit Cu-dependent oxidation of ascorbate. Thus, optimum ability of MT to inhibit Cu-redox cycling directly correlates with its ability to bind Cu. Some of this Cu, however, appears releasable following oxidation of the thiolate metal-binding clusters. We speculate that redox-dependent release of Cu from MT serves both as a mechanism for physiological delivery of Cu to specific target proteins, as well as potentiation of cellular damage during oxidative stress.