Attenuation of CD95-induced apoptosis by inorganic mercury: caspase-3 is not a direct target of low levels of Hg2

• Published in: Toxicology letters Vol. 155; no. 1; pp. 161 - 170
• Main Authors: MCCABE, Michael J, ECKLES, Kevin G, LANGDON, Margaret, CLARKSON, Thomas W, WHITEKUS, Michael J, ROSENSPIRE, Allen J
• Format: Journal Article
• Language: English
• Published: Shannon Elsevier Science 15-01-2005; Amsterdam
• Subjects: Apoptosis - drug effects; Biological and medical sciences; Blotting, Western; Caspase 3; Caspases - drug effects; Chemical and industrial products toxicology. Toxic occupational diseases; Cysteine - metabolism; fas Receptor - drug effects; fas Receptor - metabolism; Humans; Indicators and Reagents; Jurkat Cells; Medical sciences; Mercury - analysis; Mercury - toxicity; Metals and various inorganic compounds; Sulfhydryl Compounds - metabolism; Toxicology; Tumor Necrosis Factor-alpha - pharmacology; Peptidases; Target; Enzyme; Cell death; Cysteine endopeptidases; Fas Antigen; Hydrolases; CD95/Fas; Mercury; Caspase-3; Heavy metal; Apoptosis
• ISSN: 0378-4274; 1879-3169
• DOI: 10.1016/j.toxlet.2004.09.013

Exposure to environmental mercury may be a factor that contributes to idiosyncratic autoimmune disease. Studies have demonstrated that inorganic, ionic mercury (i.e., Hg2+) modulates several lymphocyte signal transduction pathways, which may be a mechanism whereby Hg2+ dysregulates the immune response. The CD95/Fas apoptotic signaling pathway, which is of critical importance in regulating peripheral tolerance, is disrupted by low and environmentally relevant concentrations of Hg2+. Activation of the cysteine protease caspase-3 is a critical component of both CD95-mediated and TNF-alpha-induced apoptosis. The present work demonstrates that Hg2+ selectively disrupts death receptor mediated caspase-3 activation, where CD95-mediated caspase-3 activation is impaired in Hg2+ treated cells; whereas TNF-alpha-induced caspase-3 activation is not. Using the fluorogenic caspase-3 substrate, Ac-DEVD-7-amino-4-methyl coumarin, to measure caspase-3 enzyme activity as well as Western blotting to track processing of the caspase-3 proenzyme, we have considered the potential direct and indirect effects of Hg2+ on caspase-3. At relatively high concentrations and in a cell-free system, Hg2+ is capable of targeting the active site cysteinyl of caspase-3 resulting in enzyme inhibition. However, at more environmentally relevant exposures, Hg2+ does not gain access in appreciable quantities to the intracellular compartment where caspase-3 resides. Collectively, these data establish that Hg2+ impairs CD95-mediated apoptosis by targeting a plasma membrane proximal signaling event. By measuring the cellular Hg2+ content following various exposure conditions, we have determined that a cellular Hg2+ burden of approximately 50 ng/10(6) cells is sufficient to impair CD95-mediated caspase-3 activation. The present study furthers an understanding of the mechanism whereby relatively low and non-cytotoxic concentrations of Hg2+ may disrupt peripheral tolerance leading to sustained autoimmune disease.