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

Toxicol Lett. 2005 Jan 15;155(1):161-70. 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.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Apoptosis / drug effects*
  • Blotting, Western
  • Caspase 3
  • Caspases / drug effects*
  • Cysteine / metabolism
  • Humans
  • Indicators and Reagents
  • Jurkat Cells
  • Mercury / analysis
  • Mercury / toxicity*
  • Sulfhydryl Compounds / metabolism
  • Tumor Necrosis Factor-alpha / pharmacology
  • fas Receptor / drug effects*
  • fas Receptor / metabolism


  • Indicators and Reagents
  • Sulfhydryl Compounds
  • Tumor Necrosis Factor-alpha
  • fas Receptor
  • CASP3 protein, human
  • Caspase 3
  • Caspases
  • Mercury
  • Cysteine