EVIDENCE THAT NITRIC OXIDE IS AN UPSTREAM MEDIATOR OF FASL-MEDIATED KERATINOCYTE APOPTOSIS IN TOXIC EPIDERMAL NECROLYSIS.

Toxic epidermal necrolysis (TEN or Lyell’s syndrome) is a severe adverse drug reaction that results in extensive necrolysis of the skin due to large scale Fas-FasL mediated keratinocyte apoptosis. The signaling pathways leading to the rapid upregulation of keratinocyte FasL expression in TEN are unknown. Here we show that FasL and inducible nitric oxide synthase (iNOS), an enzyme that leads to the production of the proinflammatory and cytotoxic molecule nitric oxide (NO), are coexpressed in increased amounts by keratinocytes in lesional skin samples of TEN. In vitro, within 8 h of exposure of keratinocytes to NO donors (NOC18 or SNP), a significant increase in FasL mRNA levels and cell surface FasL could be detected. Cell surface FasL induced by NO donors was lytically active, as within 8h both NOC 18 and SNP caused a 4-fold increase in keratinocyte apoptosis. The latter was completely abolished by the caspase-8 inhibitor ZVAD, and significantly reduced by Fas:Fc confirming that NO triggers rapid FasL-mediated keratinocyte cell death in vitro. Similarly, in an in vitro model of human skin (EpiDerm™, MatTek Corp.), NOC18 induced keratinocyte FasL expression, keratinocyte apoptosis and histopathological modifications reminiscent of those observed in early biopsy specimens of TEN. In this model, keratinocyte apoptosis could be virtually completely inhibited by the caspase-8 inhibitor Z-IETD-FMK. Taken together, our data show that iNOS via the action of NO is likely to be an upstream trigger of FasL-mediated keratinocyte apoptosis in TEN.

Apoptosis, Caspase-8 inhibitor, Cell surface FasL, Cytotoxic, EpiDerm, Fas-FasL mediated keratinocyte apoptosis, FasL, FasL mRNA, Histopathological modification, INOS, Inducible nitric oxide synthase (iNOS), Keratinocyte FasL expression, Keratinocyte apoptosis, Lyell's syndrome, Lytically active, Nitric oxide (NO), Proinflammatory, TEN, Toxic epidermal necrolysis, Z-IETD-FMK, ZVAD