This article reviews advances in the study of the molecular mechanisms for
ultraviolet (UV)-induced keratinocyte apoptosis, with particular reference
to the cytokines tumor necrosis factor-alpha (TNF-alpha) and Fas Ligand (Fa
sL). TNF-alpha and FasL induce their respective receptors and then activate
caspase enzymes that are critically involved in the apoptotic process. Thi
s activation is further amplified by intracellular mitochondria-associated
mechanisms. Using gene-targeted knockout mice lacking either the TNF-Rp55 o
r the TNF-Rp75, we have shown that TNF-alpha plays an important role in UV-
induced keratinocyte apoptosis via TNF-Rp55, TNF-Rp55 shares homology with
Fas and contains an intracellular death domain. UV seems to directly stimul
ate cross-linking of Fas, resulting in the engagement of the death machiner
y. Fas-associated death domain protein (FADD) acts as an adapter protein in
both the TNF-Rp55 and Fas death-inducing cascades and is responsible for d
ownstream signal transduction by recruiting caspases, Moreover, signaling o
f p53 contributes to the induction of apoptosis by regulating Bcl-2 family
expression and increasing surface Fas expression. In addition to induction
mechanisms of apoptosis, there are numerous inhibitory molecules that play
a role in restricting the apoptotic pathway. Thus, the ultimate determinati
on of whether or not a cell undergoes apoptosis after UV radiation is based
on the balance between agonist and antagonist pathways.