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Division of Dermatology, Sunnybrook Health Science Centre, and
Amgen Institute, Department of Medical Biophysics and Immunology, University of Toronto, Toronto, Canada
Excess exposure of skin to ultraviolet B (UVB) results in the
appearance of so-called sunburn cells. Although it has been
demonstrated that sunburn cells represent apoptotic keratinocytes, the
molecular mechanisms for UVB-induced apoptosis in keratinocytes have
not been fully elucidated. The cytokine, TNF-
, has been shown to
induce apoptosis in a variety of cell types. Since UVB induces
keratinocytes to release TNF-
, we hypothesized that TNF-
is
involved in UVB-induced apoptosis in keratinocytes. In order to confirm
this hypothesis and to further delineate which type of TNF receptor
signaling mediates the apoptosis pathway, we performed both in vivo and
in vitro experiments using gene-targeted knockout mice lacking either
the TNF p55 receptor or the TNF p75 receptor. In the in vivo study,
wild-type and mutant mice were exposed to UVB, and apoptotic
keratinocytes were detected by examining DNA fragmentation using in
situ nick-end labeling. For the in vitro experiments, keratinocytes
derived from the wild-type and mutant mice were irradiated with UVB,
and the degree of apoptosis was determined by flow cytometry, nick-end
labeling of DNA, and a DNA ladder assay. Both in vivo and in vitro
studies demonstrated that the deletion of TNF receptor p55 could
suppress UVB-induced apoptosis in keratinocytes. Our observations
support the notion that TNF-
is involved in UVB-induced keratinocyte
apoptosis, and demonstrate that p55 receptor signaling plays a pivotal
role in this event.
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