Retinoids influence growth and differentiation of keratinocytes (KCs) and are widely used for the management of
skin diseases and for prevention of nonmelanoma
skin cancer (NMSC) in predisposed patients. Here we investigated the effect of
all-trans-retinoic acid (ATRA) on KC apoptosis. When KCs were cultured in confluent monolayers for several days, they acquired resistance against UVB-induced apoptosis. In contrast, when the cells were treated with 1 micromol/L ATRA for 6 days and subsequently irradiated with different doses of UVB, they underwent massive apoptosis as assessed by morphology, expression of activated
caspase-3, and DNA fragmentation. The same effect was observed when
doxorubicin was used instead of UVB. Analysis by real-time PCR and Western blot revealed that ATRA treatment strongly increased the
mRNA and
protein expression of p53 and
caspase-3, -6, -7, and -9, which are key regulators of apoptosis. UVB irradiation of ATRA-treated cells but not of control cells led to the accumulation of p53
protein and of its target gene Noxa. Inhibition of p53 and
caspases with alpha-
pifithrin and z-Val-
Ala-Asp-fluoromethyl
ketone, respectively, blocked UVB- and
doxorubicin-induced apoptosis in ATRA-treated KCs. Analogous to the observed ATRA effects in monolayer cultures, in vitro-generated organotypic skin cultures reacted with up-regulation of p53 and proapoptotic
caspases and displayed increased sensitivity to UVB-induced apoptosis. The ability of
retinoic acid to regulate the expression of proapoptotic genes and to sensitize KCs to apoptosis may play a role in their prevention of NMSC in transplant patients and patients with
DNA-repair deficiencies.