Although studies have shown that COX-2 induction/expression can inhibit apoptosis (10
), and that COX-2 inhibition can increase apoptosis (11
), little is known about the effects of the mechanism of COX-2. In the present study, using an in vivo
model, we investigated the involvement of the PGE2
receptors in UVB-induced apoptosis and show that epidermal apoptosis in COX-2−/− mice is mitigated by EP2 and EP4 agonists. Furthermore, we showed that the EP2- and EP4-mediated reduction in apoptosis was due to increased PKA and Akt activation and subsequent increased levels of antiapoptotic p-Bad.
The tumor suppressor gene, p53, is an important regulator of cell survival following UVB treatment (35
), and studies suggest a biochemical link between COX-2 and p53 (18
). Therefore, in our initial studies, we hypothesized that increased apoptosis in COX-2−/− mice might be related to increased p53 levels and/or activity. However, UVB-induced levels of p53 and a proapoptotic effector, Bax, were similar in WT and COX-2−/− mice (). The observation that p53 and Bax were similarly induced in UVB-exposed WT and COX-2−/− mice suggests that they would contribute about equally to UVB-induced apoptosis in both genotypes. Thus, modulation of p53 due to COX-2 deficiency was not the cause of the increased apoptosis observed in UVB-exposed COX-2−/− mice.
is the major prostaglandin formed in UV-exposed skin (5
), we focused on the possible involvement of the PGE2
receptors: EP1, EP2, EP3, and EP4. We observed that UVB irradiation induced the expression of EP1, EP2, and EP4 in WT mouse skin, but that expression of EP2 and EP4 was reduced in COX-2−/− mice (). EP2 and EP4 are coupled to the Gαs, and ligand binding has been reported to increase cAMP levels leading to PKA activation and to activate Akt (12
). Akt and PKA activation can mediate prosurvival pathways through the inactivation of proapoptotic proteins including Bad (15
). Activated Akt phosphorylates Ser136
of Bad, whereas PKA phosphorylates Ser112
of Bad. The phosphorylation of Bad at Ser112
, and Ser155
inactivates its proapoptotic function by causing the dissociation of Bad from mitochondrial Bcl-2 and/or Bcl-xL and the binding of p-Bad to the 14−3−3 scaffold proteins (16
). In our studies, no differences in the levels of Bcl-2 or Bcl-xL were observed between WT and COX-2−/− mice (data not shown). However, UVB activation of both Akt and PKA and phosphorylation of Bad at both Ser136
was decreased in COX-2−/− mice () and this decreased Bad phosphorylation in COX-2−/− mice compared with WT mice is a likely cause of the increased apoptosis observed in COX-2−/− mice. In support of our in vivo
findings, it was recently reported that PGE2
protected gastric mucosal cells in vitro
from ethanol-induced apoptosis via EP2 and EP4 activation (39
). Additionally, in the mouse intestine, PGE2
activation of EP2 protected the cells from γ-radiation–induced apoptosis (40
A significant finding of the present study was that both EP2 and EP4 agonists reduced epidermal apoptosis by 50% in UVB-exposed COX-2−/− mice resulting in a level of apoptosis about equal to that observed in WT mice (). Because COX-2−/− mice showed a 2.5-fold increase in apoptosis () and COX-2−/− deficiency reduced PGE2
in UVB-exposed skin by ~35% (26
), the data suggest that the protective effects of the agonists in COX-2−/− mice are mimicking the effects of PGE2
normally generated via COX-2.
illustrates a proposed model showing the roles of p53 and EP2/EP4 in UVB-induced epidermal apoptosis. UVB irradiation induces apoptosis through the induction of p53 and Bax expression in both WT and COX-2−/− mouse skin and presumably is PGE2 receptor independent. UVB also induces COX-2 expression and COX-2–mediated PGE2 production in WT, but not COX-2−/− mice, which results in the activation of the EP2 and EP4 receptors. Activation of these receptors in WT mice increases PKA and Akt activity, which causes multisite phosphorylation of Bad and abrogates the proapoptotic function of Bad and promotes cell survival.
Although in the present paper, we focused on the roles of EP2 and EP4 in the acute epidermal effects of UVB irradiation, the roles of the PGE2
receptors in UV-induced skin tumor formation have received little attention. Tober et al. (41
) reported that an EP1 antagonist reduced UVB-induced skin tumor formation in the Skh-1 hairless mouse. Furthermore, Lee et al. (42
) reported that EP2 mRNA was increased in UVB-induced papillomas and squamous cell carcinomas, and Tober et al. (43
) reported recently that EP2 mRNA levels were significantly elevated in chronically irradiated mouse skin. Based on the effects of the EP2 and EP4 agonists on UVB-induced epidermal apoptosis observed in the present study, further studies are needed to determine the effects of EP2 and EP4 deficiency, as well as of agonists and antagonists for these receptors on UV-induced skin tumor formation during chronic UV exposure.