Role of PGE2 Receptors in Mouse Skin Cancer

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We previously reported that the prostaglandin E2 (PGE2) G protein coupled receptor (GPCR), EP2, plays important roles in mouse skin tumor development using the initiation promotion model (Chun et al., Carcinogenesis, 30:1620, 2009). Because keratinocyte proliferation is essential for skin tumor development, EP2-mediated signaling pathways that could contribute to keratinocyte proliferation were investigated. A single topical application of the EP2 agonist, butaprost, dose-dependently increased keratinocyte replication and activated EGFR and PKA; and inhibition of either EGFR or PKA (AG1478 or H89, respectively) decreased butaprost-induced keratinocyte proliferation by 70%. Because previous studies indicated that GPCR activation of EGFR involved a b-arrestin1/Src complex, the possibility of EP2 acting via this mechanism in keratinocytes was investigated. It was observed that butaprost increased Src and EGFR activation and induced b-arrestin1/Src complex formation. b-arrestin1-deficiency reduced Src and EGFR activation, confirming b-arrestin1s role in their activation. In agreement with b-arrestin1 contributing to Src and EGFR activation, studies with Src and EGFR inhibitors (PP2 and AG1478, respectively) indicated Src to be upstream of EGFR. Butaprost also induced the activation of Akt, ERK1/2, and STAT3; and either b-arrestin1-deficiency or EGFR inhibition decreased their activation, confirming roles for b-arrestin1 and EGFR in their activation. In addition to EGFR activation, butaprost also increased cAMP levels and PKA activation, as measured by p-GSK3b and p-CREB formation. The PKA inhibitor, H89, decreased butaprost-induced GSK3b, CREB, and ERK activation, but did not affect EGFR activation. Thus, our recent results indicate that EP2 contributed to mouse keratinocyte proliferation by G protein independent, barrestin1 dependent activation of EGFR, and G protein dependent activation of PKA. In addition to EP2 contributing to skin tumor formation in the initiation/promotion model (Chun et al., Carcinogenesis, 30:1620, 2009), we previously reported that EP2 played a role in an anti-apoptotiic response in UVB-exposed mouse skin (Chun et al. Cancer Res., 67:2015, 2007). Because survivin is a regulator of cell survival, the possible regulation of survivin by COX-2 and EP2 in UVB-exposed mouse skin was investigated. In wild type mouse skin UVB time-dependently increased the levels of COX-2, survivin and phosphorylated-signal transducer and activator of transcription3 (p-STAT3), a transcription factor that regulates survivin expression; and COX-2- or EP2-deficiency significantly reduced their induction. Topical application of the COX-2 selective inhibitor, celecoxib, also reduced UVB-induced survivin levels. To further investigate the roles of PGE2 and EP2 in the regulation of survivin, indomethacin was used to inhibit UVB-induced endogenous PG production. Indomethacin reduced UVB-induced survivin levels, while PGE2 and the EP2 agonist, butaprost, partially restored survivin levels. The epidermal growth factor receptor (EGFR) is a downstream effector of EP2 and inhibition of EGFR (AG1478) significantly reduced UVB induction of survivin and activation of STAT3. UVB-induced epidermal apoptosis in COX-2-/- mice was reduced by butaprost and EGFR inhibition blocked butaprosts protective effects. Furthermore, butaprost in the absence of UVB exposure time-dependently increased p-EGFR, p-STAT3 and survivin levels in nave mouse skin, whereas the EP4 agonist, PGE1 alcohol, did not significantly increase p-STAT3 or survivin levels. These data suggest that COX-2-generated PGE2 regulates survivin expression in mouse skin via an EP2-mediated EGFR/STAT3 pathway, and inhibiting the EP2/survivin pathway may provide a therapeutic strategy for the chemoprevention/chemotherapy of UVB-induced skin cancer. Recent studies have indicated that B-arrestin1 deficiency reduced skin tumor multiplicity and tumor size in the initiation/ promotion model, whereas B-arrestin2 deficiency increased tumor multiplicity and tumor sizes. The differing effects of B-arrestin1/2 deficiency are currently being investigated.