CREB1 and cancer: Considering the tumorigenic role of EP4 signaling and EP4-EGFR crosstalk, we investigated the expression of pCREB, CREB, pEGFR, EGFR, p-ERK1/2, ERK1, ERK2, EP4, and MRP4 in OVCAR-5 cancer cells following the combination treatment of BPD-based PDP and AH23848 (Figure 2A).