PTGER4 and carcinoma: Both in vivo and in vitro (Fig. 3A-G), we observed that PGE2 potently induces mucus expression, which has previously been reported to be mediated by prostaglandin E2 receptor EP4 subtype (PTGER4)-mediated mucin exocytosis in a human carcinoma cell line (Belley and Chadee, 1999).