PGD2 inhibits mouse and human hair growth through GPR44. (A) Hair growth over the course of 16 days after depilation. 15-dPGJ2 (10 μg) or vehicle was applied topically to mouse skin starting on day 8 after depilation. Data are means ± SEM (n = 3 per treatment group). *P < 0.05 compared to control. (B) Hair length 10 days after topical PGD2 (1 μg; n = 3), 15-dPGJ2 (1 μg; n = 3), or vehicle (n = 3) treatment. Data are means ± SEM. *P < 0.05; **P < 0.01. (C) Hair growth in Ptgdr (n = 8), Ptgds (n = 6), and Gpr44 (n = 11) knockout (KO) mice in response to PGD2. Data are means ± SEM and are representative of three independent experiments per mouse. *P < 0.05 compared to vehicle. (D) In vitro growth of explanted human hair follicles over the course of 7 days in culture with PGD2 (n = 3), 15-dPGJ2 (n = 3), or vehicle (n = 3). Data are means ± SEM. ***P < 0.001 compared to vehicle.