Norartocarpetin inhibits tryosinase expression by activating phospho-MAPK. (A) p-ERK, (B) p-JNK, (C) p-p38. (D) melanin content in cells treated for 24 h with MAPK inhibitors with or without 10 μM of norartocarpetin. The different superscript letters indicate significant difference at p < 0.05 as analyzed by Student’s t test. *Significantly different from that at 0 h.