Inhibitors of melanogenesis in B16 melanoma 4A5 cells from flower buds of Lawsonia inermis (Henna).

The methanolic extract of Lawsonia inermis L. (henna) showed significant inhibitory activity toward melanogenesis in B16 melanoma 4A5 cells. Among the constituents isolated from the methanolic extract, luteolin, quercetin, and (±)-eriodictyol showed stronger inhibitory activity than the reference compound, arbutin. Several structure-activity relationships of the flavonoids were suggested, and OGlc<H=OH at the 3-position, OGlc<OH at the 4'-position, and the double bond between the 2- and 3-positions were important. The active constituents suppressed tyrosinase, tyrosinase related protein (TRP)-1, and TRP-2 mRNA expression. The suppression was considered as one of the mechanisms of action. Furthermore, the methanolic extract and several constituents, including luteoloside and spiraeoside, showed anti-plasmin activity, which is considered to play a key role in UV-stimulated melanogenesis in human skin.