Takayuki Katagiri

Inulavosin, a melanogenesis inhibitor, leads to mistargeting of tyrosinase to lysosomes and accelerates its degradation.

The melanosome is a highly specialized organelle where melanin is synthesized. Tyrosinase and tyrosinase-related protein-1 (Tyrp1) are major melanosomal membrane proteins and key enzymes for melanin synthesis in melanocytes. Inulavosin, a melanogenesis inhibitor isolated from Inula nervosa (Compositae), reduced the melanin content without affecting either the enzymatic activities or the transcription of tyrosinase or Tyrp1 in B16 melanoma cells. To our knowledge, this inhibitor is previously unreported. Electron-microscopic analyses revealed that inulavosin impaired late-stage development of melanosomes (stages III and IV), in which melanin is heavily deposited. However, it did not alter the early stages of melanosomes (stages I and II), when filamentous structure is observed. Immunofluorescence analyses showed that tyrosinase, but not Tyrp1, was specifically eliminated from melanosomes in cells treated with inulavosin. Unexpectedly, inulavosin specifically accelerated the degradation of tyrosinase but not other melanosomal/lysosomal membrane proteins (Tyrp1, Pmel17, and LGP85). The degradation of tyrosinase induced by inulavosin associated with lysosomes but not the proteasome. Interestingly, lysosomal protease inhibitors restored the melanogenesis but not the targeting of tyrosinase to melanosomes in the cells treated with inulavosin. Instead, colocalization of tyrosinase with lysosome-associated membrane protein-1 at late endosomes/multivesicular bodies and lysosomes was accentuated. Taken together, inulavosin inhibits melanogenesis as a result of mistargeting of tyrosinase to lysosomes.

Melanin synthesis inhibitors from Lespedeza floribunda.

In the course of our search for new melanin synthesis inhibitors from plants, 40 new flavonoids and 11 known flavonoids were isolated from the roots of Lespedeza floribunda Bunge. The structures of the new compounds were determined by MS and NMR analyses, and the absolute configurations by CD spectra. Many of the compounds inhibited melanin synthesis in normal human epidermal melanocytes (NHEM), and compounds 3, 7, 8, 11, 16, 24, 27, 29, 33, 43, 45, and 51 were particularly inhibitory. Their activities were stronger than that of hydroquinone, which is known as a major skin-lightening drug.