Differential regulation of melanosomal proteins after hinokitiol treatment

Background: Melanogenesis is regulated by a series of enzymes under the control of microphthalmia-associated transcription factor (MITF). Objective: The aim of this study was to examine melanosome-associated protein levels in Met-Ab cells after hinokitiol treatment. Methods: We measured melanin contents and analyzed melanosome-associated protein levels using Western blot and RT-PCR analysis. Results: Hinokitiol markedly inhibited melanin synthesis and also reduced the protein levels of tyrosinase (TYR), tyrosinase-retated protein 1 (TYRP-1), tyrosinase-related protein 2 (TYRP-2) and MITF in Met-Ab cells. In addition, hinokitiol significantly increased the phosphorylations of extracellular signal-regulated kinases 1 and 2 (ERK1/2). Furthermore, reverse transcription-polymerase chain reaction (RTPCR) analysis revealed that TYR and MITF mRNA levels were significantly decreased but that levels of TYRP-1 and TYRP-2 mRNA were unaffected by hinokitiol treatment. These results suggest that hinokitiot-induced ERK phosphorylation reduces MITF and TYR transcription, and mediates the action of hinokitiot on metanogenesis. Interestingly, the mRNAs of TYRP-1 and TYRP-2 were unaffected, although the protein levels of TYRP-1 and TYRP-2 were down-regulated. Thus, the effects of hinokitiol on the transcription of TYR may differ from its effects on TYRP-1 and TYRP-2. Conclusion: Therefore, we suggest that TYRP-1 and TYRP-2 may be regulated by post-transtational degradation after hinokitiol treatment. (c) 2006 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved.