Melanocore uptake by keratinocytes occurs through phagocytosis and involves Protease-activated receptor-2 activation

In the skin epidermis, melanin is produced and stored within melanosomes in melanocytes and then transferred to keratinocytes. Different models have been proposed to explain the melanin transfer mechanism, which differ essentially in how melanin is transferred - either in a membrane-bound melanosome or as a melanosome core, i.e. melanocore. Here we investigated the endocytic route followed by melanocores and melanosomes during internalization by keratinocytes, by comparing the uptake of melanocores isolated from the supernatant of melanocyte cultures with melanosomes isolated from melanocytes. We show that inhibition of actin dynamics impairs the uptake of both melanocores and melanosomes. Moreover, depletion of critical proteins involved in actin-dependent uptake mechanisms, namely Rac1 and CtBP1/BARS, together with inhibition of Rac1-dependent signaling pathways or macropinocytosis suggest that melanocores are internalized by phagocytosis, whereas melanosomes are internalized by macropinocytosis. Furthermore, we confirmed that melanocore, but not melanosome uptake is dependent on the Protease-activated receptor-2 (PAR-2) and found that PAR-2 can be specifically activated by melanocores. As skin pigmentation was shown to be regulated by PAR-2 activation, our results further support the melanocore mechanism of melanin transfer and further refine this model, which can now be described as coupled melanocore exo/phagocytosis.