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Figure 3. Schematic summary of variations in expression of MC1R isoforms and the proposed model of their influence on constitutive and UVR-induced pigmentation. The total MC1R absolute quantity increases with total melanin content in NHM and acts on melanin synthesis via the stimulation of MITF and tyrosinase (top). At the isoform level, the most pigmented NHM maintain a high relative amount of MC1R317, which in turn increases the expression of MITF and tyrosinase to produce more melanin (middle). The opposite happens concerning MC1R350, which clearly inhibits MITF and tyrosinase and therefore melanin synthesis (bottom). However, the tanning observed after UV exposure is not due to changes in MC1R350, as its expression is not modified. Since, in the meantime, MC1R317 expression is increased upon UV-R exposure, the relative amount of MC1R350 is decreased (middle bottom).
