Disrupting the MC1R/α-MSH-pCREB-MITF Axis: Rhein-based PROTAC D16 as a Potent Melanogenesis Inhibitor

Melanin protects skin from ultraviolet rays, but excessive or misdistributed synthesis can cause issues like melasma, freckles, or melanoma. Rhein from traditional Chinese herbs shows various bioactivities, with recent structural modifications enhancing its derivatives, but its effect on melanogenesis is unreported. The study reports synthesizing and evaluating D16, Rhein-based proteolysis targeting chimera (PROTACs) utilizing pomalidomide as an E3 ligand. D16 exhibited significantly reduced cytotoxicity, with half-maximal inhibitory concentration values exceeding 100 µM in both B16-F10 melanoma and human immortalized keratinocyte cells, indicating a low level of toxicity. In addition, mechanistic studies revealed that D16 suppresses melanin production primarily through the melanocortin 1 receptor/α-melanocyte-stimulating hormone signaling pathway, with further analysis suggesting phosphorylated cyclic adenosine monophosphate-response element binding protein (pCREB) as a key target. Through pCREB degradation, D16 disrupts microphthalmia-associated transcription factor transcription, leading to reduced levels of tyrosine. Molecular docking studies further confirmed strong binding between D16 and pCREB. Animal experiment results indicated that D16 effectively suppressed melanogenesis in mice. These findings underscore the potential of D16 to treat melanin-related disorders by targeting pCREB, advancing both the therapeutic utility of PROTACs and the application of pCREB modulation in pigmentation treatments.