Toll-like receptor 4 (TLR4) has been linked to melanoma development. Signal transducer and activator of transcription 3 (STAT3) has been identified as a major oncogene in melanoma progression. Interplay between TLR4 and STAT3 in melanoma is unknown. In this study, we established the TLR4/STAT3 pathway in melanoma. We also investigated the anti- melanoma effects of natural products that target this pathway. Human melanoma tissue microarray was used to analyze the correlation between TLR4 expression and STAT3 phosphorylation/activation in melanoma. siRNA-mediated gene silencing was used to determine the individual role of TLR4, and its adaptors MYD88 and TRIF in TLR4 ligands-induced STAT3 activation. B16 STAT3β (overexpressing a dominant- negative variant of STAT3) and B16 NC (harbouring the empty vector) stable lines were used to determine the role of STAT3 in TLR4-mediated melanoma development. Anti-melanoma effects of natural products parthenolide and atractylenolide II were investigated. TLR4 expression positively correlates with STAT3 activation in human melanoma samples. TLR4 ligands activate STAT3 through MYD88 and TRIF in melanoma cells and promote melanoma development. Intratumoral activation of TLR4 increases STAT3 activation in the tumor, promotes tumor growth, angiogenesis and immunosuppressive microenvironment formation in mice. The effects mediated by activating TLR4 are weakened by suppressing STAT3 function in melanoma. Phytocompounds parthenolide and atractylenolide II inhibit the TLR4/STAT3 pathway in melanoma tumors and reduces tumor growth in melanoma-bearing mice. In summary, we for the first time found that TLR4/STAT3 signaling plays a pathogenic role in melanoma, and that natural products targeting this signaling pathway have a potential to be used as anti-melanoma agents.

Keywords: Melanoma; TLR4/STAT3 pathway; Parthenolide; Atractylenolide II