Down-Regulation of SOX2 Underlies the Inhibitory Effects of the Triphenylmethane Gentian Violet on Melanoma Cell Self-Renewal and Survival

Abstract

Human melanomas contain a population of tumor-initiating cells that are able to maintain the growth of the tumor. We previously showed that the embryonic transcription factor SOX2 is essential for self-renewal and tumorigenicity of human melanoma-initiating cells. However, targeting a transcription factor is still challenging. Gentian violet (GV) is a cationic triphenylmethane dye with potent antifungal and antibacterial activity. Recently, a combination therapy of imiquimod and GV has shown an inhibitory effect against melanoma metastases. Whether and how GV affects melanoma cells remains unknown. Here we show that GV represses melanoma stem cell self-renewal through inhibition of SOX2. Mechanistically, GV hinders EGFR activation and inhibits the signal transducer and activator of transcription-3 [(STAT3)/SOX2] axis. Importantly, we show that GV treatment decreases STAT3 phosphorylation at residue tyrosine 705, thus preventing the translocation of STAT3 into the nucleus and its binding to SOX2 promoter. In addition, GV affects melanoma cell growth by promoting mitochondrial apoptosis and G2 cell cycle arrest. This study shows that in melanoma, GV affects both the stem cell and the tumor bulk compartments, suggesting the potential use of GV in treating human melanoma alone or in combination with targeted therapy and/or immunotherapy.

Knockdown of STAT3 targets a subpopulation of invasive melanoma stem‐like cells

“…Our results are in agreement with a recent study showing the higher expression of STAT3 in a population of tumor‐initiating cells in murine melanomas (Wang et al, ) characterized by the increased expression of four stemness factors ( Oct4 , Sox2, Klf4 , and c‐Myc ). Other studies exploring the plasticity of human melanoma cells confirm our findings (Ohanna et al, ; Pietrobono et al, ; Wang et al, ). Secretome of senescent melanoma cells increased the abundance of melanoma CSCs with elevated STAT3 activation and signaling (Ohanna et al, ).…”

SOX2‐mediated upregulation of CD24 promotes adaptive resistance toward targeted therapy in melanoma

“…Likewise, in A375 SOX2 OE cells that showed strong STAT3 activation the PP2 treatment did not alter this status. 35 The authors suggested that GV suppressed STAT3 activation through an EGFRdependent mechanism. Interestingly, in SK Mel 28 cells the OE of CD24 leads to the activation of Src but not of STAT3 ( Supporting Information, Fig.…”

“…In addition, we could observe an upregulation of STAT3 already after 6 hr of vem treatment while Src was only activated after 96 hr of vem treatment (data not shown). 35 Thus, it is quite possible that diverse signalling pathways can lead to STAT3 activation which in turn can trigger SOX2 expression. 4g).…”

Regression of diffuse B‐cell lymphoma of the leg with intralesional gentian violet

“…We also propose the following biomarkers to assess the sequence of events in gentian violet‐mediated tumor regression. Sox2 is one of the Yamanaka factors in tumor stem cells, and we have recently demonstrated that gentian violet downregulates Sox2 . Immunohistochemistry for Sox2 could be performed in vivo in lymphomas treated with gentian violet.…”