Inhibition of the growth of human melanoma cells with concomitant downregulation of β-catenin signaling by a novel dietary flavonoid fisetin: Implications for chemoprevention

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The prognosis of advanced melanoma remains extremely poor in spite of treatment advances, emphasizing the need to explore additional preventive approaches.One such approach is through chemoprevention by mechanism-based dietary intervention. Along this line various flavonoids, natural components of our diet are increasingly being investigated for their chemopreventive properties. Here, we show that fisetin, a dietary flavonoid known to be present in fruits and vegetables, such as strawberries, onions, oranges, apples, peaches, grapes, kiwifruit and persimmons may be one such agent. Fisetin treatment (20-80 μM; 24 h) of 451Lu human melanoma cells resulted in (i) decreased cell viability, (ii) cell cycle arrest in the G1/S phase and (iii) down-regulation of cell cycle regulatory molecules, cdks-2, -4, and -6 in a dose-dependent manner. Interestingly, fisetin-mediated cell growth inhibition was accompanied with significant downregulation of the anti-apoptotic protein Bcl2 with marginal effects on Bax and BclXl protein expressions. Aberrant activation of the Wnt/β-catenin pathway, present in one third of melanomas, may be responsible for altered expression of genes involved in cell cycle regulation. Thus, targeted disruption of β-catenin signaling seems to be a promising strategy for chemopreventive intervention of early melanoma. We found that fisetin disrupted β-catenin signaling through (i) a significant decrease in the protein expressions of Wnt3a, Wnt5a, Dishevelled (DVL) and coreceptor LRP5/6, (ii) an increase in the expression of endogenous inhibitors of Wnt signaling such as Dickkopf (DKK)-1 and Wnt inhibitory factor (WIF)-1, (iii) upregulation of the ubiquitin-mediated proteosomal degradation of β-catenin, as evidenced by increased β-TRCP levels, (iv) decreased nuclear accumulation of β-catenin, and (v) suppression of β-catenin /TCF complex formation. In addition, protein levels of the positively regulated TCF target MITF, c-myc and cyclin D1 were found to be downregulated by fisetin. Fisetin treatment further resulted in an increase in E-cadherin levels with a concomitant decrease in N-cadherin levels indicating an enhanced sequestration of β-catenin by E-cadherin. Taken together, our data suggest that inhibition of β-catenin-regulated transcription of target genes by fisetin provides a unique opportunity to develop chemopreventive approaches for human melanoma. Moreover, based on our findings it is tempting to suggest that fisetin can both prevent the onset of melanoma and slow tumor progression.