The incidence of
melanoma continues to rise. Inspite of treatment advances, the prognosis remains grim once the disease has metastasized, emphasizing the need to explore additional therapeutic strategies. One such approach is through the use of mechanism-based dietary intervention. We previously showed that the
flavonoid fisetin inhibits
melanoma cell proliferation, in vitro and in vivo. Here, we studied
fisetin-mediated regulation of
kinases involved in
melanoma growth and progression. Time-course analysis in 3-D
melanoma constructs that transitioned from radial to vertical growth showed that
fisetin treatment resulted in significant decrease in melanocytic lesions in contrast to untreated controls that showed large
tumor nests and invading disseminated cells. Further studies in
melanoma cultures and mouse xenografts showed that
fisetin-mediated growth inhibition was associated with dephosphorylation of AKT, mTOR and
p70S6K proteins. In silico modeling indicated direct interaction of
fisetin with mTOR and
p70S6K with favorable free energy values. These findings were validated by cell-free competition assays that established binding of
fisetin to
p70S6K and mTOR while little affinity was detected with AKT.
Kinase activity studies reflected similar trend with % inhibition observed for
p70S6K and mTOR at lower doses than AKT. Our studies characterized, for the first time, the differential interactions of any botanical agent with
kinases involved in
melanoma growth and demonstrate that
fisetin inhibits mTOR and
p70S6K through direct binding while the observed inhibitory effect of
fisetin on AKT is mediated indirectly, through targeting interrelated pathways.