BackgroundThe
mitogen-activated protein-kinase pathway consisting of the kinases RAF, MEK,
and ERK is central to cell proliferation and survival and is deregulated in
more than 90% of melanomas. MEK inhibitors are currently trialled in the
clinic, but despite efficient target inhibition, cytostatic rather than
cytotoxic activity limits their efficacy.MethodsWe assessed the cytotoxicity to
MEK inhibitors (PD184352 and selumetinib) in melanoma cells by toluidine-blue
staining, caspase 3 cleavage, and melanoma-sphere growth. Western blotting and
quantitative real-time polymerase chain reaction were applied to determine
SMAD-specific E3 ubiquitin protein ligase 2 (SMURF2), PAX3, and MITF
expression. Human melanoma samples (n = 77) from various stages were analyzed
for SMURF2 and PAX3 expression. RNA interference was performed to target SMURF2
during MEK inhibition in vivo in melanoma xenografts in mice and zebrafish. All
statistical tests were two-sided.ResultsActivation of transforming growth
factor β (TGF-β) signalling sensitized melanoma cells to the cytotoxic effects
of MEK inhibition. Melanoma cells resistant to the cytotoxic effects of MEK
inhibitors counteracted TGF-β signalling through overexpression of the E3
ubiquitin ligase SMURF2, which resulted in increased expression of the
transcription factors PAX3 and MITF. High MITF expression protected melanoma
cells against MEK inhibitor cytotoxicity. Depleting SMURF2 reduced MITF expression
and substantially lowered the threshold for MEK inhibitor-induced apoptosis.
Moreover, SMURF2 depletion sensitized melanoma cells to the cytotoxic effects
of selumetinib, leading to cell death at concentrations approximately 100-fold
lower than the concentration required to induce cell death in SMURF2-expressing
cells. Mice treated with selumetinib alone at a dosage of 10mg/kg body weight
once daily produced no response, but in combination with SMURF2 depletion,
selumetinib suppressed tumor growth by 97.9% (95% confidence interval = 38.65%
to 155.50%, P = .005).ConclusionsTargeting SMURF2 may be a novel therapeutic
approach for increasing the antitumor efficacy of MEK inhibitors.
Source:
Effect of SMURF2 Targeting on Susceptibility to MEK Inhibitors in
Melanoma. Smith MP, Ferguson J, Arozarena I, Hayward
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