The 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.,We 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.,Activation 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).,Targeting SMURF2 may be a novel therapeutic approach for increasing the antitumor efficacy of MEK inhibitors.
Xeroderma pigmentosum (XP) is a rare, autosomal recessive disorder of DNA repair characterized by sun sensitivity and ultraviolet (UV) induced skin and mucous membrane cancers.,Described in 1874 by Moriz Kaposi in Vienna, nearly 100 years later James Cleaver in San Francisco reported defective DNA repair in XP cells.,This eventually provided the basis for a mechanistic link between sun exposure, DNA damage, somatic mutations and skin cancer.,XP cells were found to have defects in 7 of the proteins of the nucleotide excision repair pathway and in DNA polymerase eta.,XP cells are hypersensitive to killing by UV and XP cancers have characteristic “UV signature” mutations.,Clinical studies at NIH found a nearly 10,000-fold increase in skin cancer in XP patients under age 20 years demonstrating the substantial importance of DNA repair in cancer prevention in the general population.,About 25 % of XP patients have progressive neurological degeneration with progressive loss of neurons, probably from DNA damage induced by oxidative metabolism which kills non-dividing cells in the nervous system.,Interestingly, patients with another disorder, trichothiodystrophy have defects in some of the same genes as XP but they have primary developmental abnormalities without an increase in skin cancer.