A Common Mediator of Resistance

Science Signaling  04 Dec 2012:
Vol. 5, Issue 253, pp. ec309
DOI: 10.1126/scisignal.2003835

In lung cancers with EML4-ALK (anaplastic lymphoma kinase) translocations or mutations that activate the epidermal growth factor receptor (EGFR), inhibitors that target ALK or EGFR quickly lose their effectiveness. Huang et al. (see also Guo and Wang) performed an RNA interference (RNAi) screen in the non–small-cell lung cancer (NSCLC) cell line H3122, which is sensitive to the ALK inhibitors crizotinib and NVP-TAE684. H3122 cells expressing small hairpin RNAs (shRNAs) directed against MED12, a component of the Mediator transcriptional coactivator complex, showed resistance to crizotinib and NVP-TAE684. In addition, transfection of MED12 shRNAs into the NSCLC cell lines PC9 and H3255, which have mutations in EGFR, conferred resistance to the EGFR inhibitors gefitinib and erlotinib. ALK and EGFR activate the Ras mitogen-activated kinase (MEK)–extracellular signal–regulated kinase (ERK) pathway, and phosphorylation and activation of ERK and MEK was higher in NVP-TAE684–treated H3122 cells and in gefitinib-treated PC9 and H3255 cells expressing MED12 shRNAs than in control cells. Several chemotherapeutics target BRAF or MEK, and knockdown of MED12 in multiple cancer cell lines that are sensitive to BRAF or MEK inhibitors led to the acquisition of resistance against these inhibitors. A shRNA screen directed against the kinome in MED12-silenced H3122 cells indicated that knockdown of transforming growth factor–β receptor II (TGF-βR2) restored sensitivity to crizotinib. TGF-β treatment of multiple cancer cell lines conferred resistance to inhibitors of EGFR, BRAF, or MEK, as well as to the DNA-damaging agent cisplatin. In various cancer cell lines, MED12 silencing increased the phosphorylation and activation of the TGF-β effector SMAD2, as well as expression of various TGF-β target genes, including those implicated in epithelial-mesenchymal transition (EMT), a process that is thought to contribute to invasion and metastasis. MED12 did not inhibit TGF-β signaling by suppressing transcription of TGF-βR2. Instead, a cytoplasmic pool of MED12, which was not associated with other components of the Mediator transcriptional coactivator complex, bound to unglycosylated or partially glycosylated TGF-βR2, resulting in reduced surface localization and overall abundance of TGF-βR2. A gene signature consisting of a subset of genes showing increased expression after MED12 knockdown correlated with decreased survival and response to 5-fluorouracil chemotherapy in individuals with colorectal cancer and sensitivity to MEK inhibitors in various cancer cell lines. In MED12-silenced H3122 cells, the TGF-βR inhibitor LY2157299 resensitized these cells to crizotinib. Thus, MED12 inhibits TGF-βRII signaling through a nontranscriptional mechanism, and TGF-βRII inhibitors could be useful in treating tumors showing resistance to tyrosine kinase inhibitors.

S. Huang, M. Hölzel, T. Knijnenburg, A. Schlicker, P. Roepman, U. McDermott, M. Garnett, W. Grernrum, C. Sun, A. Prahallad, F. H. Groenendijk, L. Mittempergher, W. Nijkamp, J. Neefjes, R. Salazar, P. ten Dijke, H. Uramoto, F. Tanaka, R. L. Beijersbergen, L. F. A. Wessels, R. Bernards, MED12 controls the response to multiple cancer drugs through regulation of TGF-β receptor signaling. Cell 151, 937–950 (2012). [PubMed]

X. Guo, X.-F. Wang, A mediator lost in the war on cancer. Cell 151, 927–929 (2012). [PubMed]