Systematic identification of signaling pathways with potential to confer anticancer drug resistance

Pathway-centric screening reveals new mechanisms of drug resistance and combination therapeutic strategies. Finding New Targets for Drug-Resistant Cancers The development of drug resistance is a common problem in cancer patients. Knowing how drug resistance emerged in a tumor can inform clinical strategy. Martz et al. devised a drug screen to identify pathways of resistance when cancer cells were treated with drugs that are used in the clinic. Along with pathways known to mediate drug resistance, such as the MAPK and PI3K pathways, activation of the Notch1 pathway caused drug resistance in various types of cancer cells in culture. Inhibiting Notch1 signaling restored drug efficacy in cells in culture and in xenografts in mice. Intriguingly, Notch signaling mediated drug resistance to an estrogen receptor–targeted therapy used in breast cancer and to a kinase-targeted therapy used in melanoma, suggesting that this single pathway may be important in multiple types of drug-resistant cancers. Indeed, tumors of some patients with relapsed breast cancer or melanoma had increased markers of Notch1 signaling. In the Research Article by Winter et al. also in this issue, this screening method identified a pathway of drug resistance in bone marrow cancer. More generally, by screening entire signaling pathways instead of individual genes, the work of Martz et al. shows how we can quickly map pathways to the diverse properties of cancer cells. Cancer cells can activate diverse signaling pathways to evade the cytotoxic action of drugs. We created and screened a library of barcoded pathway-activating mutant complementary DNAs to identify those that enhanced the survival of cancer cells in the presence of 13 clinically relevant, targeted therapies. We found that activation of the RAS-MAPK (mitogen-activated protein kinase), Notch1, PI3K (phosphoinositide 3-kinase)–mTOR (mechanistic target of rapamycin), and ER (estrogen receptor) signaling pathways often conferred resistance to this selection of drugs. Activation of the Notch1 pathway promoted acquired resistance to tamoxifen (an ER-targeted therapy) in serially passaged breast cancer xenografts in mice, and treating mice with a γ-secretase inhibitor to inhibit Notch signaling restored tamoxifen sensitivity. Markers of Notch1 activity in tumor tissue correlated with resistance to tamoxifen in breast cancer patients. Similarly, activation of Notch1 signaling promoted acquired resistance to MAPK inhibitors in BRAFV600E melanoma cells in culture, and the abundance of Notch1 pathway markers was increased in tumors from a subset of melanoma patients. Thus, Notch1 signaling may be a therapeutic target in some drug-resistant breast cancers and melanomas. Additionally, multiple resistance pathways were activated in melanoma cell lines with intrinsic resistance to MAPK inhibitors, and simultaneous inhibition of these pathways synergistically induced drug sensitivity. These data illustrate the potential for systematic identification of the signaling pathways controlling drug resistance that could inform clinical strategies and drug development for multiple types of cancer. This approach may also be used to advance clinical options in other disease contexts.