Identification of Cytotoxic Drugs That Selectively Target Tumor Cells with MYC Overexpression

Expression of MYC is deregulated in a wide range of human cancers, and is often associated with aggressive disease and poorly differentiated tumor cells. Identification of compounds with selectivity for cells overexpressing MYC would hence be beneficial for the treatment of these tumors. For this purpose we used cell lines with conditional MYCN or c-MYC expression, to screen a library of 80 conventional cytotoxic compounds for their ability to reduce tumor cell viability and/or growth in a MYC dependent way. We found that 25% of the studied compounds induced apoptosis and/or inhibited proliferation in a MYC-specific manner. The activities of the majority of these were enhanced both by c-MYC or MYCN over-expression. Interestingly, these compounds were acting on distinct cellular targets, including microtubules (paclitaxel, podophyllotoxin, vinblastine) and topoisomerases (10-hydroxycamptothecin, camptothecin, daunorubicin, doxorubicin, etoposide) as well as DNA, RNA and protein synthesis and turnover (anisomycin, aphidicholin, gliotoxin, MG132, methotrexate, mitomycin C). Our data indicate that MYC overexpression sensitizes cells to disruption of specific pathways and that in most cases c-MYC and MYCN overexpression have similar effects on the responses to cytotoxic compounds. Treatment of the cells with topoisomerase I inhibitors led to down-regulation of MYC protein levels, while doxorubicin and the small molecule MYRA-A was found to disrupt MYC-Max interaction. We conclude that the MYC pathway is only targeted by a subset of conventional cytotoxic drugs currently used in the clinic. Elucidating the mechanisms underlying their specificity towards MYC may be of importance for optimizing treatment of tumors with MYC deregulation. Our data also underscores that MYC is an attractive target for novel therapies and that cellular screenings of chemical libraries can be a powerful tool for identifying compounds with a desired biological activity.

MYC基因（MYC）在多种人类癌症中发生表达失调，且常与侵袭性疾病及低分化肿瘤细胞显著相关。因此，筛选出对MYC过表达细胞具有选择性的化合物，将有助于此类肿瘤的治疗。为此，我们采用携带条件性MYCN基因（MYCN）或c-MYC基因（c-MYC）表达的细胞系，对包含80种传统细胞毒性化合物的文库进行筛选，以检测这些化合物能否以MYC依赖的方式降低肿瘤细胞活力和/或抑制其增殖。
我们发现，25%的受试化合物可通过MYC特异性方式诱导细胞凋亡和/或抑制增殖。其中大多数化合物的活性可通过c-MYC或MYCN过表达得到增强。值得注意的是，这些化合物作用于不同的细胞靶点，包括微管（紫杉醇（paclitaxel）、鬼臼毒素（podophyllotoxin）、长春碱（vinblastine））、拓扑异构酶（topoisomerases）（10-羟基喜树碱（10-hydroxycamptothecin）、喜树碱（camptothecin）、柔红霉素（daunorubicin）、多柔比星（doxorubicin）、依托泊苷（etoposide）），以及DNA、RNA和蛋白质的合成与代谢（茴香霉素（anisomycin）、阿非迪霉素（aphidicolin）、胶霉毒素（gliotoxin）、MG132、甲氨蝶呤（methotrexate）、丝裂霉素C（mitomycin C））。
我们的数据表明，MYC过表达可使细胞对特定通路的干扰更为敏感，且在大多数情况下，c-MYC与MYCN过表达对细胞毒性化合物应答的影响相似。经拓扑异构酶I抑制剂（topoisomerase I inhibitors）处理的细胞，其MYC蛋白水平会出现下调；而多柔比星与小分子MYRA-A则被证实能够破坏MYC-Max相互作用（MYC-Max interaction）。
我们得出结论：当前临床使用的传统细胞毒性药物中，仅有一部分可靶向MYC通路。阐明这类化合物对MYC的特异性作用机制，对于优化MYC失调肿瘤的治疗方案具有重要价值。本研究同时证实，MYC是新型治疗手段的极具吸引力的靶点，而基于细胞的化学文库筛选可作为识别具有预期生物学活性化合物的有力工具。