Montagut Clara, Sharma Sreenath V, Shioda Toshi, McDermott Ultan, Ulman Matthew, Ulkus Lindsey E, Dias-Santagata Dora, Stubbs Hannah, Lee Diana Y, Singh Anurag, Drew Lisa, Haber Daniel A, Settleman Jeffrey
Center for Molecular Therapeutics, Massachusetts General Hospital Cancer Center, Harvard Medical School, Charlestown, Massachusetts, USA.
Cancer Res. 2008 Jun 15;68(12):4853-61. doi: 10.1158/0008-5472.CAN-07-6787.
Activating BRAF kinase mutations arise in approximately 7% of all human tumors, and preclinical studies have validated the RAF-mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase-ERK signaling cascade as a potentially important therapeutic target in this setting. Selective RAF kinase inhibitors are currently undergoing clinical development, and based on the experience with other kinase-targeted therapeutics, it is expected that clinical responses to these agents, if observed, will lead to the eventual emergence of drug resistance in most cases. Thus, it is important to establish molecular mechanisms underlying such resistance to develop effective therapeutic strategies to overcome or prevent drug resistance. To anticipate potential mechanisms of acquired resistance to RAF inhibitors during the course of treatment, we established drug-resistant clones from a human melanoma-derived cell line harboring the recurrent V600E activating BRAF mutation, which exhibits exquisite sensitivity to AZ628, a selective RAF kinase inhibitor. We determined that elevated CRAF protein levels account for the acquisition of resistance to AZ628 in these cells, associated with a switch from BRAF to CRAF dependency in tumor cells. We also found that elevated CRAF protein levels may similarly contribute to primary insensitivity to RAF inhibition in a subset of BRAF mutant tumor cells. Interestingly, AZ628-resistant cells demonstrating either primary drug insensitivity or acquired drug resistance exhibit exquisite sensitivity to the HSP90 inhibitor geldanamycin. Geldanamycin effectively promotes the degradation of CRAF, thereby revealing a potential therapeutic strategy to overcome resistance to RAF inhibition in a subset of BRAF mutant tumors.
激活型BRAF激酶突变出现在约7%的人类肿瘤中，临床前研究已证实RAF-丝裂原活化蛋白/细胞外信号调节激酶（ERK）激酶-ERK信号级联在这种情况下是一个潜在的重要治疗靶点。选择性RAF激酶抑制剂目前正在进行临床开发，基于其他激酶靶向治疗药物的经验，预计如果观察到对这些药物的临床反应，在大多数情况下最终会出现耐药性。因此，确定这种耐药性的分子机制对于制定有效的治疗策略以克服或预防耐药性很重要。为了预测治疗过程中对RAF抑制剂获得性耐药的潜在机制，我们从一个携带复发性V600E激活型BRAF突变的人黑色素瘤衍生细胞系中建立了耐药克隆，该细胞系对选择性RAF激酶抑制剂AZ628表现出极高的敏感性。我们确定，这些细胞中CRAF蛋白水平升高是对AZ628产生耐药性的原因，这与肿瘤细胞从依赖BRAF转变为依赖CRAF有关。我们还发现，在一部分BRAF突变肿瘤细胞中，CRAF蛋白水平升高可能同样导致对RAF抑制的原发性不敏感。有趣的是，表现出原发性药物不敏感或获得性耐药的AZ628耐药细胞对HSP90抑制剂格尔德霉素表现出极高的敏感性。格尔德霉素有效地促进了CRAF的降解，从而揭示了一种在一部分BRAF突变肿瘤中克服对RAF抑制耐药性的潜在治疗策略。
