Interrogating the therapeutic impact of MYB overdose brought on by AKT inhibitor treatment in blood cancers.

正常细胞表现出由谱系相关基因驱动的细胞类型特异性分子依赖性。值得注意的是，这些依赖关系在恶性转化后仍然有效。这种现象的一个例子是在黑色素瘤中看到的，其中癌细胞仍然依赖于定义谱系的转录因子MITF （PMID:16510564）。重要的是，致癌BRAF突变（发生在大约一半的黑色素瘤病例中）使MITF功能对BRAF活性水平敏感（PMID:18628967），从而产生一种癌症特异性的倾向，即抑制BRAF通过抑制MITF功能选择性地杀死黑色素瘤细胞。矛盾的是，MITF过表达也可能在braf转化的黑素细胞中具有生长抑制作用，这表明，类似于致癌基因过量的毒性作用（PMID:26688666），过量的MITF信号输出可能对癌细胞有害。在造血和淋巴组织中，c-Myb是一个谱系定义转录因子，其功能是细胞存活所必需的。毫不奇怪，造血和淋巴系的癌细胞系对c-Myb缺失很敏感（PMID:28753431）。因此，c-Myb靶向可能代表了血红素恶性肿瘤的一种基于谱系成瘾的治疗策略。这种方法的一个明显限制是正常造血组织对c-Myb活性的固有依赖性，以及可能与c-Myb全身抑制相关的潜在毒性。然而，我们推断，如果血红素癌的致癌信号转导重新布线赋予特定的癌基因以调节c-Myb功能的新形态能力，那么这些癌基因将成为这些疾病的有吸引力的治疗靶点。PI3K通路是人类癌症中最常见的不受调控的通路之一。pi3k是一种异二聚体脂质激酶，它驱动磷酸肌苷第二信使的产生，激活包括AKT在内的许多效应蛋白。PI3K通路在人类白血病和淋巴瘤中的激活是有充分记录的（PMID:25100567,32326335），并促使许多临床试验来研究PI3K通路抑制剂的活性。有趣的是，AKT抑制剂在血液病中的应用尚未得到广泛评估（pmiid: 34858045）。我们和其他人（PMID:26989080）的初步数据表明，AKT抑制剂可以在白血病细胞系中诱导显著的细胞毒性反应（图1）。有趣的是，我们发现这些效应与c- MYB蛋白的上调相关（图2A）。PI3K6（造血细胞中PI3K活性的主要来源）抑制剂治疗后也出现了类似的效果，但MEK抑制剂曲美替尼治疗后没有出现类似效果（图2B）。总之，这些数据表明在血液恶性肿瘤中AKT激活和c-Myb调节之间存在潜在联系。与此一致的是，先前的研究表明，AKT磷酸化负调控转录因子fox01的敲低会导致MYB蛋白和MYB转录靶点的显著下调（PMID:31557894）。同样的研究表明，c- Myb的异位过表达在淋巴瘤细胞系中具有短暂的生长抑制作用。因此，我们假设血癌中由PI3K通路激活引起的分子重布线使c-Myb受到AKT的调节控制，本提案将通过机制进行研究（图3）。

Normal cells exhibit cell-type-specific molecular dependencies driven by lineage-associated genes. Notably, these dependencies remain functional following malignant transformation. One example of this phenomenon is seen in melanoma where cancer cells remain dependent on the lineage-defining transcription factor MITF (PMID:16510564). Importantly, oncogenic BRAF mutations (which occurs in approximately one half of melanoma cases) render MITF function sensitive to BRAF activity levels (PMID:18628967), thus creating a cancer-specific liability where inhibition of BRAF selectively kills melanoma cells through suppression of MITF function. Paradoxically, MITF overexpression can also have growth-inhibitory effects in BRAF-transformed melanocytes, suggesting that, akin to the toxic effects of oncogene overdose (PMID:26688666), excessive MITF signalling outputs can be detrimental to cancer cells.In hematopoietic and lymphoid tissues, c-Myb is a lineage defining transcription factor whose function is necessary for cell survival. Not surprisingly, cancer cell lines of hematopoietic and lymphoid lineages are sensitive to c-Myb depletion (PMID:28753431). Therefore, c-Myb targeting could represent a lineage-addiction-based therapeutic strategy in heme malignancies. One obvious limitation to such an approach is the inherent dependence of normal hematopoietic tissue to the activity of c-Myb, and the potential toxicities that could be associated with systemic inhibition of c-Myb. However, we reasoned that if oncogenic signal transduction rewiring in heme cancers confers specific oncogenes with a neomorphic ability to regulate c-Myb function, it would render these oncogenes attractive therapeutic targets for these diseases.The PI3K pathway is one of the most commonly deregulated pathways in human cancer. PI3Ks are heterodimeric lipid kinases that drive the generation of phosphoinositide second messengers that activate a number of effector proteins including AKT (PMID:12094235) . Activation of the PI3K pathway in human leukemias and lymphomas is well documented (PMID:25100567,32326335) and has prompted a number of clinical trials to investigate the activity of PI3K pathway inhibitors. Interestingly, AKT inhibitors have not been assessed very extensively in hematological cancers (PMID: 34858045). Our preliminary data, and that of others (PMID:26989080) show that AKT inhibitors can induce significant cytotoxic responses in leukemic cell lines (Figure 1). Interestingly, we find that these effects correlate with an upregulation of c- MYB protein (Figure 2A). Similar effects are seen following treatment with an inhibitor of PI3K6, the major source of PI3K activity in hematopoietic cells, but not when treated with the MEK inhibitor trametinib (Figure 2B). Altogether, these data suggest a potential link between AKT activation and c-Myb regulation in hematological malignancies. Consistently, it has been previously shown that knockdown of FOXO1, a transcription factor that is negatively regulated by AKT phosphorylation, causes significant downregulation of MYB protein and MYB transcription targets (PMID:31557894). The same study showed that ectopic overexpression of c- Myb had a transient growth inhibitory effect in a lymphoma cell line. We therefore hypothesise that the molecular rewiring caused by PI3K pathway activation in blood cancers renders c-Myb subject to regulatory control by AKT through mechanisms this proposal will investigate (Figure 3).