Pro-Oncogenic Role of LKB1 in NSCLC

描述（由申请人提供）：EGFR激活突变常出现在非吸烟相关肺癌中。这些肿瘤中的大多数对EGFR信号传导成瘾，并且可以用酪氨酸激酶抑制剂（TKI）如吉非替尼有效治疗。肺癌患者对吉非替尼的高敏感性与EGFR信号转导对AKT活化的依赖性密切相关，吉非替尼治疗可抑制AKT活化。最近发现，AKT功能的恢复是获得TKI耐药性的EGFR突变肺肿瘤的关键一步，一种潜在机制涉及c-MET的扩增，这绕过了对EGFR的需求，使EGFR抑制剂无效。 LKB 1通常被称为肿瘤抑制因子，因为体细胞LKB 1失活突变是吸烟相关肺癌中最常见的突变基因之一。然而，LKB 1突变很少在EGFR突变的肺癌中发现。我们发现了LKB 1在依赖EGFR信号传导的肺癌中的一种新的潜在致癌作用。我们发现LKB 1表达的抑制导致细胞凋亡，在6个细胞系中，无论是EGFR或AKT是组成型激活，但不是在两个细胞系没有EGFR或AKT激活。更重要的是，通过两种已知机制获得TKI抗性的EGFR突变细胞仍然倾向于由LKB 1耗尽诱导的凋亡。 从机制上讲，我们发现EGFR/AKT激活导致LKB 1野生型细胞中FoxO 3A在苏氨酸32（Thr 32）处的磷酸化增加，但在LKB 1缺失细胞中则不然。在具有野生型LKB 1的细胞中LKB 1的消耗导致由活化的EGFR/AKT引起的FoxO 3A的磷酸化减弱，而在LKB 1缺失细胞中LKB 1功能的恢复重新建立了EGFR/AKT介导的FoxO 3A磷酸化。在将我们的分析扩展到其他AKT靶点后，使用三种不同的同基因LKB 1敲低细胞系对和磷酸特异性抗体微阵列，我们观察到其他AKT下游靶点的磷酸化需要LKB 1，包括BAD（Ser 136），FoxO 1（Ser 319），FoxO 4（Ser 197）和GSK 32（Ser 9）。由于AKT对这些位点的磷酸化抑制了细胞凋亡，因此对LKB 1的需求表明LKB 1可能在依赖EGFR信号传导的肿瘤细胞中具有促细胞凋亡作用。 总之，我们认为LKB 1在肺癌的发展中起着双重作用。虽然它是吸烟引起的肺癌的肿瘤抑制因子，但我们假设LKB 1/AMPK通路的完整性是非吸烟相关肺癌中促凋亡蛋白磷酸化和失活的关键决定因素，这些肺癌依赖于EGFR信号传导。更好地了解LKB 1/AMPK通路在NSCLC中的这种新的、潜在的促癌作用及其在化疗敏感性中的作用可能对现有疗法（如EGFR抑制剂）的靶向临床应用产生直接影响，并可能为未来实施“个性化”治疗提供分子基础。

DESCRIPTION (provided by applicant): EGFR activation mutations are frequently present in non-smoking related lung cancer. Most of these tumors are addicted to EGFR signaling and can be effectively treated with tyrosine kinase inhibitors (TKI), such as gefitinib. High sensitivity to gefitinib in lung cancer is closely correlated with dependence on AKT activation in response to EGFR signaling, and gefitinib treatment suppresses AKT activation. The restoration of AKT function was recently found to be a key step in EGFR mutant lung tumors that acquired resistance to TKI, and one potential mechanism involves the amplification of c-MET which bypasses the requirement for EGFR, rendering EGFR inhibitors ineffective. LKB1 is commonly known as a tumor suppressor because somatic LKB1 inactivation mutation is one of the most frequently mutated genes in smoking-related lung cancer. However, LKB1 mutations were rarely found in lung cancers with EGFR mutations. We have discovered a novel, potentially oncogenic role for LKB1 in lung cancers that are addicted to EGFR signaling. We found the suppression of LKB1 expression led to apoptosis in six cell lines in which either EGFR or AKT is constitutively active, but not in two cell lines without EGFR or AKT activation. More importantly, EGFR mutant cells with acquired TKI-resistance through two known mechanisms are still prone to apoptosis induced by LKB1 depletion. Mechanistically, we found that EGFR/AKT activation led to increased phosphorylation of FoxO3A at threonine 32 (Thr32) in LKB1 wild-type cells, but not in LKB1-null cells. Depletion of LKB1 in the cells with wild- type LKB1 resulted in attenuation of that phosphorylation of FoxO3A by activated EGFR/AKT, while the restoration of LKB1 function in LKB1-null cells re-established EGFR/AKT mediated FoxO3A phosphorylation. Upon expanding our analysis to other AKT targets, using three different isogenic LKB1 knockdown cell line pairs and a phospho-specific antibody microarray, we observed that there was a requirement for LKB1 in the phosphorylation of other AKT down-stream targets, including BAD (Ser136), FoxO1 (Ser319), FoxO4 (Ser197) and GSK32 (Ser9). Because the phosphorylation of these sites by AKT suppresses apoptosis, the requirement of LKB1 suggests that LKB1 may have a pro-apoptotic role in tumor cells that are addicted to EGFR signaling. In summary, we believe LKB1 plays dual roles in lung cancer development. While it is a tumor-suppressor in lung cancer caused by smoking, we hypothesize that the integrity of the LKB1/AMPK pathway is a critical determinant of the phosphorylation and inactivation of pro-apoptotic proteins in non-smoking related lung cancers that are addicted to EGFR signaling. A better understanding of this novel, potential pro-oncogenic role of the LKB1/AMPK pathway in NSCLC and its role in chemosensitivity may have a direct impact on the targeted clinical use of existing therapies, such as EGFR inhibitors, and may provide a molecular basis for future implementation of "personalized" therapy.