The role of FOXO transcriptional factors in TSC-mediated tumorigenesis

描述(申请人提供)：结节性硬化症(TSC)是由TSC1或TSC2基因突变引起的，其特征是在各种器官形成错构瘤。然而，在TSC中恶性发展是非常罕见的。TSC1-TSC2复合体是哺乳动物雷帕霉素靶标(MTOR)的关键负性调节因子。研究表明，mTOR介导的负反馈环导致TSC相关肿瘤中Akt信号的减弱，并影响TSC缺陷细胞的存活，这可能是TSC罕见恶变的原因。然而，对于(I)介导TSC促生存功能的Akt的潜在机制和关键下游效应因子(S)以及(Ii)TSC相关肿瘤减弱型的基础仍不确定。最近对小鼠模型和人类癌症的遗传学研究有力地表明，FoxO转录因子是PI3K-AKT信号网络的关键下游效应臂，在癌症的发生发展中发挥关键作用。事实上，内皮细胞的肿瘤性转化是TSC或FoxO缺失时的一个显著表型，这表明两者之间存在上位关系。此外，Foxos定位于细胞核，在TSC缺陷的细胞和肿瘤中也很活跃，提示Foxos可能在TSC的病理生理过程中起重要作用。因此，清楚地了解TSC和FoxO之间的遗传和功能相互作用对于理解TSC的发病机制和最终的治疗至关重要。这项建议旨在阐明TSC相关肿瘤中罕见恶性肿瘤的潜在机制。我们推测，TSC的抗凋亡作用主要是通过促进Akt介导的FoxO磷酸化来抑制FoxOS的转录活性。TSC缺失后Foxos的重新激活可能导致TSC相关肿瘤的严重程度减轻，因此TSC和Foxos的复合缺失将导致更多的恶性表型。其具体目的是(1)检测Foxos和TSC在小鼠体内的联合灭活是否会加剧TSC单一突变体中观察到的肿瘤表型；(2)研究Foxos对TSC介导的原代细胞生物学活性的影响；(3)寻找可能介导TSC细胞生物学功能的直接FoxO转录靶点。这一建议将为FoxO转录因子在TSC发育中的作用和重要性提供重要的生化和生物学见解。TSC-mTOR信号对Akt信号的负反馈调节提示雷帕霉素类似物与PI3K-Akt信号抑制剂联合应用可用于TSC临床试验。鉴于FoxO在PI3K-Akt信号转导中的关键作用，我们努力寻找直接参与TSC调控的FoxO转录靶点，可能会扩大TSC的药物开发机会，或者至少识别FoxO生物标志物，为临床试验中监测抗PI3K药物的影响提供药效学标志物。公共卫生相关性：这项建议旨在研究FoxO转录因子在TSC通过结合遗传学、基因组学和细胞生物学方法介导的负反馈调控中的作用。FoxO转录因子是PI3K-AKT信号在癌症发展中的关键下游替代因子。识别具有致癌活性的有效FoxO靶点在TSC的调控中发挥重要作用，可能会扩大治疗这种疾病的药物靶点的机会。1

DESCRIPTION (provided by applicant): Tuberous Sclerosis (TSC) is caused by mutation in either TSC1 or TSC2 gene and is characterized by the formation of hamartomas in a variety of organs. However, the development of malignancy is very rare in TSC. TSC1-TSC2 complex is a critical negative regulator of the mammalian target of rapamycin (mTOR). It has been showed that an mTOR-mediated negative feedback loop leads to attenuation of Akt signaling in TSC-related tumors and compromised cell survival in TSC deficient cells, which might explain the rare progression to malignancy in TSC. However, uncertainties remain as to (i) the underlying mechanisms and key downstream effector(s) of Akt that mediate the pro-survival function of TSC and (ii) the basis for the attenuated cancer phenotype of the TSC-related tumors. Recent genetic studies from both mouse models and human cancers strongly suggest that the FoxO transcription factors represent key downstream effector arms of PI3K-AKT signaling network and play critical roles in cancer development. Indeed, neoplastic conversion of endothelial cells is a prominent phenotype upon loss of either TSC or FoxO, suggesting an epistatic relationship. Furthermore, FoxOs were shown to localize in the nucleus and active in TSC deficient cells and tumors, suggesting that FoxOs might play a critical role in TSC pathophysiology. Thus, a clear understanding the genetic and functional interactions between TSC and FoxO appears to be critical to understand TSC pathogenesis and ultimate management. This proposal aims to elucidate the underlying mechanisms of rare malignancy in TSC-related tumors. We hypothesize that the anti-apoptotic function of TSC is mainly mediated by suppression of FoxOs transcriptional activity through promoting Akt-mediated FoxO phosphorylation. Reactivation of FoxOs upon loss of TSC might contribute to the less severity of TSC-related tumors and thus compound deletion of both TSC and FoxOs would lead more malignant phenotypes. The specific aims are (1) to examine whether compound inactivation of FoxOs and TSC in the mouse would exacerbate the tumor phenotypes observed in TSC single mutants, (2) to study the impact of FoxOs on TSC-mediated cell biological activity in primary cells, and (3) to identify direct FoxO transcriptional targets which might mediate the cell biological functions of TSC. This proposal will provide important biochemical and biological insights into the role and essentiality of FoxO transcriptional factors in TSC development. The negative feedback regulation of Akt signaling by TSC-mTOR signaling suggests combined treatment of rapamycin analogues with inhibitors of PI3K-Akt signaling should be used in TSC clinical trials. Given the critical role of FoxO in PI3K-Akt signaling, our efforts to identify direct FoxO transcriptional targets involved in the regulation of TSC might expand drug development opportunities for TSC or, minimally, identify a FoxO biomarker which may provide a pharmacodynamic marker for monitoring the impact of anti-PI3K drugs in clinical trials. PUBLIC HEALTH RELEVANCE: This proposal aims to examine the role of FoxO transcriptional factors, a key downstream surrogate of PI3K-AKT signaling in cancer development, in TSC-mediated negative feedback regulation through combined genetic, genomics and cell biological approaches. The identification of validated FoxO targets with oncogenic activities playing essential roles in the regulation of TSC may expand drug target opportunities for this disease. 1