The Anoikis Effector Bit1 Displays Tumor Suppressive Function in Lung Cancer Cells

The mitochondrial Bit1 (Bcl-2 inhibitor of transcription 1) protein is a part of an apoptotic pathway that is uniquely regulated by integrin-mediated attachment. As an anoikis effector, Bit1 is released into the cytoplasm following loss of cell attachment and induces a caspase-independent form of apoptosis. Considering that anoikis resistance is a critical determinant of transformation, we hypothesized that cancer cells may circumvent the Bit1 apoptotic pathway to attain anchorage-independence and tumorigenic potential. Here, we provide the first evidence of the tumor suppressive effect of Bit1 through a mechanism involving anoikis induction in human lung adenocarcinoma derived A549 cells. Restitution of Bit1 in anoikis resistant A549 cells is sufficient to induce detachment induced-apoptosis despite defect in caspase activation and impairs their anchorage-independent growth. Conversely, stable downregulation of Bit1 in these cells significantly enhances their anoikis resistance and anchorage-independent growth. The Bit1 knockdown cells exhibit significantly enhanced tumorigenecity in vivo. It has been previously shown that the nuclear TLE1 corepressor is a putative oncogene in lung cancer, and we show here that TLE1 blocks Bit1 mediated anoikis in part by sequestering the pro-apoptotic partner of Bit1, the Amino-terminal Enhancer of Split (AES) protein, in the nucleus. Taken together, these findings suggest a tumor suppressive role of the caspase-independent anoikis effector Bit1 in lung cancer. Consistent with its role as a tumor suppressor, we have found that Bit1 is downregulated in human non-small cell lung cancer (NSCLC) tissues.

线粒体Bit1（转录1的Bcl-2抑制剂，Bcl-2 inhibitor of transcription 1）蛋白隶属于一条仅由整合素介导黏附调控的凋亡通路。作为失巢凋亡（anoikis）效应因子，Bit1在细胞失去黏附后会被释放至细胞质，并诱导半胱天冬酶（caspase）非依赖性的细胞凋亡。鉴于失巢凋亡抗性是细胞恶性转化的关键决定因素，我们提出假说：癌细胞可通过规避Bit1介导的凋亡通路，从而获得锚定非依赖性生长能力与致瘤潜能。本研究首次证实，Bit1可通过诱导失巢凋亡的机制，在人肺腺癌来源的A549细胞中发挥肿瘤抑制作用。在失巢凋亡抗性的A549细胞中恢复Bit1的表达，即便细胞存在半胱天冬酶激活缺陷，仍足以诱导失黏附诱导的细胞凋亡，并削弱其锚定非依赖性生长能力。反之，在上述细胞中稳定下调Bit1的表达，可显著增强其失巢凋亡抗性与锚定非依赖性生长能力。Bit1敲低的细胞在体内展现出显著增强的致瘤性。已有研究表明，核TLE1共抑制因子是肺癌中的推定致癌基因；本研究证实，TLE1可通过在细胞核中隔离Bit1的促凋亡伴侣——分裂氨基末端增强子（Amino-terminal Enhancer of Split，AES）蛋白，部分阻断Bit1介导的失巢凋亡。综上，这些发现表明，半胱天冬酶非依赖性的失巢凋亡效应因子Bit1在肺癌中发挥肿瘤抑制作用。与其作为肿瘤抑制因子的功能一致，我们发现Bit1在人非小细胞肺癌（non-small cell lung cancer, NSCLC）组织中表达下调。