Hypoxia tolerance of cancer cells through suppression of mTOR signaling

mTOR（哺乳动物雷帕霉素靶点）被激活，有助于癌细胞在氧合和营养丰富的条件下增殖和存活。另一方面，在缺氧和营养不良的条件下，mTOR信号被抑制，尽管这种抑制的生理作用尚未明确。本研究旨在阐明缺氧条件下mTOR信号的抑制与癌细胞存活之间的关系。首先，我们在体外研究了分子机制。在低氧敏感的结肠癌细胞系COLO 320中，即使在缺氧条件下，Rheb和eIF4E的过表达也能激活mTOR信号。导入Rheb和eIF4E的细胞对缺氧更敏感。通过抑制PI3K或mTOR避免了死亡。另一方面，在耐缺氧的胰腺癌细胞系AsPC-1中，rheb和eIF4E都不能在缺氧条件下激活mTOR信号。我们还尝试了配备有活性形式Rheb的TAT-ODD系统。经TAT-ODD-rheb处理的AsPC-1细胞在缺氧条件下不能刺激mTOR信号传导。总之，在AsPC-1细胞中，除了rheb之外，其他因子对于在缺氧条件下激活mTOR信号也是必需的。我们发现IGF刺激可导致COLO 320细胞在缺氧状态下大量凋亡，以及AsPC-1细胞在缺氧状态下大量凋亡。在低氧条件下，IGF诱导内质网应激，内质网应激，内质网应激是细胞凋亡所必需的。特别是，我们发现CHOP诱导对细胞死亡至关重要（Cancer Research, in revision）。我们还发现S6的磷酸化（mTOR激活的下游事件）在缺氧条件下比在高氧条件下受到抑制。由于我们无法在体内缺氧区激活mTOR信号，因此体内mTOR抑制的作用仍有待阐明。

mTOR (mammalian target of rapamycin) is activated and contributes to proliferation and survival of cancer cells under oxygenated and nutritionally rich conditions. On the other hand, under hypoxic and nutritionally poor conditions, mTOR signal is suppressed, although the physiological role of the suppression had not been clarified. This study aimed at elucidating the correlation between the suppression of mTOR signaling and cancer cell survival under hypoxic conditions.First, we examined the molecular mechanism in vitro. In a colon cancer cell line, COLO 320, which is hypoxia sensitive, over-expression of Rheb as well as eIF4E activated mTOR signaling even under hypoxic conditions. Rheb and eIF4E introduced cells were more sensitive to hypoxia. The death was circumvented by inhibition of PI3K or mTOR. On the other hand, in a pancreas cancer cell line, AsPC-1, which is hypoxia tolerant, neither rheb nor eIF4E could activate mTOR signaling under hypoxic conditions. We also tried TAT-ODD system equipped with active form of Rheb. AsPC-1 cells treated with TAT-ODD-rheb could not stimulate mTOR signaling under hypoxic conditions. Together, in AsPC-1 cells, other factors in addition to rheb are necessary to activate mTOR signaling under hypoxic conditions.We found that IGF stimulation generated massive apoptosis in COLO 320 cells in hypoxia as well as AsPC-1 cells in anoxia. IGF induced robust endoplasmic reticulum stress, ER stress, under hypoxic conditions, and the ER stress was necessary for apoptosis. Especially, we revealed that CHOP induction was critical for the cell death (Cancer Research, in revision). We also found that phosphorylation of S6, a downstream event of mTOR activation, was suppressed under hypoxic area compared with well oxygenized area. Since, we could not activate mTOR signaling under hypoxic area in vivo, the role of mTOR suppression in vivo remains to be elucidated.