Molecular analysis of compensatory mechanism which tumor cells exert in VEGF deficent/RipTag mouse

当我们从加州大学旧金山分校转移RipTag小鼠时，我们发现日本的肿瘤形成大大减少。已知RipTag小鼠肿瘤的形成对遗传背景很敏感。虽然我们试图通过回交C57B1/6N或DBA背景来解决这个问题，但我们没有观察到肿瘤形成的明显恢复。后来，在UCSF的原始小鼠群体中发现了同样的现象。RipTag小鼠可能发生了一些基因组变化。目前正在考虑重新建立转基因小鼠。同时，对RipTag/VEGF-/-小鼠胰腺标本进行分析，发现肿瘤残留，微血管密度较低，无明显坏死。我们发现残余肿瘤有广泛的缺氧区。这表明抑制血管生成导致缺氧区出现，肿瘤形成减少。在这些低氧肿瘤中，细胞凋亡没有明显增加，肿瘤细胞似乎通过适应低氧微环境而存活。从临床角度来看，这一结果提出了一个严重的问题，即抗血管生成治疗可能通过选择耐缺氧的癌细胞来诱导耐药。然后，我们开始研究恶性细胞缺氧耐受的分子机制。我们发现各种细胞系可以在缺氧条件下维持几周。这些肿瘤细胞是活的，但未观察到细胞增殖和死亡。我们关注的是mTOR信号，它在缺氧时被抑制，但其后果尚未明确。我们发现mTOR信号的抑制对它们在长时间缺氧条件下的生存至关重要。

While we transferred RipTag mice from university of California at San Francisco, we found profound reduction of tumor formation in Japan. Formation of tumor in RipTag mouse is known to be sensitive to the genetic background. Although we tried to resolve the problem by backcrossing it into C57B1/6N or DBA background, we did not observe remarkable recovery of tumor formation. Afterwards, it has been revealed that the same phenomenon was observed in the original mouse colony in UCSF. It is likely that some genomic change in RipTag mouse occurred. Re-establishment of the transgenic mouse is now under consideration. Meanwhile, analysis of the archive pancreas samples from RipTag/VEGF-/- mice revealed that there were residual tumors with quite low microvessel density without apparent necrosis. We found that the residual tumors contains broad hypoxic region. This finding suggests that inhibiting angiogenesis results in emergence of hypoxic region as well as reduction of tumor formation. In these hypoxic tumors, apoptosis was not significantly increased, tumor cells seems to survive by adapting to the hypoxic microenvironment. From clinical point of view, the results offered a serious issue that anti-angiogenesis therapy might induce drug resistance by selecting hypoxia tolerant cancer cells. Then, We started to investigate the molecular mechanism of hypoxia tolerance of malignant cells. We found various cell lines can be maintained in hypoxic conditions for a couple of weeks. These tumor cells were alive but no proliferation nor cell death was observed. We focused on the mTOR signaling, which is know to be suppressed in hypoxia, but the consequence has not been clarified yet. We revealed that suppression of mTOR signaling is critical for their survival under prolonged hypoxic conditions.