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

虽然我们从加州大学旧金山分校转移了RipTag小鼠，但我们发现日本的肿瘤形成显著减少。已知RipTag小鼠肿瘤的形成对遗传背景敏感。虽然我们试图通过将其回交到C57B1/6N或DBA背景中来解决问题，但我们没有观察到显著的肿瘤形成恢复。后来发现，在加州大学旧金山分校的原始小鼠群体中也观察到了同样的现象。RipTag小鼠的基因组可能发生了一些变化。目前正在考虑重新建立转基因小鼠。同时，对RipTag/VEGF-/-小鼠的存档胰腺标本进行分析，发现有残留的肿瘤，微血管密度很低，没有明显的坏死。我们发现残留的肿瘤含有广泛的缺氧区。这一发现表明，抑制血管生成会导致缺氧区的出现，并减少肿瘤的形成。在这些低氧肿瘤中，凋亡并没有明显增加，肿瘤细胞似乎是通过适应低氧微环境而存活下来的。从临床角度看，抗血管生成治疗可能通过选择耐低氧癌细胞来诱导耐药。于是，我们开始研究肿瘤细胞耐缺氧的分子机制。我们发现，不同的细胞系可以在低氧条件下维持几周。这些肿瘤细胞是活的，但没有观察到增殖和细胞死亡。我们重点研究了mTOR信号转导通路，它在低氧条件下被抑制，但其后果尚不清楚。我们发现，抑制mTOR信号对它们在长期低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.