34 A major mechanism for a cell to adapt to hypoxia is by using the HIF pathway that activates target pathways regulating the delivery of oxygen and its utility. However, as can be seen below, HIF1 also directly or indirectly regulates the expression of other genes involved in stability of the cellular genome. There are two other cellular signaling pathways in response to hypoxia. These include the mammalian target of rapamycin (mTOR) pathway and the endoplasmic reticulum stress pathway. Repression of the mTOR pathway and activation of the endoplasmic

reticulum stress pathway by hypoxia Ganetespib regulates protein synthesis through inhibition of mRNA translation.35 Although there have been only a few studies reporting the involvement of these pathways in the stability of the cellular genome, it is worthwhile to briefly review these pathways. The mTOR is a Ser/Thr protein kinase and forms mTOR complex 1 (mTORC1) with Raptor and GβL. Raptor is a scaffolding protein that mediates interaction between mTOR kinase and its substrates to promote mTOR signaling. GβL plays a role in stabilizing mTOR and Raptor binding. When cells are under nutrient- and energy-replete conditions, the mTORC1 activates

downstream INCB024360 price proteins, including ribosomal protein S6 kinase (p70S6K), Montelukast Sodium eukaryotic initiation factor 4E binding protein 1 (4E-BP1) and eukaryotic elongation factor 2 kinase

(EEF2K). Phosphorylation of these proteins promotes protein synthesis, cell growth, cell proliferation and cell metabolism.35,36 Chronic hypoxia down-regulates mTORC1 signaling through multiple pathways to maintain cellular protein synthesis levels appropriate for suboptimal conditions. Hypoxia inhibits mTORC1 signaling through the accumulation of the tuberous sclerosis protein 1 and 2 (TSC1-TSC2) complex. TSC1 stabilizes TSC2 by forming a complex with TSC2. TSC2 is a GTPase-activating protein (GAP) and regulates the Ras homolog enriched in brain (RHEB). RHEB activates mTORC1 when it is GTP-bound. Since the TSC1-TSC2 complex promotes conversion of RHEB-GTP to RHEB-GDP, this results in the cessation of mTORC1 activity.36 Accumulation of the TSC1-TSC2 complex is achieved through competitive inhibition of complex formations between 14 and 3-3 and TSC2 by DNA-damage-inducible transcript 4 (DDIT4 or REDD1). REDD1 is up-regulated by HIF1 under hypoxic conditions, binding to 14-3-3, and it dissociates TSC2 from the 14-3-3/TSC2 complex.