For instance, the mTOR downstream effector eIF4E seems to be overexpressed and functionally active in the surgical margins of HNSCC patients and has been regarded as an independent risk factor for recurrent disease [16]. in response to hypoxia through AMPK/REDD1 was deregulated, which hence might contribute to the persistent activation of the mTOR pathway in this cancer type. == Introduction == Cancer development is frequently initiated by genetic alterations affecting cell growth-promoting as well as tumor-suppressive signaling networks in conjunction with epigenetic disturbances and progresses by the Choline Chloride constant adaptation of tumor cells to microenvironmental selective pressures [1,2]. Microenvironmental stresses such as poor oxygenation or hypoxia arising in part by the structural and functional abnormalities affecting the vasculature are commonly found within the heterogeneous Choline Chloride regions of expanding solid tumors [3]. This is particularly evident in head and neck squamous cell carcinomas (HNSCC), including those of the oral cavity and pharynx, where the extent of intratumoral hypoxia plays an important prognostic factor as it relates to cancer aggressiveness, chemoradiotherapy resistance, and overall patient survival [46]. The availability of molecular oxygen also plays a critical role in controlling signaling pathways responsible for relaying Choline Chloride nutrient- and energy-sensing cues to control cellular programs commonly involved in human cancer such as cell proliferation, differentiation, and survival [7]. Among these, the Akt/mammalian target of rapamycin (mTOR) pathway seems to be a major regulator of the cellular responses to hypoxia and other microenvironmental stresses [8]. Acting as a positive central integrator of mitogenic signals, the evolutionarily conserved serine/threonine protein kinase mTOR transmits stimulatory cues for protein synthesis and cell growth by phosphorylating key downstream substrates involved Choline Chloride in protein translation, including the eukaryotic initiation factor 4E-binding protein-1 (4EBP1) and ribosomal p70 S6 kinase (S6K). This occurs after the sequential activation of phosphoinositide-3-kinase (PI3K) and the serine/threonine kinase Akt in response to a myriad of cell surface molecules including growth factors, hormones, and extracellular matrix components. However, in an attempt to preserve cellular energy consumption, mTOR activity is usually attenuated in response to cellular stresses, such as hypoxia, through activation of the tuberous sclerosis complex 1/2 (TSC1/TSC2) tumor-suppressor complex. TSC2 functions as a GTPase-activating protein antagonizing the small GTPase Rheb acting upstream Choline Chloride of mTOR. During permissive conditions, the Rheb-GAP activity of TSC2 is usually kept inactive by an Akt-driven inhibitory phosphorylation, leading to the accumulation of active GTP-bound Rheb, which in turn phosphorylates and activates mTOR [811]. These observations suggest that any aberration impinging around the TSC1/2 complex-activating network may result in an abnormally high mTOR activity. Interestingly, emerging information points to the mTOR signaling network as frequently altered in human HNSCC and its derived cell lines [1216]. Thus, it is plausible that in those human HNSCC that are dependent on aberrantly high mTOR activity, the hypoxia-driven inhibitory mechanisms responsible for down-regulating this pathway might be impaired. This possibility prompted us to explore whether hypoxic responses controlled by the TSC complex/mTOR signaling cascade may be deregulated in HNSCC. In this regard, the identification of the regulated in development and DNA damage 1 (REDD1) gene as a novel NTRK1 repressor of mTOR activity in both mammalian andDrosophilacells has provided a key component to the stress-response pathway negatively affecting mTOR function.REDD1encodes a 232-amino acid cytosolic protein (34 kDa) with unknown functional domains. REDD1-mediated inhibition of mTOR is usually stimulated in cells exposed to hypoxia as well as in cells undergoing energy stress and is dependent on the presence of a functional TSC1/TSC2 tumor-suppressor complex [1720]. An additional pathway by which energy stress can result in mTOR.