Mammalian target of rapamycin, a molecular target in squamous cell carcinomas of the head and neck

Emerging knowledge on how the dysregulated function of signaling networks contributes to the malignant growth of squamous cell carcinoma of the head and neck (HNSCC) can now be exploited to identify novel mechanism-based anticancer treatments. In this regard, we have observed that persistent activation of the serine/threonine kinase Akt is a frequent event in HNSCC, and that blockade of its upstream kinase, 3'-phosphoinositide-dependent kinase 1, potently inhibits tumor cell growth. Akt promotes cell proliferation by its ability to coordinate mitogenic signaling with energy- and nutrient-sensing pathways that control protein synthesis through the atypical serine/threonine kinase, mammalian target of rapamycin (mTOR). This kinase, in turn, phosphorylates key eukaryotic translation regulators, including p70-S6 kinase and the eukaryotic translation initiation factor, 4E binding protein 1. Indeed, we show here that aberrant accumulation of the phosphorylated active form of S6, the most downstream target of the Akt-mTOR-p70-S6 kinase pathway, is a frequent event in clinical specimens from patients with HNSCC and their derived cell lines. Of interest, this enhanced level of the phosphorylated active form of S6 was rapidly reduced in HNSCC cell lines and HNSCC xenograft models at clinically relevant doses of rapamycin, which specifically inhibits mTOR. Furthermore, we observed that rapamycin displays a potent antitumor effect in vivo, as it inhibits DNA synthesis and induces the apoptotic death of HNSCC cells, ultimately resulting in tumor regression. These findings identify the Akt-mTOR pathway as a potential therapeutic target for HNSCC, and may provide the rationale for the early clinical evaluation of rapamycin and its analogues in patients with HNSCC.