Abstract
Emerging evidence supporting the activation of the Akt-mammalian target of rapamycin (mTOR) signaling network in head and neck squamous cell carcinoma (HNSCC) progression has provided the rationale for exploring the therapeutic potential of inhibiting this pathway for HNSCC treatment. Indeed, rapamycin, a clinically relevant mTOR inhibitor, promotes the rapid regression of HNSCC-tumor xenografts in mice. However, rapamycin does not affect the growth of HNSCC cells in vitro, thus raising the possibility that, as for other cancer types, rapamycin may not target cancer cells directly but may instead act on a component of the tumor microenvironment, such as tumor-associated vasculature. Here, we used a retroinhibition approach to assess the contribution of cancer cell-autonomous actions of rapamycin to its antitumor activity in HNSCC. A rapamycin-resistant form of mTOR (mTOR-RR) was expressed in HNSCC cells while retaining the wild-type (rapamycin-sensitive) mTOR (mTOR-WT) alleles in host-derived endothelial and stromal cells. Expression of mTOR-RR prevented the decrease in phospho-S6 levels caused by rapamycin through mTOR in HNSCC cells but not in stromal cells, and rendered HNSCC xenografts completely resistant to the antitumoral activity of rapamycin. This reverse pharmacology strategy also enabled monitoring the direct consequences of inhibiting mTOR in cancer cells within the complex tumor microenvironment, which revealed that mTOR controls the accumulation of hypoxia-inducible factor-1 alpha (HIF-1 alpha) and the consequent expression of vascular endothelial growth factor and a glucose transporter, Glut-1, in HNSCC cells. These findings indicate that HNSCC cells are the primary target of rapamycin in vivo, and provide evidence that its antiangiogenic effects may represent a downstream consequence of mTOR inhibition in HNSCC cells.