Targeting mTOR in cancer: renal cell is just a beginning

The mammalian target of rapamycin (mTOR) is a key regulator of cell growth and proliferation. The mTOR pathway integrates signals from nutrients, energy status and extracellular growth factors to regulate many processes, including cell cycle progression, angiogenesis, ribosome biogenesis, and metabolism. Growth factors such as insulin-like growth factor, epidermal growth factor and vascular endothelial growth factor bind to and activate their corresponding tyrosine kinase receptors (TKR) located on the cell surface, to induce signal transduction to the nucleus. TKR induces intracellular signaling cascades via the phosphorylation of the phosphatidylinositol 3-kinase, which in turn phosphorylates Akt. Of particular interest among the Akt targets is the downstream effect on mTOR, which is responsible for protein synthesis of molecules necessary for nutrient uptake, angiogenesis, ribosome biogenesis, cell growth, and proliferation. Growing evidence suggests that mTOR deregulation is associated with many types of human cancer. The importance of mTOR signaling in tumor biology is now widely accepted. Consequently, a number of agents that selectively target mTOR are being developed for cancer treatment and currently temsirolimus and everolimus are approved for the treatment of advanced renal cell cancer. However, the therapeutic benefit of mTOR inhibitors in the clinic may vary depending on the activation state of the different components of the mTOR pathway in a given case. Therefore it seems clear that predicting sensitivity to rapamycins in different cancers will likely require assessing multiple molecular markers related to mTOR signaling pathway, such as phosphatase and tensin homolog (PTEN), phospho-Akt, cytoplasmic p27, and phospho-S6 kinase.