Fig. 16.5 Cyclosporin A and tacrolimus inhibit T-cell activation by interfering with the serine/threonine-specific phosphatase calcineurin. As shown in the upper panel, signaling via T-cell receptor-associated tyrosine kinases leads to opening of calciumrelease-activated calcium (CRAC) channels in the plasma membrane. This increases the concentration of Ca2+ in the cytoplasm and promotes calcium binding to the regulatory protein calmodulin (see Fig. 7.18). Calmodulin is activated by binding Ca2+ and can then target many downstream effector proteins such as the phosphatase calcineurin. Binding by calmodulin activates calcineurin to dephosphorylate the transcription factor NFAT (see Section 7-14), which then enters the nucleus and transcribes genes that are required for T-cell activation to progress. As shown in the lower panel, when cyclosporin A or tacrolimus or both are present, they form complexes with their immunophilin targets, cyclophilin and FK-binding protein, respectively. These complexes bind to calcineurin, preventing it from becoming activated by calmodulin, and thereby preventing the dephosphorylation of NFAT.
Fig. 16.6 Rapamycin inhibits cell growth and proliferation by selectively blocking activation of the kinase mTOR by Raptor. Rapamycin binds FK-binding protein (FKBP), the same immunophilin protein that binds to tacrolimus (FK506). The rapamycin:FKBP complex does not inhibit calcineurin, but instead blocks one of two complexes that activate mTOR, a large kinase that regulates many metabolic pathways. mTOR is activated downstream of various growth factor signaling pathways, and becomes associated with either of two proteins, Raptor (regulatory associated protein of mTOR) and Rictor (rapamycininsensitive companion of mTOR). The complex with Raptor, mTORC1, promotes cell proliferation, translation of proteins, and autophagy, and the complex with Rictor, mTORC2, influences cell adhesion and migration by controlling the actin cytoskeleton. The rapamycin:FKBP complex acts to inhibit the Raptorassociated mTORC1, and thereby selectively reduces cell growth and proliferation.
the PI 3-kinase pathway. These pathways activate AKT, which phosphorylates and inactivates a regulatory complex called TSC. This complex normally acts as an inhibitor of the small GTPase Rheb; after TSC is phosphorylated, Rheb is free to activate mTOR (see Fig. 7.22). Two distinct mTOR complexes can be formed, mTORC1 and mTORC2, which are controlled by two regulatory proteins called Raptor and Rictor, respectively, and which activate different downstream cellular pathways (Fig. 16.6). Rapamycin appears to inhibit only the mTORC1 complex, as the rapamycin:FKBP complex selectively inhibits the Raptor-dependent pathway that regulates this complex (see Fig. 16.6). Blockade of this pathway markedly reduces T-cell proliferation, arresting cells in the G1 phase of the cell cycle and promoting apoptosis. Rapamycin inhibits lymphocyte proliferation driven by growth factors such as IL-2, IL-4, and IL-6, and increases the number of regulatory T cells, perhaps because these cells use different signaling pathways from those of effector T cells. Rapamycin also selectively reduces the outgrowth of effector T cells while apparently enhancing the formation of memory T cells. Because of this, the use of rapamycin to augment T-cell memory induced by vaccines is being considered.