The immunosuppressant rapamycin interferes with G1-phase progression in lymphoid and other cell types by inhibiting the function of the mammalian target of rapamycin (mTOR). mTOR was determined to be a terminal kinase in a signaling pathway that couples mitogenic stimulation to the phosphorylation of the eukaryotic initiation factor (eIF)-4E-binding protein, PHAS-I. The rapamycin-sensitive protein kinase activity of mTOR was required for phosphorylation of PHAS-I in insulin-stimulated human embryonic kidney cells. mTOR phosphorylated PHAS-I on serine and threonine residues in vitro, and these modifications inhibited the binding of PHAS-I to eIF-4E. These studies define a role for mTOR in translational control and offer further insights into the mechanism whereby rapamycin inhibits G1-phase progression in mammalian cells.

The immunosuppressant, rapamycin, inhibits cell growth by interfering with the function of a novel kinase, termed mammalian target of rapamycin (mTOR). The putative catalytic domain of mTOR is similar to those of mammalian and yeast phosphatidylinositol (PI) 3‐kinases. This study demonstrates that mTOR is a component of a cytokine‐triggered protein kinase cascade leading to the phosphorylation of the eukaryotic initiation factor‐4E (eIF‐4E) binding protein, PHAS‐1, in activated T lymphocytes. This event promotes G1 phase progression by stimulating eIF‐4E‐dependent translation initiation. A mutant YAC‐1 T lymphoma cell line, which was selected for resistance to the growth‐inhibitory action of rapamycin, was correspondingly resistant to the suppressive effect of this drug on PHAS‐1 phosphorylation. In contrast, the PI 3‐kinase inhibitor, wortmannin, reduced the phosphorylation of PHAS‐1 in both rapamycin‐sensitive and ‐resistant T cells. At similar drug concentrations (0.1–1 microM), wortmannin irreversibly inhibited the serine‐specific autokinase activity of mTOR. The autokinase activity of mTOR was also sensitive to the structurally distinct PI 3‐kinase inhibitor, LY294002, at concentrations (1–30 microM) nearly identical to those required for inhibition of the lipid kinase activity of the mammalian p85‐p110 heterodimer. These studies indicate that the signaling functions of mTOR, and potentially those of other high molecular weight PI 3‐kinase homologs, are directly affected by cellular treatment with wortmannin or LY294002.

The immunosuppressive drug, rapamycin, interferes with an undefined signaling pathway required for the progression of G1-phase T-cells into S phase. Genetic analyses in yeast indicate that binding of rapamycin to its intracellular receptor, FKBP12, generates a toxic complex that inhibits cell growth in G1 phase. These analyses implicated two related proteins, TOR1 and TOR2, as targets of the FKBP12-rapamycin complex in yeast. In this study, we have used a glutathione S-transferase (GST)-FKBP12-rapamycin affinity matrix to isolate putative mammalian targets of rapamycin (mTOR) from tissue extracts. In the presence of rapamycin, immobilized GST-FKBP12 specifically precipitates similar high molecular mass proteins from both rat brain and murine T-lymphoma cell extracts. Binding experiments performed with rapamycin-sensitive and -resistant mutant clones derived from the YAC-1 T-lymphoma cell line demonstrate that the GST-FKBP12-rapamycin complex recovers significantly lower amounts of the candidate mTOR from rapamycin-resistant cell lines. The latter results suggest that mTOR is a relevant target of rapamycin in these cells. Finally, we report the isolation of a full-length mTOR cDNA that encodes a direct ligand for the FKBP12-rapamycin complex. The deduced amino acid sequence of mTOR displays 42 and 45% identity to those of yeast TOR1 and TOR2, respectively. These results strongly suggest that the FKBP12-rapamycin complex interacts with homologous ligands in yeast and mammalian cells and that the loss of mTOR function is directly related to the inhibitory effect of rapamycin on G1- to S-phase progression in T-lymphocytes and other sensitive cell types.

Perturbations to the well-being of tissues in plants and invertebrates generate fragments of endogenous molecules that are recognized by innate immune receptors. Vertebrates have homologous receptors on specialized cells such as dendritic cells, but whether these receptors respond to fragments of endogenous molecules is not known. We tested the idea that Toll-like receptors on dendritic cells might recognize polysaccharide fragments of heparan sulfate proteoglycan. Dendritic cells were found to mature in response to heparan sulfate as measured by costimulatory protein expression, morphology, and T lymphocyte stimulation, but this maturation was absent when Toll-like receptor 4 was mutated or inhibited. These findings suggest that Toll-like receptors in vertebrates may monitor tissue well-being by recognizing fragments of endogenous macromolecules.

The effects of insulin on the mammalian target of rapamycin, mTOR, were investigated in 3T3-L1 adipocytes. mTOR protein kinase activity was measured in immune complex assays with recombinant PHAS-I as substrate. Insulin-stimulated kinase activity was clearly observed when immunoprecipitations were conducted with the mTOR antibody, mTAb2. Insulin also increased by severalfold the 32 P content of mTOR that was determined after purifying the protein from 32 P-labeled adipocytes with rapamycin⅐FKBP12 agarose beads. Insulin affected neither the amount of mTOR immunoprecipitated nor the amount of mTOR detected by immunoblotting with mTAb2. However, the hormone markedly decreased the reactivity of mTOR with mTAb1, an antibody that activates the mTOR protein kinase. The effects of insulin on increasing mTOR protein kinase activity and on decreasing mTAb1 reactivity were abolished by incubating mTOR with protein phosphatase 1. Interestingly, the epitope for mTAb1 is located near the COOH terminus of mTOR in a 20-amino acid region that includes consensus sites for phosphorylation by protein kinase B (PKB). Experiments were performed in MER-Akt cells to investigate the role of PKB in controlling mTOR. These cells express a PKB-mutant estrogen receptor fusion protein that is activated when the cells are exposed to 4-hydroxytamoxifen. Activating PKB with 4-hydroxytamoxifen mimicked insulin by decreasing mTOR reactivity with mTAb1 and by increasing the PHAS-I kinase activity of mTOR. Our findings support the conclusion that insulin activates mTOR by promoting phosphorylation of the protein via a signaling pathway that contains PKB.

Systemic inflammatory response syndrome (SIRS) is typically associated with trauma, surgery, or acute pancreatitis. SIRS resembles sepsis, triggered by exogenous macromolecules such as LPS acting on Toll-like receptors. What triggers SIRS in the absence of infection, however, is unknown. In this study, we report that a SIRS-like response can be induced in mice by administration of soluble heparan sulfate, a glycosaminoglycan associated with nucleated cells and extracellular matrices, and by elastase, which cleaves and releases heparan sulfate proteoglycans. The ability of heparan sulfate and elastase to induce SIRS depends on functional Toll-like receptor 4, because mutant mice lacking that receptor or its function do not respond. These results provide a molecular explanation for the initiation of SIRS.

Antigen-specific T cells circulate freely and accumulate specifically at sites of antigen expression. To enhance the survival and targeting of systemically delivered viral vectors, we exploited the observation that retroviral particles adhere nonspecifically, or 'hitchhike,' to the surface of T cells. Adoptive transfer of antigen-specific T cells, loaded with viruses encoding interleukin (IL)-12 or Herpes Simplex Virus thymidine kinase (HSVtk), cured established metastatic disease where adoptive T-cell transfer alone was not effective. Productive hand off correlated with local heparanase expression either from malignant tumor cells and/or as a result of T-cell activation by antigen, providing high levels of selectivity for viral transfer to metastatic tumors in vivo. Protection, concentration and targeting of viruses by adsorption to cell carriers represent a new technique for systemic delivery of vectors, in fully immunocompetent hosts, for a variety of diseases in which delivery of genes may be therapeutically beneficial.

The role and control of the four rapamycin-sensitive phosphorylation sites that govern the association of PHAS-I with the mRNA cap-binding protein, eukaryotic initiation factor 4E (eIF4E), were investigated by using newly developed phospho-specific antibodies. Thr(P)-36/45 antibodies reacted with all three forms of PHAS-I that were resolved when cell extracts were subjected to SDS-polyacrylamide gel electrophoresis. Thr (