The mammalian target of rapamycin (mTOR) governs cell growth and proliferation by mediating the mitogen- and nutrient-dependent signal transduction that regulates messenger RNA translation. We identified phosphatidic acid (PA) as a critical component of mTOR signaling. In our study, mitogenic stimulation of mammalian cells led to a phospholipase D-dependent accumulation of cellular PA, which was required for activation of mTOR downstream effectors. PA directly interacted with the domain in mTOR that is targeted by rapamycin, and this interaction was positively correlated with mTOR's ability to activate downstream effectors. The involvement of PA in mTOR signaling reveals an important function of this lipid in signal transduction and protein synthesis, as well as a direct link between mTOR and mitogens. Furthermore, these studies suggest a potential mechanism for the in vivo actions of the immunosuppressant rapamycin.

Syncytia arising from the fusion of cells expressing the HIV‐1‐encoded Env gene with cells expressing the CD4/CXCR4 complex undergo apoptosis following the nuclear translocation of mammalian target of rapamycin (mTOR), mTOR‐mediated phosphorylation of p53 on Ser15 (p53S15), p53‐dependent upregulation of Bax and activation of the mitochondrial death pathway. p53S15 phosphorylation is only detected in syncytia in which nuclear fusion (karyogamy) has occurred. Karyogamy is secondary to a transient upregulation of cyclin B and a mitotic prophase‐like dismantling of the nuclear envelope. Inhibition of cyclin‐dependent kinase‐1 (Cdk1) prevents karyogamy, mTOR activation, p53S15 phosphorylation and apoptosis. Neutralization of p53 fails to prevent karyogamy, yet suppresses apoptosis. Peripheral blood mononuclear cells from HIV‐1‐infected patients exhibit an increase in cyclin B and mTOR expression, correlating with p53S15 phosphorylation and viral load. Cdk1 inhibition prevents the death of syncytia elicited by HIV‐1 infection of primary CD4 lymphoblasts. Thus, HIV‐1 elicits a pro‐apoptotic signal transduction pathway relying on the sequential action of cyclin B–Cdk1, mTOR and p53.

The immunosuppressant rapamycin, in complex with its cellular receptor FKBP12, targets the cellular protein FKBP12-rapamycin-associated protein/mammalian target of rapamycin/rapamycin and FKBP12 target 1 (FRAP/mTOR/RAFT1) and inhibits/delays G 1 cell cycle progression in mammalian cells. As a member of the novel phosphatidylinositol kinase-related kinase family, FRAP's kinase activity is essential for its signaling function. The FKBP12-rapamycin binding (FRB) domain in FRAP is also speculated to play an important role in FRAP function and signaling. However, the biochemical and physiological functions of FRB, as well as the mechanism for rapamycin inhibition, have been unclear. The present study focuses on investigation of FRB's role and the functional relationship between FRB domain and kinase domain in FRAP. Microinjection of purified FRB protein into human osteosarcoma MG63 cells results in a drastic blockage of the G 1 to S cell cycle progression; such a dominant negative effect is reversed by a point mutation (Trp 2027 3 Phe). The same mutation also abolishes kinase activity of FRAP without affecting ATP binding, and truncation studies suggest that upstream sequences including FRB are required for kinase activity in vitro. Given these data, we propose a model for FRAP function, in which the FRB domain is required for activation of the kinase domain, possibly through the interaction with an upstream activator. In addition, our observations provide direct evidence linking FRAP function to G 1 cell cycle progression.Mammalian cell proliferation is regulated by extracellular mitogens via multiple signal transduction pathways. One such pathway leads to the up-regulation of protein synthesis, which is essential for G 1 progression of the cell cycle (1-3). At least two proteins involved in regulating the translational machinery have been found to lie downstream of this pathway: the p70 S6 kinase (p70 s6k ) 1 (4, 5) and an eIF4E binding protein (4E-BP1) (3, 6 -8). The immunosuppressant rapamycin inhibits this pathway at a point upstream of p70 s6k (9, 10) and 4E-BP1 (11-14); this inhibition requires the presence of the cellular protein FKBP12 (15) and results in selective reduction of protein synthesis (16 -19) and G 1 arrest in a variety of mammalian cells (15), as well as in the yeast Saccharomyces cerevisiae (20,21).A major player in the rapamycin-sensitive pathway has been identified as the cellular target of rapamycin-FKBP12 complex, designated FRAP (22), RAFT1 (23), or mTOR (24). FRAP belongs to the novel family of phosphatidylinositol kinase (PIK)-related kinases which include Ataxia telangiectasia mutated. Members of this family are involved in a range of essential cellular functions, including cell cycle progression, cell cycle checkpoints, DNA repair, and DNA recombination (25-28). A kinase domain with sequence homology to lipid and protein kinases has been found at the C termini of all members in this family, and the kinase activity is crucial for the functions of these proteins. The FRAP protein is a 28...

Effects of butter and 2 types of margarine on blood lipid and lipoprotein concentrations were compared in a controlled diet study with 23 men and 23 women. Table spreads, added to a common basal diet, provided 8.3% of energy as fat. Diets averaged 34.6% of energy as fat and 15.5% as protein. Each diet was fed for 5 wk in a 3 x 3 Latin-square design. One margarine (TFA-M) approximated the average trans monoene content of trans fatty acid-containing margarines in the United States (17% trans fatty acids by dry wt). The other margarine (PUFA-M) was free of trans unsaturated fatty acids; it contained approximately twice the polyunsaturated fatty acid content of TFA-M (49% compared with 27% polyunsaturated fatty acids). The tub-type margarines had similar physical properties at ambient temperature. Fasting blood lipids and lipoproteins were determined in 2 samples taken from the subjects during the fifth week of each dietary treatment. Compared with butter, total cholesterol was 3.5% lower (P=0.009) after consumption of TFA-M and 5.4% lower (P< 0.001) after consumption of PUFA-M. Similarly, LDL cholesterol was 4.9% lower (P=0.005) and 6.7% lower (P< 0.001) after consumption of TFA-M and PUFA-M, respectively. Neither margarine differed from butter in its effect on HDL cholesterol or triacylglycerols. Thus, consumption of TFA-M or PUFA-M improved blood lipid profiles for the major lipoproteins associated with cardiovascular risk when compared with butter, with a greater improvement with PUFA-M than with TFA-M.