Phosphatidic acid-mediated mitogenic activation of mTOR signaling.

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.

Impact of hydrogenated fat on high density lipoprotein subfractions and metabolism.

Relative to saturated fatty acids, trans-fatty acids/hydrogenated fat-enriched diets have been reported to increase low density lipoprotein (LDL) cholesterol levels and either decrease or have no effect on high density lipoprotein (HDL) cholesterol levels. To better understand the effect of trans-fatty acids/hydrogenated fat on HDL cholesterol levels and metabolism, 36 subjects (female, n = 18; male, n = 18) were provided with each of three diets containing, as the major sources of fat, vegetable oil-based semiliquid margarine, traditional stick margarine, or butter for 35-day periods. LDL cholesterol levels were 155 +/- 27, 168 +/- 30, and 177 +/- 32 mg/dl after subjects followed the semiliquid margarine, stick margarine, and butter-enriched diets, respectively. HDL cholesterol levels were 43 +/- 10, 42 +/- 9, and 45 +/- 10 mg/dl, respectively. Dietary response in apolipoprotein (apo) A-I levels was similar to that in HDL cholesterol levels. HDL(2) cholesterol levels were 12 +/- 7, 11 +/- 6, and 14 +/- 7 mg/dl, respectively. There was virtually no effect of dietary fat on HDL3 cholesterol levels. The dietary perturbations had a larger effect on particles containing apoA-I only (Lp A-I) than apoA-I and A-II (Lp A-I/A-II). Cholesterol ester transfer protein (CETP) activity was 13.28 +/- 5.76, 15.74 +/- 5.41, and 14.35 +/- 4.77 mmol x h(-1) x ml(-1), respectively. Differences in CETP, phospholipid transfer protein activity, or the fractional esterification rate of cholesterol in HDL did not account for the differences observed in HDL cholesterol levels. These data suggest that the saturated fatty acid component, rather than the trans- or polyunsaturated fatty acid component, of the diets was the putative factor in modulating HDL cholesterol response.

The FKBP12-rapamycin-binding domain is required for FKBP12-rapamycin-associated protein kinase activity and G1 progression.

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 G1 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 G1 to S cell cycle progression; such a dominant negative effect is reversed by a point mutation (Trp2027 --> 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 G1 cell cycle progression.

Effects of margarine compared with those of butter on blood lipid profiles related to cardiovascular disease risk factors in normolipemic adults fed controlled diets.

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.