The yeast acyltransferase Sct1p regulates fatty acid desaturation by competing with the desaturase Ole1p.

The degree of fatty acid unsaturation, that is, the ratio of unsaturated versus saturated fatty acyl chains, determines membrane fluidity. Regulation of expression of the fatty acid desaturase Ole1p was hitherto the only known mechanism governing the degree of fatty acid unsaturation in Saccharomyces cerevisiae. We report a novel mechanism for the regulation of fatty acid desaturation that is based on competition between Ole1p and the glycerol-3-phosphate acyltransferase Sct1p/Gat2p for the common substrate C16:0-CoA. Deletion of SCT1 decreases the content of saturated fatty acids, whereas overexpression of SCT1 dramatically decreases the desaturation of fatty acids and affects phospholipid composition. Whereas overexpression of Ole1p increases desaturation, co-overexpression of Ole1p and Sct1p results in a fatty acid composition intermediate between those obtained upon overexpression of the enzymes separately. On the basis of these results, we propose that Sct1p sequesters C16:0-CoA into lipids, thereby shielding it from desaturation by Ole1p. Ta-king advantage of the growth defect conferred by overexpressing SCT1, we identified the acyltransferase Cst26p/Psi1p as a regulator of Sct1p activity by affecting the phosphorylation state and overexpression level of Sct1p. The level of Sct1p phosphorylation is increased when cells are supplemented with saturated fatty acids, demonstrating the physiological relevance of our findings.

On the interaction of fluorophore-encapsulating PEGylated lecithin liposomes with hamster and human platelets.

Polyethylene glycol (PEG)-grafted phosphatidylcholine liposomes are used as drug carriers due to their low immunogenicity and prolonged circulation time. The interaction between sterically stabilized lecithin liposomes and platelets has not been investigated before, and deserves to be subjected to scrutiny inasmuch as the uptake of liposomes by platelets could be detrimental for drug delivery and primary hemostasis. Consequently, the interaction between resting and convulxin-activated hamster and human platelets and calcein- or 5,6-carboxyfluorescein-encapsulating PEGylated liposomes composed of distearoyl- and dipalmitoyl phosphatidylcholine and PEG-derivatized distearoyl phosphatidylethanolamine was investigated by flow cytometry, confocal microscopy, and a glass capillary thrombosis model. Fluorescently labeled liposomes of the same composition were subsequently assayed in vivo after 15 and 45 min of systemic circulation. Neither resting nor activated hamster and human platelets interacted with liposomes at 0.70 mM lipid concentration. An absence of any interaction was corroborated in the in vivo experiments. Alternatively, flow cytometry assays evinced that human platelets interact with liposomes at lipid concentrations of >or=1.35 mM. These interactions were more profound for activated platelets than resting platelets. We conclude that the use of PEGylated lecithin liposomes at lipid concentrations of <1.35 mM has no detrimental impact on liposomal drug delivery based on PEGylated lecithin liposomes, but that these drug carriers may be associated with a reduced targeting efficacy or compromised primary hemostatic system when used at concentrations of >or=1.35 mM. In contrast, these drug carriers may become valuable in thrombosis- and drug delivery-related research and applications at concentrations of >or=1.35 mM.