C16 ceramide

l-α-Phosphatidic acid sodium salt (≥98%), l-α-phosphatidylcholine (≥99%), and C16-ceramide (shown
as Figure 1) were purchased
from Sigma-Aldrich. Three kinds of liposomes were prepared: (1) 80
mol % phosphatidylcholine and 20 mol % phosphatidic acid, (2) 100
mol % ceramide, and (3) 80 mol % phosphatidylcholine, 10 mol % ceramide,
and 10 mol % phosphatidic acid. They are referred to as PA/PC, CER
and PA/PC/CER, respectively.
The liposome–nanoparticle composites are
prepared by the
thin layer hydration method as described previously.^{15 (link)} Briefly, a single lipid or several lipids are dissolved
in a 1:1 (v:v) mixture of methanol and chloroform in a total concentration
of 2 mM. After evaporation of the solvents under a nitrogen stream,
the dry lipid film is rehydrated by the citrate-stabilized gold nanoparticle
solution, allowing liposomes to form. The solution is frozen in liquid
nitrogen and thawed at 37 °C several times. The lipid–nanoparticle
suspensions are then extruded through a polycarbonate membrane with
a pore size of 200 nm.

FBS, penicillin, and streptomycin were purchased from HyClone Laboratories. RPMI 1640 medium and charcoal-stripped FBS were purchased from Thermo Fisher. Dialyzed FBS was from Life Technologies. Optima ammonium acetate, ammonium hydroxide, Optima liquid chromatography–mass spectrometry (LC-MS) grade, acetonitrile, methanol, and water were purchased from Fisher Scientific. U-¹³C glucose, U-¹³C glutamine, and 2,3,3-²H serine were obtained from Cambridge Isotope Laboratories. U-¹³C palmitate, C16-ceramide, and C2-ceramide were purchased from Sigma-Aldrich.