Newly synthetic ceramide-1-phosphate analogs; their uptake, intracellular localization, and roles as an inhibitor of cytosolic phospholipase A2α and inducer of cell toxicity

Natural Ligand-Mimetic and Nonmimetic Inhibitors of the Ceramide Transport Protein CERT

Lipid transfer proteins (LTPs) are recognized as key players in the inter-organelle trafficking of lipids and are rapidly gaining attention as a novel molecular target for medicinal products. In mammalian cells, ceramide is newly synthesized in the endoplasmic reticulum (ER) and converted to sphingomyelin in the trans-Golgi regions. The ceramide transport protein CERT, a typical LTP, mediates the ER-to-Golgi transport of ceramide at an ER-distal Golgi membrane contact zone. About 20 years ago, a potent inhibitor of CERT, named (1R,3S)-HPA-12, was found by coincidence among ceramide analogs. Since then, various ceramide-resembling compounds have been found to act as CERT inhibitors. Nevertheless, the inevitable issue remains that natural ligand-mimetic compounds might directly bind both to the desired target and to various undesired targets that share the same natural ligand. To resolve this issue, a ceramide-unrelated compound named E16A, or (1S,2R)-HPCB-5, that potently inhibits the function of CERT has recently been developed, employing a series of in silico docking simulations, efficient chemical synthesis, quantitative affinity analysis, protein-ligand co-crystallography, and various in vivo assays. (1R,3S)-HPA-12 and E16A together provide a robust tool to discriminate on-target effects on CERT from off-target effects. This short review article will describe the history of the development of (1R,3S)-HPA-12 and E16A, summarize other CERT inhibitors, and discuss their possible applications.

Inverse-phosphocholine lipids: a remix of a common phospholipid

Zwitterionic inverse-phosphocholine (iPC) lipids contain headgroups with an inverted charge orientation relative to phosphocholine (PC) lipids. The iPC lipid headgroup has a quaternary amine adjacent to the bilayer interface and a phosphate that extends into the aqueous phase. Neutral iPC lipids with ethylated phosphate groups (CPe) and anionic iPC lipids nonethylated phosphate groups (CP) were synthesized. The surface potential of CPe liposomes remains negative across a broad pH range and in the presence of up to 10 mM Ca(2+). CP liposomes aggregate in the presence of Ca(2+), but at a slower rate than other anionic lipids. Hydrolysis of CP lipids by alkaline phosphatases generates a cationic lipid. CPe liposomes release encapsulated anionic carboxyfluorescein (CF) 20 times faster than PC liposomes and release uncharged glucose twice as fast as PC liposomes. As such, iPC lipids afford a unique opportunity to investigate the biophysical and bioactivity-related ramifications of a charge inversion at the bilayer surface.