L-Serine Deficiency Elicits Intracellular Accumulation of Cytotoxic Deoxysphingolipids and Lipid Body Formation.

L-serine is required to synthesize membrane lipids such as phosphatidylserine and sphingolipids. Nevertheless, it remains largely unknown how a diminished capacity to synthesize L-serine affects lipid homeostasis in cells and tissues. Here, we show that deprivation of external L-serine leads to the generation of 1-deoxysphingolipids (doxSLs), including 1-deoxysphinganine, in mouse embryonic fibroblasts (KO-MEFs) lacking D-3-phosphoglycerate dehydrogenase (Phgdh), which catalyzes the first step in the de novo synthesis of L-serine. A novel mass spectrometry-based lipidomic approach demonstrated that 1-deoxydihydroceramide was the most abundant species of doxSLs accumulated in L-serine-deprived KO-MEFs. Among normal sphingolipid species in KO-MEFs, levels of sphinganine, dihydroceramide, ceramide, and hexosylceramide were significantly reduced after deprivation of external L-serine, whereas those of sphingomyelin, sphingosine, and sphingosine 1-phosphate were retained. The synthesis of doxSLs was suppressed by supplementing the culture medium with L-serine but was potentiated by increasing the ratio of L-alanine to L-serine in the medium. Unlike with L-serine, depriving cells of external L-leucine did not promote the occurrence of doxSLs. Consistent with results obtained from KO-MEFs, brain-specific deletion of Phgdh in mice also resulted in accumulation of doxSLs in the brain. Furthermore, L-serine-deprived KO-MEFs exhibited increased formation of cytosolic lipid bodies containing doxSLs and other sphingolipids. These in vitro and in vivo studies indicate that doxSLs are generated in the presence of a high ratio of L-alanine to L-serine in cells and tissues lacking Phgdh, and de novo synthesis of L-serine is necessary to maintain normal sphingolipid homeostasis when the external supply of this amino acid is limited.

Effects of Parenteral Amino Acid Administration on the Postoperative Nutritional Status and Wound Healing of Protein-Malnourished Rats.

In Japan, parenteral nutrition (PN) solutions are frequently administered to patients in the postoperative short-term period. In these cases, amino acid-containing peripheral parenteral nutrition (PPN) solutions, amino acid-free maintenance solutions or combinations of the two are used. However, consensus regarding the most beneficial solution for these patients is lacking. Here, we examined the nutritional status and wound healing outcomes in protein-malnourished rats receiving postoperative administrations of PPN solution, maintenance solution or combinations of the two solutions. Protein malnutrition was induced in Sprague-Dawley rats by feeding an AIN-93G-based low-protein diet (5% casein) for 2 wk. After laparotomy, dorsal skin incision, and placement of a jugular vein catheter, the rats were divided into 3 groups. Each group was administered 113 kcal/kg/d, with group A receiving maintenance solutions without amino acid, group B receiving PPN with 1.5% amino acid, and group C receiving PPN with 3% amino acid. After 5 d post-operative administration, we measured the tensile strength of the wound area, skeletal muscle weights, and nutritional parameters. Significantly higher plasma nutritional parameters and gastrocnemius and extensor digitorum longus (EDL) muscle weights were observed in groups B and C than in group A. Group C exhibited significantly elevated tensile strength of the wound area along with up-regulation of type I collagen mRNA expression compared to group A. These findings demonstrate the nutritional status and wound healing benefits of short-term postoperative administration of PPN solutions containing amino acids in protein-malnourished rats.