Weanling Offspring of Dams Maintained on Serine-Deficient Diet Are Vulnerable to Oxidative Stress

Serine signaling governs metabolic homeostasis and health

Serine has functions that are involved in metabolic homeostasis and health in pathological or stressful situations. Notably, the de novo serine synthesis pathway (SSP) plays a vital role in targeted regulation of immune responses, cell proliferation, and lipid/protein metabolism. The presentation of serine residues derived from SSP may be a signal of stress and provide novel insights into the relationship between metabolic homeostasis and diseases. Here, we summarize the current trends in understanding the regulatory mechanisms of serine metabolism, discuss how serine signaling governs metabolic and antistress processes, including oxidative stress, immunity, energy and lipid metabolism, intestinal microbiota, and the neurological system. We present a possible framework by which serine metabolism maintains metabolic homeostasis and treats human diseases.

Serine Administration Improves Selenium Status, Oxidative Stress, and Mitochondrial Function in Longissimus Dorsi Muscle of Piglets with Intrauterine Growth Retardation

Intrauterine growth retardation (IUGR) causes oxidative stress in the skeletal muscle. Serine and selenoproteins are involved in anti-oxidative processes; however, whether IUGR affects selenium status and whether serine has beneficial effects remain elusive. Here, we investigated the effects of serine administration on selenium nutritional status and oxidative stress in the longissimus dorsi muscle of piglets with IUGR. Six newborn Min piglets having normal birth weight were administered saline, and 12 IUGR piglets were either administered saline or 0.8% serine. The results showed a lower selenium content in skeletal muscle in IUGR piglets, which was restored after serine administration. IUGR piglets showed a disturbed expression of genes encoding selenoproteins, with decreased expression of GPX2, GPX4, TXNRD1, and TXNRD3 and increased expression of DIO1, DIO2, SELF, SELM, SELP, and SELW. Notably, serine administration restored the expression levels of these genes. In accordance with the changes in gene expression, the activity of GPX, TXNRD, and DIO and the content of GSH and SELP were also altered, whereas serine administration restored their contents and activities. Moreover, we observed severe oxidative stress in the skeletal muscle of IUGR piglets, as indicated by decreased GSH content and increased MDA and PC content, whereas serine administration alleviated these changes. In conclusion, our results indicate that IUGR piglets showed a disturbed expression of genes encoding selenoproteins, accompanied by severe oxidative stress. Serine administration can improve selenium status, oxidative stress, and mitochondrial function in the longissimus dorsi muscle of piglets with IUGR. These results suggest that serine could potentially be used in the treatment of IUGR in piglets.

LncCCAT1 interaction protein PKM2 upregulates SREBP2 phosphorylation to promote osteosarcoma tumorigenesis by enhancing the Warburg effect and lipogenesis

Pyruvate kinase M2 (PKM2) plays an important role in the consumption of glucose and the production of lactic acid, the striking feature of cancer metabolism. The association of PKM2 with osteosarcoma (OS) has been reported but its role in OS has yet to be elucidated. To study this, PKM2‑bound RNAs in HeLa cells, a type of cancer cells widely used in the study of molecular function and mechanism, were obtained. Peak calling analysis revealed that PKM2 binds to long noncoding RNAs (lncRNAs), which are associated with cancer pathogenesis and development. Validation of the PKM2‑lncRNA interaction in the human OS cell line revealed that lncRNA colon cancer associated transcript‑1 (lncCCAT1) interacted with PKM2, which upregulated the phosphorylation of sterol regulatory element‑binding protein 2 (SREBP2). These factors promoted the Warburg effect, lipogenesis, and OS cell growth. PKM2 appears to be a key regulator in OS by binding to lncCCAT1. This further extends the biological functions of PKM2 in tumorigenesis and makes it a novel potential therapeutic for OS.

Exogenous and Endogenous Serine Deficiency Exacerbates Hepatic Lipid Accumulation

Serine is involved in the regulation of hepatic lipid metabolism. However, whether exogenous or endogenous serine deficiency affects lipid accumulation in the liver and related mechanisms is unclear. Here, we investigated the effects of serine deficiency on hepatic fat accumulation in mice fed a serine-deficient diet or in mice supplemented with the D-3-phosphoglycerate dehydrogenase (PHGDH) inhibitor NCT-503. Both treatments produced an increase in body weight and liver weight and higher triglyceride content in the liver. Both treatments also exacerbated hepatic inflammatory responses and oxidative stress. Importantly, NCT-503 supplementation significantly inhibited PHGDH activity and decreased the serine content in the liver. Dietary serine deficiency significantly affected the colonic microbiota, characterized by a decreased ratio of Firmicutes/Bacteroidetes and decreased proportion of Bifidobacterium. Dietary serine deficiency additionally resulted in significantly decreased colonic and serum acetate and butyrate levels. The collective results indicate that NCT-503 supplementation may contribute to overaccumulation of hepatic lipid, by causing hepatic serine deficiency, while dietary serine deficiency may produce similar outcomes by affecting the gut-microbiota-liver axis.

Serine Supplementation Alleviates Doxorubicin-Induced Oxidative Damage in Skeletal Muscle of Mice

Muscle weakness affects physical activity and quality of life of patients. Serine, a nutritionally non-essential amino acid has been reported to enhance protein synthesis and implicate in biosynthesis of multiple bioactive molecules. It remains unknown whether it can protect mice against oxidative stress-induced muscles weakness. This study was conducted to test the hypothesis that serine administration alleviates doxorubicin-induced oxidative damage in skeletal muscle of mice. Mice pre-treated with or without serine were intraperitoneally injected with either doxorubicin or equal volume of saline. Reactive oxygen species (ROS) accumulation, activity of antioxidant enzymes, oxidation product of protein, DNA, and lipid, activity of mitochondrial complex, and protein level of nuclear-factor-erythroid-2-related factor 2 (NRF2)/constitutive-androstane-receptor (CAR) signaling in skeletal muscle of mice were determined. Compared with the control, doxorubicin exposure led to oxidative damage as shown by increased ROS accumulation, decreased activity of antioxidant enzymes, and enhanced oxidative product of protein, DNA, and lipid in the skeletal muscle of mice. These effects of doxorubicin were associated with increased activity of complex I and reduced glutathione. Interestingly, doxorubicin-induced oxidative damage was alleviated by serine administration. Further study showed that the beneficial effect of serine was associated with enhanced NRF2/CAR signaling. Our result showed that serine attenuated doxorubicin-induced muscle weakness in mice. Serine supplementation might be a nutritional strategy to improve the function of skeletal muscle in patients exposed to doxorubicin.

Serine-to-glycine ratios in low-protein diets regulate intramuscular fat by affecting lipid metabolism and myofiber type transition in the skeletal muscle of growing-finishing pigs

Serine and glycine are 2 of the first-affected nonessential amino acids in low crude protein (CP) diets for pigs. Therefore, we explored the effects of different dietary serine-to-glycine ratios on growth performance and lipid metabolism in growing-finishing pigs. A total of 160 crossbred healthy barrows, with a similar body weight of around 59.50 kg, were randomly allotted into 1 of 5 treatments (8 pens per treatment and 4 pigs per pen). The serine-to-glycine ratios of the 5 dietary treatments were as follows: diet A (NORMAL group), 1.18:1 (16% CP); diet B (LOW group), 1.2:1 (12% CP); diet C (S2G1 group), 2:1 (12% CP); diet D (S1G2 group), 1:2 (12% CP); and diet E (S1G1 group), 1:1 (12% CP).We found that the pigs fed a low CP diet (12% CP), when maintaining serine-to-glycine ratio at 1:2 and a total amount of 1.44%, had the same average daily gain as the pigs fed a normal CP diet (16% CP) (P > 0.05), but they had increased intramuscular fat (P < 0.05). Furthermore, they exhibited higher expression of genes involved in lipid oxidation (P < 0.05), which was regulated by modulating methylation levels in the promoters of acyl-CoA oxidase 1 (ACOX1) and acyl-CoA dehydrogenase medium chain (ACADM). When compared with the pigs fed a normal CP diet, these pigs had more oxidative myofibers (P < 0.05), which were regulated by AMPK-PGC-1α and Calcineurin-MEF2/NFAT pathways in a coordinated manner. Our findings suggested that a dietary serine-to-glycine ratio of 1:2 is beneficial for improving meat quality in pigs fed a low CP diet.

Dietary Serine and Sulfate-Containing Amino Acids Related to the Nutritional Status of Selenium in Lactating Chinese Women

Previous studies suggested that serine can promote the synthesis of selenoproteins and the interaction, transformation, and replacement of serine, cysteine, and selenocysteine have been observed in the human body. This study was designed to clarify whether the dietary intakes of serine and sulfate-containing amino acids (SAAs) could directly affect the selenium (Se) nutritional status or the level of milk Se in lactating women. Breast milk and plasma samples were collected from a total of 264 lactating Chinese women when they revisited their local hospital at the 42nd day postpartum to detect the concentration of Se with ICP-MS and the content of selenoprotein P (SEPP1) and the activity of glutathione peroxidase 3 (GPX3) in the plasma by ELISA. The daily Se intake by each subject was calculated based on her own plasma Se concentration. The 24-h dietary record data for 3 consecutive days were collected to calculate their dietary intakes of protein together with each amino acid daily based on the China Food Composition Tables (CFCT). Ordinal polytomous logistic regression was applied to examine the determinants of BMI values for lactating women. For all subjects, the concentration of plasma SEPP1 and milk Se of participants with insufficient Se intake were significantly associated with the dietary intake of serine and 2 SAAs (methionine and cystine), respectively (P < 0.05). No significant correlation was found between each amino acid related to the synthesis of endogenous serine and every biomarker of the Se nutrition status in subjects with an insufficient dietary protein intake (P > 0.05). The ordinal logistic regression analysis showed that dietary protein intake (ordinal OR 1.012, 95% CI = 0.004-0.020, P = 0.002) and plasma SEPP1 (ordinal OR 0.988, 95% CI = - 0.023 to - 0.001, P = 0.036) affected the BMI value together in these lactating women. In conclusion, dietary serine and SAAs were found to directly affect the nutritional status, and both high protein intake and low plasma SEPP1 might be the health risks in these lactating Chinese women.

Serine Deficiency Exacerbates Inflammation and Oxidative Stress via Microbiota-Gut-Brain Axis in D-Galactose-Induced Aging Mice

Inflammation and oxidative stress play key roles in the process of aging and age-related diseases. Since serine availability plays important roles in the support of antioxidant and anti-inflammatory defense system, we explored whether serine deficiency affects inflammatory and oxidative status in D-galactose-induced aging mice. Male mice were randomly assigned into four groups: mice fed a basal diet, mice fed a serine- and glycine-deficient (SGD) diet, mice injected with D-galactose and fed a basal diet, and mice injected with D-galactose and fed an SGD diet. The results showed that D-galactose resulted in oxidative and inflammatory responses, while serine deficiency alone showed no such effects. However, serine deficiency significantly exacerbated oxidative stress and inflammation in D-galactose-treated mice. The composition of fecal microbiota was affected by D-galactose injection, which was characterized by decreased microbiota diversity and downregulated ratio of Firmicutes/Bacteroidetes, as well as decreased proportion of Clostridium XIVa. Furthermore, serine deficiency exacerbated these changes. Additionally, serine deficiency in combination with D-galactose injection significantly decreased fecal butyric acid content and gene expression of short-chain fatty acid transporters (Slc16a3 and Slc16a7) and receptor (Gpr109a) in the brain. Finally, serine deficiency exacerbated the decrease of expression of phosphorylated AMPK and the increase of expression of phosphorylated NFκB p65, which were caused by D-galactose injection. In conclusion, our results suggested that serine deficiency exacerbated inflammation and oxidative stress in D-galactose-induced aging mice. The involved mechanisms might be partially attributed to the changes in the microbiota-gut-brain axis affected by serine deficiency.