Folate Cofactors Regulate Serine Metabolism in Fetal Ovine Hepatocytes

Abstract

The fetal liver is the primary site of fetal serine production. The regulation of this unique fetal hepatic serine production is unknown. We hypothesized that serine production would be responsive to folate cofactor supply or hormonal regulation. To test this hypothesis, we determined the effect of key folate cofactors and insulin and glucagon on serine and glycine metabolism in primary culture of fetal ovine hepatocytes. Hepatocytes were cultured in serum-free, low-folate media [5 nM 5-methyltetrahydrofolate (THF)] with or without 50 nM 5,10-methylene-THF (MTHF) or 5-formyl-THF (FTHF). Serine and glycine production (P) and utilization (U) were determined by stable isotope dilution with [1-13 C]serine and [1-13 C]glycine for 24 h. The effect of insulin (1 M) or glucagon (1 M) was determined in a similar manner. Under basal conditions, serine P (43.2 Ϯ 5.1 mol/mg DNA per 24 h) is greater than serine U (24.1 Ϯ 3.1 mol/mg DNA per 24 h), whereas glycine U (27.3 Ϯ 3.0 mol/mg DNA per 24 h) exceeds glycine P (16.7 Ϯ 1.9 mol/mg DNA per 24 h). MTHF results in a significant decrease in serine U (16.0 Ϯ 2.7 mol/mg DNA per 24 h; p ϭ 0.02 versus low folate), with no change in serine P. FTHF reduces serine P (36.2 Ϯ 4.9 mol/mg DNA per 24 h; p ϭ 0.01), but does not alter serine U. There were no effects on glycine metabolism with 50 nM MTHF or FTHF. Serine P and U were inversely correlated whereas glycine P and U were directly correlated with the media concentration of MTHF or FTHF. Glucagon treatment increased serine U by 260 Ϯ 65% versus low folate (p ϭ 0.0004) but did not change serine P. Insulin treatment led to parallel increases in both serine P and U. Both folate cofactor availability and hormone concentrations regulate serine metabolism in the fetal liver. We speculate that serine metabolism may be a marker of fetal hepatic folate cofactor supply. (Pediatr Res 52: 589-594, 2002) Abbreviations THF, tetrahydrofolate MTHF, methylene tetrahydrofolate FTHF, formyl tetrahydrofolate SHMT, serine hydroxymethyltransferase MTHFS, methenyl tetrahydrofolate synthase MEM, minimal essential medium BrdU, 5-bromo-2'-deoxyuridine GC/MS, gas chromatography-mass spectrometry Serine occupies a unique metabolic position in the fetus. Studies in the fetal sheep demonstrated that serine is one of only three amino acids to have a net production across the fetal liver (1, 2). However, serine is poorly transported across the placenta to the fetus (3, 4). Thus, the fetus must rely on endogenous production of serine to meet the majority of its serine requirements. Serine is an important amino acid for DNA and RNA synthesis and phospholipid synthesis (5, 6). In human fetuses, serine metabolism is similar to that of fetal sheep (7,8). Indeed, human fetuses that are small-forgestational age have selectively low levels of serine and glycine, suggesting an important role for serine in fetal growth (8 -10).Little is known about the regulation of fetal hepatic serine metabolism. Others and we have demonstrated that the majority of serine produced by the ...