Role of serine biosynthesis and its utilization in the alternative pathway from glucose to glycogen during the response to insulin in cultured foetal-rat hepatocytes

Abstract

The role of serine as a possible intermediate of the alternative pathway from glucose to glycogen was investigated under basal and insulin-stimulated conditions in 18-day cultured foetal-rat hepatocytes because these cells cannot use pyruvate-derived metabolites [Bismut & Plas (1989) Biochem. J. 263, 889-895]. Incubation of cells with [U-14C]glucose for 24 h led to a release of labelled serine in the medium concomitantly with a net serine production (100 nmol/24 h per culture). The rate of [14C]serine formation (close to 3 nmol/h per culture) indicated that a large part of newly formed serine originated from glucose. When short-term experiments were performed at day 2, glycogen labelling from [U-14C]serine or [U-14C]glycine, which was increased 3-fold by insulin after 2 h, evidenced their participation as glycogenic precursors. When a double-isotope procedure with [U-14C,3-3H]glucose was used, the direct and the alternative pathways from glucose were found to contribute to glycogenesis by 75 and 25% respectively. Cycloserine (18 mM), a transaminase inhibitor, strongly inhibited glycogen labelling from [U-14C] serine while producing a 70% increase in glucose incorporation by the alternative pathway, in both the presence and the absence of insulin. The inhibitor had no effect on the direct pathway from glucose to glycogen. Supplementation with 1 mM-hydroxypyruvate, a serine-derived metabolite, did not affect direct glucose incorporation, whereas the alternative pathway was stimulated whether insulin was present or not. These results indicate that the sequence glucose----serine----glycogen is operative in cultured foetal hepatocytes. The alternative pathway interferes with hydroxypyruvate utilization, and is likely mediated by the serine aminotransferase pathway, independently of the acute glycogenic action of insulin.

Consumption and Production of Amino Acids by Insulin-Responsive Cultured Fetal Rat Hepatocytes: The Particular Case of Serine

“…Maintained in primary culture, fetal hépa tocytes have been found to be responsive to insulin for the stimulation of glycogenosis [6][7][8], which has been shown to develop under glucocorticoid control [7,9], It has been re ported that in this cell system glycogen syn thesis is partially achieved from an alternative pathway mediated by glucose conversion into serine, indicating a direct relationship be tween carbohydrate and protein metabolism [10]. The aim of the present paper was to study the behaviour of cultured fetal hépato cytes regarding the amino acids present in the extracellular medium by measuring the con centration of each amino acid between 24 and 48 h of culture.…”

Hepatic Phosphoserine Aminotransferase 1 Regulates Insulin Sensitivity in Mice via Tribbles Homolog 3

“…Serine is a nonessential amino acid, but is indispensable in the synthesis of proteins, sphingolipids, other amino acids, and nucleotides (34). Recent studies have shown that the serine synthesis pathway is involved in the development of breast cancer, tumorigenesis, cancer cell proliferation (14,15,44), and the glycogen synthesis process (16). A previous study has also shown that TRB3 expression can be induced by amino acid deprivation (45).…”

“…For example, PSAT1 regulates serine synthesis (19), and serine reportedly regulates glycogen synthesis (16). In addition, a-ketoglutaric acid, as a by-product of PSAT1 catalyzed reaction (14), stimulates insulin secretion (20).…”

“…Three steps are involved in serine synthesis (11), and PSAT1 is involved in the second step catalyzing the 3-phosphohydroxypyruvate to form L-phosphoserine (11,12). Serine contributes to the regulation of several physiological processes (13)(14)(15)(16), and, as one of a key enzymes in serine synthesis, PSAT1 has some important metabolic functions (17)(18)(19). For example, PSAT1 deficiency results in intractable seizures and acquired microcephaly (17), while increased expression of PSAT1 is associated with the development of colorectal cancer (18).…”

Folate Cofactors Regulate Serine Metabolism in Fetal Ovine Hepatocytes

“…Coincident with this change, there is an increase in the activities of the enzymes for the use of serine for gluconeogenesis (21,22). Inasmuch as induction of gluconeogenesis is linked to hormonal changes that occur at birth (23), it is possible that the change in serine metabolism at birth is in part related to changes in glucagon and insulin that occur after birth.…”