Regulatory phenomena in mammalian serine metabolism

The PHGDH enigma: Do cancer cells only need serine or also a redox modulator?

Upregulation of serine biosynthesis pathway activity is an increasingly apparent feature of many cancers. Most notably, the first rate-limiting enzyme of the pathway, phosphoglycerate dehydrogenase (PHGDH), is genomically amplified in some melanomas and breast cancers and can be transcriptionally regulated by various tumor suppressors and oncogenes. Yet emerging evidence suggests that serine-in particular, serine biosynthetic pathway activity-may promote cancer in ways beyond providing the building blocks to support cell proliferation. Here, we summarize how mammalian cells tightly control serine synthesis before discussing alternate ways in which increased serine synthetic flux through PHGDH may benefit cancer cells, such as maintenance of TCA cycle flux through alpha-ketoglutarate (αKG) and modulation of cellular redox balance. We will also provide an overview of the current landscape of therapeutics targeting serine synthesis and offer a perspective on future strategies.

Serum levels of excitatory amino acids, serine, glycine, histidine, threonine, taurine, alanine and arginine in treatment-resistant depression: modulation by treatment with antidepressants and prediction of clinical responsivity

Previous research has revealed that major depression is accompanied by disorders in excitatory amino acids, e.g. glutamate and aspartate, and alterations in serum levels of other amino acids, e.g. serine, glycine and taurine. The aim of the present study was to examine serum levels of aspartate, asparagine, glutamate, glutamine, serine, glycine, threonine, histidine, alanine, taurine and arginine in major depression patients with treatment-resistant depression (TRD). No significant differences in the serum concentrations of any of the above amino acids could be found between patients with and without TRD and normal controls. Non-responders to treatment with antidepressants during a period of 5 weeks were characterized by significantly lower serum levels of aspartate, asparagine, serine, threonine and taurine. A 5-week period of treatment with antidepressants significantly reduced the serum levels of aspartate, glutamate and taurine, and significantly increased the serum concentrations of glutamine. The results suggest that alterations in serum levels of aspartate, asparagine, serine, threonine and taurine may predict the subsequent response to treatment with antidepressants, and that the latter may modulate serum levels of excitatory amino acids and taurine.

Plasma concentrations of excitatory amino acids, serine, glycine, taurine and histidine in major depression

This study was carried out to investigate plasma levels of excitatory amino acids, such as glutamate and aspartate, and glutamine, serine, glycine, taurine and histidine in major depression. The plasma amino acids were determined by means of HPLC in 22 normal controls and 25 unmedicated patients with major depression. Major depression was characterized by higher plasma taurine levels than normal controls. Significantly lower plasma glycine values and a higher serine/glycine ratio were observed in the depressed group. No significant differences in glutamine, histidine, serine or aspartate levels could be detected between the study groups. By means of linear discriminant analysis, a highly significant separation between major depressed subjects and normal volunteers was found using glycine, glutamate and taurine as discriminatory variables. No significant relationships between any of the amino acids and severity of depression could be found. The results suggest that major depression is accompanied by perturbations in the serine/glycine ratio, excitatory amino acids, such as glutamate, and inhibitory amino acids, such as taurine.

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

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.

Control analysis of mammalian serine biosynthesis. Feedback inhibition on the final step

The flux of serine biosynthesis in the liver of the normal rabbit, and of the rat on a low protein diet, is most sensitive to the activity of phosphoserine phosphatase (flux control coefficient up to 0.97), the last of the three enzymes in the pathway after it branches from glycolysis. The concentration of the pathway product, serine, has a strong controlling influence on the flux (response coefficient up to -0.64) through feedback inhibition at this step. The pathway is therefore controlled primarily by the demand for serine rather than the supply of the pathway precursor, 3-phosphoglycerate. Under conditions where there is a lower biosynthetic flux, the flux control coefficients of the first two enzymes of the pathway are increased, and are probably dominant in the rat on a normal diet. In rabbit liver, when ethanol is used to inhibit serine biosynthesis, control can be distributed between the three enzymes, even though the reactions catalysed by the first two remain close to equilibrium. Apart from their intrinsic value in aiding the understanding of the regulation of mammalian serine metabolism, our findings illustrate the danger of assuming that there are invariant design principles in the regulation of metabolic pathways, such as feedback control on the first step after a branch.

Effect of acute ethanol on serine biosynthesis in liver

The effect of an acute intraperitoneal dose of ethanol (1 g/kg), glucose (7.2 g/kg), or the combination of the two on the metabolite pattern of the biosynthetic pathway of L-serine has been determined in rabbit liver in vivo as has the effect of 10 mM ethanol on the glucose-, fructose-, or pyruvate-stimulated accumulation of L-serine in rabbit hepatocytes in vitro. In vivo, the 50% increase in L-serine and 80% increase in L-phosphoserine content of liver following glucose injection was completely prevented by ethanol. In fact, the L-phosphoserine content fell to only 6% of the control value. In spite of these and other significant changes in the metabolite pattern of the pathway of L-serine biosynthesis (D-3-phosphoglycerate dehydrogenase, L-phosphoserine aminotransferase (PSAT), and L-phosphoserine phosphatase), the mass action ratio of the combined reactions of the first two steps remained close to their equilibrium position. As a consequence it is estimated that the tissue content of phosphohydroxypyruvate fell to less than 2% of the control value, to approximately 0.3% of its Km for the PSAT reaction. The conclusion that acute ethanol blocks L-serine biosynthesis (presumably by redox effects) was supported by the prevention or inhibition of L-serine accumulation in hepatocytes metabolizing glucose, fructose, or pyruvate. Because L-serine is an important source of one-carbon fragments, the inhibition of its biosynthesis may be another mechanism by which ethanol interferes with folate and one-carbon metabolism.

The reactions of the phosphorylated pathway of L-serine biosynthesis: thermodynamic relationships in rat liver in vivo

The effect of the induction of the enzymes of the phosphorylated pathway of L-serine biosynthesis on the thermodynamic relationships among the reactions has been determined in rat liver in vivo. The mass action ratios of the reactions involved were calculated from the concentrations of appropriate metabolites in freeze-clamped liver from animals fed normal and low-protein diets for 2 weeks. These ratios were compared with the equilibrium constants of the same reactions previously determined under physiological conditions and the results previously obtained in the rabbit. The thermodynamic relationships in the pathway were different between the normal rat and rabbit as might have been expected, because of the significantly lower activities of the L-serine biosynthetic enzymes in the former animal. Although the delta G for the overall pathway is nearly identical in the rat and rabbit (-5.8 versus -5.5 kcal/mol, respectively), the distribution of delta G among the reactions is different. The disequilibrium in the pathway in rat liver is nearly equally divided between the L-phosphoserine phosphatase (EC 3.1.3.3) step and the other two reactions [D-3-phosphoglycerate dehydrogenase (EC 1.1.1.95) and L-phosphoserine aminotransferase (EC 2.6.1.52)], whereas in rabbit the phosphatase reaction accounts for nearly the entire delta G. Feeding the rat a low protein diet, however, induced the activity of D-3-phosphoglycerate dehydrogenase 12-fold, that of L-phosphoserine aminotransferase 20-fold, and that of L-phosphoserine phosphatase 2-fold. With the induction of the pathway, L-phosphoserine appeared in the tissue, there was a more than 3-fold rise in L-serine in the liver, and the pattern of delta G in the rat liver approached that in the rabbit.

Enzymes of serine metabolism in normal, developing and neoplastic rat tissues

The cellular pattern of serine metabolism was conceptualized into four main areas of metabolic sequences: the biosynthesis of serine from intermediates of the glycolytic pathway (the so-called "phosphorylated pathway"); and alternative pathways of serine utilization initiated by serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. The known regulatory and adaptive properties of the enzymes involved in these pathways were reviewed in detail and key enzymes associated with each pathway (phosphoserine aminotransferase, serine dehydratase, serine aminotransferase, and serine hydroxymethyltransferase, respectively) were selected for further investigation. Tissue distribution studies in the rat revealed that whereas serine dehydratase and serine aminotransferase activities were largely confined to the liver, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were more broadly distributed. In particular in tissues with a high rate of cell turnover, phosphoserine aminotransferase and serine hydroxymethyltransferase activities were coordinately increased. An increase in serine hydroxymethyltransferase activity coincided temporally with the incorporation of [3-14C]serine and thymidine into DNA in normal human lymphocytes during proliferation after mitogenic stimulation by phytohemagglutinin. The evidence suggested a primarily gluconeogenic role for serine dehydratase and serine aminotransferase. Serine hydroxymethyltransferase has a role in providing glycine and one-carbon folate co-factors as precursors for nucleotide biosynthesis and in some situations serves to metabolically couple the pathway of serine biosynthesis to utilization for de novo purine and pyrimidine synthesis. Multiple enzymic forms were distinguished for serine dehydratase, serine aminotransferase and serine hydroxymethyltransferase. For serine dehydratase the two cytosolic multiple forms had no apparent functional significance; the multiple forms were catalytically unmodified by conditions promoting phosphorylation-dephosphorylation in vitro. The mitochondrial form of serine aminotransferase showed adaptive responses in gluconeogenic situations, and the hypothesis was proposed that the mitochondrial isoenzyme of serine hydroxymethyltransferase is associated together with serine aminotransferase in a pathway for gluconeogenesis from protein-derived amino acids such as glycine and hydroxyproline. The adaptive behaviour of the enzymes during the neonatal development of rat liver revealed that serine aminotransferase reached a peak in the mid-suckling period at a time when gluconeogenesis is known to be increased.(ABSTRACT TRUNCATED AT 400 WORDS)

Serine biosynthesis in human hair follicles by the phosphorylated pathway: follicular 3-phosphoglycerate dehydrogenase

The phosphorylated pathway of serine biosynthesis was demonstrated in human hair bulbs and sheaths by the formation of phosphoserine and serine from (14C)3-phosphoglyceric acid. The initial and rate limiting enzyme of the pathway, 3-phosphoglycerate dehydrogenase (3-PGDH) was demonstrated by enzyme determinations in human and rat hair follicles, human epidermis, and chicken epidermis. Follicular 3-PGDH was characterized using a sensitive fluorometric assay with NADH as a co-substrate. Monovalent cations (Na+, K+, Li+, or NH4+) were necessary for full enzyme activity. p-Hydroxymercuribenzoate inhibited activity, and activity was 3 times higher with NADH as a co-substrate than with NADPH. The apparent Km for the substrate hydroxyphosphopyruvic acid was 32.8 muM, and the apparent Km for NADH 4.8 muM similar to the Kms for other mammalian 3-PGDHs. Enzyme activity was not altered by parenteral corticosteroids, a high carbohydrate diet, low protein diet, or starvation. Enzyme activity decreased over the first 12 days of life in newborn rats. The phosphorylated pathway of serine synthesis provides a potential nondietary and nonhepatic source of serine, glycine, and their products in keratinizing tissues.

The effect of feeding with a tryptophan-free amino acid mixture on rat liver magnesium ion-activated deoxyribonucleic acid-dependent ribonucleic acid polymerase

1. The Widnell & Tata (1966) assay method for Mg(2+)-activated DNA-dependent RNA polymerase was used for initial-velocity determinations of rat liver nuclear RNA polymerase. One unit (U) of RNA polymerase was defined as that amount of enzyme required for 1 mmol of [(3)H]GMP incorporation/min at 37 degrees C. 2. Colony fed rats were found to have a mean RNA polymerase activity of 65.9muU/mg of DNA and 18h-starved rats had a mean activity of 53.2muU/mg of DNA. Longer periods of starvation did not significantly decrease RNA polymerase activity further. 3. Rats that had been starved for 18h were used for all feeding experiments. Complete and tryptophan-deficient amino acid mixtures were given by stomach tube and the animals were killed 15-120min later. The response of RNA polymerase to the feeding with the complete amino acid mixture was rapid and almost linear over the first hour of feeding, resulting in a doubling of activity. The activity was still elevated above the starvation value at 120min after feeding. The tryptophan-deficient amino acid mixture produced a much less vigorous response about 45min after the feeding, and the activity had returned to the starvation value by 120min after the feeding. 4. The response of RNA polymerase to the feeding with the complete amino acid mixture was shown to occur within a period of less than 5min to about 10min after the feeding. 5. Pretreatment of the animals with puromycin or cycloheximide was found to abolish the 15min RNA polymerase response to the feeding with the complete amino acid mixture, but the activity of the controls was unaffected. 6. The characteristics of the RNA polymerase from 18h-starved animals and animals fed with the complete or incomplete amino acid mixtures for 1h were examined. The effects of Mg(2+) ions, pH, actinomycin D and nucleoside triphosphate omissions were determined. The [Mg(2+)]- and pH-activity profiles of the RNA polymerase from the animal fed with the complete mixture appeared to differ from those of the enzyme from the other groups, but this difference is probably not significant. 7. [5-(3)H]Orotic acid incorporation by rat liver nuclei in vivo was shown to be affected by the amino acid mixtures in a similar manner to the RNA polymerase. 8. The tryptophan concentrations of plasma and liver were determined up to 120 min after feeding with the amino acid mixtures. Feeding with the complete mixture produced a rapid increase in free tryptophan concentrations in both plasma and liver, but feeding with the incomplete mixture did not alter the plasma concentration. The liver tryptophan concentration increased at about 45min after feeding with the tryptophan-deficient diet. 9. There was a good correlation between the liver tryptophan concentration and RNA polymerase activity in all groups of animals. 10. It was concluded that the rat liver nucleus responded to an increase in amino acid supply by increased synthesis of RNA as a result of synthesis of RNA polymerase de novo. The correlation of tryptophan concentration and RNA polymerase activity appears to reflect the general amino acid concentration required to support hepatic protein synthesis and to produce new RNA polymerase. This new polymerase appears to differ from the basal RNA polymerase by its rapid synthesis and destruction, which may be a means of regulating RNA synthesis by the amino acid concentration in the liver.