title: Quantification of folate metabolism using transient metabolic flux analysis.
abstract: BACKGROUND
  Systematic quantitative methodologies are needed to understand the heterogeneity of cell metabolism across cell types in normal physiology, disease, and treatment. Metabolic flux analysis (MFA) can be used to infer steady state fluxes, but it does not apply for transient dynamics. Kinetic flux profiling (KFP) can be used in the context of transient dynamics, and it is the current gold standard. However, KFP requires measurements at several time points, limiting its use in high-throughput applications.
  
  
  RESULTS
  Here we propose transient MFA (tMFA) as a cost-effective methodology to quantify metabolic fluxes using metabolomics and isotope tracing. tMFA exploits the time scale separation between the dynamics of different metabolites to obtain mathematical equations relating metabolic fluxes to metabolite concentrations and isotope fractions. We show that the isotope fractions of serine and glycine are at steady state 8 h after addition of a tracer, while those of purines and glutathione are following a transient dynamics with an approximately constant turnover rate per unit of metabolite, supporting the application of tMFA to the analysis of folate metabolism. Using tMFA, we investigate the heterogeneity of folate metabolism and the response to the antifolate methotrexate in breast cancer cells. Our analysis indicates that methotrexate not only inhibits purine synthesis but also induces an increase in the AMP/ATP ratio, activation of AMP kinase (AMPK), and the inhibition of protein and glutathione synthesis. We also find that in some cancer cells, the generation of one-carbon units from serine exceeds the biosynthetic demand.
  
  
  CONCLUSIONS
  This work validates tMFA as a cost-effective methodology to investigate cell metabolism. Using tMFA, we have shown that the effects of treatment with the antifolate methotrexate extend beyond inhibition of purine synthesis and propagate to other pathways in central metabolism.
authors.affiliation: Rutgers Cancer Institute of New Jersey, New Brunswick, NJ 08901 USA.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Tedeschi
authors.affiliation: Rutgers Cancer Institute of New Jersey, New Brunswick, NJ 08901 USA.
authors.forename: Nadine
authors.initials: N
authors.lastname: Johnson-Farley
authors.affiliation: Rutgers Cancer Institute of New Jersey, New Brunswick, NJ 08901 USA.
authors.forename: Hongxia
authors.initials: H
authors.lastname: Lin
authors.affiliation: Human Metabolome Technologies America, Boston, MA 02134 USA.
authors.forename: Laura M
authors.initials: LM
authors.lastname: Shelton
authors.affiliation: Human Metabolome Technologies, Tsuruoka, Yamagata 997-0052 Japan.
authors.forename: Takushi
authors.initials: T
authors.lastname: Ooga
authors.affiliation: Cancer Research UK Beatson Institute, Garscube Estate, Switchback Road, Glasgow, G61 1BD UK.
authors.forename: Gillian
authors.initials: G
authors.lastname: Mackay
authors.affiliation: Cancer Research UK Beatson Institute, Garscube Estate, Switchback Road, Glasgow, G61 1BD UK.
authors.forename: Niels
authors.initials: N
authors.lastname: Van Den Broek
authors.affiliation: Rutgers Cancer Institute of New Jersey, New Brunswick, NJ 08901 USA.
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Bertino
authors.affiliation: Rutgers Cancer Institute of New Jersey, New Brunswick, NJ 08901 USA ; Cancer Research UK Beatson Institute, Garscube Estate, Switchback Road, Glasgow, G61 1BD UK.
authors.forename: Alexei
authors.initials: A
authors.lastname: Vazquez
keywords: Folate metabolism; Metabolic flux analysis; Methotrexate