Multivalent ligand-receptor binding interactions in the fibroblast growth factor system produce a cooperative growth factor and heparin mechanism for receptor dimerization

The binding interactions for the three primary reactants of the fibroblast growth factor (FGF) system, basic FGF (bFGF), an FGF receptor, FGFR1, and the cofactor heparin/heparan sulfate (HS), were explored by isothermal titrating calorimetry, ultracentrifugation, and molecular modeling. The binding reactions were first dissected into three binary reactions: (1) FGFR1 + bFGF<==>FGFR1/bFGF, K1 = 41 (+/- 12) nM; (2) FGFR1 + HS<==>FGFR1/HS, K2 = 104 (+/- 17) microM; and (3) bFGF + HS<==>bFGF/HS, K3 = 470 (+/- 20) nM, where HS = low MW heparin, approximately 3 kDa. The first, binding of bFGF to FGFR1 in the absence of HS, was found to be a simple binary binding reaction that is enthalpy dominated and characterized by a single equilibrium constant, K1. The conditional reactions of bFGF and FGFR1 in the presence of heparin were then examined under conditions that saturate only the bFGF heparin site (1.5 equiv of HS/bFGF) or saturate the HS binding sites of both bFGF and FGFR1 (1.0 mM HS). Both 3-and 5-kDa low MW heparins increased the affinity for FGFR1 binding to bFGF by approximately 10-fold (Kd = 4.9 +/- 2.0 nM), relative to the reaction with no HS. In addition, HS, at a minimum of 1.5 equiv/bFGF, induced a second FGFR1 molecule to bind to another lower affinity secondary site on bFGF (K4 = 1.9 +/- 0.7 microM) in an entropy-dominated reaction to yield a quaternary complex containing two FGFR1, one bFGF, and at least one HS. Molecular weight estimates by analytical ultracentrifugation of such fully bound complexes were consistent with this proposed composition. To understand these binding reactions in terms of structural components of FGFR1, a three-dimensional model of FGFR1 was constructed using segment match modeling. Electrostatic potential calculations confirmed that an elongated cluster, approximately 15 x 35 A, of nine cationic residues focused positive potential (+2kBT) to the solvent-exposed beta-sheet A, B, E, C' surface of the D(II) domain model, strongly implicating this locus as the HS binding region of FGFR1. Structural models for HS binding to FGFR1, and HS binding to bFGF, were built individually and then assembled to juxtapose adjacent binding sites for receptor and HS on bFGF, against matching proposed growth factor and HS binding sites on FGFR1. The calorimetric binding results and the molecular modeling exercises suggest that bFGF and HS participate in a concerted bridge mechanism for the dimerization of FGFR1 in vitro and presumably for mitogenic signal transduction in vivo.(ABSTRACT TRUNCATED AT 400 WORDS)

Optimization of environment for high density Vero cell culture: effect of dissolved oxygen and nutrient supply on cell growth and changes in metabolites

This study was initiated for optimization of the environment of a technologically useful mammalian cell line for high density production. Cultures of Vero cells on microcarriers were perfused with 100%, 50%, 25% and 12.5% modified L15 media (galactose was replaced with 10 mM-fructose, with 4 mM-glutamine and 5% foetal bovine serum) in phosphate-buffered saline at either 4 or 8 vol. day-1. Cell growth, pH, dissolved oxygen, and changes in the metabolites, lactate to pyruvate and lactate to ammonia indices, demonstrated that under the conditions used in the present study, perfusion of cultures with 50% L15 medium in PBS at 8 vol. day-1 provided the optimum microenvironment for Vero cell growth. The highest cell density in the perfused cultures was 3 X 10(7) cells ml-1, which at these conditions was ten times higher than the maximum cell density (3 X 10(6) cells ml-1) obtained in a batch culture. Nutrient supply and conditioning factors were the most probable growth-limiting factors in cultures that were perfused with 12.5% and 25% L15 media, while multilayering, limitation of available oxygen, and accumulation of metabolic end products in the cellular microenvironment were the most probable causes of a density-dependent inhibition of cell growth observed under the optimized and overfed (supply of 100% L15 medium at the rate of 8 vol. day-1) culture conditions. Under the optimized environmental condition, the major source of energy was probably glutamine during the first week. However, significant utilization of fructose became evident at higher cell densities during the second week, when lactate production dramatically declined and reached an almost undetectable level, while respiration progressively assumed the predominant role in energy production. It is postulated that ‘available’ oxygen in the multicell-layered microenvironment of the optimized cultures was higher than in the overfed culture due to the greater utilization rate of oxygen for oxidation of excess nutrients in the overfed culture.

Measurement of trimethylselenonium ion in human urine

A comparison is made between two methods (ion-exchange chromatography vs a difference method) for the quantitative measurement of trimethylselenonium ion (TMSe) in human urine. It is shown that the difference method yields reliable data only if TMSe constitutes a relatively large fraction of urine selenium. Under normal conditions of selenium intake in man, accurate measurement of this important metabolite can, at present, be carried out only with the ion-exchange chromatographic procedure. Preliminary data from a human subject employing stable isotope tracer methodology are given to show that the fraction of urine selenium present as TMSe varies with the level of intake as well as other factors.

Urinary trimethylselenonium excretion by the rat: effect of level and source of selenium-75

The purpose of this study was to explore in rats the urinary metabolites of selenium (Se), by using [75Se]selenomethionine, [75Se]selenocystine, and [75Se]selenite, and to assess the effects of low and high levels of Se intake on trimethylselenonium ion (TMSe) excretion in urine. Male adult rats were adapted for 6 weeks to a commercial rat laboratory stock diet (0.25 ppm Se). They were then starved for 24 hours and given an oral dose of either low (16 micrograms Se/kg body weight) or high (1500 micrograms Se/kg body weight) Se as the test Se compounds. Appearance of radioactivity in TMSe and non-TMSe Se metabolites in urine was monitored for 48 hours. About 40% of the 75Se dose was excreted in urine. TMSe was the major urinary Se metabolite (57-69% of urinary 75Se and 16-25% of oral 75Se dose) at high, and a minor urinary Se metabolite (10% of urinary 75Se and 3-4% of oral 75Se dose) at low dose levels of Se and for all three Se test compounds. At least 80% of urinary 75Se and 26-42% of the orally administered 75Se were excreted as non-TMSe Se metabolites in urine under the latter condition. It is hypothesized that at a requirement intake of Se either a trace or no TMSe is excreted in urine, and it becomes a major excretory metabolite of Se when the dietary trace mineral intake exceeds a requirement level, probably serving as a means of detoxification.