Drug competition for intracellular triiodothyronine-binding sites

Characterizing protein domain associations by Small-molecule ligand binding

BACKGROUND

Protein domains are evolutionarily conserved building blocks for protein structure and function, which are conventionally identified based on protein sequence or structure similarity. Small molecule binding domains are of great importance for the recognition of small molecules in biological systems and drug development. Many small molecules, including drugs, have been increasingly identified to bind to multiple targets, leading to promiscuous interactions with protein domains. Thus, a large scale characterization of the protein domains and their associations with respect to small-molecule binding is of particular interest to system biology research, drug target identification, as well as drug repurposing.

METHODS

We compiled a collection of 13,822 physical interactions of small molecules and protein domains derived from the Protein Data Bank (PDB) structures. Based on the chemical similarity of these small molecules, we characterized pairwise associations of the protein domains and further investigated their global associations from a network point of view.

RESULTS

We found that protein domains, despite lack of similarity in sequence and structure, were comprehensively associated through binding the same or similar small-molecule ligands. Moreover, we identified modules in the domain network that consisted of closely related protein domains by sharing similar biochemical mechanisms, being involved in relevant biological pathways, or being regulated by the same cognate cofactors.

CONCLUSIONS

A novel protein domain relationship was identified in the context of small-molecule binding, which is complementary to those identified by traditional sequence-based or structure-based approaches. The protein domain network constructed in the present study provides a novel perspective for chemogenomic study and network pharmacology, as well as target identification for drug repurposing.

Synthesis of thyroid hormone binding proteins transthyretin and albumin by human trophoblast

CONTEXT

Mechanisms regulating materno-fetal transfer of thyroid hormone are not well understood. Modulation of trophoblast type 3 iodothyronine deiodinase (D3) may play an important role.

OBJECTIVE

The objective of this study was to investigate trophoblast thyroid hormone binding proteins that may modulate interactions between D3 and T4.

DESIGN

Placentas were obtained by informed consent from women delivering normal infants by repeat cesarean section at 38-40 wk gestation. T4 and T3 binding was examined in human placenta. Serum thyroid hormone binding proteins were identified by Western blotting, and their mRNA was examined by RT-PCR. Presence of these proteins in trophoblast was determined by immunocytochemistry and immunofluorescence. Cytosol was progressively purified to reveal additional thyroid hormone binding proteins that were identified by matrix-assisted laser desorption/ionization time of flight mass spectrometry. Effects of mefenamic acid on placental deiodination were examined by HPLC.

RESULTS

We detected high-affinity T4 and T3 binding in human placental cytosol. All three major serum-binding proteins, T4 binding globulin (TBG), transthyretin (TTR), and albumin, were present in cytosol. TTR mRNA and albumin mRNA were detected in human placenta, and TTR and albumin were identified histochemically in syncytiotrophoblasts. Neither TBG mRNA nor TBG was detected, suggesting that plasma TBG had contaminated the cytosol preparation. Low-affinity thyroid hormone binding proteins alpha-1-antitrypsin and alpha-1-acid glycoprotein were also identified. Addition of mefenamic acid, a potent inhibitor of thyroid hormone binding, to placental cytosol significantly enhanced deiodination of T4 by D3.

CONCLUSIONS

Placenta produces a series of thyroid hormone binding proteins that may modify thyroid hormone deiodination and materno-fetal thyroid hormone transport.

Cytoplasmic binding and disposition kinetics of diclofenac in the isolated perfused rat liver

1. The binding kinetics of diclofenac to hepatocellular structures were evaluated in the perfused rat liver using the multiple indicator dilution technique and a stochastic model of organ transit time density. 2. The single-pass, in situ rat liver preparation was perfused with buffer solution (containing 2% albumin) at 30 ml min(-1). Diclofenac and [(14)C]-sucrose (extracellular reference) were injected simultaneously as a bolus dose into the portal vein (six experiments in three rats). An analogous series of experiments was performed with [(14)C]-diclofenac and [(3)H]-sucrose. 3. The diclofenac outflow data were analysed using three models of intracellular distribution kinetics, assuming (1) instantaneous distribution and binding (well-mixed model), (2) 'slow' binding at specific intracellular sites after instantaneous distribution throughout the cytosol (slow binding model), and (3) 'slowing' of cytoplasmic diffusion due to instantaneous binding (slow diffusion model). 4. The slow binding model provided the best description of the data. The rate constants for cellular influx and sequestration were 0.126+/-0. 026 and 0.013+/-0.009 s(-1), respectively. The estimated ratio of cellular initial distribution volume to extracellular volume of 2.82 indicates an almost instantaneous distribution in the cellular water space, while the corresponding ratio of 5.54 estimated for the apparent tissue distribution volume suggests a relatively high hepatocellular binding. The non-instantaneous intracellular equilibration process was characterized by time constants of the binding and unbinding process of 53.8 and 49.5 s, respectively. The single-pass availability of diclofenac was 86%. The results obtained with [(14)C]-diclofenac and [(3)H]-sucrose were not statistically different.

Preferential inhibition of cytoplasmic T3 binding is associated with reduced nuclear binding in cultured cells

Previous studies from our laboratory have suggested that the nonsteroidal antiinflammatory drug, diclofenac (DCF), is a more potent competitor for T3 binding sites in cytoplasm than for those in the nucleus. In the present study we have examined the competitive potency for DCF and its effect on nuclear binding of T3 in cultured cells. DCF was a weak competitor for T3 binding sites in cytosol and nuclear extracts prepared from HepG2 cells with a potency of 21 and 295 microM, respectively. When expressed relative to T3, DCF was 135-fold more potent in cytosol than in nuclear extract. In intact cells, T3 was bound by nuclei with an affinity, Kd of 0.22 +/- 0.07 nM whereas in nuclear extract the affinity was 0.60 +/- 0.21 nM. DCF was a competitive inhibitor in both preparations but reduced the apparent affinity 4-fold in intact cells but only 2-fold in nuclear extract. In whole-cell experiments, DCF increased the rate of dissociation of T3 from cells prelabeled with hormone for 30 min. When these prelabeled cells were incubated with DCF, 0.1 mM, cell-associated T3 was significantly lower at 30 and 60 min than in cells reincubated without the drug. These data show that cellular transport mechanisms precede nuclear binding by T3 and suggest that there is a critical role for nonnuclear binding proteins in thyroid hormone action.