Figure 2. Inhibition of VEGFR-2 activation activity of VEGF byEGCG is retained after removal of unbound polyphenol using adialysis membrane. HUVECs were treated for 5 min with preincubatedbasal medium containing 25 ng/mL VEGF, 25 ng/mLVEGF and 1 M EGCG, or samples prepared by dialysis (includingthe controls). For the dialysed samples, VEGF was incubatedwith or without EGCG for 30 min at room temperature prior todialysis for 2 or 24 h at 4C. The controls were incubated in thesame conditions without dialysis. The retentate was diluted inmedium to give a final concentration of 25 ng/mL VEGF and 1 MEGCG. Phosphorylated VEGFR-2 was determined by ELISA. *p <0.05, **p < 0.01, ***p < 0.001 compared to their stimulated cells,respectively. Data expressed as mean ± SD (n = 3).and non-reducing conditions as un-treated VEGF (SupportingInformation Fig. 1). This strongly suggests that covalentlylinked VEGF-polyphenol adduct(s) were not formed and thatbinding between VEGF and the two polyphenols was the resultof (strong) non-covalent binding. We also attempted todetermine if there were differences in the pI of polyphenoltreatedVEGF and un-treated VEGF but the high intrinsicpI of VEGF (pI = 8.0–8.5) [25] precluded us from visualisingVEGF or putative VEGF adducts on the gels, presumablybecause the VEGF migrated off the top (cathodic) edgeof the gels (data not shown). We also analysed VEGF andpolyphenol-treated VEGF using MALDI-TOF MS to try anddetect changes in the mass of VEGF after polyphenol treatment.However, the mass spectra obtained for VEGF andpolyphenol-treated VEGF were very similar and no modificationswere observed (data not shown). The lack of observed increasesin the mass of VEGF post-treatment with the polyphenolsis not consistent with the formation of covalent bondsbetween the VEGF and the polyphenols, and it likely indicatesthat the non-covalent binding involved in complex formationis disrupted during ionisation such that only massescorresponding with free VEGF are observed in the MALDITOFanalysis. The combination of results obtained fromSDS-PAGE and MALDI-TOF MS do not support the notionthat the binding between VEGF and polyphenol is due to a covalentinteraction. Bearing inmind that VEGF activity was notrecovered from polyphenol-treated VEGF samples followingdialysis, it ismost likely that the polyphenol-mediated inhibitionof VEGF is the result of strong non-covalent interactionsbetween VEGF and the polyphenols.3.3 Kinetics of inhibition of VEGF activity by EGCGand dp4In order to investigate the kinetics of VEGF inhibition by thepolyphenols, VEGF was incubated with EGCG (62.5 nM) ordp4 (200 nM) for extended periods of time (1–270 min and1–120 min, respectively) after which the VEGFR-2-activatingactivity of VEGF was determined by treating HUVECs withthe polyphenol-treated VEGF (5 min). The data clearly showsthat EGCG-mediated inhibition of VEGF activity is time dependent(Fig. 3A), which is not consistent with the very rapid(<1 s) time scales over which classic freely reversible enzymesubstratecomplex equilibria are formed. Further, the EGCGdata fitted very well to a two-phasemodel (two-phase decay) inwhich an initial fast exponential decay phase (KFast = 0.1184± 0.02 319 min−1) was followed by a slow exponential decayphase (KSlow = 0.003505 ± 0.0007308 min−1). Similar resultswere obtained with dp4 but with different rates (Fig. 3B).3.4 In-silico studies of the binding of EGCG and dp4to VEGFEGCG was predicted to bind into a groove at the pole ofVEGF (Fig. 4A) with a binding affinity of −8.1 kcal/mol. dp4was predicted to bind to a region of VEGF that is adjacentto the groove that EGCG is predicted to occupy (Fig. 4B),with an affinity of −8.2 kcal/mol.We also identified potentialresidues on VEGF that EGCG or dp4 may interact with basedon the predicted most energetically favourable binding sites(Table 1). EGCG was predicted to interact with 13 residues onboth subunits of VEGF and form hydrogen bonds with threeresidues (ASP34, LYS48 and SER50). dp4 was predicted tointeract with 15 residues of VEGF including five residues viahydrogen bonds (SER50, ASN62, ASP63, GLU64, GLU67,CYS68).3.5 Effect of polyphenols on VEGF binding toendothelial cellsThe inhibitory effects of the polyphenols on the VEGFR-2activity of VEGF may, or may not be, a consequence of thepolyphenol preventing binding of VEGF to VEGFR-2 on theendothelial cells. We explored the effects of the polyphenoltreatments of VEGF on its ability to bind to the surface ofHUVECs. Using flow cytometry, we show that dp4 treatment
การแปล กรุณารอสักครู่..
