3.2 Nature of the interaction between the VEGF
protein and the polyphenols
Having demonstrated that EGCG and dp4 interact directly
with the VEGF protein, we explored the nature of this interaction
further by investigating whether it was the result of
weak or strong interactions and whether these were covalent
or non-covalent binding. First, we dialysed VEGF-polyphenol
complexes and determined whether or not VEGF recovered
its ability to activate VEGFR-2. Our data show that untreated
VEGF caused strong activation of VEGFR-2 in HUVEC after
dialysis (Fig. 2) whereas EGCG-treated VEGF did not
exhibit any VEGFR-2 phosphorylation activity post-dialysis.
Similarly, dialysed dp4-treated VEGF was unable to phosphorylate
VEGFR-2 in HUVECs (data not shown). These observations
likely indicate that the EGCG and dp4 bind tightly to
Figure 1. Apple dp4 and EGCG inhibit VEGF-induced VEGFR-2
phosphorylation by interacting with the VEGF molecule. HUVECs
were exposed (A) to 1 M apple dp4 or EGCG for 4 h before removal
of the polyphenol (washing with PBS) and the addition and
incubation of 25 ng/mL VEGF for 5 min, or (B and C) for 5 min to
1 M apple dp4 or EGCG previously incubated with 25 ng/mL
VEGF for 5 min. Phosphorylated VEGFR-2 was determined by
ELISA (A and B) and Western blot (C). ***p < 0.001 compared
to the stimulated cells. Data are expressed as mean ± SD (n = 6
(A and B)). (C) Densitometric analysis of n = 3 Western blots.
VEGF and are not released after an extended period (24 h)
of dialysis. This is consistent with a covalent or strong noncovalent
interaction, but not with an easily reversible noncovalent
interaction. Second, EGCG- and dp4-treated VEGF
samples were subjected to various forms of gel electrophoresis.
Using SDS-PAGE, polyphenol-treated VEGF exhibited
exactly the same migration properties under both reducing