Biomedical Engineering Reference
In-Depth Information
smooth muscle cells by increasing superoxide levels via NADPH oxidase acti-
vation. FGF-2 expression could also be stimulated by other factors known to
increase ROS, including endothelin-1 and transforming growth factor-b1[
78
].
3.2 ROS in Angiogenic Signaling
ROS not only initiate angiogenesis but they additionally play a critical role in
intracellular signaling during angiogenesis. In endothelial cells, NADPH oxidase
basally produces low superoxide levels, but it is stimulated acutely by growth
factors [
79
]. VEGF specifically stimulates superoxide production via Rac-1
dependent activation of NADPH oxidases [
80
]. When VEGF binds to its receptor
(primarily VEGFR2), the small GTPase Rac1 is activated. Rac1 moves to the
plasma membrane to stimulate NADPH oxidase. ROS produced by the NADPH
oxidase then inactivate the protein tyrosine phosphatases which negatively regu-
late VEGFR2 (Fig.
2
). This enhances VEGFR2 autophosphorylation and VEGF
signaling for endothelial cell proliferation and migration [
80
-
82
]. The role of ROS
in VEGF signaling is supported by studies showing that antioxidants attenuated
VEGF receptor tyrosine phosphorylation and subsequent endothelial cell prolif-
eration and migration [
80
]. In addition to communicating downstream VEGF
signaling, NADPH oxidase-produced ROS also support future growth factor
signals by inducing VEGF and FGF-2 expression [
78
,
83
].
Growth factor-stimulated primary superoxide production from NADPH oxidase
may then stimulate secondary hydrogen peroxide formation via superoxide dis-
mutases. VEGF induced manganese superoxide dismutase (MnSOD) expression
via the NAPDH oxidase pathway [
81
]. MnSOD overexpression led to increased
mitochondrial H
2
O
2
, which enhanced phosphosphoinositol-3-kinase (PI3K) sig-
naling and activated its downstream targets, including Akt. The MnSOD-derived
ROS then upregulated VEGF expression and promoted both endothelial cell
sprouting in a collagen gel as well as blood vessel formation in the chick
chorioallantoic membrane assay [
84
]. Similary to VEGF, FGF-2 induced angio-
genesis was 3-fold higher in transgenic mice with elevated Cu/Zn superoxide
dismutase. In vitro, the SOD inhibitor disulfiram inhibited hydrogen peroxide
formation and DNA synthesis in capillary endothelial cells [
85
]. These results
suggest that primary ROS may then induce secondary ROS production, which is
critical for angiogenesis.
While ROS affect a wide variety of cell components, ROS may influence
angiogenic signaling pathways by oxidizing redox-sensitive proteins. Hydrogen
peroxide reversibly oxidizes low-pKa cysteine residues in proteins such as NF-jB
[
86
], hypoxia inducible factor (HIF) [
87
], and protein tyrosine phosphatases (PTPs)
[
14
]. Hydrogen peroxide increased NF-jB binding activity, and antisense NF- jB
inhibited tube formation by H
2
O
2
[
48
,
59
]. HIF-1 activates many angiogenic genes
and pathways in response to hypoxia, in particular VEGF expression. ROS produced
by NADPH oxidases induced HIF-1 in vascular cells [
88
]. PTPs associate with
Search WWH ::
Custom Search