Biology Reference
In-Depth Information
prostacyclin analogues. Pretreatment with milrinone, a PDE3 inhibitor, restores
relaxations to prostanoids to levels similar to that seen in control fetuses. While
fetal levels of PDE3 are not altered by PPHN, PPHN suppresses the normal rise in
PDE3 expression and activity following delivery and mechanical ventilation (Chen
et al.
2009
; Lakshminrusimha et al.
2009a
). Interestingly, nitric oxide increases
PDE3 expression and activity in these lambs, demonstrating the interrelated nature
of cGMP and cAMP pathways in PPHN.
Because neonatal PPHN is associated with severe hypoxemia, the use of high
oxygen concentrations, up to 100% oxygen, is typically considered as a first-line
therapy in infants with PPHN (Farrow et al.
2005
; Tiktinsky and Morin
1993
).
However, the use of oxygen may greatly exaggerate oxidative stress in multiple
cellular compartments of the diseased vasculature. Recent data suggest that hyper-
oxia may diminish vascular responses to endogenous and exogenous nitric oxide in
both the normal and remodeled pulmonary vasculature, indicating that ROS may
inactivate NO or other enzymes responsible for mediating its vascular effects
(Lakshminrusimha et al.
2007a
,
2009b
). The mechanisms by which ROS lead to
pulmonary vasoconstriction and diminished NO responsiveness is certainly compli-
cated, but emerging data indicate multiple targets in the pulmonary vascular regu-
latory pathways, including the PDEs. We recently published that exposing normal
fetal pulmonary artery smooth muscle cells (FPASMC) to hyperoxia for 24 h leads
to decreased cGMP response to exogenous NO. We further demonstrated that
exposure to hyperoxia for 24 h increases PDE5 mRNA and protein expression as
well as increased level of PDE5 phosphorylation and activity. We also noted a
dose-response effect of hyperoxia on PDE5 activity, with a stepwise increase noted
in PDE5 activity as the cells were treated with 21, 50, and 95% O
2
(Fig.
1
).
Inhibition of the hyperoxia-induced PDE5 activity with sildenafil was sufficient to
partially rescue the cGMP response to exogenous NO, further indicating that PDE5
is a critical regulator of cGMP in the context of hyperoxia (Farrow et al.
2008a
).
As might be expected, exposure to hyperoxia for 24 h led to increased oxidative
stress within the FPASMC. As such, we hypothesized that ROS may serve as
critical mediators in the crosstalk between oxygen and PDE5. In support of that, a
single dose of an exogenous oxidant, hydrogen peroxide (H
2
O
2
), was sufficient to
induce long-lasting changes in PDE5 expression, phosphorylation, and activity,
which mirrored those seen after exposure to hyperoxia. Similarly, the changes in
PDE5 expression and activity as well as the decreased cGMP responsiveness
in hyperoxia were all reversed with pretreatment with a chemical antioxidant,
N
-acetyl-cysteine (NAC). This confirms that ROS, in general, and H
2
O
2
, in partic-
ular, are sufficient to induce significant increases in PDE5 expression and activity in
the pulmonary artery smooth muscle cell, which may promote vasoconstriction,
poor NO response, and vascular remodeling (Farrow et al.
2008a
).
In order to better understand the impact of ROS on the intact neonatal pulmonary
vasculature, we recently ventilated both healthy control and PPHN lambs immedi-
ately after birth with 100% oxygen, in order to simulate the clinical scenario seen
with a human infant with severe PPHN. Resuscitation of healthy lambs with
100% oxygen significantly exaggerated pulmonary arterial contractile responses
