Biopterin Metabolism and eNOS Expression during Hypoxic Pulmonary Hypertension in Mice

Abstract

Tetrahydrobiopterin (BH4), which fosters the formation of and stabilizes endothelial NO synthase (eNOS) as an active dimer, tightly regulates eNOS coupling / uncoupling. Moreover, studies conducted in genetically-modified models demonstrate that BH4 pulmonary deficiency is a key determinant in the pathogenesis of pulmonary hypertension. The present study thus investigates biopterin metabolism and eNOS expression, as well as the effect of sepiapterin (a precursor of BH4) and eNOS gene deletion, in a mice model of hypoxic pulmonary hypertension. In lungs, chronic hypoxia increased BH4 levels and eNOS expression, without modifying dihydrobiopterin (BH2, the oxidation product of BH4) levels, GTP cyclohydrolase-1 or dihydrofolate reductase expression (two key enzymes regulating BH4 availability). In intrapulmonary arteries, chronic hypoxia also increased expression of eNOS, but did not induce destabilisation of eNOS dimers into monomers. In hypoxic mice, sepiapterin prevented increase in right ventricular systolic pressure and right ventricular hypertrophy, whereas it modified neither remodelling nor alteration in vasomotor responses (hyper-responsiveness to phenylephrine, decrease in endothelium-dependent relaxation to acetylcholine) in intrapulmonary arteries. Finally, deletion of eNOS gene partially prevented hypoxia-induced increase in right ventricular systolic pressure, right ventricular hypertrophy and remodelling of intrapulmonary arteries. Collectively, these data demonstrate the absence of BH4/BH2 changes and eNOS dimer destabilisation, which may induce eNOS uncoupling during hypoxia-induced pulmonary hypertension. Thus, even though eNOS gene deletion and sepiapterin treatment exert protective effects on hypoxia-induced pulmonary vascular remodelling, increase on right ventricular pressure and / or right ventricular hypertrophy, these effects appear unrelated to biopterin-dependent eNOS uncoupling within pulmonary vasculature of hypoxic wild-type mice.

European contribution to the study of ROS: A summary of the findings and prospects for the future from the COST action BM1203 (EU-ROS)

“…Decreased levels of BH 4 and DHFR together with diminished • NO bioavailability were also observed in vivo in mice suffering from chronic hypoxia-induced pulmonary hypertension. While treatment with the biopterin precursor sepiapterin, which needs to be converted to BH 4 by DHFR, only marginally affected this disease [228] , treatment with FA not only enhanced • NO and BH4 bioavailability but also diminished pulmonary hypertension [225] . Since a decline in DHFR has also been reported to promote angiotensin-II induced hypertension [227] , preservation of DHFR levels appears to be an important mechanism to combat uncoupling of NOS and its associated pathologies.…”

Selective depletion of vascular EC-SOD augments chronic hypoxic pulmonary hypertension

“…These data are consistent with studies from the ductal ligation model of neonatal PH that also suggests that decreased EC-SOD activity is associated with decreased eNOS function and eNOS uncoupling. Delivery of recombinant SOD to neonatal lambs with PH reversed PH, increased GTPCH-1 expression, and increased BH 4 levels, indicating that GTPCH-1 is a redox-regulated enzyme and loss of extracellular SOD also impairs its expression and activity in this model of PH (4,9,11,12,34,35). Future studies may determine the specific cGMP-independent targets responsible for worsened PH due to increased production of O 2 ·Ϫ and decreased production of NO · following the disruption of eNOS activity.…”

Bone Morphogenetic Protein Receptor Type II Deficiency and Increased Inflammatory Cytokine Production. A Gateway to Pulmonary Arterial Hypertension

“…This contributes to an abnormal proliferative response in PASMCs and increased STAT3 signaling. Uncontrolled inflammation and increased oxidative stress are also linked to a loss of endothelial barrier integrity (37), dysregulation of the nitric oxide synthase system leading to abnormal vasoconstriction (58), and excessive DNA damage (53). It is likely that a combination of these mechanisms leads to the pulmonary hypertensive phenotype.…”