Increasing tetrahydrobiopterin in cardiomyocytes adversely affects cardiac redox state and mitochondrial function independently of changes in NO production

Abstract

Tetrahydrobiopterin (BH4) represents a potential strategy for the treatment of cardiac remodeling, fibrosis and/or diastolic dysfunction. The effects of oral treatment with BH4 (Sapropterin™ or Kuvan™) are however dose-limiting with high dose negating functional improvements. Cardiomyocyte-specific overexpression of GTP cyclohydrolase I (mGCH) increases BH4 several-fold in the heart. Using this model, we aimed to establish the cardiomyocyte-specific responses to high levels of BH4. Quantification of BH4 and BH2 in mGCH transgenic hearts showed age-based variations in BH4:BH2 ratios. Hearts of mice (<6 months) have lower BH4:BH2 ratios than hearts of older mice while both GTPCH activity and tissue ascorbate levels were higher in hearts of young than older mice. No evident changes in nitric oxide (NO) production assessed by nitrite and endogenous iron-nitrosyl complexes were detected in any of the age groups. Increased BH4 production in cardiomyocytes resulted in a significant loss of mitochondrial function. Diminished oxygen consumption and reserve capacity was verified in mitochondria isolated from hearts of 12-month old compared to 3-month old mice, even though at 12 months an improved BH4:BH2 ratio is established. Accumulation of 4-hydroxynonenal (4-HNE) and decreased glutathione levels were found in the mGCH hearts and isolated mitochondria. Taken together, our results indicate that the ratio of BH4:BH2 does not predict changes in neither NO levels nor cellular redox state in the heart. The BH4 oxidation essentially limits the capacity of cardiomyocytes to reduce oxidant stress. Cardiomyocyte with chronically high levels of BH4 show a significant decline in redox state and mitochondrial function.

The Nrf2/GCH1/BH4 Axis Ameliorates Radiation-Induced Skin Injury by Modulating the ROS Cascade

“…Conversely, inhibition of BH4 by GCH1 feedback regulatory protein promoted radiation-induced oxidative stress (Pathak et al, 2014). Diminished BH4 availability likely induces NOS uncoupling and increases O 2 À production, thereby promoting oxidative stress and aggravating radiation-induced damage to cells (Schmidt and Alp, 2007;Sethumadhavan et al, 2016). Our results showed that GCH1 expression and BH4 concentration in irradiated human skin tissue are lower than that of the nonirradiated counterparts, and the cellular NO level was decreased after radiation.…”

A novel role for endothelial tetrahydrobiopterin in mitochondrial redox balance

“…However, the specific role of BH 4 within the mitochondria is yet to be elucidated and complicated by uncertainties regarding the presence or absence of mitochondria-specific NOS. Recently, Sethumadhavan et al [21] showed that increasing BH 4 in cardiomyocytes affected redox state and mitochondrial function independent of differences in NO generation. This finding further supports our hypothesis of an NOS-independent role for BH 4 involving the mitochondria, albeit in model overexpressing GTPCH many fold.…”

“…It is therefore plausible that BH 4 is having a direct effect inside mitochondria, leading to increased O 2 - . Mitochondria are known critical sources and targets of ROS, under both physiological and pathophysiological conditions [21] , [22] and elucidating the role of BH 4 is of vital importance. Enzymatic cofactor-independent roles of BH 4 so far remain relatively unexplored, although a direct cofactor role in the mitochondria has previously been alluded to [24] , [33] .…”

Cryogenic electron paramagnetic resonance spectroscopy of flash-frozen tissue for characterization of mitochondrial disease

“…A second challenge is that many of the signals overlap, sometimes extensively. From the 24 [79,82,[108][109][110][111] . Additional well-characterized tissue-specific signals include Fe(III) from transferrin (Tf), Cu(II) from ceruloplasmin (Cp), high-and low-spin hemes from catalase, and Mn(II).…”

“…EPR as a stand-alone technique provides three pieces of information relevant to MD and other diseases and conditions with metabolic components including cancer, neurological diseases, and cardiac dysfuncti on [79,82,106,[108][109][110][111]115] . The first is a measure of the redox potential across the MRC by quantitation of signals due to oxidized and reduced redox centers.…”