Tetrahydrobiopterin compounds modulate intracellular signaling and reactive oxygen species levels in an in vitro model of ischemia-reperfusion injury

Tetrahydrobiopterin (BH₄) and 4-amino-tetrahydrobiopterin (ABH₄) prevent acute rejection after solid organ transplantation. Moreover, BH₄ also attenuates ischemiareperfusion injury (IRI). The mechanisms underlying these protective effects are poorly defined. Activation of intracellular signaling proteins, including the mitogen-activated protein kinases (MAPKs) ERK, p38, and JNK, and the excessive production of mitochondrial reactive oxygen species (ROS) are observed mainly during early reperfusion. While the role of ROS in the initiation and progression of IRI is well understood, the contribution of individual signaling pathways is less clear. Here, we tested the potential effects of BH₄ and ABH₄ on MAPK activity and mitochondrial ROS levels. During hypoxia and reoxygenation (H/R), all three MAPKs were activated during early reoxygenation in cardiomyocytes and endothelial cells. p38 and JNK activation were further augmented by BH₄ and ABH₄, whereas ERK activation was not affected. Pretreatment with BH₄ and ABH₄ reduced the basal mitochondrial ROS levels as well as the H/R-induced increase in ROS. Prolonged incubation with ABH₄, however, showed pro-apoptotic effects in cardiomyocytes. These data suggest that a protective effect of BH₄ and ABH₄ pretreatment may be attributed mainly to their antioxidant capacity. The effects on intracellular signaling are complex and warrant further investigations.