Effects of 6-formylpterin as an internal source of hydrogen peroxide on cell death of human peripheral blood leukocytes

Properties and reactivity of the folic acid and folate photoproduct 6-formylpterin

Folates (vitamin B9) are essential components of our diet and our gut microbiota. They are omnipresent in our cells and blood. Folates are necessary for DNA synthesis, methylation, and other vital bioprocesses. Folic acid (FA), as the synthetic form of folates, is largely found in supplements and fortified foods. FA and folate drugs are also extensively used as therapeutics. Therefore, we are continuously exposed to the pterin derivatives, and their photo-degradation products, such as 6-formylpterin (6-FPT) and pterin-6-carboxylic acid. During ultraviolet radiation, these two photolytic products generate reactive oxygen species (ROS) responsible for the cellular oxidative stress. 6-FPT can exhibit variable pro/anti-oxidative roles depending on the cell type and its environment (acting as a cell protector in normal cells, or as an enhancer of drug-induced cell death in cancer cells). The ROS-modulating capacity of 6-FPT is well-known, whereas its intrinsic reactivity has been much less investigated. Here, we have reviewed the properties of 6-FPT and highlighted its capacity to form covalent adducts with the ROS-scavenging drug edaravone (used to treat stroke and amyotrophic lateral sclerosis) as well as its implication in immune surveillance. 6-FPT and its analogue acetyl-6-FPT function as small molecule antigens, recognized by the major histocompatibility complex-related class I-like molecule, MR1, for presentation to mucosal-associated invariant T (MAIT) cells. As modulators of the MR1/MAIT machinery, 6-FPT derivatives could play a significant immuno-regulatory role in different diseases. This brief review shed light on the multiple properties and cellular activities of 6-FPT, well beyond its primary ROS-generating activity.

Effects of intracellular reactive oxygen species generated by 6-formylpterin on T cell functions

The intracellular generation of reactive oxygen species (ROS) by 6-formylpterin and its effects on the human T cell functions were examined in vitro. When T cells isolated from fresh blood were incubated with 6-formylpterin for 1hr, the oxygen consumption and concomitant ROS generation were observed. The incubation of T cells with 50-500microM 6-formylpterin for 24hr brought about the elevation of intracellular ROS without inducing cell death. In contrast, the incubation of T cells with exogenously administered hydrogen peroxide (H(2)O(2)) or other pterin derivatives (6-hydroxymethylpterin, pterin-6-carboxylic acid, pterin, neopterin, biopterin and folic acid) for 24hr did not cause the intracellular ROS elevation. In the T cells stimulated with mitogenic lectin phytohemagglutinin (PHA) in conjunction with phorbol myristate acetate (PMA), 6-formylpterin suppressed the NF-kappaB-dependent transcription, the production of cytokines (IFN-gamma and IL-2) and the cell proliferation. These suppressive effects of 6-formylpterin were all reversed by N-acetyl-l-cystein (NAC). However, 6-formylpterin did not inhibit the NF-kappaB-DNA binding of the nuclear extracts obtained from the PHA/PMA-stimulated T cells. Since the NF-kappaB-DNA binding assay performed in vitro merely shows the presence or absence of NF-kappaB subunit in the nuclear extracts but not guarantees the actual binding of NF-kappaB with DNA in the nucleus, these findings suggest that intracellular ROS generated by 6-formylpterin does not affect the translocation of NF-kappaB to the nucleus but that it inhibits the NF-kappaB-dependent transcription in the nucleus, resulting in the suppression of cytokine production and cell proliferation in the activated T cells.