Abstract.A stable nitroxide 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-osyl (Tempol) is widely used as an antioxidant in vitro and in vivo. In this study, we investigated the effects of Tempol on the growth of As4.1 juxtaglomerular cells in relation to cell cycle and cell death. Tempol dose-dependently decreased the growth of As4.1 cells with an IC 50 of ~1 mM at 48 h. DNA flow cytometry analysis and BrdU staining indicated that Tempol induced S phase arrest, which is accompanied by a downregulation of cyclin A. Tempol also induced apoptotic cell death, which was accompanied by the loss of mitochondrial membrane potential (MMP; ∆Ψm), an activation of caspase-3 and cleavage of poly(ADP-ribose)polymerase-1 (PARP-1). Furthermore, Tempol increased reactive oxygen species (ROS) levels, and the phosphorylation of extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK). MEK and JNK inhibitors significantly attenuated a growth inhibition in Tempol-treated As4.1 cells. In conclusion, Tempol inhibited the growth of As4.1 cells via cell cycle arrest and apoptosis. Tempol also activated ERK and JNK signaling, which was responsible for cell growth inhibition. Our present data provide useful information for the toxicological effects of Tempol in juxtaglomerular cells in relation to cell growth inhibition and cell death.
IntroductionTempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl or 4-hydroxy-tempo) is a stable piperidine nitroxide, which is widely employed in electron spin resonance spectroscopy (1). It is also known that Tempol protects cells from oxidative damage in vitro and in vivo (2). Several antioxidant mechanisms have been proposed to account for these protective effects. It may act as a mimic of superoxide dismutase (SOD) and it can reduce the formation of hydroxyl radicals either by scavenging superoxide anion or by reducing intracellular concentration of Fe(II) (3). Usually, such beneficial effects are observed up to a few µM. However, the prospect of moving it from the experimental to the clinical arena has prompted more detailed studies on their possible toxicity. Much evidence suggests that Tempol impairs ferritin synthesis and Fe metabolisms, consequently leading to cell death (4). In addition, mutagenic effects has been reported in different bacterial strains (5). Of note, Tempol as a pro-oxidant induces the cell cycle arrest and apoptosis depending on cell types (6). Therefore, in order to clarify these discrepancies among the different effects of Tempol, further study needs to be performed for re-evaluating its real biological functions and roles.The mitogen-activated protein kinases (MAPKs) are a large family of serine/threonine kinases, which are major comp onents of signaling pathways in cell proliferation, differentiation, and cell death (7). There are currently three known MAPKs: the extracellular signal-regulated kinase (ERK1/2), the c-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and the p38 (7). Each MAPK pathway has relatively different upstream activat...