Thyroxine enhancement and the role of reactive oxygen species in tadpole tail apoptosis

Our objective is to clarify the role of reactive oxygen species (ROS) in th e atrophying tail of anuran tadpoles (tail apoptosis). Changes in catalase, superoxide dismutase (SOD) and caspase activity, genomic DNA, and nitric o xide (NO) generation were investigated biochemically using Rana japonica ta dpole tails undergoing regression during thyroid hormone enhancement. DNA f ragmentation and ladder formation with concomitant shortening of tadpole ta il were induced by DL-thyroxine (T-4) in culture medium. Catalase activity was also decreased by T-4 treatment. T-4 was also found to increase NO synt hase (NOS) activity in cultured tadpole tail with concomitant increase in t he concentration of NO2- plus NO3- (NOx) in the culture medium. Additional treatment with N-monomethyl-L-arginine (NMMA), a potent inhibitor of NOS, s uppressed the enhancing effects of T-4 on tail shortening and catalase acti vity reduction. It was also found that treatment with isosorbide dinitrate (ISDN), a NO generating drug, alone also had an enhancing effect on tail sh ortening and catalase activity reduction similar to that seen with T-4. Bot h NO and an NO donor (ISDN) strongly suppressed catalase activity. Kinetic analysis revealed that catalase activity decreased and caspase-3-like activ ity increased during normal tadpole tail atrophy (apoptosis). These results suggested that T-4 enhances NO generation, thereby strongly inhibiting cat alase activity, resulting in an increase in hydrogen peroxide, and that the oxidative stress elicited by excess hydrogen peroxide might activate cyste ine-dependent aspartate-directed protease-3 (caspase-3-like protease), whic h is thought to cause DNA fragmentation, leading to apoptosis. (C) 1999 Els evier Science Inc.