Antioxidant inhibition of thymocyte apoptosis by dihydrolipoic acid

Recent findings suggest that intracellular oxidants are involved in the induction of apoptosis, and that this type of cell death can be inhibited by various thiol-containing antioxidants such as N-acetyl cysteine. To study the effects of a physiologically important thiol reductant, rat thymocytes were preincubated with either lipoic acid, dihydrolipoic acid, or lipoamide and then exposed to methylprednisolone or etoposide, two stimuli known to induce apoptosis in these cells. Dihydrolipoic acid and lipoamide both exerted an inhibitory effect on apoptosis induced by the two stimuli, while lipoic acid was inactive. Inhibition of apoptosis was evident as (a) reduced formation of condensed, pyknotic nuclei; (b) a prevention of cell shrinkage; and (c) decreased chromatin degradation. Furthermore, the depletion of reduced glutathione that occurs as thymocytes undergo apoptosis was also prevented in the presence of DHLA. Investigation of the pattern of chromatin fragmentation revealed that DNA in the antioxidant-loaded thymocytes remained above 50 kb pairs in size, indicating that inhibition by DHLA was operative at an early step in the apoptotic pathway. These results suggest that intracellular oxidation is an obligate, early component of thymocyte apoptosis.