ANTIOXIDANT INHIBITION OF THYMOCYTE APOPTOSIS BY DIHYDROLIPOIC ACID

Recent findings suggest that intracellular oxidants are involved in th e induction of apoptosis, and that this type of cell death can be inhi bited by various thiol-containing antioxidants such as N-acetyl cystei ne. To study the effects of a physiologically important thiol reductan t, rat thymocytes were preincubated with either lipoic acid, dihydroli poic acid, or lipoamide and then exposed to methylprednisolone or etop oside, two stimuli known to induce apoptosis in these cells. Dihydroli poic acid and lipoamide both exerted an inhibitory effect on apoptosis induced by the two stimuli, while lipoic acid was inactive. Inhibitio n of apoptosis was evident as (a) reduced formation of condensed, pykn otic nuclei; (b) a prevention of cell shrinkage; and (c) decreased chr omatin 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 fragmentat ion revealed that DNA in the antioxidant-loaded thymocytes remained ab ove 50 kb pairs in size, indicating that inhibition by DHLA was operat ive at an early step in the apoptotic pathway. These results suggest t hat intracellular oxidation is an obligate, early component of thymocy te apoptosis.