Endocrine Abstracts

It is well known that glucocorticoid (GC)-induced apoptosis in thymocytes is triggered via glucocorticoid receptor activation. Nevertheless, the mechanism of paradoxical promoting effect of glucocorticoids on thymocytes survival and GC -suppression of age-associated thymic involution [1] is poorly understood. It is reasonable to propose that irreversible apoptotic DNA fragmentation can be an important regulatory step in apoptosis. A series of findings suggest that activation of latent nuclear endonucleases responsible for internucleosomal DNA cleavage require caspase activities in cytoplasm and elevated influx of Ca⁺² ions into nuclei [2,3]. In this study we attempt to examine whether internucleosomal DNA cleavage in nuclei of thymocytes can be elicited independently upstream cytoplasmic circuits and whether the process per se can be affected by hydrocortisone.

Thymus gland was isolated from rats sacrificed under ether anesthesia by decapitation. DNA was prepared from isolated thymocyte nuclei subjected to incubation in different conditions. DNA fragmentation was assessed after electrophoresis and visualization by ethidium bromide staining. The results revealed that incubation of nuclei in sucrose buffered with 25 mM Tris pH 7,4, 15 mM NaCL, 60 mM KCL for 24 hours in 37°C elicit internucleosomal DNA fragmentation resembling DNA laddering in thymocytes undergoing hydrocortisone-induced apoptotis. The presence of hydrocortisone in incubation media suppresses DNA fragmentation in dose dependent manner 10^-7-10^-4. These results suggest that thymocyte nuclei possess autonomous machinery for internucleosomal DNA fragmentation activation which can be affected by hydrocortisone. We suppose that further examination of phenomenon will help better understanding regulation of apoptotic pathways and developing new approaches in pharmacological targeting of different apoptotic events.