LOSS OF CELL-CYCLE CONTROLS IN APOPTOTIC LYMPHOBLASTS OF THE BURSA OFFABRICIUS

Lymphoblasts of the normal embryonic follicles of the chicken bursa of Fabricius undergo rapid apoptosis when exposed to gamma-radiation or when cell-cell contacts are disrupted by mechanical dispersion in shor t term culture. We have observed previously that overexpression of v-m yc sensitizes preneoplastic bursal lymphoblasts to induction of cell d eath, whereas resistance to induced cell death is acquired during prog ression to neoplasia. In this study we observed extensive DNA degradat ion in the large majority of the lymphoblast population within the fir st hour after dispersion-induced apoptosis. Paradoxically these cells continued to progress into S-phase with the bulk of DNA cleavage and d eath occurring in S-phase cells (i.e., in cells with more than 2C and less than 4C DNA content). We confirmed the S phase status of apoptoti c cells by determining that detection of nuclear cyclin A in individua l cells also corresponded with detection of DNA breakage. Levels of cy clin E, cyclin E-dependent H1 histone kinase, and p53 proteins were ma intained during dispersion-induced DNA cleavage. gamma-radiation faile d either to inhibit cell cycle progression or to raise p53 levels in d ispersed bursal lymphoblasts. In intact bursal follicles low doses of gamma-radiation induced p53 whereas higher, apoptosis-inducing doses f ailed to induce p53 or prevent G1 to S-phase progression. These result s suggest that normal DNA damage-induced cell cycle checkpoint control s are lost or overridden when apoptosis is induced in bursal lymphobla sts.