DIFFERENTIAL ACTIVATION OF A CALCIUM-DEPENDENT ENDONUCLEASE IN HUMAN B-LYMPHOCYTES - ROLE IN IONOMYCIN-INDUCED APOPTOSIS

The state of B cell maturation profoundly influences the outcome, i.e. , activation, growth arrest, or programmed cell death, of a variety of stimuli, including the calcium ionophore, ionomycin. Initial studies confirmed the observation that cell lines representative of immature B cells, i.e., Burkitt lymphoma cell lines, were induced to undergo apo ptosis in response to ionomycin, whereas more mature B cell lines did not, and instead underwent cell cycle arrest in the G1 interval. To un derstand this differential outcome, we have focused on comparing the e xpression and activation of an endonuclease(s) in cells induced by ion omycin to undergo programmed cell death (Ramos) with cells resistant t o ionomycin-induced programmed cell death (Ly1). Our results demonstra ted that a low m.w. fraction of an endogenous Ca2+/Mg2+-dependent endo nuclease was activated in Ramos cells, but not in activated Ly1 cells, following the addition of ionomycin. Of interest, however, low m.w. e ndogenous endonuclease(s) activity was induced when isolated Ly1 cell nuclei were treated with exogenous calcium instead, Use of field inver sion gel electrophoresis further indicated that cleavage of DNA into l arge m.w. (>50 kbp) DNA fragments does not precede ionomycin-induced i nternucleosomal cleavage in Ramos cells or in ionomycin-resistant Ly1 cells. In summary, these data support the conclusion that ionomycin-in duced apoptosis involves the activation of a latent, low m.w., calcium -responsive endonuclease and suggest that control of endonuclease dere pression may contribute to cell-specific regulation of calcium ionopho re-induced apoptosis.