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Activation of a p53-mediated Apoptotic Pathway in Quiescent Lymphocytes after the Inhibition of DNA Repair by Fludarabine¹

Department of Experimental Therapeutics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, and The University of Texas Graduate School of Biomedical Sciences, Houston, Texas 77030

Purpose: The inhibition of UV-initiated DNA repair by 9-ß-D-arabinofuranosyl-2-fluoroadenine (F-ara-A), the nucleoside of fludarabine, induces apoptosis in quiescent human lymphocytes. The sensing and signaling mechanisms after DNA repair inhibition by F-ara-A are unknown. The purpose of this study was 2-fold: (a) determine the importance of the inhibition of DNA repair processes for F-ara-A cytotoxicity and (b) identify the apoptotic signaling mechanism(s) that respond to DNA repair inhibition by F-ara-A.

Experimental Design: Lymphocytes were treated with F-ara-A to accumulate the active triphosphate metabolite and subsequently DNA repair was activated by UV irradiation. Cell viability was quantitated with respect to the treatments alone and in combination to evaluate the actions of F-ara-A inhibition of DNA repair on p53 status and Fas death receptor ligand expression and function.

Results: Preincubation of lymphocytes with 3 µM F-ara-A inhibited DNA repair initiated by 2 J/m² UV and induced greater than additive apoptosis after 24 h. After equivalent repair inhibition with 0.1 µM aphidicolin, there was apparently lesser p53 activation and significantly less apoptosis in irradiated lymphocytes than after 3 µM F-ara-A. Blocking the incorporation of F-ara-A nucleotide into repairing DNA using 30 µM aphidicolin lowered the apoptotic response to that observed with aphidicolin and UV. p53 serine 15 phosphorylation and protein accumulation were detected 2 h after treatment. Fas and Fas ligand mRNA expression and protein levels increased significantly after repair inhibition. Neutralizing antibodies against Fas or Fas ligand significantly reduced apoptosis.

Conclusions: These results suggest that inhibition of UV-induced DNA repair by F-ara-A is critical for cytotoxicity and that induction of apoptosis may be conducted by a p53-mediated signaling mechanism to the Fas death pathway.

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