Mechanisms of Nitrosourea-Induced β-Cell Damage: Activation of Poly (ADP-Ribose) Synthetase and Cellular Distribution

Abstract

It has been hypothesized that the critical step in streptozocin (STZ)-induced beta-cell toxicity is the overactivation of the nuclear enzyme poly(ADP-ribose) synthetase resulting from DNA strand breaks. Overactivation of this enzyme leads to a lethal depletion of its substrate, NAD, in the beta-cell. However, recently it has been shown that a lethal concentration of STZ and a nontoxic concentration of its nitrosoamide moiety methylnitrosourea (MNU) damage beta-cell DNA to the same extent and cause comparable amounts of DNA strand breaks. This study was performed to determine whether STZ and MNU activate poly(ADP-ribose) synthetase to the same extent. Monolayer cultures of islet cells from neonatal rats were exposed to concentrations of MNU and STZ of 10(-3) to 10(-2) M. The results show that both chemicals caused comparable activation of the enzyme at all concentrations tested. These data demonstrate that activation of poly(ADP-ribose) synthetase alone is not the critical step in STZ-induced beta-cell toxicity. Based on this finding, it appeared possible that STZ may be selectively sequestered into some critical site in the beta-cell other than the nucleus. Therefore, studies were initiated with 14C-labeled STZ and MNU to determine whether STZ might be distributed in the beta-cell differently than MNU. Total cellular DNA and protein from both RINr (clone 38) and islet cell monolayers were separated on hydroxylapatite columns after exposure to 14C-labeled chemicals. The amount of label incorporated into each fraction was determined by liquid scintillation spectrometry, and the ratio of label incorporated in protein to that in DNA was determined.(ABSTRACT TRUNCATED AT 250 WORDS)