Changes in extranucleolar transcription during actinomycin D-induced apoptosis

Abstract

Actinomycin D (AMD) inhibits DNAdependent RNA polymerases and its selectivity depends on the concentration used; at very high concentrations it may also induce apoptosis. This study investigates the effects of different concentrations (0.01 to 1 µg/ml) of AMD on RNA transcription and maturation and on the organization of nuclear ribonucleoproteins (RNPs), and their relationship with apoptosis induction. Human HeLa cells were used as a model system. At the lowest concentration used, AMD induced the segregation of the nucleolar components and impaired r-RNA synthesis, as revealed by the decreased immunopositivity for bromouridine incorporation and for DNA/RNA hybrid molecules. The synthesis of pre-mRNAs, on the contrary, was active, while the immunolabeling of snRNP proteins and of the SC-35 splicing factor strongly decreased on perichromatin fibrils (where they are involved in co-transcriptional splicing). This suggests that the post-transcriptional maturation of extranucleolar RNAs was also affected. Moreover, still in the absence of typical late morphological or biochemical signs of apoptosis (i.e. chromatin condensation), these cells displayed the early apoptotic features, i.e. the externalization of phosphatidylserine residues on the plasma membrane and propidium iodide exclusion in vivo. At the highest concentrations of AMD used, apoptosis massively occurred, with the typical morphological events (progressive chromatin condensation, clustering of snRNPs and SC-35 splicing factor, cell blebbing). However, transcription of hnRNAs was maintained in the residual areas of diffuse chromatin up to advanced apoptotic stages. The inhibition of rRNA synthesis and the defective pre-mRNA maturation seem to be part of the apoptotic process induced by AMD.