The aminonucleoside of puromycin (AMS) is known to have a differential effect on ribosomal RNA (rRNA) synthesis and the cell-cycle traverse in normal as compared to neoplastic or virally transformed cells. In this study, AMS and its demethylated derivative, 3'-amino-3-'deoxyadenosine (3'-AmA) have been used to compare their effects on normal (IMR 90) versus transformed (AG 2804) human fibroblasts with regard to preribosomal RNA (pre-rRNA) transcription, the processing of this RNA, and structural changes in the nucleolus. The processing of pre-rRNA in normal and transformed fibroblasts treated with 3'-AmA for 4 hr was markedly depressed. However, this process did not appear to be affected by the AMS treatment of normal cells, while in transformed cells it was maximally inhibited within 4 hr of exposure to this drug. Ultrastructural changes were observed in the nucleoli of normal and transformed cells treated with 3'-AmA, whereas treatment of these cells with AMS produced alterations of nucleolar structure only in the transformed cells. These changes correlated with the onset of inhibition of pre-rRNA processing. Our findings suggest that the impairment of pre-rRNA processing produced by AMS and 3'-AmA in transformed cells and by 3'-AmA in normal cells may be due to structural disorganization of the nucleolus produced by these antimetabolites.
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