Chick-Erythrocyte Nucleus Reactivation in Heterokaryons: Suppression by Inhibitors of Proteolytic Enzymes

Abstract

Reactivation of chick-erythrocyte nuclei in heterokaryons (obtained by Sendai virus-induced fusion of chick erythrocytes with HeLa cells) is suppressed by specific inhibitors of trypsin and trypsin-like enzymes.N-a-tosyl-L-lysyl-chloromethane and N-a-tosyl-L-arginine methylester inhibit erythrocyte nuclear enlargement and suppress RNA and DNA synthesis in nuclei of erythrocytes and HeLa cells in heterokaryons at concentrations that only minimally influence individual HeLa cells or HeLa homokaryons.Although other unknown mechanisms of action cannot be formally excluded, the data are interpreted as fitting best with an intracellular site of action of the protease inhibitors studied, and as suggesting a role for cellular proteases in reactivation of chick-erythrocyte nuclei in heterokaryons.The mature chick erythrocyte contains a dormant nucleus, the replicative and transcriptional activities of its nuclear DNA having ceased during the latter stages of erythropoiesis. The nuclear chromatin of this end-stage cell is condensed and nucleoli are absent (1, 2). When chick erythrocytes are fused with active cells (e.g., HeLa cells or fibroblasts) to form heterokaryons, the erythrocyte nucleus is reactivated (3, 4). Shortly after cell fusion, the volume and dry mass of the erythrocyte nucleus increase, its tightly packed chromatin ("nuclear bodies") disperses, and RNA synthesis resumes. DNA replication begins 1-2 days after fusion, and nucleoli appear a day or two later (3-8).The basis for reactivation of the dormant erythrocyte nucleus in heterokaryons is not understood. During Sendai virus-induced cell fusion the erythrocyte lyses and erythrocyte nuclei enter the cytoplasm of their partner cells without appreciable quantities of erythrocyte cytoplasm (3-9). For this reason it is believed that factors present in partner cell cytoplasm reactivate erythrocyte nuclei (5, 6, 10). We report here that certain inhibitors of proteolytic enzymes suppress reactivation of erythrocyte nuclei in heterokaryons. In addition, HeLa nuclei, ordinarily resistant to protease inhibitors, become sensitive to them when paired with chick-erythrocyte nuclei in heterokaryons, suggesting that they are sensitized by factors released by the erythrocyte nucleus. The results implicate cellular proteases in reactivation of chick-erythrocyte nuclei in heterokaryons.Abbreviations: TLCK, N-a-tosyl-L-lysyl-chloromethane (trivial name: L-1-tosylamide-S2leucylchloromethylketone); TAME, Na-tosyl-L-arginine methylester; TPCK, N-a-tosyl-L-phenylalanylchloromethane (trivial name: L-1-tosylamide-2-phenyl-chloromethylketone); PheMeSF, phenyl-methyl sulfonyl fluoride; Me2SO, dimethylsulfoxide. MATERIALS AND METHODSChick erythrocytes were obtained from 15-to 16-day-old embryos as described (7,11). HeLa cell cultures were maintained in Eagle's medium containing 10% fetal bovine serum and antibiotics (penicillin 100 u/ml; streptomycin 100 ,ug/ ml). Heterokaryons were prepared by fusion of chick erythrocytes with HeLa cells using UV-inactivated Sendai virus (80...