The effect of micrococcal nuclease and DNAase I on bulk and 5 S RNA synthesis in isolated HeLa cell nuclei

Association of 5' CpG demethylation and altered chromatin structure in the promoter region with transcriptional activation of the multidrug resistance 1 gene in human cancer cells

Selection of human cells for resistance to vincristine or doxorubicin often induces overexpression of the multidrug resistance 1 gene (MDR1), which encodes the cell surface P-glycoprotein, as a result of gene amplification or transcriptional activation. However, the precise mechanism underlying such transcriptional activation of MDR1 remains unclear. The relation between methylation status of CpG sites in the MDR1 promoter region and transcriptional activation of MDR1 has now been investigated. The P-glycoprotein-overexpressing, multidrug-resistant KB/VJ300 and KB-C1 cells, which were established from human cancer KB3-1 cells, were examined; MDR1 is transcriptionally activated but not amplified in KB/VJ300 cells, whereas it is amplified in KB-C1 cells. Determination of the methylation status revealed that the MDR1 promoter region was hypomethylated in KB/VJ300 and KB-C1 cells, but hypermethylated in KB3-1 cells. Prior treatment of KB3-1 cells with the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine resulted in a 90-fold increase in the frequency of vincristine-resistance. Of three lines, KB/CdR-1, KB/CdR-2, and KB/CdR-3, established from KB3-1 cells after exposure to 5-aza-2'-deoxycytidine, MspI/HpaII sites in the MDR1 promoter region were hypomethylated in KB/CdR-1 and KB/CdR-2 cells, but not in KB/CdR-3 cells. MDR1 mRNA expression was detected in KB/CdR-1 and KB/CdR-2 cells, but not in KB/CdR-3 cells. The binding of YB-1 and Sp1, transcription factors implicated in MDR1 expression, in the MDR1 promoter was not affected by the methylation status of a neighboring CpG sites. The MDR1 promoter region in KB/VJ300 cells showed an increased sensitivity to DNase I compared with that in KB3-1 cells, suggesting an altered chromatin structure. The methylation status of the promoter region may plays an important role in MDR1 overexpression and in acquisition of the P-glycoprotein-mediated multidrug resistance phenotype.

DNAase I sensitivities in chromatin of the Xenopus laevis somatic and oocyte 5 S DNAs

The DNAase I sensitivities of the somatic-type 5 S DNA and oocyte-type 5 S DNA have been compared in nuclei from adult somatic tissues of Xenopus laevis. Neither of these Type III genes is expressed in mature erythrocytes and only somatic-type 5 S DNA is expressed in liver. We find that somatic-type 5 S DNA is DNAase-I-sensitive in liver nuclei and less sensitive in erythrocyte nuclei, while oocyte-type 5 S DNA is insensitive in both tissues. The DNAase I sensitivity appears to be uniform across each active somatic-type 5 S DNA repeat. Two regions slightly hypersensitive to DNAase I are found only in liver somatic-type 5 S DNA. One of these regions is within the gene, overlapping with the binding site of the transcription factor (TF III A) required for 5 S RNA synthesis. Thus, the correlation between DNAase I sensitivity and gene activity previously seen for protein-coding genes also holds for these Type III genes. Our data lead us to suggest that the fully DNAase-I-sensitive chromatin conformation on 5 S DNA requires the presence both of transcription factors and RNA polymerase.

Rapid inactivation of Ca2+, Mg2+-dependent endonuclease of rat liver nuclei after cycloheximide treatment

Ca2+, Mg2+ dependent endonuclease activity of isolated nuclei from rat liver disappeared completely within one to two hours after intraperitoneal administration of inhibitors of eukaryotic protein synthesis such as cycloheximide or puromycin. Actinomycin D, on the other hand, revealed no inhibition of the endonuclease activity, but even reversed the effect of cycloheximide by simultaneous addition.