The protooncogene c-myc is known to be associated with both cell proliferation and apoptosis. The possible cellular affects of castration on the ventral prostate gland of rat as well as the relationship to a castration induced c-myc expression were examined. Levels of c-myc mRNA in the ventral prostate gland peaked at 6 h (early induction) and 48 h (late induction) after castration, respectively. Castrationinduced DNA fragmentation was not observed at an early induction of c-myc mRNA. DNA fragmentation appeared to be testosterone-dependent. On the other hand, cellular DNA synthesis measured by [ 3 H]thymidine uptake in the ventral prostate gland was increased to maximum at 6 h after castration. These results suggest that an early induction of c-myc mRNA in ventral prostate gland after castration is closely associated with cell proliferation of the gland.
DNA topoisomerase II is a marker for the proliferation state of mammalian cells in culture, and the protein levels are markedly higher in exponentially growing cells than quiescent cells and can be downregulated by growth of the cells at high density and serum starvation. Correlation between ATF and TPA‐repressed DNA topoisomerase IIα (Topo IIα) mRNA has been investigated during TPA‐induced differentiation of HL‐60 cells. Topo IIα RNA and unknotting activity were reduced at 24 hours in TPA‐treated HL‐60 cells. The level of Topo IIα mRNA and the activity were gradually decreased in proportion to the concentration of TPA. Two DNA‐protein complexes were formed by DNA mobility shift assay when ATF‐binding site was incubated with nuclear extract prepared from TPA‐free HL‐60 cells, and the amount of ATF was vanished after TPA treatment. TPA‐repressed Topo IIα mRNA and ATF levels were partially restored after pretreatment of staurosporin. These results suggest that the reduced level of ATF may be important to the transcriptional repression of Topo IIα gene during TPA‐induced differentiation in HL‐60 cells and related to protein kinase C signal pathway.
Epidermal growth factor (EGF) is a potent mitogen for rat hepatocytes and mammalian histone synthesis is functionally and temporally coupled to DNA replication. To gain an insight on the role of EGF in the regulation of H2B histone gene expression in primary hepatocyte cultures, the binding patterns of nuclear proteins to various elements in the H2B histone gene upstream region have been investigated. EGF induced H2B histone mRNA with maximal stimulation reached at 36 hours. The induction of H2B histone mRNA was dependent on the concentration of EGF and almost reduced by actinomycin‐D pretreatment. In DNase I footprinting analysis, one nuclear factor (TATA element‐binding protein, TBP) bound at ‐20 bp (TATA element)in either the absence or presence of EGF. One DNA‐protein complex was formed by DNA mobility shift assay when TATA element was incubated with nuclear extract prepared from EGF‐free hepatocytes, and the amount of TBP was increased after EGF treatment. These results suggest that TBP may be correlated with transcriptional regulation of H2B histone gene by EGF in primary hepatocytes.
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