| Summary: | 碩士 === 國立臺灣大學 === 動物學系 === 84 === Cadmium acetate is highly toxic to the S phase cells and
induces S phase delay. In this report, we study the
accumulation and distribution of cadmium and its effects on DNA
synthesis in synchronous S phase CHO-K1 cells. The accumulation
of cadmium was time and dose dependent. The concentrations of
cadmium in cytoplasm and in nucleus were 1.7 mM and 0.7 mM
respectively when the cells were incubated with 4 and 2 microM
cadmium acetate for 8 h. Treatment of cells with 2 microM
cadmium for 5 h resulted in S phase delay, while 4 microM
cadmium completely arrested the cells at the mid-S phase.Newly
synthesized DNA was decreased when the cells were exposed to 2
microM (incubated for 8 h) or 4 microM (incubated for 5 h) of
cadmium acetate. Furthermore, the rate of DNA synthesis was
inhibited after treatment of cells with 2 microM (or 4 microM)
cadmium for 3 h (or 2 h) and the inhibition was time dependent.
These results show that the cadmium-induced DNA synthesis
inhibition and S phase arrest were well correlated with
intracellular accumulation of cadmium.By using alkaline sucrose
gradient analysis, cadmium treatment inhibited the number of
replicons but did not affect the initiation synthesis, and
indicating the DNA synthesis at elongation phase was decreased
by cadmium. The activity of thymidine kinase was decreased by 4
microM, but not 2 microM cadmium. Thus, the inhibitory effect
of thymidine kinaes activity was unlikely involved in cadmium-
induced S phase delay. We also showed that DNA topoisomerase
was not affected by cadmium in CHO-K1 cells. Cadmium also
induced apoptosis in CHO-K1 cells. When 2 microM or 4 microM of
cadmium was added to cells synchronized at S phase, nuclear
fragmentation and cell blebbing were observed lately. In
addition, DNA ladder was observed on an electrophorelic gel.
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