Cellular polyamine depletion reduces DNA synthesis in isolated lymphocyte nuclei

The accumulation of putrescine, spermidine and spermine in activated bovine lymphocytes was blocked by the combined action of two inhibitors of polyamine biosynthesis, methylglyoxal bis(guanylhydrazone) (MGBG) and alpha-methylornithine. Lymphocytes were cultured under three conditions: (1) alpha-methylornithine alone, (2) MGBG alone, or (3) alpha-methylornithine plus MGBG. DNA synthesis in nuclei isolated from these cells was reduced from control rates by approx. 10, 55 and 75%, respectively. In each case, the degree of inhibition was similar to that observed with the intact cells. Stimulation of nuclear DNA synthesis with the postnuclear supernatant fraction was not affected by polyamine depletion of the cells. Several experiments indicate that the reduced rate of in vitro DNA synthesis was caused by the lack of polyamines and not by alternate effects of the drugs. No inhibition was observed (1) when spermidine was added to inhibited cultures 12 h before harvest and nuclear isolation, (2) when the drugs were added after polyamines had accumulated, and (3) when the drugs were added directly to the in vitro assay. In addition, the degree of inhibition of in vitro DNA synthesis correlated with the degree of polyamine deficiency. These in vitro studies confirm the results obtained with whole cells and support the hypothesis that DNA synthesis is one cellular site of action of the naturally occurring polyamines.