The polyamines putrescine, spermidine, and spermine have been implicated in the regulation of both proliferation and differentiation. Spermidine is required for DNA replication, and the intracellular depletion of this polyamine can be used to distinguish cellular events related to proliferation. We have demonstrated previously that depletion of intracellular spermidine results in cytostasis of human HL-60 promyelocytes. The inhibition of HL-60 proliferation is associated with inhibition of differentiation induced by dimethyl sulfoxide, hexamethylene bisacetamide, butyric acid, and retinoic acid. The present studies extend these findings by monitoring the effects of spermidine depletion and inhibition of DNA replication on induction of HL-60 differentiation by leukocyte-conditioned medium and phorbol ester. The results demonstrate that both inducers enhance intracellular polyamine levels. However, depletion of intracellular spermidine inhibits induction of HL-60 differentiation by leukocyte-conditioned medium, but not by phorbol ester. Further, the addition of exogenous spermidine abrogates the inhibition of proliferation, and leukocyte-conditioned medium induced HL-60 differentiation, confirming the requirement of this polyamine for the expression of a differentiated phenotype. The present results and our previous findings suggest that spermidine is required for induction of HL-60 differentiation by a variety of agents, including leukocyte-conditioned medium, but that phorbol ester induces HL-60 differentiation in the absence of polyamine biosynthesis and DNA replication.

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This work was supported by USPHS Grant CA-19589 awarded by the National Cancer Institute, Department of Health and Human Services.

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