The polyamines putrescine, spermidine and spermine have been implicated in the regulation of cellular proliferation and differentiation. However, a direct role has been demonstrated only for spermidine in DNA replication. We have previously shown that spermidine is required for both murine erythroleukemia (MEL) cell proliferation and induction of differentiation. Furthermore, we have demonstrated that spermidine is required for both the induction of heme synthesis and transcription of alpha- and beta-globin mRNA. The requirement of spermidine for induction of MEL differentiation may reflect the necessity of cell division for expression of the mature phenotype. The biosynthesis of spermidine and spermine is associated with the formation of 5'-methylthioadenosine (MTA). We have recently demonstrated that MTA inhibits induction of MEL differentiation. This effect was not associated with spermidine depletion or cytostasis. MTA is a known inhibitor of S-adenosylhomosysteine hydrolase and DNA methylation and may inhibit induction of differentiation by interfering with transmethylation reactions. MTA also inhibits cyclic AMP phosphodiesterase activity and potentiates the cellular cAMP response to adenylate cyclase activators. Therefore, this naturally occurring nucleoside may be an intermediate involved in the interrelationships between cellular proliferation and specific gene expression. The proposed studies will thus extend our work on the involvement of polyamines in leukemic cell proliferation and differentiation. The initial studies will monitor the effects of MTA on MEL intracellular S-adenosylmethionine metabolism, nucleic acid methylation, and intracellular cAMP metabolism. A more precise understanding of the biochemical basis for regulating MEL commitment to differentiation will be explored by using MTA analogues and other inhibitors of S-adenosylhomocysteine hydrolase. This work should provide the basis for monitoring the effects of MTA and MTA analogues on transcription and DNA methylation of specific MEL genes including alpha-globin, beta- globin and mouse c-myc. Since previous studies have also demonstrated that spermidine is required for induction of human HL-60 leukemic cell differentiation, the proposed work will thus also monitor the effects of MTA on methylation and expression of specific genes (c-myc, c-fos) in this cell line. These studies should be important in providing a basis for understanding biochemical interrelationships between proliferation and differentiation in leukemic cells.
