Pyrimidine nucleotide and nucleic acid synthesis in human monocytes and macrophages

In most cell types, the production of deoxynucleotides is tightly coupled to the pace of cell division, and nearly all deoxynucleotides are used for semiconservative DNA synthesis. The capacity of peripheral blood monocytes and macrophages to proliferate is controversial. However, these cells have been reported to produce and release thymidine, which can serve as a precursor or regulator of DNA synthesis by lymphocytes and other cells. To determine to what extent de novo pyrimidine nucleotide synthesis is linked to cell division in peripheral blood monocytes and macrophages, compared to human U937 promonocytes and CEM lymphoblasts, we used a precise precursor-product labeling method. The results showed that in all three cell types, the pace of pyrimidine deoxynucleotide production, and of thymidylate synthesis, was in proportion to the rate of DNA synthesis. The human blood monocytes and macrophages, in contrast to U937 cells, had extraordinarily low deoxyribonucleotide pools (less than 1 pmol/10(6) cells) and synthesized neither thymidylate nor DNA de novo during 7 days culture. Colony-stimulating factors augmented RNA synthesis in monocyte-derived macrophages, and enhanced cell survival, without inducing either DNA or thymidylate synthesis. We conclude that the thymidine released by macrophages derives from dead or dying cells, and not from de novo synthesis.