Definition of the roles for iron and essential fatty acids in cell cycle progression of normal human T lymphocytes.
A serum-free cell culture system for human T lymphocytes was used to investigate the synthesis and metabolism of several important cell cycle-regulated proteins (p62c-fos, p110Rb, and p34cdc2 and its homologs) and the possible roles of iron and essential free fatty acids in regulating cell cycle progression. Following stimulation with phorbol dibutyrate (PDB) and ionomycin under serum-free conditions, resting T cells entered the cell cycle, as evidenced by a burst of synthesis of p62c-fos and an increase in the amount of the p33 homolog of the cdc2 kinase. However, in the absence of other additions, cells were arrested in the G1 phase of the cell cycle. Supplementation of the medium with two components, iron and linoleic acid (LA), permitted activated cells to progress through the G1 phase of the cycle and initiate DNA synthesis. Under these conditions p110Rb became phosphorylated and p34cdc2 was synthesized similar to T cells proliferating in normal serum-containing medium. The addition of iron, without LA, had little effect on activated cells; however, the addition of LA, in the absence of added iron, had profound effects. RNA accumulated to levels characteristic of cells at the G1/S interface, phosphorylation of p110Rb was almost complete, and p34cdc2 was synthesized, although at lower levels than in proliferating cells. However, no DNA synthesis was detected; under these conditions the cells appeared to be blocked at or near the G1/S border. Since there was a possibility that some component of the cell culture system could provide "trace" amounts of iron, and also to further delineate the role of iron in this system, cells were activated in medium containing LA and deferoxamine (10 microM), a chelator of iron. The accumulation of p34cdc2 was now reduced to nearly undetectable levels although phosphorylation of p110Rb was not substantially affected. It therefore appears that synthesis of p34cdc2 requires a low amount of iron, a finding which may define a possible regulatory point in the cell cycle for iron before its well-recognized role in regulating S phase entry by acting as a cofactor for the enzyme ribonucleotide reductase.