Stimulation of DNA synthesis and myo-inositol incorporation in mammalian cells

Phosphatidylinositol (PI) synthesis and its role in controlling the cell cycle has been investigated using fibroblasts and liver cells in culture. PI synthesis as measured by incorporation of ³H]-myo-inositol into trichloroacetic acid precipitable material during 0–60 min after serum or growth factor stimulation of serum-starved cells is increased in primary fetal rat liver cells, rat embryo fibroblasts, and 3T3 mouse cells. In contrast, growth stimulation of 3T3 cells and hepatocytes rendered quiescent in G₁ by amino acid starvation is not accompanied by increased incorporation of ³H]-myo-inositol into trichloroacetic acid precipitable material. This suggests that those cells might be arrested at a different point in G₁ than cells arrested by serum depletion. Inhibition of PI synthesis by δ-hexachlorocyclohexane (HCH), a steric analog of myo-inositol, during early times (e.g., 0–4 hr) after growth stimulation, reversibly blocks initiation of DNA synthesis in 3T3 cells. The results support the idea that increased PI synthesis in response to growth stimulation in the cell types studied here is a prerequisite for progression through G₁ and subsequent entry into S phase.