Cell cycle-specific behavior of erythropoietin

The murine erythropoietin-dependent erythroleukemia cell line, HCD-57, was employed to study the cell cycle-specific behavior of erythropoietin. Cell cycle duration for HCD-57 cells was approximately 12 hours and was uninfluenced by erythropoietin. Populations of HCD-57 cells synchronized in G₁ by centrifugal elutriation were able to pass through one complete cell cycle in the absence of erythropoietin but, thereafter, arrested in G₁ as identified by propidium iodide staining and flow cytometry. Analysis of cell cycle behavior using the metachromic dye acridine orange, however, revealed that HCD-57 cells pass through a G₀ cell cycle phase and, like serum-deprived 3T3 cells, actually arrest in G₀ when deprived of erythropoietin. Expression of the cell cycle regulatory protein p34cdc2 was invariant throughout the cell cycle in HCD-57 cells. p34cdc2 was constitutively phosphorylated in G₀ cells, and this effect was not modified by erythropoietin. Erythropoietin receptor distribution was log normal in HCD-57 cells in each phase of the cell cycle. The affinity of these surface receptors for erythropoietin was essentially invariant throughout the cell cycle, but receptor expression was upregulated in G₂M cells as compared with cells in G₁ or S phase. Taken together, these data indicate that erythropoietin has an important role in the G₀-G₁ to S phase transition but, based on receptor expression, is involved in other phases of the cell cycle as well.