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.