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 erythropoie tin. Cell cycle duration for HCD-57 cells was approximately 12 hours a nd was uninfluenced by erythropoietin. Populations of HCD-57 cells syn chronized in G(1) by centrifugal elutriation were able to pass through one complete cell cycle in the absence of erythropoietin but, thereaf ter, arrested in G(1) as identified by propidium iodide staining and f low cytometry. Analysis of cell cycle behavior using the metachromic d ye acridine orange, however, revealed that HCD-57 cells pass through a G(0) cell cycle phase and, like serum-deprived 3T3 cells, actually ar rest in G(0) when deprived of erythropoietin. Expression of the cell c ycle regulatory protein p34cdc2 was invariant throughout the cell cycl e in HCD-57 cells. p34cdc2 was constitutively phosphorylated in G(0) c ells, and this effect was not modified by erythropoietin. Erythropoiet in receptor distribution was log normal in HCD-57 cells in each phase of the cell cycle. The affinity of these surface receptors for erythro poietin was essentially invariant throughout the cell cycle, but recep tor expression was upregulated in G(2)M cells as compared with cells i n G(1) or S phase. Taken together, these data indicate that erythropoi etin has an important role in the G(0)-G(1) to S phase transition but, based on receptor expression, is involved in other phases of the cell cycle as well.