Similarities in structure and function of bovine serum erythrotropin and human insulin-like growth factor II: two fetal erythroid cell stimulating factors.

Serum erythrotropin (ET) was isolated from fetal bovine serum. Partial sequence analysis of the N-terminal portion of the peptide indicated that the first 20 amino acids were practically identical to those found in human insulin-like growth factor II (IGF II). The effect of IGF II on [3H] thymidine incorporation in cell cultures of fetal bovine liver was similar to the effect of ET. Both factors acted synergistically with erythropoietin but not with platelet derived growth factor. The stimulation of thymidine incorporation by ET and IGF II on cell cultures of fetal liver erythroid cells was at least 15 times higher than their effects on cultures of fetal calf intestine, lung and kidney cells.