The erythrotropins, new factors which stimulate thymidine incorporation and globin chain synthesis in liver erythroid cells.

A serum-free culture system of erythroid cells of fetal rabbit liver was used for the determination of the erythropoietin-like activity present in fetal calf plasma. The activity found was too high to be explained solely by the presence of erythropoietin. For this reason we decided to investigate the nature of factors present in fetal calf tissues which could have an erythropoietin-like effect in a serum-free medium. The use of fetal calf liver cells and the replacement of the 59Fe incorporation with thymidine incorporation into acid-insoluble materials improved the sensitivity of the assay. This method was used for the rapid screening for the presence of erythropoietin-like factors in reversed-phase extracts of livers, kidneys, lungs, intestines and brains from fetal calves. Intestine extracts stimulated thymidine incorporation and were therefore used as the source for the isolation by reversed-phase and gel permeation high performance liquid chromatography (HPLC) of two active factors (1 and 2). They had different retention times on reversed-phase HPLC but had very similar amino acid compositions and a molecular weight of about 3500 daltons. Both factors added to cultures of calf liver cells caused an increase of alpha and gamma globin chain synthesis equivalent to that observed in the presence of erythropoietin. However, there were important functional differences between both factors and erythropoietin in a rat liver bioassay of thymidine incorporation similar to the one described above. Factors 1 or 2 could not increase the number of cells after a 20h incubation whereas erythropoietin caused a modest increase in cell numbers. Both factors were less potent than erythropoietin in stimulating thymidine incorporation in rat liver cells. Finally, factors 1 and 2 were synergistic with erythropoietin. It is concluded that fetal calf intestine contains factors acting on erythroid cells which have different physical and biological properties from those of erythropoietin. The names erythrotropins I and II are proposed for these factors.