Importance of FSH-releasing protein and inhibin in erythrodifferentiation

Inhibin is a hypophysiotropic hormone which selectively suppresses the secretion of pituitary follicle-stimulating hormone. It has been isolated from gonadal fluids and characterized as a protein heterodimer consisting of an α subunit and one of two β subunits (β_A or β_B)^1-4. FSH-releasing protein (FRP), also named activin, is a dimer consisting of two inhibin β-chains^5-6. A factor from conditioned medium of a leukaemia cell line has been isolated which can induce mouse Friend cells to become benzidine-positive, and which shares a similar N-terminal sequence with porcine FRP⁷. In this report, we find that FRP and inhibin modulate both the induction of haemoglobin accumulation in a human erythroleukaemic cell line, K562, and the proliferation of erythroid progenitor cells in human bone marrow culture. These two proteins could constitute a novel humoral regulatory control of erythropoiesis which would involve two types of related protein dimers with functionally opposite effects.