Abstract
Activin A is a multi-functional cytokine with a potent stimulation on erythroid cell differentiation in the bone marrow. The actions of activin A are determined by a balance of the levels of activin A and its inhibitor, follistatin (FS). However, the regulation of its actions in the bone marrow has been unclear. Here we show that bone marrow-derived stromal fibroblasts are the major source of activin A and FS in the bone marrow, and that the production of activin A is enhanced by interleukin-1beta (IL-1beta) and lipopolysaccharide (LPS), whereas interferon-gamma (IFN-gamma) inhibits the secretion of activin A by stromal fibroblasts. Concomitantly, IL-1beta as well as LPS inhibits and IFN-gamma stimulates FS secretion from stromal fibroblasts. Thus, these cytokines potently regulate activin A actions by reciprocal modulation of activin A and FS secretion from stromal fibroblasts. Because activin A exhibits anti-inflammatory effects in various tissues, up-regulation of activin A actions by IL-1beta and endotoxin in the bone marrow may play a protective role against inflammatory processes as well as anaemia. The present results also suggest that the inhibitory effect of IFN-gamma on erythropoiesis is mediated at least in part by a suppression of activin A actions in bone marrow.