Expression and secretion of activin A: possible physiological and clinical implications.

Activins, ®rst isolated in 1986 from porcine follicular ̄uid (1, 2), are gonadal proteins which stimulate pituitary follicle-stimulating hormone (FSH) synthesis and secretion. However, from 1986 to the present day, several lines of research have found that activins are not only synthesized in the ovaries and testes, but also in other tissues where they function as paracrine and autocrine factors to regulate a number of processes within and outside of the reproductive axis. The evidence of expression of activin mRNA in a wide variety of tissues ± including placenta, pituitary, adrenals, spleen, bone marrow and speci®c regions of the brain ± and the diverse set of biological functions in these tissues suggested a possible role for activin as a growth factor and a cytokine (3±6). Furthermore, the availability of a suitable assay developed in the last few years has made it possible to measure activins in several biological ̄uids. In particular, the discrete amount of activin A circulating in the systemic bloodstream suggests a possible endocrine role, but up to now the possible source(s) and target(s) are still debated. The present review summarizes the various activin sources and functions in humans, and the data obtained on activin A measurement in various biological ̄uids which suggest possible physiological and clinical implications.