Activins are growth and differentiation factors belonging to the transforming growth factor-β superfamily. They are dimeric proteins, consisting of two inhibin-β subunits.
The structure of activins is highly conserved during vertebrate evolution. Activins signal through type I and type II receptor serine/threonine receptor kinases. Subsequently downstream signals such as Smad proteins are phosphorylated.
Activins are present in many tissues of the mammalian organism, where they function as autocrine and/or paracrine regulators of various physiological processes, including reproduction.
In the hypothalamus, activins are thought to stimulate the release of gonadotropin-releasing hormone.
In the pituitary, activins increase folliclestimulating hormone secretion and up-regulate gonadotropin- releasing hormon receptor expression. In the ovaries, activins regulate processes such as folliculogenesis, steroid hormone production and oocyte maturation.
During pregnancy, activin-A is also involved in the regulation of placental functions.