Activin A

Activin A, a multifunctional cytokine, is a member of TGF-β superfamily. Activin A first binds to the type II activin receptors (ActIIRA or ActRIIB) on the member surface, and then recruits and phosphorylates type I activin receptors (ActRI). Activin A primarily signal through SMAD2/3 proteins to regulate a variety of functions, including inflammation, fibrosis, and tumorigenesis. Activin is a member of transforming growth factor-β (TGF-β) superfamily consisting of two inhibin β subunits linked by disulfide bonds. Activin A is expressed widely in various tissues and cells with strong bioactivities and is the mostly studied activin^[1].
The sequence of amino acids in Activin A proteins from different species is very stable, which leads to the conclusion that in the process of evolution, Activin A has been only slightly altered, and that both in humans and in animals, its function is similar.
Activin exists in three basic molecular forms composed of two inhibin β subunits: activin A (βAβA), activin B (βBβB), and activin AB (βAβB). Activin A binds with high affinity to activin type II receptors, which recruit type I receptors and are necessary for the activation of Smad2/3 signaling. The phosphorylated ActRI activates Smad2 and Smad3, which form a complex with Smad4 to translocate to the nucleus. Activin and TGF-β share the same signaling pathway at the level of Smad2/3/4. Activin A exerts a variety of biological functions including regulation of hematopoietic cell proliferation, neuron differentiation, pituitary hormone secretion, and tissue repair. It is also involved in the process of many diseases, for example, inflammation, fibrosis and tumorigenesis^[1][2].