Activin is a protein complex, a gene within the chromosomes 2, 7, and 12 and part of the female endocrine system in humans, though it exists in both sexes. It shares a very close relationship with inhibin, in that they seem to have the completely inverse function though similar mechanisms, structures, and interactions. They’re both greatly linked to FSH, follicle-stimulating hormone, a hormone secreted by the gonadotropic cells of the anterior pituitary gland, which plays a role in regulating development, growth, pubertal maturation, and the reproductive processes of the body.
As mentioned, inhibin and activin are mostly described together because of the close pairing yet contrary roles they share with one another. Primarily, they are linked by their relationship to the hormone FSH. Amongst roles in cell proliferation, apoptosis, metabolism, homeostasis, immune response, differentiation, endocrine function and wound repair, activin enhances FSH biosynthesis and secretion. It does this through androgen secretion in the ovary and testis, which also enhances spermatogenesis in males.
Inhibin, on the other hand, inhibits FSH production. High FSH levels are greatly linked to several diseases and conditions, including premature menopause, premature ovarian aging, gonadal dysgenesis, castration, Klinefelter syndrome and more, so the regulation that inhibin provides is essential. In males, it’s primarily secreted from the Sertoli cells. Meanwhile, in females, it is produced more widely in the gonads, pituitary gland, placenta, corpus luteum and other organs besides.
Inhibin is much less well-known that activin, and its mechanisms haven’t yet been thoroughly laid out. However, the wide belief is that it competes with activin when binding to activin receptors or perhaps by binding to specific, as of yet unknown inhibin receptors. In either case, we can look to the mechanisms of activin to form a hypothesis of how the mechanisms of inhibin actually work.
Activins, on the other hand, work with two specific types of cell surface transmembrane receptors. These receptors have serine/threonine kinase activities within their cytoplasmic domains. The Type 1 receptors are ACVR1, ACVR1B and ACVR1C, while the Type 2 receptors are ACVR2A and ACVR2B. Activins work in a sequence, binding first to the Type 2 receptors, which then starts a chain reaction of recruitment, phosphorylation, and activiont of the Type 1 receptors. This then makes the activins able to bind to the Type 1 receptors. In doing so, it then interacts with the phopshorylates resulting in a wide diversity of gene expression.
Again, it’s not confirmed what the mechanisms of inhibin are. It might follow the exact same route as activins, or there may be as of yet unidentified inhibin receptors. Inhibins have been discovered more recently, so there is yet more to learn about them.
Activin has a dimeric structure, much like inhibin. In both dimers, both monomers are linked by a single disulfide bond. Activin’s monomers are both identical or near-identical beta subunits. However, inhibin differentiates itself by having one of the beta subunits identical or near-identical to activin’s, but also having an alpha subunit that’s not as closely related to the beta subunit.