Inhibin, beta B, also known as INHBB, is a protein which in humans is encoded by the INHBBgene.[5][6] INHBB is a subunit of both activin and inhibin, two closely related glycoproteins with opposing biological effects.
Function
Inhibin
Inhibins are heterodimeric glycoproteins composed of an α subunit (INHA) and one of two homologous, but distinct, β subunits (βA or βB, this protein). mRNA for the two subunits has been demonstrated in the testes of adult rats.[7] Inhibin can bind specifically to testicular interstitial cells throughout development and may be an important regulator of Leydig celltestosterone production or interstitial cell function.[8]
The inhibin beta B subunit joins the α subunit to form a pituitary FSH secretion inhibitor. Inhibin has been shown to regulate gonadal stromal cell proliferation negatively and to have tumour-suppressor activity. In addition, serum levels of inhibin have been shown to reflect the size of granulosa-cell tumors and can therefore be used as a marker for primary as well as recurrent disease. Because expression in gonadal and various extragonadal tissues may vary severalfold in a tissue-specific fashion, it is proposed that inhibin may be both a growth/differentiation factor and a hormone.
Activin
Furthermore, the beta B subunit forms a homodimer, activin B, and also joins with the beta A subunit to form a heterodimer, activin AB, both of which stimulate FSH secretion.[6]
Tissue distribution
Sections of testicular tissue from rat revealed positive immunoreactivity against anti-inhibin intensely appeared in Leydig cells.[9] In adult animals, binding of 125I inhibin was localized primarily to the interstitial compartment of the testis.[8] Also, Jin et al., (2001) reported that Leydig cells showed strong positive staining for the inhibin βA subunit in pigs testis.[10]
Receptors
In situ ligand binding studies have shown that 125I inhibin βA binds specifically to Leydig cells throughout rat testis development. These results suggest that inhibin has been considered as a regulator of Leydig cell differentiated function.[11][12] Recently, additional inhibin specific binding proteins were identified in inhibin target tissues, including pituitary and Leydig cells.[13][14] From these receptors betaglycan (the TGF-β type III receptor) and InhBP/p120 (a membrane-tethered proteoglycan) were identified as putative inhibin receptors and they are all present in Leydig cells. However, a faint positive reaction was detected in Leydig cell cytoplasm in rats treated with anise oil.[9] This may be related to the damaged Leydig cells, as a result of the decreasing of inhibin expression. This may be related to its content of safrole.
^"Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
^Burger HG, Igarashi M (April 1988). "Inhibin: definition and nomenclature, including related substances". Endocrinology. 122 (4): 1701–2. doi:10.1210/endo-122-4-1701. PMID3345731.
Schmelzer CH, Burton LE, Tamony CM, et al. (1990). "Purification and characterization of recombinant human activin B.". Biochim. Biophys. Acta. 1039 (2): 135–41. doi:10.1016/0167-4838(90)90178-I. PMID2364091.
Burger HG, Igarashi M (1988). "Inhibin: definition and nomenclature, including related substances". Endocrinology. 122 (4): 1701–2. doi:10.1210/endo-122-4-1701. PMID3345731.
Mason AJ, Niall HD, Seeburg PH (1986). "Structure of two human ovarian inhibins". Biochem. Biophys. Res. Commun. 135 (3): 957–64. doi:10.1016/0006-291X(86)91021-1. PMID3754442.