INHIBIN: UNITY IN DIVERSITY
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S. B. MOODBIDRI, S. V. GARDE, and A. R. SHETH
Historically, inhibin was thought to be a testicular hormone involved in the regulation of pituitary FSH by a negative feedback control. The ability of inhibin to preferentially suppress FSH without affecting LH triggered extensive research for its possible use as a male contraceptive, suggesting a plurality of molecular forms and a multiplicity of biological actions of this putative hormone. It also became evident that inhibin is not unique to the testis, as presumed earlier, and can even be obtained from the ovary. This has necessitated a fundamental revision of the original concept of inhibin. Unfortunately, not many perceive inhibin as a loose conglomerate of structurally dissimilar, FSH-suppressing proteins and insist on singling out a 32-kDa protein derived from ovarian follicular fluid to be designated as inhibin. This article highlights features common to two distinctly different types of inhibin: seminal inhibin and ovarian inhibin. Evidence is also provided to indicate that the term inhibin need not be specific to the ovarian protein, but encompasses proteins hitherto dismissed as inhibin-like or inhibinrelated proteins.
Key Words: Inhibin; Hormone; Fertility; Infertility.
DIVERSITY IN CHEMICAL PROPERTIES There is now a mass of information on the properties of the two inhibin preparations, and it is interesting to find close similarities between them despite their diverse physicochemical properties.
Seminal Inhibin Earlier investigators usually used human or bovine seminal plasma as the starting material for obtaining inhibin on the assumption that the testis, the organ from where inhibin originates, secretes inhibin into the seminal fluid along with its other secretions, making it a rich and easily accessible source [8, 13, 15, 16, 40, 41, 50, 511. Inhibin from human seminal plasma has been more extensively characterized than the bovine preparation. Studies on the primary structure of human seminal inhibin have revealed that it is a simple protein composed of 94 amino acid residues with a molecular weight of 10.5 kDa [43, 451. However, gel filtration and electrophoresis give an apparent molecular weight of 14-16 kDa. It is structurally similar to a sperm-coating antigen [25], and a slight structural similarity to the ovarian inhibin 0 chain has been identified in the C-terminal half of the
From the Institute for Research in Reproduction (ICMR), Parel, Bombay, India. Address correspondence to Anil R. Sheth, PhD, FASc, FMAS, Director-in-Charge, Institute for Research in Reproduction, Jehangir Merwanji Street, Parel, Bombay 400012, India. ARCHIVES OF ANDROLOGY 28: 149-157 (1992) Copyright 0 1992 by Hemisphere Publishing Corporation
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molecule [52]. Incidentally, it has been demonstrated that the biologically active core of the 10.5-kDa seminal inhibin molecule resides in the C-terminal 28 amino acid segment [l]. The 10.5-kDa seminal inhibin originates from the prostate [3, 291 and not from the testis as presumed. The human prostate has been found to contain a hundredfold greater concentration of 10.5-kDa inhibin than the human testis [53]. Further, compared to the normal prostate, the concentration and synthesis of inhibin is significantly higher in benign prostatic hypertrophy [ 11, 421. In view of this, seminal inhibin has also been identified as prostatic inhibin peptide (PIP) [48] or prostatic secretory protein-94 (PSP-94) [ 121.
Ovarian Inhibin With the realization that inhibin, earlier considered as a protein exclusive to the male, also occurs in the female, the focus shifted to the ovary and ovarian fluid as its source. Ovarian inhibin has been demonstrated to be a glycoprotein of 32 kDa consisting of two dissimilar disulfide-linked subunits designated CY (18 kDa) and 0 (14 kDa) [28, 31, 34, 381. These findings are consistent for inhibin obtained from different species and have been further substantiated by cloning of the gene encoding the protein. Testicular inhibin has also been demonstrated to be structurally similar to ovarian inhibin [2]. A 13-kDa testicular inhibin has also been reported [32]. The 32-kDa ovarian inhibin has structural homology with transforming growth factor 0 [ 101 and Mullerian duct inhibiting substance [7].
UNITY IN IMMUNOBIOLOGICAL PROPERTIES In spite of vast differences in the physicochemical characteristics of the two inhibin preparations, there exist certain striking similarities in their mode of action, tissue distribution, and immunobiological characteristics.
Mode of Action It is being increasingly realized that the endocrine effect, that is, the FSH-suppressing activity of different inhibins, is an accidental occurrence, and perhaps not very significant. More important is the autocrine or paracrine activity [46] and the fact that inhibins may act as local growth factors, playing a vital role in cellular growth, differentiation, and pathophysiolOgY.
Tissue Distribution Both 10.5-kDa seminal inhibin [47] and 32-kDa ovarian inhibin [36] are found in various tissues throughout the body, suggesting that they might also play a number of, as yet, unrecognized functions. However, the discussion here is limited to the reproductive tissues. Although the major sources of inhibin are undoubtedly the male and female gonads, the precise cell types responsible for its synthesis was a matter of debate for a while. This function, however, was later attributed to the Sertoli cells [49] of the testes and the granulosa cells [14]. It was surprising, therefore, to find 13-kDa testicular inhibin in the Leydig cells of the humans, nonhuman primates, and rodents (Fig. 1) [17-19, 321 and 32-kDa inhibin as detected by immunocytochernical methods [27, 37, 391. Further, the capability of the Leydig cells to synthesize inhibin was evident by the expression of specific mRNA for 32-kDa inhibin [33]. In this context, it was interesting to observe that following HCG administration there is an
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FIGURE 1 Immunocytochemical localization of 13-kDa testicular inhibin in (1) human testis, and (2) epididymis. The presence of inhibin in each case is indicated by an arrow. (Source. Taken from Garde et al. [20] and Veeramachaneni Rao et al. [54] with permission.)
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FIGURE 1 (Continued). Immunocytochemical localization of 10.5-kDa seminal inhibin in (3) benign hyperplastic prostate; and 32-kDa ovarian inhibin in (4) ram testis. The presence of inhibin in each case is indicated by an arrow. (Source. Taken from Garde et al. [20] and Veeramachaneni Rao et al. [54] with permission.)
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FIGURE 1 (Continued). Immunocytochemical localization of 32-kDa ovarian inhibin in (5) epididymis, and (6) prostate. The presence of inhibin in each case is indicated by an arrow. (Source. Taken from Garde et al. [20] and Veeramachaneni Rao et al. [54] with permission.)
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increase in 10.5-kDa inhibin [21] and 32-kDa inhibin [44] secretion by Leydig cells. A manyfold increase in concentration of 13-kDa inhibin [23] and 32-kDa inhibin [4] in hyperplastic Leydig cells has also been reported. Other reproductive tissues where both 10.5- and 32-kDa inhibin have been detected either by immunochemical methods or by the presence of specific mRNA are the epididymis [24, 541 and the endometrium [5, 20, 261.
Inhibin-FSH Relationship The main and most obvious common feature of the two inhibins is their FSH-suppressing activity. Both seminal and ovarian inhibin, albeit coming from two different sources, suppress the synthesis and release of FSH by rat pituitary cell cultures and suppress circulating levels of FSH in treated animals. Further, immunoneutralization of endogenous inhibin either with antibodies to 10.5-kDa seminal inhibin [22] or with antibodies to 32-kDa ovarian inhibin [9] cause an increase in circulating levels of FSH. It is becoming increasingly evident that just as inhibin controls FSH levels, FSH controls concentrations of different types of inhibins. Our studies have shown that FSH increases the biosynthesis of 10.5-kDa inhibin in dog and human prostate [48]. Similarly, McLachlan et al. [30] and Rivier and Vale [35] have shown that following HMG treatment there is an increase in circulating levels of 32-kDa inhibin. It is apparent from the above discussion that more than one type of inhibin is involved in FSH regulation and that irrespective of the type of inhibin its modulation by FSH is similar.
Inhibin as a Tumor Marker Another common feature of the two inhibin preparations, though unrelated to their biological activity, is their prospective use as tumor markers. The use of 10.5-kDa seminal inhibin as a marker for prostatic tumor was proposed as early as 1985 [6]. Several years later, 32-kDa ovarian inhibin was suggested as a tumor marker for granulosa cells and hydatidiform mole [61.
CONCLUSION Different molecular forms of inhibin may have equally important roles to play in regulating FSH. It is appropriate, therefore, to take into account the FSH-suppressing activity in its totality as represented by the whole family of inhibins, rather than the 32-kDa ovarian follicular protein alone in the context of FSH regulation.
REFERENCES 1. Arbatti NJ, Seidah NG, Rochemont J, Escber E, Sairam MR, Sheth AR, Chretien M (1985): Beta-2 inhibin is the active core of human seminal plasma beta-inhibin, sequence, synthesis and bioactivity. FEBS Lett 181:57-63 2. Bardin CW, Morris PL, Chen C-L, Shaha C, Voglmayr J, Rivier J, Spiess J, Vale WW (1987): Testicular inhibin: structure and regulation by FSH, androgens and EGF. In: Inhibin-Nonsteroidal regulation of follicle stimulating hormone secretion. Burger HG, de Kretser DM, Findlay JK, Igarashi M (Eds), Serono Symposium Publications, Vol42, New York: Raven, pp 179-190 3. Beksac MS, Khan SA, Eliasson R, Skakkebaek NE, Sheth AR, Diczfalusy E (1984): Evidence for the prostatic origin of immunoreactive inhibin-like material in human seminal plasma. Int J Androl 7:389-397
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4. Bergh A, Cajander S (1990): Immunohistochemical localization of inhibin a in the testes of normal men and in men with testicular disorders. Int J Androl 13:463-469 5. Biro JC, Eneroth P (1990): Inhibitory effect of the uterus on plasma and pituitary FSH in rats. J Endocrinol 124: 183- 189 6. Bremner WJ (1989): Inhibin: from hypothesis to clinical application. New Engl J Med 321:826-827 7. Cate RL, Mattaliano RJ, Hession C, Tizard R, Farber NM, Cheng A, Ninfa EG, Frey AZ, Gash DJ, Chow EP, Fisher RA, Bertonis JM, Torres G, Wallner BP, Ramachandran KL, Ragin RC, Manganaro TF, MacLaughlin DJ, Donahoe PK (1986): Isolation of bovine and human genes for Mullerian inhibiting substance and expression of the human gene in animal cells. Cell 45:685-698 8. Chari S, Duraiswami S, Franchimont P (1978): Isolation and characterization of inhibin from bull seminal plasma. Acta Endocrinol (kbh) 87:434-448 9. Culler MD, Wisnjewski MG, Sheppard KK, Nigro-Villar A (1988): Sex related differences in the role of inhibin in regulating FSH secretion in the rat. Proc 8th Int Congress of Endocrinology, Kyoto, Japan. Imura H, et al. (Eds), New York: Elsevier, p 137 10. Derynek R, Jarret JA, Chen EY, Eaton DH, Bell JR, Assoian RK, Roberts AB, Sporn MB, Goeddel DV (1985): Human transforming growth factor p complementary DNA sequence and expression in normal and transformed cells. Nature (London) 316:701-705 11. Doctor VM, Sheth AR, Simha MM, Arbatti NJ, Zaveri JP, Sheth NA (1986): Studies on immunocytochemical localization of inhibin like material in human prostatic tissue: comparison of its distribution in normal, benign and malignant prostates. Br J Cancer 53547-554 12. Dube JY, Frenette G, Paquin R, Chapdelaine P, Remblay J, Tremblay RR, Lazure C, Seidah N , Chretien M (1987): Isolation from human seminal plasma of an abundant 16 kDa protein originating from the prostate, its identification with a 94-residue peptide originally described as fi inhibin. J Androl 8: 182-189 13. Duraiswami S, Mahapatra S, Chari S, Daume E (1980): Characterization of (Y - inhibin from bull seminal plasma. Acta Endocrinol (kbh) 94 (Suppl 234):44-45 14. Erickson GF, Hsueh AJW (1978): Secretion of inhibin by rat granulosa cells in vitro. Endocrinology 103:19601963 15. Franchimont P, Chari S, Hegelstein MT, Duraiswami S (1975): Existence of a follicle-stimulating hormone inhibiting factor “Inhibin” in bull seminal plasma. Nature (London) 257:403-404 16. Franchimont P, Demoulin A, Verstaelen-Proyard J , Hazee-Hagelstein MT, Tunbridge WMG (1979): Identification in human seminal fluid of an inhibin-like factor which selectively regulates FSH secretion. J Reprod Fertil SUPPI26: 123-133 17. Garde SV, Moodbidri SB, Phadke AM, Sheth AR (1988): Localization of inhibin in human testes by immunoperoxidase technique. Anat Rec 222:357-361 18. Garde SV, Moodbidri SB, Sheth AR (1989): Localization of inhibin in testes of human, bonnet monkey, dog and rat by immunoperoxidase technique. Indian J Exp Biol 27:404-407 19. Garde SV, Sheth AR, Kulkarni SA (1991): Cellular distribution of inhibin in marmoset testes during development. Anat Rec 229:334-338 20. Garde SV, Sheth AR, Zaveri BJ, Shah J, Hinduja I (1991): Endometrium: an extragonadal source of inhibin Indian J Exp Biol 29:889-896 21. Hurkadli KS, Arbatti NJ, Mehta S, Sheth AR (1984): Serum inhibin levels after administration of hCG. Arch Androl 12:45-48 22. Hurkadli KS, Sheth AR (1985): Studies on immunoneutralization of inhibin: a time course study. Indian J Exp Biol 23:561-565 23. Hurkadli KS, Sheth AR, Garde SV (1989): Follicle stimulating hormone (FSH) modulating peptides: inhibin related peptides. Indian J Exp Biol 27:303-309 24. Hurkadli KS, Joseph R, Garde SV, Sheth AR (1991): De novo biosynthesis and localization of inhibin in marmoset (Callithrix jacchus) and rat epididymis. Indian J Exp Biol 29:715-720 25. Johansson J, Sheth AR, Cederlund E, Jornvall H (1984): Analysis of an inhibin preparation reveals apparent identity between a peptide with inhibin-like activity and sperm coating antigen. FEBS Lett 176:21-26 26. Kaiser M, Gibori G, Mayo KE (1990): The rat follistatin gene is highly expressed in decidual tissue. Endocrinology 126:2768-2770
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27. Lau Y, Li CH (1987): Localization of a-inhibin 31 in rodent testis and brain by immunocytochemical procedure and Western blot analysis. Biochem Biophys Res Commun 145:81-89 28. Ling N, Ying SY, Ueno N, Esch F, Denoray L, Guillemin R (1985): Isolation and partial characterization of M, 32000 protein with inhibin activity from porcine follicular fluid. Proc Natl Acad Sci USA 82:7217-7221 29. Mbikay M, Nolet S , Fournjer S, Benjannot S, Chapdelaine P, Paradis G, Dube JY, Tremblay R, Lazure C, Seidah NG, Chretien M (1987): Molecular cloning and sequence of the cDNA for a 94-amino acid seminal plasma protein secreted by the human prostate. DNA 6:23-29 30. McLachlan RI, Healdy DL, Robertson DM, Burger HG, de Kretser DM (1987): Circulating immunoreactive inhibin in the luteal phase and early gestation of women undergoing ovulation induction. Fertil Steril 48:loOl1005 31. Miyamoto K, Hasegawa Y, Fukuda M, Namura M, Igarashi M, Kaneawa K, Matsuo H (1985): Isolation of porcine follicular fluid inhibin of 32 K Dalton. Biochem Biophys Res Cornmun 129:396-403 32. Moodbidri SB, Garde SV, Sheth AR, Sheth NA, Doctor VM (1987): Purification, isolation and immunocytochemical localization of human testicular inhibin. In: Inhibins-Isolation, estimation and physiology. Sheth AR (Ed), Boca Raton, FL: CRC Press, Vol I , pp 61-67 33. Risbridger GP, Clements J, Robertson DM, Frummond AE, Muir J, Burger HG, de Kretser DM (1989): Immuno- and bioactive inhibin and inhibin a-subunit expression in rat Leydig cell cultures. Mol Cell Endocrinol 66~119-122 34. Rivier J, Spiess J, McClintock R, Vaughan J, Vale W (1985): Purification and partial characterization of inhibin from porcine follicular fluid. Biochem Biophys Res Commun 133: 120-127 35. Rivier C , Vale W (1987): Inhibin: measurement and role in the immature female rat. Endocrinology 120:16881690 36. Roberts V, Meunier H , Vaughan J, Rivier J, Rivier C, Vale W, Sawchenko P (1989): Production and regulation of inhibin subunits in pituitary gonadotrops. Endocrinology 124:552-554 37. Roberts V, Meunier H, Sawchenko PE, Vale W (1989): Differential production and regulation of inhibin subunits in rat testicular cell types. Endocrinology 125:2350-2359 38. Robertson DM, de Vos FL, Foulds LM, McLachlan RL, Burger HG, Morgan FJ, Hearn MTW, de Kretser DM (1986): Isolation of a 32 KDa form of inhibin from bovine follicular fluid. Mol Cell Endocrinol 44:271-277 39. Saha C, Morris PL, Chen CC, Vale W, Bardin CW (1989): Immunostainable inhibin subunits are in multiple types of testicular cells. Endocrinology 125: 1941-1950 40. Sairam MR (1981): Characterization of inhibin from bull seminal plasma. In: Intragonadal regulation of reproduction. Franchimont P, Channing CP (Eds), New York: Academic, pp 251-281 41. Sairam MR, Kato M, Manjunath P, Ramasharma K, Miller UM, Haung ESR, Madhwaraj HG (1982): Characteristics of inhibin: a comparative study using bull and seminal fluids and porcine follicular fluid. In: Intraovarian control mechanisms. Channing CP, Segal SJ (Eds), New York: Plenum, pp 79-91 42. Sathe VS, Sheth NA, Sheth AR, Phadke MA (1987): Biosynthesis and localization of inhibin in human prostate. The Prostate 10:33-43 43. Seidah NG, Arbatti NJ, Rochemont J, Sheth AR, Chretien M (1984): Complete amino-acid sequence of human seminal plasma inhibin. FEBS Lett 175:349-355 44. Sharpe RM, Kerr JB, Maddocks S (1988): Evidence for a role of the Leydig cells in control of the intratesticular secretion of inhibin. Mol Cell Endocrinol 60:243-247 45. Sheth AR, Arbatti NJ, Carlquist M, Jornvall H (1984): Characterization of the high molecular weight form of the human polypeptide inhibin: inhibiting secretion of FSH. FEBS Lett 165: 11-15 46. Sheth AR, Arbatti NJ (1985): Inhibin: an update. Indian J Exp Biol 23:475-484 47. Sheth NA, Shanbhag SA, Sheth AR (1987): Inhibin in gastric cancer. In: Inhibins-Isolation, estimation and physiology. Sheth AR (Ed), Boca Raton, FL: CRC Press, pp 129-139 48. Sheth AR, Hurkadli KS, Sheth NA (1990): Significance of prostatic inhibin peptide (PIP) and thyrotropin releasing hormone in pathophysiology of prostate. In: The prostate as an endocrine gland. Farnsworth WE, Ablin RJ (Eds), Boca Raton, FL: CRC Press, pp 131-148 49. Steinberger A, Steinberger E (1976): Secretion of an FSH inhibiting factor by cultured Sertoli cells. Endocrinology 99:918-921 50. Thakur AN, Vaze AY, Dattatreyamurty B, Arbatti NJ, Sheth AR (1978): Isolation and characterization of inhibin from human seminal plasma. Indian J Exp Biol 16:854-856
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51. Thakur AN, Vaze AY, Dattatreyamurty 9, Sheth AR (1981): Isolation and purification of inhibin from human
seminal plasma. Indian J Exp Biol 19:307-313 52. Ulvsbaek M, Lindstrom C, Welber H, Abrahamsson PA, Lilja H, Lundwall A (1989): Molecular cloning of a small prostate protein known as 0 microseminoprotein, PSP-94 or p inhibin and demonstration of transcripts in nongenital tissues. Biochem Biophys Res Commun 164:1310-1315 53. Vaze AY, Thakur AN, Sheth AR (1979): Development of a radioimmunoassay for human seminal plasma inhibin. J Reprod Fertil Suppl 26: 135-146 54. Veeramachaneni Rao DN, Schanbacher BD, Amann RP (1989): Immunolocalization and concentrations of inhibin 01 in the ovine testis and excurrent duct system. Biol Reprod 41:499-503
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S. B. MOODBIDRI, S. V. GARDE, and A. R. SHETH
Historically, inhibin was thought to be a testicular hormone involved in the regulation of pituitary FSH by a negative feedback control. The ability of inhibin to preferentially suppress FSH without affecting LH triggered extensive research for its possible use as a male contraceptive, suggesting a plurality of molecular forms and a multiplicity of biological actions of this putative hormone. It also became evident that inhibin is not unique to the testis, as presumed earlier, and can even be obtained from the ovary. This has necessitated a fundamental revision of the original concept of inhibin. Unfortunately, not many perceive inhibin as a loose conglomerate of structurally dissimilar, FSH-suppressing proteins and insist on singling out a 32-kDa protein derived from ovarian follicular fluid to be designated as inhibin. This article highlights features common to two distinctly different types of inhibin: seminal inhibin and ovarian inhibin. Evidence is also provided to indicate that the term inhibin need not be specific to the ovarian protein, but encompasses proteins hitherto dismissed as inhibin-like or inhibinrelated proteins.
Key Words: Inhibin; Hormone; Fertility; Infertility.
DIVERSITY IN CHEMICAL PROPERTIES There is now a mass of information on the properties of the two inhibin preparations, and it is interesting to find close similarities between them despite their diverse physicochemical properties.
Seminal Inhibin Earlier investigators usually used human or bovine seminal plasma as the starting material for obtaining inhibin on the assumption that the testis, the organ from where inhibin originates, secretes inhibin into the seminal fluid along with its other secretions, making it a rich and easily accessible source [8, 13, 15, 16, 40, 41, 50, 511. Inhibin from human seminal plasma has been more extensively characterized than the bovine preparation. Studies on the primary structure of human seminal inhibin have revealed that it is a simple protein composed of 94 amino acid residues with a molecular weight of 10.5 kDa [43, 451. However, gel filtration and electrophoresis give an apparent molecular weight of 14-16 kDa. It is structurally similar to a sperm-coating antigen [25], and a slight structural similarity to the ovarian inhibin 0 chain has been identified in the C-terminal half of the
From the Institute for Research in Reproduction (ICMR), Parel, Bombay, India. Address correspondence to Anil R. Sheth, PhD, FASc, FMAS, Director-in-Charge, Institute for Research in Reproduction, Jehangir Merwanji Street, Parel, Bombay 400012, India. ARCHIVES OF ANDROLOGY 28: 149-157 (1992) Copyright 0 1992 by Hemisphere Publishing Corporation
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molecule [52]. Incidentally, it has been demonstrated that the biologically active core of the 10.5-kDa seminal inhibin molecule resides in the C-terminal 28 amino acid segment [l]. The 10.5-kDa seminal inhibin originates from the prostate [3, 291 and not from the testis as presumed. The human prostate has been found to contain a hundredfold greater concentration of 10.5-kDa inhibin than the human testis [53]. Further, compared to the normal prostate, the concentration and synthesis of inhibin is significantly higher in benign prostatic hypertrophy [ 11, 421. In view of this, seminal inhibin has also been identified as prostatic inhibin peptide (PIP) [48] or prostatic secretory protein-94 (PSP-94) [ 121.
Ovarian Inhibin With the realization that inhibin, earlier considered as a protein exclusive to the male, also occurs in the female, the focus shifted to the ovary and ovarian fluid as its source. Ovarian inhibin has been demonstrated to be a glycoprotein of 32 kDa consisting of two dissimilar disulfide-linked subunits designated CY (18 kDa) and 0 (14 kDa) [28, 31, 34, 381. These findings are consistent for inhibin obtained from different species and have been further substantiated by cloning of the gene encoding the protein. Testicular inhibin has also been demonstrated to be structurally similar to ovarian inhibin [2]. A 13-kDa testicular inhibin has also been reported [32]. The 32-kDa ovarian inhibin has structural homology with transforming growth factor 0 [ 101 and Mullerian duct inhibiting substance [7].
UNITY IN IMMUNOBIOLOGICAL PROPERTIES In spite of vast differences in the physicochemical characteristics of the two inhibin preparations, there exist certain striking similarities in their mode of action, tissue distribution, and immunobiological characteristics.
Mode of Action It is being increasingly realized that the endocrine effect, that is, the FSH-suppressing activity of different inhibins, is an accidental occurrence, and perhaps not very significant. More important is the autocrine or paracrine activity [46] and the fact that inhibins may act as local growth factors, playing a vital role in cellular growth, differentiation, and pathophysiolOgY.
Tissue Distribution Both 10.5-kDa seminal inhibin [47] and 32-kDa ovarian inhibin [36] are found in various tissues throughout the body, suggesting that they might also play a number of, as yet, unrecognized functions. However, the discussion here is limited to the reproductive tissues. Although the major sources of inhibin are undoubtedly the male and female gonads, the precise cell types responsible for its synthesis was a matter of debate for a while. This function, however, was later attributed to the Sertoli cells [49] of the testes and the granulosa cells [14]. It was surprising, therefore, to find 13-kDa testicular inhibin in the Leydig cells of the humans, nonhuman primates, and rodents (Fig. 1) [17-19, 321 and 32-kDa inhibin as detected by immunocytochernical methods [27, 37, 391. Further, the capability of the Leydig cells to synthesize inhibin was evident by the expression of specific mRNA for 32-kDa inhibin [33]. In this context, it was interesting to observe that following HCG administration there is an
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FIGURE 1 Immunocytochemical localization of 13-kDa testicular inhibin in (1) human testis, and (2) epididymis. The presence of inhibin in each case is indicated by an arrow. (Source. Taken from Garde et al. [20] and Veeramachaneni Rao et al. [54] with permission.)
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FIGURE 1 (Continued). Immunocytochemical localization of 10.5-kDa seminal inhibin in (3) benign hyperplastic prostate; and 32-kDa ovarian inhibin in (4) ram testis. The presence of inhibin in each case is indicated by an arrow. (Source. Taken from Garde et al. [20] and Veeramachaneni Rao et al. [54] with permission.)
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lnhibin
FIGURE 1 (Continued). Immunocytochemical localization of 32-kDa ovarian inhibin in (5) epididymis, and (6) prostate. The presence of inhibin in each case is indicated by an arrow. (Source. Taken from Garde et al. [20] and Veeramachaneni Rao et al. [54] with permission.)
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increase in 10.5-kDa inhibin [21] and 32-kDa inhibin [44] secretion by Leydig cells. A manyfold increase in concentration of 13-kDa inhibin [23] and 32-kDa inhibin [4] in hyperplastic Leydig cells has also been reported. Other reproductive tissues where both 10.5- and 32-kDa inhibin have been detected either by immunochemical methods or by the presence of specific mRNA are the epididymis [24, 541 and the endometrium [5, 20, 261.
Inhibin-FSH Relationship The main and most obvious common feature of the two inhibins is their FSH-suppressing activity. Both seminal and ovarian inhibin, albeit coming from two different sources, suppress the synthesis and release of FSH by rat pituitary cell cultures and suppress circulating levels of FSH in treated animals. Further, immunoneutralization of endogenous inhibin either with antibodies to 10.5-kDa seminal inhibin [22] or with antibodies to 32-kDa ovarian inhibin [9] cause an increase in circulating levels of FSH. It is becoming increasingly evident that just as inhibin controls FSH levels, FSH controls concentrations of different types of inhibins. Our studies have shown that FSH increases the biosynthesis of 10.5-kDa inhibin in dog and human prostate [48]. Similarly, McLachlan et al. [30] and Rivier and Vale [35] have shown that following HMG treatment there is an increase in circulating levels of 32-kDa inhibin. It is apparent from the above discussion that more than one type of inhibin is involved in FSH regulation and that irrespective of the type of inhibin its modulation by FSH is similar.
Inhibin as a Tumor Marker Another common feature of the two inhibin preparations, though unrelated to their biological activity, is their prospective use as tumor markers. The use of 10.5-kDa seminal inhibin as a marker for prostatic tumor was proposed as early as 1985 [6]. Several years later, 32-kDa ovarian inhibin was suggested as a tumor marker for granulosa cells and hydatidiform mole [61.
CONCLUSION Different molecular forms of inhibin may have equally important roles to play in regulating FSH. It is appropriate, therefore, to take into account the FSH-suppressing activity in its totality as represented by the whole family of inhibins, rather than the 32-kDa ovarian follicular protein alone in the context of FSH regulation.
REFERENCES 1. Arbatti NJ, Seidah NG, Rochemont J, Escber E, Sairam MR, Sheth AR, Chretien M (1985): Beta-2 inhibin is the active core of human seminal plasma beta-inhibin, sequence, synthesis and bioactivity. FEBS Lett 181:57-63 2. Bardin CW, Morris PL, Chen C-L, Shaha C, Voglmayr J, Rivier J, Spiess J, Vale WW (1987): Testicular inhibin: structure and regulation by FSH, androgens and EGF. In: Inhibin-Nonsteroidal regulation of follicle stimulating hormone secretion. Burger HG, de Kretser DM, Findlay JK, Igarashi M (Eds), Serono Symposium Publications, Vol42, New York: Raven, pp 179-190 3. Beksac MS, Khan SA, Eliasson R, Skakkebaek NE, Sheth AR, Diczfalusy E (1984): Evidence for the prostatic origin of immunoreactive inhibin-like material in human seminal plasma. Int J Androl 7:389-397
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