Synthetic human seminal alpha-inhibin-92 selectively suppresses follicle-stimulating hormone release in vivo

Inhibin

Inhibin is a glycoprotein hormone, consisting of two dissimilar, disulphide-linked subunits, termed α (MW 20kD) and β (MW 3-15kD), which inhibits the production and/or secretion of pituitary gonadotrophins, preferentially follicle stimulating hormone (FSH). The most widely studied inhibin molecule has a molecular weight of 31-32kD, as purified and cloned from bovine, porcine, ovine, rat and human sources. Higher molecular weight forms have been identified in ovarian follicular fluids and in culture media of granulosa and Sertoli cells, and generally differ from the 31kD form in having larger α-subunits, designated by their molecular weights, e.g. α44 in 58kD inhibin. There are two forms of the β-subunit, named βA and βB and the corresponding inhibin dimers have been named inhibin A and inhibin B. Dimers of the β-subunit, which have been shown to have FSH stimulating activity, are termed activins and are designated activin A, B or AB depending on whether the dimer is a homodimer of βA or βB or a heterodimer of βA and βB (Figure 1). The major gonadal source of inhibin is the granulosa cell in the female and the Sertoli cell in the male. Other potential cellular sites of origin will be discussed below.

Hypothalamic control of gonadotropin secretion by LHRH, FSHRF, NO, cytokines, and leptin

Gonadotropin secretion by the pituitary gland is under the control of luteinizing hormone-releasing hormone (LHRH) and the putative follicle stimulating hormone-releasing factor (FSHRF). Lamprey III LHRH is a potent FSHRF in the rat and seems to be resident in the FSH controlling area of the rat hypothalamus. It is an analog of mammalian LHRH and may be the long sought FSHRF. Gonadal steroids feedback at hypothalamic and pituitary levels to either inhibit or stimulate the release of LH and FSH, which is also affected by inhibin and activin secreted by the gonads. Important control is exercised by acetylcholine, norepinephrine (NE), dopamine, serotonin, melatonin, and glutamic acid (GA). Furthermore, LH and FSH also act at the hypothalamic level to alter secretion of gonadotropins. More recently, growth factors have been shown to have an important role. Many peptides act to inhibit or increase release of LH and the sign of their action is often reversed by estrogen. A number of cytokines act at the hypothalamic level to suppress acutely the release of LH but not FSH. NE, GA, and oxytocin stimulate LHRH release by activation of neural nitric oxide synthase (nNOS). The pathway is as follows: oxytocin and/or GA activate NE neurons in the medial basal hypothalamus (MBH) that activate NOergic neurons by alpha, (alpha 1) receptors. The NO released diffuses into LHRH terminals and induces LHRH release by activation of guanylate cyclase (GC) and cyclooxygenase. NO not only controls release of LHRH bound for the pituitary, but also that which induces mating by actions in the brain stem. An exciting recent development has been the discovery of the adipocyte hormone, leptin, a cytokine related to tumor necrosis factor (TNF) alpha. In the male rat, leptin exhibits a high potency to stimulate FSH and LH release from hemipituitaries incubated in vitro, and increases the release of LHRH from MBH explants. LHRH and leptin release LH by activation of NOS in the gonadotropes. The NO released activates GC that releases cyclic GMP, which induces LH release. Leptin induces LH release in conscious, ovariectomized estrogen-primed female rats, presumably by stimulating LHRH release. At the effective dose of estrogen to activate LH release, FSH release is inhibited. Leptin may play an important role in induction of puberty and control of LHRH release in the adult as well.

Effects of alpha-inhibin-92 fragments and alpha-inhibin-92 antiserum on the control of follicle-stimulating hormone release in male rats

Alpha-inhibin-92 (alpha-IB-92) has been characterized from human seminal plasma and found to be active in suppressing FSH release in vitro and in vivo. In order to determine if smaller fragments of this 92 amino acid peptide would still be active to suppress FSH release, we have evaluated 5 of these fragments for their effects on FSH and LH release in the present study. Five alpha-IB-92 fragments (1-34, 1-46, 35-65, 35-92 and 66-92) were synthesized and injected intravenously (iv) into castrated adult rats (2 days post operation). Only fragments alpha-IB-92-(35-65) and alpha-IB-92-(66-92) significantly lowered plasma FSH, but not LH, at doses of 10 micrograms. These fragments exerted a preferential FSH-suppressing effect, but their activities were less than that of alpha-IB-92. In view of the rapid action of these peptides and the preferential FSH suppressing effect, they could be useful clinically to suppress FSH release. To determine the possible physiologic significance of alpha-IB-92, we injected antiserum raised against alpha-IB-92 into immature male rats and evaluated its effects on FSH and LH release. Normal rabbit serum (NRS) or anti-alpha-IB-92 serum was injected iv through indwelling jugular catheters into conscious, unrestrained 18 day-old male rats. Blood samples (0.2 ml) were collected at various intervals. Intravenous injection of alpha-IB-92 antiserum (0.1 ml/rat) selectively elevated plasma levels of FSH but not LH from 2-8 h post-injection in 18 day-old male rats (P < 0.01). Since immunoneutralization of alpha-IB-92 significantly elevated FSH release in immature rats, alpha-IB-92 has a physiological inhibiting role in control of FSH but not LH release at this stage of development.

Control of follicle-stimulating hormone and luteinizing hormone release by hypothalamic peptides

Lesion, stimulation, and pharmacological studies point to separate hypothalamic control of pulsatile FSH and LH secretion. LH release is controlled by a region extending from the preoptic area to the anterior and mid-median eminence, whereas FSH release is controlled by a region extending from the dorsal anterior hypothalamic area to the caudal median eminence. We have separated an FSH-releasing factor from LHRH by gel-filtration on Sephadex G-25, confirming results obtained over 25 years ago; and we are attempting its isolation in collaboration with Vale and River. In the meantime, reasoning that FSH-releasing factor might be related to LHRH, we tested many analogs of LHRH and found one that has selective FSH-releasing activity over a 50-fold dose range; however, it is relatively weak. This led us to the possibility that the GAP might be FSH-RF. Indeed, GAP1-13 has FSH but no LH-releasing activity over a 100-fold dose range; however, it is less potent than we would expect of the natural product. Substituting D-Trp-9 into the molecule to inhibit enzymatic degradation yielded a more potent and completely selective FSH-releasing peptide,24 which could be clinically useful. Alpha-inhibin-92 of Li et al. has been shown to have a highly selective dose-related suppressive action on FSH release in castrate male rats.25 Smaller fragments (35-65 and 66-92) of this molecule also possess the activity, albeit at higher doses. That this molecule may be physiologically significant is indicated by elevations in plasma FSH in immature rats obtained following intravenous injection of antisera raised against the peptide. Because of its much smaller size than that of 32-kDa alpha, beta inhibins and the lack of carbohydrate in the molecule, this can be relatively easily synthesized and might have clinical utility as an FSH release-inhibiting peptide.