Oestrogens regulate pituitary alpha2,3-sialyltransferase messenger ribonucleic acid levels in the female rat

Abstract

Follicle-stimulating hormone (FSH) is synthesized by the anterior pituitary gland in multiple molecular forms. Increased acidic/sialylated FSH charge isoforms are associated with conditions characterized by a low oestrogen output. In the present study, we analysed the dynamics of the changes in mRNA levels of the enzyme Galbeta1,3[4]GlcNAc alpha2,3-sialyltransferase (2,3-STase) (one of the enzymes that incorporate sialic acid residues into the FSH molecule) in intact and ovariectomized rats. The anterior pituitaries of 4-day regularly cyclic adult female Wistar rats were obtained at 1000 h on the days of pro-oestrus (P), oestrus (O), dioestrus 1 (D1) and dioestrus 2 (D2), at 0200 h, 1400 h, 1800 h and 2200 h on D1, at 1800 h on day of O and at 1000 h after 7, 14, 21, 28 and 45 days of oophorectomy performed on the morning of P. Total RNA was isolated from each gland and the 2,3-STase levels were measured by Northern blot hybridization analysis employing a 346-base pair cDNA probe encoding for a non-conserved amino acid sequence of the catalytic domain of the enzyme. Maximal levels of the enzyme mRNA were detected at 1000 h on D1; thereafter, they progressively decreased by 60% during the ensuing 24 h, reaching the lowest concentration values (26% of the maximally observed level on D1) at 1000 h on day of P and remaining unchanged during the morning of O. Administration of the potent oestradiol receptor antagonist ICI 182,780 at 1000 h on D1 completely reverted the time-dependent decrease in 2,3-STase mRNA levels observed during the afternoon of D1, whereas oestradiol benzoate administered at 1000 h on day of O significantly reduced the enzyme mRNA levels (to 21% of the levels detected in vehicle-treated controls). In ovariectomized rats, the alpha2,3-STase mRNA progressively increased from day 21 to day 45 post castration. Administration of oestradiol benzoate on day 28 after oophorectomy significantly reduced the 2,3-STase mRNA levels (to 36% of the levels detected in vehicle-injected controls); ICI 182,780 partially counteracted this oestradiol-mediated effect. The dynamics of these changes in 2,3-STase mRNA levels partially correlated with changes in the relative abundance of the FSH charge isoforms separated by preparative chromatofocusing of anterior pituitary extracts, particularly in glands obtained during the morning of P and O. These data demonstrate for the first time that pituitary 2,3-STase is a hormonally-regulated enzyme and that the changes in transcription and/or stability of its mRNA may be involved, in part, in the post-translational processing of the FSH molecule during certain physiological conditions.

Effect of Dihydrotestosterone on Pituitary Follicle-Stimulating Hormone Isoforms in Adult Male Rats Treated with a Gonadotropin-Releasing Hormone Antagonist

“…Similar effects have been observed after estrogen administration [23, 24]. In this regard, it has been proposed that E 2 may affect mRNA expression of the pituitary α-2,3-sialyltransferase, the enzyme responsible for the incorporation of the terminal sialic acid to the FSH carbohydrate chains [25]. In normal women, previous studies have demonstrated that the endocrine milieu modulates not only the incorporation of the terminal sialic acid to the carbohydrate chain but also the processing of the inner structure of FSH oligosaccharides [17].…”

“…FSH oligosaccharide heterogeneity and its hormonal regulation have been mainly ascribed to variations in the sialic acid content of FSH glycosylation variants [12, 13, 21, 22, 23, 24, 25, 30]. However, the hormonal regulation of FSH polymorphism is exerted not only at the level of the terminal sugar residue incorporated to the carbohydrate chain, but also in the processing of the inner structure of oligosaccharides.…”

“…Particularly, changes in the enzymes α-mannosidase II and N-acetylglucosaminyl transferase III would be the determining steps in the synthesis of complex- or hybrid-type oligosaccharides, respectively. In this regard, it has been reported that estrogens may regulate mRNA expression of the pituitary alpha-2,3-sialyltransferase, one of the enzymes responsible for the incorporation of sialic acid to the carbohydrate chains of FSH [25]. …”

In vitro biological-to-immunological ratio of serum gonadotropins throughout male puberty in children with insulin-dependent diabetes mellitus

“…This observation agrees with previous studies showing that both plasma bioactive LH concentrations and the calculated B:I LH ratio increase across male puberty [42,43], but not with studies employing monoclonal antibody-based immunoassay procedures [44,45], thus emphasizing the potential effect of differential glycosylation on the reactivity of [46][47][48][49]. The observed changes in relative in vitro bioactivity and B:I ratio of gonadotropins across puberty, may result from modifications in the carbohydrate structure of gonadotropins as documented by studies showing that both serum LH and FSH charge isoform distribution changes during early normal puberty in males [16,17], due presumably to progressive exposure to sex steroid hormones and/or GnRH [38,41,[50][51][52][53][54][55][56][57][58].…”

What is the Clinical Relevance of Follicle-Stimulating Hormone Isoforms in Fertility Treatment?

“…28 Pituitary expression of different glycosyl transferases, including 2,3 α-sialyltransferase (an enzyme that catalyses the incorporation of sialic acid residues into sugar residues attached to the FSH molecule), is down-regulated by estradiol. 29 Recent studies have shown that estradiol administered to postmenopausal women shifts pituitary FSH release to more basic FSH isoforms compared with non-treated women. 28 Thus, in addition to the fine-tuned interplay between the selected developing follicle and pituitary release of gonadotrophins, it appears that the follicle is capable also of directing the isoform profile released.…”