Low- and Fully N-Glycosylated Gonadotropins Circulating in Women With Polycystic Ovary Syndrome

Abstract

Context: A preponderance of basic luteinizing hormone (LH) molecules having elevated bioactivity was detected in the circulation of women with polycystic ovary syndrome (PCOS). Subsequent studies have shown that LH and follicle-stimulating hormone (FSH) both circulate as glycoforms differing in number of glycans: low-N-glycosylated glycoforms, LHdi and FSHtri, with high in vitro bioactivity, and fully glycosylated glycoforms, LHtri and FSHtetra, with high in vivo bioactivity.

Objective: This work aims to characterize the glycosylation patterns on circulating gonadotropin glycoforms in women with PCOS.

Methods: Serum samples, collected from 8 women with PCOS were included. The concentration, sulfonation, and sialylation of each glycoform were determined and compared with values of serum samples from healthy women: 22 women at follicular phase, 16 at midcycle, and 15 after menopause.

Results: All the women with PCOS had higher LHdi serum levels compared with those in the follicular-phase group. Median LHdi and median LHtri levels were significantly elevated in PCOS women. The percentage of LHdi was increased from 37 to 49 and that of FSHtri was decreased from 41 to 33. The LHdi, LHtri, and FSHtetra glycoforms were more sialylated and both LH glycoforms less sulfonated in women with PCOS.

Conclusion: All women with PCOS had increased serum levels of LHdi, compared with those in the follicular phase. The percentage of LHdi was increased and that of FSHtri decreased in women with PCOS. The increased LHdi leads to maintenance of the abnormal early follicular development of the polycystic ovary, and the decreased FSHtri contributes to the arrested follicle growth.

Keywords: FSH glycoforms; LH glycoforms; N-glycosylation; sialic acid; sulfonated N-acetylgalactosamine.

Figure 1.

N-glycosylation of human FSH and LH occurs in the rough endoplasmic reticulum (ER) of the gonadotrophs in the anterior pituitary gland [6-10]. Oligosaccharide precursors linked to dolichol, a specific lipid, at the cytoplasmic side of the ER membrane are flipped across the membrane bilayer to the luminal side of the ER by use of an enzyme, a flippase. An enzymatic complex in the ER membrane, termed oligosaccharyltransferase (OST), transfers the oligosaccharide precursor to a γ amino group of asparagine (–Asn-X-Thr/Ser) on the nascently translated α-subunit polypeptides and FSH and LH β-subunit polypeptides. Fully N-glycosylated α-subunits, with 2 N-glycans, combine with FSH β-subunits with 1 or 2 N-glycans, forming FSHtri and FSHtetra, respectively, and with the LH β-subunits with none or one N-glycan, forming LHdi and LHtri molecules, respectively. The FSH and LH molecules are modified posttranslationally in the Golgi apparatus within the cell where the branching of the N-glycans occurs in the medial-Golgi. Further synthesis of the glycans and their terminal decoration with the AMS residues: sialic acid (SA) and sulfonated N-acetylgalactosamine (SU) occur in the trans-Golgi. AMS, anionic monosaccharide; CMP, cytidine 5′;-monophosphate; FSH, follicle-stimulating hormone; LH, luteinizing hormone; PAPS, 3′phosphoadenyl-5′phosphosulfate; UDP, uridine diphosphate.

Figure 2.

Concentrations of low-N-glycosylated luteinizing hormone (LH) molecules, (left panel) LHdi, and of fully N-glycosylated LH molecules (right panel) LHtri in serum samples of 8 women with polycystic ovary syndrome (PCOS) compared with those at follicular phase, days 3 to 10, of 22 healthy women with normal menstrual cycles. The values are presented in relation to age of the women.