Low-glycosylated forms of both FSH and LH play major roles in the natural ovarian stimulation

Abstract

Background: The natural ovarian stimulation is mediated by four gonadotrophin glycoforms: FSHtri with three, FSHtetra with four, LHdi with two, and LHtri with three N-glycans. The aim of the study was to determine the serum concentrations of the four glycoforms and their contents of anionic monosaccharides (AMS), i.e. sialic acid (SA) and sulfonated N-acetylgalactosamine (SU) residues throughout the menstrual cycle.

Methods: Serum samples were collected from 78 healthy women with regular menstrual cycles. The serum glycoform molecules were identified by their distributions at electrophoreses. Analyses were also performed after removal of terminal SA. The hormones were measured with time-resolved sandwich fluoroimmunoassays.

Results: The concentration profiles of the four glycoforms were markedly different. FSHtri, which had a 3-fold higher biopotency than FSHtetra, had peak levels on cycle day 5 and at midcycle and nadirs on cycle days 9 and 21-23. FSHtetra had a raised level on cycle days 5-12, followed by a decrease. LHdi and LHtri had similar patterns, but the peak/nadir ratio was much more pronounced for LHdi than for LHtri, 18 versus 4. The numbers of SA residues per molecule were at a maximum around midcycle when the corresponding numbers of SU were at a minimum. The SU/SA ratio was at a minimum on cycle day 12.

Conclusion: The results indicate that the LHdi and the FSHtri molecules play major roles in the natural ovarian stimulation. The SU/SA ratios per molecule favoured a prolonged circulatory half-life of all glycoforms at the midcycle phase. The observations may lead to more successful inductions of ovulation in anovulatory women.

Keywords: FSH glycoforms; LH glycoforms; normal menstrual cycle; ovulation induction; sialic acid; sulfonated N-acetylgalactosamine.

Figure 1.

Left panel: Concentrations of FSH and LH in serum samples from 78 women with a normal menstrual cycle. Right panel: Per cent low-glycosylated forms, FSHtri and LHdi, in these 78 serum samples. Data in this and the following figures are plotted as three-day moving mean values. The day of the menstrual cycle is given and the first day indicated by a vertical hatched bar. The ovarian cycle starts on day 25 of the previous cycle and lasts to day 24 of the menstrual cycle, the end indicated by a vertical dashed line. Mean values ± SEM.

Figure 2.

Concentrations of FSHtri and FSHtetra, left panel, and their estimated biopotencies, in arbitrary units per L, right panel, during the normal menstrual cycle. See also legend to Figure 1.

Figure 3.

Concentrations of LHdi and LHtri during the normal menstrual cycle. See also legend to Figure 1.

Figure 4.

Number of anionic monosaccharide (AMS) residues per glycan on FSHtri and FSHtetra, left panel, and on LHdi and LHtri, right panel, during the normal menstrual cycle. See also legend to Figure 1.

Figure 5.

Number of sialic acid (SA) and sulfonated N-acetylgalactosamine (SU) residues per molecule on FSHtri and FSHtetra, left panel, and on LHdi and LHtri, right panel, during the normal menstrual cycle. See also legend to Figure 1.

Figure 6.

Ratios between number of sialic acid (SA) and sulfonated N-acetylgalactosamine (SU) residues per molecule on FSHtri and FSHtetra, left panel, and on LHdi and LHtri, right panel, during the normal menstrual cycle. See also legend to Figure 1.

Figure 7.

Schematic drawings of the glycosylation in the rough endoplasmic reticulum and the branching followed by the synthesis to terminal sialic acid or sulfonated GalNAc residues in the Golgi of human anterior pituitary cells. The structures of circulating glycoforms of FSH, LH, and TSH are schematically shown. (CMP = cytidine monophosphate; PAPS =3’phosphoadenyl-5’phosphosulphate; UDP = uridine diphosphate).