P-640 Dynamics of IGF signalling during the ovulatory peak in women undergoing ovarian stimulation
Authors: Bøtkjær J  A , Poulsen L  L C , Noer P  R , Grøndahl M  L , Englund A  L M , Franks S , Hardy K , Oxvig C , Andersen C  Y 
Published: July 2024

Study question

How is the IGF system expressed and regulated during the midcycle surge in women?

Summary answer

The IGF system becomes downregulated after ovulation trigger (OT) highlighted by a significant upregulation of the inhibitor, stanniocalcin-1, and a downregulation of protease-induced IGF release.

What is known already

IGF signalling is known to affect human ovarian follicular function during growth and development. Particularly during the preovulatory stage, IGF activity appears to be highly significant and has been proposed as one of the primary factors contributing to the remarkable mitotic activity observed in the granulosa cells (GCs). However, the role and regulation of the IGF system is unknown during the ovulatory peak, which is characterized by profound changes in GC function.

Study design, size, duration

Follicular fluid (FF) and GCs were collected from fifty women during the ovulatory peak from two specific time-points. One sample was obtained before oocyte pick up (OPU): before OT (T = 0 h (n = 23)), at 12 h (n = 10), 17 h (n = 6), or 32 h (n = 11) after OT, and one sample was obtained at OPU 36 h after OT (n = 50).

Participants/materials, setting, methods

Fifty women receiving fertility treatment at the Fertility Clinic, Department of Gynaecology and Obstetrics, Holbæk Hospital, Denmark, were included. For ethical reasons, only women who had developed more than eight mature follicles at their final control visit before OT were approached and asked for possible participation.

Gene expression profiles were assessed by microarray analysis of GCs. Intrafollicular levels of IGF-related proteins were assessed with immunoassays and a proteinase assay was used to quantify activity.

Main results and the role of chance

This study presents a novel insight into the dynamics of IGF bioavailability within human ovarian follicles during the midcycle surge of gonadotropins. Notably, this investigation reports a significant downregulation of genes promoting IGF signalling, i.e., IGF-2, PAPPA, and IRS1, along with a concurrent upregulation of genes associated with IGF binding and inactivation, including IGFBPs, IGF2R, and STC1. Particularly, the substantial increase in the inhibitor, stanniocalcin-1 (STC1), at both gene and protein levels likely plays a key role in the downregulation of IGF signalling. Additionally, this study reveals a marked reduction in intrafollicular PAPP-A proteolytic activity after OT, further emphasizing the importance of PAPP-A within human ovaries.

In addition, this study indicates a pronounced decline in GC mitotic activity during the midcycle surge, as evidenced by the downregulation of proliferation markers, including MKI67, E2F1, E2F8, and FOXM1, highlighting a substantial decrease in GC proliferation rate during this critical phase. Previous research has demonstrated that IGF signalling stimulates the proliferation and differentiation of cultured human GCs. Therefore, the significant changes in IGF bioavailability observed in human GCs during the ovulatory surge suggest an essential role of IGF signalling in controlling the proliferation and differentiation of GCs within the preovulatory follicle.

Limitations, reasons for caution

This study highlights the need for future functional studies to confirm a direct mechanism to account for the abrupt cessation of GC proliferation.

Differences between using hCG or GnRH agonist for final maturation of follicles were not shown, thus future studies should be powered to reveal effects of the protocols.

Wider implications of the findings

These data suggest that downregulation of IGF signalling mediated by increased STC1 expression is instrumental for the sudden cessation in GC proliferation and onset of differentiation during the ovulatory peak. Furthermore, this study implies importance of IGF-independent functions of STC1, especially during the ovulatory peak, where elevated levels are observed.

Trial registration number

Not applicable