The interleukin-1β system in the corpora lutea of pigs during early pregnancy and the estrous cycle

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Abstract

Expression of mRNAs encoding interleukin-1β (IL-1β), IL-1β receptor I (IL-1RI), IL-1 receptor accessory protein (IL-1RAcP) and IL-1 receptor antagonist (IL-1Ra), as well as synthesis of IL-1β and IL-1RI proteins, were examined in the corpus luteum (CL) during critical stages of CL activity on days 10–16 of pregnancy and 2–16 of the estrous cycle. Luteal cells were cultured in vitro with IL-1β, and the effect on release of steroid hormones was determined. Expression of the IL-1β system in the CL changed significantly during pregnancy and the estrous cycle. IL-1β, IL-1RI, and IL-1Ra mRNA levels were elevated on days 12–13, whereas IL-1RAcP mRNA was increased on days 15–16 of pregnancy. In cyclic CL, expression of IL-1β, IL-1RI, and IL-1RAcP mRNAs was increased on days 12–13. IL-1β and IL-1RI protein were highest in the CL on days 10–11 and 8–11 of pregnancy and the estrous cycle. Luteal cells harvested from gravid and cyclic CL produced IL-1β in vitro. IL-1β increased progesterone and estradiol-17β (E2) release by luteal cells on days 10–16 and 10–11 of pregnancy, respectively and on days 2–11 of the estrous cycle. IL-1β decreased the level of E2 produced by regressed CL (days 15–16). Expression of the IL-1β system in CL and IL-1β secretion from luteal cells changed depending on the status of the CL. These data show that IL-1β may be involved in intraluteal, luteotrophic regulation of CL functions in gravid and cyclic pigs.

Introduction

Interleukin-1β (IL-1β) is one of several cytokines involved in the regulation of ovarian and uterine functions (Wuttke et al., 1997, Gèrard et al., 2004, Franczak et al., 2010, Franczak et al., 2012). This cytokine enhances follicular growth, ovulation, and the formation of the corpus luteum (CL), as well as implantation and embryo development (Simón et al., 1994b, Tuo et al., 1996, Ross et al., 2003). In different species, IL-1β was found to act as a local regulator of cyclic and gravid CL activity through the modulation of luteal prostaglandins and steroid secretion (Nothnick and Pate, 1990, Kohen et al., 1999, Zmijewska et al., 2012).

In target cells, IL-1β acts throughout the IL-1 system, which consists of ligands (IL-1β and IL-1α) that use the same IL-1 receptor type-I (IL-1RI), receptor accessory protein (IL-1RAcP), and endogenous receptor antagonist (IL-1Ra) (Dinarello, 1997, Gèrard et al., 2004). The selected components of the IL-1β system have been demonstrated in murine (Simón et al., 1994a), human (Kohen et al., 1999), bovine (Petroff et al., 1999), and rabbit (Krusche et al., 2002) luteal tissue. To date, interleukin-1-like activity has been demonstrated only in ovarian follicular fluid in non-pregnant, cycling pigs (Takakura et al., 1989). However, the expression of the IL-1β system in the CL of gravid and cycling pigs remains unknown.

CLs in cycling pigs are infiltrated by macrophages (Hehnke et al., 1994) which, in addition to luteal fibroblasts, endothelial cells, and epithelial cells, are the main source of IL-1β (Gèrard et al., 2004). However, human luteal cells release IL-1β even when luteal leukocytes are removed (Castro et al., 1998). Whether porcine luteal cells are a potential source of IL-1β remains unknown.

The life span of the CL in pigs is precisely regulated during the luteal phase of the estrous cycle (days 2–16) and early pregnancy (days 10–16) (Niswender et al., 2000). The status of porcine CL is dramatically changed on days 12–13 of the estrous cycle or pregnancy, depending on the physiological state of the female. On these days, cyclic CLs are destined for regression while gravid CLs survive. The actions of an intraluteal IL-1β system may be involved in regulating these processes (Zmijewska et al., 2012).

The aim of this study was to demonstrate the expression of the IL-1β system and the action of exogenous IL-1β in the CL of early pregnant and cyclic pigs. Specifically, we have determined that (1) IL-1β, IL-1RI, IL-1RAcP and IL-1Ra mRNA expression levels change in gravid and cyclic CL (days 10–16); (2) IL-1β and IL-1R protein is expressed in gravid (days 10–16) and cyclic CL (days 2–16); (3) IL-1β is present in the culture medium of luteal cells isolated from gravid (days 10–16) and cyclic (days 2–16) CL as well as medium from cells isolated from corpus albicans (days 18–20); and (4) IL-1β acts to induce steroid release (progesterone, P4, and estradiol-17β, E2) by in vitro cultured luteal cells of gravid (days 10–16) and cyclic (days 2–16) pigs.

Section snippets

Animals, CL and corpus albicans collection

All animals were used in this study in accordance with requirements of the local Animal Ethics Committee. Pigs were sacrificed on days 10–11, 12–13, and 15–16 (n = 4/period) of pregnancy or the estrous cycle as well as on days 2–3, 8–11, and 18–20 (n = 4/period) of the estrous cycle. The ovaries were harvested directly after slaughter. For RNA and protein isolation, CLs were isolated from the ovaries, frozen in liquid nitrogen and stored at −80 °C. The remaining CL and corpus albicans were

Expression of IL-1β, IL-1RI, IL-1RAcP, IL-1Ra mRNAs, and IL-1β and IL-1RI proteins in the CL of gravid and cyclic pigs

The expression of the mRNAs examined (Fig. 1) and proteins (Fig. 2) changed in the CL of both gravid and cyclic pigs. The level of IL-1β mRNA increased significantly between pregnancy days 10–11 and 15–16. The highest levels of IL-1RI and IL-1Ra mRNA were observed on days 12–13 of pregnancy, and mRNA expression levels were correlated (r = 0.54) across the days of pregnancy studied. IL-1RAcP mRNA expression levels were similar (p > 0.05) on days 10–11 and 12–13 and increased (p < 0.05) on days 15–16

Discussion

In this study, we have shown (1) corpus luteum expression of IL-1β, IL-1RI, IL-1RAcP, and IL-1Ra mRNAs; (2) IL-1β and IL-1R protein expression in the CL; (3) in vitro basal secretion of IL-1β; and (4) IL-1β-induced secretion of P4 and E2 in luteal cells of early pregnant and cyclic pigs. Our results confirm the presence of the IL-1β system in the CL of cyclic pigs as determined previously in the CLs of other species (Simón et al., 1994a, Simón et al., 1994b, Petroff et al., 1999, Kohen et al.,

Acknowledgments

This research was funded by grant nos. N311 3998 39 (2010–2012) and N311 0685 33 (2007–2010) from the Polish Ministry of Science and Higher Education.

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