Aldosterone enhances progesterone biosynthesis regulated by bone morphogenetic protein in rat granulosa cells

Highlights

• Aldosterone (Aldo) induces FSH-induced progesterone synthesis by rat granulosa cells (GCs).

• Aldo increases progesterogenic enzyme expression by suppressing BMP-Smad signaling.

• Aldo upregulates inhibitory Smad6, while BMP-6 suppresses MR expression on rat GCs.

Abstract

Aldosterone (Aldo) is involved in various cardiovascular diseases such as hypertension and heart failure. Aldo levels are known to be increased in patients with polycystic ovary syndrome, and expression of the mineralocorticoid receptor (MR) has also been detected in the ovary. However, the effect of Aldo on reproductive function has yet to be elucidated. Here, we examined the effects of Aldo on follicular steroidogenesis using primary culture of rat granulosa cells by focusing on the ovarian bone morphogenetic protein (BMP) system acting as a luteinizing inhibitor. We found that Aldo treatment increased FSH-induced progesterone production in a concentration-responsive manner. Consistent with the effects on steroidogenesis, Aldo increased mRNA levels of progesterogenic factor and enzymes including StAR and P450scc, whereas Aldo failed to change FSH-induced estradiol and cAMP synthesis or P450arom expression by granulosa cells. Progesterone production and StAR expression induced by FSH and Aldo were reversed by co-treatment with spironolactone, suggesting the involvement of geonomic MR action. Aldo treatment attenuated Smad1/5/9 phosphorylation and Id1 transcription induced by BMP-6. Furthermore, Aldo enhanced the expression of inhibitory Smad6 in the presence of BMP-6. In addition, BMP-6 downregulated MR expression, while Aldo modulated the mRNA levels of endogenous BMP-6 and BMP type-II receptors, indicating the existence of a feedback loop between the BMP system and MR in granulosa cells. 　Collectively, the results indicated that Aldo predominantly enhances FSH-induced progesterone production by inhibiting BMP-Smad signaling, suggesting a novel role of Aldo in ovarian steroidogenesis and a functional link between MR and BMP pathways in granulosa cells.

Introduction

Aldosterone (Aldo) is a major mineralocorticoid hormone that is synthesized and secreted by the zona glomerulosa of the adrenal cortex [1]. Aldo mainly acts on the distal tubules and collecting ducts of the kidney via the mineralocorticoid receptor (MR), leading to a homeostasis of sodium and fluid balance in the body. Recently, Aldo has been shown to be involved in cardiovascular and metabolic disorders as a proinflammatory hormone [1]. In a clinical situation, systemic Aldo levels can be elevated in a variety of diseases including primary aldosteronism due to adrenal hyperfunction and secondary hyperaldosteronism caused by congestive heart failure, renal failure and liver cirrhosis.

Maintenance of the ovarian renin-angiotensin system (RAS) has been recognized to be important for normal follicular function. High levels of plasma Aldo have been reported in patients with polycystic ovary syndrome (PCOS), which causes infertility, hyperandrogenism and insulin resistance in women, suggesting that an excess of Aldo is involved in the pathogenesis of PCOS [2,3]. It has also been reported that obese women with PCOS had high total renin levels compared to the levels of angiotensin-converting enzyme activity and Aldo and that the high total renin levels are related to the development of insulin resistance in PCOS patients [4].

Evidence indicating extra-adrenal mineralocorticoid production including production in the ovary has been accumulating [[5], [6], [7]]. In ovarian tissues, the existence of functional MR has been demonstrated in various mammals [[8], [9], [10]]. A potent inhibitory action of the MR antagonist spironolactone in blocking gonadotropin-induced steroidogenesis was also shown, implying that endogenous mineralocorticoids may play a key role in initiating or maintaining periovulatory progesterone synthesis [5]. Hence, these results indicate that the ovary is constantly exposed to circulating as well as local mineralocorticoids. It has been shown that Aldo is involved in the induction of bovine oocyte maturation [9]. These findings further indicate that mineralocorticoids act not only as endocrine factors but also as autocrine/paracrine factors to regulate ovarian physiology. Despite these observations, there have been only a few studies in which the effects of mineralocorticoids on ovarian reproductive functions were examined.

In the present study, we utilized rat primary granulosa cells to elucidate the functional impact of Aldo in the ovarian steroidogenesis regulated by follicle-stimulating hormone (FSH) and ovarian growth factors. Functional interactions among FSH and various ovarian factors including activins/inhibins, bone morphogenetic proteins (BMPs) and growth differentiation factors (GDFs) have been shown to be indispensable for follicle growth and maturation [11,12], in which the BMP system regulates FSH receptor (FSHR) activity in granulosa cells, leading to suppression of progesterone synthesis and luteinization [[13], [14], [15]]. The present study uncovered a unique activity of Aldo in progesterone enhancement and its interaction with the BMP system in granulosa cells.