Dienogest increases the progesterone receptor isoform B/A ratio in patients with ovarian endometriosis

Endometriosis is an estrogen-dependent inflammatory disease that affects 6-10% of females of reproductive age [1]. It is characterized by the presence of endometrium-like tissue outside the uterine cavity, primarily on the ovaries, and represents one of the most common causes of chronic pelvic pain, dysmenorrhea and infertility [2]. The main aims of treatment are to alleviate pain and other symptoms, reduce the size of the endometriotic lesions and improve the quality of life of affected individuals. Nonsteroidal anti-inflammatory drugs are frequently used by patients with endometriosis in an attempt to relieve pelvic pain, although clinical trial evidence to support the efficacy of these agents in endometriotic patients is lacking [3]. Gonadotropin-releasing hormone (GnRH) agonists are an established therapy for endometriosis. Although GnRH agonists provide effective pain relief and reduce the progression of endometriotic implants [4], the hypoestrogenic state they induce is associated with negative effects such as accelerated bone mineral density loss, hot flashes and vaginal dryness [5]. Oral contraceptives (OCs) are widely used to treat the symptoms of endometriosis, although they are not approved for this indication in the majority of countries due to the lack of supportive trial evidence [6]. Progestins have long been used for the treatment of endometriosis to relieve pain by suppressing ovarian estrogen biosynthesis, in turn, suppressing growth and inflammation [7]. Unfortunately, the relief of pain appears to be relatively short-term [8], and approximately 9% of females with endometriosis simply do not respond to progestin therapy due to unknown reasons [9]. In fact, a general tendency for relative progesterone resistance within the eutopic and ectopic endometrium of females with endometriosis is well-documented [1, 10].

Recently, two major progesterone receptor (PR) isoforms were identified, namely PR-A and PR-B (11). PR-A is a 94-kDa protein, whereas PR-B is a 114-kDa protein that contains an additional NH2-terminal stretch of approximately 165 amino acids containing a region encoding a transactivation function. These isoforms may arise as a result of either initiation of translation from alternative sites in the same messenger RNA (mRNA) [11] or by transcription from alternative promoters [12]. Although the exact functions of each of these isoforms remain unclear, there is increasing evidence that they are functionally different [12, 13]. PR-B tends to be a stronger activator of progesterone target genes, whereas PR-A has been shown to act as a dominant repressor of PR-B [14, 15].

Endometriosis is associated with a reduced response to progesterone in both eutopic and ectopic endometrium. According to recent reports, the resistance of endometriotic tissue to progesterone, evident in both laboratory and clinical observations, can be explained by alterations in the distribution of ER and PR isoforms and dysregulation of progesterone target genes [10, 16‐18]. Because the effects of progesterone on target genes are conferred primarily by PR-B in the endometrium [19], the presence of the inhibitor isoform-A and the absence of the stimulatory isoform-B provide a possible explanation for progesterone resistance in endometriotic implants. In fact, a decreased PR-B/PR-A ratio has been demonstrated in ectopic tissue [20, 21], and recent reports suggest that the tendency toward progesterone resistance in patients with endometriosis is likely the result of the promotion of hypermethylation of PR-B, which renders PR-B either silenced or downregulated [21]. Moreover, a number of investigators have reported markedly elevated levels of ERβ and lower levels of ERα in human endometriotic tissues and primary stromal cells compared with that observed in eutopic endometrial tissues and cells [16, 22, 23]. ERβ, acting as an ERα suppressor, might contribute to the decreased PR levels and progesterone resistance observed in patients with endometriosis [24].

Dienogest (17a-cyanomethyl-17b-hydroxyestra-4,9-dien-3-one) is an oral progestin that has been systematically investigated for the treatment of endometriosis in dose-ranging [25], placebo-controlled [26, 27], active comparator-controlled [28, 29] and long-term trials [30] conducted in Europe and Japan. The main anti-endometriotic effect of dienogest has been suggested to be attributable to central inhibition of ovulation. Furthermore, direct antiproliferative effects of dienogest have been demonstrated in human eutopic endometrial stromal cells [31]. Recent studies demonstrate that dienogest inhibits the proliferation of endometriotic stromal cells [32], prostaglandin E2 production and the aromatase mRNA expression of the endometrial epithelial cell line [33]. However, there is no evidence regarding whether dienogest improves progesterone resistance in patients with endometriosis. In the present study, therefore, we examined the effects of dienogest on alterations in the ratios of PR-B to PR-A and ERβ to ERα in ectopic endometrial tissue obtained from endometriotic females.

The current study demonstrated statistically significant decreases in both PR–B and PR-A messenger RNA and proteins in ectopic endometrium derived from females with endometrioma who did not receive any medical treatment (Figure 1A, 1B, 3B, and 3F). Furthermore, the relative expressions of PR-B and PR-A were significantly lower in ectopic endometrium (Figure 1C). According to Attia et al. [20], the resistance of endometriotic tissue to progesterone, evident in both laboratory and clinical observations, can be explained by the absence of PR-B transcripts and proteins and the presence of PR-A in ectopic lesions. Similar findings have been reported in epithelial cells selected from a small number of ectopic samples [10]. Recently, independent investigators suggested that alterations in the relative expressions of PR-A and PR-B in endometrial cells may also play a pivotal role in the pathogenesis of endometriosis. A decreased PR-B/PR-A ratio has been demonstrated in ectopic tissue [5, 10], and our findings are consistent with the results of these studies.

Several investigators have reported markedly higher levels of ERβ and lower levels of ERα in human endometriotic tissues and primary stromal cells compared with that observed in eutopic endometrial tissues and cells [22, 40]. Recently, Bulun [41] reported that the levels of the nuclear receptors ERα, ERβ and PR are quite different in endometriotic tissue and endometrium. The high ERβ/ ERα ratiosin endometriotic stromal cells in turn lead to increased ERβ binding to the PR promoter and mediate downregulation of the expression of PR [21]. ERβ acts as a suppressor of ERα in both endometrial and endometriotic stromal cells by binding to regulatory elements of specific promoters of the ERα and PR genes [42]. Therefore, ERα deficiency in endometriotic patients may be responsible for the failure of E2 to induce the PR expression, thus contributing to secondary PR deficiency and progesterone resistance in females with endometriosis. Although strikingly high quantities of E2 produced via local aromatase activity are observed in endometriotic tissue [43, 44], the E2-dependent induction of PR is strongly inhibited [20]. Findings consistent with these studies were observed in the current study, which demonstrated statistically significant higher levels of ERβ and lower levels of ERα in ectopic endometrium derived from females with endometrioma (Figure 2 and Figure 3).

The endometrioma samples obtained from the patients who received dienogest or LA treatment showed a significantly higher expression of PR-B compared with that observed in the control endometrioma samples (Figure 1A and Figure 3). Although LA treatment increased the PR-A expression in the endometrioma samples, dienogest treatment did not alter the expression of PR-A (Figure 1B). Consequently, dienogest treatment improved the PR-B/PR-A ratio; however, there were no significant differences in endometrioma after LA treatment (Figure 1C). Progesterone has recently been shown to reverse E2-stimulated increases in the ERβ mRNA and protein expression in cultured hippocampal slices [45]. Our current study demonstrates that dienogest treatment alters the expression of ERβ (Figure 2B and Figure 3) and significantly decreases the ERβ/ERα ratio in endometrioma (Figure 2C). A recent report demonstrated that the promoter region of PR-B, but not PR-A, is hypermethylated in patients with endometriosis [21]. Although further studies are needed to clarify whether medical treatments might be associated with the methylation status of the PR-B promoter and to explore the mechanisms underlying the ERβ downregulation induced by dienogest, a decreased ERβ/ERα ratio in the endometriotic tissues of patients treated with dienogest may be responsible for the observed improvements in the PR expressions.

We demonstrated that dienogest improves progesterone resistance in endometrial tissue. This finding enhances understanding of the anti-endometriotic effects of dienogest. It is possible that PR-B deficiency is only the tip of the iceberg with regard to the pathogenesis of endometriosis and that numerous other molecular aberrations may also contribute to the development of resistance to hormone treatments in females with endometriosis. Although our findings may explain at least part of the mechanisms underlying the clinical improvements observed in endometriotic patients using dienogest, the normalization of the PR expression profile observed in this study suggests that dienogest may be an effective and long-term treatment for endometriosis.