Progesterone signaling in uterine fibroids: Molecular mechanisms and therapeutic opportunities.

The effects of progesterone in human cells are largely mediated by two main isoforms of the progesterone receptor (PR): PR-A (94 kDa) and PR-B (120 kDa). These receptors, products of the same gene (PGR on chromosome 11), act as ligand-dependent transcription factors. PR-A is a truncated version of PR-B, differing by 164 amino acids in the N-terminal region [ 4 ]. Beyond PR-A and PR-B, several other truncated PR isoforms (PR-C, PR-M, PR-S, PR-T) have been identified, though their roles remain unclear [ 5 ]. PRs contain four main domains: the N-terminal domain (NTD), DNA binding domain (DBD), hinge region, and ligand-binding domain (LBD), along with activation function (AF) regions critical for gene transcription [ 5 ]. The unique AF3 domain in the N-terminal domain (NTD) of PR-B enhances its transcriptional activity, leading to a stronger gene-activating function compared to PR-A, which can even act to repress PR-B activity [ 6 ]. Progesterone signaling also occurs outside the nucleus via membrane-bound receptors: membrane progesterone receptors (mPRs) and progesterone receptor membrane components (PGRMCs). These receptors facilitate rapid, non-genomic P4 signaling and belong to different receptor families, including PAQRs and MAPRs [ 7 , 8 ].

Selective progesterone receptor modulators (SPRMs) are a new class of synthetic steroid drugs that act on PRs, inducing agonistic, antagonistic or combined effects on PR signaling, depending on the cell type [ 5 ] ( Fig. 2 ). This divergent effect can be attributed to the difference in PR-A/PR-B ratio in each tissue and the affinity of each SPRM for every isoform, as well as the interaction of each SPRM with different gene co-activators or co-repressors [ 5 , 54 ]. Given the role of P4 in uterine fibroid pathogenesis, SPRMs have been suggested as a promising class of drugs for the treatment of this disease [ 54 ]. In this section, we discuss the mechanisms of action and the therapeutic effects of the main SPRMs, which include mifepristone, ulipristal acetate, asoprisnil and vilaprisan, on multiple P4-mediated effects that contribute to fibroid growth, such as cell proliferation, ECM synthesis, angiogenesis and apoptosis ( Fig. 3 and Table 1 ). Mifepristone (RU-486) is a SPRM initially developed for its anti-glucocorticoid properties. Later, it was found to bind strongly to PRs, with an affinity more than twice that of progesterone itself in human endometrial and myometrial tissues [ 5 , 55 ]. Due to its anti-glucocorticoid and anti-progesterone actions, mifepristone is FDA-approved for managing Cushing’s syndrome and, in combination with misoprostol, for early pregnancy termination [ 56 ]. However, its potential therapeutic uses have expanded to include several gynecological conditions, such as endometriosis, adenomyosis, labor induction, and uterine fibroids [ 57 ]. The mechanism of action of mifepristone in treating uterine fibroids involves multiple pathways, which collectively inhibit fibroid growth and alleviate symptoms. Mifepristone targets the insulin-like growth factor-1 (IGF-1) signaling pathway by suppressing the ERK1/2 pathway, contributing to fibroid shrinkage [ 58 ]. The drug also downregulates TGF-β and the AKT pathway, which are known to support fibroid cell growth and ECM accumulation [ 53 , 59 ]. Mifepristone promotes fibroid cell apoptosis by reducing Bcl-2 levels, thus presenting another therapeutic mechanism [ 60 ]. Furthermore, mifepristone reduces ECM synthesis by downregulating ECM-related genes, including COL1A1, fibronectin, and syndecan-1 [ 61 , 62 ]. In fibroid cells, mifepristone has been shown to block the progesterone-induced upregulation of LAT-2, an amino acid transporter with oncogenic properties [ 63 ]. The drug also increases the expression of KLF11, a tumor suppressor gene typically downregulated in fibroids, leading to further fibroid regression [ 64 ]. Additionally, recent studies have associated mifepristone treatment with upregulated expression of GSTM1, a gene linked to tumor restriction, adding another mechanism to its tumor-suppressive actions [ 65 ]. Together, these findings indicate that mifepristone efficiently promotes fibroid regression by regulating genes involved in cell proliferation, ECM deposition, and apoptosis ( Fig. 3 ). Clinically, mifepristone has been extensively studied for fibroid treatment, particularly at low doses. A 2005 study showed that daily doses of 5 mg and 10 mg mifepristone for 12 months led to significant fibroid shrinkage (48%) and symptom relief (52–53%) without excessive endometrial hyperplasia, although some regrowth occurred after treatment cessation [ 66 ]. A similar study in 2008 confirmed that a 5 mg daily dose effectively reduced fibroid size and symptoms, while additional evidence suggested 5 mg as the optimal dose when compared to lower doses [ 67 , 68 ]. A large randomized, double-blind trial conducted over 3 months with 124 patients compared daily 5 mg mifepristone to placebo, showing a 28.5% reduction in fibroid size in the mifepristone group versus a slight increase in the placebo group. Amenorrhea was significantly more common in the mifepristone group (93.1% vs. 4.3%), along with marked improvements in pelvic pain, bleeding, and quality of life [ 69 ]. Alternative regimens have also been explored. For example, vaginal mifepristone administration led to fibroid volume reduction and symptom improvement without causing significant endometrial changes [ 70 ]. Another study compared daily versus biweekly dosing and found that both regimens resulted in similar efficacy, suggesting a cost-effective alternative [ 71 ]. Overall, these clinical findings suggest mifepristone as a promising treatment option for uterine fibroids, particularly for symptom management and preoperative fibroid reduction. With its mechanisms targeting proliferation, apoptosis, and ECM synthesis, mifepristone provides an effective, non-surgical treatment alternative for symptomatic fibroids. Ulipristal acetate (UPA) (CDB-2914) is a SPRM approved by the European Medicines Agency (EMA) in 2009 and the Food and Drug Administration (FDA) in 2010 for emergency postcoital contraception as a single dose [ 5 ]. UPA functions by binding to PRs, thereby blocking the activation of PR-mediated gene transcription by endogenous P4 [ 5 ]. Unlike mifepristone, UPA has minimal effects on glucocorticoid receptors and no known activity on estrogen or mineralocorticoid receptors [ 72 ]. Due to its potent anti-progesterone effects, UPA has shown benefits in treating several gynecological conditions, especially uterine fibroids. In 2012, the EMA approved UPA for fibroid management at a daily dose of 5 mg, following positive outcomes from phase III/IV trials [ 73 ]. However, reports of severe liver complications led to its withdrawal from the market in 2020, pending further safety assessments [ 74 ]. Preclinical studies have demonstrated multiple actions of UPA, including antifibrotic, antiangiogenic, antiproliferative, and proapoptotic effects ( Fig. 3 ). UPA promotes apoptosis in fibroid cells by downregulating the anti-apoptotic protein Bcl-2 while upregulating cleaved caspase-3 and PARP expression. This effect is complemented by reduced proliferation through decreased expression of proliferating cell nuclear antigen (PCNA) [ 75 ]. More recent research has shown that UPA induces autophagy in cultured leiomyoma cells, indicated by increased levels of autophagy markers such as LC3-II, p62/SQSTM1, Atg7, and Atg4D [ 76 ]. UPA also modulates the PR isoform ratio by upregulating PR-A and downregulating PR-B expression, a mechanism that restricts fibroid growth [ 42 ]. Additionally, UPA has antiangiogenic effects, as evidenced by the downregulation of VEGF-A, VEGF-B and adrenomedullin (ADM) in leiomyoma cells, along with their receptors [ 42 , 77 ]. UPA further limits ECM synthesis through the increased activity of matrix metalloproteinases (MMP-1, MMP-2, MMP-3, and MMP-9) and decreased production of tissue inhibitors of metalloproteinases (TIMP-1 and TIMP-2) in leiomyoma cells, but not in normal myometrium [ 78 – 80 ]. Tinelli et al. identified specific molecular mechanisms of UPA in uterine fibroids, showing that UPA treatment reduced the expression of cofilin, Erk and Src phosphorylation, p27, and ezrin, while not affecting Akt phosphorylation, cyclin D1, or β-catenin levels [ 81 ]. These changes may play a role in modulating cytoskeleton remodeling and cell cycle regulation in uterine fibroids. Additional antifibrotic mechanisms include the inhibition of TGF-β signaling, evidenced by reduced levels of TGF-β3, p-TGFR2, p-Smad2, and p-Smad3 in UPA-treated fibroid tissue [ 29 , 30 ]. UPA also downregulates various profibrotic factors, including activin A, fibronectin, versican, collagen type I and II, procollagen type I, and fibrillin [ 23 , 29 , 30 , 77 , 82 ]. UPA affects osmoregulation in fibroid cells by inhibiting the transcription factor NFAT5 and the osmolyte transporters AKR1B1 and SLC5A3, resulting in decreased production of proteoglycans such as versican, aggrecan, and brevican [ 83 ]. Additionally, UPA downregulates ECM structural components, such as integrin subunit beta 4 and tenascin-C, as well as the A-kinase anchoring protein 13 (AKAP13), which influences mechanical signaling in fibroids [ 84 , 85 ]. UPA has been the focus of numerous clinical trials for fibroid treatment. A phase II study compared daily doses of 10 mg and 20 mg UPA to placebo, demonstrating fibroid volume reduction of 17% and 24% in the UPA groups, while the placebo group showed a 7% increase. Amenorrhea was achieved in 20 out of 26 women in the UPA groups, while none of the placebo-treated participants experienced amenorrhea. Quality of life (QoL) scores and hemoglobin levels also significantly improved in the UPA groups. Notably, 9 participants exhibited abnormal liver function tests, and some showed cystic glandular changes in endometrial biopsies [ 86 ]. The PEARL trials (I-IV) contributed substantial evidence on efficacy and safety of UPA in treating symptomatic uterine fibroids. In PEARL I, patients were assigned to either 5 mg or 10 mg UPA or placebo daily for 13 weeks preoperatively [ 87 ]. UPA at 5 mg and 10 mg decreased fibroid size by 21% and 12%, respectively, while placebo led to a 3% increase. Bleeding control was achieved in 91% of women on 5 mg UPA and 92% on 10 mg, compared to 19% with placebo. Amenorrhea rates were significantly higher in the UPA groups (73% and 82%) compared to placebo (6%). Although some endometrial changes were observed, they were reversible post-treatment [ 87 ]. PEARL II compared 5 mg and 10 mg daily UPA to monthly leuprolide acetate injections over 3 months [ 88 ]. Uterine bleeding control was achieved in 90% and 98% of women receiving UPA, and 89% with leuprolide acetate. While leuprolide caused greater fibroid shrinkage (47% reduction) compared to UPA (20–22%), it was associated with more hot flashes (40% vs. 10–11% with UPA). UPA, however, had a higher rate of benign endometrial changes, which were reversible post-treatment [ 88 ]. PEARL III and its extension examined the long-term efficacy of UPA [ 21 ]. Patients received up to four 3-month cycles of 10 mg UPA with intermittent norethisterone acetate (NETA) or placebo for 10 days. Fibroid volume decreased by 45% after the first cycle, and up to 72% after the fourth. Amenorrhea rates reached 79% initially and rose to 90% after multiple cycles. PAECs (benign endometrial changes) were observed but decreased after treatment, indicating UPA’s safety and efficacy over repeated cycles [ 21 ]. In PEARL IV, 451 women received two 12-week courses of either 5 mg or 10 mg UPA. By the end of treatment, amenorrhea was achieved in 62% (5 mg) and 73% (10 mg) groups, with fibroid volume reduction of 54% and 58%, respectively [ 89 ]. PAECs were more common in the 10 mg group but did not indicate malignancy. The study demonstrated that both doses were effective for symptom relief and fibroid reduction, with no fibroid regrowth observed during follow-up [ 89 ]. The PREMYA trial, a large multicenter study, involved 1473 women receiving 5 mg of UPA for 3 months [ 90 ]. At the end of the treatment, 60% of participants reported significant symptom improvement, and 38.8% required less invasive surgical interventions. QoL scores remained improved over a 12-month follow-up, suggesting long-lasting effects of UPA treatment [ 90 ]. The VENUS I and II trials, conducted primarily among African American women with higher BMI, also confirmed the efficacy of UPA [ 22 , 91 ]. In VENUS I, fibroid volume was reduced by 9.6% and 16.3% in the 5 mg and 10 mg groups, respectively, compared to a 7.2% increase in placebo. Amenorrhea rates were significantly higher in the UPA groups (47.2% and 58.3%) compared to placebo (1.8%). VENUS II demonstrated further fibroid volume reduction during a second treatment cycle, with minor adverse events, such as hot flashes and fatigue, and no endometrial malignancy [ 22 , 91 ]. The MYOMEX trial compared UPA to leuprolide acetate as preoperative treatments before myomectomy. Although leuprolide resulted in greater fibroid reduction and shorter surgery times, UPA was associated with less intense intraoperative blood loss, although it made myomectomies somewhat more challenging. Both pre-treatments led to similar improvements in QoL and bleeding patterns six months post-surgery [ 92 , 93 ]. The MYOMEX-2 trial aims to assess the long-term cost-effectiveness of UPA versus surgery, with results expected soon [ 94 ]. The effect of UPA on in vitro fertilization (IVF) outcomes was studied retrospectively, showing a 49% decrease in mean fibroid volume, with improvements in uterine cavity distortion and symptomatic relief. IVF success rates were similar in the UPA-pretreated and control groups, indicating that UPA might optimize uterine conditions for IVF candidates with fibroids [ 95 ]. The UCON study compared UPA to the levonorgestrel-releasing intrauterine system in women with fibroids and heavy bleeding. UPA achieved higher amenorrhea rates (64% vs. 25%) and significant QoL improvements without hepatotoxicity or endometrial malignancy, suggesting that it may be more effective in achieving amenorrhea than the levonorgestrel device [ 96 ]. Overall, UPA shows efficiency as a SPRM for managing symptomatic uterine fibroids. Asoprisnil (J-867) is a unique SPRM with high specificity for PRs, particularly in the endometrium [ 97 , 98 ]. Unlike other SPRMs, it belongs to the 11β-benzaldoxime substituted steroidal family and demonstrates both agonist and antagonist effects on P4 signaling by recruiting gene coactivators and corepressors when bound to PRs [ 5 , 99 ]. In rabbit uterine cells, PR binding affinity of asoprisnil is approximately three times higher than that of P4, while it exhibits medium affinity for glucocorticoid receptors, low affinity for androgen receptors, and no affinity for estrogen or mineralocorticoid receptors [ 98 ]. Initial promising preclinical data led to its development for treating uterine fibroids and endometriosis [ 98 ], but concerns over endometrial changes limited its clinical use [ 100 , 101 ]. Preclinical studies have shown that asoprisnil can selectively inhibit fibroid cell growth by downregulating growth factors like TGF-β3, EGF, and IGF-I and their receptors, while sparing normal myometrial cells [ 44 , 102 ]. It also induces apoptosis in fibroid cells, marked by reduced Bcl-2 levels, increased cleaved caspase-3 and PARP expression, and elevated TUNEL-positive cell rates [ 51 , 102 ]. Interestingly, PR-B expression is higher in asoprisnil-treated leiomyoma cells, suggesting that PR-B may contribute to selective actions of asoprisnil on fibroid tissue [ 51 ]. Additionally, asoprisnil increases ER-stress proteins and activates apoptosis-related genes such as GADD153, leading to the upregulation of pro-apoptotic proteins Bax and Bak and further reducing Bcl-2 levels [ 103 ]. Other studies have shown that asoprisnil activates the TRAIL-mediated apoptotic pathway and suppresses X-linked inhibitor of apoptosis in fibroid cells [ 104 ]. Furthermore, asoprisnil reduces ECM production in fibroids by decreasing collagen types I and III, TIMP-1, and TIMP-2, while enhancing the synthesis of ECM-degrading proteins like MMP-1 and MT1-MMP [ 32 ]. In a Phase II clinical trial with 129 women, asoprisnil at doses of 5, 10, or 25 mg daily significantly reduced fibroid-related bleeding and induced amenorrhea in a dose-dependent manner, with the highest efficacy observed in the 25 mg group, which also achieved a 36% reduction in fibroid volume [ 105 ]. However, endometrial biopsies revealed non-physiologic changes in over 50% of women on asoprisnil, and some patients developed asymptomatic ovarian cysts. Another study noted structural changes in the endometrium and decreased mitotic activity in the fibroid tissue of asoprisnil-treated women [ 106 ]. A pooled analysis of two Phase III trials in 907 women confirmed the efficacy of asoprisnil in reducing uterine bleeding, fibroid volume, and improving quality of life, with effects maintained over a 6-month follow-up [ 100 ]. However, non-physiologic endometrial changes occurred frequently in asoprisnil-treated women, and several cases of breast and endometrial cancer were observed during an extension study, raising concerns about the long-term safety of the drug [ 101 ]. These findings suggest that while asoprisnil is effective in reducing fibroid symptoms, its endometrial safety profile warrants caution, limiting its clinical utility. Vilaprisan (BAY 1002670) is a novel SPRM with potent antagonistic activity on PRs, estimated to be stronger than mifepristone or UPA [ 107 ]. Although highly selective for PRs, preclinical studies suggest it has moderate binding affinity to GR and low affinity to AR [ 107 ]. Given its strong anti-progesterone effect, vilaprisan entered clinical trials in 2017 to assess its efficacy and safety for treating uterine fibroids, demonstrating promising early-phase results. In initial Phase I trials, the effects of vilaprisan on menstrual cycle regulation were evaluated [ 108 ]. A randomized, placebo-controlled study with 73 healthy women administered doses of 0.1–5 mg daily for 12 weeks. The 2 mg dose showed significantly increased non-bleeding rates, with menstrual bleeding resuming in all women within 52 days of stopping treatment. Follicular growth was unaffected, while estrogen levels (E2) stayed above 40 pg/mL. Some participants on higher doses, particularly 5 mg, developed PAECs and reported adverse events (AEs) like headache, ovarian cyst formation, fatigue, and abdominal pain [ 108 ]. Another Phase I study evaluated the impact of vilaprisan on ovarian function [ 109 ]. Seventy women received doses between 0.5–4 mg daily for 12 weeks. Ovulation was inhibited in over 80% of women taking 1 mg or more, and E2, P4, FSH, and LH levels were markedly decreased, although follicular growth continued. Amenorrhea rates peaked at 2 mg, and endometrial thickness increased but normalized after treatment ended. As in the first study, PAECs occurred in a dose-dependent manner, affecting up to 95% of women on doses of 1 mg or more [ 109 ]. With ovarian function effects established, Phase II and III trials assessed the efficacy and safety of vilaprisan in women with fibroids and heavy menstrual bleeding. The ASTEROID 1 trial, a Phase II study of 300 women, tested daily doses of 0.5–4 mg over 12 weeks, followed by a 24-week follow-up [ 110 ]. Complete cessation of bleeding occurred in up to 60% of patients in the vilaprisan groups, while only 1.7% in the placebo group achieved this. Amenorrhea was achieved in over 83% of patients taking at least 1 mg, and median menstrual blood loss dropped to 0 mL in all vilaprisan groups. Fibroid volume decreased significantly (14.9%–41.4%) in a dose-dependent manner, with most women reporting improved symptoms. Common AEs included ovarian cysts, headache, and hot flashes, with no significant difference in AE frequency across groups. These findings suggested vilaprisan is effective in controlling fibroid-related bleeding, with 2 mg identified as the optimal dose [ 110 ]. The ASTEROID 2 trial compared vilaprisan to UPA and placebo in 100 women with fibroids and heavy menstrual bleeding [ 111 ]. At week 12, 62.9% of vilaprisan-treated women achieved complete bleeding cessation compared to none in the placebo group. Fibroid volume decreased by 29.9% with vilaprisan and 23.8% with UPA, while placebo saw a slight increase. The study indicated the comparable efficacy of vilaprisan to UPA in reducing bleeding and fibroid size, with no serious safety concerns [ 111 ]. Finally, the ASTEROID 3 trial, a Phase III study, assigned 75 women to 2 mg vilaprisan or placebo over two 12-week courses [ 112 ]. Amenorrhea was achieved in 83.3% of vilaprisan-treated patients compared to none in the placebo group. Vilaprisan effectively reduced menstrual blood loss, and although serious AEs were reported in 27.8% of participants, they did not differ significantly between groups. These results highlight the potential of vilaprisan for rapid bleeding reduction, but further trials are needed to assess long-term safety and efficacy [ 112 ].

P4 exerts its effect on fibroids mainly through interaction with classical PRs, since PR-A and PR-B expression has been shown to be higher in leiomyomas compared to normal myometrium [ 19 , 20 ]. The efficacy of selective progesterone receptor modulators (SPRMs) for the treatment of uterine fibroids is also strong indirect evidence that the interaction of P4 with classical PRs contributes to fibroid growth [ 21 , 22 ]. Research regarding the effect of mPRs on uterine leiomyomas remains limited, although a recent study examining the expression of PRs in uterine leiomyoma tissue samples suggested that both nuclear and membrane PRs contribute to leiomyoma pathobiology via distinct mechanisms [ 23 ]. It is evident that P4 and its receptors play a major role in uterine fibroid pathogenesis; however, further research is needed to understand the interactions between PRs and other regulators of tumor growth and to potentially discover new signaling pathways that can set new therapeutic targets. For instance, the crosstalk between P4 and E2 signaling has been shown, with studies indicating that E2 induced PR expression in leiomyoma cells [ 24 ]. Katzenellenbogen et al. demonstrated that E2 upregulated PR expression in breast cancer cells[ 25 ], suggesting a synergistic relationship between E2 and P4 signaling pathways. Moreover, mPRs regulate cAMP and PKA activity through Gi and Gs protein coupling, activating PI3K/AKT and MAPK pathways, while PGRMC1 may influence non-genomic P4 signaling via cAMP and protein kinase G [ 12 – 14 , 16 , 17 ]. These findings suggest that a complex regulatory network of P4 signaling may exist in fibroid pathogenesis and may influence the effectiveness of SPRMs.

Although the exact underlying mechanisms that drive ECM deposition in uterine fibroids are still under investigation, many studies have shown that P4 may play a key role through its interaction with growth factors, ECM proteins, and MMPs [ 26 ]. Transforming growth factor-β (TGF-β) is one of the major growth factors that induce fibrosis in uterine fibroids. TGF-β3 expression was shown to be increased during the secretory phase (mediated mainly by P4), indicating the importance of P4 signaling on TGF-β function [ 27 , 28 ]. Recent studies proved that TGF-β1 and TGF-β3 transcripts were markedly decreased when uterine fibroid cells were treated with ulipristal acetate, a SPRM, indirectly suggesting that P4 mediates TGF-β signaling [ 23 , 29 , 30 ]. Moreover, P4 has been found to decrease the expression of decorin, a proteoglycan that inhibits TGF-β3, suggesting an alternative mechanism of P4 induced ECM deposition [ 31 ]. The expression of MMPs and TIMPs was also observed to be upregulated in leiomyoma cells when treated with the SPRM asoprisnil, proving that P4 can induce ECM accumulation by inhibiting its degradation by metalloproteinases [ 32 ]. Additionally, P4 was reported to enhance collagen synthesis, a major ECM component, through downregulation of mi-R29b in uterine leiomyoma xenografts [ 33 ]. The four mechanisms mentioned above (increased TGF-β expression, downregulation of decorin, upregulation of MMPs, enhanced collagen synthesis) provide strong evidence of P4’s influence on abnormal ECM synthesis and remodeling in uterine leiomyomas. More extended research on the molecular mechanisms mediating these interactions, and further exploring the effect of SPRMs on ECM deposition, will provide useful information and hopefully reveal new therapeutic targets for uterine fibroid shrinkage. The vascular pattern of uterine leiomyomas is characterized by a highly vascularized area in the periphery, also known as “perifibromal vascular capsule,” which surrounds a relatively hypovascular core [ 34 ]. Although uterine fibroids are mostly hypovascular, angiogenesis plays a key role in maintaining and promoting their growth, as implicated by the efficacy of uterine artery embolization in reducing fibroid size [ 35 ]. The difference in vascular architecture between uterine fibroid tissue and normal myometrium also indicates that dysregulated angiogenesis is part of fibroid pathophysiology and is potentially mediated by dysregulated expression of angiogenic and antiangiogenic factors [ 34 , 36 ]. Many studies have established that the expression of primary angiogenic growth factors, such as VEGF, EGF, FGF, PDGF, ADM and TGF-β, is higher in human leiomyomas compared to normal myometrial tissue [ 36 ]. In a recent study, aberrant expression of HMGA2, a known driver gene mutation of uterine fibroids, was linked to increased levels of angiogenic factor and their receptors compared to MED-12 mutant leiomyoma cells, suggesting that vascular alterations in uterine fibroids may be attributed to HMGA2 overexpression [ 37 , 38 ]. Another mechanism by which leiomyoma cells exhibit altered angiogenesis is their distinct reaction to hypoxia. In other tumors, hypoxia triggers the expression of hypoxia inducible factor-1 (HIF-1), which enhances angiogenesis and cell proliferation to promote tumor survival and growth [ 39 ]. However, HIF-1α and HIF-1β transcription has been repeatedly found to be low in leiomyoma cells despite being significantly hypoxic, suggesting a lack of response to hypoxia and potentially explaining why leiomyomas exhibit diminished vascularity and lack malignant features [ 36 , 40 ]. P4 is a key mediator of uterine angiogenesis, mainly through its interaction with VEGF [ 41 ]. Although the specific mechanisms are not fully established, many studies have shown that P4 signaling also regulates angiogenesis in uterine fibroids [ 36 , 42 , 43 ]. P4 was observed to upregulate EGF expression in leiomyoma cells, while EGF transcripts significantly declined under the influence of asoprisnil, suggesting that P4 signaling regulates EGF expression and thus, angiogenesis [ 43 , 44 ]. Additionally, treatment of human leiomyoma cells with ulipristal acetate resulted in downregulation of ADM and VEGF as well as their receptors, indirectly indicating the role of P4 in promoting angiogenesis [ 42 ]. More thorough examination of the relationship between P4/PRs and angiogenic growth factors is needed to answer whether the dysregulated vascular pattern of uterine fibroids can be attributed to P4 stimulation. Several studies have supported that Bcl-2, an antiapoptotic gene, is upregulated in uterine leiomyomas and significantly contributes to their pathogenesis [ 45 – 47 ]. Moreover, Bcl-2 protein was found to be abundant in leiomyoma cells but undetectable in myometrial tissue, indicating that Bcl-2 overexpression and subsequent escaping of the apoptosis is an essential feature of uterine fibroids [ 45 ]. Bcl-2 protein expression in leiomyomas was found to rise in accordance with P4 in the secretory phase, suggesting that P4 signaling can inhibit apoptosis through Bcl-2 protein activation [ 45 ]. Furthermore, in vivo leiomyoma tissue exhibited significantly greater levels of Bcl-2 mRNA when treated with P4, and liganded PR-A was found to strongly bind to the Bcl-2 promoter, suggesting that P4 can induce Bcl-2 gene expression through its interaction with PR-A [ 48 ]. The antiapoptotic effect of P4 was also validated in a more recent study, where P4 exhibited a dose-dependent inhibitory effect on apoptosis of uterine leiomyoma tissue cultures [ 49 ]. Additionally, telapristone acetate and asoprisnil, both potent SPRMs, were found to significantly decrease Bcl-2 protein levels while increasing apoptosis markers, indirectly proving the role of P4 on apoptosis evasion [ 50 , 51 ]. Another mechanism by which P4 may promote leiomyoma cell survival is by rapid activation of the AKT pathway, an essential signaling pathway that regulates cell proliferation, differentiation and survival and is commonly overactivated in many cancers [ 52 , 53 ]. In this study, phosphorylated AKT levels in leiomyoma cells significantly increased when treated with P4, an effect not observed in matched myometrial cells [ 53 ]. Escaping apoptosis is, without a doubt, an essential part of uterine fibroid survival. Exploring other possible antiapoptotic mechanisms of P4 signaling along with more extended research on the underlying molecular interactions between P4/PRs and proapoptotic/antiapoptotic factors will help deepen the understanding of the importance of P4 on uterine fibroid cell survival and potentially lead to new therapeutic targets.

Uterine fibroids are a complex, multifactorial disease involving various pathophysiological mechanisms that contribute to fibroid cell growth and survival. P4 plays a central role in fibroid pathogenesis, activating pathways that promote leiomyoma cell proliferation, upregulating antiapoptotic factors, and disrupting vascular patterns. P4 also drives excessive ECM synthesis while inhibiting ECM resorption by suppressing matrix metalloproteinases. The recognition of P4 as a key mediator in fibroid growth has led to interest in SPRMs for treatment. Mifepristone has shown efficacy in reducing fibroid size and symptoms, but its long-term use is limited by endometrial proliferation concerns. Asoprisnil effectively controls bleeding but raises safety concerns with prolonged use. Ulipristal acetate, approved by the EMA for fibroid management, was effective in reducing fibroid size and bleeding but was suspended by the European Medicines Agency (EMA) in February 2018 due to reports of liver toxicity. In May of 2018 the EMA issued restrictive measures, and a Pharmacovigilance Risk Assessment Committee made recommendations to reduce the risks in May 2018 to resume treatment of premenopausal women with ulipristal acetate for whom surgical procedures are inappropriate or unsuccessful suspended. Ulipristal acetate was not approved for use in the U.S. Vilaprisan, a newer SPRM, has shown promise in recent trials for controlling bleeding and reducing fibroid volume. However, Bayer Healthcare Pharmaceuticals issued a pause in December 2018 based on preliminary findings from long-term rodent carcinogenicity studies. Despite the absence of findings indicating safety concerns for humans, further development was suspended due to a change in development plans. Though no SPRMs are approved for treatment of fibroids in the U.S., the class of compounds demonstrated rapid reduction in bleeding and a remarkably durable reduction in fibroid size. It is possible that new compounds could be developed, or intermittent treatment regimens could be devised with existing SPRMs to serve as a nonsurgical option for this complex disease.

Progesterone (P4) is a female sex steroid hormone primarily produced by the corpus luteum during the luteal phase of the menstrual cycle, synthesized from cholesterol and metabolized mainly in the liver. Its primary role is to transition and maintain the endometrium in a secretory state for embryo implantation and pregnancy support. P4 also contributes to lobuloalveolar development in the mammary glands and promotes osteoblastic activity in bones, preventing bone loss [ 1 ]. Beyond reproductive functions, P4 signaling is implicated in hormone-dependent diseases like endometriosis, breast cancer, and uterine fibroids [ 2 ]. Uterine fibroids, the most common benign gynecological tumors, can significantly affect fertility and quality of life depending on their size, number, and location. While estrogen was traditionally seen as the main hormonal driver, recent research highlights the role of P4 in fibroid pathogenesis. Studies show P4 regulates angiogenic factors, extracellular matrix components, growth factors, and apoptotic agents in fibroid cells [ 3 ]. Given the role of P4 in promoting fibroid cell proliferation, selective progesterone receptor modulators (SPRMs) have been explored as potential therapies. This scoping review examines how P4 and its receptors contribute to fibroid pathogenesis and summarizes key findings on the efficacy and tolerability of SPRMs in treating uterine fibroids.

Nuclear (genomic) PRs, PR-A and PR-B, mediate the classical or genomic P4 signaling pathway ( Fig. 1 ). In the absence of P4, PRs are found in the cytoplasm in an inactive state, bound to chaperone proteins. Heat shock proteins (HSP90, HSP70 and HSP40), and cochaperone molecules, HSP90-binding protein p23 and HSP70/HSP90 organizing protein (Hop), prevent PRs from entering the nucleus [ 9 ]. Upon ligation with P4, PRs undergo a series of conformational changes, which leads to their release from the chaperone proteins and their translocation to the nucleus. Inside the nucleus, PRs dimerize and the dimers bind to specific promoter regions, called progesterone receptor elements (PREs) [ 10 ]. Along with other gene specific co-regulators and transcription factors, they form a complex which can induce or inhibit the expression of target genes. A large variety of physiological responses can be initiated, depending on whether PR dimers are in the form of homodimers (AA or BB) or heterodimers (AB). Activated PRs can also bind to DNA regions not identified as PREs by interacting with specific transcription factors, such as activating protein 1 (AP-1), specificity protein 1 (Sp1), signal transducer and activator of transcription5 (STAT5), and nuclear factor kappa-lightchain-enhancer of activated B cells (NF-κB) [ 5 ]. Progesterone (P4) can activate rapid, non-genomic effects through non-classical receptors, including membrane progesterone receptors (mPRs) and progesterone receptor membrane components (PGRMCs). This signaling pathway bypasses nuclear transcription, triggering intracellular responses within minutes [ 11 ]. mPR isoforms (mPRα, mPRβ, and mPRγ) are coupled with inhibitory G (Gi) proteins, leading to decreased cAMP and protein kinase A activity, while mPRδ and mPRε are coupled with stimulatory G (Gs) proteins, increasing cAMP and PKA activity [ 12 , 13 ]. mPRs also activate PI3K/AKT and MAP kinase pathways, impacting various reproductive functions [ 12 , 14 ]. PGRMC1, part of the PGRMC family, contains a cytochrome b5 domain that interacts with CYP enzymes involved in steroidogenesis, potentially influencing P4 synthesis [ 15 ]. PGRMC1 can associate with serpine 1 mRNA binding protein (SERBP1) on the cell membrane, forming a complex that, when activated by P4, reduces intracellular Ca 2+ through increased cAMP and protein kinase G activation [ 16 , 17 ]. Additionally, PGRMC1 may enhance mPRα expression on the cell surface, indicating a cooperative role in non-genomic P4 signaling [ 18 ].