Environmental Health Perspectives Volume 108, Supplement 5, October 2000
Advances in Uterine Leiomyoma Research: The Progesterone Hypothesis
Mitchell S. Rein
North Shore Medical Center, Brigham and Women's Hospital, Harvard Medical School, Cambridge, Massachusetts, USA; Women's Health Center of the North Shore, Danvers, Massachusetts, USA
Abstract
Uterine leiomyomas are monoclonal tumors. However, the factors involved in their initiation and growth remain poorly understood. The neoplastic transformation of myometrium to leiomyoma likely involves somatic mutations of normal myometrium and the complex interactions of sex steroids and local growth factors. Traditionally, estrogen has been considered the major promoter of myoma growth. The purpose of this review is to highlight the biochemical, histologic, and clinical evidence that supports an equally important role for progesterone in the growth of uterine myomas. Biochemical studies suggest that progesterone, progestins, and the progesterone receptor modulate myoma mitotic activity. A hypothesis to explain the pathogenesis of myomas is presented.
Key words: fibroids, leiomyoma, myomas, pathogenesis, progesterone, uterus. --
Environ Health Perspect 108(suppl 5):791-793 (2000).
http://ehpnet1.niehs.nih.gov/docs/2000/suppl-5/791-793rein/abstract.html
This article is based on a presentation at the conference on Women's Health and the Environment: The Next Century--Advances in Uterine Leiomyoma Research held 7-8 October 1999 in Research Triangle Park, North Carolina, USA. An earlier version of this review was published previously [Rein MS, Barbieri RL, Friedman AJ. Progesterone: a critical role in the pathogenesis of uterine myomas. Am J Obstet Gynecol 172:14-18 (1970)].
Address correspondence to M.S. Rein, Dept. of Obstetrics and Gynecology, The North Shore Medical Center, 79 Highland Ave., Ste. 316, Salem MA 01970-2370 USA. Telephone: (978) 777-1070. Fax: (978) 774-9635. E-mail: rein@nsmc.partners.org
Received 23 February 2000; accepted August 14 2000.
Symptomatic uterine leiomyomas or myomas are a major public health problem for women of reproductive age. Uterine myomas are monoclonal tumors derived from a single myometrial cell (
1). However, the factors involved in their initiation and growth remain poorly understood. The neoplastic transformation of myometrium to leiomyoma likely involves somatic mutations of normal myometrium and the complex interactions of sex steroids and local growth factors. This review highlights a critical role for progesterone as a promoter of myoma growth. This version of a previously published review (
2) has been updated to include information supporting the progesterone hypothesis. However, whether progesterone is equally, more, or less important than estrogen as a promoter of myoma growth remains unknown.
Somatic mutation is a potential initial event in tumorigenesis. However, neoplastic transformation can occur in the absence of a somatic mutation. For example, estrogen appears to contribute to tumor formation in rodents in the absence of a somatic mutation. In addition, the possibility of other epigenetic mechanisms exists as well. Somatic mutations may include a variety of chromosomal aberrations ranging from point mutations to chromosomal loss or gain. The large chromosomal abnormalities such as translocations and deletions are often detected with standard cytogenetic karyotypes. A major goal in the rapidly emerging field of myoma biology is to identify the initiators of tumorigenesis.
The importance of somatic mutations in the pathogenesis of uterine myomas is supported by several lines of evidence. First, the independent, monoclonal origin of individual myomas suggests that somatic mutations offer a selective growth advantage to the mutated myocyte. In addition, a variety of chromosomal rearrangements (3) are associated with myomas. The most common chromosome aberrations involve chromosome bands 12q14-15 and 7q22. The heterogeneity of the cytogenetic abnormalities suggests that a number of different somatic mutations may be involved in myoma tumorigenesis. The somatic mutations in individual myomas may be the biologic basis for the differential responsiveness of individual myomas to a variety of growth-promoting agents. Clonal proliferation appears to precede the development of cytogenetic rearrangements (4). This may suggest epigenetic mechanisms contribute to tumorigenesis or, alternatively, that somatic mutations not detectable cytogenetically are the initial events in myoma tumorigenesis. This finding also explains the absence of cytogenetic abnormalities in a large portion of myoma specimens.
Traditionally, estrogen has been considered the major promoter of myoma growth. There is significant biochemical evidence to support an important role for estrogen in the stimulation of myoma growth. The long-term administration of a gonadotropin-releasing hormone (GnRH) agonist is associated with both hypoestrogenemia and a reduction in myoma volume (5). Otubu et al. (6) found the concentration of estradiol to be significantly higher in myomas than in normal myometrium. Yamamoto et al. (7) demonstrated a significantly lower conversion of estradiol to estrone in myomas compared with myometrium. Rein et al. (8) demonstrated a significantly increased concentration of estrogen receptors in myomas compared with that in autologous myometrium. Although these observations suggest that the intramyoma hormonal milieu is hyperestrogenic, there is no evidence that estrogen directly stimulates myoma growth.
The mitogenic effects of estrogen are likely mediated by other factors and their receptors. Several estrogen-regulated genes have been confirmed in uterine myomas. There is increasing evidence to suggest that estrogen stimulation of the progesterone receptor (9), epidermal growth factor (10), and insulinlike growth factor-I (11) is involved in myoma growth. Estrogen also appears to be involved in the regulation of myoma extracellular matrix. Estrogen directly stimulates collagen types I and III (12) mRNA as well as the gap junction protein connexin-43 (13.) Estrogen also stimulates local production of the parathyroid hormone-related peptide (14). The expression of these estrogen-regulated genes appears to be greater in uterine myomas than in the adjacent myometrium. On the basis of these observations, Andersen et al. (13) have suggested that hypersensitivity to estrogen may be important in the pathogenesis of myomas.
The clinical observations that have traditionally supported the estrogen hypothesis also support the hypothesis that progesterone is critically involved in the pathogenesis of uterine myomas. Similar to estrogen levels, progesterone levels are cyclically elevated during the reproductive years, significantly elevated during pregnancy, and suppressed after the menopause and during GnRH agonist therapy. Thus it is difficult to distinguish the relative importance of estrogen versus progesterone in these clinical scenarios. This review presents the biochemical, histologic, and clinical evidence that supports an important role for progesterone in the development and growth of uterine myomas.
In 1949, Segaloff et al. (
15) reported increased cellularity and mitotic activity in uterine myomas obtained from six patients treated with 20 mg progesterone daily for 30-189 days. Over the years, additional biochemical evidence has accumulated to suggest a central role for progesterone in the growth of uterine myomas. Tiltman (
16) demonstrated significantly higher mitotic activity on the basis of the number of mitoses per high-power field in myomas obtained from women treated with medroxyprogesterone acetate when compared with an untreated control group. Kawaguchi et al. (
17) investigated the role of menstrual cycle phase on the rate of myoma mitoses. Myoma specimens from 181 patients revealed that the mitotic count was significantly higher in the secretory phase (12.7/100 high-powered fields) than in the proliferative phase (3.8/100 high-powered fields) or during menses (8.3/100 high-powered fields.) The authors concluded that the increased mitotic activity in the secretory phase suggests that myoma growth is affected by progesterone. Lamminen et al. (
18) compared proliferative activity of myomas from premenopausal and postmenopausal women. A quantitative proliferation index was determined by automatic image analysis on myoma tissue sections stained with a proliferating cell nuclear antigen antibody in an immunoperoxidase procedure. The quantitative proliferation index was defined as the percentage of proliferating cell nuclear antigen-stained area in the total nuclear area. The quantitative proliferation index was significantly higher in premenopausal myoma specimens than in postmenopausal specimens. Myomas from postmenopausal women receiving either no hormone replacement or estrogen replacement alone demonstrated low proliferative activity. In contrast, myomas from postmenopausal women receiving combined estrogen and progestin replacement demonstrated a proliferative index equal to that observed in premenopausal women.
Kawaguchi et al. (19) also studied the ultrastructural features of cultured smooth muscle cells from uterine myomas and normal myometrium. Myoma and myometrial cells in estrogen and progesterone-containing medium appeared more active under the electron microscope than cells in estrogen-containing or control medium. Myoma cells exposed to estrogen and progesterone revealed an increased number of myofilaments with dense bodies, suggesting that progesterone is involved in myoma differentiation. Brandon et al. (20) recently demonstrated increased progesterone receptor mRNA expression, as well as increased progesterone receptor protein levels, in myoma tissue when compared with adjacent myometrium. These authors also demonstrated that the proliferation antigen Ki-67 was significantly elevated in myoma tissue, suggesting that amplified progesterone receptor-mediated signaling is associated with myoma growth. Harrison-Woolrych et al. (21) recently reported that epidermal growth factor mRNA production in myomas is increased only during the secretory phase of the menstrual cycle, suggesting progesterone, not estrogen, is the major mediator of myoma epidermal growth factor production. In vitro, progesterone upregulates the cell survival gene BCL2 in leiomyoma cell cultures. The increased levels of BCL2 may inhibit the normal process of programmed cell death (apoptosis), extending the potential for tumor growth (22). In summary, there is significant biochemical and histologic evidence supporting a central role for progesterone in the proliferation of myoma growth.
In 1961, Mixson and Hammond (
23) reported the treatment of 16 patients with uterine myomas with the synthetic progestin norethynodrel. The dose was the amount necessary to induce and maintain amenorrhea and ranged from 20 to 40 mg daily. Fifteen of 16 patients demonstrated significant enlargement of the uterus, as assessed by serial clinical examinations. There was some decrease in uterine size in all patients 12 weeks after discontinuation of progestin therapy. A return to pretreatment size was noted in 70% of the patients during the follow-up period. These authors concluded that norethynodrel causes rapid but reversible enlargement of uterine myomas.
More recently, several clinical trials evaluating the medical treatment of myomas with GnRH agonist plus steroid add-back strongly suggest an important role for progesterone in myoma growth. Friedman et al. (24) studied 16 patients with uterine myomas who were randomized to receive daily subcutaneous leuprolide acetate (0.5 mg) with daily medroxyprogesterone acetate (20 mg) or placebo for 6 months. Patients treated with leuprolide plus medroxyprogesterone acetate demonstrated no significant reduction in uterine volume, as assessed by ultrasonography, when compared with the 50% reduction noted among patients treated with leuprolide plus placebo. Carr et al. (25) reported on 16 women with large myomas who were randomized to receive subcutaneous leuprolide acetate (1 mg/day) plus medroxyprogesterone acetate (20 mg/day) for 12 weeks, or subcutaneous leuprolide acetate (1 mg/day) plus placebo for 12 weeks. Uterine and myoma volume were assessed with magnetic resonance imaging studies. Similar to the findings of Friedman et al. (24), there was no significant change in uterine or myoma volume during simultaneous treatment with leuprolide plus medroxyprogesterone acetate. Although significant reduction was found in patients treated with leuprolide alone, sequential treatment with medroxyprogesterone acetate resulted in a significant increase in uterine volume. Thus, medroxyprogesterone acetate appears to inhibit the ability of GnRH agonist-induced hypoestrogenism to shrink uterine myomas.
In another prospective, randomized clinical trial, Friedman et al. (26) studied 51 women over 12 months and compared intramuscular leuprolide (3.75 mg every 28 days) plus estrogen-progestin (0.75 mg estropipitate daily, 0.7 mg norethindrone on days 1-14) with intramuscular leuprolide (3.75 mg every 28 days) plus a higher dose of progestin only (10 mg norethindrone daily). In contrast to the aforementioned trials, GnRH agonist was given alone for 12 weeks, and then the steroid add-back was initiated in a sequential regimen. Mean uterine volume decreased to 64% of pretreatment size (i.e., a 36% size reduction) by treatment week 12 in both groups. After estrogen-progestin add-back, uterine volume was 69% of pretreatment size at week 24 and 75% of pretreatment volume a week 52 (p < 0.05 vs pretreatment volume). In the progestin add-back group, uterine volume increased to 86% of pretreatment size at week 24 and to 92% of pretreatment size by week 52 (p not significant vs pretreatment volume). This study suggests that high-dose norethindrone can reverse the effectiveness of GnRH agonist-induced myoma shrinkage in a dose-dependent fashion.
The observation that uterine myomas regress in response to the antiprogesterone agent RU-486 offers additional support for an important role of progesterone in the pathogenesis of uterine myomas. Murphy et al. (27) examined the effects of daily RU-486 (50 mg) for 3 months in 10 patients with uterine myomas. Amenorrhea was induced in all patients studied, and myomectomy or hysterectomy was performed in 6 of 10 patients at the end of the treatment period. Myoma volume was assessed by serial ultrasonographic examinations and revealed a 49% reduction after 12 weeks. Serum estradiol, estrone, and progesterone receptor remained unchanged from early follicular phase values. Immunohistochemical examination revealed a significant reduction in progesterone receptor but not estrogen receptor, suggesting that myoma regression may be mediated through a direct antiprogesterone effect. In summary, significant clinical evidence has accumulated to support a critical role for progesterone in the proliferation of uterine myoma volume.
Myoma tumorigenesis may involve somatic mutations of normal myometrium and the complex interactions of sex steroids and local growth factors. Traditionally, estrogen has been considered the major promoter of myoma growth. However, this review supports an equally important role for progesterone in the development and growth of uterine myomas. Figure 1 depicts a new hypothesis by applying the oncogenic theory of separate tumor initiators and promoters to explain myoma pathogenesis.
Figure 1. The initiation and growth of myomas likely involves a multistep cascade of separate tumor initiators and promoters. ER, estrogen receptor; PR, progesterone receptor. The initial neoplastic transformation of the normal myocyte involves somatic mutations. Although the initiators of the somatic mutations remain unclear, the mitogenic effect of progesterone may enhance the propagation of somatic mutations. Myoma proliferation is a result of clonal expansion and likely involves the complex interactions of estrogen, progesterone, and local growth factors. Estrogen and progesterone appear equally important as promoters of myoma growth.
Our findings suggest that progesterone, progestins, and progesterone receptor stimulate mitotic activity and promote myoma proliferation. Progestins stimulate myoma growth despite a hypoestrogenic hormonal milieu. Biochemical, pathologic, and clinical evidence have been presented to support this hypothesis. Current data suggest that progesterone is a critical sex steroid that modulates myoma mitotic activity. Our hypothesis also suggests that the rate of mitosis strongly influences the propagation of somatic mutations. Thus, in a cyclic fashion, progesterone may contribute to myoma formation by increasing the likelihood of somatic mutations and stimulating myoma growth.
The mechanism by which progestins stimulate myoma growth is not known. Our hypothesis suggests that estrogen-induced expression of progesterone receptor is one mechanism of myoma growth. This is supported by the demonstration of Sadovsky et al. (28) of estrogen-dependent progesterone receptor expression in a transformed uterine myocyte cell line. Adams et al. (9) recently studied myoma estrogen receptor and progesterone receptor during the follicular and luteal phase, and concluded that estrogen upregulates both estrogen and progesterone receptor, whereas progesterone downregulates only estrogen receptor. The activation of progesterone receptor may stimulate local growth factor production and/or growth factor receptors.
Alternatively, progestin stimulation of myoma proliferation may be due to cross-reaction with the androgen receptor. Reddy and Rose (29) reported a significant increase in 5-reductase activity in myoma compared with endometrium and myometrium, suggesting that androgens may be involved in myoma pathogenesis. Future clinical trials with steroid add-back will likely compare different doses of different progestins. These studies may help clarify the specificity of the progesterone effect, as there may be important differences in the effects of various progestins and progesterone on myoma growth.
In summary, our hypothesis suggests that the development and growth of myomas involves a multistep cascade and highlights a critical role for progesterone as a promoter of myoma growth. Future studies will be necessary to further elucidate the complex interactions and specific roles of the various initiators and promoters of myoma growth. Expanding our knowledge of the biologic characteristics of myomas may facilitate further advances in the clinical management of these common tumors.
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Last Updated: October 3, 2000