ABSTRACT
Leiomyomata, or fibroids, are benign uterine tumors of unknown etiology that develop from transformation of myometrial smooth muscle cells and/or connective tissue fibroblasts. Leiomyomata have a limited malignant trans formation (<1 %), and despite the presence of multiple tumors seen in the same uterus, they occur independently of metastatic processes. However, symptomatic leiomyomata are the major cause of abnormal uterine bleeding, pelvic pain, and pelvic masses, accounting for more than a third of all the hysterectomies per formed to relieve patients from these and other symptoms.1,2
Because leiomyomata develop during repro ductive years and regress after the menopause, ovarian steroids are considered as their key growth-promoting factors. Clinical observations indicate that a hypoestrogenic condition created by therapy with long-acting analogs of gonadotropin-releasing hormone (GnRHa) often leads to regression of leiomyoma growth. However, the side effects of GnRHa prevent pro longed therapy, and discontinuation results in rapid return of the leiomyomata to their original size. Co-therapy with low doses of cyclic or con
tinuous estrogen "add back" has been effective in reducing the side effects of GnRHa and allow ing prolonged therapy, whereas co-treatment with medroxyprogesterone acetate reverses the beneficial effect of GnRHa therapy.1 In recent years considerable progress has been made toward several new interventional and thera peutic approaches in the field of leiomyoma medical management. Preclinical and clinical assessments of selective estrogen receptor mod ulators (SERM) and selective progesterone receptor modulators (SPRM) indicate potential usefulness of these agents as additional tools in the management of leiomyoma growth.3-7
Despite the progress in medical management of leiomyomata, the identity of molecule(s) that initiate myometrial cellular transformation and subsequently regulate leiomyoma growth remains unknown.8,9 The above therapeutic interventions are known to influence ovarian steroid production and steroid receptor-medi ated signaling, but how they influence the molec ular environment of leiomyomata to bring about their regression remains unknown. In this chapter we will discuss our current understand ing of the leiomyoma molecular environment and regulation by ovarian steroids. We discuss first the importance of ovarian steroids and
molecular mechanisms of their receptor activa tion in overall gene expression, particularly in the utilization of genomic and, possibly, nongenomic pathways that are activated by ovarian steroid receptors, and their downstream signaling leading to regulation of specific target gene expression. Among these genes there is particu lar emphasis on several growth factors, cytokines, and chemokines whose expression and local actions are considered crucial in regu lating leiomyoma cell growth, angiogenesis, apoptosis, and inflammatory reactions leading to leiomyomata' fibrotic characteristics.8,9
The ovarian steroids estrogen and progesterone are central players in regulating all aspects of female reproductive activity. Together and in a coordinated fashion they act at the level of the hypothalamic-pituitary-ovarian axis and uterus, regulating neuroendocrine gonadotropin pro duction, ovulatory activity, and cyclic uterine changes during the reproductive years. In addi tion to their normal physiological functions, ovarian steroids are key to the development of uterine and breast cancers, endometriosis and leiomyoma growth, and other steroid-depend ent reproductive disorders. In this context, estro gen has received considerable attention as the major growth-promoting factor for leiomy omata, although progesterone also influences the outcome of this disorder.8,9
Ovarian steroids implement their biological actions through interaction with specific estro gen receptors (ER) and progesterone receptors (PR). ER and PR are members of a nuclear recep tor superfamily of transcription factors. Estro gen receptors consist of ERa and ERp, encoded by two distinct genes but sharing a high degree of amino acid homology in their DNA-binding (97%) and ligand-binding (58%) domains. It has become apparent that significant differences exist between ERa and ERβ in their pattern of cellular/tissue expression, ligand-binding affini ties, and response elements, resulting in differ ent transcriptional effects at the same site and gene targets. Although both ERa and ERβ are expressed throughout the female reproductive
tract, ERa is the dominant subtype in the uterus, and estrogen-induced uterotrophic responses require functional ERa.10,11 Unlike ER, the prog esterone receptors PR-A and PR-B derive from two promoters on the PR gene, with PR-B dif fering from PR-A protein because of the pres ence of an additional sequence of amino acids at its amino terminus. Both receptors differentially regulate transcriptional activation of their target gene promoters, with PR-B acting as a stronger activator. PR-A and PR-B are expressed in various reproductive tissues; however, their ratio varies depending on developmental and hormonal status. In reproductive tissues, the action of progesterone is mediated primarily through PR-B, whereas PR-A acts as a repressor of PR-B function.12,13
Upon binding to their specific ligands, ER and PR undergo conformational structural changes leading to receptor dimerization, post-transla tional modification, and binding to specific enhancer DNA elements in promoters of their specific target genes. These modifications and the recruitment of a specific group of proteins, called coregulators, are necessary to trigger changes in promoter activity of ER and PR target genes. Modulation of the expression of these genes and their products ultimately results in regulation of specific cellular activities that implement both normal physiological and pathological outcomes of tissue responses to ovarian steroids.10-18 An extensive body of evidence generated from various molecular approaches examining the physiological conse quences of deletion of steroid receptors and their coregulators has provided further insights into their functional diversity in target tissues. Female mice lacking ERa are infertile, but an ERa-mediated uterine proliferative response can be induced by epidermal growth factor in the absence of estrogen.19 This latter observation has provided an important physiological validation of a novel nongenomic action for ligandindependent activation of ERs. The classical genomic and nongenomic actions of ER and PR will be discussed in more detail. Mice lacking PR-A displayed normal mammary gland and thymus development, but showed severe uterine hyperplasia and ovarian abnormalities. Deletion of PR-B resulted in reduced mammary
ductal morphogenesis, but did not affect ovarian, uterine, or thymic responses to proges terone. Female mice lacking both PRs exhibit impaired sexual behavior and neuroendocrine gonadotropin regulation, anovulation, uterine dysfunction, and impaired ductal branching of mammary gland and thymic involution. These specific knockout mouse models have indicated isoform-and tissue-specific functions of PRs and confirm that PR-A and PR-B function as distinct transcription factors, with PR-A both necessary and sufficient to elicit the progesterone-depend ent reproductive responses required for female fertility. The selective ability of PR-A to inhibit ER-and PR-B-induced transcriptional responses is considered to contribute to antiestrogenic activities of progesterone in the uterus.20
