Dysregulation of IGF-I signaling in uterine leiomyoma.

IGF-I expression has been observed in human uterine leiomyomas. To examine whether autocrine IGF-I signaling plays a role in the growth of these tumors, we used an animal model of uterine leiomyoma (the Eker rat) to investigate regulation of IGF-I and the IGF-I receptor (IGF-IR) expression in tumors and normal myometrium. During the normal estrous cycle, myometrial IGF-I expression peaked on the day of proestrus when the rate of proliferation in this tissue is greatest. In leiomyomas, the expression of IGF-I was increased 7.5-fold compared with the age-matched normal tissue. The level of IGF-IR mRNA in both tumor and non-tumor tissues was found to inversely correlate with that of IGF-I. Changes observed in IGF-I signaling components correlated with the activation state of the signal-transducing protein insulin receptor substrate-1 (IRS-1). During diestrus and proestrus when IGF-I levels were increasing, tyrosine phosphorylation of IRS-1 was increased up to 5.7-fold in the normal myometrium relative to estrus, when IGF-I levels were the lowest. Additionally, IRS-1 phosphorylation was 4-fold greater in leiomyomas relative to age-matched normal myometrium. Autocrine stimulation of the IGF-IR may, therefore, play a role in regulating the normal growth of the myometrium, and dysregulation of IGF-I signaling could contribute to the neoplastic growth of uterine leiomyomas.

Altered hormonal responsiveness of proliferation and apoptosis during myometrial maturation and the development of uterine leiomyomas in the rat.

Uterine leiomyomas are responsive to the ovarian steroids, estrogen and progesterone; however, a mechanistic understanding of the role of these hormones in the development of this common gynecologic lesion remains to be elucidated. We have used the Eker rat uterine leiomyoma model to investigate how ovarian hormones regulate or promote the growth of these tumors. Proliferative and apoptotic rates were quantitated in normal uterine tissues and leiomyomas in response to endogenous ovarian steroids. In 2- to 4-mo-old animals, cell proliferation in the normal uterus corresponded with high serum levels of steroid hormones during the estrous cycle, and apoptosis occurred in the rat uterus in all cell types following sharp, cyclical declines in serum hormone levels. It is interesting that the responsiveness of uterine mesenchymal cells changed between 4 and 6 mo of age, with significant decreases in both proliferative and apoptotic rates observed in myometrial and stromal cells of cycling animals. Leiomyomas displayed much higher levels of proliferation than did age-matched myometrium; however, their apoptotic index was significantly decreased in comparison with normal myometrium. This disregulation between proliferative and apoptotic responses, which were tightly regulated during ovarian cycling in the normal myometrium, may contribute to the disruption of tissue homeostasis and underlie neoplastic growth of these tumors.

Efficacy of LGD1069 (Targretin), a retinoid X receptor-selective ligand, for treatment of uterine leiomyoma.

The conventional treatment of uterine leiomyomas, or fibroids, with gonadotropin-releasing hormone (GnRH) agonists is often associated with serious side effects, necessitating short-term, palliative use of this therapy. Therefore, we examined a retinoid X receptor (RXR)-selective ligand, LGD1069, as a possible treatment for leiomyoma. LGD1069 has demonstrated efficacy as a chemopreventive agent in the N-nitroso-N-methylurea (NMU)-induced rat mammary carcinoma model and is a therapeutic agent in several epithelial tumor models. Previous studies have shown that it has both antitumor effects and antiestrogenic activity in the rat uterus, suggesting the potential utility of this agent for treatment of hormonally dependent uterine fibroids. The expression of retinoid receptors in tumors and cell lines derived from leiomyomas arising in the Eker rat was confirmed by Northern analysis. After treatment for 4 months with LGD1069, the number of grossly observable tumors was substantially reduced although the total incidence of tumors, including microscopic lesions, remained unaffected, suggesting an effect of the compound on tumor growth kinetics rather than on tumor initiation. Analysis of terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) staining and determination of 5-bromo-2-deoxyuridine (BrdU) incorporation indicated that the reduction in grossly observable tumors that occurred in treated animals was mediated by a significant increase in the level of apoptosis rather than a decrease in cell proliferation. These results suggest that LGD1069 may be an effective therapeutic agent for uterine leiomyoma that may inhibit tumor growth and, consequently, alleviate the symptoms associated with this disease.

Preclinical evidence for therapeutic efficacy of selective estrogen receptor modulators for uterine leiomyoma.

OBJECTIVE: Uterine leiomyoma are the most common gynecologic neoplasm and a primary cause of hysterectomy in premenopausal women. Preclinical studies were conducted in the Eker rat model to investigate the potential efficacy of selective estrogen receptor modulators (SERMs) as therapeutic agents for this tumor. METHODS: Twelve-month-old Eker rats were randomized into five treatment arms including tamoxifen, placebo, LY 326315, vehicle, and no treatment. Additional animals received ovariectomy or sham surgery at 4 months of age to determine the effect of ovarian ablation on tumor development. The study was terminated after 2 to 4 months of treatment, and tumor incidence, size, proliferative and apoptotic indices were determined. Size and incidence data were subjected to chi-square analysis. One-way analysis of variance and Fisher's least significant difference tests were used to compare proliferative and apoptotic indices. RESULTS: Ovariectomy virtually ablated leiomyoma development, indicating that these tumors were dependent on ovarian hormones for growth and development. Treatment with tamoxifen or raloxifene analog LY 326315 reduced leiomyoma incidence by 40-60% and reduced the size of remaining tumors. The effect of SERMs on leiomyomas was mediated by a decrease in cell proliferation without a decrease in apoptotic index. CONCLUSION: SERMs have been shown to be therapeutically efficacious against breast cancer and to reduce tumor incidence in women at increased risk for this disease. The present data indicate that therapeutic efficacy may also be extended to uterine leiomyoma and demonstrate the utility of this animal model for preclinical studies to identify new therapeutic modalities.

Regulation of apoptosis in uterine leiomyomata.

Tumors developing from hormone-dependent tissues, such as the breast and prostate, have been successfully treated in the clinic by methods of hormone ablation, and the resulting tumor regression has been shown to occur at least in part by the process of apoptosis. The growth of leiomyomas arising from uterine smooth muscle cells is similarly modulated by circulating steroid hormones and has been associated with periods of increased estrogen secretion. The inhibition of ovarian hormone production by endocrine therapy often results in the regression of these tumors, but the role of apoptosis in this process has not been elucidated. Using cell lines derived from the Eker rat model of uterine leiomyoma, we have investigated the mechanism of growth inhibition by estrogen deprivation. Estrogen-depleted medium and the antiestrogen tamoxifen significantly reduced cell numbers in culture and arrested cell proliferation, but did not induce apoptosis. However, the presence of an intact apoptotic pathway was demonstrated in these cells by serum starvation. In vivo data were in agreement with in vitro results, which showed that tamoxifen treatment does not change the apoptotic rate of leiomyoma tissues. Therefore, growth modulation of leiomyomas by hormone deprivation occurs via mechanisms independent of apoptosis, indicating a fundamental difference in the response of leiomyomas to hormone deprivation from that of tumors of the breast and prostate. These data suggest that creation of a hypoestrogenic milieu within leiomyomas reduces tumor volume without inducing a concomitant increase in the rate of apoptosis, which may be responsible for the limited effectiveness of currently available medicinal therapies.