Pregnancy, parturition, and prostaglandins: defining uterine leiomyomas.

Leiomyomas, benign smooth muscle tumors of the uterus, are the most common gynecologic neoplasm in women. Studies with surgically resected human tissues and primary cultures have revealed that several genes are differentially expressed in leiomyomas compared to matched normal myometrium. An estrogen-driven pattern of gene expression in leiomyomas, similar to that seen in normal myometrium during pregnancy and parturition, is associated with a persistent inappropriate response of neoplastic myometrial smooth muscle cells to ovarian hormones. This is possibly due to aberrant expression levels or signaling via estrogen and progesterone receptors. We propose the hypothesis that uterine leiomyomas mimic a differentiated myometrial cell at pregnancy and exhibit a hypersensitivity to sex steroid hormones that prevents the cells from responding to normal apoptotic or dedifferentiation signals and from returning to a nongravid phenotype. Support of this hypothesis is derived from experimental studies in female Eker rats that develop uterine leiomyomas with many similarities to the human disease. Our hypothesis accounts for the benign nature of these tumors and their high incidence in women during the reproductive years. By identifying the factors that participate in parturition and involution of the pregnant myometrium, we may better define uterine leiomyomas and thus identify novel targets for therapeutic strategies to treat these tumors.