No REST for fibroids

Good news: A long-awaited model for uterine fibroid tumor research is upon us (1). Historically, there has been a paucity of in vivo models for the study of fibroid tumors (i.e., leiomyomas), benign female pelvic organ tumors characterized by clonal proliferation of smooth muscle cells and expression of genes involved in fibrosis and extracellular matrix (ECM) production. This is staggering, considering that prevalence rates of leiomyomata in women at 50 y of age are nearly 70% in white women and more than 80% in black women (2). This level of penetrance contributes to the high number of hysterectomies performed every year in the United States that equates to crippling medical costs reported to be greater than $2 billion annually (3, 4).

In PNAS, Varghese et al. (1) find that the loss of the RE1 silencing transcription factor (REST), a transcriptional repressor, in fibroids resulted in the up-regulation of the G protein-coupled receptor GPR10, which, when activated, increased the proliferation of cultured primary human leiomyoma cells (Fig. 1). They describe their development of a transgenic mouse model that exhibits the leiomyoma phenotype upon expression of GPR10 in uterine smooth muscle cells. This journey began by examining a database (Gene Expression Omnibus dataset GSE13319) that allows comparison of normal human myometrial samples vs. patient fibroid (i.e., leiomyoma) samples (5). They identify GPR10, a receptor known to activate PI3K/AKT signaling, as the most highly dysregulated transcript in fibroids. This observation is important because the PI3K/AKT and mammalian target of rapamycin (MTOR) signaling pathway has been previously implicated in the proliferation of leiomyomas or cells derived from leiomyomas (6–8).

Fig. 1.

Loss of REST in patient leiomyoma cells resulted in the expression of the G protein coupled receptor GPR10 and activation of the AKT/MTOR signaling pathway. Transgenic overexpression of GPR10 in mouse myometrial smooth muscle cells resulted in a phenotype characteristic of human fibroid disease.

The finding that GPR10 may be involved in the genesis of fibroids is surprising because GPR10 was identified as a receptor for prolactin-releasing peptide (PrRP) and is highly expressed in the pituitary (9). Coupled with the expression of GPR10 in patient’s leiomyoma samples was an increase in the activation of AKT and MTOR. Patient leiomyoma cells responded to treatment with a PrRP-31 peptide with an increase in PI3K/AKT/MTOR signaling and proliferation (Fig. 1), a response not evident in benign normal myometrial-derived smooth muscle cells. Despite the intriguing correlation, it is unknown whether endogenous PrRP contributes to the genesis of fibroids in vivo. Other reports link alterations in GRP10 to metabolic disorders (10), suggesting that additional factors may control the activation of GPR10. The study of Varghese et al. (1) raises many questions and offers possibilities for answering others by using the transgenic mouse model it describes.

GPR10 overexpression in the mouse results in myometrial hyperplasia with ECM deposition mimicking that observed with uterine fibroids. Unlike the human phenotype, in which multifocal lesions are common, the mouse phenotype appears to encompass the bulk of the myometrial component. There is no doubt this is attributed to the driver system allowing targeted overexpression to uterine smooth muscle cells. Despite the differences, it raises several interesting questions related to the cause of focal genesis of the lesions in the human. Adding the fact that fibroids are considered clonal, how is it that spontaneously yet simultaneously localized leiomyoma arise in or on the uteri?

The mouse model developed a pronounced uterine fibroid-like phenotype. Despite the cause and effect of GPR10 overexpression with increased deposition of ECM, the authors appreciate that this is an oversimplification based on receptor expression alone, and is likely only the beginning of an anticipated complex story. Future studies are needed to determine if knockdown or loss of REST in benign myometrial cells is sufficient to influence their proliferation rate or deposition of ECM. REST transcriptionally represses GPR10 in cell lines (11), which begs the question of whether restoration of REST expression in cultured leiomyoma cells is sufficient to reverse the aberrant proliferation rate or increased ECM deposition characteristic of leiomyoma cells. Theoretically, REST can target and suppress more than 2,000 gene promoters, including many involved in fibrosis and matrix production. It will be important to determine whether forced knockdown of REST in benign myometrial cells, which is sufficient to increase GPR10 expression, will ultimately drive the benign transformation coupled to the formation of leiomyoma. Would knockdown of REST concurrent with an overexpression of GPR10 result in a more pronounced and/or aggressive leiomyoma phenotype in the mouse? Future studies could explore whether the expression of the REST gene or the stability of the REST protein are malleable targets for therapeutic intervention.

Of considerable interest is the observation that REST can interact with MED12 (a product of the mediator complex subunit 12 gene), an important component of the cellular transcription apparatus. Somatic mutations are present in MED12 in sporadic leiomyoma and are present in between 37% and 70% of samples analyzed (12–14), providing ample evidence to suggest a role of MED12 as a potential oncogene. Importantly, these mutations are shared by uterine leiomyoma and uterine leiomyosarcoma (14), which clinically are considered very different diseases.

In this study, Varghese et al. (1) are unable to detect altered interactions between REST and MED12. It would be of interest to determine whether a gain-of-function mutation in MED12, along with loss of REST concurrent with elevated expression of GPR10, results in an overall increased risk for disease, worsened prognosis, or reduced response to current pharmacologic therapies.

Currently, the first-line therapy for women with leiomyomas is suppression of ovulation and endogenous sex hormone synthesis with various medical agents, including gonadotropin-releasing hormone

The study by Varghese et al. demonstrates that activation of GRP10 results in the activation of the P13K/AKT/MTOR signaling pathway.

agonists. Unfortunately, there is significant variability in the response to the different medical treatment options. In the event these medical strategies are unsuccessful, these benign tumors are surgically removed by myomectomy or hysterectomy. In response to the lack of success in developing reliable, tolerable, and effective medical therapies, there has been an increasing interest in the investigation of novel targets for the development of alternative treatment modalities that are effective, well-tolerated, and have a durable response. A better understanding of the signaling pathways responsible for the genesis, growth, and spread of leiomyomas might provide a potential target. To date, some specific growth factor receptor pathways have been targeted clinically [e.g., EGF receptor and transforming growth factor beta (TGFB) receptor] (15); however, more studies are needed. Likewise, selective steroid receptor modulators have been used for the treatment of fibroids (15, 16). Studies are needed to determine whether endogenous steroids and exogenous hormone therapy affect the expression and activity of REST and GPR10.

The study by Varghese et al. (1) demonstrates that activation of GPR10 results in the activation of the PI3K/AKT/MTOR signaling pathway (Fig. 1). There are a number of growth factor related receptors that, upon activation, result in PI3K/AKT/MTOR signaling and are believed to contribute to myometrial cell function (15). Although this study focuses on the AKT pathway, it seems unlikely that activation of this pathway alone results in the genesis of uterine fibroids. However, it is important to appreciate that the PI3K/AKT/MTOR pathway is influenced negatively or positively by feedback mechanisms, as well as by cross-talk with members of other well-recognized signaling pathways, including the RAS/RAF/MEK (17) and 5′ AMP-activated protein kinase (18) pathways. Nevertheless, it does not rule out the potential effectiveness of the various PI3K/AKT/MTOR inhibitors delivered individually or as part of a combination regimen. It will be important that future studies include inactivation or activation of these other pathways singularly or in combination to better appreciate the contribution or the signaling pathways activated by GPR10 to the leiomyoma phenotype.