Abstract
IntroductionIn this pre-clinical in vitro study conducted in estrogen receptor positive (ER+) breast cancer cells, we have characterized the effects of insulin-like growth factor I (IGF-1) on the cytostatic and cytotoxic action of antiestrogen treatment when used as a single agent or in combination with the antiprogestin mifepristone (MIF). Our goal was to identify new molecular targets to improve the efficacy of hormonal therapy in breast cancer patients that have a poor response to hormonal therapy, in part, due to high circulating levels of unbound insulinIGF-1.MethodsIGF-1-mediated effects on cytostasis and apoptotic cell death were determined with cell counts conducted in the presence and absence of trypan blue; enzyme-linked immunosorbent assays to determine the intracellular levels of cleaved cytokeratin 18, a marker of epithelial cancer cell apoptosis; and immunoblot analysis to determine the levels of cleaved poly-ADP ribose polymerase (PARP) and lamin A that result from caspase-dependent apoptosis. Cytotoxicity was further characterized by determination of the levels of reactive oxygen species (ROS) and the percent of mitochondrial membrane depolarization in cell populations treated with the different hormones in the presence and absence of IGF-1. Small molecule inhibitors of the dual-specificity protein kinase MEK1, MEK1 siRNA, Bim siRNA, and vectors overexpressing MEK1 wild type and mutant, dominant negative cDNA were used to identify key IGF-1 downstream prosurvival effectors.ResultsIGF-1, at physiologically relevant levels, blocked the cytotoxic action(s) of the antiestrogens 4-hydroxytamoxifen (4-OHT) and tamoxifen (TAM) when used as single agents or in combination with the antiprogestin MIF. The antiapoptotic action of IGF-1 was mediated primarily through the action of MEK1. MEK1 expression reduced the levels of ROS and mitochondrial membrane depolarization induced by the hormonal treatments via a mechanism that involved the phosphorylation and proteasomal turnover of the proapoptotic BH3-only Bcl-2 family member Bim. Importantly, small-molecule inhibitors of MEK1 circumvented the prosurvival action of IGF-1 by restoring Bim to levels that more effectively mediated apoptosis in ER+ breast cancer cells.Conclusionhis study provides strong support for the use of MEK1 inhibitors in combination with hormonal therapy to effectively affect cytostasis and activate a Bim-dependent apoptotic pathway in ER+ breast cancer cells. We discuss that MEK1 blockade may be a particularly effective treatment for women with high circulating levels of IGF-1, which have been correlated to a poor prognosis.
Highlights
In this pre-clinical in vitro study conducted in estrogen receptor positive (ER+) breast cancer cells, we have characterized the effects of insulin-like growth factor I (IGF-1) on the cytostatic and cytotoxic action of antiestrogen treatment when used as a single agent or in combination with the antiprogestin mifepristone (MIF)
Once we established that 20 ng/ml insulin-like growth factor-1 (IGF-1) maximally induced cell proliferation, while blocking cell detachment (Figure 1 and data not shown), this concentration was used in all subsequent experiments, including the experiments shown in Figure 1 (b through d), in which we further characterized the IGF-1 prosurvival action
This study has identified the IGF-1/IGF-IR/MEK prosurvival axis that exists in ER+ breast cancer cells to attenuate significantly the cytotoxic action of antiestrogen and antiprogestin treatment, with little effect on the antiproliferative action of these hormones
Summary
In this pre-clinical in vitro study conducted in estrogen receptor positive (ER+) breast cancer cells, we have characterized the effects of insulin-like growth factor I (IGF-1) on the cytostatic and cytotoxic action of antiestrogen treatment when used as a single agent or in combination with the antiprogestin mifepristone (MIF). The development of resistance to TAM and other antiestrogens occurs often in breast cancer patients and is a major clinical concern [3,5]. To understand the mechanisms of intrinsic and acquired resistance to antiestrogens, numerous in vitro studies have been conducted, and the multiple mechanisms described by these studies have been reviewed [5,6]. Even with antihormonal therapies that severely deplete the estrogenic environment of the breast cancer cells, such as aromatase inhibitors, both inherent and acquired resistance occurs [7]. The fact that antiestrogen resistance is still a major obstacle to successful antiestrogen therapy underscores the importance of investigating new therapies or identifying effective combination therapies for the treatment of ER+ breast cancer
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