While more than 70% of breast cancers express estrogen receptor-α (ER+), endocrine therapies targeting these receptors often fail. The molecular mechanisms that underlie treatment resistance remain unclear. We investigated the potential role of glucose-regulated protein 78 (GRP78) in mediating estrogen resistance. Human breast tumors showed increased GRP78 expression when compared with normal breast tissues. However, GRP78 expression was reduced in ER+ breast tumors compared with HER2-amplifed or triple-negative breast tumors. ER+ antiestrogen-resistant cells and ER+ tumors with an acquired resistant antiestrogen phenotype were both shown to overexpress GRP78, which was not observed in cases of de novo resistance. Knockdown of GRP78 restored antiestrogen sensitivity in resistant cells, and overexpression of GRP78 promoted resistance in sensitive cells. Mechanistically, GRP78 integrated multiple cellular signaling pathways to inhibit apoptosis and stimulate prosurvival autophagy, which was dependent on TSC2/AMPK-mediated mTOR inhibition but not on beclin-1. Inhibition of autophagy prevented GRP78-mediated endocrine resistance, whereas caspase inhibition abrogated the resensitization that resulted from GRP78 loss. Simultaneous knockdown of GRP78 and beclin-1 synergistically restored antiestrogen sensitivity in resistant cells. Together, our findings reveal a novel role for GRP78 in the integration of cellular signaling pathways including the unfolded protein response, apoptosis, and autophagy to determine cell fate in response to antiestrogen therapy.

How breast cancer cells respond to the stress of endocrine therapies determines whether they acquire a resistant phenotype or execute a cell death pathway. A successfully executed survival signal then requires determination of whether or not to replicate. How these cell fate decisions are regulated is unclear but evidence suggests that the signals determining these outcomes are highly integrated. Central to the final cell fate decision is signaling from the unfolded protein response, which can be activated following the sensing of stress within the endoplasmic reticulum. Duration of the response to stress is partly mediated by the duration of inositol requiring enzyme-1 (IRE1; ERN) activation following its release from heat shock protein A5 (HSPA5). The resulting signaling appears to use several B-cell lymphoma-2 (BCL2) family members to both suppress apoptosis and activate autophagy. Changes in metabolism induced by cellular stress are key components of this regulatory system, and further adaptation of the metabolome is affected in response to stress. Here we describe the unfolded protein response, autophagy and apoptosis, and how their regulation is integrated. Central topological features of the signaling network that integrate cell fate regulation and decision execution are discussed.

The mammary glands of prepubertal estrogen receptor (ER)␤؊͞؊ mice are morphologically indistinguishable from those of WT littermates. It appears that, although ER␤ is expressed in the mouse mammary gland, it is not involved in ductal growth of the gland. In this study, we examined the possibility that ER␤ has a role in the differentiated function of the mammary gland. Pregnancy is rare in ER␤؊͞؊ mice, but an intensive breeding program produced seven pregnant ER␤؊͞؊ mice, of which five did not eat their offspring and continued to successful lactation. Histomorphological comparison of lactating glands revealed that alveoli were larger and there was less secretory epithelium in ER␤؊͞؊ than in WT mice. Ultrastructural analysis showed abundant milk droplets and normal apical villi in the luminal epithelial cells, but the extracellular matrix and lamina basalis were reduced, and very frequently the interepithelial cell space was increased. Levels of the adhesion molecules, E-cadherin, connexin 32, occludin, and integrin ␣2 were reduced, and no zona occludens was detectable. In addition, there was widespread expression of the proliferation marker, Ki-67, in luminal epithelial cells in ER␤؊͞؊ but not in WT mice. These findings suggest a role for ER␤ in organization and adhesion of epithelial cells and hence for differentiated tissue morphology. We speculate that, because a reduced risk for breast cancer is conferred on women who breast-feed at an early age, ER␤ could contribute to this risk reduction by facilitating terminal differentiation of the mammary gland.lactation ͉ cadherin ͉ integrin ͉ tight junction T argeted disruption of estrogen receptor (ER)␤ in mice has revealed that this is a functional receptor in both males and females and that, in addition to its role in reproductive functions (1), it is also important in the cardiovascular (2) and central nervous systems (3). Female mice homozygous for the mutated ER␤ gene (ER␤Ϫ͞Ϫ mice) have been described as subfertile or infertile (4, 5). The ovarian defect is due to early atresia of antral follicles and failure to ovulate. As a result, corpora lutea are rare in ER␤Ϫ͞Ϫ mouse ovaries (6).Prepubertal ER␤Ϫ͞Ϫ females appear to have a normal mammary histology (5) with unaffected ductal outgrowth of the mammary gland anlage. However, because corpora lutea are rare, little progesterone is produced in the ovaries and, in contrast to their WT littermates, ER␤Ϫ͞Ϫ mammary glands fail to develop ductal side branches and alveoli after puberty. On administration of progesterone, side branching occurs, and mammary glands of ER␤Ϫ͞Ϫ mice appear morphologically indistinguishable from those of their WT littermates (7). As judged by nursing behavior and growth of their pups, ER␤Ϫ͞Ϫ mice have previously been reported to lactate effectively (5). In contrast, in female mice lacking aromatase (8) or ER␣ (9), mammary glands fail to develop beyond the prepubertal stage. From these studies, it has been concluded (5) that estrogen, acting through ER␣ but not through ER␤, is essential for norm...