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.

bAntiestrogen therapy induces the unfolded protein response (UPR) in estrogen receptor-positive (ER ؉ ) breast cancer. X-box binding protein 1 (XBP1), which exists in the transcriptionally inactive unspliced form [XBP1(U)] and the spliced active form [XBP1(S)], is a key UPR component mediating antiestrogen resistance. We now show a direct link between the XBP1 and NF-B survival pathways in driving the cell fate decisions in response to antiestrogens in ER ؉ breast cancer cells, both in vitro and in a xenograft mouse model. Using novel spliced and nonspliceable forms of XBP1, we show that XBP1(U) functions beyond being a dominant negative of XBP1(S). Both isoforms regulate NF-B activity via ER␣; XBP1(S) is more potent because it also directly regulates p65/RelA expression. These findings provide new insights into the fundamental signaling activities of spliced and unspliced XBP1 in breast cancer, establish NF-B to be a mediator of these activities, and identify XBP1 and its splicing to be novel therapeutic targets.A pproximately 70% of all newly diagnosed breast cancers express the estrogen receptor alpha (ER␣) protein. Antiestrogens, such as tamoxifen (TAM) and fulvestrant (ICI 182,780 [ICI]; Faslodex), are widely used to treat these breast cancers, but resistance, either de novo or acquired, limits their curative potential (1). More patients die from ER␣-positive (ER␣ ϩ ) breast cancer than from any other breast cancer subtype. Identifying the underlying molecular mechanisms of antiestrogen resistance remains a critical and immediate need. The unfolded protein response (UPR) that cells initiate to recover from endoplasmic reticulum (EnR) stress plays a central role in mediating antiestrogen resistance (2).EnR stress is sensed by the EnR transmembrane proteins IRE1, PERK, and ATF6; these proteins then activate downstream signaling to enhance proper protein folding, decrease the rate of protein biosynthesis, and promote cell survival in an attempt to recover homeostasis (3). Some UPR functions are prosurvival, and their expression/activation is often altered in cancer cells. We have previously shown that a key UPR effector, X-box binding protein 1 (XBP1), is involved in antiestrogen resistance in breast cancer (4). XBP1 exists in two forms, an unspliced form [XBP1(U)] and a spliced form [XBP1(S)]. The unconventional cytosolic splicing of XBP1 mRNA by IRE1␣ removes a 26-bp intron, resulting in a translational frameshift and a protein product with a distinct C terminus that can now act as a transcription factor (5, 6). The transactivation domain within the C terminus of XBP1(S) confers its transcription factor activity, whereas the C terminus of XBP1(U) lacks the transactivation domain but contains a nuclear export signal and a protein degradation domain (7).XBP1(S) is a central UPR effector, binding to EnR stress response elements (ERSEs) and unfolded protein response elements (UPREs) in the promoter of its target genes to induce the transcription of EnR chaperones and other genes involved in coping with EnR str...

The peroxisome proliferator-activated receptor-δ (PPARδ) regulates a multitude of physiological processes associated with glucose and lipid metabolism, inflammation and proliferation. One or more of these processes are potential risk factors for the ability of PPARδ agonists to promote tumorigenesis in the mammary gland. In the present study, we describe a new transgenic mouse model in which activation of PPARδ in the mammary epithelium by endogenous or synthetic ligands resulted in progressive histopathological changes that culminated in the appearance of estrogen receptor- and progesterone receptor-positive and ErbB2-negative infiltrating ductal carcinomas. Multiparous mice presented with mammary carcinomas after a latency of 12 months, and administration of the PPARδ ligand GW501516 reduced tumor latency to five months. Histopathological changes occurred concurrently with an increase in an inflammatory, invasive, metabolic and proliferative gene signature, including expression of the trophoblast gene, Plac1, beginning one week after GW501516 treatment, and remained elevated throughout tumorigenesis. The appearance of malignant changes correlated with a pronounced increase in phosphatidylcholine and lysophosphatidic acid metabolites, which coincided with activation of Akt and mTor signaling that were attenuated by treatment with the mTor inhibitor everolimus. Our findings are the first to demonstrate a direct role of PPARδ in the pathogenesis of mammary tumorigenesis, and suggest a rationale for therapeutic approaches to prevent and treat this disease.