Quickly, MCF7 cells were seeded in duplicate into 96-well plates at 5,000 cells/well. eIF3f in ER-positive breast cancer cells compared with ER-negative cells, and determined that low eIF3f levels are required for proper proliferation and survival of ER-positive MCF7 cells. The expression of eIF3f is tightly controlled by ER at the transcriptional Febantel (genomic pathway) and translational (nongenomic pathway) level. Specifically, estrogen-bound ER represses transcription of the gene, SNX13 while promoting eIF3f mRNA translation. To regulate translation, estrogen activates the mTORC1 pathway, which enhances the binding of eIF3 to the eIF4F complex and, consequently, the assembly of the 48S preinitiation complexes and protein synthesis. We observed preferential translation of mRNAs with highly structured 5-UTRs that usually encode factors involved in cell proliferation and survival (cyclin D1 and survivin). Our results underscore the importance of estrogen-ERCmediated control of eIF3f expression for the proliferation and survival of ER-positive breast cancer cells. These findings may provide rationale for the development of new therapies to treat ER-positive breast cancer. tamoxifen), Febantel promoting ER degradation (selective estrogen receptor degraders, fulvestrant), or blocking estrogen biosynthesis (aromatase inhibitors, letrozole, anastrozole, and exemestane) (2). However, their effectiveness is compromised by the emergence of intrinsic or acquired resistance in treated patients (2,C4). Therefore, better understanding of ER-positive breast Febantel cancer biology is critical to development of more effective therapeutic strategies that minimize resistance and cancer recurrence. ER is a nuclear receptor whose activity is primarily regulated by the binding of its ligand estrogen (17-estradiol). Estrogen-ER complex acts as a transcription factor that activates or represses the expression of multiple target genes (genomic pathway) (5, 6). Alternatively, extranuclear ligand-bound ER elicits rapid, stimulatory effects on cytoplasmic signal transduction pathways mediated by the mitogen-activated protein kinase (MAPK)/ERK or the phosphatidylinositol 3-kinase (PI3K)/AKT/mTOR complex 1 (mTORC1), also termed the nongenomic pathway (7). By acting through these signaling pathways, increased levels of estrogen-ER complex promote cell proliferation, cell cycle progression, survival, angiogenesis, invasion, and migration in cancer cells. Regulation of mRNA translation is critical to define the proteome, maintain homeostasis, and control cell proliferation, growth, and development. Protein synthesis occurs in four steps: initiation, elongation, termination, and ribosome recycling, with initiation being the rate-limiting phase (8). The translation initiation comprises: (PI3K and mTOR inhibitors) has shown promising results in preclinical studies and clinical trials (16,C19). However, ineffectiveness or resistance in monotherapy setting and/or toxicity in combination limited their clinical utility. Here we focus on expanding our understanding of the role of the translation initiation Febantel factor eIF3 in tumor biology. eIF3 is a large complex composed of 13 nonidentical subunits (eIF3a-m) in human cells. Current model proposes that the assembly of eIF3 starts with the interaction of eIF3a and eIF3b to form the eIF3 nucleation core. The association of eIF3g and eIF3i to eIF3b gives rise to the subcomplex known as yeast-like core (YLC). Then, the sequential interaction of the seven subunits eIF3c, eIF3e, eIF3f, eIF3h, eIF3k, eIF3l, and eIF3m with eIF3a forms the eIF3 octamer. The nonoctameric eIF3d subunit joins eIF3 complex through its binding to eIF3e (20). Overexpression of eIF3a, eIF3b, eIF3c, eIF3h, eIF3i, and eIF3m, or underexpression of eIF3e and eIF3f have been reported in several cancers, including breast tumors (12, 15, 21). First evidence supporting a role for eIF3 in cancer biology was obtained by ectopic overexpression of individual subunits in NIH3T3 cells. Ectopic expression of eIF3a, eIF3b, eIF3c, eIF3h, and eIF3i stimulates global protein synthesis and translation of mRNAs that encode growth-regulating proteins, and leads to malignant transformation. In contrast, ectopic expression of eIF3e and eIF3f inhibits protein synthesis and decreases cell growth and proliferation (22). Recent studies have demonstrated that changes in the levels of a single eIF3 subunit.