The 40 S ribosomal S6 kinase 1 (S6K1) acts downstream of mTOR (mammalian target of rapamycin) and is sensitive to inhibition by rapamycin. The chromosomal region 17q23 containing the RPS6KB1 gene is frequently amplified in breast cancer cells, leading to S6K1 overexpression. The role of S6K1 in disease development and progression is supported by the observation that S6K1 overexpression is associated with poor prognosis in breast cancer patients. However, the identity of mammary cell-specific S6K1 targets is not well understood. In this study, we report that overexpression of S6K1 endows breast cancer cells with a proliferative advantage in low serum conditions and enhanced sensitivity to rapamycin. We investigate the molecular mechanism behind this observation to show that S6K1 regulates estrogen receptor ␣ (ER␣) by phosphorylating it on serine 167, leading to transcriptional activation of ER␣. By contributing to the activation of ER␣, S6K1 promotes ER␣-mediated cell proliferation and may be a target of therapeutic intervention in breast cancer. mTOR (mammalian target of rapamycin) is a conserved protein kinase that is a key regulator of cell growth and proliferation in response to extracellular cues, including nutrient availability and growth stimuli. Rapamycin is a naturally derived inhibitor of mTOR that was revealed to be an inhibitor of cell proliferation, as manifested by its potent immunosuppressive properties and activity against solid tumors (1). The 40 S ribosomal S6 kinase 1 (S6K1) 2 is one of the best characterized downstream targets of mTOR. Rapamycin treatment results in rapid dephosphorylation and inactivation of S6K1 (2). S6K1 is an important regulator of cell size control, protein translation, and cell proliferation (3). The 40 S ribosomal protein S6 is the best characterized target of S6K1 (4). Other targets have been reported, some of which include the apoptotic protein Bad, the eukaryotic elongation factor 2 kinase, the eukaryotic translation initiation factor 4B (eIF4B), the RNA-binding protein SKAR, and the translational inhibitor PDCD4 (5-9). It appears that S6K1 regulates its targets to increase the biosynthetic capacity of the cell that is necessary for cell division (9 -11).Data suggest that S6K1 is implicated in breast cancer. S6K1 is encoded by the RPS6KB1 gene localized to the chromosomal region 17q23. Region 17q23 is amplified in several breast cancer cell lines and in ϳ30% of primary tumors (12), whereas S6K1 is overexpressed in the majority of cell lines and primary tumors with this amplification (13-17). Furthermore, the role of S6K1 in disease development and progression is supported by the observation that RPS6KB1 amplification and S6K1 overexpression are associated with poor prognosis in breast cancer patients (13,18).Interestingly, whereas RPS6KB1 is amplified in several cancer types, high level (multicopy) amplification of RPS6KB1 is limited to breast cancer (16). This suggests that S6K1 may have a specific role in regulating the growth of breast cancer cells. Overexpression...

Nucleolar channel systems (NCSs) are membranous organelles appearing transiently in the epithelial cell nuclei of postovulatory human endometrium. Their characterization and use as markers for a healthy receptive endometrium have been limited because they are only identifiable by electron microscopy. Here we describe the light microscopic detection of NCSs using immunofluorescence. Specifically, the monoclonal nuclear pore complex antibody 414 shows that NCSs are present in about half of all human endometrial epithelial cells but not in any other cell type, tissue or species. Most nuclei contain only a single NCS of uniform 1 μm diameter indicating a tightly controlled organelle. The composition of NCSs is as unique as their structure; they contain only a subset each of the proteins of nuclear pore complexes, inner nuclear membrane, nuclear lamina and endoplasmic reticulum. Validation of our robust NCS detection method on 95 endometrial biopsies defines a 6-day window, days 19-24 (±1) of an idealized 28 day cycle, wherein NCSs occur. Therefore, NCSs precede and overlap with the implantation window and serve as potential markers of uterine receptivity. The immunodetection assay, combined with the hitherto underappreciated prevalence of NCSs, now enables simple screening and further molecular and functional dissection.