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The principal cell of the kidney collecting duct is one of the most highly regulated epithelial cell types in vertebrates. The effects of hormonal, autocrine, and paracrine factors to regulate principal cell transport processes are central to the maintenance of fluid and electrolyte balance in the face of wide variations in food and water intake. In marked contrast with the epithelial cells lining the proximal tubule, the collecting duct is electrically tight, and ion and osmotic gradients can be very high. The central role of principal cells in salt and water transport is reflected by their defining transporters-the epithelial Na(+) channel (ENaC), the renal outer medullary K(+) channel, and the aquaporin 2 (AQP2) water channel. The coordinated regulation of ENaC by aldosterone, and AQP2 by arginine vasopressin (AVP) in principal cells is essential for the control of plasma Na(+) and K(+) concentrations, extracellular fluid volume, and BP. In addition to these essential hormones, additional neuronal, physical, and chemical factors influence Na(+), K(+), and water homeostasis. Notably, a variety of secreted paracrine and autocrine agents such as bradykinin, ATP, endothelin, nitric oxide, and prostaglandin E2 counterbalance and limit the natriferic effects of aldosterone and the water-retaining effects of AVP. Considerable recent progress has improved our understanding of the transporters, receptors, second messengers, and signaling events that mediate principal cell responses to changing environments in health and disease. This review primarily addresses the structure and function of the key transporters and the complex interplay of regulatory factors that modulate principal cell ion and water transport.

Hormone regulation of ion transport in the kidney tubules is essential for fluid and electrolyte homeostasis in vertebrates. A large body of evidence has suggested that transporters and channels exist in multiprotein regulatory complexes; however, relatively little is known about the composition of these complexes or their assembly. The epithelial sodium channel (ENaC) in particular is tightly regulated by the salt-regulatory hormone aldosterone, which acts at least in part by increasing expression of the serine-threonine kinase SGK1. Here we show that aldosterone induces the formation of a 1.0-1.2-MDa plasma membrane complex, which includes ENaC, SGK1, and the ENaC inhibitor Nedd4-2, a key target of SGK1. We further show that this complex contains the PDZ domain-containing protein connector enhancer of kinase suppressor of Ras isoform 3 (CNK3). CNK3 physically interacts with ENaC, Nedd4-2, and SGK1; enhances the interactions among them; and stimulates ENaC function in a PDZ domain-dependent, aldosterone-induced manner. These results strongly suggest that CNK3 is a molecular scaffold, which coordinates the assembly of a multiprotein ENaC-regulatory complex and hence plays a central role in Na(+) homeostasis.

Serum and glucocorticoid regulated kinase 1 (SGK1) has been identified as a key regulatory protein that controls a diverse set of cellular processes including sodium (Na+) homeostasis, osmoregulation, cell survival, and cell proliferation. Two other SGK isoforms, SGK2 and SGK3, have been identified, which differ most markedly from SGK1 in their N[H.sub.2]-terminal domains. We found that SGK1 and SGK3 are potent stimulators of epithelial [Na.sup.+] channel (ENaC)-dependent Na+ transport, while SGK2, which has a short N[H.sub.2] terminus, is a weak stimulator of ENaC. Further characterization of the role of the SGK1 N[H.sub.2] terminus revealed that its deletion does not affect in vitro kinase activity but profoundly limits the ability of SGK1 either to stimulate ENaC-dependent [Na.sup.+] transport or inhibit Forkhead-dependent gene transcription. The N[H.sub.2] terminus of SGK1, which shares sequence homology with the phosphoinositide 3-phosphate [PI(3)P] binding domain of SGK3, binds phosphoinositides in protein lipid overlay assays, interacting specifically with PI(3)P, PI(4)P, and PI(5)P, but not with PI(3,4,5)[P.sub.3]. Moreover, a point mutation that reduces phosphoinositide binding to the N[H.sub.2] terminus also reduces SGK1 effects on [Na.sup.+] transport and Forkhead activity. These data suggest that the N[H.sub.2] terminus, although not required for PI 3-kinase-dependent modulation of SGK1 catalytic activity, is required for multiple SGKI functions, including stimulation of ENaC and inhibition of the proapoptotic Forkhead transcription factor. Together, these observations support the idea that the N[H.sub.2]-terminal domain acts downstream of PI 3-kinase-dependent activation to target the kinase to specific cellular compartments and/or substrates, possibly through its interactions with a subset of phosphoinositides. epithelial sodium channel; Forkhead; kinase activity doi:10.1152/ajprenal.00027.2007.

Regulation of ENaC occurs at several levels. The principal hormonal regulator of ENaC, aldosterone, acts through the mineralocorticoid receptor to modulate ENaC-mediated sodium transport, and considerable attention has focused on defining the components of the early phase of this response. Two genes, SGK1 and GILZ, have now been implicated in this regulation. While the functional significance of SGK1 in mediating aldosterone effects is well established, new evidence has enhanced our understanding of the mechanisms of SGK1 action. In addition, recent work demonstrates a novel role for GILZ in the stimulation of EnaC-mediated sodium transport. Interestingly, both SGK1 and GILZ appear to negatively regulate tonic inhibition of EnaC and thus use disinhibition to propagate the rapid effects of aldosterone to increase sodium reabsorption in tight epithelia. epithelial sodium channel; Nedd4-2; 14-3-3; ERK; trafficking

Background: The mechanism by which immune cells regulate metastasis is unclear. Understanding the role of immune cells in metastasis will guide the development of treatments improving patient survival.Methods: We used syngeneic orthotopic mouse tumour models (wild-type, NOD/scid and Nude), employed knockout (CD8 and CD4) models and administered CXCL4. Tumours and lungs were analysed for cancer cells by bioluminescence, and circulating tumour cells were isolated from blood. Immunohistochemistry on the mouse tumours was performed to confirm cell type, and on a tissue microarray with 180 TNBCs for human relevance. TCGA data from over 10,000 patients were analysed as well.Results: We reveal that intratumoral immune infiltration differs between metastatic and non-metastatic tumours. The non-metastatic tumours harbour high levels of CD8+ T cells and low levels of platelets, which is reverse in metastatic tumours. During tumour progression, platelets and CXCL4 induce differentiation of monocytes into myeloid-derived suppressor cells (MDSCs), which inhibit CD8+ T-cell function. TCGA pan-cancer data confirmed that CD8lowPlatelethigh patients have a significantly lower survival probability compared to CD8highPlateletlow.Conclusions: CD8+ T cells inhibit metastasis. When the balance between CD8+ T cells and platelets is disrupted, platelets produce CXCL4, which induces MDSCs thereby inhibiting the CD8+ T-cell function. [ABSTRACT FROM AUTHOR]

Abstract Trophoblast cells of the human placenta proliferate, migrate, and invade the pregnant uterus and its vasculature in order to nourish the developing fetus, in a way that is imitated by malignant tumors. Many similarities exist between embryo implantation and the growth of cancer cells. We begin this article by reviewing decades of studies that have helped unearth the mechanisms that contribute to the tumor-like phenotype of human trophoblast cells. Interestingly, these attributes are only transient in nature, with stringent spatial and temporal confines. The importance of intrinsic molecular controls that effectively circumscribe the extent and duration of trophoblast incursion, becomes increasingly evident in abnormal pregnancies that are characterized by aberrant trophoblast proliferation/invasion. We summarize and discuss the significance of abnormalities in these regulatory mechanisms, and finally, speculate about the use of human trophoblastic cells as model systems for the study of a variety of cellular processes. While on one hand, human placental cells are bestowed with a capacity to proliferate indefinitely and invade extensively, on the other, these cells are also replete with mechanisms to regulate these tumor-like attributes and eventually progress to a senescent apoptotic state. This is therefore, a 'well-behaved' tumor. The comparison in the present review is between the invasive cytotrophoblastic cell type and the tumor cell type.

Abstract Background The lipocalin (LCN) family of structurally conserved hydrophobic ligand binding proteins is represented in all major taxonomic groups from prokaryotes to primates. The importance of lipocalins in reproduction and the similarity to known epididymal lipocalins prompted us to characterize the novel human epididymal LCN6. Methods and Results LCN6 cDNA was identified by database analysis in a comprehensive human library sequencing program. Macaca mulatta (rhesus monkey) cDNA was obtained from an epididymis cDNA library and is 93% homologous to the human. The gene is located on chromosome 9q34 adjacent LCN8 and LCN5. LCN6 amino acid sequence is most closely related to LCN5, but the LCN6 beta-barrel structure is best modeled on mouse major urinary protein 1, a pheromone binding protein. Northern blot analysis of RNAs isolated from 25 human tissues revealed predominant expression of a 1.0 kb mRNA in the epididymis. No other transcript was detected except for weak expression of a larger hybridizing mRNA in urinary bladder. Northern hybridization analysis of LCN6 mRNA expression in sham-operated, castrated and testosterone replaced rhesus monkeys suggests mRNA levels are little affected 6 days after castration. Immunohistochemical staining revealed that LCN6 protein is abundant in the caput epithelium and lumen. Immunofluorescent staining of human spermatozoa shows LCN6 located on the head and tail of spermatozoa with the highest concentration of LCN6 on the post-acrosomal region of the head, where it appeared aggregated into large patches. Conclusions LCN6 is a novel lipocalin closely related to Lcn5 and Lcn8 and these three genes are likely products of gene duplication events that predate rodent-primate divergence. Predominant expression in the epididymis and location on sperm surface are consistent with a role for LCN6 in male fertility.