ABSTRACT:The pregnane X receptor (PXR) plays crucial roles in multiple physiological processes. However, the signaling mechanisms responsible are not well defined; it is most likely that multiple functions of PXR are modulated by its phosphorylation. Therefore, we sought to determine whether mutation at a highly conserved Thr ) and show p70 S6K phosphorylation and regulation of hPXR transactivation to support the notion that phosphorylation plays important roles in regulating hPXR function.

The semicircular canal duct (SCCD) epithelium is a vestibular epithelial domain that was recently shown to actively contribute to endolymph homeostasis by Cl(-) secretion under control of beta(2)-adrenergic stimulation. By analogy to other Cl(-) secretory epithelia, we hypothesized that SCCD also provides an active absorptive pathway for Na(+) under corticosteroid control. Measurements of short-circuit current (I(sc)) demonstrated stimulation (7-24 h) by the glucocorticoids hydrocortisone (EC(50) 13 nM), corticosterone (33 nM), prednisolone (70 nM), and dexamethasone (13 nM) over physiologically and therapeutically relevant concentrations and its block by amiloride (IC(50) 470 nM) and benzamil (57 nM), inhibitors of the epithelial sodium channel (ENaC). I(sc) was also partially inhibited by basolateral ouabain and Ba(2+), indicating the participation of Na(+)-K(+)-ATPase and a K(+) channel in Na(+) transport. By contrast, aldosterone stimulated I(sc) only at unphysiologically high concentrations (EC(50) 102 nM). The action of all steroids was blocked by mifepristone (RU-486; K(d) approximately 0.3 nM) but not by spironolactone (K(d) approximately 0.7 microM). Expression of mRNA for the alpha-, beta-, and gamma-subunits of ENaC was demonstrated in the presence and absence of glucocorticoids. These findings are the first to identify SCCD in the vestibular labyrinth as a site of physiologically significant ENaC-mediated Na(+) absorption and osmotically coupled water flux. They further demonstrate regulation of Na(+) transport by natural and therapeutic glucocorticoids. The results provide for the first time an understanding of the therapeutic benefit of glucocorticoids in the treatment of Meniere's disease, a condition that is associated with increased luminal fluid volume.

Kim SH, Kim KX, Raveendran NN, Wu T, Pondugula SR, Marcus DC. Regulation of ENaC-mediated sodium transport by glucocorticoids in Reissner's membrane epithelium. 's membrane epithelium forms much of the barrier that produces and sustains the large ionic differences between cochlear endolymph and perilymph. We have reported that Reissner's membrane contributes to normal cochlear function by absorbing Na ϩ from endolymph via amiloridesensitive channels in gerbil inner ear. We used mouse Reissner's membrane to 1) identify candidate genes involved in the Na ϩ transport pathway, 2) determine whether their level of expression was regulated by the synthetic glucocorticoid dexamethasone, and 3) obtain functional evidence for the physiological importance of these genes. Transcripts were present for ␣-, ␤-, and ␥-subunits of epithelial Na ϩ channel (ENaC); corticosteroid receptors GR (glucocorticoid receptor) and MR (mineralocorticoid receptor); GR agonist regulator 11␤-hydroxysteroid dehydrogenase (HSD) type 1 (11␤-HSD1); Na ϩ transport control components SGK1, Nedd4-2, and WNKs; and K ϩ channels and Na ϩ -K ϩ -ATPase. Expression of the MR agonist regulator 11␤-HSD2 was not detected. Dexamethasone upregulated transcripts for ␣-and ␤-subunits of ENaC (ϳ6-and ϳ3-fold), KCNK1 (ϳ3-fold), 11␤-HSD1 (ϳ2-fold), SGK1 (ϳ2-fold), and WNK4 (ϳ3-fold). Transepithelial currents from the apical to the basolateral side of Reissner's membrane were sensitive to amiloride (IC50 ϳ0.7 M) and benzamil (IC50 ϳ0.1 M), but not EIPA (IC50 ϳ34 M); amilorideblocked transepithelial current was not immediately changed by forskolin/IBMX. Currents were reduced by ouabain, lowered bath Na ϩ concentration (from 150 to 120 mM), and K ϩ channel blockers (XE-991, Ba 2ϩ , and acidification from pH 7.4 to 6.5). Dexamethasone-stimulated current and gene expression were reduced by mifepristone, but not spironolactone. These molecular, pharmacological, and functional observations are consistent with Na ϩ absorption by mouse Reissner's membrane, which is mediated by apical ENaC and/or other amiloride-sensitive channels, basolateral Na ϩ -K ϩ -ATPase, and K ϩ -permeable channels and is under the control of glucocorticoids. These results provide an understanding and a molecular definition of an important transport function of Reissner's membrane epithelium in the homeostasis of cochlear endolymph.

Pregnane xenobiotic receptor (PXR) is an orphan nuclear receptor that regulates the metabolism of endobiotics and xenobiotics. PXR is promiscuous and unique in that it is activated by a diverse group of xenochemicals, including therapeutic anticancer drugs and naturally-occurring endocrine disruptors. PXR has been predominantly studied to understand its regulatory role in xenobiotic clearance in liver and intestine via induction of drug metabolizing enzymes and drug transporters. PXR, however, is widely expressed and has functional implications in other normal and malignant tissues, including breast, prostate, ovary, endometrium and bone. The differential expression of PXR and its target genes in cancer tissues has been suggested to determine the prognosis of chemotherapeutic outcome. In addition, the emerging evidence points to the implications of PXR in regulating apoptotic and antiapoptotic as well as growth factor signaling that promote tumor proliferation and metastasis. In this review, we highlight the recent progress made in understanding the role of PXR in cancer, discuss the future directions to further understand the mechanistic role of PXR in cancer, and conclude with the need to identify novel selective PXR modulators.

, which is important for transduction processes. We have recently shown that glucocorticoid receptors (GR) stimulate absorption of Na ϩ by semicircular canal duct (SCCD) epithelia. In the present study, we sought to determine the presence of genes involved in the control of the amiloride-sensitive Na ϩ transport pathway in rat SCCD epithelia and whether their level of expression was regulated by glucocorticoids using quantitative real-time RT-PCR. Transcripts were present for ␣-, ␤-, and ␥-subunits of the epithelial sodium channel (ENaC); the ␣1-, ␣3-, ␤1-, and ␤3-isoforms of Na ϩ -K ϩ -ATPase; inwardly rectifying potassium channels [IC50 of short circuit current (Isc) for Ba 2ϩ : 210 M] Kir2

Background The expression of drug-metabolizing enzymes cytochrome P450 (CYPs) is controlled by pregnane X receptor (PXR), and therefore understanding how PXR modulates CYP expression is important to minimize adverse drug interactions, one type of preventable adverse drug reaction. Objective We review the mechanisms of PXR-mediated repression of CYP expression Methods We discuss the clinical implications of CYP repression and the role of signal cross-talks, including protein-protein interactions and phosphorylation of PXR and coregulators, in inhibiting PXR and repressing CYP expression Results/conclusion Kinases such as cyclin-dependent kinase 2 (Cdk2), protein kinase A (PKA), protein kinase C (PKC), and 70kDa form of ribosomal protein S6 kinase (p70 S6K) repress CYP expression by phosphorylating and inhibiting PXR. Growth factor signaling represses CYP expression by phosphorylating and inhibiting forkhead in rhabdomyosarcoma (FKHR), a coactivator of PXR. During inflammation, nuclear factor κB (NF-κB) represses both PXR and CYP expression via protein-protein interactions with the PXR pathway.

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Pregnane X receptor (PXR) is an adopted orphan nuclear receptor that is activated by a wide-range of endobiotics and xenobiotics, including chemotherapy drugs. PXR plays a major role in the metabolism and clearance of xenobiotics and endobiotics in liver and intestine via induction of drug-metabolizing enzymes and drug-transporting proteins. However, PXR is expressed in several cancer tissues and the accumulating evidence strongly points to the differential role of PXR in cancer growth and progression as well as in chemotherapy outcome. In cancer cells, besides regulating the gene expression of enzymes and proteins involved in drug metabolism and transport, PXR also regulates other genes involved in proliferation, metastasis, apoptosis, anti-apoptosis, inflammation, and oxidative stress. In this review, we focus on the differential role of PXR in a variety of cancers, including prostate, breast, ovarian, endometrial, and colon. We also discuss the future directions to further understand the differential role of PXR in cancer, and conclude with the need to identify novel selective PXR modulators to target PXR in PXR-expressing cancers.