Pregnane X receptor (PXR) is a nuclear receptor considered to be a master xenobiotic receptor that coordinately regulates the expression of genes encoding drug-metabolizing enzymes and drug transporters to essentially detoxify and eliminate xenobiotics and endotoxins from the body. In the past several years, the function of PXR in the regulation of xenobiotic metabolism has been extensively studied, and the role of PXR as a xenobiotic sensor has been well established. It is now clear, however, that PXR plays many other roles in addition to its xenobiotic-sensing function. For instance, recent studies have discovered previously unidentified roles of PXR in inflammatory response, cell proliferation, and cell migration. PXR also contributes to the dysregulation of these processes in diseases states. These recent discoveries of the role of PXR in the physiologic and pathophysiologic conditions of other cellular processes provides the possibility of novel targets for drug discovery. This review highlights areas of PXR regulation that require further clarification and summarizes the recent progress in our understanding of the nonxenobiotic functions of PXR that can be explored for relevant therapeutic applications.

BackgroundProteins functioning in the same biological pathway tend to be transcriptionally co-regulated or form protein-protein interactions (PPI). Multiple spatially and temporally regulated events are coordinated during mitosis to achieve faithful chromosome segregation. The molecular players participating in mitosis regulation are still being unravelled experimentally or using in silico methods.ResultsAn extensive literature review has led to a compilation of 196 human centromere/kinetochore proteins, all with experimental evidence supporting the subcellular localization. Sixty-four were designated as “core” centromere/kinetochore components based on peak expression and/or well-characterized functions during mitosis. By interrogating and integrating online resources, we have mined for genes/proteins that display transcriptional co-expression or PPI with the core centromere/kinetochore components. Top-ranked hubs in either co-expression or PPI network are not only enriched with known mitosis regulators, but also contain candidates whose mitotic functions are not yet established. Experimental validation found that KIAA1377 is a novel centrosomal protein that also associates with microtubules and midbody; while TRIP13 is a novel kinetochore protein and directly interacts with mitotic checkpoint silencing protein p31comet.ConclusionsTranscriptional co-expression and PPI network analyses with known human centromere/kinetochore proteins as a query group help identify novel potential mitosis regulators.

p21-Activated serine-threonine kinase (PAK1) is implicated in breast cancer. We have shown previously that PAK1 is tyrosyl phosphorylated by prolactin (PRL)-activated Janus tyrosine kinase (JAK2). Although a role for both PRL and PAK1 in breast cancer is widely acknowledged, the mechanism remains poorly understood. In the present study, PRL-activated PAK1 stimulates the invasion of TMX2-28 human breast cancer cells through Matrigel. Three-dimensional (3D) collagen IV stimulates the secretion of the matrix proteases, metalloproteinase (MMP)-1 and -3 that is further enhanced by the PRL-dependent tyrosyl phosphorylation of PAK1. 3D collagen IV also stimulates the expression and secretion of MMP-2, but in contrast to MMP-1 and -3, PRL/PAK1 signaling down-regulates MMP-2 expression and secretion. In contrast, MMP-9 expression and secretion are stimulated by 3D collagen I, not collagen IV, and are not affected by PRL but are down-regulated by PAK1. MMP-1 and -3 are required and MMP-2 contributes to PRL-dependent invasion. ERK1/2 signaling appears to be required for the enhanced expression and secretion of MMP-1 and -3 and enhanced PRL-dependent invasion. p38 MAPK and c-Jun N-terminal kinase 1/2 pathways participate in production of MMP-1 and -3 as well as in PRL/PAK1-dependent cell invasion. Together, these data illustrate the complex interaction between the substratum and PRL/PAK1 signaling in human breast cancer cells and suggest a pivotal role for PRL-dependent PAK1 tyrosyl phosphorylation in MMP secretion.

The p21-activated serine-threonine kinase (PAK1) is activated by small GTPase-dependent and -independent mechanisms and regulates cell motility. Both PAK1 and the hormone prolactin (PRL) have been implicated in breast cancer by numerous studies. We have previously shown that the PRL-activated tyrosine kinase JAK2 (Janus tyrosine kinase 2) phosphorylates PAK1 in vivo and identified tyrosines (Tyr) 153, 201, and 285 in the PAK1 molecule as sites of JAK2 tyrosyl phosphorylation. Here, we have used human breast cancer T47D cells stably overexpressing PAK1 wild type or PAK1 Y3F mutant in which Tyr(s) 153, 201, and 285 were mutated to phenylalanines to demonstrate that phosphorylation of these three tyrosines are required for maximal PRL-dependent ruffling. In addition, phosphorylation of these three tyrosines is required for increased migration of T47D cells in response to PRL as assessed by two independent motility assays. Finally, we show that PAK1 phosphorylates serine (Ser) 2152 of the actin-binding protein filamin A to a greater extent when PAK1 is tyrosyl phosphorylated by JAK2. Down-regulation of PAK1 or filamin A abolishes the effect of PRL on cell migration. Thus, our data presented here bring some insight into the mechanism of PRL-stimulated motility of breast cancer cells.

The human cytochrome P450 (CYP) enzymes CYP3A4 and CYP3A5 metabolize most drugs and have high similarities in their structure and substrate preference. Whereas CYP3A4 is predominantly expressed in the liver, CYP3A5 is upregulated in cancer, contributing to drug resistance. Selective inhibitors of CYP3A5 are, therefore, critical to validating it as a therapeutic target. Here we report clobetasol propionate (clobetasol) as a potent and selective CYP3A5 inhibitor identified by high-throughput screening using enzymatic and cell-based assays. Molecular dynamics simulations suggest a close proximity of clobetasol to the heme in CYP3A5 but not in CYP3A4. UV–visible spectroscopy and electron paramagnetic resonance analyses confirmed the formation of an inhibitory type I heme–clobetasol complex in CYP3A5 but not in CYP3A4, thus explaining the CYP3A5 selectivity of clobetasol. Our results provide a structural basis for selective CYP3A5 inhibition, along with mechanistic insights, and highlight clobetasol as an important chemical tool for target validation.