Old dance with a new partner: EGF receptor as the phenobarbital receptor mediating Cyp2B expression

Proteomic Analysis of Proteins Related to Defense Responses in Arabidopsis Plants Transformed with the rolB Oncogene

During Agrobacterium rhizogenes-plant interaction, the rolB gene is transferred into the plant genome and is stably inherited in the plant's offspring. Among the numerous effects of rolB on plant metabolism, including the activation of secondary metabolism, its effect on plant defense systems has not been sufficiently studied. In this work, we performed a proteomic analysis of rolB-expressing Arabidopsis thaliana plants with particular focus on defense proteins. We found a total of 77 overexpressed proteins and 64 underexpressed proteins in rolB-transformed plants using two-dimensional gel electrophoresis and MALDI mass spectrometry. In the rolB-transformed plants, we found a reduced amount of scaffold proteins RACK1A, RACK1B, and RACK1C, which are known as receptors for activated C-kinase 1. The proteomic analysis showed that rolB could suppress the plant immune system by suppressing the RNA-binding proteins GRP7, CP29B, and CP31B, which action are similar to the action of type-III bacterial effectors. At the same time, rolB plants induce the massive biosynthesis of protective proteins VSP1 and VSP2, as well as pathogenesis-related protein PR-4, which are markers of the activated jasmonate pathway. The increased contents of glutathione-S-transferases F6, F2, F10, U19, and DHAR1 and the osmotin-like defense protein OSM34 were found. The defense-associated protein PCaP1, which is required for oligogalacturonide-induced priming and immunity, was upregulated. Moreover, rolB-transformed plants showed the activation of all components of the PYK10 defense complex that is involved in the metabolism of glucosinolates. We hypothesized that various defense systems activated by rolB protect the host plant from competing phytopathogens and created an effective ecological niche for A. rhizogenes. A RolB → RACK1A signaling module was proposed that might exert most of the rolB-mediated effects on plant physiology. Our proteomics data are available via ProteomeXchange with identifier PXD037959.

Nuclear receptor phosphorylation in xenobiotic signal transduction

Nuclear pregnane X receptor (PXR, NR1I2) and constitutive active/androstane receptor (CAR, NR1I3) are nuclear receptors characterized in 1998 by their capability to respond to xenobiotics and activate cytochrome P450 (CYP) genes. An anti-epileptic drug, phenobarbital (PB), activates CAR and its target CYP2B genes, whereas PXR is activated by drugs such as rifampicin and statins for the CYP3A genes. Inevitably, both nuclear receptors have been investigated as ligand-activated nuclear receptors by identifying and characterizing xenobiotics and therapeutics that directly bind CAR and/or PXR to activate them. However, PB, which does not bind CAR directly, presented an alternative research avenue for an indirect ligand-mediated nuclear receptor activation mechanism: phosphorylation-mediated signal regulation. This review summarizes phosphorylation-based mechanisms utilized by xenobiotics to elicit cell signaling. First, the review presents how PB activates CAR (and other nuclear receptors) through a conserved phosphorylation motif located between two zinc fingers within its DNA-binding domain. PB-regulated phosphorylation at this motif enables nuclear receptors to form communication networks, integrating their functions. Next, the review discusses xenobiotic-induced PXR activation in the absence of the conserved DNA-binding domain phosphorylation motif. In this case, phosphorylation occurs at a motif located within the ligand-binding domain to transduce cell signaling that regulates hepatic energy metabolism. Finally, the review delves into the implications of xenobiotic-induced signaling through phosphorylation in disease development and progression.

Phase 0 of the Xenobiotic Response: Nuclear Receptors and Other Transcription Factors as a First Step in Protection from Xenobiotics

This mini-review examines the crucial importance of transcription factors as a first line of defense in the detoxication of xenobiotics. Key transcription factors that recognize xenobiotics or xenobiotic-induced stress such as reactive oxygen species (ROS), include AhR, PXR, CAR, MTF, Nrf2, NF-κB, and AP-1. These transcription factors constitute a significant portion of the pathways induced by toxicants as they regulate phase I-III detoxication enzymes and transporters as well as other protective proteins such as heat shock proteins, chaperones, and anti-oxidants. Because they are often the first line of defense and induce phase I-III metabolism, could these transcription factors be considered the phase 0 of xenobiotic response?

Loss of hepatocyte ERBB3 but not EGFR impairs hepatocarcinogenesis

Epidermal growth factor receptor (EGFR) and ERBB3 have been implicated in hepatocellular carcinogenesis (HCC). However, it is not known whether altering the activity of either EGFR or ERBB3 affects HCC development. We now show that Egfr(Dsk5) mutant mice, which have a gain-of-function allele that increases basal EGFR kinase activity, develop spontaneous HCC by 10 mo of age. Their tumors show increased activation of EGFR, ERBB2, and ERBB3 as well as AKT and ERK1,2. Hepatocyte-specific models of EGFR and ERBB3 gene ablation were generated to evaluate how the loss of these genes affected tumor progression. Loss of either receptor tyrosine kinase did not alter liver development or regenerative liver growth following carbon tetrachloride injection. However, using a well-characterized model of HCC in which N-nitrosodiethylamine is injected into 14-day-old mice, we discovered that loss of hepatocellular ERBB3 but not EGFR, which occurred after tumor initiation, retarded liver tumor formation and cell proliferation. We found no evidence that this was due to increased apoptosis or diminished phosphatidylinositol-3-kinase activity in the ERBB3-null cells. However, the relative amount of phospho-STAT3 was diminished in tumors derived from these mice, suggesting that ERBB3 may promote HCC through STAT3 activation.

Threonine-290 regulates nuclear translocation of the human pregnane X receptor through its phosphorylation/dephosphorylation by Ca2+/calmodulin-dependent protein kinase II and protein phosphatase 1

The human pregnane X receptor (hPXR) is recognized as a xenobiotic-sensing nuclear receptor that transcriptionally regulates the gene expression of drug-metabolizing enzymes and transporters. Our study elucidates the mechanism by which the localization of hPXR is regulated through threonine-290. A phosphomimetic mutation at threonine-290 (T290D) retained hPXR in the cytoplasm of HepG2, HuH6, and SW480 cells in vitro and the mouse liver in vivo even after treatment with rifampicin, and a phosphodeficient mutation (T290A) translocated from the cytoplasm to the nucleus as the wild-type hPXR. The amount of the unphosphorylated wild-type yellow fluorescent protein-hPXR fusion protein but not the T290A mutant increased on Phos-tag gels in response to stimulations with rifampicin and cyclin-dependent kinase 2 inhibitor roscovitine, and a marked increase was observed in the unphosphorylated levels of the T290A mutant in nontreated cells. The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN93 [2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine)] and transfection with anti-CaMKII small-interfering RNA (siRNA) enhanced the unphosphorylated levels of the wild-type protein. CaMKII directly phosphorylated the threonine-290 of hPXR, and the T290A mutant conferred resistance to CaMKII. The protein phosphatase (PP) inhibitor okadaic acid (100 nM) and transfection with anti-PP1 siRNA but not anti-PP2A siRNA led to reduced expression of CYP3A4 mRNA. After the rifampicin and roscovitine stimulations, PP1 was recruited to the wild-type hPXR but not the T290A mutant. These results suggest that phosphorylation at threonine-290 by CaMKII may impair the function of hPXR by repressing its translocation to the nucleus, and dephosphorylation by PP1 is necessary for the xenobiotic-dependent nuclear translocation of hPXR.