Edward E. Schmidt

Cytoprotective Nrf2 Pathway Is Induced In Chronically Txnrd 1-Deficient Hepatocytes

Background Metabolically active cells require robust mechanisms to combat oxidative stress. The cytoplasmic thioredoxin reductase/thioredoxin (Txnrd1/Txn1) system maintains reduced protein dithiols and provides electrons to some cellular reductases, including peroxiredoxins. Principal Findings Here we generated mice in which the txnrd1 gene, encoding Txnrd1, was specifically disrupted in all parenchymal hepatocytes. Txnrd1-deficient livers exhibited a transcriptome response in which 56 mRNAs were induced and 12 were repressed. Based on the global hybridization profile, this represented only 0.3% of the liver transcriptome. Since most liver mRNAs were unaffected, compensatory responses were evidently effective. Nuclear pre-mRNA levels indicated the response was transcriptional. Twenty-one of the induced genes contained known antioxidant response elements (AREs), which are binding sites for the oxidative and chemical stress-induced transcription factor Nrf2. Txnrd1-deficient livers showed increased accumulation of nuclear Nrf2 protein and chromatin immunoprecipitation on the endogenous nqo1 and aox1 promoters in fibroblasts indicated that Txnrd1 ablation triggered in vivo assembly of Nrf2 on each. Conclusions Chronic deletion of Txnrd1 results in induction of the Nrf2 pathway, which contributes to an effective compensatory response.

The role of the transcriptional activator protein DBP in circadian liver gene expression

Summary DBP, a liver-enriched transcriptional activator protein of the leucine zipper protein family, accumulates according to a very strong circadian rhythm (amplitude approx. 1000-fold). In rat parenchymal hepatocytes, the protein is barely detectable during the morning hours. At about 2 p.m., DBP levels begin to rise, reach maximal levels at 8 p.m. and decline sharply during the night. This rhythm is free-running: it persists with regard to both its amplitude and phase in the absence of external time cues, such as daily dark/light switches. Also, fasting of rats for several days influences neither the amplitude nor the phase of circadian DBP expression. Since the levels of DBP mRNA and nascent transcripts also oscillate with a strong amplitude, circadian DBP expression is transcriptionally controlled. While DBP mRNA fluctuates with a similar phase and amplitude in most tissues examined, DBP protein accumulates to high concentrations only in liver nuclei. Hence, at least in nonhepatic tissues, cyclic DBP transcription is unlikely to be controlled by a positive and/or negative feedback mechanism involving DBP itself. More likely, the circadian DBP expression is governed by hormones whose peripheral concentrations also oscillate during the day. Several lines of evidence suggest a pivotal role of glucocorticoid hormones in establishing the DBP cycle. Two genes whose mRNAs and protein products accumulate according to a strong circadian rhythm with a phase compatible with regulation by DBP encode enzymes with key functions in cholesterol metabolism: HMG-coA reductase is the rate-limiting enzyme in cholesterol synthesis; cholesterol 7-α hydroxylase performs the rate-limiting step in the conversion of cholesterol to bile acid. DBP may thus be involved in regulating cholesterol homeostasis.

A novel persulfide detection method reveals protein persulfide- and polysulfide-reducing functions of thioredoxin and glutathione systems.

Protein Persulfide Detection Protocol reveals vital roles for thioredoxin and glutathione systems in maintaining sulfane sulfur homeostasis in cells and in vivo. Hydrogen sulfide signaling involves persulfide formation at specific protein Cys residues. However, overcoming current methodological challenges in persulfide detection and elucidation of Cys regeneration mechanisms from persulfides are prerequisites for constructing a bona fide signaling model. We here establish a novel, highly specific protein persulfide detection protocol, ProPerDP, with which we quantify 1.52 ± 0.6 and 11.6 ± 6.9 μg/mg protein steady-state protein persulfide concentrations in human embryonic kidney 293 (HEK293) cells and mouse liver, respectively. Upon treatment with polysulfides, HEK293 and A549 cells exhibited increased protein persulfidation. Deletion of the sulfide-producing cystathionine-γ-lyase or cystathionine-β-synthase enzymes in yeast diminished protein persulfide levels, thereby corroborating their involvement in protein persulfidation processes. We here establish that thioredoxin (Trx) and glutathione (GSH) systems can independently catalyze reductions of inorganic polysulfides and protein persulfides. Increased endogenous persulfide levels and protein persulfidation following polysulfide treatment in thioredoxin reductase-1 (TrxR1) or thioredoxin-related protein of 14 kDa (TRP14) knockdown HEK293 cells indicated that these enzymes constitute a potent regeneration system of Cys residues from persulfides in a cellular context. Furthermore, TrxR1-deficient cells were less viable upon treatment with toxic amounts of polysulfides compared to control cells. Emphasizing the dominant role of cytosolic disulfide reduction systems in maintaining sulfane sulfur homeostasis in vivo, protein persulfide levels were markedly elevated in mouse livers where hepatocytes lack both TrxR1 and glutathione reductase (TR/GR-null). The different persulfide patterns observed in wild-type, GR-null, and TR/GR-null livers suggest distinct roles for the Trx and GSH systems in regulating subsets of protein persulfides and thereby fine-tuning sulfide signaling pathways.

Selenoprotein Gene Nomenclature

The human genome contains 25 genes coding for selenocysteine-containing proteins (selenoproteins). These proteins are involved in a variety of functions, most notably redox homeostasis. Selenoprotein enzymes with known functions are designated according to these functions: TXNRD1, TXNRD2, and TXNRD3 (thioredoxin reductases), GPX1, GPX2, GPX3, GPX4, and GPX6 (glutathione peroxidases), DIO1, DIO2, and DIO3 (iodothyronine deiodinases), MSRB1 (methionine sulfoxide reductase B1), and SEPHS2 (selenophosphate synthetase 2). Selenoproteins without known functions have traditionally been denoted by SEL or SEP symbols. However, these symbols are sometimes ambiguous and conflict with the approved nomenclature for several other genes. Therefore, there is a need to implement a rational and coherent nomenclature system for selenoprotein-encoding genes. Our solution is to use the root symbol SELENO followed by a letter. This nomenclature applies to SELENOF (selenoprotein F, the 15-kDa selenoprotein, SEP15), SELENOH (selenoprotein H, SELH, C11orf31), SELENOI (selenoprotein I, SELI, EPT1), SELENOK (selenoprotein K, SELK), SELENOM (selenoprotein M, SELM), SELENON (selenoprotein N, SEPN1, SELN), SELENOO (selenoprotein O, SELO), SELENOP (selenoprotein P, SeP, SEPP1, SELP), SELENOS (selenoprotein S, SELS, SEPS1, VIMP), SELENOT (selenoprotein T, SELT), SELENOV (selenoprotein V, SELV), and SELENOW (selenoprotein W, SELW, SEPW1). This system, approved by the HUGO Gene Nomenclature Committee, also resolves conflicting, missing, and ambiguous designations for selenoprotein genes and is applicable to selenoproteins across vertebrates.

Transcriptional promiscuity in testes

Numerous genes are expressed preferentially in spermatids, the haploid germ cells of the testis. Although some have defined, spermatid-specific roles, the expression and function of the products of most of these genes are not restricted to spermatids [1]. Examples include transcription factors that have defined functions elsewhere, kinases, metabolic enzymes, and so on. Recently, another group of proteins has been added to this list — the components of the basal RNA polymerase II (pol II) transcription machinery [2]. Recent reports suggest that the pol II machinery pre-assembles into a holoenzyme complex [3,4]. The levels of the components of this complex coordinately increase 30to 100-fold during the early haploid stages of spermatogenesis ([2] and unpublished observations). Thus, early spermatid nuclei may have much higher concentrations of holoenzyme than do somatic cells. In this light, it is interesting to consider what would be the consequences of increasing holoenzyme levels by two orders of magnitude. All models of transcriptional regulation to date include the assumption that the basal transcription machinery is constant and limiting; differences in the activity of a promoter between cell types have been presumed to result solely from differences in the ‘attractiveness’ of that promoter for a fixed concentration of pol II machinery. Promoter attractiveness can be regulated by altering DNA methylation, chromatin structure or transcription factor assemblages [5–7]. But changes in the concentration of holoenzyme might also alter the rate of transcription initiation at a promoter. If the relative activities of the three promoters depicted in Figure 1a are plotted as a function of holoenzyme concentrations, each promoter gives a different activity curve (Fig. 1c). Thus, changes in holoenzyme concentration can affect the activities of various promoters differentially. For example, raising the holoenzyme concentration from 1.0 to 100 arbitrary units activates promoter C negligibly, whereas promoter B, which is a weak promoter at a holoenzyme concentration of 1.0, is now activated to the point of being nearly as strong as promoter C. Interestingly, at elevated holoenzyme concentrations, some DNA sequences that do not normally act as promoters — for example, promoter A in the figure — may now promote very strong transcription. Is there any experimental evidence to support such a model? A recent paper [4] on mammalian RNA polymerase II holoenzyme showed that promoters which require transcription factors for maximal activity in vitro using crude nuclear extracts (which have relatively low holoenzyme concentrations) were highly active in the absence of such factors when supplied with a high concentration of purified holoenzyme. The addition of transcription factors did not further activate transcription, suggesting that at high holoenzyme concentrations, weak and strong promoters are similarly active. In other studies [3], purified yeast holoenzyme was shown to be responsive to activators. Figure 1c shows how, depending on holoenzyme concentrations, promoters might exhibit great differences in their response to activators. What are the implications of this model for spermatid-specific gene expression? The model suggests that many genes which are expressed in spermatids might not require spermatid-specific transcription factors for their activation. Rather, early spermatids, by having elevated holoenzyme concentrations, may provide a permissive environment for transcription initiation. Thus, poor promoters in particular should have elevated activity in early spermatids (Fig. 1c). The model is consistent with the large number of genes that are expressed in the testis, and with the observation that testis-specific expression frequently involves both up-regulation of existing promoters and recruitment of additional promoters [1]. In contrast, genes for abundant spermatid-specific proteins have additional regulatory mechanisms to ensure that the difference in expression between spermatids and other cell types is orders of magnitude greater still [1,8]. The model proposed here does not suggest that genes which are expressed in spermatids, including those with known functions elsewhere, are expressed simply ‘by 768 Current Biology 1996, Vol 6 No 7

Spermatid-specific Overexpression of the TATA-binding Protein Gene Involves Recruitment of Two Potent Testis-specific Promoters

The gene encoding the TATA-binding protein, TBP, is highly overexpressed during the haploid stages of spermatogenesis in rodents. RNase protection analyses for mRNAs containing the previously identified first, second, and eighth exons suggested that most TBP mRNAs in testis did not initiate at the first exon used in somatic cells (here designated exon 1C). Using a sensitive ligation-mediated cDNA amplification method, 5′ end variants of TBP mRNA were identified, and the corresponding cDNAs were cloned from liver and testis. In liver, a single promoter/first exon is used to generate a steady-state level of roughly five molecules of TBP mRNA per diploid cell equivalent. In testis, we detect modest up-regulation of the somatic promoter and recruitment of at least five other promoters. Three of the alternative promoter/first exons, including 1C and two of the testis-specific promoter/first exons, 1D and 1E, contribute roughly equivalent amounts of mRNA which, in sum, account for greater than 90% of all TBP mRNA in testis. As a result, round spermatids contain an estimated 1000 TBP mRNA molecules per haploid cell. Testis TBP mRNA also exhibits several low abundance 5′ end splicing variants; however, all detected TBP mRNA leader sequences splice onto the common exon 2 and are expected to initiate translation at the same site within exon 2. The precise locations of the three major initiation exons are mapped on the gene. The identification of the strong testis-specific promoter/first exons will be important for understanding spermatid-specific tbp gene regulation.

Estrogen controls the survival of BRCA1-deficient cells via a PI3K–NRF2-regulated pathway

Significance Our establishment of a connection between the phosphatidylinositol 3-kinase (PI3K) and NRF2 pathways provides the basis for the tissue specificity of BRCA1-related cancers. Because BRCA1 is a vital component of the intracellular machinery maintaining genomic stability, BRCA1 functions as a major tumor suppressor in cells of all types. However, BRCA1-related cancers occur overwhelmingly in breasts and ovaries. Our work demonstrates that estrogen (E2) acts via the PI3K–AKT pathway to regulate NRF2 activation in BRCA1-deficient cells, resulting in the induction of antioxidant genes that protect the cell from reactive oxygen species-induced death. BRCA1-deficient cells are thus allowed to survive and may accumulate mutations, such as loss of PTEN, that perpetuate NRF2 activation. Our work supports the emerging clinical strategy of treating BRCA1-related cancers with PI3K inhibitors. Mutations in the tumor suppressor BRCA1 predispose women to breast and ovarian cancers. The mechanism underlying the tissue-specific nature of BRCA1’s tumor suppression is obscure. We previously showed that the antioxidant pathway regulated by the transcription factor NRF2 is defective in BRCA1-deficient cells. Reactivation of NRF2 through silencing of its negative regulator KEAP1 permitted the survival of BRCA1-null cells. Here we show that estrogen (E2) increases the expression of NRF2-dependent antioxidant genes in various E2-responsive cell types. Like NRF2 accumulation triggered by oxidative stress, E2-induced NRF2 accumulation depends on phosphatidylinositol 3-kinase–AKT activation. Pretreatment of mammary epithelial cells (MECs) with the phosphatidylinositol 3-kinase inhibitor BKM120 abolishes the capacity of E2 to increase NRF2 protein and transcriptional activity. In vivo the survival defect of BRCA1-deficient MECs is rescued by the rise in E2 levels associated with pregnancy. Furthermore, exogenous E2 administration stimulates the growth of BRCA1-deficient mammary tumors in the fat pads of male mice. Our work elucidates the basis of the tissue specificity of BRCA1-related tumor predisposition, and explains why oophorectomy significantly reduces breast cancer risk and recurrence in women carrying BRCA1 mutations.

Hepatocyte DNA replication in growing liver requires either glutathione or a single allele of txnrd1.

Ribonucleotide reductase (RNR) activity requires an electron donor, which in bacteria, yeast, and plants is usually either reduced thioredoxin (Trx) or reduced glutaredoxin. Mice lacking glutathione reductase are viable and, although mice lacking thioredoxin reductase 1 (TrxR1) are embryonic-lethal, several studies have shown that mouse cells lacking the txnrd1 gene, encoding TrxR1, can proliferate normally. To better understand the in vivo electron donor requirements for mammalian RNR, we here investigated whether replication of TrxR1-deficient hepatocytes in mouse livers either employed an alternative source of Trx-reducing activity or, instead, solely relied upon the glutathione (GSH) pathway. Neither normal nor genetically TrxR1-deficient livers expressed substantial levels of mRNA splice forms encoding cytosolic variants of TrxR2, and the TrxR1-deficient livers showed severely diminished total TrxR activity, making it unlikely that any alternative TrxR enzyme activities complemented the genetic TrxR1 deficiency. To test whether the GSH pathway was required for replication, GSH levels were depleted by administration of buthionine sulfoximine (BSO) to juvenile mice. In controls not receiving BSO, replicative indexes were similar in hepatocytes having two, one, or no functional alleles of txnrd1. After BSO treatment, hepatocytes containing either two or one copies of this gene were also normal. However, hepatocytes completely lacking a functional txnrd1 gene exhibited severely reduced replicative indexes after GSH depletion. We conclude that hepatocyte proliferation in vivo requires either GSH or at least one functional allele of txnrd1, demonstrating that either the GSH- or the TrxR1-dependent redox pathway can independently support hepatocyte proliferation during liver growth.

Hepatocytes lacking thioredoxin reductase 1 have normal replicative potential during development and regeneration

Cells require ribonucleotide reductase (RNR) activity for DNA replication. In bacteria, electrons can flow from NADPH to RNR by either a thioredoxin-reductase- or a glutathione-reductase-dependent route. Yeast and plants artificially lacking thioredoxin reductases exhibit a slow-growth phenotype, suggesting glutathione-reductase-dependent routes are poor at supporting DNA replication in these organisms. We have studied proliferation of thioredoxin-reductase-1 (Txnrd1)-deficient hepatocytes in mice. During development and regeneration, normal mice and mice having Txnrd1-deficient hepatocytes exhibited similar liver growth rates. Proportions of hepatocytes that immunostained for PCNA, phosphohistone H3 or incorporated BrdU were also similar, indicating livers of either genotype had similar levels of proliferative, S and M phase hepatocytes, respectively. Replication was blocked by hydroxyurea, confirming that RNR activity was required by Txnrd1-deficient hepatocytes. Regenerative thymidine incorporation was similar in normal and Txnrd1-deficient livers, further indicating that DNA synthesis was unaffected. Using genetic chimeras in which a fluorescently marked subset of hepatocytes was Txnrd1-deficient while others were not, we found that the multigenerational contributions of both hepatocyte types to development and to liver regeneration were indistinguishable. We conclude that, in mouse hepatocytes, a Txnrd1-independent route for the supply of electrons to RNR can fully support DNA replication and normal proliferative growth.

Cre activity in fetal albCre mouse hepatocytes: Utility for developmental studies.

The albCre transgene, having Cre recombinase driven by the serum albumin (alb) gene promoter, is commonly used to generate adult mice having reliable hepatocyte‐specific recombination of loxP‐flanked (“floxed”) alleles. Based on previous studies, it has been unclear whether albCre transgenes are also reliable in fetal and juvenile mice. Perinatal liver undergoes a dynamic transition from being predominantly hematopoietic to predominantly hepatic. We evaluated Cre activity during this transition in albCre mice using a sensitive two‐color fluorescent reporter system. From fetal through adult stages, in situ patterns of Cre‐dependent recombination of the reporter closely matched expression of endogenous Alb mRNA or protein, indicating most or all hepatocytes, including those in fetal and juvenile livers, had expressed Cre and recombined the reporter. Our results indicate the albCre transgene is effective in converting simple floxed alleles in fetal and neonatal mice and is an appropriate tool for studies on hepatocyte development. genesis 47:789–792, 2009.