The role of Sp1 and AP-2 in basal and protein kinase A--induced expression of mitochondrial serine:pyruvate aminotransferase in hepatocytes

Epigenomic and transcriptional profiling identifies impaired glyoxylate detoxification in NAFLD as a risk factor for hyperoxaluria

Epigenetic modifications (e.g. DNA methylation) in NAFLD and their contribution to disease progression and extrahepatic complications are poorly explored. Here, we use an integrated epigenome and transcriptome analysis of mouse NAFLD hepatocytes and identify alterations in glyoxylate metabolism, a pathway relevant in kidney damage via oxalate release-a harmful waste product and kidney stone-promoting factor. Downregulation and hypermethylation of alanine-glyoxylate aminotransferase (Agxt), which detoxifies glyoxylate, preventing excessive oxalate accumulation, is accompanied by increased oxalate formation after metabolism of the precursor hydroxyproline. Viral-mediated Agxt transfer or inhibiting hydroxyproline catabolism rescues excessive oxalate release. In human steatotic hepatocytes, AGXT is also downregulated and hypermethylated, and in NAFLD adolescents, steatosis severity correlates with urinary oxalate excretion. Thus, this work identifies a reduced capacity of the steatotic liver to detoxify glyoxylate, triggering elevated oxalate, and provides a mechanistic explanation for the increased risk of kidney stones and chronic kidney disease in NAFLD patients.

Molecular mechanism of monoamine oxidase A gene regulation under inflammation and ischemia-like conditions: key roles of the transcription factors GATA2, Sp1 and TBP

Monoamine oxidase A (MAOA) plays important roles in the pathogenesis of several neurological and cardiovascular disorders. The mechanism of transcriptional regulation of MAOA under basal and pathological conditions, however, remains incompletely understood. Here, we report systematic identification and characterization of cis elements and transcription factors that govern the expression of MAOA gene. Extensive computational analysis of MAOA promoter, followed by 5'-promoter deletion/reporter assays, revealed that the -71/-40 bp domain was sufficient for its basal transcription. Gel-shift and chromatin immunoprecipitation assays provided evidence of interactions of the transcription factors GATA-binding protein 2 (GATA2), Sp1 and TATA-binding protein (TBP) with this proximal promoter region. Consistently, over-expression of GATA2, Sp1 and TBP augmented MAOA promoter activity in a coordinated manner. In corroboration, siRNA-mediated down-regulation of GATA2/Sp1/TBP repressed the endogenous MAOA expression as well as transfected MAOA promoter activity. Tumor necrosis factor-α and forskolin activated MAOA transcription that was reversed by Sp1 siRNA; in support, tumor necrosis factor-α- and forskolin-induced activities were enhanced by ectopic over-expression of Sp1. On the other hand, MAOA transcription was diminished upon exposure of neuroblasts or cardiac myoblasts to ischemia-like conditions because of reduced binding of GATA2/Sp1/TBP with MAOA promoter. In conclusion, this study revealed previously unknown roles of GATA2, Sp1 and TBP in modulating MAOA expression under basal as well as pathophysiological conditions such as inflammation and ischemia, thus providing new insights into the molecular basis of aberrant MAOA expression in neuronal/cardiovascular disease states. Dysregulation of monoamine oxidase A (MAOA) have been implicated in several behavioral and neuronal disease states. Here, we identified three crucial transcription factors (GATA2, Sp1 and TBP) that regulate MAOA gene expression in a coordinated manner. Aberrant MAOA expression under pathophysiological conditions including inflammation and ischemia is mediated by altered binding of GATA2/Sp1/TBP with MAOA proximal promoter. Thus, these findings provide new insights into pathogenesis of several common diseases. GATA2, GATA-binding protein 2; Sp1, specificity protein 1; TBP, TATA-binding protein.

Activator protein-2α functions as a master regulator of multiple transcription factors in the mouse liver

AIM

  Activator protein 2α (AP-2α) belongs to the AP-2 family of transcription factors that are involved in the regulation of cell proliferation, differentiation, apoptosis and carcinogenesis and has been suggested to function as a tumor suppressor in many cancers. However, the physiological role of AP-2α in hepatocytes is unknown. The present study is to characterize the expression and function of AP-2α in the liver of conscience mouse.

METHODS

  Exogenous AP-2α was overexpressed in the mouse liver by in vivo gene delivery and changes in transcription factor expression were identified by using protein-DNA arrays and immunoblotting.

RESULTS

  Western blotting and protein/DNA arrays showed that AP-2α is expressed in the nuclei of mouse hepatocytes. Overexpression of AP-2αin vivo significantly suppressed transcription factors AP-1, CREB and c-Myc, and markedly increased CBF, c-Myb, NF-1, Pax-5, RXR, Smad3/4, TR(DR-4), USF-1 and GATA. Among all GATA proteins, only GATA-4 level was dramatically elevated and there was a concomitant loss of phospho-GATA-4. Corresponding changes were detected in upstream kinases Akt, GSK-3β and PKA, which regulates the phosphorylation status and stability of GATA-4 protein.

CONCLUSIONS

  AP-2α is expressed in mouse hepatocytes and it acts as a master regulator of numerous transcription factors in the liver.

Studies on a unique organelle localization of a liver enzyme, serine:pyruvate (or alanine:glyoxylate) aminotransferase

Serine:pyruvate (or alanine:glyoxylate) aminotransferase (SPT or AGT) in the liver is unique in that its subcellular distribution is entirely peroxisomal in man and herbivores, and largely mitochondrial in carnivores. In rats, this enzyme is located in both mitochondria and peroxisomes and only the mitochondrial activity is markedly induced by glucagon. The mechanism of the species-specific dual organelle localization is either transcription of the gene from two different start sites or loss of upstream translation initiation ATG codon by mutations. In herbivores, peroxisomal localization of SPT appears to be indispensable to prevent excessive oxalate production by removing glyoxylate, an immediate precursor of oxalate, formed from glycolate in this organelle. In carnivores, its mitochondrial localization appears to be needed to metabolize glyoxylate formed from L-hydroxyproline in mitochondria. In addition, SPT contributes substantially to gluconeogenesis from serine in rabbit, human and dog livers, irrespective of its mitochondrial or peroxisomal localization. (Communicated by Shigetada Nakanishi, M.J.A.).

Diet and the epigenetic (re)programming of phenotypic differences in behavior

Phenotypic diversity is shaped by both genetic and epigenetic mechanisms that program tissue specific patterns of gene expression. Cells, including neurons, undergo massive epigenetic reprogramming during development through modifications to chromatin structure, and by covalent modifications of the DNA through methylation. There is evidence that these changes are sensitive to environmental influences such as maternal behavior and diet, leading to sustained differences in phenotype. For example, natural variations in maternal behavior in the rat that influence stress reactivity in offspring induce long-term changes in gene expression, including in the glucocorticoid receptor, that are associated with altered histone acetylation, DNA methylation, and NGFI-A transcription factor binding. These effects can be reversed by early postnatal cross-fostering, and by pharmacological manipulations in adulthood, including Trichostatin A (TSA) and L-methionine administration, that influence the epigenetic status of critical loci in the brain. Because levels of methionine are influenced by diet, these effects suggest that diet could contribute significantly to this behavioral plasticity. Recent data suggest that similar mechanisms could influence human behavior and mental health. Epidemiological data suggest indeed that dietary changes in methyl contents could affect DNA methylation and gene expression programming. Nutritional restriction during gestation could affect epigenetic programming in the brain. These findings provide evidence for a stable yet dynamic epigenome capable of regulating phenotypic plasticity through epigenetic programming.

The social environment and the epigenome

The genome is programmed by the epigenome. Two of the fundamental components of the epigenome are chromatin structure and covalent modification of the DNA molecule itself by methylation. DNA methylation patterns are sculpted during development and it has been a long held belief that they remain stable after birth in somatic tissues. Recent data suggest that DNA methylation is dynamic later in life in postmitotic cells such as neurons and thus potentially responsive to different environmental stimuli throughout life. We hypothesize a mechanism linking the social environment early in life and long-term epigenetic programming of behavior and responsiveness to stress and health status later in life. We will also discuss the prospect that the epigenetic equilibrium remains responsive throughout life and that therefore environmental triggers could play a role in generating interindividual differences in human behavior later in life. We speculate that exposures to different environmental toxins alters long-established epigenetic programs in the brain as well as other tissues leading to late-onset disease.

Transcriptional induction of Smurf2 ubiquitin ligase by TGF-β

Smad ubiquitination regulatory factor 2 (Smurf2), a ubiquitin ligase for Smads, plays critical roles in the regulation of transforming growth factor-beta (TGF-beta)-Smad signaling via ubiquitin-dependent degradation of Smad2 and Smad7. We found that TGF-beta stimulates Smurf2 expression. TGF-beta activated the Smurf2 promoter in a TGF-beta responsive cell lines, whereas IL-1alpha, PDGF and epidermal growth factor did not. TGF-beta-mediated Smurf2 promoter activation was inhibited by Smad7 or an activin receptor-like kinase 5 inhibitor but not by dominant negative Smad or disruption of Smad-binding elements in the promoter. Moreover, inhibition of the phosphatidil inositol 3 kinase (PI3K)/Akt pathway suppressed TGF-beta-mediated Smurf2 induction. These results suggest that TGF-beta stimulates Smurf2 expression by Smad-independent pathway such as PI3K/Akt pathway via TGF-beta receptor.

Involvement of AP-2 binding sites in regulation of human beta-glucuronidase

The lysosomal hydrolase beta-glucuronidase (beta-gluc) can be used for the bioactivation of non-toxic glucuronide prodrugs of anticancer agents. The enzyme is present at high levels in many tumours and hence may lead to an enhanced drug targeting by tumour-selective release of the active anticancer drug. Individual expression and regulation of this enzyme is one factor modulating the bioactivation of glucuronide prodrugs. Nevertheless, in contrast to murine beta-gluc, which is inducible by androgens, the human enzyme has been regarded as an unregulated housekeeping gene due to a lacking TATA box and high G+C contents within the putative promotor sequence. Despite these facts, we were able to demonstrate downregulation of human beta-gluc expression by the calcium ionophore A23187 and the calcium ATPase inhibitor thapsigargin in the human hepatoma cell line HepG2. However, cis-acting elements responsible for this regulation have not yet been identified. We therefore characterised the 5'-untranslated region of the human beta-gluc gene using transient transfection assays with promotor-luciferase constructs in HepG2 cells and cloned fragments between 3,770 bp and 107 bp. A23187 reduced the beta-gluc promotor activity. This effect disappeared using fragments smaller than 356 bp. Using site-directed in vitro mutagenesis and gel-electrophoretic-mobility shift assays, we found evidence of an involvement of transcription factor activating protein-2 (AP-2) binding sites on the regulation of human beta-glucuronidase by A23187. Our studies provide a basis for the understanding of the transcriptional regulation of the human beta-glucuronidase gene and could be useful for the optimisation of glucuronide prodrug therapy.

Identification of a prostaglandin-responsive element in the Na,K-ATPase beta 1 promoter that is regulated by cAMP and Ca2+. Evidence for an interactive role of cAMP regulatory element-binding protein and Sp1

The Na,K-ATPase is a transmembrane protein responsible for maintaining electrochemical gradients across the plasma membrane in all mammalian cells, a process that is subject to regulation at the transcriptional as well as post-transcriptional level. Included among physiologic regulators in the kidney are prostaglandins. Previously, we demonstrated that prostaglandin E(1) (PGE(1)) increases the activity and expression of the Na,K-ATPase in Madin-Darby canine kidney cells (Taub, M., Borsick, M., Geisel, J., Matlhagela, K., Rajkhowa, T., and Allen, C. (2004) Exp. Cell Res. 299, 1-14; Taub, M. L., Wang, Y., Yang, I. S., Fiorella, P., and Lee, S. M. (1992) J. Cell. Physiol. 151, 337-346). In this work, we present evidence that transcription of the Na,K-ATPase beta(1) subunit is stimulated by PGE(1), an effect that may be mediated through the cAMP and Ca(2+) pathways. Transient transfection studies using 5'-deletion mutants of the human beta(1) subunit promoter indicated that region -100 to -92 containing the sequence AGTCCCTGC (a prostaglandin-responsive element (PGRE)) is required to elicit the stimulatory effects of PGE(1), 8-bromo-cAMP, phorbol 12-myristate 13-acetate, and okadaic acid. Electrophoretic mobility shift assays indicated that both the cAMP regulatory element-binding protein (CREB) and Sp1 bind to the PGRE present within this region of the beta(1) subunit promoter. The involvement of the PGRE and Sp1 sites in regulation by PGE(1) was further confirmed by the increased PGE(1) stimulation that was observed following insertion of the PGRE into a promoter/luciferase construct containing a portion of a heterologous promoter and the fibronectin promoter with four GC boxes. Further evidence suggesting an interaction between Sp1 and CREB was obtained from experiments conducted with pLuc-MCS-beta 72-167, which contains region -167 to -72 in the human beta(1) subunit promoter. The PGE(1) stimulation observed in Madin-Darby canine kidney cells transiently transfected with pLuc-MCS-beta 72-167 was reduced when the two GC boxes immediately upstream from the PGRE were translocated farther upstream. Also consistent with an interaction between CREB and Sp1 are the results of our immunoprecipitation studies indicating that CREB co-immunoprecipitated with Sp1 when an antibody against CREB, Sp1, or the CREB-binding protein was used.

Regulation of sphingosine kinase 1 gene expression by protein kinase C in a human leukemia cell line, MEG-O1

The prolonged treatment with phorbol 12-myristate 13-acetate (PMA) of a human megakaryoblastic leukemia cell line, MEG-O1, induced increase of sphingosine kinase (SPHK) enzyme activity and SPHK1 protein expression as well as SPHK1 message. Protein kinase C (PKC) inhibitor prevented the PMA-induced SPHK1 gene expression. To elucidate the regulatory mechanism of this gene expression, we examined the promoter area (distal to the first exon) and its binding proteins. Luciferase analyses showed that the area of 300 bp from the first exon was sufficient for PMA-responsiveness, and that specificity protein 1 (Sp1)- and two activator protein 2 (AP-2)-binding motifs within this area were necessary for responsiveness. Inhibitors for PKC and MEK1 decreased this PMA-induced promoter activity. Electrophoresis mobility shift assay (EMSA) showed that Sp1 protein was originally bound to the Sp1 site and that two additional bands bound to the two AP-2 motifs were observed only when stimulated with PMA in MEG-O1 cells. The appearance of these bands resulted from binding to an unknown protein rather than AP-2. These results indicated that PMA up-regulates SPHK1 gene expression through PMA-responsive elements of the 5' promoter area of the gene, and suggested that PMA-mediated SPHK1 gene expression would be mediated via PKC- and ERK-dependent signal transduction pathway by binding the transcription factor to AP-2 motifs.

Control of oxalate formation from L-hydroxyproline in liver mitochondria

Serine:pyruvate/alanine:glyoxylate aminotransferase (SPT/AGT) is largely located in mitochondria in carnivores, whereas it is entirely found within peroxisomes in herbivores and humans. In rat liver, SPT/AGT is found in both of these organelles, and only the mitochondrial enzyme is markedly induced by glucagon. Although SPT/AGT is a bifunctional enzyme involved in the metabolism of both L-serine and glyoxylate, its contribution to L-serine metabolism is independent of mitochondrial or peroxisomal localization (Xue HH et al., J Biol Chem 274: 16028-16033, 1999). Therefore, the species-specific and food habit-dependent organelle distribution might be required for proper metabolism of glyoxylate at the subcellular site of its formation. Glyoxylate formation from glycolate and that from L-hydroxyproline have been shown to occur in peroxisomes and mitochondria, respectively. The present study found that urinary excretion of oxalate was markedly increased when a large dose of L-hydroxyproline or glycolate was administered to rats. Oxalate formation from L-hydroxyproline but not that from glycolate was significantly reduced when mitochondrial SPT/AGT had been induced by glucagon. The hydroxyproline content of collagen is 10 to 13%, and collagen accounts for about 30% of total animal protein; therefore, these results suggest that an important role of mitochondrial SPT/AGT in carnivores is to convert L-hydroxyproline-derived glyoxylate into glycine in situ, preventing undesirable overflow into the production of oxalate.