Inhibition of extracellular signal-regulated kinase (ERK) signaling participates in repression of nuclear factor (NF)-kappaB activity by glucocorticoids.

Glucocorticoid (GC) effects are mediated via the GC-receptor (GR), which either stimulates or represses gene expression. Repression of target genes often involves negative cross-talk between the GR and other transcription factors e.g. NF-kappaB, important for gene activation. Using HEK293 cells we here describe that repression of NF-kappaB requires functions of the GR that are dependent on the signaling pathways employed to activate NF-kappaB. While a GR mutant was able to repress NF-kappaB activity following activation by TNFalpha, it did not so following activation by the phorbol ester TPA. In these cells, TPA stimulation but not TNFalpha, activated extracellular signal-regulated kinase (ERK). We demonstrated that the ability of the dexamethasone activated GR mutant to repress TPA-induced NF-kappaB activity was restored in conjunction with ERK1/2 inhibition. Previous reports have shown GC-mediated inhibition of ERK1/2 phosphorylation to involve GC induction of MAPK phosphatase-1 (MKP-1). Here, we demonstrated that the GRR488Q mutant was incapable of inducing gene expression of endogenous MKP-1 following dexamethasone treatment, in contrast to the GRwt. However, TPA treatment alone resulted in much stronger MKP-1 expression in both GRwt and GRR488Q containing cells than that of dexamethasone suggesting that the inability of GRR488Q to inhibit TPA-induced NF-kappaB activity did not involve a lack of MKP-1 expression. In line with this, RNAi targeted towards MKP-1 did not abolish or inhibit the ability of the GRwt to repress NF-kappaB activity. Importantly, we observed no difference in activated ERK1/2 (phospho-ERK1/2) expression over time between GRwt and GRR488Q containing cells following co-treatment with TPA and dexamethasone. Based on these results we suggest that GRwt does not directly regulate ERK1/2 but rather alters ERK1/2-mediated effects allowing it to repress NF-kappaB activity, a capacity lacked by the GRR488Q mutant.

Detection of the mitochondrial and catalytically active alanine aminotransferase in human tissues and plasma.

Serum alanine aminotransferase (ALT) is used as a clinical marker of hepatotoxicity. Three forms of human ALT have been identified, ALT1 and 2 and an alternative splice variant of ALT2 (herein called ALT2_2). The standard ALT activity assay does not discriminate between ALT from different organs, or the isoforms measured in the plasma. Here, we show that ALT1 and 2 possess similar enzymatic activity for alanine and pyruvate but with different Km and kcat values, while recombinant ALT2_2 protein does not possess any enzymatic activity. Isolation of organelles from cultured human skeletal muscle cells, showed localisation of ALT2 to the mitochondrial fraction and endoplasmatic reticulum (ER), but not to the cytosol. In human hepatocytes, on the other hand, ALT1 was only localised to the cytosol and ER, with no detection in mitochondria. ALT2 was not detected in cultured human hepatocytes, liver extract or tissue using Western blotting or immunohistochemistry. The islet of Langerhans and cardiomyocytes were other examples of cells with high expression of catalytic ALT2. A clinical method for selective measurement of ALT1 and 2 in human plasma is described, and both ALT1 and 2 were immunoprecipitated from human plasma and structurally detected using Western blotting techniques.

Identification of target genes involved in the antiproliferative effect of glucocorticoids reveals a role for nuclear factor-(kappa)B repression.

Glucocorticoid hormones (GCs) exert an antiproliferative effect on most cells. However, the molecular mechanism is still largely unclear. We investigated the antiproliferative mechanism by GCs in human embryonic kidney 293 cells with stably introduced glucocorticoid receptor (GR) mutants that discriminate between cross-talk with nuclear factor-(kappa)B (NF-(kappa)B) and activator protein-1 signaling, transactivation and transrepression, and antiproliferative vs. non-antiproliferative responses. Using the GR mutants, we here demonstrate a correlation between repression of NF-(kappa)B signaling and antiproliferative response. Gene expression profiling of endogenous genes in cells containing mutant GRs identified a limited number of genes that correlated with the antiproliferative response. This included a GC-mediated up-regulation of the NF-(kappa)B-inhibitory protein I(kappa)B(alpha), in line with repression of NF-(kappa)B signaling being important in the GC-mediated antiproliferative response. Interestingly, the GC-stimulated expression of I(kappa)B(alpha) was a direct effect despite the inability of the GR mutant to transactivate through a GC-responsive element. Selective expression of I(kappa)B(alpha) in human embryonic kidney 293 cells resulted in a decreased percentage of cells in the S/G2/M phase and impaired cell proliferation. These results demonstrate that GC-mediated inhibition of NF-(kappa)B is an important mechanism in the antiproliferative response to GCs.

Isoform-specific alanine aminotransferase measurement can distinguish hepatic from extrahepatic injury in humans.

Serum alanine aminotransferase (ALT) is used as a clinical marker to detect hepatic damage and hepatoxicity. Two isoforms of ALT have been identified, ALT1 and ALT2, which have identical enzymatic capacities and are detected simultaneously in human serum/plasma using classical clinical chemical assays. Differences exist in the expression patterns of the ALT1 and ALT2 proteins in different organs which suggest that changes in the proportion of ALT1 and ALT2 in plasma may arise and reflect damage to different human organs. However, this has not been previously studied due to the lack of a selective methodology that can quantify both ALT1 and ALT2 isoforms in the total ALT activity normally measured in clinical samples. To the best of our knowledge, our current study reveals for the first time, that under 3 different conditions of liver damage (non-alcoholic fatty liver disease, hepatitis C and during liver surgery) the leakage of ALT1 activity into plasma greatly exceeds that of ALT2, and that the measurement of ALT1 during liver damage is equal to the measurement of total ALT activity. By contrast, during skeletal muscle injury, induced in volunteers by physical exertion, the leakage of ALT2 exceeds that of ALT1 and the proportion of circulating ALT isoforms changes accordingly. The ALT isoform changes occurring in plasma reflect previously demonstrated relative contents of ALT1 and ALT2 activities in human liver and skeletal muscle. These data suggest that assessing the percentage contribution of ALT1 and ALT2 activities to total ALT activity in plasma may distinguish hepatic from extrahepatic injury using the same standard analytical platform.

PPARalpha regulates the hepatotoxic biomarker alanine aminotransferase (ALT1) gene expression in human hepatocytes.

In this work, we investigated a potential mechanism behind the observation of increased aminotransferase levels in a phase I clinical trial using a lipid-lowering drug, the peroxisome proliferator-activated receptor (PPAR) alpha agonist, AZD4619. In healthy volunteers treated with AZD4619, serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities were elevated without an increase in other markers for liver injury. These increases in serum aminotransferases have previously been reported in some patients receiving another PPARalpha agonist, fenofibrate. In subsequent in vitro studies, we observed increased expression of ALT1 protein and mRNA in human hepatocytes after treatment with fenofibric acid. The PPAR effect on ALT1 expression was shown to act through a direct transcriptional mechanism involving at least one PPAR response element (PPRE) in the proximal ALT1 promoter, while no effect of fenofibrate and AZD4619 was observed on the ALT2 promoter. Binding of PPARs to the PPRE located at -574 bp from the transcriptional start site was confirmed on both synthetic oligonucleotides and DNA in hepatocytes. These data show that intracellular ALT expression is regulated by PPAR agonists and that this mechanism might contribute to increased ALT activity in serum.

Isoforms of alanine aminotransferases in human tissues and serum--differential tissue expression using novel antibodies.

Serum alanine aminotransferase (ALT) is used as a clinical marker of hepatotoxicity. Two forms of ALT have been identified, ALT1 and ALT2, encoded by separate genes. The cellular and tissue distribution of the different ALT proteins has not been characterized in humans, and their relative contribution to serum is unknown. Here, we describe the development of novel isoenzyme specific ALT1 and ALT2 antibodies and the expression of the enzymes in human cells and organs. In normal human tissue, high expression of ALT1 was found in liver, skeletal muscle and kidney and low levels in heart muscle and not detectable in pancreas. High ALT2 reactivity was detected in heart and skeletal muscle, while no ALT2 expression was found in liver or kidney. Using immunohistochemistry, strong ALT1 reactivity was found in hepatocytes, renaltubular epithelial cells and in salivary gland epithelial cells, while ALT2 was expressed in adrenal gland cortex, neuronal cell bodies, cardiac myocytes, skeletal muscle fibers and endocrine pancreas. Immunoprecipitation using ALT antibodies on normal human serums showed ALT1 to be mainly responsible for basal ALT activity. Together, the results points to a differential expression of ALT1 and ALT2 in human organs and substantiate a need for investigations regarding the possible impacts on ALT measurements.

Cross-talk between glucocorticoid and retinoic acid signals involving glucocorticoid receptor interaction with the homoeodomain protein Pbx1.

Glucocorticoid (GC) signalling influences the response of the cell to a number of other signals via a mechanism referred to as 'cross-talk'. This cross-talk may act at several levels, including an interaction between the transcription factors involved in the signalling pathways. In the present paper, we demonstrate a novel functional interaction between GC and all- trans -retinoic acid (RA) signalling. We show that, in P19 embryonal carcinoma cells, GCs potentiate RA-induced expression of the murine Hoxb -1 gene through an autoregulatory element, b1-ARE, recognized by the Pbx1 and HOXB1 homoeodomain proteins. The synergistic effect of GC did not involve GC receptor (GR) binding to the b1-ARE, and the GC-GR complex alone was unable to activate transcription via the element. Furthermore, the ability of the GR to transactivate was not required, excluding expression of a GC-induced protein as the mechanism for the GC/RA synergy. Additional transfection experiments showed that the Pbx1/HOXB1 heterodimer was the target for the GC effect. Furthermore, functional dissection of the GR demonstrated that the DNA-binding domain (DBD) of the GR was required for the synergy. A physical interaction between the GR and Pbx1 proteins was demonstrated in vivo by co-immunoprecipitation experiments. These results are compatible with a model in which the GC/RA synergy is mediated by a direct interaction between the GR and Pbx1. On the basis of the ubiquitous expression of both GR and Pbx1, a number of genes regulated by Pbx are likely to be important targets for GC-mediated 'cross-talk'.