Svetlana Ivanovic-Matic

STAT3/NFκB Interplay in the Regulation of α2‐Macroglobulin Gene Expression During Rat Liver Development and the Acute Phase Response

The synthesis of alpha‐2‐macroglobulin (α2M) is low in adult rat liver and elevated in fetal liver. During the acute‐phase (AP) response it becomes significantly increased in both adult and fetal liver. In this work, the cross talk of STAT3 and NF‐κB transcription factors during α2M gene expression was analysed. Using immunoblotting, their cellular compartmentalization was examined by comparing the cytoplasmic levels of STAT3 and NF‐κB with their active equivalents, the 86 and 91 kDa isoforms and p65‐subunit, respectively, in the nuclear extract and nuclear matrix. Different partitioning dynamics of the transcription factors were observed. At the level of protein‐DNA interactions, studied by α2M promoter affinity chromatography, it was established that different ratios of promoter‐binding STAT3 isoforms participated in elevated hepatic transcription in the basal state fetus and the AP‐adult, but only the 91 kDa isoform in the AP‐fetus. Unchanged levels of DNA‐bound p65 in the control and AP‐fetus suggest that it participated in constitutive transcription. The promoter‐binding of p65 observed in the AP‐adult suggests that it was involved in transcriptional stimulation of α2M expression. The selective enrichment of the AP‐adult nuclear matrix with promoter‐binding STAT3 disclosed the importance of this association in the induction of transcription. Protein‐protein interactions were examined by co‐immunoprecipitation. Interactions between the 86 kDa STAT3 isoform and p65 that were observed in the control and AP‐fetus and of both the 86 and 91 kDa STAT3 isoforms with p65 in the AP‐adult, suggest that protein‐protein interactions were functionally connected to increased transcription. We concluded that α2M gene expression is driven by developmental‐ and AP‐related mechanisms that rely on STAT3/NF‐κB interplay. IUBMB Life, 59: 170‐178, 2007

The protein composition of the hepatocyte nuclear matrix is differentiation-stage specific.

The protein composition of hepatocyte nuclear matrices was examined in rats from the 16th day of gestation to 75 days after birth (adult). An overall increase in size of the nuclear matrix was accompanied by quantitative and qualitative changes in its protein content. Quantitative changes of the major proteins of the peripheral lamina surrounding the isolated nuclear matrix were detected. By Western analysis we established that in pre- and postnatal nuclear matrices the relative concentrations of lamin C were greater than lamin A. After birth, the relative concentrations of both lamins progressively increased. In the adult nuclear matrix, the concentration of lamin A was greater than lamin C. In contrast, the relative concentrations of lamin B remained unchanged throughout development and growth. The relative concentrations of two nuclear matrix-associated regulatory proteins studied changed with development and growth: transcription factor C/EBPalpha isoforms, which were detected during the gestation period, increased notably after the first postnatal day, attaining a maximum at the adult stage; the high concentrations of the proliferating cell nuclear antigen (PCNA) perceptibly decreased after the 21st prenatal day. Changes in the composition of the nuclear matrix protein suggest that this structure coordinates nuclear functioning during cell differentiation.

The acute phase response of rats to soman intoxication

The capacity of an organophosphate to elicit the acute phase response (APR) was assessed by studying the effects of acute soman intoxication on two major processes which characterize inflammation, cytokine production in macrophages and the expression of acute phase protein (APP) genes in the liver. It was established that the concentration of lymphostimulatory substances secreted by the macrophages of soman-intoxicated rats was increased to a level characteristic of the primary inflammatory reaction. Macrophage activation was followed by increased transcription rates of APP genes and the corresponding mRNA and protein synthesis in the liver. The pattern of the DNA-protein complexes obtained with nuclear extracts and the cis-element of the rat haptoglobin gene in the gel-retardation assay suggested that the molecular events which underlie the expression of APP genes of intoxicated rats are similar to those that occur during the APR. From these findings we concluded that soman intoxication was a metabolic injury which elicited the typical APR.

Association of the glucocorticoid receptor with STAT3, C/EBPβ, and the hormone-responsive element within the rat haptoglobin gene promoter during the acute phase response

Upregulation of haptoglobin (Hp) expression in the rat during the acute phase (AP) response is the result of synergistic effects of IL‐6–, IL‐1β–, and corticosterone‐activated signaling pathways. IL‐6 signaling terminates in cis–trans interactions of the Hp gene hormone‐responsive element (HRE) with transcription factors STAT3 and C/EBPβ. The aim of this study was to examine the unresolved molecular mechanism of glucocorticoid action. A 3‐fold rise in serum corticosterone at 2 and 4 h of the AP response induced by turpentine administration preceded a 2.3‐fold increase in the rate of Hp gene transcription at 12 h that was accompanied by a 4.8‐fold increase in glucocorticoid receptor (GR), the appearance of an 86‐kDa STAT3 isoform and 3.9‐, 1.9‐, and 1.7‐fold increased amounts of 91‐kDa STAT3, 35‐ and 42‐kDa C/EBPβ isoforms in the nucleus. These events resulted in 4.6‐ and 2.5‐fold increased Hp levels in the liver and serum at 24 h. HRE affinity chromatography and immunoblot analysis revealed that maximal occupancy of the HRE with GR, STAT3, and C/EBPβ at 12 h correlated with increased transcriptional activity of the Hp gene. Coimmunoprecipitation experiments showed that activated GR established de novo interaction with STAT3 isoforms while GR–C/EBPβ interactions observed during basal transcription increased during the AP response. Computer analysis of the HRE disclosed two potential GR‐binding sites: one overlapping STAT3, another adjacent to a C/EBPβ‐binding site. This finding and the experimental results suggest that activated GR through direct interactions with STAT3 and C/EBPβ, participates in Hp gene upregulation as a transcriptional coactivator.

C/EBPα AND C/EBPβ ARE PERSISTENTLY ASSOCIATED WITH THE RAT LIVER NUCLEAR MATRIX THROUGHOUT DEVELOPMENT AND THE ACUTE PHASE RESPONSE

The partitioning of C/EBPα and C/EBPβ on the nuclear matrix structure was examined during the different transcriptional activities accompanying liver development and the acute phase (AP) response. The presence of C/EBPα and C/EBPβ was established on the nuclear matrix. Their relative concentrations on the matrix always reflected the developmental stage‐ and AP‐related fluctuations observed in the nuclear extract. Thus, they progressively increased as development proceeded, whereas during the AP response, C/EBPα decreased and C/EBPβ increased. In addition, the levels of both transcription factors were always notably higher in the nuclear matrix than in the extracts. We conclude that the observed changes and overall enrichment of the nuclear matrix with regulatory proteins is a reflection of the importance of such interactions for the in vivo functioning of C/EBP proteins.

IDENTIFICATION OF NUCLEAR MATRIX AND ASSOCIATED PROTEINS THAT BIND THE HAPTOGLOBIN GENE CIS-ELEMENT

To identify the major nuclear matrix proteins that bind to the rat haptoglobin gene cis‐element, we isolated a soluble nuclear matrix protein fraction and analysed it by gel retardation. Two major DNA‐binding proteins exhibiting different types of protein‐DNA interactions were detected: a DNA sequence‐specific 32‐kDa isoform of transcription factor C/EBPbeta, and a nuclear matrix protein p55 that bound to the DNA nonspecifically. During increased transcription of the haptoglobin gene in the course of the acute‐phase reaction, the DNA‐binding affinities and concentrations of these proteins in the soluble nuclear matrix fraction were increased. These data lend further evidence that the nuclear matrix is an active support structure that localizes gene regulatory proteins and participates in transcriptional regulation.

Properties of nuclear matrix proteins that bind the 5' flanking region of the haptoglobin gene are changed during the acute-phase response.

It was previously shown that during the acute‐phase response‐induced elevated transcription of the rat haptoglobin gene, protein p55 and the lamins mediate the increased binding of restriction fragment II (−541/−146) via a 38 bp adenine tract lying 147bp upstream from the haptoglobin gene cis element (−165/−56), to scaffold type II‐like nuclear matrices. Here we show that the fragment II binding pattern to ≥40 kDa proteins of nuclear matrices analogous to type I scaffolds is more complex. Fragment II bound conspicuously to a 40 and less so to p55, a 60 kDa protein and the lamins of control matrices. During increased gene transcription, it bound prominently to p55, the lamins, and less so to 45 and 52 kDa proteins. This was accompanied by the tenacious binding of the DNA to isolated nuclear matrices in vitro and post‐translational modifications of certain matrix proteins. The lamins and p55 demonstrated a greater N‐acetylglucosamine/sialic acid content and p55 an increased in vitro phosphorylation by a nuclear matrix‐associated cyclic‐nucleotide‐independent kinase. The acute‐phase response also caused an increased partitioning of p55 with the nuclear matrix. It was concluded that, as a result of a molecular remodelling of the nuclear matrix at the point of contact with chromatin, the nature of its association with region II DNA changed during elevated haptoglobin gene expression.

C/EBPα and C/EBPβ Regulate Haptoglobin Gene Expression during Rat Liver Development and the Acute-phase Response

The participation of C/EBPα and C/EBPβ in the transcriptional regulation of the haptoglobin (Hp) gene throughout liver development and the acute-phase (AP) response was examined. Western immunoblot analysis revealed that the relative concentrations of C/EBPα and C/EBPβ increased during differentiation in two nuclear protein fractions – the nuclear extract and nuclear matrix. The AP reaction was accompanied by a decrease of the relative concentration of C/EBPα and an increase of C/EBPβ during development in both protein fractions. Using Western analysis after DNA-affinity chromatography it was observed that a 45 kDa C/EBPα isoform displayed a binding affinity towards the Hp gene hormone responsive element (HRE) in both pre- and postnatal livers. In the course of the AP response DNA binding of the 45 kDa isoform was detected only in the adult, when its binding affinity decreased. The 35 kDa C/EBPβ isoform exhibited a binding affinity towards the Hp HRE after the second week from birth, whereas the AP response promoted an enhanced binding of 35 kDa isoform after the first postnatal week. These results indicate that Hp gene transcription is regulated by C/EBPα during normal liver development, whereas C/EBPβ is involved in the AP regulation during the later phase of differentiation and in the adult.

Acute-phase related binding ability of p53 for the hormone response element of the haptoglobin gene in adult rats.

Interaction between transcription factor p53 and the hormone response element (HRE) of the haptoglobin (Hp) gene in adult rat liver was studied. We detected a sequence homologous to the p53 consensus DNA‐binding site in the regulatory promoter element of the Hp gene. DNA‐affinity chromatography, followed by Western immunoblot analysis with an antibody to p53 indicated that components of the nuclear extract possessed the same antigen determinants as p53. While p53 was identified in both control and acute‐phase (AP) samples, DNA‐binding affinity for the Hp gene HRE was detected only in the nuclear extract prepared from rats undergoing the AP response. Whether either as an inducible or as a constitutive transcription factor, p53 could be involved in the transcriptional regulation of the Hp gene in adult rat liver.

Oxidative stress-dependent contribution of HMGB1 to the interplay between apoptosis and autophagy in diabetic rat liver

The progression of oxidative stress, resulting cell damage, and cell death underlies the etiology of liver damage/dysfunction as a complication of diabetes. High-mobility group box 1 (HMGB1) protein, a chromatin-binding nuclear protein and damage-associated molecular pattern molecule, is integral to oxidative stress and signaling pathways regulating cell death and cell survival. We previously found that in streptozotocin (STZ)-induced diabetic rats, reduction of oxidative stress after melatonin administration lowered necrotic cell death and increased expression of HMGB1 and hepatocellular damage. In the present study, we examined whether alleviation of diabetes-attendant oxidative stress and ensuing change in HMGB1 expression influence the dynamic equilibrium between apoptosis/autophagy and liver damage. We observed that elevated HMGB1 protein levels in diabetic rat liver accompanied increased interactions of HMGB1 with TLR4 and RAGE, and activation of the intrinsic apoptotic pathway and Beclin 1-dependent autophagy. The absence of p62 degradation in diabetic rat liver pointed to defective autophagy which was responsible for lower autophagosome/autophagolysosome formation and an increased apoptosis/autophagy ratio. Compared to diabetic rats, in melatonin-treated diabetic rats, the structure of liver cells was preserved, HMGB1/TLR4 interaction and downstream apoptotic signaling were significantly reduced, HMGB1/Beclin 1 colocalization and interactions were augmented and Beclin 1-mediated autophagy, mithophagy in particular, were increased. We concluded that in mild oxidative stress, HMGB1 is cytoprotective, whereas in intense oxidative stress, HMGB1 actions promote cell death and liver damage. Since reduced HMGB1 binds to RAGE but not to TLR4, redox modification of HMGB1 as a mechanism regulating the cross-talk between apoptosis and autophagy in diabetes is discussed.