T1 is a c-Fos- and FosB-responsive gene which is induced by growth factors through multiple signal transduction pathways

Runx2 mediated Induction of Novel Targets ST2 and Runx3 Leads to Cooperative Regulation of Hypertrophic Differentiation in ATDC5 Chondrocytes

Knowledge concerning expression and function of Suppression of Tumorigenicity 2 (ST2) in chondrocytes is at present, limited. Analysis of murine growth plates and ATDC5 chondrocytes indicated peak expression of the ST2 transmembrane receptor (ST2L) and soluble (sST2) isoforms during the hypertrophic differentiation concomitant with the expression of the hypertrophic markers Collagen X (Col X), Runx2 and MMP-13. Gain- and loss-of-function experiments in ATDC5 and primary human growth plate chondrocytes (PHCs), confirmed regulation of ST2 by the key transcription factor Runx2, indicating ST2 to be a novel Runx2 target. ST2 knock-out mice (ST2−/−) exhibited noticeable hypertrophic zone (HZ) reduction in murine growth plates, accompanied by lower expression of Col X and Osteocalcin (OSC) compared to wild-type (WT) mice. Likewise, ST2 knockdown resulted in decreased Col X expression and downregulation of OSC and Vascular Endothelial Growth Factor (VEGF) in ATDC5 cells. The ST2 suppression was also associated with upregulation of the proliferative stage markers Sox9 and Collagen II (Col II), indicating ST2 to be a new regulator of ATDC5 chondrocyte differentiation. Runx3 was, furthermore, identified as a novel Runx2 target in chondrocytes. This study suggests that Runx2 mediates ST2 and Runx3 induction to cooperatively regulate hypertrophic differentiation of ATDC5 chondrocytes.

STAT3 and ERK pathways are involved in cell growth stimulation of the ST2/IL1RL1 promoter

The ST2 gene was originally identified as a primary responsive gene induced by stimulation with growth factors and by oncogenic stress. The ST2 gene harbors two distinct promoters - a distal promoter and a proximal promoter. In this study, we identified a novel type of serum-responsive element in the ST2 proximal promoter using reporter gene analysis; this element includes a possible responsive element for STAT family proteins. Indeed, enforced expression of constitutively active STAT3 activated this promoter element and induced the expression of ST2 gene products. Furthermore, an oncogenic Ras (G12V) mutant also caused the expression of ST2 gene products by utilizing the proximal promoter. We also clarified that activation of the ST2 promoter by either growth stimulation or oncogenic Ras was suppressed by the inhibitors for STAT3 and ERK pathways. Our observations strongly suggest the importance of STAT family and ERK pathways for the induction of ST2 gene products by cell growth stimulation.

The biological paths of IL-1 family members IL-18 and IL-33

Cytokines are key mediators of the immune system, and few have been more thoroughly studied than those of the IL-1 family. IL-1α and IL-1β are the founding members and now celebrate 25 years since their cloning. In that time, IL-1-directed research has illuminated many aspects of cytokine biology and innate immunity. The family is now recognized to include 11 total members, including IL-18 and IL-33, which are the topic of this review. These two inflammatory cytokines are expressed broadly, and their actions influence a variety of physiologic responses involved in inflammation and immunity. The purpose of this article is not to provide an exhaustive review of IL-18 and IL-33 but rather, to summarize what is known about their key functions and to provide perspective on their similarities and differences.

IL-33: a tissue derived cytokine pathway involved in allergic inflammation and asthma

Besides classic T cell-derived T-helper type 2 (Th2) cytokines such as IL-4, IL-5 and IL-13, tissue-produced cytokines such as thymic stromal lymphopoietin, IL-25 and IL-33 are now recognized as important contributors to allergic inflammation. IL-33 is produced by various tissue dwelling cells and broadly enhances allergic inflammation through its effects on hematopoietic cell types. The environmental or endogenous triggers that provoke IL-33 cellular release may be associated with infection, inflammation or tissue damage. This review summarizes the wide range of documented IL-33 activities on human cellular mediators of inflammation as well as genetic evidence that IL-33 contributes to disease. Finally, there will be a discussion of still unanswered questions regarding the mechanisms by which cytokine activity is generated and IL-33's relationship with other Th2-associated cytokines.

Growth state-dependent binding of USF-1 to a proximal promoter E box element in the rat plasminogen activator inhibitor type 1 gene

Induced PAI-1 gene expression in renal epithelial (NRK-52E, clone EC-1) cells occurs as part of the immediate-early response to serum. PAI-1 transcripts are maximally expressed early in G(1) (within 4 h of serum addition to quiescent EC-1 cells) and then subsequently decline to basal levels prior to entry into DNA synthetic phase. Comparative analysis of PAI-1 mRNA abundance and de novo-synthesized thiolated RNA in quiescent cells, as well as at 4 h (early G(1)) and 20 h (late G(2)) postserum addition, in conjunction with RNA decay measurements indicated that PAI-1 gene regulation upon growth activation was predominantly transcriptional. An E box motif (CACGTG), important in the induced expression of some growth state-dependent genes, mapped to nucleotides -160 to -165 upstream of the transcription start site in the PAI-1 proximal promoter. Mobility-shift assessments, using a 18-bp deoxyoligonucleotide construct containing the E box within the context of PAI-1-specific flanking sequences, confirmed binding of EC-1 nuclear protein(s) to this probe and, specifically, to the E box hexanucleotide site. The specificity of this protein-probe interaction was verified by competition analyses with double-stranded DNA constructs that included E box deoxyoligonucleotides with non-PAI-1 flanking bases, mutant E box sequences incapable of binding NRK nuclear proteins, and unrelated (i.e., AP-1) target motifs. Extract immunodepletion and supershift/complex-blocking experiments identified one PAI-1 E box-binding protein to be upstream stimulatory factor-1 (USF-1), a member of the HLH family of transcription factors. Mutation of the CACGTG site to TCCGTG in an 18-bp PAI-1 probe inhibited the formation of USF-1-containing complexes confirming that an intact E box motif at -160 to -165 bp in the PAI-1 promoter and, in particular, the CA residues at -165 and -164 are essential for USF-1 binding. Incorporation of this 2 bp change into a reporter construct containing 764 bp of the proximal PAI-1 "promoter" ligated to a CAT gene effectively reduced (by 74%) CAT activity in cycling cells. An intact E box motif at nucleotides -160 to -165 in the PAI-1 promoter, thus, is an important functional element in the regulation of PAI-1 transcriptional activity in renal cells.

The absence of interleukin 1 receptor-related T1/ST2 does not affect T helper cell type 2 development and its effector function

T1/ST2, an orphan receptor with homology with the interleukin (IL)-1 receptor family, is expressed constitutively and stably on the surface of T helper type 2 (Th2) cells, but not on Th1 cells. T1/ST2 is also expressed on mast cells, which are critical for Th2-mediated effector responses. To evaluate whether T1/ST2 is required for Th2 responses and mast cell function, we have generated T1/ST2-deficient (T1/ST2(-/-)) mice and examined the roles of T1/ST2. Naive CD4(+) T cells isolated from T1/ST2(-/-) mice developed to Th2 cells in response to IL-4 in vitro. T1/ST2(-/-) mice showed normal Th2 responses after infection with the helminthic parasite Nippostrongylus brasiliensis as well as in the mouse model of allergen-induced airway inflammation. In addition, differentiation and function of bone marrow-derived cultured mast cells were unaffected. These findings demonstrate that T1/ST2 does not play an essential role in development and function of Th2 cells and mast cells.

Post-transcriptional regulation of the arginine transporter Cat-1 by amino acid availability

The regulation of the high affinity cationic amino acid transporter (Cat-1) by amino acid availability has been studied. In C6 glioma and NRK kidney cells, cat-1 mRNA levels increased 3.8-18-fold following 2 h of amino acid starvation. The transcription rate of the cat-1 gene remained unchanged during amino acid starvation, suggesting a post-transcriptional mechanism of regulation. This mechanism was investigated by expressing a cat-1 mRNA from a tetracycline-regulated promoter. The cat-1 mRNA contained 1.9 kilobase pairs (kb) of coding sequence, 4.5 kb of 3'-untranslated region, and 80 base pairs of 5'-untranslated region. The full-length (7.9 kb) mRNA increased 5-fold in amino acid-depleted cells. However, a 3.4-kb species that results from the usage of an alternative polyadenylation site was not induced, suggesting that the cat-1 mRNA was stabilized by cis-acting RNA sequences within the 3'-UTR. Transcription and protein synthesis were required for the increase in full-length cat-1 mRNA level. Because omission of amino acids from the cell culture medium leads to a substantial decrease in protein synthesis, the translation of the increased cat-1 mRNA was assessed in amino acid-depleted cells. Western blot analysis demonstrated that cat-1 mRNA and protein levels changed in parallel. The increase in protein level was significantly lower than the increase in mRNA level, supporting the conclusion that cat-1 mRNA is inefficiently translated when the supply of amino acids is limited, relative to amino acid-fed cells. Finally, y(+)-mediated transport of arginine in amino acid-fed and -starved cells paralleled Cat-1 protein levels. We conclude that the cat-1 gene is subject to adaptive regulation by amino acid availability. Amino acid depletion initiates molecular events that lead to increased cat-1 mRNA stability. This causes an increase in Cat-1 protein, and y(+) transport once amino acids become available.

Attenuated expression of the serum responsive T1 gene in ras transformed fibroblasts due to the inhibition of c-fos gene activity

The T1 gene encodes a protein, which shares homology with the IL-1 receptors. In fibroblasts, T1 is induced by growth factors and in response to the onset of oncogene expression. The c-fos gene is transiently activated in these situations and was shown to be the major mediator of T1 gene induction. In contrast, the sustained expression of a ras oncogene in NIH3T3 cells resulted in the downregulation of basal T1 gene activity and the attenuation of T1 gene induction in response to mitogenic signals. Likewise, the immediate early genes encoding c-Fos, FosB, and Fra-2 are repressed in these cells. T1 gene repression could be overcome by the forced expression of c-fos in ras transformed fibroblasts. Thus, the lack of c-fos gene expression is the likely cause for ras mediated T1 gene repression. Fra-1, in contrast to the other three members of the Fos family, is permanently synthesized in high amounts in ras transformed NIH3T3 fibroblasts. We show that AP-1, which is abundant in these cells throughout the whole cell cycle, consists predominantly of Fra-1/c-Jun and Fra1/JunD heterodimers. We provide evidence that Fra1/c-Jun heterodimers are responsible for the repression of c-fos gene induction following serum stimulation. The introduction of a dominant negative version of c-Jun into ras transformed fibroblasts was able to rescue c-fos gene induction in response to serum stimulation, further demonstrating that AP-1 is indeed involved in c-fos gene repression. We conclude that oncogenic ras mediates the activation of the fra-1 gene which results in elevated AP-1 activity throughout the cell cycle. Fra-1 containing AP-1 complexes repress the c-fos and possibly other immediate early genes thereby preventing the induction of certain delayed early genes such as the T1 gene in response to mitogenic stimulation.

The IL-1 Receptor-Related T1 Antigen Is Expressed on Immature and Mature Mast Cells and on Fetal Blood Mast Cell Progenitors

Expression of the T1 gene, also known as ST2, DER4, and Fit-1, has been shown to be associated with cell proliferation. It gives rise to two different mRNAs that encode a receptor-like protein and a soluble molecule representing the ectodomain of the receptor form. Although T1 is a member of the IL-1R family, its biologic function is currently unknown. In this study, we have analyzed the expression of the T1 surface Ag in murine hemopoietic organs. Mast cells (MCs) were shown to be the only identifiable cell lineage that expressed T1 at high levels. T1 expression was found on cultured bone marrow-derived immature MCs. Similarly, freshly isolated connective tissue-type MCs from the i.p. cavity were also shown to express high levels of T1. Interestingly, the earliest detectable committed MC precursor isolated from fetal blood (FB) at day 15.5 of gestation, but not circulating hemopoietic stem cells in FB, also expresses high level of T1. Since FB promastocytes lack expression of the high affinity IgE receptor (FcεRI), T1 expression precedes expression of FcεRI in MC ontogeny. The finding that the T1 Ag is selectively expressed at several stages during development of the MC lineage suggests that this cell surface molecule, in combination with the well-established markers c-Kit and FcεRI, should be valuable for studying the MC lineage.

GATA-Dependent expression of the interleukin-1 receptor-related T1 gene in mast cells

The murine delayed-early serum-responsive gene T1 encodes glycoproteins of the interleukin-1 receptor family. Transcriptional initiation in fibroblasts is regulated by c-Fos and gives rise to a rare 5-kb mRNA and an abundant 2.7-kb mRNA. These transcripts are translated into a receptor-like membrane-anchored protein and a secreted protein consisting only of the ectodomain. In mast cells, T1 gene transcription is initiated 10.5 kb further upstream than in fibroblasts and gives rise predominantly to the 5-kb transcript under normal growth conditions. Here we demonstrate that calcium ionophore stimulation of mast cells resulted in an upregulation of T1 gene expression and a switch from the long to the short T1 transcript. This was paralleled by the disappearance of the receptor-type T1 protein on the mast cell surface and the secretion of large amounts of the truncated T1 protein. c-Fos and a T1 enhancer, which have previously been identified to be essential for T1 expression in fibroblasts, were not required for calcium ionophore-mediated T1 gene upregulation. Overexpression of the transcription factor GATA-1 in mast cells caused elevated T1 synthesis. Three GATA elements were identified in the minimal GATA-responsive mast cell promoter. Mutational analysis revealed that all three GATA elements are involved in T1 gene expression. Point mutations within the middle GATA element eliminated promoter activity completely, while mutations of the distal and proximal GATA binding sites reduced promoter strength by factors of 2 and 5, respectively. Exogenous expression of GATA-1 was not sufficient to activate the mast cell-specific promoter in NIH 3T3 fibroblasts.

Expression analysis of the soluble and membrane-associated forms of the interleukin-1 receptor-related T1 protein in primary mast cells and fibroblasts

The murine T1 gene encodes a membrane-bound glycoprotein (T1M) and a soluble variant (T1S) which represents the ectodomain of the receptor-type form. T1 is an orphan receptor belonging to the interleukin-1 receptor family. Its biological function is currently unknown. We analyze the expression of the two T1 proteins in mast cells and fibroblasts by using a set of monoclonal antibodies (MAb) that specifically recognize the extracellular portion of the T1 receptor. To generate anti-T1 MAbs, we immunized Lewis rats with a eukaryotically expressed chimeric protein consisting of the T1-receptor ectodomain fused to a human immunoglobulin domain. The two MAbs DJ4 and DJ8 were shown to specifically detect the murine T1M protein on the surface of primary IL-3-dependent bone marrow-derived mast cells as shown by flow cytometry and immunohistochemistry. Both antibodies were also capable of immunoprecipitating the membrane-associated 110-120 kDa T1M protein from mast cell lysates. In serum-stimulated but not in quiescent NIH3T3 fibroblasts, DJ4 and DJ8 MAbs detected both the soluble T1S protein as a 45-65 kDa band on SDS polyacrylamide gels as well as the membrane-bound 95 kDa T1M protein. The T1M protein in fibroblasts was less abundantly expressed and exhibited a lower molecular weight than the mast cell-produced T1M, probably as a consequence of different protein glycosylation. The MAbs described here represent highly specific reagents and valuable tools that should facilitate the establishment of the murine T1 protein expression pattern thus contributing to the solution of the question of its function.

Expression of ST2, an interleukin-1 receptor homologue, is induced by proinflammatory stimuli

ST2/T1 is an orphan receptor highly homologous to the IL-1 receptor. Using ST2 cDNA, ST2 specific primers, and a polyclonal antibody generated against ST2, the expression of mRNA and protein corresponding to both the soluble and membrane anchored forms of ST2 was studied. ST2 mRNAs were ubiquitously expressed in all the human tissues examined and were induced by cytokines and phorbol esters. Three different species of mRNAs were observed in different human cells and tissues. In contrast, only two species of ST2 mRNAs were observed in murine Balb/c-3T3 cells and no ST2 mRNA was seen in most tissues of normal mice. However, in a murine model where mouse ears are exposed to UVB irradiation leading to inflammation, ST2 mRNA was expressed 48 h post UV exposure. Similarly, in Balb/c-3T3 cells, the expression of soluble ST2 mRNA and protein was induced by pro-inflammatory stimuli such as TNF, IL-1alpha, IL-1beta and PMA in both exponentially growing and quiescent cells. The expression of the membrane ST2, however, remained constant. These data suggest a role for ST2 in inflammation.

Transcription of the interleukin-1 receptor-related T1 gene is initiated at different promoters in mast cells and fibroblasts

The delayed early serum response gene T1 encodes glycoproteins of the immunoglobulin superfamily with significant sequence similarity to the type 1 interleukin-1 receptor. The T1 gene is transcribed in fibroblasts into an abundant 2.7-kilobase (kb) and a rare 5-kb mRNA in response to proliferation-inducing stimuli. It gives predominantly rise to the longer transcript in the bone marrow of adult mice and in cultured mast cells. Alternative 3' processing is responsible for the two mRNA forms. The short transcript encodes a secreted protein with marked similarity to the extracellular domain of the interleukin-1 receptor, whereas the long mRNA is translated into a protein with an additional putative transmembrane and an intracellular domain. Here we demonstrate that T1 transcription in mast cells and fibroblasts initiates at two different start sites which are 10.5 kb apart. The alternative first exons are both spliced to exon 2 which contains the translation start site. Northern blot analysis and primer extension experiments revealed that promoter usage is strictly cell type-specific. T1 transcription in mast cells is initiated exclusively at the distal promoter, whereas in fibroblasts both the short and the long T1 mRNA start at the proximal promoter. Two GATA-1 elements were identified in the 5'-flanking region of the mast cell-specific distal exon 1.

The existence of a growth-specific DNA binding factor for the promoter region of mouse ST2 gene

A comparison of the 5'-flanking regions of human and mouse ST2 genes revealed the presence of two highly conserved DNA sequences. The promoter activity assay with a luciferase gene as a reporter showed that the deletion of the upstream conserved region diminished the transcriptional activity in growing BALB/c-3T3 cells. By electrophoretic mobility-shift analysis, the presence of a factor that binds to the positive regulatory region of the mouse ST2 gene was found in growing but not in quiescent BALB/c-3T3 cells. These results suggest the functional importance of this conserved region and the requirement of a binding factor for the expression of the ST2 gene.