Retinoic acid induction of human tissue-type plasminogen activator gene expression via a direct repeat element (DR5) located at -7 kilobases

Functional inhibition of cancer stemness-related protein DPP4 rescues tyrosine kinase inhibitor resistance in renal cell carcinoma

Tyrosine kinase inhibitors (TKIs) are used as targeted drugs for advanced renal cell carcinoma (RCC), although most cases eventually progress by acquiring resistance. Cancer stemness plays critical roles in tumor aggressiveness and therapeutic resistance, and dipeptidyl peptidase IV (DPP4) has been recently identified as a cancer stemness-related protein. A question arises whether DPP4 contributes to TKI efficacy in RCC. We established patient-derived RCC spheroids and showed that DPP4 expression is associated with stemness-related gene expression. TKI sunitinib resistance was rescued by DPP4 inhibition using sitagliptin or specific siRNAs in RCC cells and tumors. DPP4 expression can be inducible by retinoic acid and repressed by ALDH1A inhibition. Among type 2 diabetes patients with clinical RCC tumors, higher TKI efficacy is observed in those bearing DPP4^high tumors treated with DPP4 inhibitors. This study provides new insights into TKI resistance and drug repositioning of DPP4 inhibitor as a promising strategy for advanced RCC.

ALDH1A1 in patient-derived bladder cancer spheroids activates retinoic acid signaling leading to TUBB3 overexpression and tumor progression

Acquired chemoresistance is a critical issue for advanced bladder cancer patients during long-term treatment. Recent studies reveal that a fraction of tumor cells with enhanced tumor-initiating potential, or cancer stem-like cells (CSCs), may particularly contribute to acquired chemoresistance and recurrence. Thus, CSC characterization will be the first step towards understanding the mechanisms underlying advanced disease. Here we generated long-term patient-derived cancer cells (PDCs) from bladder cancer patient specimens in spheroid culture, which is favorable for CSC enrichment. Pathological features of bladder cancer PDCs and PDC-dependent patient-derived xenografts (PDXs) were basically similar to those of their corresponding patients' specimens. Notably, CSC marker aldehyde dehydrogenase 1A1 (ALDH1A1), a critical enzyme that synthesizes retinoic acid (RA), was abundantly expressed in PDCs. ALDH1A1 inhibitors and shRNAs repressed both PDC proliferation and spheroid formation, whereas all-trans RA could rescue ALDH1A1 shRNA-suppressed spheroid formation. ALDH inhibitor also reduced the in vivo growth of PDC-derived xenografts. ALDH1A1 knockdown study showed that tubulin beta III (TUBB3) was one of the downregulated genes in PDCs. We identified functional RA response elements in TUBB3 promoter, whose transcriptional activities were substantially activated by RA. Clinical survival database reveals that TUBB3 expression may associate with poor prognosis in bladder cancer patients. Moreover, TUBB3 knockdown was sufficient to suppress PDC proliferation and spheroid formation. Taken together, our results indicate that ALDH1A1 and its putative downstream target TUBB3 are overexpressed in bladder cancer, and those molecules could be applied to alternative diagnostic and therapeutic options for advanced disease.

Allelic imbalance of tissue-type plasminogen activator (t-PA) gene expression in human brain tissue

We have identified a single-nucleotide polymorphism (SNP) in the t-PA enhancer (-7351C>T), which is associated with endothelial t-PA release in vivo. In vitro studies demonstrated that this SNP is functional at the level of transcription. In the brain, t-PA has been implicated in both physiologic and pathophysiologic processes. The aim of the present study was to examine the effect of the t-PA –7351C>T SNP on t-PA gene expression in human brain tissue. Allelic mRNA expression was measured in heterozygous post-mortem brain tissues using quantitative TaqMan genotyping assay. Protein-DNA interactions were assessed using electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP). Significantly higher levels of t-PA mRNA were generated from chromosomes that harboured the wild-type –7351C allele, as compared to those generated from the mutant T allele (for the hippocampus, C to T allelic ratio of ~1.3, p=0.010, n=12; and for the cortex, C to T allelic ratio of ~1.2, p=0.017, n=12). EMSA showed reduced neuronal and astrocytic nuclear protein binding affinity to the T allele, and identified Sp1 and Sp3 as the major transcription factors that bound to the –7351 site. ChIP analyses confirmed that Sp1 recognises this site in intact cells. In conclusion, the t-PA –7351C>T SNP affects t-PA gene expression in human brain tissue. This finding might have clinical implications for neurological conditions associated with enhanced t-PA levels, such as in the acute phase of cerebral ischaemia, and also for stroke recovery.

Effect of Regulatory Element DNA Methylation on Tissue-Type Plasminogen Activator Gene Expression

Expression of the tissue-type plasminogen activator gene (t-PA; gene name PLAT) is regulated, in part, by epigenetic mechanisms. We investigated the relationship between PLAT methylation and PLAT expression in five primary human cell types and six transformed cell lines. CpG methylation was analyzed in the proximal PLAT gene promoter and near the multihormone responsive enhancer (MHRE) -7.3 kilobase pairs upstream of the PLAT transcriptional start site (TSS, -7.3 kb). In Bowes melanoma cells, the PLAT promoter and the MHRE were fully unmethylated and t-PA secretion was extremely high. In other cell types the region from -647 to -366 was fully methylated, whereas an unmethylated stretch of DNA from -121 to +94 was required but not sufficient for detectable t-PA mRNA and t-PA secretion. DNA methylation near the MHRE was not correlated with t-PA secretion. Specific methylation of the PLAT promoter region -151 to +151, inserted into a firefly luciferase reporter gene, abolished reporter gene activity. The region -121 to + 94 contains two well-described regulatory elements, a PMA-responsive element (CRE) near -106 and a GC-rich region containing an Sp1 binding site near +59. Methylation of double-stranded DNA oligonucleotides containing the CRE or the GC-rich region had little or no effect on transcription factor binding. Methylated CpGs may attract co-repressor complexes that contain histone deacetylases (HDAC). However, reporter gene activity of methylated plasmids was not restored by the HDAC inhibitor trichostatin. In conclusion, efficient PLAT gene expression requires a short stretch of unmethylated CpG sites in the proximal promoter.

Dependence of Proximal GC Boxes and Binding Transcription Factors in the Regulation of Basal and Valproic Acid-Induced Expression of t-PA

Objective. Endothelial tissue-type plasminogen activator (t-PA) release is a pivotal response to protect the circulation from occluding thrombosis. We have shown that the t-PA gene is epigenetically regulated and greatly induced by the histone deacetylase (HDAC) inhibitor valproic acid (VPA). We now investigated involvement of known t-PA promoter regulatory elements and evaluated dependence of potential interacting transcription factors/cofactors. Methods. A reporter vector with an insert, separately mutated at either the t-PA promoter CRE or GC box II or GC box III elements, was transfected into HT-1080 and HUVECs and challenged with VPA. HUVECs were targeted with siRNA against histone acetyl transferases (HAT) and selected transcription factors from the Sp/KLF family. Results. An intact VPA-response was observed with CRE mutated constructs, whereas mutation of GC boxes II and III reduced the magnitude of the induction by 54 and 79% in HT-1080 and 49 and 50% in HUVECs, respectively. An attenuated induction of t-PA mRNA was observed after Sp2, Sp4, and KLF5 depletion. KLF2 and p300 (HAT) were identified as positive regulators of basal t-PA expression and Sp4 and KLF9 as repressors. Conclusion. VPA-induced t-PA expression is dependent on the proximal GC boxes in the t-PA promoter and may involve interactions with Sp2, Sp4, and KLF5.

Dynamic Enhancer Methylation--A Previously Unrecognized Switch for Tissue-Type Plasminogen Activator Expression

Tissue-type plasminogen activator (t-PA), which is synthesized in the endothelial cells lining the blood vessel walls, is a key player in the fibrinolytic system protecting the circulation against occluding thrombus formation. Although classical gene regulation has been quite extensively studied in order to understand the mechanisms behind t-PA regulation, epigenetics, including DNA methylation, still is a largely unexplored field. The aim of this study was to establish the methylation pattern in the t-PA promoter and enhancer in non-cultured compared to cultured human umbilical vein endothelial cells (HUVECs), and to simultaneously examine the level of t-PA gene expression. Bisulphite sequencing was used to evaluate the methylation status, and real-time RT-PCR to determine the gene expression level. While the t-PA promoter was stably unmethylated, we surprisingly observed a rapid reduction in the amount of methylation in the enhancer during cell culturing. This demethylation was in strong negative correlation with a pronounced (by a factor of approximately 25) increase in t-PA gene expression levels. In this study, we show that the methylation level in the t-PA enhancer appears to act as a previously unrecognized switch controlling t-PA expression. Our findings, which suggest that DNA methylation is quite dynamic, have implications also for the interpretation of cell culture experiments in general, as well as in a wider biological context.

Role of Retinoic Acid-Metabolizing Cytochrome P450s, CYP26, in Inflammation and Cancer

Vitamin A (retinol) and its active metabolite, all-trans-retinoic acid (atRA), play critical roles in regulating the differentiation, growth, and migration of immune cells. Similarly, as critical signaling molecules in the regulation of the cell cycle, retinoids are important in cancers. Concentrations of atRA are tightly regulated in tissues, predominantly by the availability of retinol, synthesis of atRA by ALDH1A enzymes and metabolism and clearance of atRA by CYP26 enzymes. The ALDH1A and CYP26 enzymes are expressed in several cell types in the immune system and in cancer cells. In the immune system, the ALDH1A and CYP26 enzymes appear to modulate RA concentrations. Consequently, alterations in the activity of ALDH1A and CYP26 enzymes are expected to change disease outcomes in inflammation. There is increasing evidence from various disease models of intestinal and skin inflammation that treatment with atRA has a positive effect on disease markers. However, whether aberrant atRA concentrations or atRA synthesis and metabolism play a role in inflammatory disease development and progression is not well understood. In cancers, especially in acute promyelocytic leukemia and neuroblastoma, increasing intracellular concentrations of atRA appears to provide clinical benefit. Inhibition of the CYP26 enzymes to increase atRA concentrations and combat therapy resistance has been pursued as a drug target in these cancers. This chapter covers the current knowledge of how atRA and retinol regulate the immune system and inflammation, how retinol and atRA metabolism is altered in inflammation and cancer, and what roles atRA-metabolizing enzymes have in immune responses and cancers.

Human tissue-type plasminogen activator

Tissue-type plasminogen activator (t-PA ) plays an important role in the removal of intravascular fibrin deposits and has several physiological roles and pathological activities in the brain. Its production by many other cell types suggests that t-PA has additional functions outside the vascular and central nervous system. Activity of t-PA is regulated at the level of its gene transcription, its mRNA stability and translation, its storage and regulated release, its interaction with cofactors that enhance its activity, its inhibition by inhibitors such as plasminogen activator inhibitor type 1 or neuroserpin, and its removal by clearance receptors. Gene transcription of t-PA is modulated by a large number of hormones, growth factors, cytokines or drugs and t-PA gene responses may be tissue-specific. The aim of this review is to summarise current knowledge on t-PA function and regulation of its pericellular activity, with an emphasis on regulation of its gene expression.

Retinoid metabolism and its effects on the vasculature

Retinoids, the metabolically-active structural derivatives of vitamin A, are critical signaling molecules in many fundamental biological processes including cell survival, proliferation and differentiation. Emerging evidence, both clinical and molecular, implicates retinoids in atherosclerosis and other vasculoproliferative disorders such as restenosis. Although the data from clinical trials examining effect of vitamin A and vitamin precursors on cardiac events have been contradictory, this data does suggest that retinoids do influence fundamental processes relevant to atherosclerosis. Preclinical animal model and cellular studies support these concepts. Retinoids exhibit complex effects on proliferation, growth, differentiation and migration of vascular smooth muscle cells (VSMC), including responses to injury and atherosclerosis. Retinoids also appear to exert important inhibitory effects on thrombosis and inflammatory responses relevant to atherogenesis. Recent studies suggest retinoids may also be involved in vascular calcification and endothelial function, for example, by modulating nitric oxide pathways. In addition, established retinoid effects on lipid metabolism and adipogenesis may indirectly influence inflammation and atherosclerosis. Collectively, these observations underscore the scope and complexity of retinoid effects relevant to vascular disease. Additional studies are needed to elucidate how context and metabolite-specific retinoid effects affect atherosclerosis. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Retinoids regulate human amniotic tissue-type plasminogen activator gene by a two-step mechanism

The collagenolytic effects of the tissue-type plasminogen activator (t-PA) leading to extracellular matrix degradation are clearly involved in the physiopathology of human foetal membranes rupture. Nevertheless, the regulation of t-PA gene expression in extraembryonic developmental contexts remains unknown. The aim of our study is to propose the retinoic acids (RAs) as molecular regulators of t-PA expression in foetal membranes. RA induced t-PA mRNA and proteins in a time-dependent manner in amniotic membrane explants and Wistar Institute Susan Hayflick (WISH) cells. Furthermore, the use of cycloheximide revealed a two-step regulation of t-PA gene. Gene reporter assays confirmed that the RA-induced t-PA gene expression occurred through interactions of retinoid receptors (RARs and RXRs) with a DR5 response element located at –7 kb from the transcription site. Site-directed mutagenesis of this region of the t-PA promoter showed that SP1 factor was also retinoid-mediated induction, and immunoprecipitation assays revealed that SP1 and RAR/RXR interacted physically. Chromatin immunoprecipitation demonstrated that interactions between RARs, RXRs and t-PA promoter were time dependent: RAR-α/RXR-α bound DR5 motif before and up to 12 hrs of RA exposure, and RAR-β/RXR-α bound DR5 response element after 12 hrs of RA treatment. Finally, experiments using shRNA and RAR-β-specific antagonist revealed that reducing RAR-β induction decreased t-PA induction. Altogether, our results established that the RA-mediated regulation of t-PA in human foetal membranes occurred through two steps, with a major role played by RAR-β.

Towards retinoid therapy for Alzheimer's disease

Alzheimer's disease(AD) is associated with a variety of pathophysiological features, including amyloid plaques, inflammation, immunological changes, cell death and regeneration processes, altered neurotransmission, and age-related changes. Retinoic acid receptors (RARs) and retinoids are relevant to all of these. Here we review the pathology, pharmacology, and biochemistry of AD in relation to RARs and retinoids, and we suggest that retinoids are candidate drugs for treatment of AD.

Retinoids and activation of PKC induce tissue-type plasminogen activator expression and storage in human astrocytes

BACKGROUND

Emerging data demonstrate important roles for tissue-type plasminogen activator (t-PA) in the central nervous system (CNS). In contrast to endothelial cells, little is known about the regulation of t-PA gene expression and secretion in astrocytes.

OBJECTIVES

The aims of the present study were to investigate whether t-PA gene expression is regulated by retinoids and the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) in human astrocytes, and to study whether t-PA is stored and subject to regulated release from these cells, as with endothelial cells.

METHODS

Native human astrocytes were treated with RA and/or PMA. mRNA was quantified by real-time RT-PCR and protein secretion determined by ELISA. Intracellular t-PA immunoreactivity in astrocytes was examined by immunocyto- and histochemistry.

RESULTS

RA and/or PMA induced a time-dependent increase in t-PA mRNA and protein levels in astrocytes, reaching 10-fold after combined treatment. This was associated with increased amounts of t-PA storage in intracellular granular structures. Both forskolin and histamine induced regulated release of t-PA. The presence of t-PA in reactive astrocytes was confirmed in human brain tissue.

CONCLUSIONS

These data show that RA and PKC activation induce a strong up-regulation of t-PA expression in astrocytes, and increased intracellular storage pools. Moreover, a regulated release of t-PA can be induced from these cells. This raises the possibility that astrocytes contribute to the regulation of extracellular t-PA levels in the CNS.

Vitamin A: a drug for prevention of restenosis/reocclusion after percutaneous coronary intervention?

The re-establishment of adequate blood flow in a vessel with a reduced lumen due to an atherosclerotic plaque by percutaneous vascular intervention is a well established procedure. However, the long-term outcome of such interventions is negatively influenced by the development of intimal hyperplasia/restenosis. Although extensively researched, this still represents a significant clinical problem. Retinoids, i.e. natural and synthetic derivates of vitamin A, represent a potential therapeutic compound, since they have been shown to influence the vast majority of processes that ultimately lead to reocclusion of the injured vessel. Retinoids exert their effects at the transcriptional level through their nuclear receptors. Targeting multiple processes, i.e. proliferation, migration, extracellular matrix composition and cell differentiation, as well as coagulation/fibrinolysis, should increase their future role in the prevention of restenosis. The purpose of this review is to summarize the diverse effects of retinoids on pathobiological and biological processes activated at sites of vascular injury with particular emphasis on intimal hyperplasia/restenosis after endovascular interventions.

Effects of dietary vitamin A intake on acrosin- and plasminogen-activator activity of ram spermatozoa

Acrosin and plasminogen activators are proteolytic enzymes of ram spermatozoa that play an essential role in the induction of the acrosome reaction, as well as the binding of spermatozoa to the oocyte and their penetration through the layers that surround the oocyte. Since vitamin A can alter gene expression in various tissues, testis included, this study was undertaken to evaluate the possible effect of vitamin A intake on acrosin- and plasminogen-activator activity. During a 20-week experiment, 15 rams of the Greek breed Karagouniki, divided to three groups, received different amounts of vitamin A per os in retinyl acetate capsules (group A, controls, 12,500 iu/animal per day; group B, 50,000 iu/animal per day; group C, 0 iu/animal per day up to the 13th week, then 150,000 iu/animal per day until the end of the experiment). Acrosin- and plasminogen-activator activity were determined by spectrophotometric methods. Vitamin A was determined in blood plasma by HPLC. No statistical differences were detected regarding the body weight of the rams or the qualitative and quantitative parameters of their ejaculate throughout the whole experiment. No statistically significant alterations of enzyme activity were detected in group B. In group C, both enzyme activities started declining in week 9. Compared with controls, maximum reduction for acrosin was 49% on week 11 and for plasminogen activators 51% in week 14. Activities returned to normal rates after vitamin A re-supplementation. To date, the main result of vitamin A deficiency was known to be arrest of spermatogenesis and testicular degeneration. A new role for vitamin A may be suggested, since it can influence factors related to male reproductive ability before spermatogenesis is affected.

In vivo and in vitro analysis of the human tissue-type plasminogen activator gene promoter in neuroblastomal cell lines: evidence for a functional upstream kappaB element

Besides its well-established role in wound healing and fibrinolysis, tissue-type plasminogen activator (t-PA) has been shown to contribute to cognitive processes and memory formation within the central nervous system, and to promote glutamate receptor-mediated excitotoxicity. The t-PA gene is expressed and regulated in neuronal cells but the regulatory transcriptional processes directing this expression are still poorly characterized. We have used DNase I-hypersensitivity mapping and in vivo foot printing to identify putative regulatory elements and transcription factor binding sites in two human neuroblastomal (KELLY and SK-N-SH) and one human glioblastomal (SNB-19) cell lines. Hypersensitive sites were found in the proximal promoter region of all cell lines, and within the first exon for KELLY and SNB-19 cells. Mapping of methylation-protected residues in vivo detected a cluster of protected residues corresponding to a cAMP response element (CRE) and Sp1 sites in the proximal promoter previously shown to be essential for basal expression in other cell types. Protected residues were also found at other sites, notably a kappaB element at position bp -3081 to -3072 that was partly protected in KELLY and SNB-19 cells. Analysis of transfected reporter constructs in KELLY and SNB-19 cells confirmed that this particular element is functionally significant in the transactivation of the t-PA promoter in both cell types. This study defines, by in vivo and in vitro methods, a previously undescribed kappaB site in the t-PA gene promoter that influences t-PA expression in neuronal cells.

Characterization of the murine orphan G-protein-coupled receptor gene Rai3 and its regulation by retinoic acid

A retinoic acid-inducible gene (RAI3) encoding an orphan G-protein-coupled receptor is regulated during embryonal carcinoma differentiation. To elucidate the mechanisms that mediate its regulation, we isolated and characterized the mouse gene (Rai3) and promoter region. Rai3 spans about 18 kb containing four exons separated by three introns and maps to mouse chromosome 6 near D6mit25. A functional analysis of the Rai3 gene promoter revealed that the proximal region harbors most of the elements necessary for its regulation, including GC boxes and Sp1-, AP1-, and AP2-binding sites. A functional retinoic acid response element direct repeat of two novel motifs separated by a 5-bp spacer (5'-TGTCCCtcggtTCACCC-3') was identified at -64 bp upstream of the transcription start site using the methods of promoter truncation, electrophoretic mobility shift assay, and mutation analysis. These findings provide strong evidence that Rai3 is regulated directly by retinoic acid via its receptors.

Control elements between -9.5 and -3.0 kb in the human tissue-type plasminogen activator gene promoter direct spatial and inducible expression to the murine brain

Tissue-type plasminogen activator (t-PA) participates in the control of synaptic plasticity and memory formation in the central nervous system (CNS). Transgenic mice harbouring either 9.5, 3.0 or 1.4 kb of the human t-PA promoter fused to the LacZ reporter gene were used to assess t-PA promoter-directed expression in vivo. The 9.5 kb t-PA promoter directed expression to the brain, most notably to the dentate gyrus, superior colliculus, hippocampus, thalamus and piriform cortex. Staining was also observed in the retrosplenial and somatosensory cortex. The 3.0 kb t-PA promoter directed generalized and poorly defined expression to the cortex and hippocampus, while the 1.4 kb t-PA promoter directed expression selectively to the medial habenula. Intravenous administration of lipopolysaccharide into mice harbouring the 9.5 kb t-PA promoter resulted in an increase in reporter gene activity in the lateral orbital cortex and thalamus. Results of in vitro transfection experiments of NT2 cells with a series of t-PA promoter deletion constructs confirmed the presence of regulatory elements throughout the 9.5 kb promoter region. Finally, we describe a cis-acting element related to the NFAT recognition site that provides a protein-binding site and which may play a role in the selective expression of the 1.4 t-PA promoter in the medial habenula. These results indicate that elements between -3.0 and -9.5 kb of the t-PA promoter confer constitutive and inducible expression to specific regions of the CNS.

Retinoic Acid Upregulates the Plasminogen Activator System in Human Epidermal Keratinocytes

The activation of the proteolytic plasminogen activator system is important for the re-epithelialization of skin wounds. Keratinocytes synthesize and secrete the urokinase-type plasminogen activator, which binds to its specific receptor on keratinocytes. Receptor-bound urokinase-type plasminogen activator efficiently activates cell surface bound plasminogen. This results in pericellular proteolysis, which facilitates keratinocyte migration. Urokinase-type plasminogen activator activity is specifically controlled by plasminogen activator inhibitor-1 and -2. As retinoids have been reported to accelerate epithelialization of skin wounds in animal studies and clinical settings, we investigated the effects of all-trans retinoic acid on the plasminogen activator system in human epidermal keratinocytes. As tested in a chromogenic plasminogen activation assay, incubation with 10 microM all-trans retinoic acid caused a marked induction of cell-associated plasminogen activity after 24 h, and this induction was blocked by neutralizing anti-urokinase-type plasminogen activator antibodies, but not anti-tissue-type plasminogen activator antibodies. All-trans retinoic acid lead to a strong increase in urokinase-type plasminogen activator (enzyme-linked immunosorbent assay) and urokinase-type plasminogen activator receptor cell surface expression (flow cytometry) after 24 h. At this time-point, tissue-type plasminogen activator and plasminogen activator inhibitor-1 and -2 proteins were not or only slightly increased. Northern blot analyses revealed that all-trans retinoic acid caused an early and short-lived increase of plasminogen activator inhibitor-1, but a prolonged induction of urokinase-type plasminogen activator and urokinase-type plasminogen activator receptor mRNA levels. Collectively, these data suggest that all-trans retinoic acid activates the plasminogen activator system in human epidermal keratinocytes by differentially regulating activating and inhibiting components. The activation of the plasminogen activator system may be one mechanism by which all-trans retinoic acid exerts beneficial effects in cutaneous wound healing.

Ectopic expression of the cAMP-responsive element binding protein inhibits phorbol ester-mediated induction of tissue-type plasminogen activator gene expression

The human tissue-type plasminogen activator (t-PA) gene is regulated in a cell-type dependent manner. The t-PA gene is transcriptionally induced by the phorbol ester PMA in HeLa cells, but suppressed by PMA in HT-1080 cells. A cAMP responsive element (tPACRE) and a Sp-1 site located within the proximal t-PA gene promoter are functionally important in both cell systems. HeLa and HT-1080 cells contain a different repertoire of factors that associate with the tPACRE. In HT-1080 cells, CREB and c-Jun are the two major t-PACRE binding proteins identified, while activating transcription factor 2 (ATF-2) is a predominant t-PACRE binding protein in HeLa cells. To determine whether alteration in the distribution of tPACRE binding proteins would influence the differential regulation of the t-PA gene in these cells, the tPACRE binding profiles in these two cell systems were manipulated by over expressing ATF-2 in HT-1080 cells and CREB in HeLa cells. Supershift experiments confirmed that the overexpression of these factors resulted in binding to the tPACRE site. However, the presence of ATF-2 in HT-1080 cells did not affect either constitutive or PMA-mediated suppression of the endogenous t-PA gene. In contrast, enforced tPACRE-binding activity of CREB in HeLa cells significantly reduced the magnitude of PMA-mediated induction of t-PA mRNA in HeLa cells. These results indicate that the introduction of CREB into HeLa cells disrupts the regulation of the t-PA gene.

Regulation of retinoic acid signaling in the embryonic nervous system: a master differentiation factor

This review describes some of the properties of retinoic acid (RA) in its functions as a locally synthesized differentiation factor for the developing nervous system. The emphasis is on the characterization of the metabolic enzymes that synthesize and inactivate RA, and which determine local RA concentrations. These enzymes create regions of autocrine and paracrine RA signaling in the embryo. One mechanism by which RA can act as a differentiation agent is through the induction of growth factors and their receptors. Induction of growth factor receptors in neural progenitor cells can lead to growth factor dependency, and the consequent developmental fate of the cell will depend on the local availability of growth factors. Because RA activates the early events of cell differentiation, which then induce context-specific differentiation programs, RA may be called a master differentiation factor.

1,25-Dihydroxyvitamin D(3) induction of the tissue-type plasminogen activator gene is mediated through its multihormone-responsive enhancer

Tissue-type plasminogen activator (t-PA) is a positive modulator of the plasminogen-plasmin system, which is involved in bone remodeling. In the present study, 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] was found to stimulate t-PA gene expression in ROS17/2.8 osteosarcoma cells. Transient transfection analysis and in vitro DNA binding studies identified two vitamin D-responsive elements (VDRE) in the human t-PA enhancer. The first VDRE (bp -7175 to -7146) comprised an inverted palindrome separated by 9 bp (IP9) overlapping a palindrome separated by 3 bp. The second VDRE (bp -7315 to -7302) is an IP2 element overlapping the previously identified retinoic acid-responsive element. 1,25(OH)(2)D(3) treatment of primary osteoblasts derived from t-PAlacZ transgenic mice containing 9 kb of 5' sequence of the human t-PA gene increased the number of lacZ-positive cells, fitting with the probability model of enhancer function.

Hormone-activated nuclear receptors inhibit the stimulation of the JNK and ERK signalling pathways in endothelial cells

Glucocorticoid hormones, retinoids, and vitamin D3 display anti-angiogenic activity in tumor-bearing animals. However, despite their in vivo effect on the tumor vasculature little is known about their mechanism of action. Here we show that the synthetic glucocorticoid dexamethasone (Dex) and retinoic acid (RA) inhibit the activation of c-Jun N-terminal kinase (JNK) and extracellular-regulated kinase (ERK) signalling pathways by the pro-angiogenic agents tumor necrosis factor and vascular endothelial growth factor in endothelial cells. In contrast, Dex and RA failed to inhibit the activation of the p38 mitogen-activated protein kinase cascade. As a number of pro-angiogenic factors activate AP-1 transcription factor via the JNK and ERK pathways, our results suggest that the antagonism with AP-1 may underlie at least partially the anti-angiogenic effect of glucocorticoids and retinoids.

Retinoic acid receptor antagonist BMS453 inhibits the growth of normal and malignant breast cells without activating RAR-dependent gene expression

To elucidate the role of RAR-dependent gene transcription in inhibiting breast cell growth, we have investigated the ability of retinoids to suppress growth of normal, immortal, and malignant breast cells. We compared the ability of all trans retinoic acid (atRA) to activate retinoid receptors in normal, immortal, and malignant breast cells, with its ability to inhibit the growth of these cells. Our studies demonstrate that normal breast cells are more sensitive to the growth inhibitory effect of atRA than are immortal nonmalignant breast cells and breast cancer cells. atRA activated RAR-dependent gene transcription in both atRA-sensitive and -resistant breast cells as determined by transfection of a RARE-containing reporter gene. These results demonstrate that activation of RAR-dependent gene transcription by atRA is not sufficient to inhibit growth in atRA-resistant breast cancer cells. To determine whether activation of RAR-dependent gene transcription by atRA is necessary for growth inhibition, we tested the growth suppressive effect of a retinoid (BMS453) which binds RAR receptors and transrepresses AP-1 but does not activate RAR-dependent gene expression. This retinoid inhibited the growth of normal breast cells (HMEC and 184) and T47D breast cancer cells. Breast cancer cells which were resistant to atRA, were also resistant to BMS453. Normal human breast cells were most sensitive to the anti-proliferative effects of BMS453. These results indicate that in some breast cells RAR-dependent transactivation is not necessary for retinoids to inhibit growth. Instead, retinoids may suppress growth by inhibiting transcription factors such as AP-1 through transcription factor crosstalk.

Involvement of Sp1 in basal and retinoic acid induced transcription of the human tissue-type plasminogen activator gene

Transcription of the human tissue-type plasminogen activator (t-PA) gene is regulated by a multi-hormonal responsive enhancer at -7 kb. Transient co-transfections of Drosophila SL2 and human HT1080 fibrosarcoma cells with t-PA reporter constructs showed that Sp1 and Sp3 activate the t-PA promoter. Moreover Sp1 (but not Sp3) binding to the promoter is involved in induction by retinoic acid (RA), a response mediated through the enhancer. The role of Sp1 is specific, since mutation of the CRE element in the promoter did not affect response to RA. In contrast, the glucocorticoid induction mediated by the enhancer is independent of these Sp1 and CRE elements.

Synergistic transcriptional activation of the mouse urokinase plasminogen activator (uPA) gene and of its enhancer activator protein 1 (AP1) site by cAMP and retinoic acid

We have investigated the mechanism whereby all-trans retinoic acid (tRA) potentiates the 8-bromo-cAMP (8-BrcAMP)-dependent transcription of the urokinase plasminogen activator (uPA) gene in SC115 mouse mammary carcinoma cells. Photoaffinity labelling experiments showed that tRA did not alter the cellular content of cAMP-dependent protein kinase regulatory subunits I and II. In agreement with this, nuclear run-on analysis in the presence of the translational inhibitor puromycin demonstrated that the effect of 8-BrcAMP and its potentiation by tRA were independent of protein synthesis. A transiently transfected 6.6 kb uPA 5'-flanking region-chloramphenicol acetyltransferase (CAT) fusion gene mimicked the response of the endogenous uPA gene. Thus 1 mM 8-BrcAMP induced a 100-200% increase in CAT content, 100 nM tRA had no effect and 100 nM tRA+1 mM 8-BrcAMP induced a 300-500% increase in cells co-transfected with tRA receptor and/or 9-cis-RA receptor. Analysis of 5'-deleted constructs showed that the tRA effect required at least two cis regions: -2657 to -2186, encompassing the 100 bp uPA enhancer, and -709 to -324, which exhibited silencing activity. Neither region contained a tRA-response element-like motif. Because tRA receptor and 9-cis-RA receptor interact with activator protein 1 (AP1), we tested whether tRA regulated the uPA enhancer AP1 site in the presence of 8-BrcAMP. We found that a dimer of this site fused to a minimal uPA-CAT fusion gene was responsive to 1 mM 8-BrcAMP (100% CAT increase), not responsive to 100 nM tRA, and synergistically responsive to 100 nM tRA+1 mM 8-BrcAMP (240% CAT increase) in cells co-transfected with Fos and Jun. Synergistic activation of the same construct and of the 6.6 kb uPA-CAT fusion gene was also obtained using tRA and 100 nM PMA. We conclude that multiple cis elements, probably including the uPA enhancer AP1 site, mediate the tRA potentiation of uPA transcription.

Retinoic acid-enhanced invasion through reconstituted basement membrane by human SK-N-SH neuroblastoma cells involves membrane-associated tissue-type plasminogen activator

Al-trans retinoic acid (RA) enhanced human, S-type, SK-N-SH neuroblastoma cell invasion of reconstituted basement membrane in vitro but did not induce terminal differentiation of this cell line. In contrast to basal invasion, which was urokinase (uPA)- and plasmin-dependent, RA-enhanced invasion was dependent on tissue-type plasminogen activator (t-PA) and plasmin activity. Neither basal nor RA-enhanced invasion involved TIMP-2 inhibitable metalloproteinases. Enhanced invasion was associated with the induction of t-PA expression, increased expression of the putative t-PA receptor amphoterin, increased association of t-PA with cell membranes and increased net membrane-associated PA activity. Enhanced invasion was not associated with significant changes in the expression of uPA or its membrane receptor UPAR; plasminogen activator inhibitors PAI-1 and PAI-2; metalloproteinases MMP-1, MMP-2, MMP-3, MMP-9 and membrane type MMP1; or tissue inhibitors of metalloproteinases TIMP-1 and TIMP-2. RA stimulated the association of t-PA with the external cell membrane surface, which could be inhibited by heparin sulphate but not by mannose sugars or chelators of divalent cations, consistent with a role for amphoterin. Our data indicate that RA can promote the malignant behavior of S-type neuroblastoma cells refractory to RA-mediated terminal differentiation by enhancing their basement membrane invasive capacity. We suggest that this results from the action of a novel, RA-regulated mechanism involving stimulation of t-PA expression and its association with the cell membrane leading to increased PA-dependent matrix degradation.

Specific organization of the negative response elements for retinoic acid and thyroid hormone receptors in keratin gene family

Retinoic acid and thyroid hormone are important regulators of epidermal growth, differentiation, and homeostasis. Retinoic acid is extensively used in the treatment of many epidermal disorders ranging from wrinkles to skin cancers. Retinoic acid and thyroid hormone directly control the transcription of differentiation-specific genes including keratins. Their effect is mediated through nuclear receptors RAR and T3R. We have previously identified the response element in the K14 gene, K14RARE/TRE, to which these receptors bind, and found that it consists of a cluster of five half-sites with variable spacing and orientation. To determine whether this specific structure is found in other keratin genes, we have mapped and analyzed the RARE/TRE elements in three additional epidermal keratin genes: K5, K6, and K17. We used three different approaches to identify these elements: co-transfection of promoter deletion constructs, gel-shift assays, and site-specific mutagenesis. We localized the RARE/TRE elements relatively close to the TATA box in all three promoters. All three RARE/TRE elements have a similar structural organization: they consist of clusters of 3-6 half-sites with variable spacing and orientation. This means that the clustered structure of the RARE/TREs is a common characteristic for keratin genes. RARE and TRE in the K5 promoter are adjacent to each other whereas in the K17 promoter they overlap. All three keratin REs bind specifically both RAR and T3R in gel-shift assays. Interestingly, addition of ligand to the receptor changes the binding pattern ofthe T3R from homodimer to monomer, reflecting the change in regulation from induction to inhibition.

Characterisation of the rat tissue-type plasminogen activator gene promoter -- identification of a TAAT-containing promoter element

Tissue-type plasminogen activator (tPA) activates plasminogen to the active protease plasmin and is implicated in many biological processes that require extracellular proteolysis. In rat ovarian cells, gonadotropins induce the tPA gene by a cAMP-dependent pathway and this induction correlates with the time of follicular rupture. We have previously identified several promoter elements within the first 621 bp of the rat tPA promoter that are important for constitutive and cAMP-induced expression of the gene, including a cAMP responsive element (CRE), a nuclear factor 1 (NF1) element, a SP1-binding site and a G+C-rich box. In this report we have extended our study by analysing promoter constructs, ranging in size from 7.7 kb to 135 bp fused to the luciferase reporter gene. Transient transfection analysis of rat granulosa cells and human 293 cells, reveal that the proximal 268 bp of the promoter is enough to confer high basal and cAMP-induced expression of the gene. At position -162 to -172, between the previously identified CRE and NF1 sites, a novel TAAT-containing promoter element was identified. Mutational inactivation of the TAAT motif indicates that this element is important for both constitutive and cAMP-induced expression of the gene, and for the binding of a presumably novel nuclear factor that we have termed tPA promoter factor-1 (tPF-1).

Differences in metabolism and isomerization of all-trans-retinoic acid and 9-cis-retinoic acid between human endothelial cells and hepatocytes

Retinoic acid stimulates the expression of tissue-type plasminogen activator (t-PA) in vascular endothelial cells in vitro and enhances t-PA levels in plasma and tissues in vivo. Compared with the in vivo situation, high retinoic acid concentrations are required to induce optimally t-PA expression in vitro. These findings led us to study retinoic acid metabolism in cultured human endothelial cells. For comparison, these studies were also performed in the human hepatoma cell line, HepG2, and key experiments were repeated with human primary hepatocytes. Both hepatocyte cultures gave very similar results. Human endothelial cells were shown to possess an active retinoic acid metabolizing capacity, which is quantitatively comparable to that of hepatocytes, but different from that of hepatocytes in several qualitative aspects. Our results demonstrate that all-trans-retinoic acid is quickly metabolized by both endothelial cells and hepatocytes. All-trans-retinoic acid induces its own metabolism in endothelial cells but not in hepatocytes. 9-cis-Retinoic acid is degraded slowly by endothelial cells, whereas hepatocytes metabolize 9-cis-retinoic acid very quickly. Furthermore, our data show that hepatocytes, but not endothelial cells, detectably isomerise all-trans-retinoic acid to 9-cis-retinoic acid and vice versa. In both endothelial cells and hepatocytes all-trans-retinoic acid metabolism was inhibitable by the cytochrome P-450 inhibitors liarozole (10 microM) and ketoconazole (10 microM), albeit to different extents and with different specificities. In the presence of the most potent retinoic acid metabolism inhibitor in endothelial cells, liarozole, at least 10-fold lower all-trans-retinoic acid concentrations were required than in the absence of the inhibitor to obtain the same induction of t-PA. In conclusion, our results clearly demonstrate that all-trans-retinoic acid and 9-cis retinoic acid are actively but differently metabolized and isomerised by human endothelial cells and hepatocytes. The rapid metabolism of retinoic acid explains the relatively high concentrations of retinoic acid required to induce t-PA in cultured endothelial cells.

9,13-di-cis-Retinoic acid induces the production of tPA and activation of latent TGF-beta via RAR alpha in a human liver stellate cell line, LI90

We studied the mechanism by which 9,13-di-cis-retinoic acid (9,13dcRA), a novel and endogenous stereoisomer of all-trans-RA, induces TGF-beta formation in a human liver stellate cell line, LI90. 9,13dcRA induced the expression of RAR alpha and RARbeta, enhanced the production of tissue-type plasminogen activator (tPA), thereby, surface plasmin levels, and induced the activation of latent TGF-beta. Similar effects were obtained with RAR alpha-selective retinoid, but not with RARbeta- or RARgamma-selective retinoid, and the induction was inhibited by RAR alpha-selective antagonist. These results suggest that 9,13dcRA up-regulates tPA expression, resulting in the formation of TGF-beta by LI90 cells, at least in part, via induction and activation of RAR alpha.

Changes in rat corneal matrix metalloproteinases and serine proteinases under vitamin A deficiency

PURPOSE

Vitamin A deficiency alters the transparency of the cornea due to epithelial cell keratinization and increases the susceptibility of the cornea to ulceration. The purpose of this study was to determine the effect of vitamin A deficiency on rat corneal matrix metalloproteinases and serine proteinases.

METHODS

Four dietary groups of male WAG/RijMCW rats were prepared: (1) Vitamin A deficient rats were raised on a casein-based retinoid deficient diet; (2) Retinol repleted rats were raised on the retinoid deficient diet. On the eighty-sixth day on this diet, the rats were fed retinyl palmitate and then given free access to the retinyl palmitate-supplemented control diet; (3) The weight-matched, pair-fed rats were restricted in their intake of the retinyl palmitate-supplemented diet so that their weight gain matched that of the A-rats; (4) The non-restricted rats were given free access to the retinyl palmitate-supplemented diet. The animals were killed at the late plateau stage for weight of the deficiency (102-106 days). Zymography was used to study proteinases in the corneal extracts.

RESULTS

Vitamin A deficient and control rat corneas contain multiple matrix metalloproteinases and serine proteinases. The matrix metalloproteinases at 90/92 kDa (gelatinase B) and 66/63/57 kDa (gelatinase A) were significantly decreased in the corneas of the vitamin A deficient rats relative to the control corneas. Corneas from the four groups of rats contained 76, 45, 38, 28 and 22 kDa proteinases that cleaved casein. Only the vitamin A deficient corneas contained a 50 kDa casein cleaving enzyme. The 76, 45, 38 and 28 kDa serine proteinases were significantly lower in the vitamin A deficient corneas. The major 22 kDa enzyme was not altered by the deficiency. All casein cleaving proteinases were inhibited by phenylmethylsulfonyl fluoride and chymostatin except for a minor 76 kDa band. The activity of this band was not altered by inhibitors for the other classes of proteinases, ethylenediaminetetraacetic acid, E-64 or pepstatin. The concentrations of the 61, 52 and 40 kDa plasminogen activators were not altered by the deficiency.

CONCLUSIONS

Alterations in corneal proteinases under vitamin A deficiency conditions may be involved in the characteristic changes observed in the cornea under vitamin A deficiency conditions: decreased exfoliation of epithelial cells, increased levels of keratofibrils in the corneal keratocytes, increased stromal keratocyte degradation and increased susceptibility towards ulceration.

Role of insulin-like growth factor binding protein-2 and its limited proteolysis in neuroblastoma cell proliferation: modulation by transforming growth factor-beta and retinoic acid

Insulin-like growth factor (IGF) binding proteins (IGFBPs) modulate IGF action at cellular level through inhibition or, alternatively, potentiation, where their limited proteolysis is a contributory mechanism. Under basal conditions, neuroblastoma cells secrete IGFs (essentially IGF-II), IGFBPs (IGFBP-4 and predominantly IGFBP-2 that is partially proteolysed), and proteases, including tissue-type plasminogen (PLG) activator, whose activity is inhibited by PLG activator inhibitor-1. Neuroblastoma cells were used to investigate the influence of the plasmin system, transforming growth factor-beta retinoic acid on cell growth and the IGF system. In cells treated with 5 micrograms/ml PLG, proliferation was stimulated, an effect that was inhibited in the presence of either alpha IR-3 (which blocks the type 1 IGF receptor) or anti-IGF-II antibodies. There was a parallel increase in IGFBP-2 proteolysis, which resulted in a 5-fold loss of affinity for IGF-II. In the presence of 1 ng/ml transforming growth factor-beta, PLG-induced mitogenesis and IGFBP-2 proteolysis were reduced, and Northern blot analysis revealed increased PLG activator inhibitor-1 mRNA. Conversely, with 2 microM retinoic acid, the mitogenic effect of PLG, IGFBP-2 proteolysis, and tissue-type PLG activator mRNAs were increased. Therefore, IGF-II mediates autocrine proliferation in neuroblastoma cells under the control of IGFBPs secreted by the cells, its bioavailability being enhanced as a result of plasmin-induced IGFBP-2 proteolysis.

Identification of a multihormone responsive enhancer far upstream from the human tissue-type plasminogen activator gene

A 2.4-kilobase (kb) DNA fragment, located 7.1 kb upstream from the human tissue-type plasminogen activator (t-PA) gene (t-PA2.4), acts as an enhancer which is activated by glucocorticoids, progesterone, androgens, and mineralocorticoids. Transient expression of t-PA-chloramphenicol acetyltransferase reporter constructs in HT1080 human fibrosarcoma cells identified a glucocorticoid responsive unit with four functional binding sites for the glucocorticoid receptor, located between bp -7,501 and -7,974. The region from bp -7,145 to -9,578 (t-PA2.4) was found to confer a cooperative induction by dexamethasone and all-trans-retinoic acid (RA) to its homologous and a heterologous promoter, irrespective of its orientation. The minimal enhancer, defined by progressive deletion analysis, comprised the region from -7.1 to -8.0 kb (t-PA0.9) and encompassed the glucocorticoid responsive unit and the previously identified RA-responsive element located at -7.3 kb (Bulens, F., Ibañez-Tallon, I., Van Acker, P., De Vriese, A., Nelles, L., Belayew, A., and Collen, D. (1995) J. Biol. Chem. 270, 7167-7175). The amplitude of the synergistic response to dexamethasone and RA increased by reducing the distance between the enhancer and the proximal t-PA promoter. The synergistic interaction was also observed between the aldosterone and the RA receptors. It is postulated that the t-PA0.9 enhancer might play a role in the hormonal regulation of the expression of human t-PA in vivo.

Differential binding of cAMP-responsive-element (CRE)-binding protein-1 and activating transcription factor-2 to a CRE-like element in the human tissue-type plasminogen activator (t-PA) gene promoter correlates with opposite regulation of t-PA by phorbol ester in HT-1080 and HeLa cells

The human tissue-type plasminogen activator gene (t-PA) is induced by the phorbol ester, phorbol 12-myristate 13-acetate (PMA), in HeLa cells. Previous studies in transfected HeLa cells identified two cis-acting regulatory elements within the t-PA gene promoter responsible for both constitutive and PMA-inducible expression. One element differs from the consensus cAMP response element (CRE) by a single nucleotide substitution (referred to in this report as t-PACRE) and another which bears similarity to the AP-2 recognition sequence. In HT-1080 fibrosarcoma cells, t-PA mRNA levels are expressed at higher constitutive levels and are suppressed by PMA. Nuclear run-on transcription experiments indicate that PMA-mediated suppression of t-PA in these cells is associated with a decrease in t-PA gene template activity. We designed experiments to determine whether nuclear t-PACRE or AP-2-like binding proteins were differentially expressed in HeLa and HT-1080 cells and, accordingly, if these could be correlated with the opposite effect of PMA on t-PA expression. Band shift analyses indicated that the migration profiles of HeLa and HT-1080 nuclear proteins interacting with the AP-2-like site were indistinguishable; however, those produced with the t-PACRE binding site were qualitatively and quantitatively distinct. The distribution of t-PACRE binding proteins in these cells was investigated in a supershift assay using specific antibodies against members of the fos/jun and CRE-binding protein (CREB)/activating transcription factor (ATF) families. In HT-1080 cells, CREB-1 was the most prominent t-PACRE-binding activity detected and was greatly increased in cells treated with PMA. In contrast, CREB-1 activity was absent in HeLa cells, but antibodies specific for ATF-2 produced a marked supershifted complex which was unaffected by PMA treatment. Since CREB-1 can repress transcription of other target genes (including c-jun) via association with identical cis-acting CRE-like sequences, we suggest that the mechanism for the transcriptional down-regulation of t-PA by PMA in HT-1080 cells requires CREB-1 binding to the t-PACRE while ATF-2, by associating with the same site, plays a role in PMA-mediated induction of t-PA in HeLa cells.