Significance of Increased Tissue Transglutaminase in Hormone Refractory Prostate Cancer

The progression of hormone responsive to hormone refractory prostate cancer poses a major clinical challenge in the successful treatment of prostate cancer. The hormone refractory prostate cancer cells exhibit resistance not only to castrate levels of testosterone, but also to other therapeutic modalities and hence become lethal. Currently, there is no effective treatment available for managing this cancer. These observations underscore the urgency to investigate mechanism(s) that contribute to the progression of hormone-responsive to hormone-refractory prostate cancer and to target them for improved clinical outcomes. Tissue transglutaminase (TG2) is a multifunctional pro-inflammatory protein involved in diverse physiological processes such as inflammation, tissue repair, and wound healing. Its expression is also implicated in pathological conditions such as cancer and fibrosis. Interestingly, we found that the androgen-independent prostate cancer cell lines, which lacked androgen receptor (AR) expression, contained high basal levels of tissue transglutaminase. Inversely, the cell lines that expressed androgen receptor lacked transglutaminase expression. This attracted our attention to investigate the possible role this protein may play in the progression of prostate cancer, especially in view of recent observations that its expression is linked with increased invasion, metastasis, and drug resistance in multiple cancer cell types. The results we obtained were rather surprising and revealed that stable expression of tissue transglutaminase in androgen-sensitive LNCaP prostate cancer cells rendered these cells independent of androgen for growth and survival by silencing the AR expression. The AR silencing in TG2 expressing cells (TG2-infected LNCaP and PC-3 cells) was due to TG2-induced activation of the inflammatory nuclear transcription factor-kB (NF-kB). Thus, TG2 induced NF-kB was found to directly bind to the AR promoter. Importantly, TG2 protein was specifically recruited to the AR promoter in complex with the p65 subunit of NF-kB. Moreover, TG2 expressing LNCaP and PC-3 cells exhibited epithelial-to-mesenchymal transition, as evidenced by gain of mesenchymal (such as fibronectin, vimentin, etc.) and loss of epithelial markers (such as E-cadherin, b-catenin). Taken together, these results suggested a new function for TG2 and revealed a novel mechanism that is responsible for the progression of prostate cancer to the aggressive hormone-refractory phenotype.

The molecular mechanism of retinoic acid action: Regulation of tissue transglutaminase gene expression

Retinoic acid is a small lipophilic molecule that exerts profound effects on the growth and differentiation of both normal and transformed cells. It is also a natural morphogen that is critical in the development of embryonic structures. The molecular effects of retinoic acid involve alterations in the expression of several proteins and these changes are presumably mediated in part by alterations in gene expression. For instance, retinoic acid causes a rapid induction of tissue transglutaminase, an enzyme involved in protein cross-linking. The molecular mechanisms responsible for the effects of retinoic acid on gene expression have not been characterized. To approach this question, I have isolated and characterized tissue transglutaminase of cDNA clones. The deduced amino acid sequences of tissue transglutaminase and of factor XIIIa showed a relatively high degree of homology in their putative calcium binding domains.^ To explore the mechanism of induction of this enzyme, both primary (macrophages) and cultured cells (Swiss 3T3-C2 and CHO fibroblasts) were used. I found that retinoic acid is a general inducer of tissue transglutaminase mRNA in these cells. In murine peritoneal macrophages retinoic acid causes a rapid accumulation of this mRNA and this effect is independent of concurrent protein synthesis. The retinoic acid effect is not mediated by a post-transcriptional increase in the stability of the tissue transglutaminase mRNA, but appears to involve an increase in the transcription rate of the tissue transglutaminase gene. This provides the first example of regulation by retinoic acid of a specific gene, supporting the hypothesis that these molecules act by directly regulating the transcriptional activity of specific genes. A molecular model for the effects of retinoic acid on the expression of genes linked to cellular proliferation and differentiation is proposed. ^

Molecular basis of retinoid action: The role of retinoic acid receptors in retinoic acid-induced tissue transglutaminase expression

Retinoic acid has profound effects on the cellular growth and differentiation of a variety of cells. However, the molecular basis of retinoic acid action has, until recently, not been well understood. The identification of retinoic acid receptors which bear a high degree of homology to members of the steroid receptor super-family has dramatically altered our understanding of the biology of retinoids. The focus of this dissertation has been toward identification of retinoic acid binding proteins responsible for the effects of this molecule on gene expression.^ We have characterized in detail the retinoic acid-dependent induction of tissue transglutaminase gene expression in a myeloid cell line, human promyelocytic leukemia cells (HL-60 cells). Using cDNA probes specific for tissue transglutaminase, we have determined that the retinoic acid induced increase in enzyme level is due to an increase in the level of tissue transglutaminase mRNA. We have used this model as a probe to investigate the molecular basis of retinoid regulated gene expression.^ This thesis demonstrates that retinoic acid receptors are expressed in cells which induce tissue transglutaminase expression in response to retinoic acid. In Hl-60 cells retinoic acid-induced transglutaminase expression is associated with saturable nuclear retonic acid binding. Transcripts for both the alpha and beta forms of the retinoic acid receptors can be detected in these cells. Pretreatment of HL-60 cells with agents that potentiate retinoic acid-induced transglutaminase expression also modestly induced the alpha form of the retinoic acid receptor. Studies in macrophages and umbilical vein endothelial cells have also associated expression of the beta form of the retinoic acid with retinoic acid induced tissue transglutaminase expression.^ To investigate directly if retinoic acid receptors regulate retinoic acid-induced tissue transglutaminase expression we developed a series of stably transfected Balb-c 3T3 cells expressing different levels of the beta or gamma form of the retinoic acid receptor. These studies indicated that either the beta or gamma receptor can stimulate endogenous tissue transglutaminase expression in response to retinoic acid. These are among the first studies in the steroid field to describe regulation of an endogenous gene by a transfected receptor. ^

Characterization of the human tissue transglutaminase gene promoter

Transglutaminases are a family of calcium-dependent enzymes, that catalyze the covalent cross-linking of proteins by forming $\varepsilon(\gamma$-glutamyl)lysine isopeptide bonds. In order to investigate the molecular mechanisms regulating the expression of the tissue transglutaminase gene and to determine its biological functions, the goal of this research has been to clone and characterize the human tissue transglutaminase promoter. Thirteen clones of the tissue transglutaminase gene were obtained from the screening of a human placental genomic DNA library. A 1.74 Kb fragment derived from DNA located immediately upstream of the translation start site was subcloned and sequenced. Sequence analysis of this DNA fragment revealed that it contains a TATA box (TATAA), a CAAT box (GGACAAT), and a series of potential transcription factor binding sites and hormone response elements. Four regions of significant homology, a GC-rich region, a TG-rich region, an AG-rich region, and HR1, were identified by aligning 1.8 Kb of DNA flanking the human, mouse, and guinea pig tissue transglutaminase genes.^ To measure promoter activity, we subcloned the 1.74 Kb fragment of the tissue transglutaminase gene into a luciferase reporter vector to generate transglutaminase promoter/luciferase reporter constructs. Transfection experiments showed that this DNA segment includes a functional promoter with high constitutive activity. Deletion analysis revealed that the SP1 sites or corresponding sequences contribute to this activity. We investigated the role of DNA methylation in regulating the activity of the promoter and found that in vitro methylation of tissue transglutaminase promoter/luciferase reporter constructs suppressed their basal activity. Methylation of the promoter is inversely correlated with the expression of the tissue transglutaminase gene in vivo. These results suggest that DNA methylation may be one of the mechanisms regulating the expression of the gene. The tumor suppressor gene product p53 was also shown to inhibit the activity of the promoter, suggesting that induction of the tissue transglutaminase gene is not involved in the p53-dependent programmed cell death pathway. Although retinoids regulate the expression of the tissue transglutaminase gene in vivo, retinoid-inducible activity can not be identified in 3.7 Kb of DNA 5$\sp\prime$ to the tissue transglutaminase gene.^ The structure of the 5$\sp\prime$ end of the tissue transglutaminase gene was mapped. Alignment analysis of the human tissue transglutaminase gene with other human transglutaminases showed that tissue transglutaminase is the simplest member of transglutaminase superfamily. Transglutaminase genes show a conserved core of exons and introns but diverse N-terminuses and promoters. These observations suggest that key regulatory sequences and promoter elements have been appended upstream of the core transglutaminase gene to generate the diversity of regulated expression and regulated activity characteristic of the transglutaminase gene family. ^

THE MECHANISM OF RETINOIC ACID REGULATION OF TISSUE TRANSGLUTAMINASE GENE EXPRESSION

Retinoic acid regulates cellular growth and differentiation by altering the expression of specific sets of genes, but the molecular mechanism by which this is achieved is unknown. We have used the rapid induction of a specific enzyme, tissue transglutaminase in mouse macrophages, human leukemia cells and a variety of other cell types to study the regulation of gene expression by retinoic acid. Soluble retinoic acid binding proteins, such as cellular Retinoic Acid Binding Protein (cRABP), have been proposed as specific mediators of retinoic acid regulation of gene expression. This thesis demonstrates the lack of cRABP in a number of cell lines which are sensitive to retinoic acid regulation of tissue transglutaminase expression. These cells are also devoid of other soluble retinoic acid binding activity. The level of retinoic acid binding activity that could have been detected (6 fmol) is far below that of most cells and tissues which are sensitive to the effects of retinoic acid on growth and differentiation. A mouse melanoma cell line, S91-C2, was found to contain an unusual retinoic acid binding protein which has a lower affinity for retinoic acid than mouse tissue cRABP and also behaves differently on gel filtration HPLC chromatography.^ The induction of tissue transglutaminase by retinoic acid in macrophages is specifically inhibited by pertussis toxin. Pertussis toxin ADP-riblosylates membrane GTP-binding proteins such as N(,i) and interferes with signalling from plasma membrane receptors to regulatory enzymes. Pertussis toxin inhibition of transglutaminase induction is due to inhibition of tissue transglutaminase mRNA accumulation and is paralleled by the ADP-ribosylation of a 41,000 dalton macrophage membrane protein. It is concluded that soluble retinoic acid binding proteins are not essential for retinoic acid induction of tissue transglutaminase and that a membrane GTP-binding protein is closely linked to the sensitive response of macrophages to retinoic acid. ^

Cloning and characterization of the full length cDNA and promoter of mouse tissue transglutaminase

Transglutaminases are a family of enzymes that catalyze the covalent cross-linking of proteins through the formation of $\varepsilon$-($\gamma$-glutaminyl)-lysyl isopeptide bonds. Tissue transglutaminase (Tgase) is an intracellular enzyme which is expressed in terminally differentiated and senescent cells and also in cells undergoing apoptotic cell death. To characterize this enzyme and examine its relationship with other members of the transglutaminase family, cDNAs, the first two exons of the gene and 2 kb of the 5$\sp\prime$ flanking region, including the promoter, were isolated. The full length Tgase transcript consists of 66 bp of 5$\sp\prime$-UTR (untranslated) sequence, an open reading frame which encodes 686 amino acids and 1400 bp of 3$\sp\prime$-UTR sequence. Alignment of the deduced Tgase protein sequence with that of other transglutaminases revealed regions of strong homology, particularly in the active site region.^ The Tgase cDNA was used to isolate and characterize a genomic clone encompassing the 5$\sp\prime$ end of the mouse Tgase gene. The transcription start site was defined using genomic and cDNA clones coupled with S1 protection analysis and anchored PCR. This clone includes 2.3 kb upstream of the transcription start site and two exons that contain the first 256 nucleotides of the mouse Tgase cDNA sequence. The exon intron boundaries have been mapped and compared with the exon intron boundaries of three members of the transglutaminase family: human factor XIIIa, the human keratinocyte transglutaminase and human erythrocyte band 4.1. Tissue Tgase exon II is similar to comparable exons of these genes. However, exon I bears no resemblance with any of the other transglutaminase amino terminus exons.^ Previous work in our laboratory has shown that the transcription of the Tgase gene is directly controlled by retinoic acid and retinoic acid receptors. To identify the region of the Tgase gene responsible for regulating its expression, fragments of the Tgase promoter and 5$\sp\prime$-flanking region were cloned into the chloramphenicol actetyl transferase (CAT) reporter constructs. Transient transfection experiments with these constructs demonstrated that the upstream region of Tgase is a functional promoter which contains a retinoid response element within a 1573 nucleotide region spanning nucleotides $-$252 to $-$1825. ^

Significance of tissue transglutaminase expression in multidrug resistant MCF -7 human breast cancer cells

Tissue transglutaminase (tTGase) is an enzyme that catalyzes the posttranslational modification of proteins via Ca2+-dependent cross-linking reactions. In this study, we extended our earlier observation that tTGase is highly expressed in MCF-7 human breast carcinoma cells selected for the multidrug resistance phenotype (MCF-7/DOX). To directly assess the involvement of tTGase in drug resistance, parental MCF-7 (MCF-7/WT) cells were transfected with cDNAs encoding either a catalytically active (wildtype) or inactive (mutant) tTGase protein. Expression of wildtype tTGase led to spontaneous apoptosis in MCF-7/WT cells, while the mutant tTGase was tolerated by the cells but did not confer resistance to doxorubicin. Analysis of calcium by a spectrofluorometric technique revealed that MCF-7/DOX cells exhibit a defective mechanism in intracellular calcium mobilization, which may play a role in preventing the in situ activation of tTGase and thus allowing the cells to grow despite expressing this enzyme. An elevation in intracellular calcium by treatment with the calcium ionophore A23187 induced rapid and substantial apoptosis in MCF-7/DOX cells as determined by morphological and biochemical criteria. Pretreatment of MCF-7/DOX cells with a tTGase-specific inhibitor (monodansylcadaverine) suppressed A12387-induced apoptosis, suggesting the possible involvement of tTGase-catalyzed protein cross-linking activity. A23187-induced apoptosis in MCF-7/DOX cells was further characterized by PARP cleavage and activation of downstream caspases (-3, -6, and -7). Another interesting aspect of tTGase/A23187-induced apoptosis in MCF-7/DOX cells was that these cells failed to show any prototypic changes associated with the mitochondrial (altered membrane potential, cytochrome c release, caspase-9 activation), receptor-induced (Bid cleavage), or endoplasmic reticulum-stressed (caspase-12 activation) apoptotic pathways. In summary, our data demonstrate that, despite being highly resistant to conventional chemotherapeutic drugs, MCF-7/DOX cells are highly sensitive to apoptosis induced by increased intracellular calcium. We conclude that tTGase does not play a direct role in doxorubicin resistance in MCF-7/DOX cells, but may play a role in enhancing the sensitivity of these cells to undergo apoptosis. ^

Structure-function relation in retinoid regulated gene expression

Retinoids are known to inhibit proliferation of and induce terminal differentiation of many normal and transformed cells. It has been postulated that retinoids exert their effect by altering gene expression. HL-60 cells and macrophages both respond to retinoic acid action by the rapid induction of the enzyme tissue transglutaminase. The induction has been shown to be due to increased transcription of the transglutaminase gene. The first part of the dissertation studied the structure-function relationship of retinoid-regulated transglutaminase induction, differentiation and proliferation in HL-60 cells using retinoid analogs. The results indicated strict structural constraints and a strong structure-function correlation between transglutaminase induction and differentiation; those retinoids that induced transglutaminase also induced differentiation, those analogs that did not induce transglutaminase could not induce differentiation. The ability of the retinoids to induce transglutaminase in HL-60 cells was paralleled in macrophages. However, the antiproliferative effect of the retinoids displayed less stringent structural constraints than their differentiation- and transglutaminase-inducing properties. Specifically all the retinoids were able to inhibit proliferation to varying extents. It is concluded that the induction of transglutaminase and of differentiation by retinoids is mediated by receptors. While receptor mediation cannot be entirely ruled out, with the current data no definitive statement can be made about the antiproliferative activity of retinoids. Also, the concordance in the ability of the retinoids to induce transglutaminase and the ability to induce differentiation of HL-60 cells suggests that the former is an early response of the cells to retinoids and differentiation a later consequence on the same pathway. Using the induction of transglutaminase as an index of the direct, or primary, effect of retinoids on gene expression, the second part of the dissertation investigates, by 2D gel electrophoresis, the alteration in the rates of synthesis of other proteins in macrophages and HL-60 cells in response to short incubations with retinoic acid. Any changes in parallel with transglutaminase were taken to indicate proteins directly under the control of retinoic acid. It is concluded that retinoic acid regulates the expression of a circumscribed set of genes in a cell-specific manner. The results support the hypothesis that retinoids exert their multiple effects on myeloid cells, in part, by receptor-mediated alternations in gene expression. ^

New aspects of estrogen action in the endometrium: Reciprocal interplay with retinoic acid pathway and a novel estrogen-regulated gene

The uterine endometrium is a major target for the estrogen. However, the molecular basis of estrogen action in the endometrium is largely unknown. I have used two approaches to study the effects of estrogen on the endometrium. One approach involved the study of the interaction between estrogen and retinoic acid (RA) pathways in the endometrium. I have demonstrated that estrogen administration to rodents and estrogen replacement therapy (ERT) in postmenopausal women selectively induced the endometrial expression of retinaldehyde dehydrogenase II (RALDH2), a critical enzyme of RA biosynthesis. RALDH2 was expressed exclusively in the stromal cells, especially in the stroma adjacent to the luminal and glandular epithelia. The induction of RALDH2 by estrogen required estrogen receptor and occurred via a direct increase in RALDH2 transcription. Among the three RA receptors, estrogen selectively induced the expression of RARα. In parallel, estrogen also increased the utilization of all-trans retinol (the substrate for RA biosynthesis) and the expression of two RA-regulated marker genes, cellular retinoic acid binding protein II (CRABP2) and tissue transglutaminase (tTG) in the endometrium. Thus estrogen coordinately upregulated both the production and signaling of RA in both the rodent and human endometrium. This coordinate upregulation of RA system appeared to play a role in counterbalancing the stimulatory effects of estrogen on the endometrium, since the depletion of endogenous RA in mice led to an increase in estrogen-stimulated stromal proliferation and endometrial Akt phosphorylation. In addition, I have also used a systematic approach (DNA microarray) to categorize genes and pathways affected by the ERT in the endometrium of postmenopausal women and identified a novel estrogen-regulated gene EIG121. EIG121 was exclusively expressed in the glandular epithelial cells of the endometrium and induced by estrogen in vivo and in cultured cell lines. Compared with the normal endometrium, EIG121 was highly overexpressed in type 1 endometrial cancer, but profoundly suppressed in type 2 endometrial tumors. Taken together, these studies suggested that estrogen regulates the expression of many genes of both the pro-proliferative and anti-proliferative pathways and the abnormality of these pathways may increase the risks for estrogen-dependent endometrial hyperplasia and endometrial cancer. ^