"Tissue" transglutaminase is specifically expressed in neonatal rat liver cells undergoing apoptosis upon epidermal growth factor-stimulation

Lead Nitrate Induces Inflammation and Apoptosis in Rat Lungs Through the Activation of NF-κB and AhR Signaling Pathways

Lead (Pb) is one of the most frequent hazardous air contaminants, where the lungs are particularly vulnerable to its toxicity. However, the Pb distribution and its impact on lung inflammation/apoptosis and particularly the involvement of nuclear factor kappa B (NF-κB) and aryl hydrocarbon receptor (AhR) signaling pathways in Pb-induced lung toxicity have not yet been fully investigated. Adult male Wistar albino rats were exposed to Pb nitrate 25, 50, and 100 mg/kg b.w. orally for 3 days. The histopathological changes of several rat organs were analyzed using hematoxylin and eosin staining. The concentrations of Pb ion in different organ tissues were quantified using inductive coupled plasma mass spectrometry, while gas chromatography-mass spectrometry was used to identify organic compounds. The changes in the mRNA and protein expression levels of inflammatory and apoptotic genes in response to Pb exposure were quantified by using RT-PCR and Western blot analyses, respectively. Treatment of rats with Pb for three consecutive days significantly increased the accumulation of Pb in lung tissues causing severe interstitial inflammation. Pb treatment also increased the percentage of lung apoptotic cells and modulated apoptotic genes (Bc2, p53, and TGF-α), inflammatory markers (IL-4, IL-10, TNF-α), and oxidative stress biomarkers (iNOS, CYP1A1, EphX) in rat lung tissues. These effects were associated with a significant increase in organic compounds, such as 3-nitrotyrosine and myeloperoxidase, and some inorganic elements, such as selenium. Importantly, the Pb-induced lung inflammation and apoptosis were associated with a proportional increase in the expression of NF-κB and AhR mRNAs and proteins. These findings clearly show that Pb induces severe inflammation and apoptosis in rat lungs and suggest that NF-κB and AhR may play a role in Pb-induced lung toxicity.

Regulation of Apoptotic Cell Clearance During Resolution of Inflammation

Programmed cell death (apoptosis) has an important role in the maintenance of tissue homeostasis as well as the progression and ultimate resolution of inflammation. During apoptosis, the cell undergoes morphological and biochemical changes [e.g., phosphatidylserine (PtdSer) exposure, caspase activation, changes in mitochondrial membrane potential and DNA cleavage] that act to shut down cellular function and mark the cell for phagocytic clearance. Tissue phagocytes bind and internalize apoptotic cells, bodies, and vesicles, providing a mechanism for the safe disposal of apoptotic material. Phagocytic removal of apoptotic cells before they undergo secondary necrosis reduces the potential for bystander damage to adjacent tissue and importantly initiates signaling pathways within the phagocytic cell that act to dampen inflammation. In a pathological context, excessive apoptosis or failure to clear apoptotic material results in secondary necrosis with the release of pro-inflammatory intracellular contents. In this review, we consider some of the mechanisms by which phagocytosis of apoptotic cells can be controlled. We suggest that matching apoptotic cell load with the capacity for apoptotic cell clearance within tissues may be important for therapeutic strategies that target the apoptotic process for treatment of inflammatory disease.

Curcumin (Diferulolylmethane) Reduces Transglutaminase 2 Overexpression Induced by Retinoic Acid in Human Nervous Cell Lines

OBJECTIVES

Curcumin, a naturally occurring compound derived from turmeric (Curcuma longa) has long been suggested to have strong therapeutic or preventive potential against human diseases because of its antioxidative, anticancerous, and anti-inflammatory effects. Curcumin is known to exert anti-inflammatory effects by interrupting NF-κB signaling at multiple levels. Many observations indicate that curcumin shows its valuable potential by inhibiting the activity of I-κB kinase. Transglutaminase 2 (TG2) expression is increased in inflammatory diseases. Data in the literature suggest that this enzyme activates the proinflammatory transcriptional factor NF-κB by inducing the polymerization of its inhibitory subunit I-κBα, which in turn results in the dissociation of NF-κB and its translocation to the nucleus, where it is capable of upregulating host inflammatory genes. Interestingly, NF-κB regulatory response elements are also present in the TG2 promoter, suggesting a possible role for this pathway in the mechanism responsible for chronic inflammation. On the basis of these literature data, our objective was to analyze the effects of curcumin on TG2 expression in human nervous cell lines.

METHODS

Human nervous cell lines were treated with curcumin alone or in association with retinoic acid in order to induce TG2 overexpression. TG2 levels were analyzed by Western blot and real-time PCR analyses.

RESULTS

Curcumin was able to downregulate the expression of TG2 in human nervous cell lines, which was also the case after treatment with retinoic acid.

CONCLUSIONS

These results suggest a possible use of curcumin in reducing TG2 overexpression in human nervous cells.

Transglutaminase as a therapeutic target for celiac disease

INTRODUCTION

The only current treatment for celiac disease is a strict gluten-free diet. The ubiquitous presence of gluten in groceries, however, makes the diet burdensome and difficult to maintain, and alternative treatment options are thus needed. Here, the important role of transglutaminase 2 (TG2) in the pathogenesis of celiac disease makes it an attractive target for drug development.

AREAS COVERED

The present paper gives an overview of TG2 and addresses its significance in the pathogenesis of celiac disease. Moreover, the article summarizes preclinical studies performed with TG2 inhibitors and scrutinizes issues related to this therapeutic approach.

EXPERT OPINION

Activation of TG2 in the intestinal mucosa is central in celiac disease pathogenesis and researchers have therefore suggested TG2 inhibitors as a potential therapeutic approach. However, a prerequisite for such a drug is that it should be specific for TG2 and not affect the activity of other members of the transglutaminase family. Such compounds have already been introduced and tested in vitro, but a major obstacle to further development is the lack of a well-defined animal model for celiac disease. Nonetheless, with encouraging results in preclinical studies clinical trials with TG2 inhibitors are eagerly awaited.

Increase in endoplasmic reticulum-associated tissue transglutaminase and enzymatic activation in a cellular model of Parkinson's disease

Parkinson's disease (PD) is characterized by accumulation of α-synuclein aggregates and degeneration of melanized, catecholaminergic neurons. The tissue transglutaminase (tTG) enzyme catalyzes molecular protein cross-linking. In PD, tTG levels are increased and cross-linking has been identified as an important factor in α-synuclein aggregation. In our quest to link tTGs distribution in the human brain to the hallmarks of PD pathology, we recently reported that catecholaminergic neurons in PD disease-affected brain areas display typical endoplasmic reticulum (ER) granules showing tTG immunoreactivity. In the present study, we set out to elucidate the nature of the interaction between tTG and the ER in PD pathogenesis, using retinoic-acid differentiated SH-SY5Y cells exposed to the PD-mimetic 1-methyl-4-phenylpyridinium (MPP(+)). Alike our observations in PD brain, MPP(+)-treated cells displayed typical TG-positive granules, that were also induced by other PD mimetics and by ER-stress inducing toxins. Additional immunocytochemical and biochemical investigation revealed that tTG is indeed associated to the ER, in particular at the cytoplasmic face of the ER. Upon MPP(+) exposure, additional recruitment of tTG toward the ER was found. In addition, we observed that MPP(+)-induced tTG activity results in transamidation of ER membrane proteins, like calnexin. Our data provide strong evidence for a, so far unrecognized, localization of tTG at the ER, at least in catecholaminergic neurons, and suggests that in PD activation of tTG may have a direct impact on ER function, in particular via post-translational modification of ER membrane proteins.

Dihydroisoxazole analogs for labeling and visualization of catalytically active transglutaminase 2

We report the synthesis and preliminary characterization of "clickable" inhibitors of human transglutaminase 2 (TG2). These inhibitors possess the 3-halo-4,5-dihydroisoxazole warhead along with bioorthogonal groups such as azide or alkyne moieties that enable subsequent covalent modification with fluorophores. Their mechanism for inhibition of TG2 is based on halide displacement, resulting in the formation of a stable imino thioether. Inhibition assays against recombinant human TG2 revealed that some of the clickable inhibitors prepared in this study have comparable specificity as benchmark dihydroisoxazole inhibitors reported earlier. At low micromolar concentrations they completely inhibited transiently activated TG2 in a WI-38 fibroblast scratch assay and could subsequently be used to visualize the active enzyme in situ. The potential use of these inhibitors to probe the role of TG2 in celiac sprue as well as other diseases is discussed.

TG2 transamidating activity acts as a reostat controlling the interplay between apoptosis and autophagy

Tissue transglutaminase (TG2) activity has been implicated in inflammatory disease processes such as Celiac disease, infectious diseases, cancer, and neurodegenerative diseases, such as Huntington's disease. Furthermore, four distinct biochemical activities have been described for TG2 including protein crosslinking via transamidation, GTPase, kinase and protein disulfide isomerase activities. Although the enzyme plays a complex role in the regulation of cell death and autophagy, the molecular mechanisms and the putative biochemical activity involved in each is unclear. Therefore, the goal of the present study was to determine how TG2 modulates autophagy and/or apoptosis and which of its biochemical activities is involved in those processes. To address this question, immortalized embryonic fibroblasts obtained from TG2 knock-out mice were reconstituted with either wild-type TG2 or TG2 lacking its transamidating activity and these were subjected to different treatments to induce autophagy or apoptosis. We found that knock out of the endogenous TG2 resulted in a significant exacerbation of caspase 3 activity and PARP cleavage in MEF cells subjected to apoptotic stimuli. Interestingly, the same cells showed the accumulation of LC3 II isoform following autophagy induction. These findings strongly suggest that TG2 transamidating activity plays a protective role in the response of MEF cells to death stimuli, because the expression of the wild-type TG2, but not its transamidation inactive C277S mutant, resulted in a suppression of caspase 3 as well as PARP cleavage upon apoptosis induction. Additionally, the same mutant was unable to catalyze the final steps in autophagosome formation during autophagy. Our findings clearly indicate that the TG2 transamidating activity is the primary biochemical function involved in the physiological regulation of both apoptosis and autophagy. These data also indicate that TG2 is a key regulator of cross-talk between autophagy and apoptosis.

Negative energy balance and hepatic gene expression patterns in high-yielding dairy cows during the early postpartum period: a global approach

In high-yielding dairy cows the liver undergoes extensive physiological and biochemical changes during the early postpartum period in an effort to re-establish metabolic homeostasis and to counteract the adverse effects of negative energy balance (NEB). These adaptations are likely to be mediated by significant alterations in hepatic gene expression. To gain new insights into these events an energy balance model was created using differential feeding and milking regimes to produce two groups of cows with either a mild (MNEB) or severe NEB (SNEB) status. Cows were slaughtered and liver tissues collected on days 6–7 of the first follicular wave postpartum. Using an Affymetrix 23k oligonucleotide bovine array to determine global gene expression in hepatic tissue of these cows, we found a total of 416 genes (189 up- and 227 downregulated) to be altered by SNEB. Network analysis using Ingenuity Pathway Analysis revealed that SNEB was associated with widespread changes in gene expression classified into 36 gene networks including those associated with lipid metabolism, connective tissue development and function, cell signaling, cell cycle, and metabolic diseases, the three most significant of which are discussed in detail. SNEB cows displayed reduced expression of transcription activators and signal transducers that regulate the expression of genes and gene networks associated with cell signaling and tissue repair. These alterations are linked with increased expression of abnormal cell cycle and cellular proliferation associated pathways. This study provides new information and insights on the effect of SNEB on gene expression in high-yielding Holstein Friesian dairy cows in the early postpartum period.

Gene Expression Studies Reveal That DNA Damage, Vascular Perturbation, and Inflammation Contribute to the Pathogenesis of Carbonyl Sulfide Neurotoxicity

Carbonyl sulfide (COS) is an odorless gas that produces highly reproducible lesions in the central nervous system. In the present study, the time course for the development of the neurotoxicological lesions was defined and the gene expression changes occurring in the posterior colliculus upon exposure to COS were characterized. Fischer 344 rats were exposed to 0 or 500 ppm COS for one, two, three, four, five, eight, or ten days, six hours per day. On days 1 and 2, no morphological changes were detected; on day 3, 10/10 (100%) rats had necrosis in the posterior colliculi; and on day 4 and later, necrosis was observed in numerous areas of the brain. Important gene expression changes occurring in the posterior colliculi after one or two days of COS exposure that were predictive of the subsequent morphological findings included up-regulation of genes associated with DNA damage and G1/S checkpoint regulation (KLF4, BTG2, GADD45g), apoptosis (TGM2, GADD45g, RIPK3), and vascular mediators (ADAMTS, CTGF, CYR61, VEGFC). Proinflammatory mediators (CCL2, CEBPD) were up-regulated prior to increases in expression of the astrocytic marker GFAP and macrophage marker CSF2rb1. These gene expression findings were predictive of later CNS lesions caused by COS exposure and serve as a model for future investigations into the mechanisms of disease in the central nervous system.

Culture of HepG2 liver cells on three dimensional polystyrene scaffolds enhances cell structure and function during toxicological challenge

Cultured cells are dramatically affected by the micro-environment in which they are grown. In this study, we have investigated whether HepG2 liver cells grown in three dimensional (3-D) cultures cope more effectively with the known cytotoxic agent, methotrexate, than their counterparts grown on traditional two dimensional (2-D) flat plastic surfaces. To enable 3-D growth of HepG2 cells in vitro, we cultured cells on 3-D porous polystyrene scaffolds previously developed in our laboratories. HepG2 cells grown in 3-D displayed excellent morphological characteristics and formed numerous bile canaliculi that were seldom seen in cultures grown on 2-D surfaces. The function of liver cells grown on 3-D supports was significantly enhanced compared to activity of cells grown on 2-D standard plasticware. Unlike their 2-D counterparts, 3-D cultures were less susceptible to lower concentrations of methotrexate. Cells grown in 3-D maintained their structural integrity, possessed greater viability, were less susceptible to cell death at higher levels of the cytotoxin compared to 2-D cultures, and appeared to respond to the drug in a manner more comparable to its known activity in vivo. Our results suggest that hepatotoxicity testing using 3-D cultures might be more likely to reflect true physiological responses to cytotoxic compounds than existing models that rely on 2-D culture systems. This technology has potential applications for toxicity testing and drug screening.

The effects of the dual 5alpha-reductase inhibitor dutasteride on localized prostate cancer--results from a 4-month pre-radical prostatectomy study

BACKGROUND

As dihydrotestosterone (DHT) is the most potent androgen in the prostate, inhibition of the 5alpha-reductase isoenzymes, which convert testosterone to DHT, could be an appropriate target for the treatment of prostate cancer.

METHODS

Eighty-one men with clinically localized prostate cancer received daily dutasteride 3.5 or 0.5 mg, or no therapy for 4 months before radical prostatectomy. Histopathological assessments were conducted on prostatectomy specimens.

RESULTS

Treatment with dutasteride was associated with reductions in serum and intraprostatic DHT of >or=90%, and a decrease in total prostate and tumor volumes. No effect of dutasteride was noted on Gleason grade. Histopathological effects on benign tissue were similar but less prominent than those seen with androgen ablation, whereas there was no significant difference in cancer histology among the groups.

CONCLUSIONS

Dutasteride treatment results in similar but less marked changes compared with androgen ablation.

Complex formation between tissue transglutaminase II (tTG) and vascular endothelial growth factor receptor 2 (VEGFR-2): Proposed mechanism for modulation of endothelial cell response to VEGF

We have recently demonstrated that thrombin-activated FXIII (FXIIIA-subunit), a plasma transglutaminase, activates VEGFR-2 by crosslinking it with the alpha(v)beta(3) integrin on the surface of endothelial cells (EC), thereby stimulating angiogenesis. Tissue transglutaminase (tTG), which is functionally and structurally related to FXIIIA, is expressed by numerous cell types, among them EC. However, its role in EC function has not been fully characterized. In the present study, we investigated the potential involvement of tTG in angiogenesis. Using co-immunoprecipitation and immunofluorescent staining experiments, we observed that tTG forms a complex with VEGFR-2 on the cell surface and within the cytoplasm of EC. Stimulation of EC with VEGF resulted in translocation of the tTG-VEGFR-2 complex from the cytoplasm to the nucleus. In VEGF-treated cells, tTG-VEGFR-2 interaction resulted in incorporation of VEGFR-2 into high molecular weight crosslinked complex (es), as revealed by an antibody against gamma-glutamyl-epsilon-lysine isopeptide bond. tTG -VEGFR-2 association was inhibited by a specific VEGFR-2 protein tyrosine kinase inhibitor (PTKI ), as well as by cystamine, inhibitor of the transglutaminase activity of tTG, but not by bacitracin which inhibits the protein-disulfide isomerase (PDI) activity of tTG. Furthermore, cystamine completely abolished the VEGF-induced nuclear translocation of the tTG-VEGFR-2 complex. Blockade of the crosslinking activity of tTG by cystamine enhanced VEGF-induced migration of EC in Boyden chamber by 31% (P < 0.02), and prolonged VEGF-induced signaling response, as demonstrated by sustained activation of the MAP kinase ERK. Taken together, our findings suggest that endothelial cell tTG might be involved in modulation of the cellular response to VEGF by forming an intracellular complex with VEGFR-2, and mediating its translocation into the nucleus upon VEGF stimulation.

Effect of retinoic acid-induced transglutaminase on cell growth in regenerating liver

Transglutaminase 2 (TG2) is implicated in the inhibitory regulation of the hepatocyte growth in vitro. In vivo, however, the role of TG2 in liver regeneration after partial hepatectomy (PH) is almost unknown. A dramatic increase of TG2 expression and activation is induced by retinoic acid (RA). Here we show the effect of the RA-induced overexpression of TG2 on liver regeneration after PH. Regenerating rat liver was prepared by 70% PH. RA was intraperitoneally injected immediately after PH. TG2 activity was determined by incorporation of 14C-putrescine into dimethylcasein. Cell cycle was evaluated for incorporation of BrdU into hepatocytes and detected by a flow cytometric analysis. The treatment of RA greatly increased TG2 activity at 1 day after PH. At that time, DNA synthesis was significantly reduced by the treatment of RA. The recovery of liver weight after PH was significantly delayed by the treatment of RA. These results suggested that TG2 was involved in growth capacity in regenerating rat liver after PH.

Tissue transglutaminase is a multifunctional BH3-only protein

Tissue transglutaminase (TG2) protein accumulates to high levels in cells during early stages of apoptosis both in vivo and in vitro. The analysis of the TG2 primary sequence showed the presence of an eight amino acid domain, sharing 70% identity with the Bcl-2 family BH3 domain. Cell-permeable peptides, mimicking the domain sequence, were able to induce Bax conformational change and translocation to mitochondria, mitochondrial depolarization, release of cytochrome c, and cell death. Moreover, we found that the TG2-BH3 peptides as well as TG2 itself were able to interact with the pro-apoptotic Bcl-2 family member Bax, but not with anti-apoptotic members Bcl-2 and Bcl-X(L). Mutants in the TG2-BH3 domain failed to sensitize cells toward apoptosis. In TG2-overexpressing cells about half of the protein is localized on the outer mitochondrial membrane where, upon cell death induction, it cross-links many protein substrates including Bax. TG2 is the first member of a new subgroup of multifunctional BH3-only proteins showing a large mass size (80 kDa) and enzymatic activity.

Expression of apoptosis-related proteins in rat with induced colitis

BACKGROUND AND AIMS

Inflammatory bowel diseases (IBD) evoke a damage-repair process accompanied by the activation of apoptotic genes. Data on transglutaminase (TG) expression in apoptotic cells in inflamed colonic epithelium has not been reported, although TG cross-links proteins within typical apoptotic bodies in various cell lines. In an experimental model of colitis we investigated the expression of different markers of apoptosis related to the degree and development of colonic inflammation.

METHODS

Two studies were performed: (a) Colitis was induced by the administration of 2,4,6-trinitrobenzen sulfonic acid (TNBS) at a dose of 10 or 20 mg per rat in 50% ethanol, and the rats were killed 1 week later; (b) Colitis was induced by 20 mg TNBS and the rats were killed 3 days, 1, 2, and 4 weeks thereafter. The colon of rats was macroscopically assessed, and biopsies were histologically assessed and immunoprobed for FasL, FasR, p53 and tTG. Cell death was detected by TUNEL, and TG activity was assayed on colon homogenates.

RESULTS

Study A: According to enhanced TUNEL positivity, FasR/FasL and p53 expression increased depending on the severity of the colitis. Study B showed increased p53 expression at day 3 while FasR/FasL coexpression peaked at 1 week. In both studies tTG was mainly expressed in the extracellular matrix of damaged tissue and in the submucosa.

CONCLUSIONS

Our findings suggest that expression of apoptosis markers is related to the degree of colitis and show that apoptosis is sustained by both p53 and FasR/FasL pathways, depending on the phase of colitis development. Moreover, the lack of TG staining in typical apoptotic bodies may account for a perturbation of the cross-linked apoptotic envelope that may be an important determinant in the development of immune response in ulcerative colitis.

Neglected opportunities in apoptosis research

Interest in apoptosis is currently growing rapidly, and previous articles in this series in TCB have described a number of recent advances in the field. Certain aspects of the process, however, have been relatively neglected. In this article, John Kerr draws attention to some of them, with the hope that he may indicate potentially productive areas for future research.

Intracellular localization and activity state of tissue transglutaminase differentially impacts cell death

Tissue transglutaminase (tTG) is a unique member of the transglutaminase family as it is both a transamidating enzyme and a GTPase. In the cell tTG is mostly cytosolic, however it is also found in the nucleus and associated with the plasma membrane. tTG can be proapoptotic, however anti-apoptotic activities of the enzyme have also been reported. To determine how the intracellular localization and transamidating activity of tTG modulates its effects on apoptosis, HEK293 cells were transiently transfected with tTG or [C277S]tTG (which lacks transamidating activity) constructs that were targeted to different intracellular compartments. Apoptosis was induced by thapsigargin treatment, which results in increased intracellular calcium concentrations. Cytosolic tTG was pro-apoptotic, while nuclear localization of [C277S]tTG attenuated apoptosis. Membrane-targeted tTG had neither pro- nor anti-apoptotic functions. This finding indicates for the first time that intracellular localization is an important determinant of the effect of tTG on apoptosis. Previous studies have suggested that tTG may modulate retinoblastoma (Rb) protein, an important suppressor of apoptosis. tTG interacted with Rb and after induction of apoptosis, the interaction of nuclear-targeted [C277S]tTG with Rb was increased significantly concomitant with an attenuation of apoptosis. In contrast, the interaction of nuclear-targeted tTG with Rb was significantly decreased and apoptosis was not attenuated. These data suggest that tTG protects cells against apoptosis in response to stimuli that do not result in increased transamidating activity by translocating to the nucleus, and that complexing with Rb may be an important aspect of the protective effects of tTG.

Role of tissue transglutaminase in GTP depletion-induced apoptosis of insulin-secreting (HIT-T15) cells

The role of tissue transglutaminase (tTG), a calcium-dependent and GTP-modulated enzyme, in apoptotic death induced by GTP depletion in islet beta-cells was investigated. GTP depletion and apoptosis were induced by mycophenolic acid (MPA) in insulin-secreting HIT-T15 cells. MPA treatment increased in situ tTG activity (but not protein levels) in a dose- and time-dependent manner in parallel with the induction of apoptosis. MPA-induced increases of both tTG activity and apoptosis were entirely blocked by co-provision of guanosine but not adenosine. MPA-enhanced tTG activity could be substantially reduced by co-exposure to monodansylcadaverine or putrescine (tTG inhibitors), and largely blocked by lowering free Ca(2+) concentrations in the culture medium. However, MPA-induced cell death was either not changed or was only slightly reduced under these conditions. By contrast, a pan-caspase inhibitor (Z-VAD-FMK) entirely prevented apoptosis induced by MPA, but did not block the enhanced tTG activity, indicating that GTP depletion can induce apoptosis and activate tTG either independently or as part of a cascade of events involving caspases. Importantly, the morphological changes accompanying apoptosis could be markedly prevented by tTG inhibitors. These findings suggest that the effect of the marked increase in tTG activity in GTP depletion-induced apoptosis of insulin-secreting cells may be restricted to some terminal morphological changes.

Proliferation and apoptosis in the developing human neocortex

The cell kinetics of the developing central nervous system (CNS) is determined by both proliferation and apoptosis. In the human neocortex at week 6 of gestation, proliferation is confined to the ventricular zone, where mitotic figures and nuclear immunoreactivity for proliferating cell nuclear antigen (PCNA) are detectable. Cell division is symmetric, with both daughter cells reentering mitosis. At week 7, the subventricular zone, a secondary proliferative zone, appears. It mainly gives rise to local circuit neurons and glial cells. Around week 12, the ventricular and subventricular zones are thickest, and the nuclear PCNA label is strongest, indicating that proliferation peaks at this stage. Thereafter, asymmetric division becomes the predominant mode of proliferation, with one daughter cell reentering mitosis and the other one migrating out. Towards late gestation, the ventricular and subventricular zones almost completely disappear and proliferation shifts towards the intermediate and subplate zones, where mainly glial cells are generated. A remnant of the subventricular zone with proliferative activity persists into adulthood. In general, proliferation follows a latero-medial gradient in the neocortex lasting longer in its lateral parts. Apoptotic nuclei have been detected around week 5, occurring in low numbers in the ventricular zone at this stage. Apoptotic cell death increases around midgestation and then spreads throughout all cortical layers, with most dying cells located in the ventricular and subventricular zones. This spatial distribution of apoptosis extends into late gestation. During the early postnatal period, most apoptotic cells are still located in the subcortical layers. During early embryonic development, proliferation and apoptosis are closely related, and are probably regulated by common regulators. In the late fetal and early postnatal periods, when proliferation has considerably declined in all cortical layers, apoptosis may occur in neurons whose sprouting axons do not find their targets.

Transglutaminase overexpression sensitizes neuronal cell lines to apoptosis by increasing mitochondrial membrane potential and cellular oxidative stress

'Tissue' transglutaminase (tTG) selectively accumulates in cells undergoing apoptosis both in vivo and in vitro. Considering the central role played by mitochondria in apoptosis, we investigated the relationships existing amongst tTG expression, apoptosis and mitochondrial function. To this aim we studied the mechanisms of apoptosis in a neuronal cell line (SK-N-BE (2)) in which the tTG-expression was driven by a constitutive promoter. Furthermore, a tet-off inducible promoter was also used in 3T3 fibroblastic cells used as control. Both cell lines, when expressing tTG, appeared 'sensitized' to apoptosis. Strikingly, we found major differences in the morphological features of mitochondria among cell lines in the absence of apoptotic stimuli. In addition, these ultrastructural characteristics were associated with specific functional features: (i) constitutively hyperpolarized mitochondria and (ii) increased reactive oxygen intermediates production. Importantly, after mitochondrial-mediated apoptosis by staurosporine, a rapid loss of mitochondrial membrane potential was found in tTG cells only. Taken together, these results seem to suggest that, via hyperpolarization, tTG might act as a 'sensitizer' towards apoptotic stimuli specifically targeted to mitochondria. These results could also be of pathogenetic relevance for those diseases that are characterized by increased tTG and apoptotic rate together with impaired mitochondrial function, e.g. in some neurodegenerative disease.

The clearance of apoptotic cells in the liver is mediated by the asialoglycoprotein receptor

Apoptosing cells are actively phagocytosed in parenchymal tissues, thus preventing the inflammatory reaction which could derive from their slow uncontrolled degradation. The molecular mechanisms by which an apoptotic cell is recognized and taken up are largely unknown. We propose that the recognition of apoptotic hepatocytes is mediated by the sugar recognition systems of the liver, particularly the asialoglycoprotein receptor (ASGP-R). The results presented here demonstrated the participation of ASGP-R in the removal of apoptotic parenchymal cells, and indicate a new perspective for the understanding of its physiological role.

Putative nucleotide binding sites of guinea pig liver transglutaminase

Three peptides corresponding to glycine-rich internal sequences of the guinea pig liver transglutaminase molecule were synthesized. These were peptide 1 (amino acid residues 520-544), peptide 2 (amino acid residues 345-367) and peptide 3 (amino acid residues 45-69). All of the synthetic peptides demonstrated significant binding ability for both ATP and GTP. Peptide 1 was the best protector of transglutaminase activity from both ATP and GTP inhibition, while peptides 2 and 3 protected the activity only from GTP inhibition. The data shown here lead us to propose putative binding site(s) for ATP and GTP guinea pig liver transglutaminase.

Cell cycle kinetics, tissue transglutaminase and programmed cell death (apoptosis)

Studies were undertaken on a highly metastatic hamster fibrosarcoma cell line with a view to assessing whether cells entering into apoptosis, measured by counting the number of transglutaminase mediated detergent insoluble envelopes, has any synchrony with a particular phase of the cell cycle. A double exposure of thymidine was used to block cells in early S-phase. Flow cytometry in combination with [3H]thymidine incorporation into DNA was used to assess the degree of synchrony and progression through the different phases of cell cycle. The apoptotic index was found to be at its maximum in mid-S-phase. Measurement of transglutaminase activity in each phase of the cell cycle indicated that the specific activity was also at its greatest during mid S-phase. The level of enzyme was relatively unchanged throughout the cell cycle indicating that the regulation of transglutaminase activity occurs primarily through effects on catalytic activity rather than enzyme synthesis.

Pattern of expression of c-Myc, Max and Bin1 in human anagen hair follicles

BACKGROUND

We recently reported the presence of c-Myc immunoreactivity in two distinct regions of the inner root sheath (IRS) of human anagen hair follicles; they corresponded to the regions where keratinocytes of Henle's and Huxley's layers enter the terminal differentiation phase that will lead to their exfoliation in the pilary canal. These regions were denoted lower (LR) ring and upper ring (UR).

OBJECTIVES

To extend these observations to other genes connected to c-Myc and specifically to Max and Bin1. Max is the best known heterodimeric partner of c-Myc, interacting with its C-terminal domain, and Bin1 is an adaptor protein interacting with its N-terminal domain.

METHODS

Human anagen hair follicles were processed for c-Myc, Max and Bin1 immunohistochemistry and immunofluorescence. The presence of different isoforms of Bin1 was evaluated by Western blot analysis.

RESULTS

Analysis of sections cut in several planes, including tangential, demonstrated the presence of a third ring of c-Myc-positive cells (intermediate ring; IR) in the cuticle of the IRS corresponding to the region where this thin layer undergoes keratinization. Max immunoreactivity was observed in the three layers of the IRS starting in the lower bulbar region and ending in each of them at the level of the corresponding c-Myc-positive ring. Bin1 immunoreactivity was clearly distinguished only in Huxley's layer and in the cuticle, starting in some cells below the UR and terminating at the level of the latter. The companion layer of the outer root sheath was also labelled up to the infundibular region. Max and Bin1 immunostaining were less consistently observed in other skin adnexae and in the epidermis.

CONCLUSIONS

The results indicate that the asynchronous differentiation along the axis of the hair follicle of the different layers of the IRS and of the companion layer involves the expression of different genes that are interrelated in the so-called 'Myc network'. The specific localization of c-Myc in the IRS only at the level of the discrete and limited regions of the three rings appears to be the hallmark of the switch from differentiation to terminal differentiation/cell deletion.

Prostatic involution in men taking finasteride is associated with elevated levels of insulin-like growth factor-binding proteins (IGFBPs)-2, -4, and -5

BACKGROUND

Insulin-like growth factor-binding proteins (IGFBPs)-2, -4, and -5 are associated with upregulation of apoptosis in the ovary. The purpose of this study was to assess the roles of IGF-I and IGFBPs during involution of the prostate. Frozen and fixed tissue was collected by transurethral prostatectomy from Caucasian men, aged 52-82 years, scheduled for prostatectomy for benign prostatic hyperplasia, who took either placebo (n = 7) or the 5alpha-reductase inhibitor finasteride for 6 days to 6 years (n = 15) prior to surgery.

METHODS

Intraprostatic androgen levels were measured by radioimmunoassay. Tissues were immunostained for IGF-I and IGFBP-2, -3, -4, and -5, and staining was quantitated by computerized image analysis. Serial sections were stained for markers of apoptosis (TUNEL and tissue transglutaminase) and IGFBP-2, -4, or -5.

RESULTS

IGF-I staining was significantly decreased in the medium-term (18-43 days) treatment group and remained so for the duration of the study (P = 0.026). IGFBP-3 staining was unchanged in the early and medium-term treatment groups; however, a transient earlier rise in the level of this proapoptotic protein cannot be ruled out. The percentage of epithelial cell area staining positively for IGFBP-2 increased significantly, from 1.6 +/- 0.5 in the placebo group to 12.0 +/- 2.0 (P < 0.0001), and 7.6 +/- 1.9 (P = 0.003) in the short (6-13 days) and medium-term treatment groups, respectively. IGFBP-4 staining increased from 2.2 +/- 0.6 to 9.8 +/- 1.9 (P < 0.0001) and 7.4 +/- 1.2 (P = 0.004) in the short and medium-term groups, respectively, and IGFBP-5 staining increased from 0.2 +/- 0.1 to 3.8 +/- 2.0 (P = 0.004) in the medium-term group. The results from serial sections showed that IGFBP-2 and -4 costained with markers of apoptosis, while IGFBP-5 did not.

CONCLUSIONS

These results indicate that IGFBP-2, -4, and -5 are associated with prostatic involution. Because of the timing and distribution of expression, we hypothesize that IGFBP-2 and -4 have a role as signals for apoptosis, but that IGFBP-5 likely does not.

The utility of tissue transglutaminase as a marker of apoptosis during treatment and progression of prostate cancer

PURPOSE

To determine the extent of cell proliferation and apoptosis during treatment and progression of prostate cancer and to determine whether staining for tissue transglutaminase is a better histological marker than TUNEL for neoadjuvant androgen ablation treatment of localized prostate cancer.

MATERIALS AND METHODS

Immunocytochemistry techniques were used on archival prostate tissue from four groups of men: 14 men with BPH, 18 men with untreated, localized prostate cancer, 21 men with localized prostate cancer who received neoadjuvant hormone therapy prior to prostatectomy and 18 men with metastatic androgen-independent prostate cancer. Cell proliferation was evaluated by staining for the Ki67 nuclear antigen, and apoptosis was evaluated by staining for DNA fragmentation (TUNEL technique) and tissue transglutaminase (tTG). Image analysis was used to quantitate the results.

RESULTS

TUNEL staining increased by 37% in localized prostate cancer compared with BPH, with a further increase of 43% seen after neoadjuvant therapy, although variation was such that neither was statistically significant. In androgen-independent cancer, TUNEL staining was decreased compared with neoadjuvant hormone treated cancer (p = 0.02). Staining for tTG was not increased in untreated prostate cancer compared with BPH; however, staining more than doubled after neoadjuvant therapy, compared with untreated prostate cancer (p = 0.04). Staining for tTG was markedly decreased in androgen-independent cancer (p = 0.07 compared with BPH and p = 0.0004 compared with neoadjuvant hormone treated cancer). Ki67 immunoreactivity did not significantly change in localized prostate cancer, either before or after neoadjuvant therapy, compared with BPH, but it more than doubled in androgen-independent prostate cancer (p = 0.07 compared with BPH and p = 0.05 compared with untreated prostate cancer).

CONCLUSIONS

This study shows that cell proliferation increases and apoptosis decreases as prostate cancer progresses to androgen independence, and, that of the markers used in this study, tissue transglutaminase most accurately reflects the anticipated effect of neoadjuvant hormone therapy on localized prostate cancer. An assessment of these parameters provides a valuable tool for appraising new prostate cancer therapies.

Inhibition of "tissue" transglutaminase increases cell survival by preventing apoptosis

Treatment of the human promonocytic cell line U937 with all-trans-retinoic acid (RA) commits these cells to apoptosis, which can be triggered by simply increasing intracellular calcium levels by the ionophore A23187. RA treatment of U937 cells is characterized by a decrease in Bcl-2 and marked induction of "tissue" transglutaminase (tTG) gene expression. In this study, we show that the inhibition of tTG expression in U937 cells undergoing apoptosis prevents their death. In fact, U937 cell-derived clones transfected with the human tTG gene in the antisense orientation showed a pronounced decrease in apoptosis induced by several stimuli. These findings demonstrate that the Ca(2+)-dependent irreversible cross-linking of intracellular proteins catalyzed by tTG represents an important biochemical event in the gene-regulated cell death in monoblasts. In addition, our data indicate that the apoptotic program in promonocytic cells is strictly regulated by RA and that a key role is played by the free intracellular calcium concentration.

Enhanced Fas/CD95-mediated apoptosis by epidermal growth factor in human endometrial epithelial cells

Epidermal growth factor (EGF) has been reported to regulate apoptosis in various cell lineages. Throughout the menstrual cycle overexpression of the EGF receptor in the secretory epithelium and constitutive expression of EGF in all types of endometrial cells were identified by immunohistochemical study of normal human endometrial tissues. However, it is not known whether EGF also regulates endometrial apoptosis. This study examined the regulatory functions of EGF in endometrial apoptosis by using a human endometrial epithelial cell line HHUA which is susceptible to Fas-mediated apoptosis. Although EGF alone did not affect the cell growth of HHUA, EGF pretreatment of HHUA enhanced Fas-mediated growth suppression and Fas-mediated DNA fragmentation in the cells. Flowcytometric analyses demonstrated that EGF did not induce Fas expression on the cell surface while expressions of the EGF receptor were down-regulated. These results suggest that EGF may enhance apoptotic susceptibility of the endometrial epithelium, especially in the secretory epithelium.

'Tissue' transglutaminase release from apoptotic cells into extracellular matrix during human liver fibrogenesis

Enhanced apoptosis characterizes several pathologies affecting human liver. This study sought to determine whether apoptosis is involved in the formation of fibrotic lesions occurring in hepatic disease. The expression of Bcl-2 was analysed, and of 'tissue' transglutaminase (tTG), a cross-linking enzyme which recent evidence suggests plays a role in the formation of fibrotic lesions in experimental settings. Regardless of the degree of liver injury, tTG abnormally accumulated in the liver cells adjacent to fibrotic tissue. Many cells showing DNA fragmentation and morphological features of apoptosis were also observed near fibrotic lesions. Bcl-2 was detected predominantly in infiltrating lymphocytes within the liver tissue. Marked staining for both tTG protein and chromatin was also observed in the acellular fibrotic tissue, which suggested an active release of intracellular macromolecules from the dying cells into the extracellular matrix. This study indicates that fibrogenesis in the liver is associated with the release of tTG from dying cells. By cross-linking extracellular matrix proteins, this enzyme might play a role in the formation of fibrotic lesions.

Fasting-induced apoptosis in rat liver is blocked by cycloheximide

The effect of cycloheximide (CH) on the fasting-induced changes of rat liver cell and protein turnover has been investigated. Late starvation phase (3-4-day-fasting period) was characterised by a decrease in liver weight and protein and DNA content. The loss of DNA was not related to liver cell necrosis but due not only to depression of cell proliferation as shown by the drop in the labelling index but also induction of apoptosis. This type of apoptosis was documented by the increase in the apoptotic index (cells labelled by TUNEL) and transglutaminase activity as well as by DNA fragmentation. The liver cells of fasted rats appeared smaller as shown by the higher cell density and DNA/protein ratio than in controls. Females were more resistant to fasting-induced apoptosis than males. A single dose of CH, a drug primary known as inhibitor of protein synthesis, induced or enhanced apoptosis in fed and 2-days fasted male rats, respectively, without any sign of cell necrosis. On the contrary, the administration of repeated doses of CH blocked apoptosis induced by fasting. CH "froze" protein and DNA content as well as apoptotic process at the level of 2 days-fasted rats. While fasting-induced liver protein loss resulted from a marked reduction in protein synthesis with a slight decrease in degradation, repeated treatment with CH virtually blocked protein loss by abolishing protein catabolism. These data suggest a direct relationship between the catabolic side of protein turnover and the apoptotic process.

Expression of phosphatidylethanolamine N-methyltransferase in Yoshida ascites hepatoma cells and the livers of host rats

Previous studies have implicated phosphatidylethanolamine N-methyltransferase-2 (PEMT2) in the regulation of non-neoplastic liver growth [Tessitore,L., Cui,Z. and Vance,E. (1997) Biochem. J., 322, 151-154]. We have now investigated whether or not PEMT2 is also involved in the control of proliferation of hepatoma cells growing in an animal and cell death by apoptosis in the liver of tumor-bearing rats. PEMT activity was barely detectable and PEMT2 protein was absent in hepatoma cells growing exponentially in vivo whereas CTP:phosphocholine cytidylyltransferase (CT) activity and expression were high. The lack of PEMT2 corresponded with the absence of its mRNA. Both PEMT2 protein and mRNA appeared when cells entered the stationary phase of tumor growth and, in parallel, CT expression decreased. The host liver first became hyperplastic and exhibited a slight increase in CT activity and decrease in PEMT2 expression. During the stationary phase of hepatoma growth the host liver regressed and eventually became hypoplastic following induction of apoptosis. The appearance of apoptosis in the host liver was associated with a marked reduction in both CT activity and expression as well as an enhancement of PEMT activity and PEMT2 expression. McArdle RH7777 hepatoma cells underwent apoptosis when transfected with cDNA for PEMT2. The evidence supports the proposal that PEMT2 may have a role in the regulation of 'in vivo' hepatoma and hepatocyte cell division as well as hepatocyte cell death by apoptosis.

Hormonal control of "tissue" transglutaminase induction during programmed cell death in frog liver

In this study, we show that sex hormones (testosterone, estradiol, and progesterone) act as physiological modulators of programmed cell death (PCD) during the frog liver involution observed postvitellogenesis. PCD in parenchymal cells is paralleled by the specific induction of the "tissue" transglutaminase (tTG) gene. tTG protein specifically accumulates in hepatocytes showing the morphological features of apoptosis. The hormone-dependent increase of both PCD and tTG was reproduced in ovariectomized frogs. Treatment of castrated animals with testosterone, estradiol, and progesterone inhibited the induction of both tTG and PCD, thus indicating that in vivo the drop in the circulating sex hormone is the signal favoring the involution phase of the maternal frog liver after mating. Although an affinity-purified polyclonal antibody raised against mammalian transglutaminase reacts in frog liver with a 55- to 60-kDa protein, concomitant with the onset of PCD, tTG cleavage products were detected, suggesting a proteolytic processing of the enzyme protein. These results represent the first evidence indicating that the physiological involution occurring postvitellogenesis of frog liver takes place by programmed cell death and that this, together with the concomitant induction of tTG gene expression, is regulated by sex hormones.

Review: The role of glutathione in the regulation of apoptosis

Recent advances in the study of the regulation of cell death by apoptosis suggest that changes in mitochondrial permeability frequently precede the development of morphological features such as chromatin condensation, phosphatidylserine inversion of the outer cell membrane and the activation of endonucleases. There is evidence that this permeability transition is associated with, and may be regulated by, changes in the intracellular redox potential. The role of the tripeptide thiol glutathione in the regulation of apoptosis-associated redox changes and the control of mitochondrial membrane permeability is reviewed in this article.

Identification of 'tissue' transglutaminase binding proteins in neural cells committed to apoptosis

Overexpression of 'tissue' transglutaminase (tTG) in the human neuroblastoma cells increases spontaneous apoptosis and renders these cells highly susceptible to death induced by various stimuli. We used immunoprecipitation to identify cellular proteins that interact specifically with tTG in SK-N-BE(2) -derived stable transfectants. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed that tTG binding proteins have molecular masses of 110, 50, 22, 14, and 12 kDa. Microsequencing and computer search analyses allowed us to identify these polypeptides as the beta-tubulin (50 kDa), the histone H2B (14 kDa), and two GST P1-1-truncated forms (22 and 12 kDa). The specificity of the interaction between tTG and these proteins was confirmed by competing tTG binding with purified enzyme and by detecting tTG in immunoprecipitates obtained using beta-tubulin or GST P1-1 mAb's. Here we demonstrate that the GST P1-1 acts as an efficient acyl donor as well as acceptor tTG substrate both in cells and in vitro. The tTG-catalyzed polymerization of GST P1-1 leads to its functional inactivation and is competitively inhibited by GSH. By contrast, the tTG-beta-tubulin interaction does not result in the cross-linking of this cytoskeletal protein, which suggests that microtubules act as the anchorage site for tTG and GST P1-1 interaction.

How the ovarian follicle of Podarcis sicula recycles the DNA of its nurse, regressing follicle cells

In Podarcis sicula specialized follicle cells send reserve materials to the previtellogenic oocyte via intercellular bridges. Immediately before the onset of vitellogenesis this transferring becomes particularly massive so that the cell volume significantly reduces, meanwhile in the nucleus the morphological alterations typical of apoptosis appear. To clarify why these follicle cells are not simply fully resorbed by the oocyte and to determine whether their DNA is discarded or recycled, we carried out a series of morphological and biochemical investigations. The finding that large macromolecular scaffolds are formed and that these are able to retain the DNA until it is extensively cut by two different endonucleases suggests that regression of the follicle cells is programmed and that the fate of their DNA is strictly controlled. Following its genetical neutralization via fragmentation, the DNA is apparently recycled by being transferred into the oocyte via the intercellular bridges, that, in fact, remain open until the very late stages of cell regression. The small DNA fragments reaching the oocyte cytoplasm would not interfere with meiosis completion but could significantly contribute to the stock of reserve materials to the advantage of the growing oocyte and/or developing embryo.

Camptothecin induces differentiation, tissue transglutaminase and apoptosis in cultured keratinocytes

Cultured normal human adult keratinocytes were exposed to (S)-(+)- camptothecin over the concentration range 10(-5) to 10(-10) M. The dose-dependent inhibition of growth was recorded using cell counting. The induction of terminal differentiation was demonstrated by the relative increase in squamous and cornified cells, and the concomitant decrease in small, proliferative cells, with an overall decrease in total cell numbers on going from 10(-10) to 10(-6) M concentration of the drug. The induction of apoptosis was studied by assay of two types of transglutaminase, "tissue" and "keratinocyte", and by assay of histone-linked mono- and oligonucleosomes. Induction of apoptosis was accompanied with increase in "tissue" transglutaminase and in the amount of nucleosomes, the latter being indicative of endonuclease activity. This activity was sharply increased at a camptothecin concentration of 10(-5) M, and may have been facilitated by "tissue" transglutaminase at lower concentrations. The data suggest that camptothecin restricts keratinocyte growth by several mechanisms including apoptosis and emphasize its possible use in topical therapy for psoriasis.

Tissue transglutaminase is not increased during apoptosis of HT-1080 human fibrosarcoma cells

Tissue transglutaminase (tTGase), a cytosolic enzyme which catalyzes the covalent cross-linking of proteins, is thought to be involved in the apoptosis. Here, we tested whether tTGase is involved during HT-1080 fibrosarcoma cell apoptosis induced by the YIGSR (Tyr-Ile-Gly-Ser-Arg) peptide. This sequence is derived from the laminin alpha1 chain, and its potency is increased by the formation of a 16mer polymerization using a lysine tree structure. Cells were treated with several different concentrations of Ac-Y 16 for 16 hours, and apoptosis was increased in dose-dependent manner. When assayed by incorporation of [14C] putrescine into succinylated casein, total transglutaminase activity was decreased in parallel with the change in the number of attached cells. Western blot analysis using polyclonal antibody against tTGase showed that the tTGase protein level had not been significantly changed when equal amounts of the protein were applied. To confirm this result, we induced apoptosis of these cells by coating the tissue culture plates with non-adhesive poly-hydroxyethyl methacrylate (HEMA). Western blot analysis showed that the tTGase protein level did not change during this process of apoptosis. Although it has been suggested that tTGase is involved in the process of apoptosis of various cells in vitro and in vivo, our data demonstrate that tTGase is not involved in the process of apoptosis of HT-1080 human fibrosarcoma cell induced by either Ac-Y 16 or a non-adhesive culture surface.

Apoptosis in the human inner ear. Detection by in situ end-labeling of fragmented DNA and correlation with other markers

The aim of this study was to obtain baseline data on the recently described special form of single cell death, apoptosis, in normal human inner ears. For this purpose, in situ end-labeling of the fragmented DNA was applied, in conjunction with apoptosis-related markers, to detect cellular elements showing programmed cell death in decalcified and paraffin-embedded tissues. Over 20 specimens were analyzed which were obtained from autopsy cases with no history of acoustic lesions confirmed by histopathology. Based on staining results, we saw no apoptotic signs in the majority of normal adult inner ears. An apoptotic cell captured in the Reissner's membrane of the cochlea from an old patient may, however, indicate an age-related subtle cell loss with the process of apoptosis. Nevertheless, the fact that more apoptosis was not found in our cases suggests that this phenomenon does not contribute significantly to the tissue homeostasis in the adult inner ear under normal conditions. These data are in accordance with our immunohistochemical findings on the p53 nucleoprotein, and proliferating cell nuclear antigen expression since there was no staining in any of the cellular elements, including the mesenchymal cells. This reflects a stationary and stable condition of cells of the vestibular and the cochlear structures, probably to maintain their integrity and the fine sensory functions. As opposed to the above findings, during inner ear development, the epithelial cells lining the cochlear lumen, the ossifying cartilage of the temporal bone, and the mesenchymal cells show different degrees of proliferation in combination with single cell death as signs of maturation of the vestibular and the cochlear apparatus. In addition, apoptosis has been demonstrated in cells of the cochlear stria vascularis from an adult patient treated with high doses of cisplatin, vinblastine and bleomycin prior to death. Furthermore, a wide range of apoptosis could be induced experimentally in a normal ear by an external perfusion of actinomycin D (ActD), which is known to produce programmed cell death in many cell types of different origins. The potential role of cytostatic agents in the apoptotic process of the inner ear needs, however, to be confirmed in large-scale specimens from patients treated with genotoxins. The fact, however, that apoptotic cells are also seen in association with ActD indicates that the fine sensory structure of the cochlea may also be a target for certain chemotherapeutic agents when administered in high doses.

Differences in transglutaminase mRNA after polyamine depletion in two cell lines

Polyamines serve as natural substrates for the transglutaminase that catalyzes covalent cross-linking of proteins and is involved in cellular adhesion and proliferation. This study tests the hypothesis that intracellular polyamines play a role in the regulation of transglutaminase expression in rat small intestinal crypt cells (IEC-6 cell line) and human colon carcinoma cells (Caco-2 cell line). Treatment with alpha-difluoromethylornithine (DFMO; a specific inhibitor of polyamine synthesis) significantly depleted the cellular polyamines putrescine, spermidine, and spermine in both cell lines. In IEC-6 cells, polyamine depletion was associated with a decrease in the levels of transglutaminase mRNA. In Caco-2 cells, however, polyamine depletion significantly increased the levels of transglutaminase mRNA and enzyme activity. In both cell lines, ornithine decarboxylase mRNA levels increased and protooncogene c-myc mRNA decreased in the presence of DFMO. Addition of polyamines to cells treated with DFMO reversed the effect of DFMO on the levels of mRNA for these genes in both lines. There was no significant change in the stability of transglutaminase mRNA between control and DFMO-treated IEC-6 cells. In contrast, the half-life of mRNA for transglutaminase in Caco-2 cells was dramatically increased after polyamine depletion. Spermidine, when given together with DFMO, completely prevented increased half-life of transglutaminase mRNA in Caco-2 cells. These results indicate that 1) expression of transglutaminase requires polyamines in IEC-6 cells but is inhibited by these agents in Caco-2 cells, 2) polyamines modulate transglutaminase expression at the level of mRNA through different pathways in these two cell lines, and 3) posttranscriptional regulation plays a major role in the induction of transglutaminase mRNA in polyamine-deficient Caco-2 cells.

Cell death/apoptosis: normal, chemically induced, and teratogenic effect

Cell death is an integral part of a variety of biological processes including cell proliferation, differentiation, and morphogenesis. We review here the morphological and biochemical nature as well as the genetic basis for cell death during normal and abnormal development. Most often referred to in normal development as programmed cell death, this controlled process determines the size, patterning, and function of many tissues. The importance of its proper genetic regulation is demonstrated by the discovery of cell death-specific genes and the several disorders including cancer and teratogenesis that result from repression or enhancement of cell death. In our studies we employed the developing mouse limb, which provides a defined window of active cell death, to elucidate mechanisms of cell death. We have developed markers that reveal in the developing normal limb an apoptotic morphology with phagocytosis and DNA fragmentation. In the limb deformity mutant Hammertoe there is a defective (restricted) cell death pattern, but the morphology remains apoptotic. By the use of these markers, we were able to observe that the teratogen retinoic acid produced enhanced apoptotic cell death. Most interestingly, retinoic acid-induced cell death in the Hammertoe mutant resulted in correction of the mutant phenotype. Future studies will determine the relationship between exogenous agents and endogenous signaling pathways as well as indicate how these interactions can alter the fate of a given cell and potentially ameliorate a genetic abnormality.

Tissue transglutaminase-dependent posttranslational modification of the retinoblastoma gene product in promonocytic cells undergoing apoptosis

The retinoblastoma gene product (pRB) plays an important role in controlling both cell release from the G1 phase and apoptosis. We show here that in the early phases of apoptosis, pRB is posttranslationally modified by a tissue transglutaminase (tTG)-catalyzed reaction. In fact, by employing a novel haptenized lysis synthetic substrate which allows the isolation of glutaminyl-tTG substrates in vivo, we identified pRB as a potential tTG substrate in U937 cells undergoing apoptosis. In keeping with this finding, we showed that apoptosis of U937 cells is characterized by the rapid disappearance of the 105,000- to 110,000-molecular-weight pRB forms concomitantly with the appearance of a smear of immunoreactive products with a molecular weight of greater than 250,000. The shift in pRB molecular weight was reproduced by adding exogenous purified tTG to extracts obtained from viable U937 cells and was prevented by dansylcadaverine, a potent enzyme inhibitor. The effect of the pRB posttranslational modification during apoptosis was investigated by determining the E2F-1 levels and by isolating and characterizing pRB-null clones from U937 cells. Notably, the lack of pRB in these U937-derived clones renders these p53-null cells highly resistant to apoptosis induced by serum withdrawal, calphostin C, and ceramide. Taken together, these data suggest that tTG, acting on the pRB protein, might play an important role in the cell progression through the death program.

Retinoic acid receptors alpha and gamma mediate the induction of "tissue" transglutaminase activity and apoptosis in human neuroblastoma cells

All-trans retinoic acid (RA) reduces human neuroblastoma growth by inducing either differentiation or apoptosis. The apoptotic program in these cells is regulated by RA and is paralleled by the transcriptional induction of "tissue" transglutaminase (tTG). tTG is a protein cross-linking enzyme, which specifically accumulates in cells undergoing apoptosis in various in vivo and in vitro systems. In neuroblastoma cells, tTG is detected exclusively in the cells expressing the S-type phenotype and showing an increased apoptosis. The present study was undertaken to identify the retinoid receptors which are involved in the regulation of tTG and apoptosis as well as in the in vitro neuronal differentiation of the human SK-N-BE(2) neuroblastoma cell line. We have previously characterized the retinoid acid receptors expressed in this cell line. In the present study, by using synthetic retinoids selectively activating RAR/RXR isoforms, we have identified the RAR/RXR receptors involved in the induction of either apoptosis or differentiation. We have also studied the effect of the selective RA analogs on tTG activity. We observed that while RARalpha- and RARgamma-selective retinoids alone were able to induce tTG activity, only the combined stimulation of both RARalpha and RARgamma induced apoptosis. Conversely, several combinations of RAR/RXR closely mimicked the differentiation effects observed with all-trans retinoic acid. These results indicate that, at variance with differentiation, the induction of apoptosis in human SK-N-BE(2) neuroblastoma cells is under the specific control of RARalpha and RARgamma. These data seem relevant for the reported ability of RARgamma to suppress the clinically malignant tumor phenotype in patients.

Identification of cytoplasmic actin as an abundant glutaminyl substrate for tissue transglutaminase in HL-60 and U937 cells undergoing apoptosis

A lysine derivative, 3-[Nalpha[Nepsilon-[2', 4'-dinitrophenyl]-amino-n-hexanoyl-L-lysylamido]-propane-1-ol, a novel amine substrate of transglutaminases, was synthesized and delivered into intact HL-60 and U937 human leukemia cells to probe the function of the intracellular enzyme. The novel substrate compound was covalently incorporated into intracellular proteins in these cells expressing high levels of tissue transglutaminase and undergoing apoptosis following the induction of their differentiation with dimethyl sulfoxide and retinoic acid. Immunoaffinity purification and microsequencing of labeled proteins identified cytoplasmic actin as the main endogenous glutaminyl substrate in these cells. As shown by confocal image analysis, cells revealed distinct labeling of the microfilament meshwork structures by the novel compound as the result of the intracellular action of transglutaminase.