Transcription regulatory elements of the first intron control human transglutaminase type I gene expression in epidermal keratinocytes

The EP1 subtype of prostaglandin E2 receptor: role in keratinocyte differentiation and expression in non-melanoma skin cancer

We have previously demonstrated that the EP1 subtype of PGE2 receptor is expressed in the differentiated compartment of normal human epidermis and is coupled to intracellular calcium mobilization. We therefore hypothesized that the EP1 receptor is coupled to keratinocyte differentiation. In in vitro studies, radioligand binding, RT-PCR, immunoblot and receptor agonist-induced second messenger studies demonstrate that the EP1 receptor is up-regulated by high cell density in human keratinocytes and this up-regulation precedes corneocyte formation. Moreover, two different EP1 receptor antagonists, SC51322 and AH6809, both inhibited corneocyte formation. SC51322 also inhibited the induction of differentiation-specific proteins, cytokeratin K10 and epidermal transglutaminase. We next examined the immunolocalization of the EP1 receptor in non-melanoma skin cancer in humans. Well-differentiated SCCs exhibited significantly greater membrane staining, while spindle cell carcinomas and BCCs had significantly decreased membrane staining compared with normal epidermis. This data supports a role for the EP1 receptor in regulating keratinocyte differentiation.

Transglutaminases and neurodegeneration

Transglutaminases (TGs) are Ca2+-dependent enzymes that catalyze a variety of modifications of glutaminyl (Q) residues. In the brain, these modifications include the covalent attachment of a number of amine-bearing compounds, including lysyl (K) residues and polyamines, which serve to either regulate enzyme activity or attach the TG substrates to biological matrices. Aberrant TG activity is thought to contribute to Alzheimer disease, Parkinson disease, Huntington disease, and supranuclear palsy. Strategies designed to interfere with TG activity have some benefit in animal models of Huntington and Parkinson diseases. The following review summarizes the involvement of TGs in neurodegenerative diseases and discusses the possible use of selective inhibitors as therapeutic agents in these diseases.

Analysis of intronic conserved elements indicates that functional complexity might represent a major source of negative selection on non-coding sequences

The non-coding portion of human genome is punctuated by a large number of multispecies conserved sequence (MCS) elements with largely unknown function. We demonstrate that MCSs are unevenly distributed in human introns with the majority of relatively short introns (< 9 kb long) displaying no or a few MCSs and that MCS density reaching up to 10% of total size in longer introns. After correction for intron length, MCSs were found to be enriched within genes involved in development and transcription, whereas depleted in immune response loci. Moreover, many central nervous system tissues show a preferential expression of MCS-rich genes and MCS enrichment significantly correlates with gene functional complexity in terms of distinct protein domains. Analysis of human-mouse orthologous pairs indicated a significant association between intronic MCS density and conservation of protein sequence, promoter regions and untranslated sequences. Moreover, MCS density correlates with the predicted occurrence of human-mouse conserved alternative splicing events. These observations suggest that evolution acts on human genes as integrated units of coding and regulatory capacity and that functional complexity might represent a major source of negative selection on non-coding sequences. To substantiate our result, we also searched previously experimentally identified intronic regulatory elements and indicate that about half of these sequences map to an MCS; in particular, support to the notion whereby mutations in MCSs can result in human genetic diseases is provided, because three previously identified intronic pathological variations were found to occur within MCSs, and human disease and cancer genes were found significantly enriched in MCSs.

A novel transglutaminase activator forms a complex with type 1 transglutaminase

Type I transglutaminase is a plasma membrane-anchored intracellular protein-protein crosslinking enzyme that is responsible for assembly of the keratinocyte cornified envelope during terminal keratinocyte differentiation. We recently described a novel protein, TIG3, that when expressed in keratinocytes causes increased transglutaminase activity and keratinocyte cell death. However, the mechanism of activation of transglutaminase by TIG3 is not known. We now extend our previous study and show that full-length TIG3 forms a complex with type I transglutaminase that is demonstrated by TIG3-transglutaminase co-precipitation. We also demonstrate that treating TIG3-expressing cells with monodansyl cadaverine, a competitive transglutaminase substrate, attenuates the TIG3-dependent response, suggesting that transglutaminase is an important mediator of TIG3 action. These findings suggest that TIG3 forms a complex with transglutaminase resulting in transglutaminase activation and that transglutaminase activity is required for the TIG3-dependent biological response.

Elevated epsilon-(gamma-glutamyl)lysine in human diabetic nephropathy results from increased expression and cellular release of tissue transglutaminase

INTRODUCTION

Diabetic nephropathy (DN) is the leading cause of chronic kidney failure, however the mechanisms underlying the characteristic expansion of the extracellular matrix (ECM) in diabetic kidneys remain controversial and unclear. In non-diabetic kidney scarring the protein crosslinking enzyme tissue transglutaminase (tTg) has been implicated in this process by the formation of increased epsilon-(gamma-glutamyl)lysine bonds between ECM components in both experimental and human disease. Studies in db+/db+ diabetic mice and in streptozotocin-treated rats have suggested a similar mechanism, although the relevance of this to human disease has not been addressed.

METHODS

We have undertaken a retrospective analysis of renal biopsies from 16 DN patients with type 2 diabetes mellitus using an immunohistochemical and immunofluorescence approach, with tTg and epsilon-(gamma-glutamyl)lysine crosslink quantified by confocal microscopy.

RESULTS

Immunofluorescent analysis of human biopsies (confocal microscopy) showed increases in levels of tTg (+1,266%, p < 0.001) and epsilon-(gamma-glutamyl)lysine (+486%, p < 0.001) in kidneys with DN compared to normal. Changes were predominantly in the extracellular periglomerular and peritubular areas. tTg staining correlated with epsilon-(gamma-glutamyl)lysine (r = 0.615, p < 0.01) and renal scarring (Masson's trichrome, r = 0.728, p < 0.001). Significant changes in epsilon-(gamma-glutamyl)lysine were also noted intracellularly in some (< or =5%) tubular epithelial cells. This is consistent with cells undergoing a novel transglutaminase-mediated cell death process in response to Ca2+ influx and subsequent activation of intracellular tTg.

CONCLUSION

Changes in tTg and epsilon-(gamma-glutamyl)lysine occur in human DN. Cellular export of tTg may therefore be a factor in the perpetuation of DN by crosslinking and stabilisation of the ECM, while intracellular activation may lead to cell death contributing towards tubular atrophy.

Transglutaminase activity and transglutaminase mRNA transcripts in gerbil brain ischemia

Brain injury of the ischemia/reperfusion type induces neuronal damage, mainly by excitatory amino acid release, intracellular Ca(2+) overload and reactive oxygen species production. We have previously demonstrated that glutamate exposure increased transglutaminase activity and transglutaminase 2 expression in cultured cerebellar granule cells and astrocytes. The aim of this study is to evaluate changes in transglutaminase activity and expression using a gerbil model of global cerebral ischemia. Moreover, the distribution and amounts of different transglutaminase isoforms were examined. Transglutaminase activity was measured by incorporation of [(3)H]putrescine into dimethylcasein throughout 48 h of reperfusion following a 3 min occlusion. Compared to sham-operated brains, significant increases were found in the ischemic hippocampus at 24 h of reperfusion, while minor changes were observed in the cortex. RT-PCR demonstrated the presence of significant mRNA amounts of transglutaminase 2 and transglutaminase 1, both in the hippocampus and the cerebral cortex, while low levels were found for transglutaminase 3 transcripts. Interestingly, transglutaminase 2 and transglutaminase 1 mRNAs were 4-fold and 2-fold increased, respectively, in the ischemic hippocampus after 24 h of reperfusion. Western blot analysis of transglutaminase 2 expression confirmed a strong up-regulation in the ischemic hippocampus. However, it is possible to hypothesize that different expression rates of transglutaminase isoforms may be dependent on different responsiveness of their transcription regulatory elements to intracellular calcium overload following excitotoxic cell injury. Our results suggest that increases in transglutaminases may be part of the tissue stress response in global brain ischemia.

Transglutaminases: crosslinking enzymes with pleiotropic functions

Blood coagulation, skin-barrier formation, hardening of the fertilization envelope, extracellular-matrix assembly and other important biological processes are dependent on the rapid generation of covalent crosslinks between proteins. These reactions--which are catalysed by transglutaminases--endow the resulting supramolecular structure with extra rigidity and resistance against proteolytic degradation. Some transglutaminases function as molecular switches in cytoskeletal scaffolding and modulate protein-protein interactions. Having knowledge of these enzymes is essential for understanding the aetiologies of diverse hereditary diseases of the blood and skin, and various autoimmune, inflammatory and degenerative conditions.

The human monocarboxylate transporter, MCT1: genomic organization and promoter analysis

Uptake of butyrate across the colonocyte luminal membrane is mediated by the monocarboxylate transporter isoform 1 (MCT1). We have demonstrated previously that expression of human colonic MCT1 is responsive to butyrate, and that this involves the dual control of MCT1 gene transcription and stability of the MCT1 transcript. Here we describe the structural organization of the human MCT1 gene, and report the isolation and characterization of the MCT1 gene promoter. The MCT1 gene spans approximately 44 kb, and is organized as 5 exons intervened by 4 introns. The first of these introns is located in the 5'-UTR-encoding DNA, spans >26 kb, and thus accounts for approximately 60% of the entire transcription unit. Analysis of a 1.5 kb fragment of the MCT1 5'-flanking region, shows an absence of the classical TATA-Box motif. However, the region contains potential binding sites for a variety of transcription factors with known association with butyrate's action in the colon. In transient transfections the 5'-flanking region drives high-level expression of a luciferase reporter-gene in cells that endogenously express MCT1. Deletion analyses indicate that the cis-acting elements necessary for basal transcription of MCT1 are contained within the -70/+213 proximal sequence of the promoter.

Human homeobox HOXA7 regulates keratinocyte transglutaminase type 1 and inhibits differentiation

Keratinocyte proliferation and differentiation result from expression of specific groups of genes regulated by unique combinations of transcription factors. To better understand these regulatory processes, we studied HOXA7 expression and its regulation of differentiation-specific keratinocyte genes. We isolated the homeobox transcription factor HOXA7 from keratinocytes through binding to a differentiation-dependent viral enhancer and analyzed its effect on endogenous differentiation-dependent genes, primarily transglutaminase 1. HOXA7 overexpression repressed transglutaminase 1-reporter activity. HOXA7 message markedly decreased, and transglutaminase RNA increased, upon phorbol ester-induced differentiation, in a protein kinase C-dependent manner. Overexpression of HOXA7 attenuated the transglutaminase 1 induction by phorbol ester, demonstrating that HOXA7 expression is inversely related to keratinocyte differentiation, and to transglutaminase 1 expression. Antisense HOXA7 expression activated transglutaminase 1, involucrin, and keratin 10 message and protein levels, demonstrating that endogenous HOXA7 down-regulates multiple differentiation-specific keratinocyte genes. In keeping with these observations, epidermal growth factor receptor activation stimulated HOXA7 expression. HOX genes function in groups, and we found that HOXA5 and HOXB7 were also down-regulated by phorbol ester. These results provide the first example of protein kinase C-mediated homeobox gene regulation in keratinocytes, and new evidence that HOXA7, potentially in conjunction with HOXA5 and HOXAB7, silences differentiation-specific genes during keratinocyte proliferation, that are then released from inhibition in response to differentiation signals.

Identification, by homozygosity mapping, of a novel locus for autosomal recessive congenital ichthyosis on chromosome 17p, and evidence for further genetic heterogeneity

Autosomal recessive congenital ichthyosis (ARCI) comprises a group of severe disorders of keratinization, characterized by variable erythema and skin scaling. It is known for its high degree of genetic and clinical heterogeneity. Mutations in the gene for keratinocyte transglutaminase (TGM1) on chromosome 14q11 were shown in patients with ARCI, and a second locus was described, on chromosome 2q, in families from northern Africa. Three other loci for ARCI, on chromosomes 3p and 19p, were identified recently. We have embarked on a whole-genome scan for further loci for ARCI in four families from Germany, Turkey, and the United Arab Emirates. A novel ARCI locus was identified on chromosome 17p, between the markers at D17S938 and D17S1856, with a maximum LOD score of 3.38, at maximum recombination fraction 0.00, at D17S945, under heterogeneity. This locus is linked to the disease in the Turkish family and in the German family. Extensive genealogical studies revealed that the parents of the German patients with ARCI were eighth cousins. By homozygosity mapping, the localization of the gene could then be refined to the 8.4-cM interval between D17S938 and D17S1879. It could be shown, however, that ARCI in the two Arab families is linked neither to the new locus on chromosome 17p nor to one of the five loci known previously. Our findings give evidence of further genetic heterogeneity that is not linked to distinctive phenotypes.