Differential expression of human cornifin alpha and beta in squamous differentiating epithelial tissues and several skin lesions

Thermostable Proteins from HaCaT Keratinocytes Identify a Wide Breadth of Intrinsically Disordered Proteins and Candidates for Liquid-Liquid Phase Separation

Intrinsically disordered proteins (IDPs) move through an ensemble of conformations which allows multitudinous roles within a cell. Keratinocytes, the predominant cell type in mammalian epidermis, have had only a few individual proteins assessed for intrinsic disorder and its possible contribution to liquid-liquid phase separation (LLPS), especially in regard to what functions or structures these proteins provide. We took a holistic approach to keratinocyte IDPs starting with enrichment via the isolation of thermostable proteins. The keratinocyte protein involucrin, known for its resistance to heat denaturation, served as a marker. It and other thermostable proteins were identified by liquid chromatography tandem mass spectrometry and subjected to extensive bioinformatic analysis covering gene ontology, intrinsic disorder, and potential for LLPS. Numerous proteins unique to keratinocytes and other proteins with shared expression in multiple cell types were identified to have IDP traits (e.g., compositional bias, nucleic acid binding, and repeat motifs). Among keratinocyte-specific proteins, many that co-assemble with involucrin into the cell-specific structure known as the cornified envelope scored highly for intrinsic disorder and potential for LLPS. This suggests intrinsic disorder and LLPS are previously unrecognized traits for assembly of the cornified envelope, echoing the contribution of intrinsic disorder and LLPS to more widely encountered features such as stress granules and PML bodies.

Increased expression of SPRR1A is associated with a poor prognosis in pancreatic ductal adenocarcinoma

OBJECTIVES

Small proline-rich protein 1A (SPRR1A) is recognized as a squamous differentiation marker but is also upregulated in some non-squamous cancers. However, its expression in pancreatic ductal adenocarcinoma (PDAC) has not been investigated. This study elucidated the expression of SPRR1A in PDAC and its effect on the prognosis and malignant behavior of PDAC.

METHODS

We examined the SPRR1A expression by immunohistochemistry in 86 surgical PDAC cases and revealed the relationship between its expression and the prognosis of the PDAC patients. Furthermore, we overexpressed SPRR1A in pancreatic cancer cell lines (PK-1 and Panc-1) and assessed the phenotype and gene expression changes in vitro.

RESULTS

Among the 84 cases, excluding 2 with squamous differentiation, 31 (36.9%) had a high SPRR1A expression. The overall survival (median 22.1 months vs. 33.6 months, p = 0.0357) and recurrence-free survival (median 10.7 months vs. 15.5 months, p = 0.0298) were significantly lower in the high-SPRR1A-expression group than in the low-SPRR1A-expression group. A multivariate analysis indicated that a high SPRR1A expression (HR 1.706, 95% CI 1.018 to 2.862, p = 0.0427) and residual tumor status (HR 2.687, 95% CI 1.487 to 4.855, p = 0.00106) were independent prognostic factors. The analysis of TCGA transcriptome data demonstrated that the high-SPRR1A-expression group had a significantly worse prognosis than the low-SPRR1A-expression group, which supported our data. SPRR1A overexpression in PK-1 and Panc-1 did not result in remarkable changes to in vitro phenotypes, such as the cell proliferation, chemo-resistance, EMT, migration or global gene expression.

CONCLUSION

Increased expression of SPRR1A is associated with a poor prognosis in PDAC and may serve as a novel prognostic marker. However, our in vitro study suggests that the SPRR1A expression may be a consequence, not a cause, of the aggressive behavior of PDAC.

Searching for New Molecular Targets for Oral Squamous Cell Carcinoma with a View to Clinical Implementation of Precision Medicine

Head and neck cancer, including oral squamous cell carcinoma (OSCC), is the eighth most common malignancy globally and is characterized by local invasiveness and high nodal metastatic potential. The OSCC incidence is also increasing, and the number of deaths is also rising steadily in Japan. The development of molecular markers to eradicate OSCC is an urgent issue for humankind. The increase in OSCC despite the declining smoking rate may be due to several viral infections through various sexual activities and the involvement of previously unfocused carcinogens, and genetic alterations in individual patients are considered to be more complicated. Given this situation, it is difficult to combat OSCC with conventional radiotherapy and chemotherapy using cell-killing anticancer drugs alone, and the development of precision medicine, which aims to provide tailor-made medicine based on the genetic background of each patient, is gaining attention. In this review article, the current status of the comprehensive search for driver genes and biomarkers in OSCC will be briefly described, and some of the candidates for novel markers of OSCC that were found will be outlined.

Identification of oral squamous cell carcinoma markers MUC2 and SPRR1B downstream of TANGO

PURPOSE

Transport and Golgi organization protein 1 (TANGO) promotes angiogenesis and lymphangiogenesis in oral squamous cell carcinoma (OSCC). To elucidate the underlying mechanisms, this study aims to identify and characterize elements downstream of TANGO that mediate its involvement in OSCC.

METHODS

In this study, microarray analysis compared gene expression between control and TANGO-repressed HSC3 cells. Protein expression in 213 OSCC tissue samples was analyzed immunohistochemically.

RESULTS

TANGO repression decreased or increased expression of Mucin 20 (MUC20) and small proline-rich protein 1B (SPRR1B), respectively. MUC20 increased the growth and invasiveness of OSCC cells via altered matrix metalloproteinase (MMP)-2 and E-cadherin expression and c-met phosphorylation. MUC20 induced angiogenesis and lymphangiogenesis by activating vascular endothelial growth factors A and C. In well-differentiated OSCC, SPRR1B expression was high (P = 0.0091) and correlated with keratinization markers and promoted proliferation by inducing mitogen-activated protein kinase p38 phosphorylation. MUC20 expression correlated significantly with clinical stage (P = 0.0024), lymph node metastasis (P = 0.0036), and number of blood and lymph vessels (P < 0.0001). MUC20-expressing cases had a significantly worse prognosis than non-expressing cases (P < 0.0001).

CONCLUSION

MUC20 and SPRR1B located downstream of TANGO may be useful molecular markers for OSCC.

Contribution of transglutaminases and their substrate proteins to the formation of cornified cell envelope in oral mucosal epithelium

Cornified envelope formation is crucial for the final differentiation of keratinized epithelium. However, the mechanisms of cornified envelope formation in the oral epithelium remain unclear. The aim of this study was to clarify the differences in the distribution and expression of cornified envelope related proteins and genes between keratinized and non-keratinized oral epithelia. We immunohistochemically investigated the distribution patterns of transglutaminase 1 (TG1), transglutaminase 3 (TG3), and their substrate proteins involucrin (IVL), loricrin (LOR), and small proline rich proteins (SPRs), in 19 keratinized and 14 non-keratinized oral epithelium samples. TG1 and TG3 mRNA levels were investigated in both types of epithelium by real time reverse transcription polymerase chain reaction (RT-PCR) using paraffin-embedded specimens. Data were analyzed to identify factors involved in cornified envelope formation. We demonstrate that 11 localization patterns show statistically significant differences between keratinized and non-keratinized oral epithelia. These factors clearly drove the separation of the two groups during cluster analysis. TG1 mRNA levels in keratinized oral epithelium were significantly higher than those in non-keratinized oral epithelium. In conclusion, the characteristic distribution of transglutaminases and their substrates and the mRNA levels of TG1 can regulate cornified envelope formation in keratinized oral epithelium, together with the contribution of TG3 first reported in this paper.

High SPRR1A expression is associated with poor survival in patients with colon cancer

High expression of small proline-rich protein 1A (SPRR1A) has been shown to be associated with tumor prognosis; however, the association between SPRR1A expression and colon cancer prognosis remains unclear. The present study sought to evaluate the association between SPRR1A expression and the clinicopathological characteristics of colon cancer, and to examine its potential prognostic value. A total of 114 patients with colon cancer were included. SPRR1A expression was evaluated by immunohistochemical staining, and the association between SPRR1A expression and clinicopathological parameters was analyzed. The prognostic value of SPRR1A was analyzed by Cox regression analysis, the Oncomine database and the R2 platform. SPRR1A expression was significantly increased in cancerous tissues compared with that in adjacent non-cancerous tissues. SPPRR1A expression was significantly associated with lymph node invasion. High SPRR1A expression was significantly associated with worse overall and disease-free survival rate. Cox regression analysis revealed that T stage, pathological N stage and high SPRR1A expression remained independent predictors for overall survival rate. The Oncomine database analysis demonstrated that SPRR1A mRNA expression levels were significantly increased in colorectal cancer tissues compared with those in adjacent non-cancerous tissues, and high SPRR1A expression was associated with a significantly worse event- and relapse-free survival time in the R2 platform. The data indicate that SPRR1A may serve as a potential biomarker for the prognosis of colon cancer.

The combined use of EFS, GPX2, and SPRR1A expression could distinguish favorable from poor clinical outcome among epithelial-like head and neck carcinoma subtypes

BACKGROUND

We aimed at identifying molecular markers predictive of clinical outcome in patients with head and neck cancer based on the expression profile of cells showing epithelial-like (EL) or mesenchymal-like (ML) phenotypes.

MATERIALS AND METHODS

We analyzed the association between EL and ML cells and migration, drug resistance, or tumor growth. The differential gene expression profile between cell types was used to build a model to stratify patients according to survival.

RESULTS

EL cells were sensitive to cisplatin and cetuximab, showed low migration, and generated squamous differentiated tumors in mouse. A differential 93-gene expression signature between ML and EL cells was used to build a three-gene (EFS, GPX2, and SPRR1A) survival model by analyzing the RNA-seq data of the TCGA-HNSC project. Its prognostic value was confirmed in two independent cohorts.

CONCLUSION

EFS, GPX2, and SPRR1A are prognostic markers able to distinguish clinical outcome among subtypes sharing an EL phenotype.

Molecular Changes in the Nasal Cavity after N, N-dimethyl-p-toluidine Exposure

N, N-dimethyl-p-toluidine (DMPT; Cas No. 99-97-8), an accelerant for methyl methacrylate monomers in medical devices, is a nasal cavity carcinogen according to a 2-yr cancer study of male and female F344/N rats, with the nasal tumors arising from the transitional cell epithelium. In this study, we exposed male F344/N rats for 5 days to DMPT (0, 1, 6, 20, 60, or 120 mg/kg [oral gavage]) to explore the early changes in the nasal cavity after short-term exposure. Lesions occurred in the nasal cavity including hyperplasia of transitional cell epithelium (60 and 120 mg/kg). Nasal tissue was rapidly removed and preserved for subsequent laser capture microdissection and isolation of the transitional cell epithelium (0 and 120 mg/kg) for transcriptomic studies. DMPT transitional cell epithelium gene transcript patterns were characteristic of an antioxidative damage response (e.g., Akr7a3, Maff, and Mgst3), cell proliferation, and decrease in signals for apoptosis. The transcripts of amino acid transporters were upregulated (e.g., Slc7a11). The DMPT nasal transcript expression pattern was similar to that found in the rat nasal cavity after formaldehyde exposure, with over 1,000 transcripts in common. Molecular changes in the nasal cavity after DMPT exposure suggest that oxidative damage is a mechanism of the DMPT toxic and/or carcinogenic effects.

Clinical significance of SPRR1A expression in progesterone receptor-positive breast cancer

Small proline-rich repeat protein 1A (SPRR1A) is a marker for terminal squamous cell differentiation. Previous studies showed that SPRR1A expression increases in squamous cell carcinoma of the skin, but decreases in esophageal squamous cell carcinoma. This study focuses on the expression of SPRR1A protein in breast cancers (BCs) in China. A total of 111 patients with histologically confirmed BC, who underwent radical surgery between January 2006 and September 2007 in China Medical University, were enrolled. The relationship between SPRR1A expression and clinicopathological factors as well as BC prognoses was also determined. Overall, SPRR1A expression was detected in more than half of the BC specimens by immunohistochemistry (56/111, 53.8%), but there was no significant difference between age groups (≥50 vs. <50 years) in terms of SPRR1A expression (P = 0.915), as well as no differences between SPRR1A expression and the clinical stage (0-I vs. II-III) or nodal status (P = 0.234 and 0.632, respectively). Moreover, human epidermal growth factor receptor 2 overexpression was not correlated with SPRR1A expression, whereas Ki67 was associated with SPRR1A expression (P = 0.155 and 0.028, respectively). Interestingly, SPRR1A expression was significantly associated with progesterone receptor-positive (P = 0.010) rather than estrogen receptor-positive (0.778) BCs. The 5-year survival rate in patients did not differ with the presence or absence of SPRR1A expression (P = 0.753), whereas the combination of SPRR1A expression, progesterone receptor status, and menopausal status allowed identification of a subgroup of BC patients with a good long-term prognosis. Thus, the SPRR1A status might play an important role in the prognosis of postmenopausal breast carcinoma patients, especially that of progesterone receptor-positive subgroups.

Comparison of molecular signatures from multiple skin diseases identifies mechanisms of immunopathogenesis

The ability to obtain gene expression profiles from human disease specimens provides an opportunity to identify relevant gene pathways, but is limited by the absence of data sets spanning a broad range of conditions. Here, we analyzed publicly available microarray data from 16 diverse skin conditions in order to gain insight into disease pathogenesis. Unsupervised hierarchical clustering separated samples by disease and common cellular and molecular pathways. Disease specific signatures were leveraged to build a multi-disease classifier which predicted the diagnosis of publicly and prospectively collected expression profiles with 93% accuracy. In one sample, the molecular classifier differed from the initial clinical diagnosis and correctly predicted the eventual diagnosis as the clinical presentation evolved. Finally, integration of interferon (IFN) regulated gene programs with the skin database revealed a significant inverse correlation between IFN–β and IFN–γ programs across all conditions. Our study provides an integrative approach to the study of gene signatures from multiple skin conditions, elucidating mechanisms of disease pathogenesis. Additionally, these studies provide a framework for developing tools for personalized medicine towards the precise prediction, prevention, and treatment of disease on an individual level.

Unique Keratinization Process in Psoriasis: Late Differentiation Markers Are Abolished Because of the Premature Cell Death

The keratinization process in psoriasis is a unique phenomenon. We have proposed an organized system for keratinization in psoriasis based on the recognition of early and late differentiation markers combined with premature cell death. The early differentiation markers, such as involucrin, small proline-rich proteins (SPRR), cystatin A and transglutaminase l, are more conspicuously expressed in psoriasis, while the late differentiation markers, such as profilaggrin and loricrin, are abolished. Keratinization markers that are not observed in the normal epidermis are also detected; these include SKALP/elafin as well as K6 and K16. With a markedly diminished turnover time, the psoriatic epidermis rapidly synthesizes differentiation markers that are mostly under the control of the protein kinase C-AP1 transcriptional control system. Because of the premature cell death, however, the late differentiation markers are not expressed. During the improvement of the lesion and the therefore longer turnover time, the late differentiation markers rapidly catch up to reveal their expression. This explains the rapid appearance of keratohyalin granules (profilaggrin) in the healing lesion of psoriasis. Thus the keratinization process in psoriasis can be explained by the accelerated keratinization combined with premature cell death. The keratinization process in psoriasis is unique, because both accelerated keratinization and premature cell death co-exist, resulting in the disappearance of late differentiation markers such as profilaggrin and loricrin. It is interesting to note that the premature cell death is also under the control of protein kinase C signaling.

Comparative shotgun proteomics using spectral count data and quasi-likelihood modeling

Shotgun proteomics provides the most powerful analytical platform for global inventory of complex proteomes using liquid chromatography−tandem mass spectrometry (LC−MS/MS) and allows a global analysis of protein changes. Nevertheless, sampling of complex proteomes by current shotgun proteomics platforms is incomplete, and this contributes to variability in assessment of peptide and protein inventories by spectral counting approaches. Thus, shotgun proteomics data pose challenges in comparing proteomes from different biological states. We developed an analysis strategy using quasi-likelihood Generalized Linear Modeling (GLM), included in a graphical interface software package (QuasiTel) that reads standard output from protein assemblies created by IDPicker, an HTML-based user interface to query shotgun proteomic data sets. This approach was compared to four other statistical analysis strategies: Student t test, Wilcoxon rank test, Fisher’s Exact test, and Poisson-based GLM. We analyzed the performance of these tests to identify differences in protein levels based on spectral counts in a shotgun data set in which equimolar amounts of 48 human proteins were spiked at different levels into whole yeast lysates. Both GLM approaches and the Fisher Exact test performed adequately, each with their unique limitations. We subsequently compared the proteomes of normal tonsil epithelium and HNSCC using this approach and identified 86 proteins with differential spectral counts between normal tonsil epithelium and HNSCC. We selected 18 proteins from this comparison for verification of protein levels between the individual normal and tumor tissues using liquid chromatography−multiple reaction monitoring mass spectrometry (LC−MRM-MS). This analysis confirmed the magnitude and direction of the protein expression differences in all 6 proteins for which reliable data could be obtained. Our analysis demonstrates that shotgun proteomic data sets from different tissue phenotypes are sufficiently rich in quantitative information and that statistically significant differences in proteins spectral counts reflect the underlying biology of the samples.

Shotgun proteomics provides the most powerful analytical platform for global inventory of complex proteomes but incomplete sampling poses challenges in comparing protein inventories by spectral counting approaches. We developed a statistical method based on quasi-likelihood modeling and demonstrate that it compares favorably to other statistical tests. Statistically significant spectral count differences were confirmed by MRM demonstrating that the observed protein level differences reflect the underlying biology of the samples.

The surprising composition of the salivary proteome of preterm human newborn

Saliva is a body fluid of a unique composition devoted to protect the mouth cavity and the digestive tract. Our high performance liquid chromatography (HPLC)-electrospray ionization-MS analysis of the acidic soluble fraction of saliva from preterm human newborn surprisingly revealed more than 40 protein masses often undetected in adult saliva. We were able to identify the following proteins: stefin A and stefin B, S100A7 (two isoforms), S100A8, S100A9 (four isoforms), S100A11, S100A12, small proline-rich protein 3 (two isoforms), lysozyme C, thymosins β(4) and β(10), antileukoproteinase, histone H1c, and α and γ globins. The average mass value reported in international data banks was often incongruent with our experimental results mostly because of post-translational modifications of the proteins, e.g. acetylation of the N-terminal residue. A quantitative label-free MS analysis showed protein levels altered in relation to the postconceptional age and suggested coordinate and hierarchical functions for these proteins during development. In summary, this study shows for the first time that analysis of these proteins in saliva of preterm newborns might represent a noninvasive way to obtain precious information of the molecular mechanisms of development of human fetal oral structures.

Regulatory role for Krüppel-like zinc-finger protein Gli-similar 1 (Glis1) in PMA-treated and psoriatic epidermis

In this study, we analyze the expression and potential function of the Krüppel-like zinc-finger protein Gli-similar protein 1 (Glis1) in normal and inflammatory skin and in the differentiation of epidermal keratinocytes. Glis1 mRNA is not expressed in normal human epidermis, but is significantly induced in psoriatic epidermis and in mouse skin upon treatment with the tumor promoter phorbol-12-myristate-13-acetate (PMA). The expression of Glis1 is restricted to the suprabasal layers. These observations suggest that Glis1 expression is associated with hyperplastic, inflammatory epidermis. Consistent with these findings, Glis1 mRNA is not expressed in undifferentiated or differentiated normal human epidermal keratinocytes (NHEK) in culture, but is dramatically induced after the addition of PMA or interferon gamma. A similar induction of Glis1 mRNA by PMA treatment was observed in the immortalized epidermal keratinocyte cell line NHEK-HPV, whereas PMA did not induce Glis1 in HaCaT cells or in several squamous cell carcinoma cell lines. To obtain insight into its function, Glis1 and a C-terminal deletion mutant Glis1DeltaC were expressed in NHEK-HPV cells and changes in epidermal differentiation and gene expression examined. Microarray analysis revealed that Glis1DeltaC promoted PMA-induced epidermal differentiation, as indicated by increased expression of many differentiation-specific genes. This, in association with its induction in psoriasis, suggests that transcriptional factor Glis1 is involved in the regulation of aberrant differentiation observed in psoriatic epidermis.

Microarray analysis of gene expression in cultured skin substitutes compared with native human skin

Cultured skin substitutes (CSS), prepared using keratinocytes, fibroblasts, and biopolymers, can facilitate closure of massive burn wounds by increasing the availability of autologous tissue for grafting. But because they contain only two cell types, skin substitutes cannot replace all of the functions of native human skin. To better understand the physiological and molecular differences between CSS and native skin, we undertook a comprehensive analysis of gene expression in native skin, cultured keratinocytes, cultured fibroblasts, and skin substitutes using Affymetrix gene chip microarrays. Hierarchical tree clustering identified six major clusters of coordinately regulated genes, using a list of 1030 genes that were the most differentially expressed between groups. These clusters correspond to biomarker pools representing expression signatures for native skin, fibroblasts, keratinocytes, and cultured skin. The expression analysis revealed that entire clusters of genes were either up- or downregulated upon combination of fibroblasts and keratinocytes in cultured skin grafts. Further, several categories of genes were overexpressed in CSS compared with native skin, including genes associated with hyperproliferative skin or activated keratinocytes. The observed pattern of expression indicates that CSS in vitro, which display a well-differentiated epidermal layer, exhibit a hyperproliferative phenotype similar to wounded native skin.

Different consequences of beta1 integrin deletion in neonatal and adult mouse epidermis reveal a context-dependent role of integrins in regulating proliferation, differentiation, and intercellular communication

There are conflicting reports of the consequences of deleting beta1 integrins from the epidermis of transgenic mice. Epidermal thinning with normal differentiation and lack of inflammation has been observed; conversely, epidermal thickening, abnormal differentiation, and dermal fibrosis can occur. beta1 integrin deletion results in decreased epidermal proliferation, yet on wounding the proliferative defect is overcome. To distinguish primary from secondary consequences of beta1 integrin loss, we compared epidermal beta1 deletion at E14.5 via K5Cre and 4-hydroxy-tamoxifen induced deletion in adulthood via K14CreER. As reported previously, there was dermo-epidermal splitting, inflammation, reduced proliferation, and hair follicle and sebaceous gland loss in 30-d-old K5Cre beta1-null mice. These changes were not observed 30 d after beta1 integrin deletion in adult epidermis, however, and there were no changes in the hair follicle stem cell compartment. Deletion in adult epidermis revealed a previously unreported correlation between the level of beta1 integrins and proliferation in the interfollicular epidermis that was remarkably consistent with human epidermis. In addition, the number of melanocytes in interfollicular epidermis was greatly increased. Our results highlight the context-dependent effects of beta1 integrin deletion and suggest that inflammation may be responsible for some of the K5Cre beta1-null phenotype.

Enhanced susceptibility of staggerer (RORαsg/sg) mice to lipopolysaccharide-induced lung inflammation

The retinoid-related orphan receptor α (RORα), a member of the ROR subfamily of nuclear receptors, has been implicated in the control of a number of physiological processes, including the regulation of several immune functions. To study the potential role of RORα in the regulation of innate immune responses in vivo, we analyzed the induction of airway inflammation in response to lipopolysaccharide (LPS) challenge in wild-type and staggerer (RORαsg/sg) mice, a natural mutant strain lacking RORα expression. Examination of hematoxylin and eosin-stained lung sections showed that RORαsg/sg mice displayed a higher degree of LPS-induced inflammation than wild-type mice. Bronchoalveolar lavage (BAL) was performed at 3, 16, and 24 h after LPS exposure to monitor the increase in inflammatory cells and the level of several cytokines/chemokines. The increased susceptibility of RORαsg/sg mice to LPS-induced airway inflammation correlated with a higher number of total cells and neutrophils in BAL fluids from LPS-treated RORαsg/sg mice compared with those from LPS-treated wild-type mice. In addition, IL-1β, IL-6, and macrophage inflammatory protein-2 were appreciably more elevated in BAL fluids from LPS-treated RORαsg/sg mice compared with those from LPS-treated wild-type mice. The enhanced susceptibility of RORαsg/sg mice appeared not to be due to a repression of IκBα expression. Our observations indicate that RORαsg/sg mice are more susceptible to LPS-induced airway inflammation and are in agreement with the hypothesis that RORα functions as a negative regulator of LPS-induced inflammatory responses.

Gene expression profiles in psoriasis: analysis of impact of body site location and clinical severity

BACKGROUND

Psoriasis is characterized by symmetry of plaques and modulation of multiple genes within those plaques.

OBJECTIVES

We compared gene expression profiles of plaques of psoriasis at different anatomical sites for both symmetrical and asymmetrical disease to ascertain whether the same genes were expressed.

METHODS

Gene expression profiles were analysed in biopsies from lesional and uninvolved skin from two groups of patients with either predominantly symmetrical or truncal plaques of psoriasis vulgaris, and from normal skin of healthy volunteers. Genomic analyses were performed using cDNA array and kinetically monitored reverse transcriptase-initiated polymerase chain reaction (kRT-PCR) approaches. A cluster of genes upregulated in involved psoriasis skin as compared with normal skin was identified using each of these two technologies.

RESULTS

Clustering of patients based on their gene expression profile did not reveal any correlation with family history of psoriasis, age at onset or association of psoriasis with arthritis. There was no difference in gene expression profile between the type (symmetrical vs. truncal) or location (left vs. right side of body) of psoriatic plaques. Gene expression profiles of involved psoriatic skin analysed by kRT-PCR analysis did correlate with both global (Psoriasis Area and Severity Index) and local (erythema, desquamation and plaque elevation) clinical severity.

CONCLUSIONS

These results indicate that it may be feasible to analyse the molecular effects of pharmacological agents on psoriatic skin in 'minizone' protocols, that the obtained data can be correlated with clinical severity and that plaques of psoriasis in the same individual express the same genes.

Expression of activated MEK1 in differentiating epidermal cells is sufficient to generate hyperproliferative and inflammatory skin lesions

Epidermal activation of Erk MAPK is observed in human psoriatic lesions and in a mouse model of psoriasis in which beta1 integrins are expressed in the suprabasal epidermal layers. Constitutive activation of the upstream kinase MEK1 causes hyperproliferation and perturbed differentiation of human keratinocytes in culture. It is not known, however, whether Erk activation in differentiating keratinocytes is sufficient to trigger hyperproliferation of basal keratinocytes and a skin inflammatory infiltrate. To investigate this, we expressed constitutively active MEK1 in the suprabasal epidermal layers of transgenic mice. Proliferation in the epidermal basal layer was stimulated and epidermal terminal differentiation was perturbed. Some older mice also developed papillomas. There was a large increase in T lymphocytes, dendritic cells, and neutrophils in the skin. The effects of suprabasal MEK1 on basal keratinocytes and leukocytes, cells that were transgene negative, suggested that MEK1 activity might stimulate cytokine release. Transgenic keratinocytes expressed elevated IL-1alpha and crossing the mice with mice overexpressing the IL-1 receptor in the epidermal basal layer led to exacerbated hyperproliferation and inflammation. These data suggest that activation of MEK1 downstream of beta1 integrins plays an important role in epidermal hyperproliferation and skin inflammation.

Role of melanoma chondroitin sulphate proteoglycan in patterning stem cells in human interfollicular epidermis

Human interfollicular epidermis is renewed by stem cells that are clustered in the basal layer in a patterned, non-random distribution. Stem cells can be distinguished from other keratinocytes by high expression of β1 integrins and lack of expression of terminal differentiation markers; they divide infrequently in vivo but form actively growing colonies in culture. In a search for additional stem cell markers, we observed heterogeneous epidermal expression of melanoma chondroitin sulphate proteoglycan (MCSP). MCSP was expressed by those keratinocytes with the highest β1 integrin levels. In interfollicular epidermis, expression was confined to non-cycling cells and,in culture, to self-renewing clones. However, fluorescence-activated cell sorting on the basis of MCSP and β1 integrin expression gave no more enrichment for clonogenic keratinocytes than sorting for β1 integrins alone. To interfere with endogenous MCSP, we retrovirally infected keratinocytes with a chimera of the CD8 extracellular domain and the MCSP cytoplasmic domain. CD8/MCSP did not affect keratinocyte proliferation or differentiation but the cohesiveness of keratinocytes in isolated clones or reconstituted epidermal sheets was greatly reduced. CD8/MCSP caused stem cell progeny to scatter without differentiating. CD8/MCSP did not alter keratinocyte motility but disturbed cadherin-mediated cell-cell adhesion and the cortical actin cytoskeleton, effects that could be mimicked by inhibiting Rho. We conclude that MCSP is a novel marker for epidermal stem cells that contributes to their patterned distribution by promoting stem cell clustering.

Evidence that Myc activation depletes the epidermal stem cell compartment by modulating adhesive interactions with the local microenvironment

Activation of Myc (c-Myc) causes epidermal cells to exit the stem cell compartment and differentiate into sebocytes and interfollicular epidermis at the expense of the hair lineages. To investigate how Myc exerts these effects we analysed the transcription of more than 10000 genes following Myc activation in the basal layer of mouse epidermis for 1 or 4 days. The major classes of induced genes were involved in synthesis and processing of RNA and proteins, in cell proliferation and in differentiation. More than 40% of the downregulated genes encoded cell adhesion and cytoskeleton proteins. Repression of these genes resulted in profound changes in the adhesive and motile behaviour of keratinocytes. Myc activation inhibited cell motility and wound healing, correlating with decreased expression of a large number of extracellular matrix proteins. Cell adhesion and spreading were also impaired, and this correlated with decreased expression of the alpha6beta4 integrin, decreased formation of hemidesmosomes and decreased assembly of the actomyosin cytoskeleton. We propose that Myc stimulates exit from the stem cell compartment by reducing adhesive interactions with the local microenvironment or niche, and that the failure of hair differentiation reflects an inability of keratinocytes to migrate along the outer root sheath to receive hair inductive stimuli.

Regulation of interleukin-1alpha expression by integrins and epidermal growth factor receptor in keratinocytes from a mouse model of inflammatory skin disease

Transgenic mice expressing beta1 integrins in the suprabasal epidermal layers have sporadic skin hyperproliferation and inflammation correlated with activation of extracellular signal-regulated kinase (Erk) mitogen-activated protein kinase and increased interleukin (IL)-1alpha production. We investigated the link between aberrant integrin expression, Erk activation, and expression of IL-1alpha. Transgenic keratinocytes had higher basal Erk activity and IL-1alpha levels than nontransgenic controls and were more sensitive to stimulation of Erk activity and IL-1alpha production by IL-1alpha, 12-O-tetradecanoylphorbol-13-acetate (TPA), epidermal growth factor (EGF), and serum. Inhibition of Erk in transgenic keratinocytes reduced basal IL-1alpha levels and the stimulation of IL-1alpha production by serum or phorbol ester, demonstrating that Erk could regulate IL-1alpha expression. TPA or IL-1alpha treatment resulted in rapid down-regulation of the EGF receptor in transgenic cells, indicative of transactivation. Inhibition of transactivation blocked basal and TPA or IL-1alpha induced Erk activation, but not IkappaBalpha degradation, and abolished increased IL-1alpha production in transgenic cells. In transgene-negative cells, constitutive activation of IL-1-dependent signaling by wild type or kinase-dead IRAK1 stimulated IL-1alpha production independent of Erk. We conclude that suprabasal integrin expression leads to Erk activation and increased IL-1alpha expression by potentiating activation of the EGF receptor. These results provide a mechanism by which aberrant integrin expression triggers epidermal hyperproliferation and inflammation.

A tumor-associated beta 1 integrin mutation that abrogates epithelial differentiation control

SCC4 human keratinocytes are derived from a squamous cell carcinoma of the tongue and undergo very little spontaneous differentiation. Introduction of a wild-type beta 1 integrin subunit into SCC4 cells stimulates differentiation, suggesting either that the cells have a defect in the integrin signaling pathways that control differentiation or that the beta1 subunit itself is defective. Here we describe a heterozygous mutation in the SCC4 beta 1 subunit. The mutation, T188I, maps to the I-like domain. It results in constitutive activation of ligand binding, irrespective of the partner alpha subunit, in solid phase assays with recombinant protein and in living cells. The mutation promotes cell spreading, but not proliferation, motility, or invasiveness. It results in sustained activation of Erk MAPK independent of cell spreading. When introduced into SCC4 keratinocytes, the wild-type beta1 integrin stimulates differentiation, whereas the mutant is inactive. Activation of beta 1 integrins in normal keratinocytes also suppresses differentiation. These results establish, for the first time, mutation as a mechanism by which integrins can contribute to neoplasia, because the degree of differentiation in epithelial cancers is inversely correlated with prognosis. They also provide new insights into how integrins regulate keratinocyte differentiation.

Origin of the epidermal calcium gradient: regulation by barrier status and role of active vs passive mechanisms

Mammalian epidermis displays a characteristic calcium gradient, with low calcium levels in the lower, basal, and spinous epidermal layers, whereas calcium levels increase progressively towards the outer stratum granulosum, and declining again in the stratum corneum. As the calcium gradient disappears after acute permeability barrier disruption, and returns after 6 h in parallel with barrier recovery, barrier function (through restriction of transcutaneous water movement) could regulate the formation of the epidermal calcium gradient. Two types of experiments confirmed the role of barrier status in regulating the calcium gradient: (i) either a vapor-permeable membrane (Gore-Tex) or an emollient (Vaseline), applied after acute barrier disruption, immediately restored barrier function, while accelerating the return of the calcium gradient, and (ii) in contrast, applications of lovastatin, a cholesterol synthesis inhibitor, which delayed barrier recovery and retarded the return of the calcium gradient. We next asked whether the calcium gradient is formed/maintained by passive and/or active mechanisms. Previous studies have demonstrated that cold exposure (4 degrees C) blocks permeability barrier recovery after acute disruption. Here, we abrogated the barrier with tape-stripping, and then compared barrier recovery and restoration of the calcium gradient in hairless mice exposed to 4 degrees C external temperatures, with and without occlusion with Gore-Tex. Although low levels of returned calcium throughout the epidermis, acutely disrupted, unoccluded, cold-exposed sites showed neither barrier recovery nor reappearance of the calcium gradient at 5 h. In contrast, acutely disrupted, cold-exposed sites, covered with Gore-Tex, likewise displayed little barrier recovery, but the calcium gradient largely returned by 3 h. These results show that (i) barrier status regulates formation of the calcium gradient, and (ii) passive processes alone can account for the formation/maintenance of the calcium gradient.

Differential expression profiling of head and neck squamous carcinoma: significance in their phenotypic and biological classification

The genetic events associated with the development and progression of head and neck squamous carcinoma (HNSC) are largely unknown. We analysed 12 matched pairs of histologically normal squamous mucosa and tumor specimens from six conventional and six phenotypic variants HNSC to define the differentially expressed genes in these tumors. Parallel expression analysis of 8055 unique genes was performed, and the level of the hybridization signal for each gene was measured after normalization. Hierarchical cluster analysis of the expressed genes showed distinct inter- and intra-tumoral patterns in and between conventional squamous carcinoma and squamous carcinoma variants. We also identified 26 (0.32%) differentially expressed genes that were consistently different between matched pairs of normal and tumor specimens; a selected set of the overexpressed genes was validated using real-time quantitative RT-PCR. The majority of the genes were associated with differentiation and proliferation. Our study defines a set of genes that could form the basis for the construction of limited HNSC targeted expression array and in-depth studies and further highlights gene profile differences that may be useful in pathobiologic classification of HNSC.

Modulations in epidermal calcium regulate the expression of differentiation-specific markers

Mammalian epidermis normally displays a distinctive calcium gradient, with low levels in the basal/spinous layers and high levels in the stratum granulosum. Although changes in stratum granulosum calcium regulate the lamellar body secretory response to permeability barrier alterations, whether modulations in calcium also regulate the expression of differentiation-specific proteins in vivo remains unknown. As acute barrier perturbations reduce calcium levels in stratum granulosum, we studied the regulation of murine epidermal differentiation after loss of calcium accompanying acute barrier disruption and by exposure of such acutely perturbed skin sites to either low (0.03 M) or high (1.8 M) calcium. Three hours after acute barrier disruption, coincident with reduced calcium and ultrastructural evidence of accelerated lamellar body secretion, both northern analyses and in situ hybridization revealed decreased mRNA levels for loricrin, profilaggrin, and involucrin in the outer epidermis, but protein levels did not change significantly. Moreover, exposure of acutely disrupted skin sites to low calcium solutions sustained the reduction in mRNA levels, whereas exposure to high calcium solutions restored normal mRNA levels (blocked by the L-type calcium channel inhibitor, nifedipine). Finally, with prolonged exposure to a low (<10% relative humidity) or high (>80% relative humidity) humidity, calcium levels increased and declined, respectively. Accordingly, mRNA and protein levels of the differentiation-specific markers increased and decreased at low and high relative humidity, respectively. These results provide direct evidence that acute and sustained fluctuations in epidermal calcium regulate expression of differentiation-specific proteins in vivo, and demonstrate that modulations in epidermal calcium coordinately regulate events late in epidermal differentiation that together form the barrier.

Activation of the esophagin promoter during esophageal epithelial cell differentiation

Esophagin is a member of the small proline-rich protein family of cell envelope precursor proteins, which are expressed during squamous cell differentiation. Esophagin is expressed at high levels in normal esophageal epithelium, but its expression is absent from esophageal squamous cell carcinomas and adenocarcinomas. Moreover, loss of esophagin expression is present in areas of dysplasia or normal mucosa adjacent to carcinomas, suggesting that absence of esophagin may constitute a harbinger of early esophageal malignant transformation. A greater understanding of transcriptional control of esophagin may provide valuable insights into esophageal malignancy. Therefore, this study was undertaken in order to isolate and carry out initial characterization of a functional promoter for esophagin. A genomic clone containing esophagin was isolated and sequenced, including 2.7 kb of the esophagin promoter region. Esophagin expression was studied in response to various treatments of primary cultured human esophageal epithelial cells and squamous cell carcinoma cell lines. Calcium was the strongest inducer of the endogenous esophagin promoter, with induction occurring at 12-72 hours. In primary cultured esophageal epithelial cells, a region spanning 116 bp upstream of the transcriptional start site to 45 bp downstream was sufficient to direct low, basal, in vitro esophagin expression. However, responsiveness of primary esophageal cells to calcium required inclusion of promoter elements 1688 bp upstream of the transcriptional start site. Site-directed mutagenesis studies suggested a putative role for C/EBP-beta, OCT-1, and OCT-3 transcription factor binding sites in the minimal promoter region. In conjunction with published human in vivo studies, these data support the hypothesis that esophagin is a biomarker of esophageal squamous cell differentiation and provide an in vitro model to evaluate regulatory factors involved in this differentiation process.

Transcriptional analyses of Barrett's metaplasia and normal upper GI mucosae

Over the last two decades, the incidence of esophageal adenocarcinoma (EA) has increased dramatically in the US and Western Europe. It has been shown that EAs evolve from premalignant Barrett's esophagus (BE) tissue by a process of clonal expansion and evolution. However, the molecular phenotype of the premalignant metaplasia, and its relationship to those of the normal upper gastrointestinal (GI) mucosae, including gastric, duodenal, and squamous epithelium of the esophagus, has not been systematically characterized. Therefore, we used oligonucleotide-based microarrays to characterize gene expression profiles in each of these tissues. The similarity of BE to each of the normal tissues was compared using a series of computational approaches. Our analyses included esophageal squamous epithelium, which is present at the same anatomic site and exposed to similar conditions as Barrett's epithelium, duodenum that shares morphologic similarity to Barrett's epithelium, and adjacent gastric epithelium. There was a clear distinction among the expression profiles of gastric, duodenal, and squamous epithelium whereas the BE profiles showed considerable overlap with normal tissues. Furthermore, we identified clusters of genes that are specific to each of the tissues, to the Barrett's metaplastic epithelia, and a cluster of genes that was distinct between squamous and non-squamous epithelia.

A role for mitogen-activated protein kinase activation by integrins in the pathogenesis of psoriasis

In normal epidermis, beta1 integrin expression is confined to the basal layer, whereas in hyperproliferative epidermis, integrins are also expressed in the suprabasal layers. Transgenic mice in which integrins are expressed suprabasally via the involucrin promoter have a sporadic psoriatic phenotype; however, the mechanism by which integrins contribute to the pathogenesis of psoriasis is unknown. We observed activation of mitogen-activated protein kinase (MAPK) in basal and suprabasal keratinocytes of human and transgenic mouse psoriatic lesions and healing mouse skin wounds, correlating in each case with suprabasal integrin expression. Phenotypically normal human and transgenic mouse epidermis did not contain activated MAPK. Transgene-positive keratinocytes produced more IL-1alpha than controls did, and keratinocyte MAPK could be activated by ligation of suprabasal integrins or treatment with IL-1alpha. Constitutive activation of MAPK increased the growth rate of human keratinocytes and delayed the onset of terminal differentiation, recreating many of the histological features of psoriatic epidermis. We propose that activation of MAPK by integrins, either directly or through increased IL-1alpha production, is responsible for epidermal hyperproliferation in psoriasis and wound healing, and that the sporadic phenotype of the transgenic mice may reflect the complex mechanisms by which IL-1 release and responsiveness are controlled in skin.

Expression of small proline rich proteins in neoplastic and inflammatory skin diseases

BACKGROUND

The formation of the cornified cell envelope (CE) during the late stages of epidermal differentiation is essential for epidermal barrier function and protects the body against environmental attack and water loss. Formation of the CE involves the replacement of the plasma membrane by cross-linkage of precursor proteins such as involucrin, small proline rich proteins (SPRR) and loricrin. In normal epidermis, SPRR1 is restricted to appendages, SPRR2 is also expressed in interfollicular areas, while SPRR3 is completely absent; the latter is most abundant in oral epithelium. This differential expression indicates an important part for SPRRs in specific barrier requirements, and reflects their importance in the biomechanical properties of the CE.

OBJECTIVES

We report here on the expression of SPRR1, SPRR2 and SPRR3 in a wide range of cutaneous neoplastic and inflammatory diseases.

METHODS

We used immunohistochemistry; in addition, Northern blot analysis of malignant tumours was performed.

RESULTS

Increased suprabasal expression of SPRR1 and SPRR2, but no SPRR3 expression, was noted in inflammatory dermatoses with orthokeratotic and parakeratotic squamous differentiation. By contrast, differentiating epidermal tumours such as Bowen's disease, keratoacanthoma and squamous cell carcinoma expressed SPRR3.

CONCLUSIONS

As SPRRs were originally cloned on the basis of their expression in ultraviolet light-irradiated keratinocytes, the expression of SPRR3 in actinic lesions is of interest, and might serve as a diagnostic tool.

Gene expression profiles in squamous cell carcinomas of the oral cavity: use of laser capture microdissection for the construction and analysis of stage-specific cDNA libraries

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer among men in the developed world affecting the oral cavity, salivary glands, larynx and pharynx. Utilizing tissue from patients with HNSCC, we sought to systematically identify and catalog genes expressed in HNSCC progression. Here, we demonstrate the successful use of laser capture microdissection for procuring pure populations of cells from patient tissue sets comprised of oral squamous cell carcinomas (OSCCs) and matching normal tissue. From the estimated 5000 cells procured for each sample, we were able to extract total RNA (14.7-18.6 ng) of sufficient quality to transcribe GAPDH by reverse transcriptase-polymerase chain reaction (RT-PCR). The RNA was used for the synthesis of blunt-ended, double-strand complementary DNAs (cDNAs) by oligo (dT)-mediated reverse transcription, followed by addition of linkers. Primers specific for these linkers with uracil deglycosylase-compatible ends were used to amplify these cDNAs by PCR and the product was subcloned into the pAMP10 cloning vector. Ninety-six clones from each of six libraries were randomly sequenced and results indicated that 76-96% of the inserts represent either anonymous expressed sequence tags (ESTs) (25-48%), known genes (9-29%) or novel sequences (27-51%), respectively, with very little redundancy. These results demonstrate that high quality, representative cDNA libraries can be generated from microdissected OSCC tissue. Furthermore, these finding suggest the existence of at least 132 novel genes expressed in our cDNA libraries, which may have a role in the pathogenesis of HNSCC, and may represent novel markers for early detection as well as targets for pharmacological intervention in this disease.

Linear IgA disease with IgA antibodies directed against 200- and 280-kDa epidermal antigens

We report an 80-year-old man with the lamina lucida type of linear IgA disease, with IgA autoantibodies reactive with 200-kDa and 280-kDa epidermal proteins. The patient presented with widespread bullous lesions on his trunk and extremities without mucosal involvement. Histopathology of lesional skin showed a subepidermal blister with papillary microabscesses of neutrophils and a few eosinophils. Direct immunofluorescence microscopy of perilesional skin showed linear deposits of IgA and C3 at the basement membrane zone. The patient's serum contained IgA autoantibodies that bound exclusively to the epidermal side of 1 mol L-1 NaCl split skin as determined by indirect immunofluorescence microscopy. Circulating IgA autoantibodies to 200- and 280-kDa antigens were detected in the patient's serum by immunoblot analysis using extracts from normal human epidermis and human epidermal keratinocytes. These two antibodies, eluted from individual nitrocellulose membranes, reacted with the epidermal side of 1 mol L-1 NaCl split skin on indirect immunofluorescence microscopy, and bound to hemidesmosomes as determined by immunoperoxidase electron microscopy. This observation suggests the presence of hitherto uncharacterized 200- and 280-kDa hemidesmosomal proteins distinct from BPAG1, BPAG2 and beta4 integrin as target antigens in linear IgA disease.

Ordered structure acquisition by the N- and C-terminal domains of the small proline-rich 3 protein

The cell envelope (CE) is a vital structure for barrier function in terminally differentiated dead stratified squamous epithelia. It is assembled by transglutaminase (TGase) cross-linking of several proteins, including hSPR3 in certain specialized epithelia normally subjected to mechanical trauma. Biochemical studies show that hSPR3 serves as a complete substrate for TGase1, TGase2, and TGase3. Multiple adjacent glutamines and lysines of only head-and-tail domain sequences are used by each enzyme for cross-linking. Structural data suggest that the hSPR3 central repeats, as well as hSPR1 and hSPR 2, are highly flexible and mobile; thus, the TGases might not be able to recognize the residues localized on the repeats as adequate substrate. To investigate this hypothesis further and to complete the structural investigation of hSPR3, we performed circular dichroism (CD) studies on peptides corresponding to the N- and C-terminal domain. CD spectra have also been carried out in the presence of different concentrations of the structure-promoting agent cosolvent trifluoroethanol (TFE), which mimics a partial hydrophobic environment found in vivo in or next to the membrane. In fact, this agent increases the dielectric constant of water proportionally, depending on its concentration, and confers structuring properties to the solution, to peptides and proteins that have a structuring propensity. The results indicate that in both the N-terminal and C-terminal, peptides acquire a more ordered structure as a function of the TFE concentration in water. This ability of both N- and C-terminal domain to acquire a more stable ordered conformation might be relevant for SPR3 to act as substrate of TGases. Indeed, only the N- and C-terminus is cross-linked by TGase1 and 3.

Initiation of assembly of the cell envelope barrier structure of stratified squamous epithelia

The cell envelope (CE) is a specialized structure that is important for barrier function in terminally differentiated stratified squamous epithelia. The CE is formed inside the plasma membrane and becomes insoluble as a result of cross-linking of constituent proteins by isopeptide bonds formed by transglutaminases. To investigate the earliest stages of assembly of the CE, we have studied human epidermal keratinocytes induced to terminally differentiate in submerged liquid culture as a model system for epithelia in general. CEs were harvested from 2-, 3-, 5-, or 7-d cultured cells and examined by 1) immunogold electron microscopy using antibodies to known CE or other junctional proteins and 2) amino acid sequencing of cross-linked peptides derived by proteolysis of CEs. Our data document that CE assembly is initiated along the plasma membrane between desmosomes by head-to-tail and head-to-head cross-linking of involucrin to itself and to envoplakin and perhaps periplakin. Essentially only one lysine and two glutamine residues of involucrin and two glutamines of envoplakin were used initially. In CEs of 3-d cultured cells, involucrin, envoplakin, and small proline-rich proteins were physically located at desmosomes and had become cross-linked to desmoplakin, and in 5-d CEs, these three proteins had formed a continuous layer extending uniformly along the cell periphery. By this time >15 residues of involucrin were used for cross-linking. The CEs of 7-d cells contain significant amounts of the protein loricrin, typically expressed at a later stage of CE assembly. Together, these data stress the importance of juxtaposition of membranes, transglutaminases, and involucrin and envoplakin in the initiation of CE assembly of stratified squamous epithelia.

Expression, regulation, and function of the SPR family of proteins. A review

The small, proline-rich (SPR) genes consist of three subclasses closely linked on human chromosome 1, a region referred to as the epidermal differentiation complex. SPR genes consist of two exons, with the second exon containing the entire open reading frame. SPRs are expressed in all squamous tissues of the skin, scalp, footpad, vaginal epithelia, and most of the epithelial lining of the digestive tract, including the lip, tongue, esophagus, and forestomach. Although SPR1 is absent in normal mucociliary epithelium of the respiratory tract, epithelia that undergo squamous differentiation in response to vitamin-A deficiency or to injury owing to exposure to environmental toxicants express SPR1. High levels of SPR1 are detected in various diseases and cancers of the skin or respiratory epithelia and in nonkeratinizing papillary adenocarcinomas. SPR expression can be regulated by transcriptional factors, by posttranscriptional factors, or by factors that affect SPR1 mRNA translation or protein turnover. Furthermore, regulation can be affected by the state of cell proliferation. The presence of SPR1 in most of these epithelia, and the absence of SPR3 in normal skin, suggest that these subclasses have distinct functions. Various approaches to the study of the cross-linked envelope (CE) components in identifying SPR1 and SPR2 and in suggesting that SPRs are one of the precursor proteins of the CE. However, expression of SPR1 in nonsquamous tissues and cell lines indicates a function not associated with squamous differentiation. Several studies have demonstrated that SPR1 antibodies react with nuclear proteins and that SPR1 is expressed in cells before entering the G0 phase of the cell cycle. Future studies should clarify the role of SPRs by modifying their contents in CE, and should identify SPR-associated proteins to clarify the cell growth-related role of SPR1.

Transglutaminase cross-linking properties of the small proline-rich 1 family of cornified cell envelope proteins. Integration with loricrin

Small proline-rich 1 (SPR1) proteins are important for barrier function in stratified squamous epithelia. To explore their properties, we expressed in bacteria a recombinant human SPR1 protein and isolated native SPR1 proteins from cultured mouse keratinocytes. By circular dichroism, they possess no alpha or beta structure but have some organized structure associated with their central peptide repeat domain. The transglutaminase (TGase) 1 and 3 enzymes use the SPR1 proteins as complete substrates in vitro but in different ways: head domain A sequences at the amino terminus were used preferentially for cross-linking by TGase 3, whereas those in head domain B sequences were used for cross-linking by TGase 1. The TGase 2 enzyme cross-linked SPR1 proteins poorly. Together with our data base of 141 examples of in vivo cross-links between SPRs and loricrin, this means that both TGase 1 and 3 are required for cross-linking SPR1 proteins in epithelia in vivo. Double in vitro cross-linking experiments suggest that oligomerization of SPR1 into large polymers can occur only by further TGase 1 cross-linking of an initial TGase 3 reaction. Accordingly, we propose that TGase 3 first cross-links loricrin and SPRs together to form small interchain oligomers, which are then permanently affixed to the developing CE by further cross-linking by the TGase 1 enzyme. This is consistent with the known consequences of diminished barrier function in TGase 1 deficiency models.

Mouse Sprr2 genes: a clustered family of genes showing differential expression in epithelial tissues

Small proline-rich (SPR) proteins are structural components of the cornified cell envelope of stratified squamous epithelia. They are subdivided into three families, i.e., SPR1, SPR2, and SPR3, of which the SPR2 family is the most complex. To understand the significance of this complexity, we have isolated 11 mouse Sprr2 genes, constructed a provisional physical map of the Sprr2 locus on mouse Chromosome 3, and examined the expression patterns of the Sprr2 genes in mouse epithelial tissues. The 11 Sprr2 sequences are highly conserved with a central domain containing a variable number of repeats. In situ hybridization showed the Sprr2 expression to be confined to epithelia. RT-PCR using primers specific for each of the 11 Sprr2 members demonstrated varying degrees of expression among the individual Sprr2 members in different tissues. The correlation between the physical location of the genes in the Sprr2 locus and their expression patterns suggests multiple levels of controlled expression.

AP-1 and ets transcription factors regulate the expression of the human SPRR1A keratinocyte terminal differentiation marker

The 173-base pair proximal promoter of SPRR1A is necessary and sufficient for regulated expression in primary keratinocytes induced to differentiate either by increasing extracellular calcium or by 12-O-tetradecanoylphorbol-13-acetate (TPA) treatment. Whereas calcium-induced expression depends both on an AP-1 and an Ets binding site in this region, responsiveness to TPA resides mainly (but not exclusively) on the Ets element, indicating that Ets factors are important targets for protein kinase C signaling during keratinocyte terminal differentiation. This conclusion is further substantiated by the finding that expression of ESE-1, an Ets transcription factor involved in SPRR regulation, is also induced by TPA, with kinetics similar to SPRR1A. The strict AP-1 requirement in SPRR1A for calcium-induced differentiation is not found for SPRR2A, despite the presence of an identical AP-1 consensus binding site in this gene. Binding site swapping indicates that both the nucleotides flanking the TGAGTCA core sequence and the global promoter context are essential in determining the contribution of AP-1 factors in gene expression during keratinocyte terminal differentiation. In the distal SPRR1A promoter region, a complex arrangement of positive and negative regulatory elements, which are only conditionally needed for promoter activity, are likely involved in gene-specific fine-tuning of the expression of this member of the SPRR gene family.

Structural and transglutaminase substrate properties of the small proline-rich 2 family of cornified cell envelope proteins

The small proline-rich (SPR) proteins are components of the cornified cell envelope of stratified squamous epithelia and become cross-linked to other proteins by transglutaminases (TGases). The SPR2 family is the most complex, as it consists of several differentially expressed members of the same size. To explore their physical and cross-linking properties, we have expressed in bacteria a human SPR2 family member, and purified it to homogeneity. By circular dichroism, it possesses no alpha or beta structure but has some organized structure associated with the central peptide repeat domain. The TGase 1, 2, and 3 enzymes expressed in epithelia use the recombinant SPR2 protein as a complete substrate in vitro, but with widely differing kinetic efficiencies, and in different ways. With TGase 1, only one glutamine on the head domain and one lysine on the tail domain were used for limited interchain cross-linking. With TGase 3, multiple head and tail domain residues were used for extensive interchain cross-linking. The total usage of glutamine and lysine residues in vitro by TGase 3 was similar to that seen in earlier in vivo studies. We conclude that SPR2 proteins are cross-linked in epithelia primarily by the TGase 3 enzyme, a minor extent by TGase 1, and probably not by TGase 2.

Small proline-rich proteins are cross-bridging proteins in the cornified cell envelopes of stratified squamous epithelia

The cornified cell envelope (CE) is a specialized structure which contributes barrier function to stratified squamous epithelial cells. It is composed of an amalgam of several structural proteins that are rendered insoluble by isopeptide bond crosslinking by transglutaminases. One set of the structural proteins present in CEs of most such epithelia are the small proline rich (SPR) proteins, which are a family of about 12 related structural proteins. We have recovered a large number of peptides containing isopeptide crosslinks, including 236 involving SPR proteins, following proteolysis of CEs isolated from foreskin epidermal tissue and cultured epidermal keratinocytes. Analysis of this database has provided novel information on their function. First, we found that SPRs became crosslinked to many other structural proteins within the CE. Second, multiple glutamine and lysine residues located only on the amino- and carboxy-termini of the SPR proteins were involved in crosslinking, so that the two ends are functionally equivalent. Third, the SPRs functioned as cross-bridging proteins, by directly adjoining other CE structural proteins. In the specialized case of the epidermal CE, the SPRs cross-bridged between loricrin. In cultured keratinocytes which make little loricrin and serve as a model for internal stratified squamous epithelia, the SPRs formed extensive cross-bridges among themselves. Thus SPRs are ubiquitous cross-bridging proteins whose differential expression patterns apparently reflect specific barrier requirements of different epithelia.

Biochemical evidence that small proline-rich proteins and trichohyalin function in epithelia by modulation of the biomechanical properties of their cornified cell envelopes

The cornified cell envelope (CE) is a specialized structure involved in barrier function in stratified squamous epithelia, and is assembled by transglutaminase cross-linking of several proteins. Murine forestomach epithelium undergoes particularly rigorous mechanical trauma, and these CEs contain the highest known content of small proline-rich proteins (SPRs). Sequencing analyses of these CEs revealed that SPRs function as cross-bridgers by joining other proteins by use of multiple adjacent glutamines and lysines on only the amino and carboxyl termini and in functionally non-polar ways. Forestomach CEs also use trichohyalin as a novel cross-bridging protein. We performed mathematical modeling of amino acid compositions of the CEs of mouse and human epidermis of different body sites. Although the sum of loricrin + SPRs was conserved, the amount of SPRs varied in relation to the presumed physical requirements of the tissues. Our data suggest that SPRs could serve as modifiers of a composite CE material composed of mostly loricrin; we propose that increasing amounts of cross-bridging SPRs modify the structure of the CE, just as cross-linking proteins strengthen other types of tissues. In this way, different epithelia may use varying amounts of the cross-bridging SPRs to alter the biomechanical properties of the tissue in accordance with specific physical requirements and functions.

Structural organization of cornified cell envelopes and alterations in inherited skin disorders

The cornified cell envelope is a highly insoluble and extremely tough structure formed beneath the cell membrane during terminal differentiation of keratinocytes. Its main function is to provide human skin with a protective barrier against the environment. Sequential cross-linking of several integral components catalyzed by transglutaminases leads to a gradual increase in the thickness of the envelope and underscores its rigidity. Key structural players in this cross-linking process include involucrin, loricrin, SPRRs, elafin, cystatin A, S100 family proteins, and some desmosomal proteins. The recent identification of genetic skin diseases with mutations in the genes encoding some of these proteins, including transglutaminase 1 and loricrin, has disclosed that abnormal cornified cell envelope synthesis is significantly involved in the pathophysiology of certain inherited keratodermas and reflects perturbations in the complex, yet highly orderly process of cornified cell envelope formation in normal skin biology.