An unusual expression of a squamous cell marker, small proline-rich protein gene, in tracheobronchial epithelium: differential regulation and gene mapping

The non-human primate as a model for studying COPD and asthma

This review evaluates the current status of information regarding the nonhuman primate as an experimental model for defining mechanisms of chronic airways disease in humans, using the concept of the epithelial-mesenchymal trophic unit (EMTU) as a basis for comparison with other laboratory species. All of the cellular and acellular compartments within the walls of tracheobronchial airways which interact as the EMTU are present throughout the airway tree in human and nonhuman primates. The epithelial compartment contains mucous goblet and basal cells in the surface epithelium and submucosal glands within the wall. The interstitial compartment of primates has a prominent subepithelial basement membrane zone (BMZ) with an attenuated fibroblast sheath and cartilage throughout the tree. In primates, there is an extensive transition zone between distal conducting airways and lung parenchyma composed of numerous generations of respiratory bronchioles. None of these features are characteristic of intrapulmonary airways in rodents, whose airways do share ciliated cells, smooth muscle cells, nerve networks, vasculature and inflammatory cell populations with primates. While the numbers of intrapulmonary airway branches are similar for most mammals, branching patterns, which dictate distribution of inhaled materials, are more uniform (dichotomous) in primates and less so (monopodial) in rodents. Development of tracheobronchial airways (both differentiation of the EMTU and overall growth) occurs over an extensive postnatal period (months to years) in primates and a comparably shorter time period (2-3 weeks) in rodents. As with allergic airways disease in humans, experimental exposure of nonhuman primates to a known human allergen, house dust mite, produces extensive remodeling of all compartments of the EMTU: mucous goblet cell hyperplasia, epithelial sloughing, basement membrane zone (BMZ) thickening and reorganization, altered attenuated fibroblast function, subepithelial fibrosis and smooth muscle thickening. Experimental allergic airways disease in nonhuman primates also shares other features with asthmatic humans: positive skin test to allergen; allergen-specific circulating IgE; airway hyper responsiveness to allergen, histamine and methacholine; increased eosinophils, IGE positive cells and mucins in airway exudate; and migratory leukocyte accumulations in the airway wall and lumen. Experimental exposure of nonhuman primates to reactive gases, such as ozone, produces the chronic respiratory bronchiolitis and other airway alterations associated with restricted airflow and chronic respiratory bronchiolitis characteristic of COPD in young smokers. We conclude that nonhuman primate models are appropriate for defining mechanisms as they relate to allergic airways disease and COPD in humans.

A molecular signature of epithelial host defense: comparative gene expression analysis of cultured bronchial epithelial cells and keratinocytes

BACKGROUND

Epithelia are barrier-forming tissues that protect the organism against external noxious stimuli. Despite the similarity in function of epithelia, only few common protective mechanisms that are employed by these tissues have been systematically studied. Comparative analysis of genome-wide expression profiles generated by means of Serial Analysis of Gene Expression (SAGE) is a powerful approach to yield further insight into epithelial host defense mechanisms. We performed an extensive comparative analysis of previously published SAGE data sets of two types of epithelial cells, namely bronchial epithelial cells and keratinocytes, in which the response to pro-inflammatory cytokines was assessed. These data sets were used to elucidate a common denominator in epithelial host defense.

RESULTS

Bronchial epithelial cells and keratinocytes were found to have a high degree of overlap in gene expression. Using an in silico approach, an epithelial-specific molecular signature of gene expression was identified in bronchial epithelial cells and keratinocytes comprising of family members of keratins, small proline-rich proteins and proteinase inhibitors. Whereas some of the identified genes were known to be involved in inflammation, the majority of the signature represented genes that were previously not associated with host defense. Using polymerase chain reaction, presence of expression of selected tissue-specific genes was validated.

CONCLUSION

Our comparative analysis of gene transcription reveals that bronchial epithelial cells and keratinocytes both express a subset of genes that is likely to be essential in epithelial barrier formation in these cell types. The expression of these genes is specific for bronchial epithelial cells and keratinocytes and is not seen in non-epithelial cells. We show that bronchial epithelial cells, similar to keratinocytes, express components that are able to form a cross-linked protein envelope that may contribute to an effective barrier against noxious stimuli and pathogens.

SPRR1B overexpression enhances entry of cells into the G0 phase of the cell cycle

Many studies have established the role of SPRR1B during squamous differentiation of skin and respiratory epithelial cells. However, its role in nonsquamous cells is largely unknown. We reported that expression of SPRR1B in Chinese hamster ovary (CHO) cells is increased as they enter the G0 phase of the cell cycle. The purpose of this study was to further investigate the SPRR1B expression pattern in nonsquamous tumors and to study its role in these cells. Expression of SPRR1B was detected by Northern blotting in a higher percentage of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone-induced compared with beryllium metal-induced rat lung adenocarcinomas. In situ hybridizations confirmed that SPRR1B is expressed in individual or clusters of cells of nonsquamous cells from mouse, rat, and human adenocarcinomas. The same pattern of expression was observed in adenocarcinomas formed in nude mice from cell lines established from adenocarcinomas. SPRR1B expression was downregulated in the cell lines derived from adenocarcinoma when cells were enriched in G0 at low confluence. Tetraploidy was induced in CHO, mouse, and human tumor cell lines by stably overexpressing SPRR1B, whereas control cells showed no change in ploidy. Inducible expression of this protein for shorter periods using the ecdyson system did not affect growth rate or the ploidy of CHO cells but accelerated entry into G0/G1 compared with controls. These findings indicate that SPRR1B is likely coupled primarily to signals responsible for withdrawal from the proliferative state rather than the final stages of cellular quiescence and that its overexpression for prolonged periods may disrupt the normal progression of mitosis.

Application of high-density DNA microarray to study smoke- and hydrogen peroxide-induced injury and repair in human bronchial epithelial cells

Recent advances in high-density DNA microarray technique allow the possibility to analyze thousands of genes simultaneously for their differential gene expression patterns in various biologic processes. Through clustering analysis and pattern recognition, the significance of these differentially expressed genes can be recognized and correlated with the biologic events that may take place inside the cell and tissue. High-density DNA microarray nylon membranes were used to explore gene expression and regulation associated with smoke- and hydrogen peroxide-induced injury and repair in differentiated human bronchial epithelial cells in vitro. At least three phases of change in gene expression could be recognized. The first phase seems to be an immediate event in response to oxidant injury. This phase includes the induction of bcl-2 and mdm2 genes that are involved in the regulation of apoptosis, and the mitogen-activated protein kinase phosphatase 1 that functions as a regulator for various mitogen-activated protein kinase activities. The second phase, usually 5 h later, includes the induction of various stress proteins and ubiquitin, which are important in providing the chaperone mechanism and the turnover of damaged macromolecules. The third phase, which is 5 to 10 h later, includes the induction of genes that seem to be involved in reducing oxidative stress by metabolizing the cellular level of reactive oxygen species. In this phase, enzymes associated with tissue and cell remodeling are also elevated. These results demonstrated a complex gene expression array by bronchial epithelial cells in response to a single insult of oxidants that are relevant to environmental pollutants.

BMK1 (ERK5) regulates squamous differentiation marker SPRR1B transcription in Clara-like H441 cells

Various toxicants and carcinogens upregulate the expression of small proline-rich protein 1B (SPRR1B), a squamous differentiation marker, in bronchial epithelial cells both in vivo and in vitro. We have recently shown that phorbol 13-myristate 12-acetate (PMA)-stimulated SPRR1B transcription in Clara-like H441 cells is mainly mediated by activator protein-1 (AP-1) and c-Jun N-terminal kinase-1 (JNK1). Though mitogen-activated protein kinase (MAPK) kinase (MEK)-1/2 pathway inhibitors strongly suppressed both basal and PMA-inducible SPRR1B transcription, overexpression of dominant negative (dn) forms of extracellular signal-regulated kinase (ERK)-1 and/or -2 did not have any significant effect indicating the involvement of another ERK-like MAPK in this pathway. Here, we report for the first time the involvement of ERK5 in PMA-inducible SPRR1B transcription in H441 cells. PMA significantly induced ERK5 activation in H441 cells. Overexpression of dn-ERK5 strongly suppressed both basal and PMA-inducible SPRR1B transcription, whereas wild-type ERK5 upregulated it. Consistent with this, a mutant form of MEK-5, an upstream activator of ERK5, strongly suppressed PMA-inducible promoter activity. However, coexpression of c-Jun restored promoter activation suppressed by dn-ERK5. Thus, in addition to JNK1, the activation of MEK5-ERK5 MAPK pathway probably plays a pivotal role in transcriptional regulation of AP-1-mediated SPRR1B expression in the distal bronchiolar region.

Development of high-density DNA microarray membrane for profiling smoke- and hydrogen peroxide-induced genes in a human bronchial epithelial cell line

Development of the high-density DNA microarray technique permits the analysis of thousands of genes simultaneously for their differential expression patterns in various biological processes. Through clustering analysis and pattern recognition, the significance of differentially expressed genes can be recognized and correlated with biological events that may take place inside the cell and tissue. With this notion in mind, high-density DNA microarray nylon membrane with colorimetry detection was used to profile the expression of smoke- and hydrogen peroxide-inducible genes in a human bronchial epithelial cell line, HBE1. On the basis of the time course of expression, at least three phases of change in gene expression could be recognized. The first phase is an immediate event in response to oxidant injury. This phase includes induction of the bcl-2 and mdm-2 genes, which are involved in the regulation of apoptosis, and the mitogen-activated protein (MAP) kinase phosphatase 1 (MKP-1) gene, that functions as a regulator of various mitogen-activated protein kinase activities. The second phase, usually 5 h later, includes the induction of various stress proteins and ubiquitin, which are important in providing the chaperone mechanism and the turnover of damaged macromolecules. The third phase, which is 5-10 h later, includes the induction of genes that are apparently involved in reducing oxidative stress by metabolizing reactive oxygen species. In this phase, enzymes associated with tissue and cell remodeling are also elevated. These results demonstrate a complex gene expression array by bronchial epithelial cells in response to the insult of oxidants that are relevant to environmental pollutants.

Phorbol ester-induced expression of airway squamous cell differentiation marker, SPRR1B, is regulated by protein kinase Cdelta /Ras/MEKK1/MKK1-dependent/AP-1 signal transduction pathway

The transcriptional induction of SPRR1B by phorbol 12-myristate 13-acetate (PMA) is mainly mediated by the first -152-base pair 5'-flanking region containing two functional AP-1 sites. In this study, we have analyzed the signaling pathways that mediate the induction in tracheobronchial epithelial cells. PKC inhibitor ablated PMA-stimulated expression of endogenous SPRR1B and reporter gene expression driven by SPRR1B promoter. PKC activator promoted the transcription. The dominant negative protein kinase Cdelta (dn-PKCdelta) and rottlerin (PKCdelta inhibitor) completely suppressed PMA-stimulated promoter activity. dn-Ras or dn-MEKK1 inhibited PMA-stimulated promoter activity, while their corresponding constitutively active mutants augmented it. dn-c-Raf-1 did not have any effect on reporter gene expression. Since MEKK1 activates multiple parallel pathways, we examined involvement of JNK/SAPK, p38, and MKK1 in promoter regulation. Co-expression of the dominant negative forms of MKK4, MKK7, JNK/SAPK, MKK3, MKK6, or p38alpha did not suppress PMA-stimulated reporter gene expression. However, MKK1 inhibitors UO126 and PD98095 suppressed gene expression. Consistent with this, expression of dn-MKK1 strongly suppressed PMA-stimulated promoter activity, while the constitutively active MKK1 augmented it. However, MKK1-mediated induction of SPRR1B probably does not depend on extracellular signal-regulated kinases 1 and 2, suggesting the requirement of another kinase(s). dn-c-Jun mutants abolished PMA-stimulated expression supporting an important role for AP-1 proteins in SPRR1B expression. Together, these results suggest that a PKCdelta/Ras/MEKK1/MKK1-dependent/AP-1 pathway regulates the PMA-inducible expression of the SPRR1B in tracheobronchial epithelial cells.

Small proline-rich protein 1 is the major component of the cell envelope of normal human oral keratinocytes

Oral keratinocytes of buccal and gingival tissues undergo a terminal differentiation program to form a protective epithelial barrier as non-keratinized or parakeratinized stratified cells. We have examined the protein composition of cell envelopes (CEs) from normal human buccal and gingival tissues as well as keratinocytes from normal human gingival cells grown in culture. Biochemical and sequencing analyses reveal that the CEs contain 60-70% small proline-rich protein 1a/b (SPR1a/b), together with smaller amounts of involucrin, annexin I and several other known CE proteins. The data imply a specialized role for SPR1 proteins in the unique barrier function requirements of oral epithelia.

A novel mechanism of retinoic acid-enhanced interleukin-8 gene expression in airway epithelium

A 3- to 8-fold stimulation of interleukin (IL)-8 gene expression by all-trans-retinoic acid (ATRA) was demonstrated in primary cultures of human and monkey tracheobronchial epithelial cells and BEAS-2B serum-sensitive cell line. The effect of ATRA on IL-8 gene expression is dose- and time-dependent. Using cycloheximide, it was observed that new protein synthesis was required for the stimulation. ATRA had no effect on IL-8 messenger RNA stability. A difference in nuclear run-on activity suggests that a transcriptional mechanism is involved in ATRA-enhanced IL-8 gene expression. Promoter-reporter gene transfection studies demonstrated ATRA enhanced IL-8 promoter activity, especially when cells were cotransfected with retinoic acid nuclear receptor-alpha expression vector. Deletion and site-directed mutagenesis analysis revealed the involvement of nuclear factor (NF)-kappaB binding site of the IL-8 gene in ATRA-enhanced promoter activity. Electrophoretic mobility shift assay (EMSA) demonstrated that ATRA enhanced DNA-NF-kappaB complex formation, especially with the p65 subunit. Western blot analysis demonstrated that ATRA did not enhance the protein amount of both the p50 and the p65 subunits in the nuclei. Because ATRA also enhances thioredoxin (TRX) gene expression, the effect of TRX on IL-8 gene expression was examined. IL-8 promoter activity was enhanced in transfected cells by the addition of TRX protein. Treatment of nuclear extracts with TRX also enhanced DNA- NF-kappaB complex formation as observed by EMSA, particularly the p65 subunit. Taking these data together, a novel mechanism is proposed in which ATRA activates promoter activity of IL-8 gene through TRX-dependent NF-kappaB activation.

Distinct roles for amino- and carboxyl-terminal sequences of SPRR1 protein in the formation of cross-linked envelopes of conducting airway epithelial cells

The small proline-rich protein, SPRR1, is a marker gene whose expression in conducting airway epithelium is elevated under a variety of conditions that enhance squamous differentiation. The purpose of this study is to elucidate the nature of the SPRR1 sequence involved in cross-linked envelope formation in a tissue/cell type, such as conducting airway epithelium, that normally does not express squamous function except after injury or maintenance in culture. For this, a Flag-SPRR1 fusion protein expression system has been developed. Using the liposome-mediated gene transfer technique on passage 1 culture of human tracheobronchial epithelial (TBE) cells, the Flag-SPRR1 fusion protein can be expressed and detected immunologically by both anti-Flag and anti-SPRR1 antibodies. The incorporation of Flag-SPRR1 fusion protein into cross-linked envelopes can be demonstrated when transfected human passage 1 TBE cultures are treated with phorbol 12-myristate 13-acetate and high calcium (1.5 mM). By deletion and site-directed mutagenesis, two distinct roles of the amino- and carboxyl-terminal sequences of SPRR1 have been demonstrated. First, we demonstrated that the amino-terminal sequence of SPRR1 protein is required for the incorporation of the fusion protein into cross-linked envelopes, whereas a deletion on the carboxyl-terminal region or on the middle repetitive unit has no effect. Interestingly, insertion of a 24-amino acid peptide of monkey MUC2 repetitive sequence in the amino-terminus of SPRR1 protein had a stimulatory effect. Site-directed mutagenesis on the following amino acid residues, Lys(7), Gln(88), and Lys(89), which were found previously to participate in the cross-linked envelope formation of keratinocytes, had no detrimental effect on the incorporation. However, mutations on Gln clusters, such as Gln(4)-Gln(6) and Gln(22)-Gln(25), had detrimental effects on the incorporation. These results suggest an amino-terminal sequence-dependent and multiple cross-linked sites for the incorporation of Flag-SPRR1 fusion protein into cross-linked envelopes of cultured human TBE cells. Second, we demonstrated that the carboxyl terminus of SPRR1 protein is required for a high level of Flag-fusion protein expression. A deletion in the carboxyl region or a mutation on the last lysine residue of the carboxyl end had a detrimental effect on the level of Flag-SPRR1 fusion protein expressed in transfected cells. In contrast, there was only a slight decrease in the level of expression if the amino-terminus was deleted. Interestingly, the efficiency for fusion protein to incorporate into cross-linked envelopes was elevated by the mutation at the carboxyl end. These results suggest distinct roles, perhaps coordinately, for both amino- and carboxyl-terminal sequences in the regulation of the life cycle of SPRR1 protein in cultured TBE cells.

Expression and regulation of a molecular marker, SPR1, in multistep bronchial carcinogenesis

A small proline-rich protein, SPR1, is overexpressed in squamous metaplasia of bronchial epithelium. We studied the expression and regulation of SPR1 in a series of human bronchial epithelial cell lines representing a model of multistep bronchial carcinogenesis. These cell lines included a primary culture of tracheobronchial epithelial cells (HTBE), a papilloma virus-transformed tracheobronchial epithelial cell line (HBE1), a cell line selected from HBE1 by a tobacco carcinogen and a phorbol ester (HBE1-C), a simian virus-transformed bronchial epithelial cell line (BEAS-2B), and a lung carcinoma cell line (H460). Different tumorigenic potentials of these cell lines were indicated by graded levels of telomerase activity. Concomitant with squamous transformation, there was an increase in SPR1 expression in HTBE, HBE1, and HBE1-C that was reversible by vitamin A. With progression of tumorigenicity, there was a marked reduction in SPR1 expression in BEAS-2B and a total loss of expression in H460. In these latter cell lines representing advanced malignant transformation, there was a loss of up- and downregulation, respectively, by the phorbol ester and vitamin A. Transfection study with chimeric constructs of the SPR1 promoter and a reporter gene showed that the dysregulation of SPR1 expression in malignant transformation was a result of perturbation of the basal and enhancer elements of the first 162 nucleotides in the 5'-flanking promoter region of the SPR1 gene. These findings suggest an association of transcriptional dysregulation of the SPR1 gene with multistep bronchial carcinogenesis.

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.

Structure and organization of the genes encoding mouse small proline-rich proteins, mSPRR1A and 1B

Two genomic clones comprising the entire coding sequence of mouse SPRR1A and 1B genes were isolated and sequenced. Sequence analysis of the 1A and 1B genomic clones indicated that both genes contain two exons separated by an intron slightly larger than 1.1kb in size (1147 and 1152nt, respectively). This type of genomic structure is identical to the counterpart of human SPRR1 gene and other genes encoding for cornified envelope proteins. Primer extension analysis using 1A and 1B gene-specific primers indicates that 1A gene is expressed in squamous tissues such as skin and esophagus, whereas 1B gene is expressed in papilloma tumors but not in squamous tissues. The first 300nt of 5'-flanking region of the mouse SPRR 1A and 1B genes reveal an overall approximately 50% identity to the human counterpart. However, there is a high degree of identity (?75%) at the promoter region containing a TATA box and TRE/TRE-like motifs. Both TATA and TRE/TRE-like motifs are almost identical in sequence and positions to those found in the counterpart of human promoter. Using transient transfection for the analysis of promoter activity, we observed that both 1A and 1B 5'-flanking regions contain the promoter activity to direct the expression of the reporter gene, chloramphenicol acetyltransferase, in airway epithelial cells in a fashion similar to that observed in the human counterpart. These results indicate a conserved nature of genetic structure and regulation of SPRR1 gene expression between human and mouse.

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.

Loss of spr1 expression measurable by quantitative RT-PCR in human bronchogenic carcinoma cell lines

Expression of the small, proline-rich protein (spr1) squamous differentiation marker was measured in five cultured normal and 12 malignant human bronchial epithelial cell (BEC) populations by quantitative reverse transcriptase polymerase chain reaction (RT-PCR). Whereas spr1 expression was quantifiable and inducible in all five cultured normal cell populations, in all 12 carcinoma cell lines evaluated it was neither quantifiable nor inducible. Primers spanning the entire spr1 coding sequence amplified full-length PCR product from genomic DNA; therefore, large deletions in the coding region were not responsible for the loss of expression measurable by RT-PCR. This is the first molecular genetic marker reported that distinguishes all normal from all carcinoma cell populations evaluated. Because the spr1 protein is a component of the crosslinked envelope that forms during the squamous differentiation process, we hypothesize that the apparent loss of spr1 gene expression disrupts mechanisms for terminal squamous differentiation in the bronchial epithelium, thereby contributing to malignant transformation.

A small proline-rich protein, spr1: specific marker for squamous lung carcinoma

We have identified a small proline-rich protein, spr1, which is a sensitive and specific marker for distinguishing squamous cancer from the other cell types of lung carcinoma. A rabbit antiserum against a 15-amino-acid peptide of the C-terminus of spr1 was prepared. The specificity of this antiserum was demonstrated in normal squamous tissues by Western blotting and immunohistochemical analysis. Expression of spr1 in 63 cases of formalin-fixed and paraffin-embedded human bronchogenic carcinoma was studied by immunohistochemical staining. For these 63 cases of bronchogenic carcinoma previously diagnosed by hematoxylin/eosin (H/E) staining, the number of spr1-positive cases/total number of H/E cases of each cell type of lung cancer were as follows: 20/20 of squamous carcinoma, 2/18 of adenocarcinoma, 4/14 of large-cell carcinoma, and 0/11 of small-cell lung carcinoma. Squamous differentiation evidenced by spr1 expression was substantiated by the presence of squamous features observed under transmission electron microscopy (TEM). We conclude that spr1 is a sensitive and specific marker for squamous bronchogenic carcinoma.

Arsenate suppression of human keratinocyte programming

The human keratinocyte line SCC-9 has been used as a model for arsenate-induced perturbations of differentiation. Growth of these cells in 10 microM arsenate permitted the cultures to reach confluence, but prevented expression of 6 markers of suprabasal differentiation (involucrin, loricrin, filaggrin, spr 1, keratin 1 and keratin 10) as assayed by Northern blotting. By contrast, only slight alterations in mRNA levels were observed for one differentiation marker (keratinocyte transglutaminase) and for keratin 5, keratin 14, AP2 or glyceraldehyde phosphate dehydrogenase. The transition metal oxyanions vanadate and chromate had essentially the same suppressive effect on these markers as arsenate, while chronic treatment with tetradecanoylphorbol acetate was generally less effective in suppressing differentiation. To determine whether the previously observed arsenate-mediated alteration in AP1 and AP2 activities could account for the suppression of involucrin, a promoter analysis was conducted. Putative AP1 and AP2 response elements were identified in regions important for transcriptional activity of the 5'-flanking DNA. Mutations in two AP1 sites and one AP2 site were observed to decrease promoter activity significantly, and in combination, to reduce it to approximately 10% of that conferred by the native sequence. These results lend support to the working hypothesis that arsenate suppresses involucrin expression, and, more generally, keratinocyte programming, by altering the transcription factors AP1 and AP2.

Inhibition of TGF-alpha gene expression by vitamin A in airway epithelium

The autocrine/paracrine growth mechanism has been implicated in the regulation of bronchial epithelial cell proliferation. By inhibiting the expression of the transforming growth factor-alpha (TGF-alpha) gene product, vitamin A is able to suppress the proliferation of tracheobronchial epithelial cells in culture. Similar repressions in TGF-alpha mRNA levels by retinol were observed in airway explant cultures and in a cell line immortalized from normal human bronchial epithelial cells. Both the nuclear run-on transcriptional assay and the transfection study with the chimeric construct of the TGF-alpha promoter and chloramphenicol acetyltransferase reporter gene partly suggest a transcriptional downregulation mechanism of TGF-alpha gene expression by the retinol treatment; however, this inhibition at the transcriptional level cannot account for the total inhibition at the mRNA level. These results suggest that a downregulation of the expression of the TGF-alpha gene at the transcriptional and post-transcriptional levels by vitamin A may precede the essential event associated with the homeostasis of normal conducting airway epithelium.

Expression of the spr1 gene in cultured tracheal epithelial cells and its regulation by retinoids before and after confluence

The absence of vitamin A or vitamin A derivatives in culture media promotes squamous cell differentiation of tracheobronchial epithelial cells. This is especially true for the expression of a small proline-rich protein (20K; 98 amino acids) in pig trachea epithelial cells. Multigene families encode different small proline-rich proteins in different species, and these proteins are possible markers for squamous cell differentiation. 20K mRNA and 20K protein were detected in cells within 4 and 5 days in culture, respectively, when cells reached about 50% confluence, and expression increase 12-fold during cell proliferation until cells reached 100% confluence. Arotinoid (10(-9)M), a synthetic retinoid, essentially totally inhibited expression of 20K mRNA in proliferating tracheobronchial cells within 3 days of treatment while 20K protein levels were only decreased 4-fold after 5 days. However, if cells were exposed to arotinoid 3 days after reaching confluent growth, the levels of either 20K mRNA or 20K protein were unchanged. Cells exposed to arotinoid from the onset of culturing, and then removal of the retinoid from proliferating cells resulted in the expression of 20K mRNA and protein after 4 and 5 days as observed previously. 20K mRNA was not detected in cells that had been continuously exposed to arotinoid from the start of culture until 3 days post confluence, even 10 days following removal of arotinoid. Our results strongly suggest that the growth phase and state of cell differentiation greatly affect the response of these epithelial cells to vitamin A derivatives.

Sequence and expression patterns of mouse SPR1: Correlation of expression with epithelial function

A final event in the terminal differentiation of stratified squamous epithelia is the formation of a cornified cell envelope, which is a complex of several proteins cross-linked together by transglutaminases. One set of proteins is the family of small proline rich (SPR) proteins. In human foreskin epidermal cell envelopes, SPRs serve as cross-bridging proteins among the more abundant loricrin. In order to study further their evolution and expression, we have isolated and sequenced cDNAs encoding two mouse SPR1 proteins, SPR1a and SPR1b Comparative sequence analysis showed the preservation of the overall structure of mammalian SPR1 proteins with highly conserved termini and a central peptide domain repeated 13 (SPE1a) or seven (SPR1b) times. Tissues obtained from mouse fetal, newborn, and adult skin were tested by Northern blot analyses, in situ hybridization and immunohistochemistry using an antibody raised to a synthetic peptide corresponding to the C terminus of the SPR1a protein. Skin expression was first detected in fetal periderm in anagen hair follicles of newborn and older mice, and in the thickened epidermis of the lip and footpad, but no signal was detected in interfollicular trunk epidermis. High levels of SPR1a expression were found in epithelia from the forestomach and penis, and in benign squamous papillomas. Other epithelia expressing SPR1a include the tongue, esophagus, and vagina. Whenever detected, SPR1a positive staining was present in the spinous and granular layers. In the forestomach and papillomas, the periphery of cells in the cornified layer was also stained. Our results suggest that SPR1a participates widely in the construction of cell envelopes in cornifying epithelia characterized by either increased thickness or a requirement for extreme flexibility. Based on its likely function as a cross-bridging protein in cell envelopes, we conclude that the mechanical attributes of cell envelopes may be determined in part by the SPR1 content, in accordance with the specific function of the epithelium.

Expression of human squamous cell differentiation marker, SPR1, in tracheobronchial epithelium depends on JUN and TRE motifs

Tracheobronchial epithelial (TBE) cells that normally do not express the squamous cell differentiation marker gene, SPR1, can be induced to produce it by 12-O-tetradecanoylphorbol-13-acetate (TPA). The regulation of SPR1 gene expression by TPA occurs, in part, at the transcriptional level in primary human and monkey TBE cells. Using a transient transfection assay, we observed that TPA stimulates the activity of the reporter gene, chloramphenicol acetyltransferase, by 2-4-fold in transfected TBE cells. However, this chloramphenicol acetyltransferase activity is cell type-specific with significantly less activity in transformed epithelial cell lines and no activity in non-epithelial cell types. TPA-dependent stimulation can also be demonstrated by co-transfection with plasmid DNAs that overexpress the JUN family of proteins, especially c-JUN. Overexpression of c-JUN and TPA treatment synergistically stimulate the SPR1 promoter activity by more than 40-fold. Deletion analysis of the promoter region demonstrates that the DNA fragment of the first 98 base pairs of the 5'-flanking region contains the basal promoter activity, while the region between -162 and -96 contains the cis-enhancer elements for both the basal and TPA/c-JUN-stimulating promoter activities. This observation is supported by in vivo genomic footprinting studies that reveal persistent protections in the following motifs of this region: -141 TRE, -131 GT, -123 ETS-like, and -111 TRE-like motifs and in the enhanced protections in -141 TRE and -111 TRE-like motifs in cells after the TPA treatment. Site-directed mutagenesis in this region demonstrates the involvement of both -141 TRE and -111 TRE-like motifs in TPA/c-JUN-dependent stimulation as well as enhanced basal transcriptional activity. However, it is primarily the -111 TRE-like motif that is involved in the mediation of the enhanced basal promoter activity of the human SPR1 gene. These results are further supported by gel mobility shift assays that demonstrate the involvement of c-JUN and these TRE motifs in the formation of the DNA-protein complex.

Two nuclear proteins in tracheal epithelial cells are recognized by antibodies specific to a squamous differentiation marker, sprI

In cell-free translations of RNA from primary cultures of pig trachea surface epithelial cells we observed that a mRNA encoding a 20 kDa proline-rich protein (sPRP) was dramatically induced during culturing (Tesfaigzi et al., 1990, Biochem. Biophys. Res. Commun., 172:M1304-1309). This mRNA was not detected in tracheal tissue or in epithelial cells prior to culturing. Antisera were raised to synthetic peptide sequences corresponding to 23 amino acids on the C-terminus (C23-antiserum) and 29 amino acids on the N-terminus (N29 antiserum) of sPRP. On Western blot analysis, C23 antiserum reacted with a 20 kDa protein in cytosolic extracts from pig tracheal cells maintained in culture for 4 days. The reaction with the 20 kDa protein was inhibited by adding C23 peptide. Two nuclear proteins (66 and 70 kDa) obtained by micrococcal nuclease treatment of tracheal cell nuclei were detected on Western blots with C23 antiserum. These proteins were present in cells both before and after culturing. Sucrose gradient fractionation indicated that these nuclear proteins are associated with chromatin. Small amounts of the 66 and 70 kDa proteins were obtained from nuclear matrix fractions. These nuclear proteins also reacted with N29 antiserum. Since these proteins share similar epitopes with the N- and C-termini of sPRP, it is likely that the 20 kDa protein (sPRP) is part of these proteins. However, purification of the nuclear proteins followed by an amino acid sequence analysis is necessary to clarify whether sPRP is part of these proteins.

Molecular cloning, characterization, and genetic mapping of the cDNA coding for a novel secretory protein of mouse. Demonstration of alternative splicing in skin and cartilage

A novel 85-kDa protein secreted by the mouse stromal osteogenic cell line MN7 was identified using two-dimensional polyacrylamide gel electrophoresis (Mathieu, E., Meheus, L., Raymackers, J., and Merregaert, J. (1994) J. Bone Miner. Res. 9, 903-913). Degenerate primers were used to isolate the cDNA coding for this protein. The full-length cDNA clone is 1.9 kilobases (kb) and codes for a protein of 559 amino acid residues. The DNA and deduced amino acid sequences have no counterparts in public data bases, but a structural similarity involving typical cysteine doublets can be observed to serum albumin family proteins and to Endo16 (a calcium-binding protein of sea urchin). Northern blot analysis revealed the presence of a 1.9-kb transcript in various tissues, and a shorter transcript of 1.5 kb, derived by alternative splicing in tail, front paw and skin of embryonic mice. The gene for the p85 protein, termed Ecm1 (for extracellular matrix protein 1), is a single-copy gene, which was localized to the region on mouse chromosome 3 known to contain at least one locus associated with developmental disorders of the skin, soft coat (soc). Alternative splicing may serve as a mechanism for generating functional diversity in the Ecm1 gene.

The small proline-rich proteins constitute a multigene family of differentially regulated cornified cell envelope precursor proteins

Loricrin, involucrin, small proline-rich protein (SPRR)1, SPRR2, and SPRR3 genes are located within a cluster of 1.5 Mbp on chromosome 1q21 and most likely evolved from a common ancestor. Monospecific polyclonal antibodies and cDNA probes were produced to investigate SPRR transcripts and proteins. SPRR expression was restricted to terminally differentiating squamous cells, preferentially located at the cell periphery, and immunoreactivity was greatly reduced in cells with a mature cornified cell envelope. Furthermore, detectable SPRR2 and SPRR3 levels were strongly increased in differentiating keratinocyte cultures after addition of LTB-2, a specific inhibitor of transglutaminases, suggesting that they are precursor proteins of the cornified cell envelope. In normal epidermis, SPRR1 was restricted to appendageal areas, SPRR2 was expressed coherently, and SPRR3 was completely absent. In the upper digestive tract, SPRR1 was expressed in sublingual and tongue epithelium, SPRR2 was mostly restricted to lingual papillae, and SPRR3 was abundant in oral and esophageal epithelium. In psoriatic epidermis, SPRR1 and SPRR2 were expressed at much higher levels than in normal epidermis. Addition of 10(-7) M retinoic acid to cultured differentiating keratinocytes significantly down-regulated the expression of SPRR2 and SPRR3 transcripts and slightly decreased that of SPRR1. Thus, SPRR1, SPRR2, and SPRR3 are differentially expressed in vivo and in vitro, suggesting that the SPRR multigene family evolved to serve as highly specialized cornified cell envelope precursor proteins in stratified epithelia.

Differential expression of protease inhibitor and small proline-rich protein genes between normal human oral tissue and odontogenic keratocysts

The technique of differential hybridization was used to compare gene transcription between normal oral mucosa and odontogenic keratocyst lining. Protease inhibitors, elafin and stefin-B as well as beta-actin and two epithelial-specific small proline-rich (spr) proteins, which we have named SPRC and SPRK and which are distinct from salivary proline-rich proteins, were differentially expressed. Increased abundance of alpha I(I) collagen and elafin transcripts was demonstrated in the keratocyst, with decreased abundance of stefin B, SPRC and cytokeratins 4 and 13 transcripts compared to normal palatal mucosa. The deduced protein sequences of SPRC and SPRK were described and compared, and the relative abundance of their respective cDNAs in palatal and keratocyst libraries determined. Identification of factors controlling transcription of these genes could advance our understanding of the development of odontogenic keratocysts.

Expression of a squamous cell marker, the spr1 gene, is posttranscriptionally down-regulated by retinol in airway epithelium

Vitamin A (retinol) is required for the normal mucociliary differentiation of respiratory epithelium. A depletion of vitamin A promotes squamous cell metaplasia. To understand how vitamin A suppresses squamous cell differentiation, the expression of a squamous cell differentiation marker, the small proline-rich protein gene (spr1), was studied in cultured monkey tracheobronchial epithelial (TBE) cells. The expression of the spr1 gene was inhibited about 40 fold by retinol. The mRNA levels of the spr1 gene started to decline within 6 h of retinol treatment and reached a minimum level after 7 days. The inhibition by retinol was concentration dependent and did not require concurrent protein synthesis. The inhibition of the spr1 mRNA by retinol was not due to a decrease in the transcription rate of its gene but due to a decrease in its stability, as determined by nuclear run-on assays and mRNA half-life measurement, respectively. This result was further supported by a DNA transfection study using a chimeric construct containing the spr1 promoter region and the chloramphenicol acetyltransferase (CAT) reporter gene. The CAT activity in transfected cells was not inhibited by retinol. These results suggest that spr1 gene expression is posttranscriptionally down-regulated by retinol.

A small proline-rich protein regulated by vitamin A in tracheal epithelial cells is induced in lung tumors

In cell-free translations of RNA from primary cultures of pig trachea surface epithelial cells, we observed that a 20 kD proline-rich protein (sPRP) is induced during culturing (Biochem. Biophys. Res. Commun. 1990; 172:1304-1309). Subsequently, a cDNA encoding sPRP has been cloned from pig tracheal cell mRNA and sequenced. This cDNA shows a high similarity to cDNAs cloned from monkey tracheal cells cultured in vitamin A-free medium and from UV-irradiated human epidermal keratinocytes. Amino acid sequences from these cDNAs are exceptionally rich in proline, glutamine, cysteine, and lysine but contain no aromatic amino acids. Two repeats of 12 amino acids on the N-terminus are followed by multiple 8 amino acid repeats. When compared with monkey trachea and human keratinocyte cDNAs, the sPRP cDNA from pig trachea has an additional 24 bp nucleotide repeat. Antiserum raised to a synthetic peptide (23 amino acids) on the C-terminus of sPRP (C23-antiserum) reacted with the 20 kD sPRP in immunoprecipitations from cell-free translations. On Northern blot analysis, sPRP cDNA hybridized to RNAs of similar sizes in tracheal cells from cat, rabbit, and lamb. sPRP was not detected in tracheal cells that were cultured with 10(-9) M arotinoid. Since sPRP is considered a putative squamous cell differentiation marker, experiments using lung tumors were performed. sPRP mRNA levels were dramatically increased in squamous lung tumors that were induced by injecting hamsters with 4-(methylnitrosamino)- 1-(3-pyridyl)-1-butanone, a tobacco-specific nitrosamine. In situ hybridization with tissue sections prepared from these lung tumors revealed that cells around the keratin pearls contained high levels of sPRP mRNA.

cDNA cloning of a hnRNP A1 isoform and its regulation by retinol in monkey tracheobronchial epithelial cells

An isoform of the hnRNP A1 was cloned from a cDNA library of monkey tracheobronchial epithelial (TBE) cells by differential hybridization. The cDNA clone MT77 has an insert of 1756 base pairs and the DNA sequence shares high homology to both human A1 alpha-type and beta-type isoforms with the exception of several differences in the coding and noncoding regions. Like the other two isoforms, MT77 has two polyadenylation sites. A probe prepared from the MT77 clone hybridizes to two message bands at 1.4 and 1.8 kb. Both messages were found in a polysomal preparation, suggesting that both messages are used in A1 protein synthesis. The expression of the A1 gene in monkey TBE cells is stimulated by vitamin A (retinol). The results of nuclear run-on transcriptional assays suggest that this stimulation occurs at the transcriptional level. Furthermore, this effect is not prevented, but superinduced, by cycloheximide. These results suggest that vitamin A may be directly involved in regulating A1 transcription through a mechanism similar to the interactions between the retinoic acid responsive elements and the nuclear receptors of retinoic acid.