Variable expression of retinoic acid receptor (RAR beta) mRNA in human oral and epidermal keratinocytes; relation to keratin 19 expression and keratinization potential

PRMT1 Inhibition Selectively Targets BNC1-Dependent Proliferation, but not Migration in Squamous Cell Carcinoma

Squamous Cell Carcinoma (SCC) develops in stratified epithelial tissues and demonstrates frequent alterations in transcriptional regulators. We sought to discover SCC-specific transcriptional programs and identified the transcription factor Basonuclin 1 (BNC1) as highly expressed in SCC compared to other tumor types. RNA-seq and ChIP-seq analysis identified pro-proliferative genes activated by BNC1 in SCC cells and keratinocytes. Inhibition of BNC1 in SCC cells suppressed proliferation and increased migration via FRA1. In contrast, BNC1 reduction in keratinocytes caused differentiation, which was abrogated by IRF6 knockdown, leading to increased migration. Protein interactome analysis identified PRMT1 as a co-activator of BNC1-dependent proliferative genes. Inhibition of PRMT1 resulted in a dose-dependent reduction in SCC cell proliferation without increasing migration. Importantly, therapeutic inhibition of PRMT1 in SCC xenografts significantly reduced tumor size, resembling functional effects of BNC1 knockdown. Together, we identify BNC1-PRMT1 as an SCC-lineage specific transcriptional axis that promotes cancer growth, which can be therapeutically targeted to inhibit SCC tumorigenesis.

Binding of NAD+-Glycohydrolase to Streptolysin O Stabilizes Both Toxins and Promotes Virulence of Group A Streptococcus

The globally dominant, invasive M1T1 strain of group A Streptococcus (GAS) harbors polymorphisms in the promoter region of an operon that contains the genes encoding streptolysin O (SLO) and NAD⁺-glycohydrolase (NADase), resulting in high-level expression of these toxins. While both toxins have been shown experimentally to contribute to pathogenesis, many GAS isolates lack detectable NADase activity. DNA sequencing of such strains has revealed that reduced or absent enzymatic activity can be associated with a variety of point mutations in nga, the gene encoding NADase; a commonly observed polymorphism associated with near-complete abrogation of activity is a substitution of aspartic acid for glycine at position 330 (G330D). However, nga has not been observed to contain early termination codons or mutations that would result in a truncated protein, even when the gene product contains missense mutations that abrogate enzymatic activity. It has been suggested that NADase that lacks NAD-glycohydrolase activity retains an as-yet-unidentified inherent cytotoxicity to mammalian cells and thus is still a potent virulence factor. We now show that expression of NADase, either enzymatically active or inactive, augments SLO-mediated toxicity for keratinocytes. In culture supernatants, SLO and NADase are mutually interdependent for protein stability. We demonstrate that the two proteins interact in solution and that both the translocation domain and catalytic domain of NADase are required for maximal binding between the two toxins. We conclude that binding of NADase to SLO stabilizes both toxins, thereby enhancing GAS virulence.

The global increase in invasive GAS infections in the 1980s was associated with the emergence of an M1T1 clone that harbors a 36-kb pathogenicity island, which codes for increased expression of toxins SLO and NADase. Polymorphisms in NADase that render it catalytically inactive can be detected in clinical isolates, including invasive strains. However, such isolates continue to produce full-length NADase. The rationale for this observation is not completely understood. This study characterizes the binding interaction between NADase and SLO and reports that the expression of each toxin is crucial for maximal expression and stability of the other. By this mechanism, the presence of both toxins increases toxicity to keratinocytes and is predicted to enhance GAS survival in the human host. These observations provide an explanation for conservation of full-length NADase expression even when it lacks enzymatic activity and suggest a critical role for binding of NADase to SLO in GAS pathogenesis.

NAD+-Glycohydrolase Promotes Intracellular Survival of Group A Streptococcus

A global increase in invasive infections due to group A Streptococcus (S. pyogenes or GAS) has been observed since the 1980s, associated with emergence of a clonal group of strains of the M1T1 serotype. Among other virulence attributes, the M1T1 clone secretes NAD⁺-glycohydrolase (NADase). When GAS binds to epithelial cells in vitro, NADase is translocated into the cytosol in a process mediated by streptolysin O (SLO), and expression of these two toxins is associated with enhanced GAS intracellular survival. Because SLO is required for NADase translocation, it has been difficult to distinguish pathogenic effects of NADase from those of SLO. To resolve the effects of the two proteins, we made use of anthrax toxin as an alternative means to deliver NADase to host cells, independently of SLO. We developed a novel method for purification of enzymatically active NADase fused to an amino-terminal fragment of anthrax toxin lethal factor (LFn-NADase) that exploits the avid, reversible binding of NADase to its endogenous inhibitor. LFn-NADase was translocated across a synthetic lipid bilayer in vitro in the presence of anthrax toxin protective antigen in a pH-dependent manner. Exposure of human oropharyngeal keratinocytes to LFn-NADase in the presence of protective antigen resulted in cytosolic delivery of NADase activity, inhibition of protein synthesis, and cell death, whereas a similar construct of an enzymatically inactive point mutant had no effect. Anthrax toxin-mediated delivery of NADase in an amount comparable to that observed during in vitro infection with live GAS rescued the defective intracellular survival of NADase-deficient GAS and increased the survival of SLO-deficient GAS. Confocal microscopy demonstrated that delivery of LFn-NADase prevented intracellular trafficking of NADase-deficient GAS to lysosomes. We conclude that NADase mediates cytotoxicity and promotes intracellular survival of GAS in host cells.

Invasive infections due to group A Streptococcus (S. pyogenes or GAS) have become more frequent since the 1980s due, in part, to the emergence and global spread of closely related strains of the M1T1 serotype. A feature of this clonal group is the production of a secreted enzyme, NAD⁺-glycohydrolase (NADase), which has been suggested to contribute to GAS virulence by intoxication of host cells. For NADase to exert its toxic effects, it must be translocated into the host cell by a second GAS protein, streptolysin O (SLO). SLO is a pore-forming toxin that damages cell membranes in addition to its role in translocating NADase. In order to distinguish effects of NADase on host cell biology from those of SLO, we used components of anthrax toxin to deliver NADase to human throat epithelial cells, independently of SLO. Introduction of NADase into GAS-infected cells increased the intracellular survival of GAS lacking NADase or SLO, and the increase in bacterial survival correlated with inhibition of intracellular trafficking of GAS to lysosomes that mediate bacterial killing. The results support an important role for NADase in enhancing GAS survival in human epithelial cells, a phenomenon that may contribute to GAS colonization and disease.

Fibroblast heterogeneity and its implications for engineering organotypic skin models in vitro

Advances in cell culture methods, multidisciplinary research, clinical need to replace lost skin tissues and regulatory need to replace animal models with alternative test methods has led to development of three dimensional models of human skin. In general, these in vitro models of skin consist of keratinocytes cultured over fibroblast-populated dermal matrices. Accumulating evidences indicate that mesenchyme-derived signals are essential for epidermal morphogenesis, homeostasis and differentiation. Various studies show that fibroblasts isolated from different tissues in the body are dynamic in nature and are morphologically and functionally heterogeneous subpopulations. Further, these differences seem to be dictated by the local biological and physical microenvironment the fibroblasts reside resulting in "positional identity or memory". Furthermore, the heterogeneity among the fibroblasts play a critical role in scarless wound healing and complete restoration of native tissue architecture in fetus and oral mucosa; and excessive scar formation in diseased states like keloids and hypertrophic scars. In this review, we summarize current concepts about the heterogeneity among fibroblasts and their role in various wound healing environments. Further, we contemplate how the insights on fibroblast heterogeneity could be applied for the development of next generation organotypic skin models.

Retinoid receptors in human esophageal squamous cell carcinoma: retinoid X receptor as a potent prognostic factor

Retinoids regulate cell proliferation and differentiation in normal and neoplastic tissue. These effects are mainly mediated by two types of nuclear retinoid receptors, retinoic acid receptors (RAR) and retinoid X receptors (RXR). RXR have been demonstrated to play important roles in esophageal carcinoma, but the expression of RXRβ and RXRγ has not been examined in esophagus. Therefore, we examined the immunoreactivity of all subtypes of RAR and RXR in 53 non-neoplastic esophageal epithelium and 74 esophageal squamous cell carcinoma tissues. In non-neoplastic epithelium RARβ immunoreactivity was marked in the basal layer and weak in the suprabasal layer, but immunoreactivity of other retinoid receptors was detected in both of layers. In addition, the status of RARβ and RXRβ immunoreactivity inversely correlated with that of lymph node metastasis (P= 0.0477 and P= 0.0034, respectively); decreased RXRβ immunoreactivity of carcinoma cells was positively associated with adverse clinical outcome of the patients (P= 0.0187). These findings all indicate the important roles of retinoid receptors, especially, RXR in the esophageal squamous cell carcinoma.

Streptolysin O inhibits clathrin-dependent internalization of group A Streptococcus

Group A Streptococcus (GAS) can be internalized by epithelial cells, including keratinocytes from human skin or pharyngeal epithelium. Internalization of GAS by epithelial cells has been postulated both to play a role in host defense and to provide a sanctuary site for GAS survival. The cholesterol-binding cytolysin streptolysin O (SLO) appears to enhance virulence in part by inhibiting GAS internalization by human keratinocytes and by disrupting the lysosomal degradation of internalized GAS. We now report that low-level production of SLO by an inducible expression system reduced GAS internalization by keratinocytes. Induced SLO expression also prevented lysosomal colocalization with intracellular bacteria and acidification of GAS-containing vacuoles. Exogenous recombinant SLO mimicked the inhibitory effect of SLO secretion on GAS entry but not that on colocalization with the lysosomal marker LAMP-1, implying that disruption of lysosomal degradation requires intracellular secretion of SLO. The internalization of SLO-negative GAS was blocked by the depletion of host cell cholesterol and by the inhibition or knocking down of the expression of clathrin or dynamin. SLO also inhibited the cellular uptake of other cargos that are internalized by clathrin-mediated uptake or by macropinocytosis. We conclude that SLO interferes with the internalization of GAS through local perturbation of the keratinocyte cell membrane and disruption of a clathrin-dependent uptake pathway.

The role of retinoid deficiency and estrogens as cofactors in cervical cancer

Persistent infection with high-risk human papillomaviruses (HR-HPVs) is involved in cervical cancer (CC), a major cause of cancer mortality worldwide. Infection occurs primarily at the transformation zone (TZ), the most estrogen- and retinoid-sensitive region of the cervix. Development of CC affects a small percentage of HR-HPV-infected women and often takes decades after infection, suggesting that HR-HPV is a necessary but not sufficient cause of CC. Thus, other cofactors are necessary for progression from cervical HR-HPV infection to cancer such as long-term use of hormonal contraceptives, multiparity, smoking, as well as micronutrient depletion and in particular retinoid deficiency, which alters epithelial differentiation, cellular growth and apoptosis of malignant cells. Therefore, early detection of HR-HPV and management of precancerous lesions together with a profound understanding of additional risk factors could be a strategy to avoid this disease. In this review we focus on the synergic effect of estrogens, retinoid deficiency and HR-HPVs in the development of CC. These risk factors may act in concert to induce neoplastic transformation in squamous epithelium of the cervix, setting the stage for secondary genetic or epigenetic events leading to cervical cancer.

Retinoid-responsive transcriptional changes in epidermal keratinocytes

Retinoids (RA) have been used as therapeutic agents for numerous skin diseases, from psoriasis to acne and wrinkles. While RA is known to inhibit keratinocyte differentiation, the molecular effects of RA in epidermis have not been comprehensively defined. To identify the transcriptional targets of RA in primary human epidermal keratinocytes, we compared the transcriptional profiles of cells grown in the presence or absence of all-trans retinoic acid for 1, 4, 24, 48, and 72 h, using large DNA microarrays. As expected, RA suppresses the protein markers of cornification; however the genes responsible for biosynthesis of epidermal lipids, long-chain fatty acids, cholesterol, and sphingolipids, are also suppressed. Importantly, the pathways of RA synthesis, esterification and metabolism are activated by RA; therefore, RA regulates its own bioavailability. Unexpectedly, RA regulates many genes associated with the cell cycle and programmed cell death. This led us to reveal novel effects of RA on keratinocyte proliferation and apoptosis. The response to RA is very fast: 315 genes were regulated already after 1 h. More than one-third of RA-regulated genes function in signal transduction and regulation of transcription. Using in silico analysis, we identified a set of over-represented transcription factor binding sites in the RA-regulated genes. Many psoriasis-related genes are regulated by RA, some induced, others suppressed. These results comprehensively document the transcriptional changes caused by RA in keratinocytes, add new insights into the molecular mechanism influenced by RA in the epidermis and demonstrate the hypothesis-generating power of DNA microarray analysis.

Enhancement of streptolysin O activity and intrinsic cytotoxic effects of the group A streptococcal toxin, NAD-glycohydrolase

Streptolysin O (SLO) is a cholesterol-dependent cytolysin produced by the important human pathogen, group A Streptococcus (Streptococcus pyogenes or GAS). In addition to its cytolytic activity, SLO mediates the translocation of GAS NAD-glycohydrolase (NADase) into human epithelial cells in vitro. Production of both NADase and SLO is associated with augmented host cell injury beyond that produced by SLO alone, but the mechanism of enhanced cytotoxicity is not known. We have now shown that expression of NADase together with SLO dramatically enhanced the lytic activity of GAS culture supernatants for erythrocytes but had no effect on SLO-mediated poration of synthetic cholesterol-rich liposomes. This result revealed a previously unknown contribution of NADase to the cytolytic activity associated with GAS production of SLO. Purified recombinant SLO bound NADase in vitro, supporting a specific, physical interaction of the two proteins. Exposure of human keratinocytes to wild-type GAS, but not to a NADase-deficient mutant strain, resulted in profound depletion of cellular NAD+ and ATP. Furthermore, expression of recombinant GAS NADase in yeast, in the absence of SLO, induced growth arrest, depletion of NAD+ and ATP, and cell death. These findings have provided evidence that the augmentation of SLO-mediated cytotoxicity by NADase is a consequence of depletion of host cell energy stores through the enzymatic action of NADase. Together, the results have provided mechanistic insight into the cytotoxic effects of a unique bipartite bacterial toxin.

Role of retinoic acid receptors in squamous-cell carcinoma in human esophagus

Background

Worldwide, cancer in the esophagus ranks among the 10 most common cancers. Alterations of retinoic acid receptors (e.g. RARα, β, γ, and RXRα, β, γ) expression is considered to play an important role in development of squamous-cell carcinoma (SCC), which is the most common esophageal cancer. Alcohol consumption and smoking, which can alter retinoic acid receptor levels, have been identified as key risk factors in the development of carcinoma in the aero-digestive tract. Therefore, the aim of the present study was to evaluate protein levels of retinoic acid receptors (i.e. RARα, β, γ, and RXRβ) in esophageal SCC and surrounding normal tissue of patients with untreated SCC and controls.

Methods

All study participants completed a questionnaire concerning smoking and alcohol drinking habits as well as anthropometrical parameters. Protein levels of RARα, β, γ, and RXRβ were determined by Western Blot in normal esophageal tissue and tissue obtained from SCC of 21 patients with newly diagnosed esophageal SCC and normal esophageal tissue of 10 controls.

Results

Protein levels of RARγ were significantly lower by ~68% in SCC compared to normal surrounding tissue in patients with SCC that smoked and/or consumed elevated amounts of alcohol. Furthermore, RARα protein levels were significantly lower (~- 45%) in SCC in comparison to normal esophageal mucosa in patients with elevated alcohol intake. When comparing protein levels of retinoic acid receptors between normal tissue of patients with SCC and controls, RARγ protein levels were found to be significantly higher (~2.7-fold) in normal esophageal tissue of SCC patients than in esophageal tissue obtained from controls. No differences were found for RARα, β, and RXRβ protein levels between normal esophageal tissue of patients and that of controls.

Conclusion

In conclusion, results of the present study suggest that alterations of retinoic acid receptors protein may contribute in the development of SCC in esophagus and that in some patients life style (e.g. smoking and alcohol consumption) may be a critical component in the alteration of retinoic acid receptor levels in esophagus.

Extracellular group A Streptococcus induces keratinocyte apoptosis by dysregulating calcium signalling

Group A Streptococcus (GAS) colonizes the oropharynx and damaged skin. To cause local infection or severe invasive syndromes the bacteria must gain access into deeper tissues. Host cell death may facilitate this process. GAS internalization has been identified to induce apoptosis. We now report an alternate mechanism of GAS-mediated apoptosis of primary human keratinocytes, initiated by extracellular GAS and involving dysregulation of intracellular calcium to produce endoplasmic reticulum stress. Two bacterial virulence factors are required for effective induction of apoptosis by extracellular GAS: (i) hyaluronic acid capsule that inhibits bacterial internalization and (ii) secreted cytolysin, streptolysin O (SLO), that forms transmembrane pores that permit extracellular calcium influx into the cytosol. Induction of keratinocyte apoptosis by wild-type GAS was accompanied by cell detachment and loss of epithelial integrity, a phenomenon not observed with GAS deficient in capsule or SLO. We propose that cell signalling initiated by extracellular GAS compromises the epithelial barrier by inducing premature keratinocyte differentiation and apoptosis, thereby facilitating GAS invasion of deeper tissues.

Hyperacetylation enhances the growth-inhibitory effect of all-trans retinoic acid by the restoration of retinoic acid receptor beta expression in head and neck squamous carcinoma (HNSCC) cells

The chemotherapeutic effects of all-trans-retinoic acid (atRA) are mediated by the retinoic acid receptor beta (RARbeta), but RARbeta expression is reduced in a number of head and neck carcinoma (HNSCC) cells which causes resistance to RA treatment in half the patients with HNSCC. The possible mechanism for the reduced RARbeta expression has been suggested as the methylation of the CpG islands adjacent to the RA response elements (RARE) in the RARbeta promoter and the loss of histone acetylation. The suppressed RARbeta expression can be reactivated by a demethylating agent (5-aza-2'-deoxycytidine, 5-AzaC) or a histone deacetylase inhibitor (trichostatin A, TSA). Therefore, we sought to determine if the restoration of RARbeta activity, or a combination of these drugs, could restore the sensitivity to RA in RARbeta-negative HNSCC cells with an epigenetically methylated RARbeta promoter region. SqCC/Y1 cells resistant to atRA showed methylated and unmethylated forms in the RARbeta promoter region. RARbeta expression of these cells was restored by 5-AzaC or TSA treatment. Also, treatment with TSA and atRA combined synergistically increased the growth-inhibitory effect and highly induced the transcriptional activation of the RARbeta promoter compared to atRA treatment in HNSCC cells. Additionally, TSA alone and the combination 5-AzaC and TSA increased lysine-9 (Lys-9) acetylation and Lys-4 methylation of the first exon at the RARbeta gene, while decreasing the methylation of Lys-9 in the HNSCC cells.

Cytolysin-dependent evasion of lysosomal killing

Local host defenses limit proliferation and systemic spread of pathogenic bacteria from sites of mucosal colonization. For pathogens such as streptococci that fail to grow intracellularly, internalization and killing by epithelial cells contribute to the control of bacterial growth and dissemination. Here, we show that group A Streptococcus (GAS), the agent of streptococcal sore throat and invasive soft tissue infections, evades internalization and intracellular killing by pharyngeal epithelial cells. Production of the cholesterol-binding cytotoxin streptolysin O (SLO) prevented internalization of GAS into lysosomes. In striking contrast, GAS rendered defective in production of SLO were internalized directly or rapidly transported into lysosomes, where they were killed by a pH-dependent mechanism. Because SLO is the prototype of cholesterol-dependent cytolysins produced by many Gram-positive bacteria, cytolysin-mediated evasion of lysosomal killing may be a general mechanism to protect such pathogens from clearance by host epithelial cells.

Acyclic retinoid activates retinoic acid receptor β and induces transcriptional activation of p21CIP1 in HepG2 human hepatoma cells

Acyclic retinoid (ACR), a novel synthetic retinoid, has recently been demonstrated by us to inhibit the in vitro growth of human hepatoma cells, and this effect was associated with decreased expression of cell cycle-related molecules. These results, taken together with previous in vitro and clinical studies with ACR, suggest that this agent may be useful in the chemoprevention and therapy of hepatoma and possibly other human malignancies. In the present study, we further examined the molecular effects of ACR on the HepG2 human hepatoma cell line, focusing on the expression of nuclear retinoid receptors and the cell cycle inhibitor protein p21CIP1. Reverse transcription-PCR assays and Western blot analyses indicated that these cells express retinoic acid receptors (RARs) α, β, and γ, retinoid X receptors (RXRs) α and β, and peroxisome proliferator-activated receptors (PPAR) γ mRNA. Treatment with ACR caused a rapid induction within 3 h of RARβ mRNA and the related protein, but there was no significant change in the levels of the mRNA or proteins for RARs α and γ, RXRs α and β, and PPARγ. There was also a rapid increase in p21CIP1 mRNA and protein in HepG2 cells treated with ACR, and this induction occurred via a p53-independent mechanism. In transient transfection reporter assays, we cotransfected the retinoic acid response element-chloramphenicol acetyltransferase (CAT) reporter gene into HepG2 cells together with a RARβ expression vector. RARβ expression markedly stimulated CAT activity (up to about 4-fold) after the addition of ACR. However, CAT activity in the presence of ACR was only about 2-fold higher than that in the absence of ACR, when cells were cotransfected with RARs α and γ or RXRα. These findings suggest that the growth inhibitory effects of ACR are mediated at least in part through RARβ and that both RARβ and p21CIP1 play critical roles in the molecular mechanisms of growth inhibition induced by ACR.

Retinoic acid receptors and cancer

Retinoids have been shown to inhibit the growth of many human tumor cells. Although the exact molecular mechanism of retinoid-mediated growth suppression remains known, the importance of the retinoic acid receptors (RARs) and retinoid X receptors (RXRs) has been in established a number of tumor cell models. We wanted to determine if modulation of RAR/RXR function would alter the retinoid sensitivity of oral squamous carcinoma cells (SCCs). Growth of SCCs was significantly suppressed by treatment with either all-trans retinoic acid (RA) or the synthetic, conformationally restricted RAR-gamma-selective retinoids SR 11254 and SR 11389. In contrast, stable oral SCC clones that constitutively overexpressed the mouse dominant negative mutant, RAR-beta (R269Q), were shown to exhibit reduced RAR/RXR transcriptional transactivation function and reduced sensitivity to growth inhibition by RA, SR 11254 and SR 11389. Likewise, the RAR-gamma antagonist SR 11253 was found to block the ability of SR 11254 and SR 11389 to inhibit SCC growth. These results indicate that modulation of RAR function through the use of either an RAR-gamma-selective antagonist or a pan-RAR dominant negative mutant significantly alters the growth inhibitory response of oral SCCs to retinoids.

NAD+-glycohydrolase acts as an intracellular toxin to enhance the extracellular survival of group A streptococci

Group A streptococci (GAS) produce several secreted products that are thought to enhance pathogenicity by facilitating spread of the organisms through host tissues. Two such products, streptolysin O (SLO) and NAD+-glycohydrolase, appear to be functionally linked, in that SLO is required for transfer of NAD+-glycohydrolase into epithelial cells. However, the effects of NAD+-glycohydrolase on host cells are largely unexplored. We now report that SLO-mediated delivery of NAD+-glycohydrolase to the cytoplasm of human keratinocytes results in major changes in host cell biology that enhance GAS pathogenicity. We derived isogenic mutant strains deficient in the expression of SLO, NAD+-glycohydrolase or both proteins in the background of a virulent, M-type 3 strain of GAS. All three mutant strains were internalized by human keratinocytes more rapidly and in higher numbers than were organisms from the wild-type strain. Association of the mutant strains with keratinocytes also resulted in reduced cytotoxicity and reduced keratinocyte apoptosis compared with wild-type GAS. These results support a model in which NAD+-glycohydrolase contributes to GAS pathogenesis by modulating host cell signalling pathways to inhibit GAS internalization, to augment SLO-mediated cytotoxicity and to induce keratinocyte apoptosis. We conclude that NAD+-glycohydrolase is a novel type of bacterial toxin that acts intracellularly in the infected host to enhance the survival and proliferation of an extracellular pathogen.

Group A Streptococcus tissue invasion by CD44-mediated cell signalling

Streptococcus pyogenes (also known as group A Streptococcus, GAS), the agent of streptococcal sore throat and invasive soft-tissue infections, attaches to human pharyngeal or skin epithelial cells through specific recognition of its hyaluronic acid capsular polysaccharide by the hyaluronic-acid-binding protein CD44 (refs 1, 2). Because ligation of CD44 by hyaluronic acid can induce epithelial cell movement on extracellular matrix, we investigated whether molecular mimicry by the GAS hyaluronic acid capsule might induce similar cellular responses. Here we show that CD44-dependent GAS binding to polarized monolayers of human keratinocytes induced marked cytoskeletal rearrangements manifested by membrane ruffling and disruption of intercellular junctions. Transduction of the signal induced by GAS binding to CD44 on the keratinocyte surface involved Rac1 and the cytoskeleton linker protein ezrin, as well as tyrosine phosphorylation of cellular proteins. Studies of bacterial translocation in two models of human skin indicated that cell signalling triggered by interaction of the GAS capsule with CD44 opened intercellular junctions and promoted tissue penetration by GAS through a paracellular route. These results support a model of host cytoskeleton manipulation and tissue invasion by an extracellular bacterial pathogen.

Regulation of keratin expression by ultraviolet radiation: differential and specific effects of ultraviolet B and ultraviolet a exposure

Skin, the most superficial tissue of our body, is the first target of environmental stimuli, among which is solar ultraviolet radiation. Very little is known about the regulation of keratin gene expression by ultraviolet radiation, however, although (i) it is well established that ultraviolet exposure is involved in skin cancers and photoaging and (ii) keratins represent the major epidermal proteins. The aim of this study was to analyze the regulation of human keratin gene expression under ultraviolet B (290-320 nm) or ultraviolet A (320-400 nm) irradiation using a panel of constructs comprising different human keratin promoters cloned upstream of a chloramphenicol acetyl transferase reporter gene and transfected into normal epidermal keratinocytes. By this approach, we demonstrated that ultraviolet B upregulated the transcription of keratin 19 gene and to a lesser extent the keratin 6, keratin 5, and keratin 14 genes. The DNA sequence responsible for keratin 19 induction was localized between -130 and +1. In contrast to ultraviolet B, ultraviolet A irradiation induced only an increase in keratin 17, showing a differential gene regulation between these two ultraviolet ranges. The induction of keratin 19 was confirmed by studying the endogenous protein in keratinocytes in classical cultures as well as in skin reconstructed in vitro and normal human skin. These data show for the first time that keratin gene expression is regulated by ultraviolet radiation at the transcriptional level with a specificity regarding the ultraviolet domain of solar light.

Differential effects of chromosome 3p deletion on the expression of the putative tumor suppressor RAR beta and on retinoid resistance in human squamous carcinoma cells

Retinoids' effects on cell growth and differentiation are mediated by nuclear retinoid receptors, which are ligand-activated transcription enhancing factors. Because the expression of the retinoic acid receptor beta (RARbeta) gene, which is located on chromosome 3p24, is diminished in premalignant and malignant tissues it has been proposed that it acts as a tumor suppressor. To test the hypothesis that RARbeta loss leads to retinoid resistance, we studied several karyotyped head and neck squamous carcinoma (HNSCC) cell lines (UMSCC-17A, -17B, -22A, -22B, and -38) with deletion of one chromosome 3p arm. RARbeta mRNA was neither detected nor induced by retinoic acid in these cells, whereas it was expressed and induced by retinoic acid in two other HNSCC cell lines (1483 and 183) without 3p deletion. Methylation of the RARbeta gene promoter was detected in the 17B and 22B cells that failed to express RARbeta but no methylation was found in 183A cells that did express RARbeta mRNA. Responsiveness of HNSCC cells to several retinoids in assays of growth inhibition and colony formation, was rank ordered as: 22B>1483>38>183>17B. Additionally, retinoid response elements were transactivated in 22B more efficiently than in 17B cells. These results indicate that loss of RARbeta expression does not necessarily lead to loss of growth inhibition by retinoids or to a block of retinoid signaling.

Keratin 19 downregulation by oral squamous cell carcinoma lines increases invasive potential

Squamous cell carcinoma (SCC) of the head and neck is the sixth most frequent cancer worldwide. The survival rate is among the lowest of the major cancers and has not improved significantly in the past two decades. Extensive local invasion and regional lymph node metastasis are, in large part, responsible for the poor clinical outcome of these tumors. Keratin intermediate filaments are the most abundant cytoskeletal proteins in SCCs and regulate the migration of normal and transformed epithelial cells. Previous studies have shown that expression of the 40-kDa keratin K19 is dysregulated in SCCs arising from oral epithelium. Immunohistochemical experiments demonstrated that, while normal epithelium and dysplastic lesions expressed abundant K19 protein, invasive SCCs exhibited a patchy or negative staining pattern. We subsequently determined that K19 expression was consistently downregulated in seven SCC lines compared with normal epithelium. We therefore wanted to determine if K19 downregulation affected the invasive phenotype of these cells. We found that SCC lines which do not express K19 are significantly more invasive in vitro than those which retain expression of this gene. Stable expression of the K19 cDNA in K19 negative cell lines altered cell morphology and intercellular adhesiveness, and significantly decreased the number of cells able to migrate through a reconstituted basement membrane. Reduced invasiveness was not due to decreased metalloproteinase activity in the K19-expressing clones. We conclude that K19 overexpression in oral SCCs decreases their invasive potential by diminishing migratory capability.

Expression of retinoic acid receptor beta is associated with inhibition of keratinization in human head and neck squamous carcinoma cells

The preventive effects of retinoids on oral carcinogenesis may be related to their ability to modulate the growth and differentiation of human oral squamous epithelial cells. Nuclear retinoid receptors (RAR alpha, beta, and gamma, and RXR alpha, beta, and gamma) may mediate these effects by regulating gene transcription. The removal of serum from the growth medium of two head and neck squamous cell carcinoma lines 1483 and SqCC/Y1 resulted in a decrease in RAR beta mRNA level and concurrent increases in the expression of the keratin K1 and transglutaminase type I (TGase I), which are markers of differentiation of keratinizing squamous epithelial cells. All-trans-retinoic acid (tRA) or 13-cis-RA increased RAR beta and decreased K1 and TGase I mRNA levels in serum-free medium. Transcriptional activation of reporter genes by means of retinoid response elements (RARE and RXRE) indicated that the RXR-RAR pathway predominates over the RXR homodimer pathway in the 1483 cells. Among several synthetic retinoids with preference for binding to specific nuclear retinoid receptors, those that induced RAR beta also suppressed K1. The inverse association between RAR beta expression and K1 and TGase I levels implicates this receptor in suppression of keratinization in oral epithelial cells.

Repression of transactivation of the retinoic acid receptor beta2 promoter in human breast cancer cells

In previous studies, we have shown that the expression of retinoic acid receptor beta2 (RARbeta2) is altered in certain breast cancer cell lines. To investigate the mechanism responsible for this change, we studied in detail the RARbeta2 promoter in cell lines which demonstrated altered expression and compared these results to cell lines in which RARbeta2 was expressed normally. Direct DNA sequencing failed to identify alterations in the sequences of the known response elements in the cell lines manifesting altered expression patterns. By contrast, electrophoretic mobility shift studies of the proteins binding to these response elements demonstrated striking differences in the cells in which expression was altered, when compared to patterns seen in normal cells. Moreover, transient transfection studies using constructs of the RARbeta2 promoter demonstrated an absence of transactivation in the lines in which the expression of this gene was altered. These data suggest that the mechanism responsible for loss of induction of RARbeta2 in breast tumor cells is, at least in part, transcriptional repression.

Expression of a stable articular cartilage phenotype without evidence of hypertrophy by adult human articular chondrocytes in vitro

Chondrocytes that were isolated from adult human articular cartilage changed phenotype during monolayer tissue culture, as characterized by a fibroblastic morphology and cellular proliferation. Increased proliferation was accompanied by downregulation of the cartilage-specific extracellular matrix proteoglycan, aggrecan, by cessation of type-II collagen expression, and by upregulation of type-I collagen and versican. This phenomenon observed in monolayer was reversible after the transfer of cells to a suspension culture system. The transfer of chondrocytes to suspension culture in alginate beads resulted in the rapid upregulation of aggrecan and type-II collagen and the downregulation of expression of versican and type-I collagen. Type-X collagen and osteopontin, markers of chondrocyte hypertrophy and commitment to endochondral ossification, were not expressed by adult articular chondrocytes cultured in alginate, even after 5 months. In contrast, type-X collagen was expressed within 2 weeks in a population of cells derived from a fetal growth plate. The inability of adult articular chondrocytes to express markers of chondrocyte hypertrophy has underscored the fundamental distinction between the differentiation pathways that lead to articular cartilage or to bone. Adult articular chondrocytes expressed only hyaline articular cartilage markers without evidence of hypertrophy.

Chicken keratin-19: cloning of cDNA and analysis of expression in the chicken embryonic gut

From many recent studies, it has been argued that keratins (cytokeratins) play important roles in the morphogenesis and differentiation of organ development. To learn the role of keratin in digestive tract development, a cDNA of the chicken homolog of keratin-19 (GK-19) was cloned and its expression pattern was analyzed in the digestive tract of chicken embryos. The GK-19 full-length sequence was approximately 1.6 kb and showed more than 80% similarity to human and mouse keratin-19. The result of in situ hybridization with the proventriculus (glandular stomach) of different developmental stages showed that GK-19 expression disappeared specifically in the glandular epithelium from day 6 to day 9 of incubation. Furthermore, GK-19 was localized in the notochord, floor plate, anterior lobe of the pituitary gland and mesonephros. These results suggest the possibility that GK-19 may have multiple roles in organogenesis during embryogenesis.

Incomplete differentiation of fetal keratinocytes in the skin equivalent leads to the default pathway of apoptosis

The program of epidermal morphogenesis and differentiation changes dramatically during development of human fetal skin. Keratinocytes derived from fetal basal cells at early stages undergo only an incomplete keratinization and are eventually replaced by newly formed fetal keratinocytes that complete the terminal differentiation program and form the first stratum granulosum and stratum corneum. Once established, this program is reiterated throughout life, as the epidermis continually renews. To test the developmental potential of early fetal keratinocytes, we have cultured them in the physiologic skin equivalent (SE) system in the presence of varied retinoic acid (RA) concentrations and have compared them to neonatal keratinocytes cultured under the same conditions. The responses of fetal and neonatal SEs have been characterized by analysis of epidermal morphology, the presence and distribution of RA-responsive and differentiation-specific keratins and filaggrin, proliferation, and apoptosis. Our study shows that fetal basal keratinocytes already are programmed to form the granular layer and incomplete stratum corneum, even when isolated from a stage prior to formation of these layers. Fetal keratinocytes respond differently than neonatal keratinocytes to RA in terms of modulation of both epidermal morphology and expression of differentiation markers. Modulation of RA-responsive K1 and K19 occurs in both fetal SE and neonatal SE but the fetal keratinocyte responds at lower RA concentrations in the medium. In contrast, fetal keratinocytes appear to be less responsive than neonatal keratinocytes in terms of filaggrin expression and stratum corneum formation. These differences in the differentiation and RA response in vitro may be related to inherent stage-specific differences between fetal and neonatal keratinocytes in RA-signaling pathways including expression of the retinoic acid receptor, RARbeta. Furthermore, high rates of apoptosis in the fetal SE suggest that apoptosis is the default pathway that is taken in the absence of complete keratinocyte differentiation.

Hyaluronic acid capsule and the role of streptococcal entry into keratinocytes in invasive skin infection

It has been suggested that entry of pathogenic bacteria, including streptococci, into epithelial cells may represent an early stage of invasive infections. We found that poorly encapsulated wild-type strains and unencapsulated mutants of group A Streptococcus entered cultured human keratinocytes with high efficiency, while strains that produced large amounts of hyaluronic acid capsule did not, regardless of M-protein type or clinical source of the isolate. However, encapsulated streptococci produced extensive local necrosis and systemic infection in a mouse model of skin infection, while an isogenic acapsular strain did not. The results implicate the hyaluronic acid capsule as a virulence factor in soft tissue infection. Entry of poorly encapsulated group A Streptococcus into human epithelial cells does not appear to represent an initial step in invasive disease; rather, the capacity of encapsulated strains to avoid uptake by epithelial cells is associated with enhanced virulence in skin and soft tissue infection.

A limited role for retinoic acid and retinoic acid receptors RAR alpha and RAR beta in regulating keratin 19 expression and keratinization in oral and epidermal keratinocytes

Different types of stratified squamous epithelia-for example, the "orthokeratinized" epidermis, the "parakeratinized" gingiva, and the "nonkeratinized" oral lining mucosal epithelia-are formed by intrinsically distinct keratinocyte subtypes. These subtypes exhibit characteristic patterns of keratin protein expression in vivo and in culture. Keratin 19 is an informative subtype-specific marker because the basal cells of only nonkeratinizing epithelia express K19 in vivo and in culture. Epidermal keratinocytes normally do not express K19, but can be induced to do so in culture by retinoic acid (RA). Keratinocyte subtypes express the retinoic acid receptor (RAR) beta at levels roughly correlated with their level of K19 expression in culture and their potential for forming a nonkeratinized epithelium in vivo. We tested the hypothesis that the level of RAR beta expressed by a keratinocyte determines its K19 expression and its form of suprabasal differentiation. Normal human epidermal and gingival keratinocytes stably overexpressing either RAR beta or RAR alpha were generated by defective retroviral transduction. Overexpression of either receptor enhanced the RA inducibility of K19 in conventional culture, in that the proportion of the transductants becoming K19+ in response to RA was markedly increased compared with controls. The pattern of differentiation of the epithelium formed in organotypic culture, assessed by basal K19 and suprabasal K1, K4, and filaggrin expression, however, was unaltered by RAR overexpression. Thus, the susceptibility of keratinocytes to regulation of K19 expression by retinoids is conditional, and levels of neither RAR beta nor RAR alpha are limiting to the intrinsic mechanism that specifies alternate differentiation pathways for stratified squamous epithelia.

Cell, tissue and organ culture as in vitro models to study the biology of squamous cell carcinomas of the head and neck

In vitro models are currently being used to study head and neck squamous cell carcinoma (HNSCC). Several hundred HNSCC cell lines have been established by various investigators and used to study a broad spectrum of questions related to head and neck cancer. The head and neck model with respect to multistage carcinogenesis is now complete. Several techniques exist for the culture of normal epithelial cells from the upper aerodigestive tract (UADT). The biology of these UADT cells (oral cavity, oropharynx, hypopharynx and larynx) is being studied. Successful culture of premalignant lesions (dysplastic mucosa, leukoplakia, erythroplakia) has resulted in establishment of a limited number of premalignant cell lines and cell cultures. HPV infection of normal oral epithelial cells for immortalization (approximately premalignant cells) coupled with transformation with carcinogens (malignant cells) has established an experimental model for progression. Two in vivo models for oral carcinogenesis, the 7,12 dimethylbenz(a)anthracene-induced hamster cheek pouch model and the 4-nitroquinoline-N-oxide rat oral model, have been established in culture. Thus, multistage carcinogenesis models have been established from both human tissues and animal models and include cultures of normal, premalignant and malignant cells. Culture techniques for growing dissociated primary tumor cells for short term experimental analysis are being used. The culture of normal or tumor tissue as organ/explant cultures allows for the maintenance of normal cell-cell and cell-matrix interaction, but limits experimentation since these cultures cannot be propagated. Several three dimensional model systems are being used to obtain this histological complexity but allow for experimentation. The ability to culture normal, premalignant and malignant cells coupled with the use of a variety of culture techniques, should allow for the continued growth and experimentation in head and neck cancer research.

Retinoid actions and implications for prevention and therapy of oral cancer

Squamous cell carcinomas of the oral cavity can be preceded by clinically obvious premalignant changes, and they have a high rate of incidence of development of second primary tumors. Recent studies suggest retinoids not only for the treatment of oral eukoplakia, but also for the prevention of second primaries. Although retinoids are promising therapeutic agents, their therapeutic potential has been limited by their undesirable side-effects. A complete network of nuclear receptors has now been identified that mediate the action of retinoids and can interfere directly with cell proliferation signals by interacting with transcription factors. It has recently been shown that retinoids with receptor-selective activities can be obtained that are likely to have fewer side-effects because of their restricted biologic activities.

Retinoid receptors and keratinocytes

In 1987, a tremendous boost in our understanding of the action of dietary vitamin A occurred with the discovery and characterization of nuclear receptors for retinoic acid, the active form of the vitamin, in the laboratories of P. Chambon and R. Evans. They have shown that the nuclear receptors are ligand-activated transcription factors capable of specific gene regulation. Since that discovery, it has been determined that there are at least six retinoic acid receptors belonging to two families, RARs and RXRs, that they are differentially expressed in various mammalian tissues, and that they act as homo- and heterodimers interacting with other ligand-activated nuclear receptors. The domain structure of the receptors has been described, and their DNA-binding, ligand-binding, dimerization, and transcriptional activation regions characterized. Among the most important retinoid-regulated genes are the homeobox proteins, regulatory transcription factors which are responsible for body axis formation, patterning, limb formation, and other crucial processes during development. Retinoic acid and its receptors also regulate many differentiation markers which are particularly important in stratified epithelia, such as skin and oral epithelia. Our increased understanding led to improved therapy of a large number of skin disorders, ranging from acne to wrinkles and including epidermal and oral carcinomas.

Suppression of retinoic acid receptor-beta in premalignant oral lesions and its up-regulation by isotretinoin

BACKGROUND

Retinoids are effective in the treatment and prevention of certain human cancers. Most of their actions are thought to result from changes in gene expression mediated by nuclear retinoic acid receptors and retinoid X receptors. We conducted a study to determine whether the expression of these receptors was altered in premalignant oral lesions and, if so, whether their expression could be restored by treatment with isotretinoin.

METHODS

We performed in situ hybridization of retinoic acid receptors and retinoid X receptors using antisense riboprobes in specimens of oral mucosa from 7 normal subjects and specimens of premalignant oral lesions from 52 patients before treatment with isotretinoin and from 39 of the 52 patients after three months of treatment.

RESULTS

All the normal specimens expressed retinoic acid receptor-beta messenger RNA (mRNA). In contrast, retinoic acid receptor-beta mRNA was detected in only 21 of the 52 premalignant oral lesions (P = 0.003). Thirty-five of the 39 specimens available for evaluation after treatment expressed retinoic acid receptor-beta mRNA (P < 0.001). All normal and premalignant specimens expressed similar levels of mRNA for retinoic acid receptor-alpha and retinoic acid receptor-gamma and the three types of retinoic X receptors, alpha, beta, and gamma. The levels of retinoic acid receptor-beta mRNA increased in the specimens from 18 of the 22 patients who had responses to isotretinoin and in 8 of the 17 specimens from the patients without responses (P = 0.04).

CONCLUSIONS

The expression of retinoic acid receptor-beta mRNA is selectively lost in premalignant oral lesions and can be restored by treatment with isotretinoin. Restoration of the expression of retinoic acid receptor-beta mRNA is associated with a clinical response. Retinoic acid receptor-beta may have a role in mediating the response to retinoids and may be a useful intermediate biologic marker in trials of these agents for the prevention of oral carcinogenesis.

Retinoic acid regulates oral epithelial differentiation by two mechanisms

The effect of retinoic acid (RA) concentration on differentiation of oral keratinocytes and the influence of fibroblasts on RA-dependent regulation were investigated in a lifted culture system. Keratinocyte differentiation was assessed by morphology, immunohistochemistry and immunoblotting. Filaggrin/profilaggrin and keratin 1 were used as biochemical markers for cornified epithelium and keratins 13 and 19 as markers for noncornified epithelium. Cultured oral keratinocytes in RA-free conditions differentiated in a manner that closely resembled the differentiation pattern of gingival epithelia in vivo. Increasing RA concentrations altered the in vivo-like terminal differentiation of oral keratinocytes by disruption of organized stratification, inhibition of filaggrin/profilaggrin and K1 expression, and stimulation of K13 and K19 expression. Differentiation of keratinocytes from both cornified and noncornified regions of the oral cavity varied in a similar manner in response to added RA, with the exception of K19 expression. K19 was consistently expressed at higher levels in keratinocytes originating from noncornified epithelial as compared to those from cornified epithelia. The level of RA regulation was ultimately dependent on the type of fibroblasts underlying the epithelial cells. Homologous fibroblasts rendered the oral keratinocytes less sensitive to the effects of RA than skin fibroblasts. In addition, at a given RA concentration, fibroblasts from cornified oral mucosa potentiated keratinocyte expression of RA sensitive markers of keratinization as compared to the influence exerted by fibroblasts originating from noncornified oral mucosa. These results indicate that the RA regulation of oral epithelial differentiation is mediated by two separate mechanisms: a direct, RA concentration-dependent effect, and an indirect, fibroblast-mediated effect.

Retinoic acid regulates oral epithelial differentiation by two mechanisms

The effect of retinoic acid (RA) concentration on differentiation of oral keratinocytes and the influence of fibroblasts on RA-dependent regulation were investigated in a lifted culture system. Keratinocyte differentiation was assessed by morphology, immunohistochemistry and immunoblotting. Filaggrin/profilaggrin and keratin 1 were used as biochemical markers for cornified epithelium and keratins 13 and 19 as markers for noncornified epithelium. Cultured oral keratinocytes in RA-free conditions differentiated in a manner that closely resembled the differentiation pattern of gingival epithelia in vivo. Increasing RA concentrations altered the in vivo-like terminal differentiation of oral keratinocytes by disruption of organized stratification, inhibition of filaggrin/profilaggrin and K1 expression, and stimulation of K13 and K19 expression. Differentiation of keratinocytes from both cornified and noncornified regions of the oral cavity varied in a similar manner in response to added RA, with the exception of K19 expression. K19 was consistently expressed at higher levels in keratinocytes originating from noncornified epithelia as compared to those from cornified epithelia. The level of RA regulation was ultimately dependent on the type of fibroblasts underlying the epithelial cells. Homologous fibroblasts rendered the oral keratinocytes less sensitive to the effects of RA than skin fibroblasts. In addition, at a given RA concentration, fibroblasts from cornified oral mucosa potentiated keratinocyte expression of RA sensitive markers of keratinization as compared to the influence exerted by fibroblasts originating from noncornified oral mucosa. These results indicate that the RA regulation of oral epithelial differentiation is mediated by two separate mechanisms: a direct, RA concentration-dependent effect, and an indirect, fibroblast-mediated effect.

Biology and chemoprevention of head and neck cancer

Head and neck cancer remains a common cause of mortality and morbidity in the United States and throughout the world. In spite of advances in the management of patients with advanced disease, overall survival in this group remains poor. Furthermore, although cancer mortality is lower in patients with early-stage disease, treatment results in significant morbidity, and these patients also face the risk of developing a second primary tumor. Chemoprevention is an innovative approach to decrease overall cancer morbidity and mortality using substances that are capable of preventing cancer progression. Head and neck cancer is an excellent model for chemoprevention, as its biology is consistent with the two concepts important for the development of chemoprevention strategies: field cancerization and multistep carcinogenesis. Several classes of compounds have been evaluated in chemoprevention trials. The most frequently studied agents, the retinoids, were found frequently to induce remissions in patients with oral leukoplakia. Furthermore, retinoids prevented progression to malignancy in one randomized maintenance study. Other agents, including beta-carotene and vitamin E, have been found also to have activity in the management of oral leukoplakia. However, the clinical role of chemopreventive agents in reducing cancer mortality remains to be defined. Two studies, one in head and neck cancer and one in lung cancer, have shown the ability of retinoids to prevent the development of second primary tumors. Current large randomized trials are defining the effectiveness of these agents in reducing the mortality of aerodigestive tract tumors in individuals at high risk.

The establishment and continuous subculturing of normal human adult hepatocytes: expression of differentiated liver functions

The use of normal adult liver hepatocytes in cell culture for biochemical, toxicological and pharmacological studies has been greatly limited owing to the loss of replicative capacity and differentiated liver function. This is contrary to the ability of the liver to regenerate following injury in vivo. This suggests that liver "stem" or "transitional" hepatocytes exist that upon proper stimulus divide and differentiate into mature hepatocytes. In this study we report the establishment and culture of hepatocytes from normal human adult liver, which: (1) possess replicative capacity sufficient to subpassage 12-15 times (27-37 cumulative population doublings); (2) can be cryopreserved for subsequent use without loss of replicative capacity; and (3) upon differentiation in culture synthesize albumin and keratin 18 and metabolize benzo[a]pyrene. The ability of these cells to divide or express differentiated functions appears to be due to a number of cellular, biochemical and physical characteristics that are present during the primary establishment and subsequent growth phases of the cell cultures. Disassociation of cells from excess liver tissue was best achieved by combining the mechanical action of the Stomacher with very low amounts of proteolytic enzymes and EGTA. The cell lines appeared to grow best when established and subpassaged in an mALPHA medium supplemented with insulin, hydrocortisone, transferrin, epithelial growth factor and fetal bovine serum (prescreened for human hepatocyte cell growth). The seeding density and cell-cell contact in culture appeared to be important for both cell division and expression of liver function. When cells were seeded at a low density and subpassaged before confluency, the cells continued to divide. Albumin and keratin 18 synthesis occurred primarily in tightly packed cell clusters. When cells were seeded at a high density, near confluency, albumin and keratin 18 synthesis occurred uniformly in all of the cells of the culture and the culture metabolized benzo[a]pyrene to water-soluble metabolites, which covalently bound to cellular DNA. This appearance of liver functions was consistent with the "transition" of hepatocytes to a terminally differentiated state. Nonhepatic markers, i.e., alpha-fetoprotein, factor VIII and gamma-glutamyl transpeptidase activity were not expressed in cells cultured at either low or high density. Thus, the data presented here indicate that normal human adult liver hepatocytes, once established in culture, can be subpassaged to a high number of population doublings, cryopreserved for later use, and modulated to express differentiated liver functions.

Modulation of mammary carcinoma cell phenotype and keratin expression patterns by retinoic acid

Immunohistochemical and biochemical procedures were used to study the influence of retinoic acid (RA) on cellular expression and distribution of cytokeratins (CKs) in feline mammary carcinoma cells. These cells were grown in vitro as established cell lines (K248C and K266) and in vivo as xenografts in athymic mice. The results were compared with the distribution of CKs in normal feline mammary gland and in a series of invasive mammary carcinomas previously probed with a panel of monoclonal antibodies specific for individual CKs. Coexpression of CKs of both major mammary gland cell types (myoepithelial cells, MECs, CKs 5/14 positive, and luminal epithelial cells, LECs CKs8/18 positive) by K248C and K266 cells, suggested a stem cell-like character of both cell lines. RA increased CK19 expression in both cell lines and CK19 was also present in tumors developed in nude mice from both RA untreated (CK19 negative) and RA-treated (CK19 positive) K248C and K266 cells. In addition, RA had cell line specific effects as well. RA treatment induced differentiation of K248C cells to more mature LEC-like cells and this change was accompanied by the loss of the MEC keratins CKs 5/14. Under the same culture conditions however, RA treatment did not induce morphological changes in the K266 cell line and the expression of CKs 5/14 was not significantly reduced. These findings suggest that the modulation of CK19 and CKs 5/14 expression observed in mammary carcinoma cells upon RA treatment might be regulated through different pathways.

Retinoic acid mediates post-transcriptional regulation of keratin 19 mRNA levels

Stratified squamous epithelia have been shown to preferentially express a site-specific pattern of keratin intermediate filaments. Retinoic acid (RA) is known to modulate expression of the basal cell keratins K19 and K5. Expression of these genes is dependent on extracellular RA concentration. We have found that K19 mRNA levels increase over time in cultured keratinocytes exposed to elevated concentrations of RA. K5 mRNA levels decrease in response to RA in a similar fashion. The observed changes in K5 message are primarily the result of RA-induced alterations in gene transcription. However, the RA-mediated induction of K19 mRNA is not the result of increased transcription but is primarily due to enhanced mRNA stability. These results suggest that an RA-dependent post-transcriptional mechanism modulates K19 intermediate filament expression in stratified squamous epithelia.

Squamous cell differentiation markers in normal, premalignant, and malignant epithelium: effects of retinoids

Vitamin A and some of its analogs (retinoids) maintain normal differentiation of epithelial tissues by preventing aberrant squamous differentiation of cells in nonkeratinizing epithelia. They can also reverse squamous metaplasia, which develops in vivo during vitamin A deficiency. These effects are the result of the ability of retinoids to suppress the expression of genes associated with squamous differentiation (e.g., transglutaminase type I, loricrin, involucrin, filaggrin, and keratin K1). In addition, retinoids reverse keratinizing premalignant lesions in the oral cavity, and inhibit the growth and squamous differentiation of head and neck squamous cell carcinomas (HNSCCs) in vitro. Nuclear retinoic acid receptors, which function as DNA-binding, trans-acting, transcription-modulating factors, are considered to be the proximate mediators of the effects of retinoids on gene expression and may mediate the re-regulation of aberrant differentiation and growth of premalignant and some malignant cells, thereby suppressing the development of head and neck cancer.