Autocrine regulation of keratinocytes: the emerging role of heparin-binding, epidermal growth factor-related growth factors

Epidermal growth factor receptor in adult retinal neurons of rat, mouse, and human

During development, the epidermal growth factor receptor (EGFR) regulates proliferation and differentiation of many types of cells, including precursors of neurons and glia. In the adult, EGFR continues to drive the growth and differentiation of epithelial cells but is absent from glia in the CNS. However, the localization and functions of EGFR in adult neurons are not well defined. By using immunohistochemistry and Western blotting, we have identified EGFR and its ligands in adult retinal ganglion cells in the normal rat, mouse, and human retina. EGFR and its ligands were also present in certain other adult retinal neurons, for example, horizontal cells and amacrine cells, and had different distribution patterns among these species. In addition, we found that EGFR was expressed in the rat retinal ganglion cell line RGC-5. One of the EGFR ligands, EGF, caused a cell shape change and increased neurofilament phosphorylation in RGC-5 cells. The expression of EGFR in postmitotic, terminally differentiated adult retinal neurons suggests that EGFR has pleiotropic functions. In addition to the conventional mitogenic role in adult epithelial cells, EGFR must serve a different, nonmitogenic function in adult neurons. Our work localizes EGFR and its ligands in the adult retinas of several species as a step toward investigating the nonmitogenic functions of EGFR in adult neurons.

HB-EGF promotes epithelial cell migration in eyelid development

Heparin-binding EGF-like growth factor (HB-EGF) is a member of the EGF family of growth factors that binds to and activates the EGF receptor (EGFR)and ERBB4. Here, we show that HB-EGF-EGFR signaling is involved in eyelid development. HB-EGF expression is restricted to the tip of the leading edge of the migrating epithelium during eyelid closure in late gestation mouse embryos. Both HB-EGF null (HBdel/del) and secretion-deficient(HBuc/uc) mutant embryos exhibited delayed eyelid closure, owing to slower leading edge extension and reduced actin bundle formation in migrating epithelial cells. No changes in cell proliferation were observed in these embryos. In addition, activation of EGFR and ERK was decreased in HBdel/del eyelids. Crosses between HBdel/del mice and waved 2 mice, a hypomorphic EGFR mutant strain, indicate that HB-EGF and EGFR interact genetically in eyelid closure. Together with our data showing that embryos treated with an EGFR-specific kinase inhibitor phenocopy HBdel/del embryos, these data indicate that EGFR mediates HB-EGF-dependent eyelid closure. Finally, analysis of eyelid closure in TGFα-null mice and in HB-EGF and TGFα double null mice revealed that HB-EGF and TGFα contribute equally to and function synergistically in this process. These results indicate that soluble HB-EGF secreted from the tip of the leading edge activates the EGFR and ERK pathway, and that synergy with TGFα is required for leading edge extension in epithelial sheet migration during eyelid closure.

Preservation of phenotype in an organotypic cell culture model of a recessive keratinization defect of Norfolk terrier dogs

The purpose of this study is to reproduce in vitro a recessive keratinization defect of Norfolk terrier dogs characterized by a lack of keratin 10 (K10) production. Keratinocytes from skin biopsy samples of four normal dogs and two affected dogs were cultured organotypically with growth factor-supplemented media in order to stimulate cornification. The cultured epidermis from the normal dogs closely resembled the normal epidermis in vivo and cornified. The cultured epidermis from the affected dogs displayed many phenotypic alterations identified in skin biopsies from dogs with this heritable defect. Immunohistochemistry and immunoblotting showed a marked decrease in K10 from the cultures of the affected keratinocytes, compared to that in K10 from the cultures of the normal keratinocytes. Real-time reverse transcription polymerase chain reaction quantitation showed a 31-fold decrease in K10, a 1.75-fold increase in K1 and a 136-fold increase in K2e between the affected and the normal epidermis. Organotypic keratinocytes showed a 241-fold decrease in K10, a 31-fold decrease in K1 and a 1467-fold decrease in K2e between the affected and normal cultures. Although in vitro keratin expression did not precisely simulate in vivo, the morphology of the normal and the affected epidermis was largely preserved; thus, this culture system may provide an alternative to in vivo investigations for cutaneous research involving cornification.

Cell death-induced activation of epidermal growth factor receptor in keratinocytes: implications for restricting epidermal damage in dermatitis

Recent findings have implicated Fas/Fas ligand (FasL) in mediating the death of keratinocytes in spongiotic lesions. We asked whether dying keratinocytes could potentially initiate a protective response of the skin to limit the destruction of the epidermis in the spongiotic areas. In addition to apoptosis, treatment of keratinocyte cultures in vitro with FasL triggers a profound phoshorylation of the epidermal growth factor receptor (EGFR) and of its downstream effectors ERK and protein kinase B (PKB/Akt). Using a variety of inhibitors and blocking antibodies, we demonstrated that: (i) apoptosis is required for the generation of the signal(s) leading to the activation of EGFR, ERK, and Akt; (ii) the activation of EGFR, ERK, and Akt by FasL is indeed mediated by its bona fide receptor Fas; (iii) the activation of EGFR is essential for the subsequent activation of ERK and Akt; and (iv) apoptotic keratinocytes secrete soluble EGFR ligands (including amphiregulin) that are processed from membrane-bound proligand forms by metalloproteinase(s). Our findings demonstrate a potential mechanism for the restriction and repair of spongiotic damage in eczemas.

BP-1107 [{2-[4-(2,4-Dioxo-thiazolidin-5-ylmethyl)-phenoxy]-ethyl}-methyl-amide]: A Novel Synthetic Thiazolidinedione That Inhibits Epidermal Hyperplasia in Psoriatic Skin-Severe-Combined Immunodeficient Mouse Transplants after Topical Application

Recent studies have demonstrated that orally administered thiazolidinedione ligands of the peroxisome proliferator-activated receptor-gamma can ameliorate clinical features of psoriasis in humans. Thiazolidinediones also inhibit the proliferation of psoriatic keratinocytes in monolayer and organ culture, and at least one of these agents (troglitazone) inhibits epidermal hyperplasia of human psoriatic skin transplanted to severe-combined immunodeficient (SCID) mice. In the present study, we show that a novel, synthetic, thiazoladinedione derivative, BP-1107 ({2-[4-(2,4-dioxo-thiazolidin-5-ylmethyl)-phenoxy]-ethyl}-methyl-amide), is capable of inhibiting psoriatic hyperplasia in the SCID mouse transplant model after topical application. Like other thiazolidinediones, BP-1107 inhibits proliferation of rapidly growing keratinocytes in monolayer culture, but compared with these agents, the effective dose of BP-1107 needed to suppress keratinocyte proliferation is much lower. Concentrations of BP-1107 that effectively inhibit keratinocyte function have no detrimental effect on dermal fibroblasts. These data suggest that effective topical antipsoriatic therapy may be provided with this agent.

Biological roles of APP in the epidermis

The amyloid precursor protein (APP) was initially detected in cells of the central nervous system where it is considered to be involved in the pathogenesis of Alzheimer's disease. However, APP is also found in peripheral organs with exceptionally strong expression in the mammalian epidermis where it fulfils a variety of distinct biological roles. Full length APP appears to facilitate keratinocyte adhesion due to its ability to interact with the extracellular matrix. The C-terminus of APP also serves as adapter protein for binding the motor protein kinesin thereby mediating the centripetal transport of melanosomes in epidermal melanocytes. By the action of alpha-secretase sAPPalpha, the soluble N-terminal portion of APP, is released. sAPPalpha has been shown to be a potent epidermal growth factor thus stimulating proliferation and migration of keratinocytes as well as the exocytic release of melanin by melanocytes. The release of sAPPalpha can be almost completely blocked by inhibiting alpha-secretase with hydroxamic acid-based zinc metalloproteinase inhibitors. In hyperproliferative keratinocytes from psoriatic skin this inhibition results in normalized growth.

Amphiregulin and epidermal hyperplasia: amphiregulin is required to maintain the psoriatic phenotype of human skin grafts on severe combined immunodeficient mice

Overexpression of amphiregulin has been shown to induce psoriasiform changes in the skin of transgenic mice shortly after birth. Therefore, amphiregulin has been suggested as a target for anti-psoriatic therapy. To test this theory, a humanized monoclonal antibody capable of neutralizing human amphiregulin was examined for anti-proliferative effects in the human skin-severe combined immunodeficient (SCID) mouse transplant model. The anti-amphiregulin antibody reduced epidermal thickness of transplanted psoriatic skin and also inhibited the hyperplastic response that developed in nonpsoriatic skin after transplantation. The same antibody also suppressed keratinocyte proliferation in monolayer culture in a dose-dependent manner. Under the same conditions in which keratinocyte proliferation was inhibited, the antibody had little effect on proliferation of human dermal fibroblasts and no effect on type I procollagen production by these cells. Taken together, these data indicate an important role for amphiregulin in psoriatic hyperplasia and suggest that inhibition of amphiregulin activity could be an efficacious therapeutic strategy for psoriasis. These data also suggest that the hyperplastic response occurring in nonpsoriatic human skin on transplantation to the SCID mouse is mediated, in large part, by amphiregulin.

Possible relations between the polymorphisms of the cytokines IL-19, IL-20 and IL-24 and plaque-type psoriasis

The aim of present study was to elucidate the role of the interleukin (IL)-24 gene in predicting risk for plaque-type psoriasis and to describe the linkage disequilibrium (LD) pattern emerging from the genes of IL-19, IL-20 and IL-24. Genes encoding IL-19, IL-20 and IL-24 locate in the region q32 of chromosome 1. The association between the single-nucleotide polymorphisms (SNPs) or haplotypes of the IL-24 gene and the susceptibility of psoriasis was not found. However, a significant protective effect of the combined haplotype CAAAC of IL-20 and IL-24 genes against plaque-type psoriasis was established (OR 0.154). Protective effect against psoriasis was also observed with haplotype TGGGT (OR 0.591) and haplotype CGAGT (OR 0.457). Performing a comprehensive analysis using the data regarding SNPs of IL-24 gene together with the previously published data regarding IL-19 and IL-20 SNPs, we identified two haplotype blocks within the region q32 of chromosome 1. The main result of the present study is that while the IL-19/IL-20 extended haplotype CACCGGAA is a significant susceptibility factor for psoriasis (previous study), IL-20/IL-24 haplotypes CAAAC, TGGGT and CGAGT have a significant protective effect. Nevertheless, family-based studies are required to confirm the impact of IL-19, IL-20 and IL-24 genes in the genetic predisposition for psoriasis.

Antagonism of Epidermal Growth Factor Receptor Tyrosine Kinase Ameliorates the Psoriatic Phenotype in Organ-Cultured Skin

Psoriatic plaque skin incubated for eight days in organ culture in the presence of a potent epidermal growth factor (EGF) receptor tyrosine kinase (RTK) antagonist reverted to a more normal histological appearance, while untreated psoriatic plaque skin retained histological features associated with the psoriatic phenotype. In concomitant studies it was shown that the EGF-RTK antagonist had no significant effect on histological features of non-psoriatic skin and no effect on dermal function, i.e. elaboration of both type I procollagen and matrix metalloproteinase-1 (MMP-1; interstitial collagenase). When human epidermal keratinocytes were treated with the EGF-RTK antagonist in monolayer culture, growth inhibition was seen (ED(50) = approximately 0.06 microM). When dermal fibroblasts were exposed to the EGF-RTK antagonist in monolayer culture, proliferation, MMP-1 and type I procollagen production were essentially unaffected at concentrations which interfered with keratinocyte growth (up to 1 microM). The capacity of the EGF-RTK antagonist to modulate the histological features of psoriatic skin in organ culture under conditions in which normal skin architecture and dermal function are largely unaffected suggests a potential for anti-psoriatic therapy.

TGF-α Regulates TLR Expression and Function on Epidermal Keratinocytes

The expression of TLRs on epithelial cells provides a first line of defense against invading pathogens. We investigated the regulated expression and function of TLR5 and TLR9 on human keratinocytes, because we found by immunohistochemistry that these TLRs are expressed in distinct layers of the epidermis. We found that TGF-alpha, a growth and differentiation factor that is present during wound healing and in psoriasis, increased the expression of both TLR5 and TLR9 on keratinocytes. In addition, TGF-alpha regulated the function of TLR5 and TLR9, because activation with their respective ligands enhanced the production of IL-8 and human beta-defensins. These findings provide evidence that TGF-alpha up-regulates TLR expression and function, augmenting host defense mechanisms at epithelial surfaces.

Amphiregulin expression in human mast cells and its effect on the primary human lung fibroblasts

BACKGROUND

Amphiregulin is a member of the epidermal growth factor family and has been shown to stimulate the proliferation of human keratinocytes in an autocrine manner.

OBJECTIVE

The aim of the present study was to examine the expression change of growth factors, especially amphiregulin, in human mast cells induced by IgE cross-linking.

METHODS

Microarray analysis and RT-PCR were used to analyze the gene expression profile of human cord blood-derived mast cells (CBMCs) stimulated with IgE cross-linking. Protein secretion in the supernatants of CBMCs was measured by means of ELISA. Double-immunofluorescence staining was used to analyze the expression in the lung mast cells.

RESULTS

Of the 64 different growth factor genes analyzed, 5 were found to be substantially upregulated. Among them, amphiregulin mRNA was induced by 44-fold in CBMCs on activation through IgE cross-linking. Secretion of amphiregulin protein was evident in CBMCs 8 hours after stimulation. Amphiregulin was also expressed in human lung mast cells from patients with asthma, as demonstrated by means of double-immunofluorescence staining. Amphiregulin promoted the proliferation of the primary human lung fibroblasts, and amphiregulin-treated primary human lung fibroblasts showed a marked increase in the expression of c-fos , a proto-oncogene that facilitates or is required for the proliferation of a wide variety of cells.

CONCLUSION

Human CBMCs secreted amphiregulin on IgE cross-linking, and the amphiregulin induced proliferation of primary human lung fibroblasts. These data suggest that local release of amphiregulin by human mast cells could play an important role in lung fibrosis by promoting the proliferation of primary human lung fibroblasts.

The human hyaluronan synthase 2 gene is a primary retinoic acid and epidermal growth factor responding gene

Hyaluronan is an abundant and rapidly turned over matrix molecule between the vital cell layers of the epidermis and subject to large concentration changes associated with keratinocyte proliferation, migration, and differentiation induced by paracrine and endocrine factors like epidermal growth factor (EGF) and all-trans-retinoic acid (RA). We found that in REK cells EGF and all-trans-RA up-regulated hyaluronan synthase 2 (Has2) gene expression within 2 h 4-fold each and in HaCaT human immortal keratinocytes 8- and 33-fold, respectively. The first 10 kb of the human Has2 promoter were scanned in silico and in vitro for potential response elements of signal transducer and activator of transcription (STAT) or RA receptor (RAR) proteins. We identified a STAT-response element in the proximal promoter region and confirmed its functionality in response to EGF by chromatin immunoprecipitation (ChIP) assays. Direct in vitro binding of RARs to four RARE candidates within the Has2 promoter could not be observed at stringent gel shift conditions, but reporter gene assays demonstrated functionality of a complex of two of these RAREs located approximately 1200 bp upstream of the transcription start site. Moreover, ChIP assays using antibodies against nine nuclear proteins monitored all-trans-RA-dependent binding of RAR, retinoid X receptor, mediator protein, and RNA polymerase II and also histone 4 acetylation to a promoter region containing the complex RARE. Taken together, the human Has2 gene is a potent primary EGF and all-trans-RA responding gene with a complex regulation.

Suprabasal expression of human amphiregulin in the epidermis of transgenic mice induces a severe, early-onset, psoriasis-like skin pathology: expression of amphiregulin in the basal epidermis is also associated with synovitis

The expression of amphiregulin (AR) in the basal epidermis of transgenic mice [keratin 14 promoter AR gene (K14-ARGE)] has been previously shown to induce an early-onset and severe skin pathology, with many similarities to psoriasis. In this study, it is demonstrated that involucrin enhancer/promoter-dependent expression of human AR (INV-AR) in the suprabasal epidermis of transgenic mice also produces a cutaneous psoriasis-like phenotype. INV-AR mice possess a limited lifespan and scaling, papillomatous, erythematous skin with partial alopecia. INV-AR mouse histopathology also revealed epidermal hyperkeratosis, parakeratosis, acanthosis, and an exaggerated dermal vasculature. A dermal and epidermal infiltrate was also evident and consisted of both neutrophils and CD3(+) T lymphocytes. The histology of synovial joints in both the INV-AR mice and the K14-ARGE mice of our previous investigation was examined. The histologic examination revealed that 3-week-old INV-AR transgenic mice displayed normal knee joint histology, while 2- to 3-week-old K14-ARGE transgenic mice frequently displayed synovitis, as exemplified by the presence of a mixed leukocytic infiltration, increased vascularization, and enhanced deposition of fibrous matrix in the knee synovium. These results demonstrate that AR overexpression in both the basal and suprabasal epidermis of transgenic mice induces a phenotype that mimics cutaneous psoriasis, while basal AR expression is also associated with synovial inflammation, a precursor to the psoriasis-associated arthropathy, psoriatic arthritis. Collectively, the results implicate epidermal AR expression as a possible mediator of innate cutaneous immunity and epidermal proliferation and also as a potential trigger of both cutaneous psoriasis and psoriatic arthritis.

Stat3 links activated keratinocytes and immunocytes required for development of psoriasis in a novel transgenic mouse model

Here we report that epidermal keratinocytes in psoriatic lesions are characterized by activated Stat3. Transgenic mice with keratinocytes expressing a constitutively active Stat3 (K5.Stat3C mice) develop a skin phenotype either spontaneously, or in response to wounding, that closely resembles psoriasis. Keratinocytes from K5.Stat3C mice show upregulation of several molecules linked to the pathogenesis of psoriasis. In addition, the development of psoriatic lesions in K5.Stat3C mice requires cooperation between Stat3 activation in keratinocytes and activated T cells. Finally, abrogation of Stat3 function by a decoy oligonucleotide inhibits the onset and reverses established psoriatic lesions in K5.Stat3C mice. Thus, targeting Stat3 may be potentially therapeutic in the treatment of psoriasis.

A novel benzodiazepine selectively inhibits keratinocyte proliferation and reduces retinoid-induced epidermal hyperplasia in organ-cultured human skin

Bz-423 is a new benzodiazepine that has cytotoxic and cytostatic effects against a number of cell types in culture, and recent studies have shown efficacy in experimental lupus models in rodents. The present study demonstrates that treating human skin in organ culture with Bz-423 suppresses retinoid-induced epidermal hyperplasia. Bz-423 suppresses hyperplasia in organ culture at concentrations that also inhibit keratinocyte proliferation in monolayer culture but that are not cytotoxic for keratinocytes and do not inhibit fibroblast growth. Under conditions in which keratinocyte proliferation is inhibited, there is no measurable effect on epidermal growth factor receptor activation, but there is reduced signaling at the level of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase. Suppression of keratinocyte growth by Bz-423 is associated with generation of intracellular oxidants. However, antioxidant treatment reduces keratinocyte cytotoxicity that occurs at high concentrations of Bz-423, but it does not inhibit growth suppression. Together, these data suggest that Bz-423 has the potential to limit the untoward effects associated with topical retinoid treatment, and in addition, may have therapeutic effects against other forms of epidermal hyperplasia.

Normalized proliferation of normal and psoriatic keratinocytes by suppression of sAPPalpha-release

The soluble form of the beta-amyloid precursor protein (sAPPalpha) is known to function in the autocrine regulation of epidermal growth and repair. Here we show that its proteolytic release by alpha-secretase in normal human keratinocytes is susceptible to hydroxamic-acid-based zinc metalloproteinase inhibitors and suppressed by these inhibitors by 80%-90%. As various other growth factors participate in regulating epidermal growth we investigated whether the inhibitor-induced sAPPalpha-deficiency would affect keratinocyte proliferation. At optimal inhibitor concentrations the suppression of sAPPalpha-release was followed by a decline in proliferation by 50%-60%, indicating that sAPPalpha is a major growth factor that cannot be compensated for by other growth factors. This finding was the basis for the treatment of human lesional psoriatic keratinocytes with these inhibitors, which resulted in the normalization of their increased proliferation rates. The reversibility of these effects and the lack of toxicity underline the value of these inhibitors and suggest their therapeutic application in psoriatic skin diseases.

Cholesterol Depletion Upregulates Involucrin Expression in Epidermal Keratinocytes Through Activation of p38

Cholesterol has been recently suggested to regulate the early steps of keratinocyte differentiation through lipid rafts. In many cell types, depletion of cholesterol activates signaling proteins like epidermal growth factor receptor (EGFR), human epidermal growth factor receptor 2 (HER2), or extracellular signal-regulated kinase (ERK) known to affect cell differentiation. In this study, we explored the effects of cholesterol depletion on the phenotype of cultured keratinocytes, using a treatment with methyl-beta-cyclodextrin (MbetaCD) to extract cholesterol and a treatment with lovastatin to inhibit cholesterol neosynthesis. Analysis of the expression of differentiation marker genes in early differentiating confluent cultures reveals that cholesterol depletion induces downregulation of keratin 14 (K14) and keratin 10 (K10) and upregulation of involucrin. MbetaCD treatment induces phosphorylation of EGFR, HER2, and ERK, but not HER3. Inhibition of EGFR with PD153035 impairs the MbetaCD-induced phosphorylation of EGFR, HER2, and ERK, but does not impair the alteration of K14, K10, or involucrin gene expression, indicating that other signaling proteins regulate this phenomenon. p38 has been suggested to regulate the expression of involucrin during keratinocyte differentiation. We found that MbetaCD treatment induces a prolonged phosphorylation of p38 in general and p38alpha in particular. An inhibition of p38 with PD169316 impairs the upregulation of involucrin mRNAs by a treatment with MbetaCD, but not by a p38delta-activating TPA treatment, which might suggest that cholesterol depletion alters involucrin gene expression through activation of p38alpha/beta.

The leucine-rich repeat protein LRIG1 is a negative regulator of ErbB family receptor tyrosine kinases

The molecular mechanisms by which mammalian receptor tyrosine kinases are negatively regulated remain largely unexplored. Previous genetic and biochemical studies indicate that Kekkon-1, a transmembrane protein containing leucine-rich repeats and an immunoglobulin-like domain in its extracellular region, acts as a feedback negative regulator of epidermal growth factor (EGF) receptor signaling in Drosophila melanogaster development. Here we tested whether the related human LRIG1 (also called Lig-1) protein can act as a negative regulator of EGF receptor and its relatives, ErbB2, ErbB3, and ErbB4. We observed that in co-transfected 293T cells, LRIG1 forms a complex with each of the ErbB receptors independent of growth factor binding. We further observed that co-expression of LRIG1 with EGF receptor suppresses cellular receptor levels, shortens receptor half-life, and enhances ligand-stimulated receptor ubiquitination. Finally, we observed that co-expression of LRIG1 suppresses EGF-stimulated transformation of NIH3T3 fibroblasts and that the inducible expression of LRIG1 in PC3 prostate tumor cells suppresses EGF- and neuregulin-1-stimulated cell cycle progression. Our observations indicate that LRIG1 is a negative regulator of the ErbB family of receptor tyrosine kinases and suggest that LRIG1-mediated receptor ubiquitination and degradation may contribute to the suppression of ErbB receptor function.

Autocrine extracellular signal-regulated kinase (ERK) activation in normal human keratinocytes: metalloproteinase-mediated release of amphiregulin triggers signaling from ErbB1 to ERK

ErbB signaling through extracellular signal-regulated kinase (ERK) has been implicated in regulating the expression of ErbB ligands in hyperproliferative skin disorders and wound healing. Here, we characterize the process of autocrine ERK activation in cultured normal human keratinocytes (NHKs) subjected to growth factor (GF) deprivation. Basal ERK phosphorylation was lower after 48 h than after 24 h of GF deprivation, and lowest at 30-60 min after an additional medium change. ERK phosphorylation was markedly increased by low concentrations of epidermal growth factor (EGF) (0.2-1 ng/ml) that provoked only a limited increase in ErbB1 tyrosine phosphorylation and internalization. Basal ErbB tyrosine phosphorylation and ERK phosphorylation were inhibited by two different ErbB receptor tyrosine kinase inhibitors, by the ErbB1-specific neutralizing monoclonal antibody 225 IgG, by two different metalloproteinase inhibitors, and by neutralizing antibodies against amphiregulin (AR). In contrast, these responses were unaffected by neutralizing antibodies against other ErbB1 ligands or the ErbB2 inhibitors geldanamycin and AG825. The time course of autocrine ERK phosphorylation correlated with the appearance of soluble AR, and two different metalloproteinase inhibitors blocked AR release. These results define an amphiregulin- and ErbB1-dependent mechanism by which autocrine ERK activation is maintained in NHKs, even when ErbB1 autophosphorylation and internalization are limited.

Requirement of the epidermal growth factor receptor in renal epithelial cell proliferation and migration

We showed that renal proximal tubular cells (RPTC) can proliferate and migrate following plating and oxidant or mechanical injury in the absence of exogenous growth factors; however, the mechanisms of this response remain unclear. We examined whether epidermal growth factor receptor (EGFR) signaling is activated following plating and mechanical injury and mediates RPTC proliferation and migration. EGFR, Akt [a target of phosphoinositide-3-kinase (PI3K)], and ERK1/2 were activated after plating and mechanical injury, and their phosphorylation was further enhanced by addition of exogenous EGF. Inactivation of the EGFR with the selective inhibitor AG-1478 completely blocked phosphorylation of EGFR, Akt, and ERK1/2 and blocked cell proliferation and migration after plating and injury. Inhibition of PI3K with LY-294002 blocked Akt phosphorylation and proliferation, whereas U-0126 blocked ERK1/2 phosphorylation but had no effect on proliferation. Furthermore, p38 was phosphorylated following mechanical injury and the p38 inhibitor SB-203580 blocked p38 phosphorylation and cell migration. In contrast, neither PI3K nor ERK1/2 inhibition blocked cell migration. These results show that EGFR activation is required for RPTC proliferation and migration and that proliferation is mediated by PI3K, whereas migration is mediated by p38.

Azasugar-Based MMP/ADAM Inhibitors as Antipsoriatic Agents

As a part of synthetic studies on MMP (matrix metalloproteinase)/ADAM (a disintegrin and metalloproteinase) inhibitors, we have preliminarily communicated that azasugar-based compound 1a exhibited a potential inhibitory activity on some metalloprotease-catalyzed proteolytic reactions. To find promising candidates for the topical treatment of psoriasis, we investigated stability in aqueous solution of compound 1a and its derivative 1b and then optimized the P1' substuent (2-5). In the present study, we synthesized novel derivatives of compound 1a and evaluated their inhibitory activity toward MMP-1, -3, and -9, TACE, and HB-EGF shedding, from a viewpoint of versatility of azasugars as a functional scaffold. As a result, it was found that compound 1b demonstrated desirable inhibitory activity as an antipsoriatic agent, and some of the derivatives showed selective inhibitory activity. In addition, it was found that compound 1b exhibited a significant therapeutic effect on a mouse TPA-induced epidermal hyperplasia model. Therefore, compound 1b could become a promising candidate as a practical antipsoriatic agent.

Membrane-associated HB-EGF modulates HGF-induced cellular responses in MDCK cells

In MDCK cells, hepatocyte growth factor/scatter factor (HGF/SF) induces epithelial cell dissociation, scattering, migration, growth and formation of branched tubular structures. By contrast, these cells neither scatter nor form tubular structures in response to the epidermal growth factor (EGF) family of growth factors. Heparin-binding EGF-like growth factor (HB-EGF) is a member of the EGF family of growth factors and is synthesized as a membrane-associated precursor molecule (proHB-EGF). ProHB-EGF is proteolytically cleaved to release a soluble ligand (sHB-EGF) that activates the EGF receptor. Although recent studies suggest possible physiological functions, the role of proHB-EGF remains largely undefined. Using MDCK cells stably expressing proHB-EGF, a noncleavable deletion mutant of proHB-EGF or soluble HB-EGF, we show that epithelial cell functions differ depending on the form of HB-EGF being expressed. Expression of noncleavable membrane-anchored HB-EGF promoted cell-matrix and cell-cell interactions and decreased cell migration, HGF/SF-induced cell scattering and formation of tubular structures. By contrast, expression of soluble HB-EGF induced increased cell migration, decreased cell-matrix and cell-cell interactions and promoted the development of long unbranched tubular structures in response to HGF/SF. These findings suggest that HB-EGF can not only modulate HGF/SF-induced cellular responses in MDCK cells but also that membrane-bound HB-EGF and soluble HB-EGF give rise to distinctly different effects on cell-cell and cell-extracellular matrix interactions.

Rosiglitazone inhibits proliferation, motility, and matrix metalloproteinase production in keratinocytes

This study was undertaken to evaluate the effects of thiazolidinediones (TZD) on keratinocyte proliferation, motility, and matrix metalloproteinase (MMP) production. Rosiglitazone (a potent TZD) inhibited both proliferation and motility as well as elaboration of MMP-1 and MMP-9. Inhibition was obtained with keratinocytes in monolayer culture and human skin in organ culture. There were significant concentration-response differences in sensitivity of the three keratinocyte responses to treatment with rosiglitazone. In contrast to keratinocytes, dermal fibroblasts were resistant to the effects of rosiglitazone. Treatment of keratinocytes with rosiglitazone did not suppress epidermal growth factor receptor autophosphorylation, but inhibited signaling through the extracellular regulated kinase mitogen-activated protein kinase pathway without a concomitant effect on pathways that lead to c-jun activation. Pioglitazone, another TZD, also suppressed keratinocyte proliferation, although it was less effective than rosiglitazone. An experimental TZD (BP-1107) inhibited keratinocyte proliferation at a much lower concentration than either rosiglitazone or pioglitazone. Because enhanced keratinocyte motility and increased MMP production as well as increased keratinocyte proliferation are thought to contribute to the phenotype of psoriatic lesional skin, we propose that interference with these keratinocyte responses contributes to the previously reported antipsoriatic activity of TZD.

Solar ultraviolet radiation as a trigger of cell signal transduction

Ultraviolet light radiation in sunlight is known to cause major alterations in growth and differentiation patterns of exposed human tissues. The specific effects depend on the wavelengths and doses of the light, and the nature of the exposed tissue. Both growth inhibition and proliferation are observed, as well as inflammation and immune suppression. Whereas in the clinical setting, these responses may be beneficial, for example, in the treatment of psoriasis and atopic dermatitis, as an environmental toxicant, ultraviolet light can induce significant tissue damage. Thus, in the eye, ultraviolet light causes cataracts, while in the skin, it induces premature aging and the development of cancer. Although ultraviolet light can damage many tissue components including membrane phospholipids, proteins, and nucleic acids, it is now recognized that many of its cellular effects are due to alterations in growth factor- and cytokine-mediated signal transduction pathways leading to aberrant gene expression. It is generally thought that reactive oxygen intermediates are mediators of some of the damage induced by ultraviolet light. Generated when ultraviolet light is absorbed by endogenous photosensitizers in the presence of molecular oxygen, reactive oxygen intermediates and their metabolites induce damage by reacting with cellular electrophiles, some of which can directly initiate cell signaling processes. In an additional layer of complexity, ultraviolet light-damaged nucleic acids initiate signaling during the activation of repair processes. Thus, mechanisms by which solar ultraviolet radiation triggers cell signal transduction are multifactorial. The present review summarizes some of the mechanisms by which ultraviolet light alters signaling pathways as well as the genes important in the beneficial and toxic effects of ultraviolet light.

Amphiregulin overexpression results in rapidly growing keratinocytic tumors: an in vivo xenograft model of keratoacanthoma

Keratoacanthoma (KA) is a variant of cutaneous squamous cell carcinoma (SCC) known for rapid growth and potential for involution. Little is known about the basis for the rapid growth because of the dearth of model systems. We hypothesized that amphiregulin (AR), a keratinocyte autocrine growth factor, had a significant role. Using immunohistochemistry, we compared 21 KA, 6 conventional SCC, and 6 basal cell carcinomas (BCC) for AR expression. All KA were positive for AR, the majority with strong immunoreactivity. The SCC were positive (5 of 6), with generally weak staining; no BCC were positive. We developed laboratory model systems to study AR overexpression in keratinocytes and its role in the pathogenesis of KA. A retroviral transduction strategy was used to overexpress AR in the HaCaT keratinocyte-like cell line. The AR overexpressing cells (HaCaT-AR) displayed autonomous proliferation in serum-free media when compared with controls (HaCaT-NIE). To develop an in vivo model, xenografts of HaCaT-AR and HaCaT-NIE were grown on SCID mice. The HaCaT-NIE cells formed thin tumors resembling conventional SCC. The HaCaT-AR cells formed rapidly growing tumors with AR expression similar to KA. HaCaT-AR cells may represent a new system for the further evaluation of KA.

Epidermal homeostasis: the role of the growth hormone and insulin-like growth factor systems

GH and IGF-I and -II were first identified by their endocrine activity. Specifically, IGF-I was found to mediate the linear growth-promoting actions of GH. It is now evident that these two growth factor systems also exert widespread activity throughout the body and that their actions are not always interconnected. The literature highlights the importance of the GH and IGF systems in normal skin homeostasis, including dermal/epidermal cross-talk. GH activity, sometimes mediated via IGF-I, is primarily evident in the dermis, particularly affecting collagen synthesis. In contrast, IGF action is an important feature of the dermal and epidermal compartments, predominantly enhancing cell proliferation, survival, and migration. The locally expressed IGF binding proteins play significant and complex roles, primarily via modulation of IGF actions. Disturbances in GH and IGF signaling pathways are implicated in the pathophysiology of several skin perturbations, particularly those exhibiting epidermal hyperplasia (e.g., psoriasis, carcinomas). Additionally, many studies emphasize the potential use of both growth factors in the treatment of skin wounds; for example, burn patients. This overview concerns the role and mechanisms of action of the GH and IGF systems in skin and maintenance of epidermal integrity in both health and disease.

The invasive phenotype of Shigella flexneri directs a distinct gene expression pattern in the human intestinal epithelial cell line Caco-2

Invasion of the human colonic epithelium by Shigella flexneri causes inflammation that disrupts the intestinal barrier. Invaded intestinal epithelial cells are the major source of mediators recruiting the inflammatory infiltrate. To better characterize the global response of intestinal epithelial cells to Shigella invasion, Caco-2 cells were infected by an invasive isolate of S. flexneri 5a, and their transcriptome was analyzed by Affymetrix (Santa Clara, CA) microarrays (12,000 genes) and compared with these elicited by a non-invasive Shigella mutant and tumor necrosis factor (TNF)-alpha. The invasive and non-invasive strains enhanced transcription of a common pattern of 240 genes, among which genes encoding isoforms of cytochrome P-450 were induced. These genes were not induced by TNF-alpha. Conversely, both the invasive strain and TNF-alpha induced a common set of 18 genes, mainly encoding proinflammatory molecules. They also induced specific sets of genes. The transcriptome induced by the invasive strain was characterized by the induction of early genes (i.e. expressed within the first 45 min of invasion) and late genes (i.e. after 60 min of invasion) whose pattern was strongly biased toward stimulation of granulopoiesis, chemoattraction, activation, and adherence of polymorphonuclear leukocytes. When compared with a non-invasive Shigella and TNF-alpha, invasive Shigella induced a narrow transcriptome that seems to program infected epithelial cells to recruit a mucosal polymorphonuclear leukocyte to infiltrate. Dramatic increase in IL-8 gene transcription points to this chemokine as the major molecule orchestrating mucosal inflammation in shigellosis.

Epidermal growth factor as a biologic switch in hair growth cycle

The hair growth cycle consists of three stages known as the anagen (growing), catagen (involution), and telogen (resting) phases. This cyclical growth of hair is regulated by a diversity of growth factors. Although normal expression of both epidermal growth factor and its receptor (EGFR) in the outer root sheath is down-regulated with the completion of follicular growth, here we show that continuous expression of epidermal growth factor in hair follicles of transgenic mice arrested follicular development at the final stage of morphogenesis. Data from immunoprecipitation and immunoblotting showed that epidermal growth factor signals through EGFR/ErbB2 heterodimers in skin. Furthermore, topical application of tyrphostin AG1478 or AG825, specific inhibitors of EGFR and ErbB2, respectively, completely inhibited new hair growth in wild type mice but not in transgenic mice. When the transgenic mice were crossed with waved-2 mice, which possess a lower kinase activity of EGFR, the hair phenotype was rescued in the offspring. Taken together, these data suggest that EGFR signaling is indispensable for the initiation of hair growth. On the other hand, continuous expression of epidermal growth factor prevents entry into the catagen phase. We propose that epidermal growth factor functions as a biologic switch that is turned on and off in hair follicles at the beginning and end of the anagen phase of the hair cycle, guarding the entry to and exit from the anagen phase.

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

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

Oral erosive/ulcerative lichen planus: preliminary findings in an open trial of sulodexide compared with cyclosporine (ciclosporin) therapy

OBJECTIVE

To study the effect of the heparinoid sulodexide systemically, compared with topical cyclosporine (ciclosporin), on chronic oral erosive/ulcerative lichen planus.

STUDY DESIGN

An open nonrandomized trial was conducted in two groups of 10 Italian patients with lichen planus, with subjective assessment of pain and assessment of ulceration amelioration by nonblinded clinicians.

RESULTS

Comparable pain relief and amelioration of erosions/ulcers were seen in patients on sulodexide and in those on ciclosporin, but with faster healing in those on sulodexide.

CONCLUSIONS

Sulodexide appears to be as effective, and perhaps more effective, than topical ciclosporin in the therapy of oral lichen planus, and is less expensive, but full double-blind placebo-controlled studies are required.

Synthesis and structure-activity relationships of 5,6,7,8-tetrahydropyrido[3,4-b]pyrazine-based hydroxamic acids as HB-EGF shedding inhibitors

HB-EGF Shedding inhibitors have been expected to become effective medicines for skin diseases caused by the proliferation of keratinocytes. In order to discover novel HB-EGF shedding inhibitors and clarify their structure-activity relationships, 5,6,7,8-tetrahydronaphthylidine-based hydroxamic acid and 5,6,7,8-tetrahydropyrido[3,4-b]pyrazine-based hydroxamic acids have been synthesized. Among the synthesized compounds, the ethoxyethoxy derivative 3o and the methoxypropoxy derivative 3p exhibited much more potent HB-EGF shedding inhibitory activity than CGS 27023A. The structural modification of 5,6,7,8-tetrahydropyrido[3,4-b]pyrazine-based hydroxamic acids enabled us to establish the following structure-activity relationships; the existence of the hydroxamic acid, the sulfonamide, and the phenyl moieties are crucial for a potent HB-EGF shedding inhibitory activity, and the stereochemistry of the alpha carbon of hydroxamic acid is also important. In addition, from the comparison of their HB-EGF shedding inhibitory activities with their MMPs inhibitory activities, we found that the S1' pocket of the responsible enzyme for HB-EGF shedding is deep unlike that of MMP-1.

11. Allergic and immunologic diseases of the skin

Many skin diseases have an inflammatory or immune component, and anti-inflammatory drugs comprise a major portion of a dermatologist's therapeutic armamentarium. Although causes of most of these diseases remain obscure, mechanisms of lesion formation and explanations for symptoms are increasingly well documented. These developments, coupled with the expected availability of novel selective immunomodulatory agents, herald a new era for immunodermatology. Patients with psoriasis, allergic contact dermatitis, atopic dermatitis, urticaria, and autoantibody-mediated blistering diseases are among those who are likely to benefit from advances in the understanding of disease pathogenesis and the emergence of immunotherapeutics.

Inhibition of apoptosis by amphiregulin via an insulin-like growth factor-1 receptor-dependent pathway in non-small cell lung cancer cell lines

Several abnormalities in the insulin-like growth factor-1 (IGF1) and erbB receptors pathways stimulate the growth and survival of lung cancer cells, but their mechanisms of action and cooperation are poorly understood. In this report, we have identified a new mechanism of apoptosis inhibition by amphiregulin through an IGF1-dependent survival pathway in non-small cell lung cancer (NSCLC) cells: amphiregulin activates the IGF1 receptor that in turn induces the secretion of amphiregulin and IGF1. In the absence of serum, the NSCLC cell line H358 resists apoptosis and secretes factors protecting the NSCLC cell line H322 from serum deprivation apoptosis. IGF1 receptor inhibitor AG1024 as well as epidermal growth factor receptor inhibitors AG556 and ZD1839 restore apoptosis in H322 cells cultured in H358-conditioned medium. Accordingly, the anti-apoptotic activity of H358-conditioned medium is completely abolished after incubation with anti-amphiregulin neutralizing antibody and only partially with anti-IGF1 neutralizing antibody. H358-conditioned medium and amphiregulin induce IGF1 receptor phosphorylation in H322 cells, which is prevented by anti-amphiregulin neutralizing antibody but not by AG556 or ZD1839. H358 cells secrete a high level of amphiregulin that, in combination with IGF1, prevents serum deprivation apoptosis. Finally, IGF1 receptor inhibitor blocks amphiregulin and IGF1 release by H358 cells.

Dual role of protein kinase C on mitogen-activated protein kinase activation and human keratinocyte proliferation

Subconfluent normal human keratinocytes exhibit autonomous (autocrine growth factor driven) proliferation and express the specific markers for keratinocyte proliferation K#5 and K#14. In keratinocyte autocrine culture, the exogenously added epidermal growth factor (EGF) has no effect on cell proliferation and mitogen-activated protein kinase (MAPK) activity. PD98059 inhibits MAPK pathway and autocrine keratinocyte proliferation. Staurosporine and Gö6976 strongly inhibit autonomous keratinocyte proliferation. In contrast, Gö6983 (which does not inhibit PKC micro ) inhibits only 20% of autocrine keratinocyte proliferation. Staurosporine inhibits MAPK activity, whereas Gö6976 and Gö6983 strongly increase it. We have concluded that MAPK, PKC micro and probably PKCalpha take part in autocrine keratinocyte proliferation. The effect of Gö6976 and Gö6983 on MAPK activity could be explained by the inhibition of PKC-dependent MAPK-phosphatase expression. The effect of staurosporine could be explained by its paradoxical action (activation) on protein kinase C (PKC) in keratinocytes.

The cellular proliferative phase of the wound repair process

The proliferative--or new-tissue formation--phase of wound healing is complex. This article examines the changes that occur to cells during this stage and the effect on the extracellular matrix environment.

Physiological and retinoid-induced proliferations of epidermis basal keratinocytes are differently controlled

To investigate the roles of retinoic acid (RA) receptors (RARs) in the physiology of epidermis that does not express RAR beta, conditional spatio-temporally controlled somatic mutagenesis was used to selectively ablate RAR alpha in keratinocytes of RAR gamma-null mice. Keratinocyte proliferation was maintained in adult mouse epidermis lacking both RAR alpha and RAR gamma, as well as in RAR beta-null mice. All RAR-mediated signalling pathways are therefore dispensable in epidermis for homeostatic keratinocyte renewal. However, topical treatment of mouse skin with selective retinoids indicated that RXR/RAR gamma heterodimers, in which RXR transcriptional activity was subordinated to that of its RAR gamma partner, were required for retinoid-induced epidermal hyperplasia, whereas RXR homodimers and RXR/RAR alpha heterodimers were not involved. RA-induced keratinocyte proliferation was studied in mutant mice in which RXR alpha, RXR alpha and RAR alpha, RAR gamma, or RXR alpha and RAR gamma genes were specifically disrupted in either basal or suprabasal keratinocytes. We demonstrate that the topical retinoid signal is transduced by RXR alpha/RAR gamma heterodimers in suprabasal keratinocytes, which, in turn, stimulate proliferation of basal keratinocytes via a paracrine signal that may be heparin-binding EGF-like growth factor.

New type of metalloproteinase inhibitor: design and synthesis of new phosphonamide-based hydroxamic acids

A series of phosphonamide-based hydroxamate derivatives were synthesized, and the inhibitory activities were evaluated against various metalloproteinases in order to clarify its selectivity profile. Among the four diastereomeric isomers resulting from the chirality at the C-3 and P atoms, the compound with a (R,R)-configuration both at the C-3 position and the phosphorus atom was found to be potently active, while the other diastereomeric isomers were almost inactive. A number of (R,R)-compounds synthesized here exhibited broad spectrum activities with nanomolar K(i) values against MMP-1, -3, -9, and TACE and also showed nanomolar IC(50) values against HB-EGF shedding in a cell-based inhibition assay. The modeling study using X-ray structure of MMP-3 suggested the possible binding mode of the phosphonamide-based inhibitors.

New strategy for antedrug application: development of metalloproteinase inhibitors as antipsoriatic drugs

Phosphonamide-based inhibitors were synthesized and evaluated for the inhibitory activities against the shedding of epidermal growth factors, amphiregulin and heparin-binding EGF-like growth factor, that would participate in the development of psoriasis. All compounds exhibited excellent inhibitory activities for these EGF sheddings; however, they also inhibited matrix metalloproteinases (MMPs). To avoid adverse effects reported by the clinical development of MMP inhibitors, the antedrug concept was introduced. Among the phosphonamide inhibitors, the 2,2,2-trifluoroethyl ester 8d and 2,2-difluoroethyl ester 8c showed rapid decomposition in human plasma, which is an essential property for the antedrug. Topical applications of these compounds significantly suppressed TPA-induced epidermal hyperplasia in murin skin, a model of psoriasis. These results suggested that the phosphonamide-based inhibitors have a therapeutic potential for the treatment of psoriasis as an antedrug application.

A three-dimensional quantitative structure-activity relationship study of heparin-binding epidermal growth factor shedding inhibitors using comparative molecular field analysis

Despite the lack of structural information on the heparin-binding (HB) epidermal growth factor (EGF) shedding putative target enzyme, the design of potent HB-EGF shedding inhibitors has been attempted by means of comparative molecular field analysis (CoMFA), a well-established 3D-QSAR technique. Two different binding modes, obtained by docking a flexible representative into the MMP-3 and TACE target enzymes, were considered as alignment rules for an in-house data set of 50 HB-EGF shedding inhibitors. CoMFA models were derived with the standard steric, electrostatic, and Bohacek and McMartin's H-bond molecular fields. These fields were used individually or in combination. For both alignments, the H-bond field alone yielded the best statistical models. From the analysis of the CoMFA contours, ideas for testing the size of the S1' pocket and suggestions for the design of new inhibitors came forward, resulting in the synthesis and testing of four new inhibitors. Three of four compounds turned out to possess from good (IC(50) = 0.56 and 0.60 microM) to excellent (IC(50) = 0.13 microM) inhibitory activity. The hypothesis that, upon binding, the S1' pocket in the vicinity of the R(1) benzene ring must be narrow in size was confirmed by the weak activity (IC(50) = 1.1 microM) of the fourth compound. The experimental profile of these new inhibitors does suggest the MMP-3 alignment as the most plausible one for HB-EGF shedding inhibitors.

Matrix-independent survival of human keratinocytes through an EGF receptor/MAPK-kinase-dependent pathway

Normal epithelial cells undergo apoptosis when they are denied contact with the extracellular matrix, in a process termed "anoikis." Conversely, malignant epithelial cells typically acquire anchorage independence, i.e., the capacity to survive and grow in the absence of matrix interaction. Here we asked the question whether anoikis is affected by signaling through the EGF receptor (EGFR). We focused on the EGFR because EGFR signaling is frequently deregulated in malignant epithelial cells. We demonstrate that EGFR activation markedly alleviated the requirement of matrix engagement for survival of primary and immortalized human keratinocytes in suspension culture. Protection of epithelial cells through EGFR activation against anoikis was associated with and required sustained MAPK phosphorylation during the early phase of suspension culture. Interestingly, high levels of MAPK phosphorylation were not only required for EGFR-mediated protection against anoikis but also occurred as a consequence of caspase activation at later stages of suspension culture. These results demonstrate that EGFR activation contributes to anchorage-independent epithelial cell survival and identify MAPK activation as an important mechanism in this process.

The proamphiregulin cytoplasmic domain is required for basolateral sorting, but is not essential for constitutive or stimulus-induced processing in polarized Madin-Darby canine kidney cells

In this study, the role of the amphiregulin precursor (pro-AR) cytoplasmic domain in the basolateral sorting and cell-surface processing of pro-AR in polarized epithelial cells has been investigated using Madin-Darby canine kidney cells stably expressing various human pro-AR forms. Our results demonstrate that newly synthesized wild-type pro-AR (50 kDa) is delivered directly to the basolateral membrane domain with >95% efficiency, where it is sequentially cleaved within the ectodomain to release several soluble amphiregulin (AR) forms. Analyses of a pro-AR cytoplasmic domain truncation mutant (ARTL27) and two pro-AR secretory mutants (ARsec184 and ARsec190) indicated that the pro-AR cytoplasmic domain is not required for efficient delivery to the plasma membrane, but does contain essential basolateral sorting information. We show that the pro-AR cytoplasmic domain truncation mutant (ARTL27) is not sorted in polarized Madin-Darby canine kidney cells, with approximately 65% of the newly synthesized protein delivered to the apical cell surface. Under base-line conditions, ARTL27 was preferentially cleaved from the basolateral surface with 4-fold greater efficiency compared with cleavage from the apical membrane domain. However, ARTL27 ectodomain cleavage could be stimulated equivalently from either membrane domain by a variety of different stimuli. The metalloprotease inhibitor BB-94 could inhibit both base-line and stimulus-induced ectodomain cleavage of wild-type pro-AR and ARTL27. These results indicate that the pro-AR cytoplasmic domain is required for basolateral sorting, but is not essential for ectodomain processing. Preferential constitutive cleavage of ARTL27 from the basolateral cell surface also suggests that the metalloprotease activity involved in base-line and stimulus-induced ARTL27 ectodomain cleavage may be regulated differently in the apical and basolateral membrane domains of polarized epithelial cells.

Keratinocyte differentiation inversely regulates the expression of involucrin and transforming growth factor beta1

Extensive skin loss from a variety of conditions such as severe thermal injury is associated with significant functional morbidity and mortality. In recent years, the healing quality has been improved for patients who suffer burns due in part to the usage of skin replacement mainly prepared from multi-layered sheets of cultured keratinocytes. Although it is known that keratinocytes are a rich source of wound healing promoting factors such as transforming growth factor-beta1 (TGF-beta1), it is not clear whether differentiated keratinocytes in a multi-layer form release this multi-functional growth factor and has any functional influence on dermal fibroblasts. This study examined the hypothesis that keratinocytes in mono- and multi-layer forms express different levels of TGF-beta1. To address this hypothesis, keratinocytes were grown in serum free medium (KSFM) supplemented with bovine pituitary extract (50 microg/ml) and EGF (5 microg/ml). When cells reached confluency, conditioned medium was removed and replaced with 50% KSFM with no additives and 50% DMEM without serum and cells were allowed to form multi-layers and differentiate. The conditioned medium was then collected every 48 h up to 24 days and the level of TGF-beta1 and the efficacy of a keratinocyte released fibroblast mitogenic factor were evaluated by ELISA and (3)H-thymidine incorporation, respectively. Northern analysis was also employed to evaluate the expression of TGF-beta1, involucrin, TIMP-1, and 18 S ribosomal RNA in keratinocytes at different times of the onset of differentiation. The microscopic morphology of keratinocytes at different times of induction of cell differentiation showed detachments (nodules) of many regions of keratinocyte sheet from culture substratum within 1-2 weeks. The numbers and sizes of these nodules were increased as the process of keratinocyte differentiation proceed. The results of TGF-beta1 evaluation revealed that mono-layers of cultured keratinocytes which were round, attached, and proliferating in KSFM + BPE and EGF containing medium released a significantly higher level of TGF-beta1 (196 +/- 58 pg /ml) relative to those grown in multi-layer forms (28 +/- 7.8 pg/ml). A longitudinal experiment was then conducted and the results showed that cells on the onset of differentiation released even greater level of TGF-beta1 (388 +/- 53 pg/ml) relative to those grown in KSFM + BPE and EGF. This finding was consistent with the expression of TGF-beta1 mRNA evaluated in keratinocytes grown in test medium for various duration. In general, the level of TGF-beta1 protein and mRNA gradually reduced to its lowest level within 12 days of growing cells in our test medium. When aliquots of the collected keratinocyte conditioned medium were added to dermal fibroblasts, the level of (3)H-thymidine incorporation increased only in those cells receiving aliquots of conditioned medium containing high levels of TGF-beta1. When involucrin was used as a differentiation marker for keratinocytes at different time points, the highest level of involucrin mRNA expression was found at the later stage of cell differentiation. In conclusion, high involucrin expressing differentiated keratinocytes seem to be quiescent in releasing both TGF-beta1 and a fibroblast mitogenic factor.

Differential utilization and localization of ErbB receptor tyrosine kinases in skin compared to normal and malignant keratinocytes

Induction of heparin-binding epidermal growth factor-like growth factor (HB-EGF) mRNA in mouse skin organ culture was blocked by two pan-ErbB receptor tyrosine kinase (RTK) inhibitors but not by genetic ablation of ErbB1, suggesting involvement of multiple ErbB species in skin physiology. Human skin, cultured normal keratinocytes, and A431 skin carcinoma cells expressed ErbB1, ErbB2, and ErbB3, but not ErbB4. Skin and A431 cells expressed more ErbB3 than did keratinocytes. Despite strong expression of ErbB2 and ErbB3, heregulin was inactive in stimulating tyrosine phosphorylation in A431 cells. In contrast, it was highly active in MDA-MB-453 breast carcinoma cells. ErbB2 displayed punctate cytoplasmic staining in A431 and keratinocytes, compared to strong cell surface staining in MDA-MB-453. In skin, ErbB2 was cytoplasmic in basal keratinocytes, assuming a cell surface pattern in the upper suprabasal layers. In contrast, ErbB1 retained a cell surface distribution in all epidermal layers. Keratinocyte proliferation in culture was found to be ErbB1-RTK-dependent, using a selective inhibitor. These results suggest that in skin keratinocytes, ErbB2 transduces ligand-dependent differentiation signals, whereas ErbB1 transduces ligand-dependent proliferation/survival signals. Intracellular sequestration of ErbB2 may contribute to the malignant phenotype of A431 cells, by allowing them to respond to ErbB1-dependent growth/survival signals, while evading ErbB2-dependent differentiation signals.

Quantitative analysis of the EGF receptor autocrine system reveals cryptic regulation of cell response by ligand capture

Autocrine signaling is important in normal tissue physiology as well as pathological conditions. It is difficult to analyze these systems, however, because they are both self-contained and recursive. To understand how parameters such as ligand production and receptor expression influence autocrine activity, we investigated a human epidermal growth factor/epidermal growth factor receptor (EGF/EGFR) loop engineered into mouse B82 fibroblasts. We varied the level of ligand production using the tet-off expression system and used metalloprotease inhibitors to modulate ligand release. Receptor expression was varied using antagonistic blocking antibodies. We compared autocrine ligand release with receptor activation using a microphysiometer-based assay and analyzed our data using a quantitative model of ligand release and receptor dynamics. We found that the activity of our autocrine system could be described in terms of a simple ratio between the rate of ligand production (VLT) and the rate of receptor production (VR). At a VLT/VR ratio of <0.3, essentially no ligand was found in the extracellular medium, but a significant number of cell receptors (30-40%) were occupied. As the VLT/VR ratio increased from 0.3 towards unity, receptor occupancy increased and significant amounts of ligand appeared in the medium. Above a VLT/VR ratio of 1.0, receptor occupancy approached saturation and most of the released ligand was lost into the medium. Analysis of human mammary epithelial cells showed that a VLT/VR ratio of <5×10−4was sufficient to evoke >20% of a maximal proliferative response. This demonstrates that natural autocrine systems can be active even when no ligand appears in the extracellular medium.