Activation of erbB2 and c-src in phorbol ester-treated mouse epidermis: possible role in mouse skin tumor promotion

Loss of DLX3 tumor suppressive function promotes progression of SCC through EGFR-ERBB2 pathway

Cutaneous squamous cell carcinoma (cSCC) ranks second in the frequency of all skin cancers. The balance between keratinocyte proliferation and differentiation is disrupted in the pathological development of cSCC. DLX3 is a homeobox transcription factor which plays pivotal roles in embryonic development and epidermal homeostasis. To investigate the impact of DLX3 expression on cSCC prognosis, we carried out clinicopathologic analysis of DLX3 expression which showed statistical correlation between tumors of higher pathologic grade and levels of DLX3 protein expression. Further, Kaplan-Meier survival curve analysis demonstrated that low DLX3 expression correlated with poor patient survival. To model the function of Dlx3 in skin tumorigenesis, a two-stage dimethylbenzanthracene (DMBA)/12-O-tetradecanoylphorbol 13-acetate (TPA) study was performed on mice genetically depleted of Dlx3 in skin epithelium (Dlx3cKO). Dlx3cKO mice developed significantly more tumors, with more rapid tumorigenesis compared to control mice. In Dlx3cKO mice treated only with DMBA, tumors developed after ~16 weeks suggesting that loss of Dlx3 has a tumor promoting effect. Whole transcriptome analysis of tumor and skin tissue from our mouse model revealed spontaneous activation of the EGFR-ERBB2 pathway in the absence of Dlx3. Together, our findings from human and mouse model system support a tumor suppressive function for DLX3 in skin and underscore the efficacy of therapeutic approaches that target EGFR-ERBB2 pathway.

SMAD7 in keratinocytes promotes skin carcinogenesis by activating ATM-dependent DNA repair and an EGFR-mediated cell proliferation pathway

SMA- and MAD-related protein 7 (SMAD7) is a general inhibitor of transforming growth factor-β (TGF-β) signaling that acts through interaction and degradation of TGF-β receptors. SMAD7 has been demonstrated to be transcriptionally upregulated in chemical-induced skin tumors and TGF-β-treated normal keratinocytes. To evaluate the function of SMAD7 in skin carcinogenesis in vivo, Smad7 transgenic mice that specifically express either wild-type (WT) SMAD7 (TG-Smad7-WT) or mutant SMAD7 (TG-Smad7-MT) in keratinocytes, as well as Smad7 keratinocyte-specific knockout (Smad72f/2f-K14Cre) mice, were subjected to chemical-induced skin carcinogenesis. WT-SMAD7-expressing transgenic mice showed significantly greater papilloma formation than did non-TG control and Smad7-MT mice. The expression of WT-SMAD7 attenuated DNA damage-induced apoptosis in epidermal keratinocytes by stimulating the ATM-dependent DNA repair pathway. Nonetheless, overexpression of WT-SMAD7 caused a susceptibility to 12-O-tetradecanoylphorbol-13-acetate-induced epidermal hyperproliferation through activation of epidermal growth factor (EGF) signaling. In agreement with the transgenic mouse data, keratinocyte-specific deletion of SMAD7 markedly suppressed the tumor formation by inhibiting ATM and epidermal growth factor receptor (EGFR) signaling. Moreover, specific inhibition of EGFR signaling attenuated the hyperproliferation and tumor formation in TG-Smad7-WT mice. Taken together, these data support a novel role for SMAD7 as a tumor promoter in skin carcinogenesis where SMAD7 stimulates the DNA repair pathway and EGFR signaling activation.

Smad7 Modulates Epidermal Growth Factor Receptor Turnover through Sequestration of c-Cbl

Epidermal growth factor (EGF) regulates various cellular events, including proliferation, differentiation, migration, and tumorigenesis. For the maintenance of homeostasis, EGF signaling should be tightly regulated to prevent the aberrant activation. Smad7 has been known as inhibitory Smad that blocks the signal transduction of transforming growth factor β. In the process of cell proliferation or transformation, Smad7 has been shown the opposite activities as a promoter or suppressor depending on cell types or microenvironments. We found that the overexpression of Smad7 in human HaCaT keratinocyte cells and mouse skin tissues elevated EGF receptor (EGFR) activity by impairing ligand-induced ubiquitination and degradation of activated receptor, which is induced by the E3 ubiquitin ligase c-Cbl. The C-terminal MH2 region but not MH1 region of Smad7 is critical for interaction with c-Cbl to inhibit the ubiquitination of EGFR. Interestingly, wild-type Smad7, but not Smad6 or mutant Smad7, destabilized the EGF-induced complex formation of c-Cbl and EGFR. These data suggest a novel role for Smad7 as a promoter for prolonging the EGFR signal in keratinocyte and skin tissue by reducing its ligand-induced ubiquitination and degradation.

Smad7 Modulates Epidermal Growth Factor Receptor Turnover through Sequestration of c-Cbl

Epidermal growth factor (EGF) regulates various cellular events, including proliferation, differentiation, migration, and tumorigenesis. For the maintenance of homeostasis, EGF signaling should be tightly regulated to prevent the aberrant activation. Smad7 has been known as inhibitory Smad that blocks the signal transduction of transforming growth factor β. In the process of cell proliferation or transformation, Smad7 has been shown the opposite activities as a promoter or suppressor depending on cell types or microenvironments. We found that the overexpression of Smad7 in human HaCaT keratinocyte cells and mouse skin tissues elevated EGF receptor (EGFR) activity by impairing ligand-induced ubiquitination and degradation of activated receptor, which is induced by the E3 ubiquitin ligase c-Cbl. The C-terminal MH2 region but not MH1 region of Smad7 is critical for interaction with c-Cbl to inhibit the ubiquitination of EGFR. Interestingly, wild-type Smad7, but not Smad6 or mutant Smad7, destabilized the EGF-induced complex formation of c-Cbl and EGFR. These data suggest a novel role for Smad7 as a promoter for prolonging the EGFR signal in keratinocyte and skin tissue by reducing its ligand-induced ubiquitination and degradation.

Erbb2 up-regulation of ADAM12 expression accelerates skin cancer progression

Solar ultraviolet (UV) radiation can cause severe damage to the skin and is the primary cause of most skin cancer. UV radiation causes DNA damage leading to mutations and also activates the Erbb2/HER2 receptor through indirect mechanisms involving reactive oxygen species. We hypothesized that Erbb2 activation accelerates the malignant progression of UV-induced skin cancer. Following the induction of benign squamous papillomas by UV exposure of v-ras(Ha) transgenic Tg.AC mice, mice were treated topically with the Erbb2 inhibitor AG825 and tumor progression monitored. AG825 treatment reduced tumor volume, increased tumor regression, and delayed the development of malignant squamous cell carcinoma (SCC). Progression to malignancy was associated with increased Erbb2 and ADAM12 (A Disintegin And Metalloproteinase 12) transcripts and protein, while inhibition of Erbb2 blocked the increase in ADAM12 message upon malignant progression. Similarly, human SCC and SCC cell lines had increased ADAM12 protein and transcripts when compared to normal controls. To determine whether Erbb2 up-regulation of ADAM12 contributed to malignant progression of skin cancer, Erbb2 expression was modulated in cultured SCC cells using forced over-expression or siRNA targeting, demonstrating up-regulation of ADAM12 by Erbb2. Furthermore, ADAM12 transfection or siRNA targeting revealed that ADAM12 increased both the migration and invasion of cutaneous SCC cells. Collectively, these results suggest Erbb2 up-regulation of ADAM12 as a novel mechanism contributing to the malignant progression of UV-induced skin cancer. Inhibition of Erbb2/HER2 reduced tumor burden, increased tumor regression, and delayed the progression of benign skin tumors to malignant SCC in UV-exposed mice. Inhibition of Erbb2 suppressed the increase in metalloproteinase ADAM12 expression in skin tumors, which in turn increased migration and tumor cell invasiveness.

Molecular mechanisms of mouse skin tumor promotion

Multiple molecular mechanisms are involved in the promotion of skin carcinogenesis. Induction of sustained proliferation and epidermal hyperplasia by direct activation of mitotic signaling pathways or indirectly in response to chronic wounding and/or inflammation, or due to a block in terminal differentiation or resistance to apoptosis is necessary to allow clonal expansion of initiated cells with DNA mutations to form skin tumors. The mitotic pathways include activation of epidermal growth factor receptor and Ras/Raf/mitogen-activated protein kinase signaling. Chronic inflammation results in inflammatory cell secretion of growth factors and cytokines such as tumor necrosis factor-α and interleukins, as well as production of reactive oxygen species, all of which can stimulate proliferation. Persistent activation of these pathways leads to tumor promotion.

Impact of mTORC1 inhibition on keratinocyte proliferation during skin tumor promotion in wild-type and BK5.AktWT mice

In this study, we examined the impact of rapamycin on mTORC1 signaling during 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced keratinocyte proliferation and skin tumor promotion in both wild-type (FVB/N) and BK5.Akt(WT) mice. TPA activated mTORC1 signaling in a time-dependent manner in cultured primary mouse keratinocytes and a mouse keratinocyte cell line. Early activation (15-30 min) of mTORC1 signaling induced by TPA was mediated in part by PKC activation, whereas later activation (2-4 h) was mediated by activation of EGFR and Akt. BK5.Akt(WT) transgenic mice, where Akt1 is overexpressed in basal epidermis, are highly sensitive to TPA-induced epidermal proliferation and two-stage skin carcinogenesis. Targeting mTORC1 with rapamycin effectively inhibited TPA-induced epidermal hyperplasia and hyperproliferation as well as tumor promotion in a dose-dependent manner in both wild-type and BK5.Akt(WT) mice. A significant expansion (∼threefold) of the label retaining cell (LRC) population per hair follicle was observed in BK5.Akt(WT) mice compared to FVB/N mice. There was also a significant increase in K15 expressing cells in the hair follicle of transgenic mice that coincided with expression of phospho-Akt, phospho-S6K, and phospho-PRAS40, suggesting an important role of mTORC1 signaling in bulge-region keratinocyte stem cell (KSC) homeostasis. After 2 weeks of TPA treatment, LRCs had moved upward into the interfollicular epidermis from the bulge region of both wild-type and BK5.Akt(WT) mice. TPA-mediated LRC proliferation and migration was significantly inhibited by rapamycin. Collectively, the current data indicate that signaling through mTORC1 contributes significantly to the process of skin tumor promotion through effects on proliferation of the target cells for tumor development.

In vivo functional studies of tumor-specific retrogene NanogP8 in transgenic animals

The current study was undertaken to investigate potential oncogenic functions of NanogP8, a tumor-specific retrogene homolog of Nanog (expressed in pluripotent cells), in transgenic animal models. To this end, human primary prostate tumor-derived NanogP8 was targeted to the cytokeratin 14 (K14) cellular compartment, and two lines of K14-NanogP8 mice were derived. The line 1 animals, expressing high levels of NanogP8, experienced perinatal lethality and developmental abnormalities in multiple organs, including the skin, tongue, eye, and thymus in surviving animals. On postnatal day 5 transgenic skin, for example, there was increased c-Myc expression and Ki-67(+) cells accompanied by profound abnormalities in skin development such as thickened interfollicular epidermis and dermis and lack of hypodermis and sebaceous glands. The line 3 mice, expressing low levels of NanogP8, were grossly normal except cataract development by 4-6 mo of age. Surprisingly, both lines of mice do not develop spontaneous tumors related to transgene expression. Even more unexpectedly, high levels of NanogP8 expression in L1 mice actually inhibited tumor development in a two-stage chemical carcinogenesis model. Mechanistic studies revealed that constitutive NanogP8 overexpression in adult L1 mice reduced CD34(+)α6(+) and Lrig-1(+) bulge stem cells, impaired keratinocyte migration, and repressed the expression of many stem cell-associated genes, including Bmp5, Fgfr2, Jmjd1a, and Jun. Our study, for the first time, indicates that transgenically expressed human NanogP8 is biologically functional, but suggests that high levels of NanogP8 may disrupt normal developmental programs and inhibit tumor development by depleting stem cells.

A wound-induced keratin inhibits Src activity during keratinocyte migration and tissue repair

Keratin 6 negatively regulates Src kinase activity and the migratory potential of skin keratinocytes during wound repair.

Injury to the epidermis triggers an elaborate homeostatic response resulting in tissue repair and recovery of the vital barrier function. The type II keratins 6a and 6b (K6a and K6b) are among the genes induced early on in wound-proximal keratinocytes and maintained during reepithelialization. Paradoxically, genetic ablation of K6a and K6b results in enhanced keratinocyte migration. In this paper, we show that this trait results from activation of Src kinase and key Src substrates that promote cell migration. Endogenous Src physically associated with keratin proteins in keratinocytes in a K6-dependent fashion. Purified Src bound K6-containing filaments via its SH2 domain in a novel phosphorylation-independent manner, resulting in kinase inhibition. K6 protein was enriched in the detergent-resistant membrane (DRM), a key site of Src inhibition, and DRMs from K6-null keratinocytes were depleted of both keratin and Src. We conclude that K6 negatively regulates Src kinase activity and the migratory potential of skin keratinocytes during wound repair. Our findings may also be important in related contexts such as cancer.

Cyanidin-3-glucoside suppresses B[a]PDE-induced cyclooxygenase-2 expression by directly inhibiting Fyn kinase activity

Benzo[a]pyrene-7,8-diol-9,10-epoxide (B[a]PDE) is a well-known carcinogen that is associated with skin cancer. Abnormal expression of cyclooxygenase-2 (COX-2) is an important mediator in inflammation and tumor promotion. We investigated the inhibitory effect of cyanidin-3-glucoside (C3G), an anthocyanin present in fruits, on B[a]PDE-induced COX-2 expression in mouse epidermal JB6 P+ cells. Pretreatment with C3G resulted in the reduction of B[a]PDE-induced expression of COX-2 and COX-2 promoter activity. The activation of activator protein-1 (AP-1) and nuclear factor-κB (NF-κB) induced by B[a]PDE was also attenuated by C3G. C3G attenuated the B[a]PDE-induced phosphorylation of MEK, MKK4, Akt, and mitogen-activated protein kinases (MAPKs), but no effect on the phosphorylation of the upstream MAPK regulator Fyn. However, kinase assays demonstrated that C3G suppressed Fyn kinase activity and C3G directly binds Fyn kinase noncompetitively with ATP. By using PP2, a pharmacological inhibitor for SFKs, we showed that Fyn kinase regulates B[a]PDE-induced COX-2 expression by activating MAPKs, AP-1 and NF-κB. These results suggest that C3G suppresses B[a]PDE-induced COX-2 expression mainly by blocking the activation of the Fyn signaling pathway, which may contribute to its chemopreventive potential.

Synergistic effect of hyaluronate fragments in retinaldehyde-induced skin hyperplasia which is a Cd44-dependent phenomenon

BACKGROUND

CD44 is a polymorphic proteoglycan and functions as the principal cell-surface receptor for hyaluronate (HA). Heparin-binding epidermal growth factor (HB-EGF) activation of keratinocyte erbB receptors has been proposed to mediate retinoid-induced epidermal hyperplasia. We have recently shown that intermediate size HA fragments (HAFi) reverse skin atrophy by a CD44-dependent mechanism.

METHODOLOGY AND PRINCIPAL FINDINGS

Treatment of primary mouse keratinocyte cultures with retinaldehyde (RAL) resulted in the most significant increase in keratinocyte proliferation when compared with other retinoids, retinoic acid, retinol or retinoyl palmitate. RAL and HAFi showed a more significant increase in keratinocyte proliferation than RAL or HAFi alone. No proliferation with RAL was observed in CD44-/- keratinocytes. HA synthesis inhibitor, 4-methylumbelliferone inhibited the proliferative effect of RAL. HB-EGF, erbB1, and tissue inhibitor of MMP-3 blocking antibodies abrogated the RAL- or RAL- and HAFi-induced keratinocyte proliferation. Topical application of RAL or RAL and HAFi for 3 days caused a significant epidermal hyperplasia in the back skin of wild-type mice but not in CD44-/- mice. Topical RAL and HAFi increased epidermal CD44 expression, and the epidermal and dermal HA. RAL induced the expression of active HB-EGF and erbB1. However, treatment with RAL and HAFi showed a more significant increase in pro-HB-EGF when compared to RAL or HAFi treatments alone. We then topically applied RAL and HAFi twice a day to the forearm skin of elderly dermatoporosis patients. After 1 month of treatment, we observed a significant clinical improvement.

CONCLUSIONS AND SIGNIFICANCE

Our results indicate that (i) RAL-induced in vitro and in vivo keratinocyte proliferation is a CD44-dependent phenomenon and requires the presence of HA, HB-EGF, erbB1 and MMPs, (ii) RAL and HAFi show a synergy in vitro and in vivo in mouse skin, and (iii) the combination of RAL and HAFi seems to have an important therapeutic effect in dermatoporosis.

Dynamic deterministic effects propagation networks: learning signalling pathways from longitudinal protein array data

Motivation: Network modelling in systems biology has become an important tool to study molecular interactions in cancer research, because understanding the interplay of proteins is necessary for developing novel drugs and therapies. De novo reconstruction of signalling pathways from data allows to unravel interactions between proteins and make qualitative statements on possible aberrations of the cellular regulatory program. We present a new method for reconstructing signalling networks from time course experiments after external perturbation and show an application of the method to data measuring abundance of phosphorylated proteins in a human breast cancer cell line, generated on reverse phase protein arrays.

Results: Signalling dynamics is modelled using active and passive states for each protein at each timepoint. A fixed signal propagation scheme generates a set of possible state transitions on a discrete timescale for a given network hypothesis, reducing the number of theoretically reachable states. A likelihood score is proposed, describing the probability of measurements given the states of the proteins over time. The optimal sequence of state transitions is found via a hidden Markov model and network structure search is performed using a genetic algorithm that optimizes the overall likelihood of a population of candidate networks. Our method shows increased performance compared with two different dynamical Bayesian network approaches. For our real data, we were able to find several known signalling cascades from the ERBB signalling pathway.

Availability: Dynamic deterministic effects propagation networks is implemented in the R programming language and available at http://www.dkfz.de/mga2/ddepn/

Contact:c.bender@dkfz.de

Dual inhibition of both the epidermal growth factor receptor and erbB2 effectively inhibits the promotion of skin tumors during two-stage carcinogenesis

The erbB family of receptor tyrosine kinases are known to play important roles in normal epithelial development and epithelial neoplasia. Considerable evidence also suggests that signaling through the epidermal growth factor receptor (EGFR) plays an important role in multistage skin carcinogenesis in mice; however, less is known about the role of erbB2. In this study, to further examine the role of both erbB2 and EGFR in epithelial carcinogenesis, we examined the effect of a dual erbB2/EGFR tyrosine kinase inhibitor, GW2974, given in the diet on skin tumor promotion during two-stage carcinogenesis in wild-type and BK5.erbB2 mice. In BK5.erbB2 mice, erbB2 is overexpressed in the basal layer of epidermis and leads to heightened sensitivity to skin tumor development. GW2974 effectively inhibited skin tumor promotion by 12-O-tetradecanoylphorbol-13-acetate in wild-type and BK5.erbB2 mice, although a more marked effect was seen in BK5.erbB2 mice. In addition, this inhibitory effect was reversible when GW2974 treatment was withdrawn. GW2974 inhibited 12-O-tetradecanoylphorbol-13-acetate-induced epidermal hyperproliferation, which correlated with reduced activation of both the EGFR and erbB2. These results support the hypothesis that both the EGFR and erbB2 play an important role in the development of skin tumors during two-stage skin carcinogenesis, especially during the tumor promotion stage. Furthermore, the marked sensitivity of BK5.erbB2 mice to the inhibitory effects of GW2974 during tumor promotion suggest greater efficacy for this compound when erbB2 is overexpressed or amplified as an early event in the carcinogenic process.

Growth factor signaling pathways as targets for prevention of epithelial carcinogenesis

Growth factor receptor (GFR) signaling controls epithelial cell growth by responding to various endogenous or exogenous stimuli and subsequently activating downstream signaling pathways including Stat3, PI3K/Akt/mTOR, MAPK, and c-Src. Environmental chemical toxicants and UVB irradiation cause enhanced and prolonged activation of GFR signaling and downstream pathways that contributes to epithelial cancer development including skin cancer. Recent studies, especially those with tissue-specific transgenic mouse models, have demonstrated that GFRs and their downstream signaling pathways contribute to all three stages of epithelial carcinogenesis by regulating a wide variety of biological functions including proliferation, apoptosis, angiogenesis, cell adhesion, and migration. Inhibiting these signaling pathways early in the carcinogenic process results in reduced cell proliferation and survival, leading to decreased tumor formation. Collectively, these studies suggest that GFR signaling and subsequent downstream signaling pathways are potential targets for the prevention of epithelial cancers including skin cancer.

Fyn kinase is a direct molecular target of delphinidin for the inhibition of cyclooxygenase-2 expression induced by tumor necrosis factor-alpha

Tumor necrosis factor (TNF)-alpha-mediated cyclooxygenase (COX)-2 expression plays key roles in inflammation and tumorigenesis, particularly skin carcinogenesis, and hence targeting the TNF-alpha-mediated signaling pathway might be a promising strategy for developing chemopreventive agents against skin cancer and other skin disorders. Here we report that Fyn kinase - one of the members of the nonreceptor protein tyrosine kinase family - is involved in TNF-alpha-induced COX-2 expression, and that delphinidin - a major anthocyanidin present in red wine and berries - inhibits these effects by directly inhibiting Fyn kinase activity. Delphinidin strongly inhibited TNF-alpha-induced COX-2 expression in JB6 P+ mouse epidermal (JB6 P+) cells, whereas two other major phenolic compounds (resveratrol and gallic acid) did not exert significant inhibitory effects. Delphinidin inhibited the TNF-alpha-induced phosphorylations of JNK, p38 MAP kinase, Akt, p90RSK, MSK1, and ERK, and subsequently blocked the activation of the eukaryotic transcription factors AP-1 and NF-kappaB. Kinase and pull-down assay data revealed that delphinidin inhibited Fyn kinase activity and directly bound with Fyn kinase noncompetitively with ATP. By using PP2 (a commercial inhibitor of Fyn kinase) and siRNA-Fyn, we confirmed that Fyn kinase is involved in TNF-alpha-induced COX-2 expression mainly by down-regulating JNK in JB6 P+ cells. Together these findings suggest that the targeted inhibition of Fyn kinase activity and COX-2 expression by delphinidin contributes to the chemopreventive potential of red wine and berries.

Multiple signaling pathways are responsible for prostaglandin E2-induced murine keratinocyte proliferation

Although prostaglandin E2 (PGE2) has been shown by pharmacologic and genetic studies to be important in skin cancer, the molecular mechanism(s) by which it contributes to tumor growth is not well understood. In this study, we investigated the mechanisms by which PGE2 stimulates murine keratinocyte proliferation using in vitro and in vivo models. In primary mouse keratinocyte cultures, PGE2 activated the epidermal growth factor receptor (EGFR) and its downstream signaling pathways as well as increased cyclic AMP (cAMP) production and activated the cAMP response element binding protein (CREB). EGFR activation was not significantly inhibited by pretreatment with a c-src inhibitor (PP2), nor by a protein kinase A inhibitor (H-89). However, PGE2-stimulated extracellularly regulated kinase 1/2 (ERK1/2) activation was completely blocked by EGFR, ERK1/2, and phosphatidylinositol 3-kinase (PI3K) pathway inhibitors. In addition, these inhibitors attenuated the PGE2-induced proliferation, nuclear factor-kappa B, activator protein-1 (AP-1), and CREB binding to the promoter regions of the cyclin D1 and vascular endothelial growth factor (VEGF) genes and expression of cyclin D1 and VEGF in primary mouse keratinocytes. Similarly, in vivo, we found that WT mice treated with PGE2 and untreated cyclooxygenase-2-overexpressing transgenic mice had higher levels of cell proliferation and expression of cyclin D1 and VEGF, as well as higher levels of activated EGFR, nuclear factor-kappa B, AP-1, and CREB, than vehicle-treated WT mice. Our findings provide evidence for a link between cyclooxygenase-2 overexpression and EGFR-, ERK-, PI3K-, cAMP-mediated cell proliferation, and the tumor-promoting activity of PGE2 in mouse skin.

Beyond wavy hairs: the epidermal growth factor receptor and its ligands in skin biology and pathology

The epidermal growth factor receptor (EGFR) network, including its seven ligands and four related receptors, represents one of the most complex signaling systems in biology. In many tissues, including the skin and its appendages (notoriously the hair follicles), its correct function is necessary for proper development and tissue homeostasis, and its deregulation rapidly results in defects in cellular proliferation and differentiation. The consequences are impaired wound healing, development of psoriasis-like lesions, structural and functional defects of the hair follicles, and tumorigenesis. In addition to in vitro experiments and data from clinical studies, several genetically modified mouse models displaying alterations in the interfollicular skin and hair follicles attributable to mutations in components of the EGFR system have been reported. These animals, in many cases representing bona fide models of known human diseases, have been seminal in the study of the role of EGFR and its ligands in the skin and its appendages. In this review, we take the multiple phenotypes of these animal models as a basis to summarize and discuss the effects elicited by members of the EGFR system in diverse aspects of skin biology and pathology, including cellular proliferation and differentiation, wound healing, hair follicle morphogenesis, and tumorigenesis.

Reduced susceptibility to two-stage skin carcinogenesis in mice with low circulating insulin-like growth factor I levels

Calorie restriction has been shown to inhibit epithelial carcinogenesis and this method of dietary restriction reduces many circulating proteins, including insulin-like growth factor I (IGF-I). Previously, we identified a relationship between elevated tissue IGF-I levels and enhanced susceptibility to chemically induced skin tumorigenesis. In this study, liver IGF-I-deficient (LID) mice, which have a 75% reduction in serum IGF-I, were subjected to the standard two-stage skin carcinogenesis protocol using 7,12-dimethylbenz(a)anthracene as the initiator and 12-O-tetradecanoylphorbol-13-acetate (TPA) as the promoter. We observed a significant reduction in epidermal thickness and labeling index in LID mice treated with either vehicle or TPA. A significant decrease in both tumor incidence and tumor multiplicity was observed in LID mice undergoing two-stage skin carcinogenesis relative to wild-type littermates. Western blot analyses of epidermal extracts revealed reduced activation of both the epidermal growth factor and IGF-I receptors in response to TPA treatment in LID mice. In addition, reduced activation of both Akt and the mammalian target of rapamycin (mTOR) was observed in LID mice following TPA treatment relative to wild-type controls. Signaling downstream of mTOR was also reduced. These data suggest a possible mechanism whereby reduced circulating IGF-I leads to attenuated activation of the Akt and mTOR signaling pathways, and thus, diminished epidermal response to tumor promotion, and ultimately, two-stage skin carcinogenesis. The current data also suggest that reduced circulating IGF-I levels which occur as a result of calorie restriction may lead to the inhibition of skin tumorigenesis, at least in part, by a similar mechanism.

Activation of epidermal akt by diverse mouse skin tumor promoters

Akt is a serine/threonine kinase involved in a variety of cellular responses, including cell proliferation and cell survival. Recent studies from our laboratory suggest that Akt signaling may play an important role in skin tumor promotion. To explore this premise, we examined epidermal Akt activation and signaling in response to chemically diverse skin tumor promoters. Mice received single or multiple applications of 12-O-tetradecanoylphorbol-13-acetate (TPA), okadaic acid, or chrysarobin. All three tumor promoters were able to activate epidermal Akt as early as 1 h after treatment. Activation of Akt following tumor promoter treatment led to enhanced downstream signaling, including hyperphosphorylation of glycogen synthase kinase-3beta and Bad. Structure activity studies with phorbol ester analogues revealed that the magnitude of activation paralleled tumor-promoting activity. In cultured primary keratinocytes, TPA treatment also led to activation of Akt. Activation of the epidermal growth factor receptor (EGFR) seemed to underlie the ability of TPA to activate Akt as both PD153035, an inhibitor of EGFR, and GW2974, a dual-specific inhibitor of both EGFR and erbB2, were able to effectively reduce TPA-induced Akt phosphorylation as well as TPA-stimulated EGFR and erbB2 tyrosine phosphorylation in a dose-dependent manner. Furthermore, inhibition of protein kinase C (PKC) activity blocked TPA-stimulated heparin-binding EGF production and EGFR transactivation. Inhibition of PKC also led to a decreased association of Akt with the PP2A catalytic subunit, leading to increased Akt phosphorylation. However, combination of EGFR inhibitor and PKC inhibitor completely abrogated TPA-induced activation of Akt. Collectively, the current results support the hypothesis that elevated Akt activity and subsequent activation of downstream signaling pathways contribute significantly to skin tumor promotion. In addition, signaling through the EGFR via EGFR homodimers or EGFR/erbB2 heterodimers may be the primary event leading to Akt activation during tumor promotion in mouse skin.

Induction of ErbB2 by ultraviolet A irradiation: potential role in malignant transformation of keratinocytes

Ultraviolet (UV) A (320-400 nm), which constitutes more than 90% of UV radiation in the sunlight that reaches the earth's surface, is considered a major cause of human skin photo-aging and skin cancer. Exposure of keratinocytes to UVA has previously been reported to lead to the activation of a variety of epidermal growth factor receptors (EGFR), including ErbB2, and ErbB2 activation is involved in skin tumor development. Here, we demonstrate that ErbB2 expression is enhanced by low-energy UVA (300-3000 mJ/cm(2)) irradiation in the skin tissues of both hairless mice and HaCaT keratinocytes. Luciferase reporter-gene activity using the 756-bp flanking region of the human erbB2 gene was increased by UVA irradiation. UVA irradiation also selectively increased the levels of activator protein (AP)-2 alpha, but not AP-2 beta and AP-2 gamma. The increase in the reporter gene activity of HaCaT cells exposed to UVA was abolished by mutation of the two AP-2 binding sites in the promoter region of the erbB2 gene. Inhibition of cAMP-dependent protein kinase caused complete blockage of ErbB2 induction and AP-2 alpha activation by UVA irradiation. Finally, we reveal that pre-exposure of HaCaT cells to UVA potentiates EGF-inducible anchorage-independent growth of the keratinocytes, which is significantly suppressed by ErbB2 inhibition. These results support the hypothesis that UVA enhances the expression of ErbB2 via cAMP- and protein kinase-dependent AP-2 alpha activation in keratinocytes, which may serve as a key mechanistic basis for the malignant transformation of keratinocytes exposed to UVA irradiation.

Multiple mechanisms of Erbb2 action after ultraviolet irradiation of the skin

Ultraviolet (UV) irradiation causes multiple pathologic changes in the skin including inflammation, immune suppression, photoaging, and cancer. Effects of UV irradiation include the activation of numerous signal transduction pathways, including the mitogen-activated protein kinases (MAPK), and the activation of transcription factors such as nuclear factor kappa B (NFkappaB). These responses alter gene expression in a manner that resembles the response to growth factors known as the "UV response". The UV response alters the kinetics of cell division and cell death allowing the skin to recover from the DNA damage caused by UV exposure. UV irradiation also rapidly activates epidermal growth factor receptor (EGFR) family members, including Erbb2 (human epithelial growth factor receptor 2 (HER2)/neu), through the generation of reactive oxygen species. Erbb2, a protooncogene that is activated in many types of cancer and associated with aggressive and chemotherapeutic-resistant disease, is expressed in both follicular and epidermal keratinocytes within the skin. However, the physiological functions of Erbb2 in the skin and its role in the UV response are largely unknown. In this review, evidence that Erbb2 is influential in modulating the response of the skin to UV will be presented. Erbb2 alters the expression of regulatory genes controlling inflammation, angiogenesis, cell division, apoptosis, cell adhesion, and migration following UV irradiation. In addition, Erbb2 dampens UV-induced S-phase arrest, augments inflammation in response to UV irradiation, and suppresses UV-induced apoptosis. In summary, the evidence presented herein links UV-induced Erbb2 activation to many of the effects of UV and implicates Erbb2 in UV-induced carcinogenesis.

Differential ErbB1 signaling in squamous cell versus basal cell carcinoma of the skin

In this study, we examined ErbB1 signaling in human basal and squamous cell carcinomas (BCC and SCC) of the skin in vivo. We used enzyme-linked immunosorbent assay, laser capture microdissection-coupled real-time reverse transcriptase-polymerase chain reaction, and immunohistochemistry to assess expression and activation levels of ErbB1 protein, ligands, and potential downstream effectors, in BCC and SCC tumors, stroma, and adjacent epidermis. Although total ErbB1 protein and mRNA were similar in cancerous and normal skin, we found that ErbB1 activation (phospho-Tyr(1068)) was greater in bulk SCC versus BCC or normal skin. In addition, three ErbB1 ligand transcripts (amphiregulin, heparin-binding epidermal growth factor-like growth factor, and transforming growth factor-alpha) were up-regulated in tumor cells of SCC but not BCC. Expression of these ligands was also increased in asymptomatic epidermis adjacent to both SCC and BCC, relative to normal skin. Interestingly, betacellulin transcript levels were inversely regulated compared with the other ligands. Consistently, downstream ErbB1 effectors (Erk1/2 and Akt) were activated in tumor cells of SCC but not of BCC and in adjacent epidermis of both BCC and SCC. These results demonstrate that ErbB1 signaling is hyperactive in tumor cells of SCC but not of BCC and in nearby asymptomatic epidermis of both tumor types. Our results suggest that targeting ErbB1 signaling might be of benefit in the treatment of SCC.

Hyaluronate fragments reverse skin atrophy by a CD44-dependent mechanism

BACKGROUND

Skin atrophy is a common manifestation of aging and is frequently accompanied by ulceration and delayed wound healing. With an increasingly aging patient population, management of skin atrophy is becoming a major challenge in the clinic, particularly in light of the fact that there are no effective therapeutic options at present.

METHODS AND FINDINGS

Atrophic skin displays a decreased hyaluronate (HA) content and expression of the major cell-surface hyaluronate receptor, CD44. In an effort to develop a therapeutic strategy for skin atrophy, we addressed the effect of topical administration of defined-size HA fragments (HAF) on skin trophicity. Treatment of primary keratinocyte cultures with intermediate-size HAF (HAFi; 50,000-400,000 Da) but not with small-size HAF (HAFs; <50,000 Da) or large-size HAF (HAFl; >400,000 Da) induced wild-type (wt) but not CD44-deficient (CD44-/-) keratinocyte proliferation. Topical application of HAFi caused marked epidermal hyperplasia in wt but not in CD44-/- mice, and significant skin thickening in patients with age- or corticosteroid-related skin atrophy. The effect of HAFi on keratinocyte proliferation was abrogated by antibodies against heparin-binding epidermal growth factor (HB-EGF) and its receptor, erbB1, which form a complex with a particular isoform of CD44 (CD44v3), and by tissue inhibitor of metalloproteinase-3 (TIMP-3).

CONCLUSIONS

Our observations provide a novel CD44-dependent mechanism for HA oligosaccharide-induced keratinocyte proliferation and suggest that topical HAFi application may provide an attractive therapeutic option in human skin atrophy.

Epidermal growth factor receptor dimerization status determines skin toxicity to HER-kinase targeted therapies

Skin toxicity, a common drug-related adverse event observed in cancer patients treated with epidermal growth factor receptor (EGFR)-directed therapies is rarely seen with therapies targeting HER2. This study reports the significance of the EGFR and HER2 dimerization status in skin with regard to these dermatologic side effects. We demonstrate the differential effect of HER-directed therapies on the ligand driven activation status of EGFR, HER2 and MAPK in normal human epidermal keratinocytes. EGFR-directed therapies, such as gefitinib and cetuximab, inhibited ligand-induced activation of EGFR and MAPK in human keratinocytes. Pertuzumab, an antibody interfering with functional HER2 heterodimerization, failed to block ligand-induced HER signaling in primary keratinocytes. Using a novel proximity-based dimerization assay (eTag™) we show that EGFR homodimers are the predominant HER dimer pair in normal primary kertinocytes and in normal skin tissue from 16 patients with solid malignancies. The presence of [p]EGFR and [p]MAPK, but the absence of [p]HER2, demonstrates productive signaling via EGFR but not HER2 in human skin. These data illustrate the importance of the EGFR dimerization partner in human skin and suggests that inhibition of EGFR homodimer signaling rather than EGFR/HER2 heterodimer signaling maybe the key molecular event determining dermatologic toxicity discrepancies observed between EGFR-targeted versus HER2-targeted therapies.

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.

Soluble Form of Heparin-binding EGF-like Growth Factor Contributes to Retinoic Acid-induced Epidermal Hyperplasia

Heparin-binding EGF-like growth factor (HB-EGF), a member of the EGF-family, is thought to be important for keratinocyte functions. HB-EGF is first synthesized as a membrane-anchored form, and its soluble form is released by ectodomain shedding. Here we investigate the role of HB-EGF in epidermal hyperplasia induced by all-trans retinoic acid (tRA) treatment. HB-EGF is normally expressed in epidermis of normal adult mice at very low levels, but topical tRA treatment results in epidermal hyperplasia, concomitant with the strong induction of HB-EGF expression in the suprabasal layer. tRA-induced epidermal hyperplasia was reduced both in the keratinocyte-specific HB-EGF null mice (K5-HB(del/del)) and knock-in mice expressing the uncleavable mutant form of HB-EGF (HB(uc/uc)), as compared with wild-type HB-EGF knock-in mice (HB(lox/lox)). Among ErbB tyrosine kinase receptors, EGF receptor (EGFR) and ErbB2 were selectively activated by tRA treatment in skin from wild-type mice, while the activation of these ErbB receptors was significantly reduced in the skin of HB-EGF null mice. These results indicate that expression of HB-EGF and generation of its soluble form, followed by activation of EGFR and ErbB2, are pivotal processes in tRA-induced epidermal hyperplasia.

Src family kinase inhibitors block amphiregulin-mediated autocrine ErbB signaling in normal human keratinocytes

c-Src potentiates proliferation, survival, and invasiveness in response to epidermal growth factor (EGF) in human mammary carcinoma cells. Tyrosine (Tyr) 845 of ErbB1 is phosphorylated by Src and has been implicated in control of malignant behavior. Although several lines of evidence also suggest important interactions of ErbB and Src family kinase signaling in normal epithelial cells, little is known about the mechanism of this interaction. Studying normal human keratinocytes (NHKs), here we demonstrate strong expression of the Src family kinases Src, Yes, and Fyn; Src family kinase-dependent stimulation of Tyr 845 by EGF; and potent inhibition of NHK proliferation and migration by two Src family kinase inhibitors PP1 and PD173952. EGF-stimulated extracellular signal-regulated kinase (ERK) phosphorylation occurred at much lower concentrations of EGF than required to phosphorylate Tyr 845. Moreover, the effect of Src family kinase inhibitors on EGF-stimulated ERK phosphorylation was transient, prompting a search for other targets of Src family kinase action. By enzyme-linked immunosorbent assay analysis, we found that three different Src family kinase inhibitors [6-(2,6-dichlorophenyl)-8-methyl-2-(4-morpholin-4-ylphenylamino)-8H-pyrido[2,3-d]pyrimidin-7-one (PD173952), 4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP1), and 2-oxo-3-(4,5,6,7-tetrahydro-1H-indol-2-ylmethylene)-2,3-dihydro-1H-indole-5-sulfonic acid dimethylamide (SU6656)] markedly inhibited elaboration of soluble amphiregulin by NHKs. The ErbB inhibitor PD158780 and the mitogen-activated protein kinase kinase inhibitor U0126 also markedly inhibited NHK proliferation, migration, and amphiregulin production. Together, these observations demonstrate that one or more Src family kinases act upstream as well as downstream of ErbB1 to promote amphiregulin-dependent autocrine stimulation of NHKs and suggest that autocrine NHK proliferation is more dependent upon ERK activation than upon Tyr 845 phosphorylation.

Protein kinase Calpha activates c-Src and induces podosome formation via AFAP-110

We report that the actin filament-associated protein AFAP-110 is required to mediate protein kinase Calpha (PKCalpha) activation of the nonreceptor tyrosine kinase c-Src and the subsequent formation of podosomes. Immunofluorescence analysis demonstrated that activation of PKCalpha by phorbol 12-myristate 13-acetate (PMA), or ectopic expression of constitutively activated PKCalpha, directs AFAP-110 to colocalize with and bind to the c-Src SH3 domain, resulting in activation of the tyrosine kinase. Activation of c-Src then directs the formation of podosomes, which contain cortactin, AFAP-110, actin, and c-Src. In a cell line (CaOV3) that has very little or no detectable AFAP-110, PMA treatment was unable to activate c-Src or effect podosome formation. Ectopic expression of AFAP-110 in CaOV3 cells rescued PKCalpha-mediated activation of c-Src and elevated tyrosine phosphorylation levels and subsequent formation of podosomes. Neither expression of activated PKCalpha nor treatment with PMA was able to induce these changes in CAOV3 cells expressing mutant forms of AFAP-110 that are unable to bind to, or colocalize with, c-Src. We hypothesize that one major function of AFAP-110 is to relay signals from PKCalpha that direct the activation of c-Src and the formation of podosomes.

Development of transgenic mice that inducibly express an active form of c-Src in the epidermis

Recent work from our laboratory has shown that elevated src kinase activity enhances tumor promotion, malignant progression, and metastasis during multistage skin carcinogenesis. In this study, we have generated "gene-switch" src(530) transgenic mice to further analyze the role of this nonreceptor tyrosine kinase in multistage carcinogenesis. Target transgenic mice that have an activated form of the human c-src (src(530)) gene fused with GAL4 binding sites upstream of the thymidine kinase (TK) promoter were generated. Two lines of epidermis-specific transactivator mice were used that targeted the expression of GLVPc or GLp65 transactivators, fusion molecules containing a truncated progesterone receptor with a GAl4-DNA binding domain, with either a mouse loricrin (ML) or human keratin 14 (HK14) promoter, respectively. The transactivator mice (ML.GLVPc or HK14.GLp65) and the target mice (TK.src(530)) were mated to generate bitransgenic mice, and src(530) transgene expression was induced by topical application of RU486 (mifepristone, a progesterone receptor antagonist). In both ML.GLVPc/TK.src(530) and HK14.GLp65/TK.src(530) bitransgenic mice, histological analysis revealed that only the bitransgenic mice had marked epidermal hyperplasia and hyperkeratosis after treatment with RU486. Neither the nontransgenic mice nor the mice hemizygous for either the transactivator transgene or the target transgene alone showed any response to treatment with RU486. In addition, no differences were observed in the skin of the bitransgenic mice versus nontransgenic littermates without treatment of RU486. Interestingly, in HK14.GLp65/TK.src(530) bitransgenic mice, squamous cell carcinomas (SCCs) arose along the periphery of the area of the punch biopsies in 25% of the bitransgenic mice several weeks after taking the biopsy and subsequent to RU486 treatment. Collectively, the data support a role of c-src activation in epidermal hyperproliferation. Furthermore, the data support the conclusion that src activation can substitute for an initiating event in the presence of a tumor promoting stimulus (i.e., wounding). Finally, inducible src(530) transgenic mice provide a new tool for dissecting the role of src activation in multistage carcinogenesis by allowing temporal control of the expression of this oncogene.

Phospholipase C delta-4 overexpression upregulates ErbB1/2 expression, Erk signaling pathway, and proliferation in MCF-7 cells

BACKGROUND

The expression of the rodent phosphoinositide-specific phospholipase C delta-4 (PLCdelta4) has been found to be elevated upon mitogenic stimulation and expression analysis have linked the upregulation of PLCdelta4 expression with rapid proliferation in certain rat transformed cell lines. The human homologue of PLCdelta4 has not been extensively characterized. Accordingly, we investigate the effects of overexpression of human PLCdelta4 on cell signaling and proliferation in this study.

RESULTS

The cDNA for human PLCdelta4 has been isolated and expressed ectopically in breast cancer MCF-7 cells. Overexpression of PLCdelta4 selectively activates protein kinase C-phi and upregulates the expression of epidermal growth factor receptors EGFR/erbB1 and HER2/erbB2, leading to constitutive activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway in MCF-7 cells. MCF-7 cells stably expressing PLCdelta4 demonstrates several phenotypes of transformation, such as rapid proliferation in low serum, formation of colonies in soft agar, and capacity to form densely packed spheroids in low-attachment plates. The growth signaling responses induced by PLCdelta4 are not reversible by siRNA.

CONCLUSION

Overexpression or dysregulated expression of PLCdelta4 may initiate oncogenesis in certain tissues through upregulation of ErbB expression and activation of ERK pathway. Since the growth responses induced by PLCdelta4 are not reversible, PLCdelta4 itself is not a suitable drug target, but enzymes in pathways activated by PLCdelta4 are potential therapeutic targets for oncogenic intervention.

Epidermal Growth Factor Receptor-Mediated Activation of Stat3 during Multistage Skin Carcinogenesis

In the present study, we have investigated the possible role of signal transducers and activators of transcription (STATs), particularly Stat3, in mouse skin tumor promotion and multistage carcinogenesis. Stat1, Stat3, and Stat5 were activated in mouse epidermis after treatment with different classes of tumor promoters, including 12-O-tetradecanoylphorbol-13-acetate (TPA), okadaic acid, and chrysarobin. In addition, Stat1, Stat3, and Stat5 were constitutively activated in skin tumors generated by the two-stage carcinogenesis regimen using 7,12-dimethylbenz(a)anthracene as initiator and TPA as promoter. Several approaches were used to examine the possible role of epidermal growth factor receptor (EGFR) in modulating Stat3 activity during tumor promotion. In primary cultures of mouse keratinocytes, addition of exogenous EGF led to activation of Stat3 as shown by an elevation in tyrosine phosphorylation and nuclear translocation. In epidermis of transgenic mice expressing transforming growth factor alpha under control of the keratin 14 promoter, Stat3 was constitutively activated. Abrogation of EGFR function in mouse epidermis using an EGFR kinase inhibitor or by overexpressing a dominant negative form of EGFR led to a reduction in Stat3 activation in response to TPA treatment. Immunoprecipitation analyses using lysates from TPA-treated epidermis and skin papillomas showed enhanced interaction between the EGFR and Stat3. Finally, Stat3 deficiency in mouse epidermis significantly reduced the proliferative response after TPA treatment. Collectively, the current results suggest that Stat3 activation may be a critical event during mouse skin tumor promotion, possibly through regulation of keratinocyte proliferation. In addition, Stat3 activation in tumor promoter-treated epidermis and in skin papillomas may occur, at least in part, via interaction with and phosphorylation by the EGFR. Finally, constitutive activation of Stat3 in both papillomas and squamous cell carcinomas suggest a role in both the development of autonomous growth and the progression of epithelial tumors in mouse skin.

Inhibition of the epidermal growth factor receptor suppresses telomerase activity in HSC-1 human cutaneous squamous cell carcinoma cells

Activation of telomerase, which stabilizes the telomere length of chromosomes, is crucial for the continued growth or progression of cancer cells. In a previous study, we showed that telomerase is frequently activated in skin tumors. Because epidermal growth factor plays an important role during the tumorigenesis of epithelial tissue, we have now examined the role of epidermal growth factor signaling in regulating telomerase activity using HSC-1 human cutaneous squamous cell carcinoma cells. Treatment of HSC-1 cells with AG 1478, an inhibitor of the epidermal growth factor receptor, or with a neutralizing antibody to the epidermal growth factor receptor, significantly suppressed their telomerase activity, in association with inhibiting their growth. The suppression of telomerase activity was obvious at day 3 and was maximal at day 5 after treatment with AG 1478. The suppression of telomerase activity correlated with the decreased expression of human telomerase catalytic subunit (hTERT) mRNA, the rate-limiting determinant of its enzyme activity. The expression of c-Myc and of Sp1 proteins, transcription factors for hTERT, were also suppressed by AG 1478 in HSC-1 cells, but the expression of Ets-2 protein, another transcription factor, was not affected. The expression of Mad-1, a competitor of c-Myc, was increased. Inhibition of ERK, Src, or Akt suppressed telomerase activity in HSC-1 cells, but to a lesser extent than did treatment with AG 1478. Serum starvation suppressed telomerase activity, but addition of epidermal growth factor or transforming growth factor alpha did not increase it, indicating the involvement of other epidermal growth factor receptor ligands in the activation of telomerase in HSC-1 cells. These data indicate that blockade of the epidermal growth factor receptor might be effective in inhibiting telomerase activity of squamous cell carcinomas, which would lead to the suppression of tumor growth.

Protein kinase C delta induces Src kinase activity via activation of the protein tyrosine phosphatase PTP alpha

Previously we have shown that protein kinase C (PKC)-mediated reorganization of the actin cytoskeleton in smooth muscle cells is transmitted by the non-receptor tyrosine kinase, Src. Several authors have described how 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulation of cells results in an increase of Src activity, but the mechanism of the PKC-mediated Src activation is unknown. Using PKC isozymes purified from Spodoptera frugiperda insect cells, we show here that PKC is not able to activate Src directly. Our data reveal that the PKC-dependent Src activation occurs via the activation of the protein tyrosine phosphatase (PTP) PTP alpha. PTP alpha becomes activated in vivo after TPA stimulation. Further, we show that PKC delta phosphorylates and activates only PTP alpha in vitro but not any other of the TPA-responsive PKC isozymes that are expressed in A7r5 rat aortic smooth muscle cells. To further substantiate our data, we show that cells lacking PKC delta have a markedly reduced PTP alpha and Src activity after 12-O-tetradecanoylphorbol-13-acetate stimulation. These data support a model in which the main mechanism of 12-O-tetradecanoylphorbol-13-acetate-induced Src activation is the direct phosphorylation and activation of PTP alpha by PKC delta, which in turn dephosphorylates and activates Src.

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.

Keratinocyte growth inhibition by high-dose epidermal growth factor is mediated by transforming growth factor beta autoinduction: a negative feedback mechanism for keratinocyte growth

The epidermal growth factor receptor and its ligands initiate a major signaling pathway that regulates keratinocyte growth in an autocrine manner. It is well known that high doses of epidermal growth factor receptor ligands inhibit keratinocyte growth. Recently, signal transducers and activators of transcription 1-dependent p21Waf1/Cip1 induction were reported to be involved in high-dose epidermal growth factor-dependent cell growth arrest in the A431 squamous cell carcinoma cell line; however, transfection of dominant-negative signal transducers and activators of transcription 1 adenovirus vector did not block epidermal growth factor-induced growth inhibition in normal human keratinocytes. As transforming growth factor beta is a potent inhibitor of keratinocyte proliferation, we hypothesized that transforming growth factor beta contributes to epidermal growth factor-mediated keratinocyte growth inhibition. Epidermal growth factor concentrations of 10 ng per ml enhanced transforming growth factor beta1 mRNA expression from 3 to 6 h poststimulation. Enzyme-linked immunosorbent assay analysis detected 150 pg per ml of transforming growth factor beta1 in the culture medium of keratinocytes incubated with 10 and 100 ng per ml epidermal growth factor, whereas 0.1 and 1.0 ng per ml epidermal growth factor slightly enhance transforming growth factor beta1 production. Epidermal growth factor (100 ng per ml) upregulated luciferase activity of p3TP-lux, which contains three tandem transforming growth factor beta-Smad signaling responsive elements, 6-fold compared with unstimulated cells. The epidermal growth factor-dependent induction of p3TP-lux luciferase activity was disrupted by transfection of the dominant negative form of transforming growth factor beta type I receptor adenovirus vector (AxdnALK5), which suggests that epidermal growth factor-induced transforming growth factor beta acts in an autocrine manner in keratinocytes. Moreover, transfection of AxdnALK5 completely blocked the growth inhibition induced by 100 ng per ml of epidermal growth factor in normal keratinocytes. These data demonstrate that an autocrine transforming growth factor beta1-ALK5 pathway is a negative feedback mechanism for epidermal growth factor-induced normal human keratinocyte growth.

Protein kinase C induces actin reorganization via a Src- and Rho-dependent pathway

We have investigated the mechanism of PKC-induced actin reorganization in A7r5 vascular smooth muscle cells. PKC activation by 12-O-tetradecanoylphorbol-13-acetate induces the disassembly of actin stress fibers concomitant with the appearance of membrane ruffles. PKC also induces rapid tyrosine phosphorylation in these cells. As we could show, utilizing the Src-specific inhibitor PP2 and a kinase-deficient c-Src mutant, actin reorganization is dependent on PKC-induced Src activation. Subsequently, the activity of the small G-protein RhoA is decreased, whereas Rac and Cdc42 activities remain unchanged. Disassembly of actin stress fibers could also be observed using the Rho kinase-specific inhibitor Y-27632, indicating that the decrease in RhoA activity on its own is responsible for actin reorganization. In addition, we show that tyrosine phosphorylation of p190RhoGAP is increased upon 12-O-tetradecanoylphorbol-13-acetate stimulation, directly linking Src activation to a decrease in RhoA activity. Our data provide substantial evidence for a model elucidating the molecular mechanisms of PKC-induced actin rearrangements.

Src family kinases phosphorylate protein kinase C delta on tyrosine residues and modify the neoplastic phenotype of skin keratinocytes

Protein kinase C delta (PKC delta) is tyrosine-phosphorylated and catalytically inactive in mouse keratinocytes transformed by a ras oncogene. In several other model systems, Src kinases are upstream regulators of PKC delta. To examine this relationship in epidermal carcinogenesis, v-ras transformed mouse keratinocytes were treated with a selective Src kinase inhibitor (PD 173958). PD 173958 decreased autophosphorylation of Src, Fyn, and Lyn kinases and prevented tyrosine phosphorylation of the Src kinase substrate p120. PD 173958 also prevented PKC delta tyrosine phosphorylation and activated PKC delta as detected by membrane translocation. Expression of keratinocyte differentiation markers increased in PD 173958-treated v-ras-keratinocytes, and fluid-filled domes emerged, indicative of tight junction formation. Antisense PKC delta or bryostatin 1 inhibited dome formation, while overexpression of PKC delta in the presence of PD 173958 enhanced the formation of domes. Plasmids encoding phenylalanine mutants of PKC delta tyrosine residues 64 and 565 induced domes in the absence of PD 173958, while phenylalanine mutants of tyrosine residues 52, 155, and 187 were inactive. Thus, Src kinase mediated post-translational modification of PKC delta on specific tyrosine residues in ras-transformed mouse keratinocytes inactivates PKC delta and contributes to alterations in the differentiated phenotype and tight junction formation associated with neoplasia.

Overexpression of a constitutively active form of c-src in skin epidermis increases sensitivity to tumor promotion by 12-O-tetradecanoylphorbol-13-acetate

Transgenic mice were developed to study the role of c-src in epithelial tumorigenesis through targeted expression of a constitutively active form of murine c-src (src(529)). Src(529) was targeted to the interfollicular epidermis with the human keratin 1 (HK1) promoter. The skin phenotype of these mice was characterized by exaggerated epidermal hyperplasia and hyperkeratosis within the first week after birth. The severity of this phenotype correlated with overall src kinase activity, both of which subsided with age. Treatment of adult HK1.src(529) transgenic mice with the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate resulted in an increase in epidermal hyperplasia and labeling index significantly greater than that seen in nontransgenic littermates. In addition, HK1.src(529) transgenic mice developed papillomas earlier and in significantly greater numbers compared with nontransgenic littermates in a standard initiation-promotion experiment. The data support the hypothesis that activation of c-src kinase plays a role in skin tumor promotion.

Human EGF receptor (HER) family and heregulin members are differentially expressed in epidermal keratinocytes and modulate differentiation

Human EGF receptor (HER), also designated HER1, is an activatable tyrosine kinase receptor. HER1 activation regulates cell growth and differentiation in epidermal keratinocytes. Expression of other HER family members was investigated in human keratinocytes cultured under autocrine conditions. HER2 and HER3 are expressed and upregulated by confluence, concurrent with induction of epidermal differentiation. HER4 is not expressed by keratinocytes. Maximum expression of the cognate ligand, heregulin, is observed in subconfluent keratinocytes, and the expression of both heregulin alpha and beta isoforms is downregulated with confluence. Recombinant heregulin isoforms have no effect on colony formation and keratinocyte proliferation, but heregulin beta activates tyrosine phosphorylation of HER2 and HER3, with no effect on HER1, in confluent differentiating keratinocyte cultures. Also, heregulin beta increases HER2/HER3 heterodimerization under those conditions. Treatment of confluent cultures by heregulin beta correlates with cell signaling and inhibition of epidermal differentiation. Together, HER2, HER3, and heregulin constitute a potential autocrine-paracrine system involved in epidermal homeostasis and repair, as well as in hyperproliferative pathologies.

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.

Tob-mediated cross-talk between MARCKS phosphorylation and ErbB-2 activation

The biochemical path for the activation of ErbB-2 by PKC activator was investigated in MDA-MB-231 human breast cancer cells. We found that PMA-induced phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) increased its binding with Tob that exerts an anti-proliferative effect through the binding with ErbB-2. The phosphorylation site domain (PSD) of MARCKS was relevant to its interaction with Tob. Decreased binding of Tob with ErbB-2 and subsequent activation of ErbB-2 were observed in MDA-MB-231 cells in response to PMA treatment. The present study proposes that MARCKS phosphorylation by PKC removes Tob from ErbB-2 by increasing its binding affinity with Tob, and thereby activates the ErbB-2 mediated signal transduction.

Constitutive expression of erbB2 in epidermis of transgenic mice results in epidermal hyperproliferation and spontaneous skin tumor development

The erbB family of receptor tyrosine kinases, which consists of the epidermal growth factor receptor (EGFr/erbB1), erbB2 (neu), erbB3 and erbB4, has been shown to be important for both normal development as well as neoplasia. The expression of rat erbB2 was targeted to the basal layer of mouse epidermis with the bovine keratin 5 promoter. Overexpression of wild type rat erbB2 in the basal layer of epidermis led to alopecia, follicular hyperplasia and sebaceous gland enlargement as well as hyperplasia of the interfollicular epidermis. Spontaneous papillomas, some of which converted to squamous cell carcinomas, arose in homozygous erbB2 transgenic mice as early as 6 weeks of age with >90% incidence by 6 months. Analysis of several proliferation/differentiation markers indicated that erbB2 overexpression led to epidermal hyperproliferation and a possible delay in epidermal differentiation. Transgenic mice were also hypersensitive to the proliferative effects of the skin tumor promoter, 12-0-tetradecanoylphorbol-13-acetate (TPA) and were more sensitive to two-stage carcinogenesis. Elevations in EGFr and erbB2 protein as well as erbB2:EGFr and erbB2:erbB3 heterodimers were observed in skin of the erbB2 transgenic mice. Phosphotyrosine levels of the EGFr, erbB2 and erbB3 proteins were also elevated. These results indicate an important role for erbB2 signaling in epidermal growth, development and neoplasia. Oncogene (2000) 19, 4243 - 4254

Alteration of Egr-1 mRNA during multistage carcinogenesis in mouse skin

Immediate early genes, including fos, jun, and early growth response-1 (Egr-1), are induced during cellular response to changes in extracellular environment. These immediate early genes are believed to mediate processes of cell growth and differentiation. In particular, Egr-1 is induced during mitogenic stimulation of a variety of cell types, including fibroblasts, B cells, and epithelial cells. In the present study, we examined Egr-1 gene expression during multistage carcinogenesis in mouse skin. After a single topical treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) to SENCAR mouse skin, Egr-1 mRNA was induced, and maximal induction was observed at 2 h in both epidermis and dermis. Induction of Egr-1 mRNA by TPA was inhibited by fluocinolone acetonide, a potent inhibitor of tumor promotion by TPA. Egr-1 mRNA was present in primary keratinocytes derived from adult SENCAR mice. The keratinocyte cultures were maintained in low Ca(2+) medium, and Egr-1 mRNA levels became significantly elevated after the cultures were switched to high Ca(2+) medium. Additionally, a large proportion of primary papillomas and carcinomas generated from SENCAR mice by standard initiation-promotion regimens exhibited elevated Egr-1 mRNA compared with normal epidermis. Taken together, these data suggest a possible role of Egr-1 during multistage carcinogenesis in mouse skin.

Cellular functions regulated by Src family kinases

Src family protein tyrosine kinases are activated following engagement of many different classes of cellular receptors and participate in signaling pathways that control a diverse spectrum of receptor-induced biological activities. While several of these kinases have evolved to play distinct roles in specific receptor pathways, there is considerable redundancy in the functions of these kinases, both with respect to the receptor pathways that activate these kinases and the downstream effectors that mediate their biological activities. This chapter reviews the evidence implicating Src family kinases in specific receptor pathways and describes the mechanisms leading to their activation, the targets that interact with these kinases, and the biological events that they regulate.