Abstract 2172: Anti-oncogenic role of peroxisome proliferator-activated receptor-α/β (PPARδ/α) involves inhibition of mitosis and regulation of Hras1-induced senescence

Ligand activation of PPARδ/α inhibits chemically-induced skin tumorigenesis and Pparα/α-null mice exhibit enhanced chemically-induced skin tumorigenesis compared to wild-type mice. The mechanism that mediates inhibition of skin carcinogenesis by PPARδ/α is still being unraveled and was examined in this study. Ligand activation of PPARδ/α caused a negative selection against cells expressing higher levels of HRAS by inducing a mitotic block. Mitosis-related genes that are predominantly regulated by E2F were induced to a higher level in HRAS-expressing Pparα/α-null keratinocytes as compared to HRAS-expressing wild-type keratinocytes. Ligand activated PPARδ/α repressed expression of these genes by direct binding with p130/p107, facilitating nuclear translocation, and increasing promoter recruitment of p130/p107. In addition, co-treatment with a PPARδ/α ligand and various mitosis inhibitors increases the efficacy of increasing G2/M arrest. PPARδ/α is also required for HRAS-induced senescence and suppression of malignant conversion of benign papillomas. HRAS-expressing Pparα/α-null keratinocytes evade senescence by utilizing the PI3K-AKT rather than the MEK-ERK pathway. PPARδ/α positively regulates MEK-ERK pathway by maintaining RASGRP1 levels and negatively regulating the PI3K-AKT pathway by repressing ILK and PDPK1 expression. Heightened AKT activity increases endoplasmic reticulum (ER) stress in Pparα/α-null keratinocytes, which in turn helps maintain higher AKT activity, leading to evasion of senescence. In addition, enhanced HRAS activity increases the synthesis of endogenous PPARδ/α ligands, thus providing a PPARδ/α-dependent cell-autonomous mechanism to suppress HRAS-induced carcinogenesis. (Supported by CA124533).

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2172. doi:1538-7445.AM2012-2172

Pharmacologic inhibition of ALK5 causes selective induction of terminal differentiation in mouse keratinocytes expressing oncogenic HRAS

TGFβ has both tumor suppressive and oncogenic roles in cancer development. We previously showed that SB431542 (SB), a small molecule inhibitor of the TGFβ type I receptor (ALK5) kinase, suppressed benign epidermal tumor formation but enhanced malignant conversion. Here, we show that SB treatment of primary K5rTA/tetORASV12G bitransgenic keratinocytes did not alter HRASV12G-induced keratinocyte hyperproliferation. However, continuous SB treatment significantly enhanced HRASV12G-induced cornified envelope formation and cell death linked to increased expression of enzymes transglutaminase (TGM) 1 and TGM3 and constituents of the cornified envelope small proline-rich protein (SPR) 1A and SPR2H. In contrast, TGFβ1 suppressed cornified envelope formation in HRASV12G keratinocytes. Similar results were obtained in HRASV12G transgenic mice treated topically with SB or by coexpressing TGFβ1 and HRASV12G in the epidermis. Despite significant cell death, SB-resistant HRASV12G keratinocytes repopulated the primary culture that had overcome HRas-induced senescence. These cells expressed reduced levels of p16(ink4a) and were growth stimulated by SB but remained sensitive to a calcium-induced growth arrest. Together these results suggest that differential responsiveness to cornification may represent a mechanism by which pharmacologic blockade of TGFβ signaling can inhibit the outgrowth of preneoplastic lesions but may cause a more progressed phenotype in a separate keratinocyte population.

Abstract 1227: Anti-oncogenic role of Peroxisome proliferator-activated receptor- β/Δ (Pparβ/Δ) involves regulation of viral Hras1 (v-Hras1)-induced senescence and endoplasmic reticulum (ER) stress in mouse primary keratinocytes

Pparβ/Δ-null mice exhibit enhanced chemically-induced skin tumorigenesis compared to wild-type counterparts. Since an oncogenic mutation of Harvey sarcoma ras virus gene (v-Hras1) is a critical event in chemical carcinogenesis, the role of PPARβ/Δ in mouse primary keratinocytes infected with a retrovirus encoding viral Hras1 (v-Hras1) gene was examined. While v-Hras1 induced senescence in wild-type mouse primary keratinocytes, the level of v-Hras1-induced senescence was significantly reduced in Pparβ/Δ-null keratinocytes, which was due in part to decreased phospho-MEK and phospho-ERK during the early stage of v-Hras1 expression. In addition, while total v-Hras1 protein level was comparable between wild-type and Pparβ/Δ-null keratinocytes, the level of the GTP-bound form of v-Hras1 was significantly decreased in Pparβ/Δ-null keratinocytes from 4 day to 7 day post v-Hras1 infection. Surprisingly, the decreased activity of v-Hras1 did not lead to a decreased level of phospho-AKT in Pparβ/Δ-null keratinocytes. However, Pparβ/Δ-null keratinocytes were more dependent upon PI3K-AKT pathway for maintenance of proliferation and evasion of senescence. Differentially expressed genes between wild-type and Pparβ/Δ-null keratinocytes that are involved in the regulation of senescence were determined by cDNA microarray analysis. Differential expressions of both positive and negative regulators of RAS pathway were observed between wild-type and Pparβ/Δ-null keratinocytes following v-Hras1 infection. Notably, the mRNA encoding CDK inhibitor p27Kip1 was lower in Pparβ/Δ-null keratinocytes, In addition, Pparβ/Δ-null keratinocytes showed enhanced endoplasmic reticulum (ER) stress and unfolded protein response (UPR) post v-Hras1 infection, characterized by enhanced vacuolation and higher levels of major ER stress markers. However, the enhanced ER stress did not lead to apoptosis in Pparβ/Δ-null keratinocytes. These results suggest an anti-oncogenic role of PPARβ/Δ in v-Hras1-transformed mouse primary keratinocytes that involves both the promotion of anti-tumorigenic senescence and attenuation of pro-tumorigenic ER stress and UPR. (Supported by CA124533)

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1227. doi:10.1158/1538-7445.AM2011-1227

Use of a TGFbeta type I receptor inhibitor in mouse skin carcinogenesis reveals a dual role for TGFbeta signaling in tumor promotion and progression

Pharmacological inhibitors of the transforming growth factor β (TGFβ) type I receptor (ALK5) have shown promise in blocking growth of xenotransplanted cancer cell lines but the effect on a multistage cancer model is not known. To test this, we treated mouse skin with SB431542 (SB), a well-characterized ALK5 inhibitor, during a two-stage skin carcinogenesis assay. Topical SB significantly reduced the total number, incidence and size of papillomas compared with 12-O-tetradecanoylphorbol 13-acetate (TPA) promotion alone, and this was linked to increased epidermal apoptosis, decreased proliferation and decreased cutaneous inflammation during promotion. In contrast, the frequency of conversion to squamous cell carcinoma (SCC) was 2-fold higher in papillomas treated with SB. Although there was no difference in tumor cell proliferation in early premalignant lesions, those that formed after SB treatment exhibited reduced squamous differentiation and an altered inflammatory microenvironment similar to SCC. In an inducible epidermal RAS transgenic model, treatment with SB enhanced proliferation and cutaneous inflammation in skin but decreased expression of keratin 1 and increased expression of simple epithelial keratin 18, markers of premalignant progression. In agreement with increased frequency of progression in the multistage model, SB treatment resulted in increased tumor formation with a more malignant phenotype following long-term RAS induction. In contrast to the current paradigm for TGFβ in carcinogenesis, these results demonstrate that cutaneous TGFβ signaling enables promotion of benign tumors but suppresses premalignant progression through context-dependent regulation of epidermal homeostasis and inflammation.

Tubular overexpression of transforming growth factor-beta1 induces autophagy and fibrosis but not mesenchymal transition of renal epithelial cells

We recently showed in a tetracycline-controlled transgenic mouse model that overexpression of transforming growth factor (TGF)-beta1 in renal tubules induces widespread peritubular fibrosis and focal degeneration of nephrons. In the present study we have analyzed the mechanisms underlying these phenomena. The initial response to tubular cell-derived TGF-beta1 consisted of a robust proliferation of peritubular cells and deposition of collagen. On sustained expression, nephrons degenerated in a focal pattern. This process started with tubular dedifferentiation and proceeded to total decomposition of tubular cells by autophagy. The final outcome was empty collapsed remnants of tubular basement membrane embedded into a dense collagenous fibrous tissue. The corresponding glomeruli survived as atubular remnants. Thus, TGF-beta1 driven autophagy may represent a novel mechanism of tubular decomposition. The fibrosis seen in between intact tubules and in areas of tubular decomposition resulted from myofibroblasts that were derived from local fibroblasts. No evidence was found for a transition of tubular cells into myofibroblasts. Neither tracing of injured tubules in electron micrographs nor genetic tagging of tubular epithelial cells revealed cells transgressing the tubular basement membrane. In conclusion, overexpression of TGF-beta1 in renal tubules in vivo induces interstitial proliferation, tubular autophagy, and fibrosis, but not epithelial-to-mesenchymal transition.

TGFbeta1-induced inflammation in premalignant epidermal squamous lesions requires IL-17

Overexpression of transforming growth factor-beta1 (TGFbeta1) in the normal epidermis can provoke an inflammatory response, but whether this occurs within a developing tumor is not clear. To test this, we used an inducible transgenic mouse to overexpress TGFbeta1 in premalignant squamous lesions. Within 48 hours of TGFbeta1 induction, there was an increase in IL-17 production by both CD4(+) and gammadelta(+) T cells, together with increased expression of T-helper-17 (Th17)-polarizing cytokines. Induction of TGFbeta1 in premalignant primary keratinocytes elevated the expression of proinflammatory and Th17-polarizing cytokines, and the keratinocyte-conditioned media caused IL-17 production by naive T cells that was dependent on T-cell TGFbeta1 signaling. Microarray analysis showed significant upregulation of proinflammatory genes 2 days after TGFbeta1 induction, and this was followed by increased MPO(+), F4/80(+), and CD8(+) cells in tumors, increased CD8(+) effectors and IFNgamma(+) cells in skin-draining LNs, and tumor regression. In parallel, the percentage of tumor CD11b(+)Ly6G(+) neutrophils was reduced. Neutralization of IL-17 blocked TGFbeta1-induced CD11b(+) Ly6G(-) tumor infiltration but did not alter the reduction of neutrophils or tumor regression. Thus, TGFbeta1 overexpression causes IL-17-dependent and IL-17-independent changes in the premalignant tumor inflammatory microenvironment.

Transforming growth factor beta1 enhances tumor promotion in mouse skin carcinogenesis

Transforming growth factor beta1 (TGFbeta1) expression is elevated by tumor promoters in the mouse skin, but its role in tumor promotion has not been well defined. To investigate this, we have compared TGFbeta1+/+ and +/- mice in a two-stage skin chemical carcinogenesis protocol. Surprisingly, TGFbeta1+/- mice had fewer number and incidence of benign papillomas, reduced epidermal and tumor cell proliferation and reduced epidermal TGFbeta1 and nuclear p-Smad2 localization in response to the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) compared with TGFbeta1+/+ mice. Maximal TPA activation of protein kinase C (PKCalpha) as measured by activity assays and activation of target genes and induction of cornified envelopes correlated with TGFbeta1 gene dosage in keratinocytes and addition of exogenous TGFbeta1 restored the cornification defect in TGFbeta1+/- keratinocytes. Similarly, inhibition of ALK5-suppressed TPA-mediated PKCalpha activation suggesting that physiological levels of TGFbeta1 are required for maximal activation of PKC-dependent mitogenic responses. Paradoxically, the TPA-induced inflammatory response was greater in TGFbeta1+/- skin, but TGFbeta1+/+ papillomas had more tumor infiltrating myeloperoxidase-positive cells and pro-inflammatory gene expression was elevated in v-ras(Ha)-transduced TGFbeta1+/+ but not TGFbeta1+/- keratinocytes. Thus, ras activation switches TGFbeta1 to a pro-inflammatory cytokine. Despite this differential proliferative and inflammatory response to TPA and enhanced papilloma formation in the TGFbeta1+/+ mice, the frequency of malignant conversion was reduced compared with TGFbeta1+/- mice. Therefore, TGFbeta1 promotes benign tumors by modifying tumor promoter-induced cell proliferation and inflammation but retains a suppressive function for malignant conversion.

The transcriptional regulatory function of p53 is essential for suppression of mouse skin carcinogenesis and can be dissociated from effects on TGF-beta-mediated growth regulation

Transcriptional regulation by p53 is critical for p53-mediated tumour suppression; however, p53-mediated transactivation has been dissociated from p53-mediated biological processes including apoptosis, DNA repair, and differentiation. We compared the effects of a mutant allele, p53(QS - val135), containing a double mutation in the amino-terminus abrogating transactivation activity and a modification at amino acid 135 partially affecting DNA binding, to complete loss of p53. We applied in vitro endpoints correlated with epithelial tumourigenesis and an in vivo assay of tumour phenotype to assess whether loss of p53-mediated transcriptional regulation underlies the malignant phenotype of p53(-/-)/v-ras(Ha)-overexpressing keratinocytes. Transactivation deficiency of p53QS-val135 was confirmed by reporter gene assays in fibroblasts and differentiating keratinocytes. Ras oncogene-induced senescence was lost in both p53(QS - val135/QS - val135) and p53(-/-) keratinocytes. Similarly, p53(QS - val135/QS - val135), like p53(-/-), cooperated with v-ras(Ha) to enhance malignant conversion. The tumours arising in p53(QS - val135/QS - val135) keratinocytes displayed strong nuclear p53 expression; thus, the p53(QS - val135) allele was maintained and the deficient transactivation function of the expressed p53QS mutant protein was supported by absence of p21(waf1) in these tumours. The p53(QS - val135) allele did not confer a dominant-negative phenotype, as p53(+/QS - val135) keratinocytes senesced normally in response to v-ras(Ha) expression and formed benign tumours. While p53(-/-) keratinocytes displayed diminished response to TGF-beta, p53(QS - val135/QS - val135) and p53(+/+) keratinocytes responded equivalently, indicating that the requirement for p53 in maximizing TGF-beta-mediated growth regulation is independent of its transactivation domain and that the ability of keratinocytes to respond to TGF-beta is insufficient to suppress the malignant phenotype in this model. Furthermore, TGF-beta enhances p53QS-induced activation of a dual p53-TGF-beta responsive reporter in a keratinocyte cell line. These findings support an essential role for p53-mediated transcriptional regulation in suppressing malignancies arising from ras-induced skin tumours, consistent with previous findings in spontaneous carcinogenesis in other organs, and highlight the potential importance of senescence for tumour suppression in vivo.

Tumor suppressor and oncogene actions of TGFbeta1 occur early in skin carcinogenesis and are mediated by Smad3

Interactions between TGFbeta1 and ras signaling pathways play an important role in cancer development. Here we show that in primary mouse keratinocytes, v-ras(Ha) does not block the early biochemical events of TGFbeta1 signal transduction but does alter global TGFbeta1 mediated gene expression in a gene specific manner. Expression of Smad3 dependent TGFbeta1 early response genes and the TGFbeta1 cytostatic gene expression response were not altered by v-ras(Ha) consistent with an intact TGFbeta1 growth arrest. However, TGFbeta1 and v-ras(Ha) cause significant alteration in genes regulating matrix remodeling as the TGFbeta1 induction of extracellular matrix genes was blocked by v-ras(Ha) but specific matrix proteases associated with cancer progression were elevated. Smad3 deletion in keratinocytes repressed normal differentiation maker expression and caused expression of Keratin 8 a simple epithelial keratin and marker of malignant conversion. Smad3 was required for the TGFbeta1 cytostatic response in v-ras(Ha) keratinocytes, but also for protease induction, keratinocyte attachment and migration. These results show that pro-oncogenic activities of TGFbeta1 can occur early in carcinogenesis before loss of its tumor suppressive function and that selective regulation rather than complete inactivation of Smad3 function may be crucial for tumor progression.

Pathological responses to oncogenic Hedgehog signaling in skin are dependent on canonical Wnt/beta3-catenin signaling

Constitutive Hedgehog (Hh) signaling underlies several human tumors, including basal cell carcinoma (BCC) and basaloid follicular hamartoma in skin. Intriguingly, superficial BCCs arise as de novo epithelial buds resembling embryonic hair germs, collections of epidermal cells whose development is regulated by canonical Wnt/beta-catenin signaling. Similar to embryonic hair germs, human BCC buds showed increased levels of cytoplasmic and nuclear beta-catenin and expressed early hair follicle lineage markers. We also detected canonical Wnt/ beta-catenin signaling in epithelial buds and hamartomas from mice expressing an oncogene, M2SMO, leading to constitutive Hh signaling in skin. Conditional overexpression of the Wnt pathway antagonist Dkk1 in M2SMO-expressing mice potently inhibited epithelial bud and hamartoma development without affecting Hh signaling. Our findings uncover a hitherto unknown requirement for ligand-driven, canonical Wnt/ beta-catenin signaling for Hh pathway-driven tumorigenesis, identify a new pharmacological target for these neoplasms and establish the molecular basis for the well-known similarity between early superficial BCCs and embryonic hair germs.

An efficient and versatile system for acute and chronic modulation of renal tubular function in transgenic mice

We describe a transgenic mouse line, Pax8-rtTA, which, under control of the mouse Pax8 promoter, directs high levels of expression of the reverse tetracycline-dependent transactivator (rtTA) to all proximal and distal tubules and the entire collecting duct system of both embryonic and adult kidneys. Using crosses of Pax8-rtTA mice with tetracycline-responsive c-MYC mice, we established a new, inducible model of polycystic kidney disease that can mimic adult onset and that shows progression to renal malignant disease. When targeting the expression of transforming growth factor beta-1 to the kidney, we avoided early lethality by discontinuous treatment and successfully established an inducible model of renal fibrosis. Finally, a conditional knockout of the gene encoding tuberous sclerosis complex-1 was achieved, which resulted in the early outgrowth of giant polycystic kidneys reminiscent of autosomal recessive polycystic kidney disease. These experiments establish Pax8-rtTA mice as a powerful tool for modeling renal diseases in transgenic mice.

Transgenic expression of interleukin-13 in the skin induces a pruritic dermatitis and skin remodeling

IL-13 has been implicated in the pathogenesis of allergic diseases, including atopic dermatitis (AD). However, a direct role of IL-13 in AD has not been established. We aimed to develop an inducible transgenic model in which IL-13 can be expressed in the skin and to define the resulting dermal phenotype and mechanisms involved. The keratin 5 promoter was used with a tetracycline-inducible system to target IL-13 to the skin. The clinical manifestations, dermal histology, cytokine gene regulation, and systemic immune responses in the transgenic mice were assessed. IL-13 was produced exclusively in the skin and caused a chronic inflammatory phenotype characterized by xerosis and pruritic eczematous lesions; dermal infiltration of CD4+ T cells, mast cells, eosinophils, macrophages, and Langerhans cells; upregulation of chemokine and cytokine genes, including thymic stromal lymphopoietin; and skin remodeling with fibrosis and increased vasculature. The dermal phenotype was accompanied by elevated serum total IgE and IgG1 and increased production of IL-4 and IL-13 by CD4+ cells from lymphoid tissues and peripheral blood mononuclear cells. IL-13 is a potent stimulator of dermal inflammation and remodeling and this transgenic model of AD is a good tool for investigating the underlying mechanisms in the pathogenesis of AD.

Ligand activation of peroxisome proliferator-activated receptor beta/delta (PPAR beta/delta) inhibits chemically induced skin tumorigenesis

Peroxisome proliferator-activated receptor (PPAR)beta/delta-null mice exhibit enhanced tumorigenesis in a two-stage chemical carcinogenesis model as compared with wild-type mice. Previous work showed that ligand activation of PPARbeta/delta induces terminal differentiation and inhibits proliferation of primary keratinocytes, and this effect does not occur in the absence of PPARbeta/delta expression. In the present studies, the effect of ligand activation of PPARbeta/delta on skin tumorigenesis was examined using both in vivo and ex vivo skin carcinogenesis models. Inhibition of chemically induced skin tumorigenesis was observed in wild-type mice administered GW0742, and this effect was likely the result of ligand-induced terminal differentiation and inhibition of replicative DNA synthesis. These effects were not found in similarly treated PPARbeta/delta-null mice. Ligand activation of PPARbeta/delta also inhibited cell proliferation and induced terminal differentiation in initiated/neoplastic keratinocyte cell lines representing different stages of skin carcinogenesis. These studies suggest that topical administration of PPARbeta/delta ligands may be useful as both a chemopreventive and/or a chemotherapeutic approach to inhibit skin cancer.

Cripto-1 alters keratinocyte differentiation via blockade of transforming growth factor-beta1 signaling: role in skin carcinogenesis

Cripto-1 is an epidermal growth factor-Cripto/FRL1/Cryptic family member that plays a role in early embryogenesis as a coreceptor for Nodal and is overexpressed in human tumors. Here we report that in the two-stage mouse skin carcinogenesis model, Cripto-1 is highly up-regulated in tumor promoter-treated normal skin and in benign papillomas. Treatment of primary mouse keratinocytes with Cripto-1 stimulated proliferation and induced expression of keratin 8 but blocked induction of the normal epidermal differentiation marker keratin 1, changes that are hallmarks of tumor progression in squamous cancer. Chemical or genetic blockade of the transforming growth factor (TGF)-beta1 signaling pathway using the ALK5 kinase inhibitor SB431542 and dominant negative TGF-beta type II receptor, respectively, had similar effects on keratinocyte differentiation. Our results show that Cripto-1 could block TGF-beta1 receptor binding, phosphorylation of Smad2 and Smad3, TGF-beta-responsive luciferase reporter activity, and TGF-beta1-mediated senescence of keratinocytes. We suggest that inhibition of TGF-beta1 by Cripto-1 may play an important role in altering the differentiation state of keratinocytes and promoting outgrowth of squamous tumors in the mouse epidermis.

Endothelin 3 Induces Skin Pigmentation in a Keratin-Driven Inducible Mouse Model

Endothelin 3 (Edn3) encodes a ligand important to developing neural crest cells and is allelic to the spontaneous mouse mutation occurring at the lethal spotting (ls) locus. Edn3(ls/ls) mutants exhibit a spotted phenotype due to reduced numbers of neural crest-derived melanocyte precursors in the skin. In this study, we show that when Edn3 is driven by the keratin 5 promoter and thereby placed proximal to melanocyte lineage cells, adult mice manifest pigmented skin harboring dermal melanocytes. Using a tetracycline inducible system, we show that the postnatal expression of Edn3 is required to maintain these dermal melanocytes, and that early expression of the Edn3 transgene is important to the onset of the hyperpigmentation phenotype. Crosses into Edn3(ls/ls) mutants demonstrate that the Edn3 transgene expression does not fully compensate for the endogenous expression pattern. Crosses into tyrosine kinase receptor Kit(Wv) mutants indicate that Edn3 can partially compensate for Kit's role in early development. Crosses into A(y) mutant mice considerably darkened their yellow coat color suggesting a previously unreported role for endothelin signaling in pigment switching. These results demonstrate that exogenous Edn3 affects both precursors and differentiated melanocytes, leading to a phenotype with characteristics similar to the human skin condition dermal melanocytosis.

Multiple forms of TGF-beta: distinct promoters and differential expression

There are now five known distinct isoforms of TGF-beta with 64-82% identity. Of these, only TGF-beta 1, 2 and 3 thus far have been demonstrated to be expressed in mammalian tissues; TGF-beta 4 has been described only in chicken and TGF-beta 5 only in frog. Although the biological activities of these five isoforms of TGF-beta are indistinguishable in most in vitro assays their sites of synthesis and localization in vivo are often distinct. Expression of the various isoforms is differentially controlled both in vivo, as in development, and in vitro after treatment of cells with steroids, such as oestrogen or tamoxifen, or with retinoids. To investigate the basis of these observations we have cloned and characterized the promoters for the human TGF-beta 1, 2 and 3 genes. Significant differences have been found: whereas the TGF-beta 1 promoter has no TATAA box and is regulated principally by AP-1 sites, both the TGF-beta 2 and 3 promoters have TATAA boxes as well as AP-2 sites and cAMP-responsive elements. Accordingly, TGF-beta 1 gene expression is induced strongly by phorbol esters whereas that of TGF-beta 2 and 3 is induced by forskolin, an activator of adenylate cyclase. Expression of TGF-beta 2 and 3 is often coordinately regulated in vivo in a pattern distinct from that of TGF-beta 1.

Context-dependent regulation of cutaneous immunological responses by TGFbeta1 and its role in skin carcinogenesis

Transforming growth factor beta1 (TGFbeta1) signaling plays a critical role in skin carcinogenesis. While most studies have focused on TGFbeta1 signaling and response in keratinocytes, it is now becoming clear that the interaction of keratinocyte-derived TGFbeta1 with cells of the immune system has an equally important role in tumor development. Tumors form within the context of innate and adaptive immune responses and studies in skin and skin carcinogenesis models have provided important insight into the impact of context-dependent pro-inflammatory and immunosuppressive actions of TGFbeta1 on tumor development. Indeed, the paradigm of TGFbeta1 duality is clearly evident in its ability to both promote and inhibit inflammatory responses. Recent studies have begun to shed new light on the molecular basis for these actions and to provide insight into how these may contribute to context-dependent effects of TGFbeta1 on carcinogenesis in the skin and other epithelial tissues.

Expression profile of skin papillomas with high cancer risk displays a unique genetic signature that clusters with squamous cell carcinomas and predicts risk for malignant conversion

Chemical induction of squamous tumors in the mouse skin induces multiple benign papillomas: high-frequency terminally benign low-risk papillomas and low-frequency high-risk papillomas, the putative precursor lesions to squamous cell carcinoma (SCC). We have compared the gene expression profile of twenty different early low- and high-risk papillomas with normal skin and SCC. Unsupervised clustering of 514 differentially expressed genes (P<0.001) showed that 9/10 high-risk papillomas clustered with SCC, while 1/10 clustered with low-risk papillomas, and this correlated with keratin markers of tumor progression. Prediction analysis for microarrays (PAM) identified 87 genes that distinguished the two papilloma classes, and a majority of these had a similar expression pattern in both high-risk papillomas and SCC. Additional classifier algorithms generated a gene list that correctly classified unknown benign tumors as low- or high-risk concordant with promotion protocol and keratin profiling. Reduced expression of immune function genes characterized the high-risk papillomas and SCC. Immunohistochemistry confirmed reduced T-cell number in high-risk papillomas, suggesting that reduced adaptive immunity defines papillomas that progress to SCC. These results demonstrate that murine premalignant lesions can be segregated into subgroups by gene expression patterns that correlate with risk for malignant conversion, and suggest a paradigm for generating diagnostic biomarkers for human premalignant lesions with unknown individual risk for malignant conversion.

Inhibition of transforming growth factor-beta1 signaling attenuates ataxia telangiectasia mutated activity in response to genotoxic stress

Ionizing radiation causes DNA damage that elicits a cellular program of damage control coordinated by the kinase activity of ataxia telangiectasia mutated protein (ATM). Transforming growth factor beta (TGFbeta)-1, which is activated by radiation, is a potent and pleiotropic mediator of physiologic and pathologic processes. Here we show that TGFbeta inhibition impedes the canonical cellular DNA damage stress response. Irradiated Tgfbeta1 null murine epithelial cells or human epithelial cells treated with a small-molecule inhibitor of TGFbeta type I receptor kinase exhibit decreased phosphorylation of Chk2, Rad17, and p53; reduced gammaH2AX radiation-induced foci; and increased radiosensitivity compared with TGFbeta competent cells. We determined that loss of TGFbeta signaling in epithelial cells truncated ATM autophosphorylation and significantly reduced its kinase activity, without affecting protein abundance. Addition of TGFbeta restored functional ATM and downstream DNA damage responses. These data reveal a heretofore undetected critical link between the microenvironment and ATM, which directs epithelial cell stress responses, cell fate, and tissue integrity. Thus, Tgfbeta1, in addition to its role in homoeostatic growth control, plays a complex role in regulating responses to genotoxic stress, the failure of which would contribute to the development of cancer; conversely, inhibiting TGFbeta may be used to advantage in cancer therapy.

Tissue homeostasis and the control of the neoplastic phenotype in epithelial cancers

Neoplastic cells at various stages of tumor progression may remain dormant for many years. The suppression of the neoplastic phenotype and tumor outgrowth depends on close contact of neoplastic cells with surrounding normal cells. This review examines the nature of these contacts primarily in models for skin cancer induction. Junctional complexes, membrane associated growth factors and their receptors, and paracrine mechanisms likely contribute to this state of tumor cell dormancy. Understanding these mechanisms will be important in primary cancer prevention and for counteracting recurrences in cancer survivors.

Involvement of Smad signaling in sphingosine 1-phosphate-mediated biological responses of keratinocytes

The lysophospholipid sphingosine 1-phosphate and the cytokine-transforming growth factor beta are both released from degranulating platelets at wound sites, suggesting a broad spectrum of effects involved in wound healing. Interestingly, both of these molecules have been previously shown to induce chemotaxis but to strongly inhibit the growth of keratinocytes, while stimulating the proliferation of fibroblasts. In contrast to sphingosine 1-phosphate, the signaling cascade of the growth factor has been extensively examined. Specifically, Smad3 has been shown to be an essential mediator of transforming growth factor beta-dependent chemotaxis of keratinocytes and mediates, in part, its growth-inhibitory effect. Here we show that sphingosine 1-phosphate, independently of transforming growth factor beta secretion, induces a rapid phosphorylation of Smad3 on its C-terminal serine motif and induces its partnering with Smad4 and the translocation of the complex into the nucleus. Moreover, sphingosine 1-phosphate fails to induce chemotaxis or inhibit the growth of Smad3-deficient keratinocytes, suggesting that Smad3 plays an unexpected functional role as a new target in sphingosine 1-phosphate signaling. Both sphingosine 1-phosphate receptors and the transforming growth factor beta-type I receptor serine/threonine kinase are essential for activation of Smad3 by this lysophospholipid and the dependent biological responses, indicating a novel cross-talk between serine/threonine kinase receptors and G-protein coupled receptors.

TGFbeta1, back to the future: revisiting its role as a transforming growth factor

TGFbeta1 was initially identified in culture media from transformed cells as part of a factor that could produce a transformed phenotype in a nontransformed cell line. Subsequently this activity was separated into TGFbeta and TGFalpha an EGF receptor ligand. With the discovery that TGFbeta1 was a potent growth inhibitor of epithelial cells, and the identification of inactivating mutations within the TGFbeta1 signaling pathway in cancers it became clear that TGFbeta1 signaling is a tumor suppressor pathway for early stages of cancer. However many human carcinomas overexpress TGFbeta1 and this is associated with poor patient prognosis and increased frequency of metastasis. Similar results have been obtained with tumor cell lines and experimental animal models. Thus stage specific duality of function is the emerging paradigm for the role of TGFbeta1 in cancer. This review will focus on the evidence for TGFbeta1 as a tumor promoting and metastasis factor and examine the biological and molecular basis for these effects. It is proposed that the switch from tumor suppressor to oncogene reflects genetic or epigenetic alterations in signaling pathways in tumor cells that alter the readout from the TGFbeta1 pathway.

Smad3 regulates senescence and malignant conversion in a mouse multistage skin carcinogenesis model

Transforming growth factor beta (TGF-beta) is a growth-inhibitory cytokine for epithelial cells. In the mouse multistage skin carcinogenesis model, defects in TGF-beta 1 signaling reduce senescence in vitro and accelerate malignant progression in vivo. However, the precise postreceptor signaling pathways and specific roles played by Smad proteins in this process have not been defined. Here we show that senescence of v-ras(Ha)-transduced Smad3 null keratinocytes is delayed, whereas overexpression of Smad3, but not Smad2 or Smad4, induced senescence. The TGF-beta 1 target genes c-myc and p15(ink4b) were deregulated in the absence of Smad3. When transplanted to a graft site on nude mice, the v-ras(Ha)-transduced Smad3 null keratinocytes underwent rapid conversion from benign papilloma to malignant carcinoma, whereas wild-type keratinocytes predominantly formed papillomas. These results link Smad3-mediated regulation of growth control genes to senescence in vitro and tumor suppression in vivo.

Mice lacking Smad3 show accelerated wound healing and an impaired local inflammatory response

The generation of animals lacking SMAD proteins, which transduce signals from transforming growth factor-beta (TGF-beta), has made it possible to explore the contribution of the SMAD proteins to TGF-beta activity in vivo. Here we report that, in contrast to predictions made on the basis of the ability of exogenous TGF-beta to improve wound healing, Smad3-null (Smad3ex8/ex8) mice paradoxically show accelerated cutaneous wound healing compared with wild-type mice, characterized by an increased rate of re-epithelialization and significantly reduced local infiltration of monocytes. Smad3ex8/ex8 keratinocytes show altered patterns of growth and migration, and Smad3ex8/ex8 monocytes exhibit a selectively blunted chemotactic response to TGF-beta. These data are, to our knowledge, the first to implicate Smad3 in specific pathways of tissue repair and in the modulation of keratinocyte and monocyte function in vivo.

Reduced skin tumor development in cyclin D1-deficient mice highlights the oncogenic ras pathway in vivo

Cyclin D1 is part of a cell cycle control node consistently deregulated in most human cancers. However, studies with cyclin D1-null mice indicate that it is dispensable for normal mouse development as well as cell growth in culture. Here, we provide evidence that ras-mediated tumorigenesis depends on signaling pathways that act preferentially through cyclin D1. Cyclin D1 expression and the activity of its associated kinase are up-regulated in keratinocytes in response to oncogenic ras. Furthermore, cyclin D1 deficiency results in up to an 80% decrease in the development of squamous tumors generated through either grafting of retroviral ras-transduced keratinocytes, phorbol ester treatment of ras transgenic mice, or two-stage carcinogenesis.