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Squamous Cell Carcinoma of the Skin: Current Strategies for Treatment and Prevention Michael A. Bachelora and David M. Owensa,b,* a
Departments of Dermatology and Pathology, College of Physicians & Surgeons, bColumbia University, New York, NY 10032, USA Abstract: Non-melanoma skin cancer (NMSC) is the most frequent malignancy in the United States, with over one million new cases reported annually. Approximately 80% of NMSC are basal cell carcinomas (BCC), while the remaining 20% of NMSC are squamous cell carcinomas (SCC). BCC and SCC commonly arise in regions of the skin subjected to chronic sun exposure implicating ultraviolet radiation (UV)-induced cellular damage as the primary causative agent. While surgical excision is an effective treatment of NMSC, strategies aimed at NMSC prevention have not been exploited clinically. The application of sunscreens has been the primary means of prevention by physically blocking the absorption of UV radiation. However, sunscreens have had limited success in decreasing NMSC incidence overall, necessitating more targeted strategies against UV damage. In this review we will discuss the relevance of recently identified UVinduced cellular changes, including induction of reactive oxygen species, immune modulation, and damage to mitochondrial DNA to serve as potential targets for the chemoprevention of NMSC. Finally, we will discuss the potential of genes required for the maintenance of epithelial stem cells in the skin as therapeutic targets for cutaneous cancer.
Key Words: Non melanoma skin cancer, ultraviolet radiation, reactive oxygen species, inflammation, stem cell, keratinocyte. INTRODUCTION Nonmelanoma skin cancer (NMSC) is the most common malignancy in fair-skinned populations. In the United States alone, it is estimated that 1.2 million new cases of NMSC are diagnosed each year, accounting for 40% of all new cancer cases [1,2]. In addition, another one million cases of NMSC are estimated to go unreported each year 1. The vast majority of NMSC are comprised of basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), which arise primarily on sun-exposed regions of the body implicating ultraviolet (UV) radiation as the main etiological agent [3-5]. BCC and SCC comprise 80% and 16% of all skin cancers, respectively, with malignant melanomas comprising 4%. Based on their distinct histopathological features and expression patterns of differentiation lineage markers, SCC are thought to originate from the epidermis and BCC from the hair follicle [6] (Fig. 1). Skin cancer incidence has been increasing in recent years as the population ages and higher levels of UV radiation reach the surface of the Earth as a result of ozone depletion. Even minor changes in the ozone are thought to have a substantial impact on UV carcinogenesis as a 1% decrease in column ozone has been calculated to result in a 1.56% increase in carcinogenic UV radiation penetrance and a concomitant increase in NMSC by 2.7% [7]. Current prevention of NMSC involves limiting excessive exposure to UV, wearing protective clothing and the use of sunscreens. However,
*Address correspondence to this author at the Columbia University Medical Center, Herbert Irving Comprehensive Cancer Center, 1130 St. Nicholas Ave, Room 321A, New York, New York 10032, USA; Tel: (212) 851-4544; Fax: (212) 851-4540; E-mail:
[email protected]
1
American Cancer Society, Cancer Facts & Figures, 2008 1573-3947/09 $55.00+.00
these approaches have remained unsuccessful in impacting the overall incidence of NMSC. In light of the inability of primary prevention strategies to remain effective, and the high frequency of recurrence in patients diagnosed with BCC and SCC, more customized therapies may be necessary to adequately target these lesions. BCC are slow growing lesions that rarely metastasize, although these tumors can have significant morbidity due to local invasiveness. Contrastingly, SCC are highly invasive lesions with a much greater propensity for metastasis. [8,9]. The reported incidence of metastatic SCC varies ranging from 0.5 to 16%, and metastatic SCC account for 1200 deaths annually [10]. Metastatic SCC are difficult to control in spite of aggressive therapeutic intervention, including surgery, and/or radiation. Moreover, metastatic SCC patients also have a greater likelihood of developing additional skin lesions within five years of initial diagnosis [11,12]. Compounding this problem is the lack of reliable diagnostic predictors of SCC metastatic potential, which currently includes tumors size (larger than 2 cm), poor differentiation, and degree of invasiveness [13,14]. In addition to exposure to damaging sunlight, other factors also increase the risk for developing NMSC. For example, a high incidence of cutaneous SCC is associated with patients receiving post-operative immunosuppressive drugs following organ transplant. These transplant recipient patients also have a rate of metastasis incidence twice that of SCC patients that are not immunosuppressed [15]. An 8-10 fold increased risk for developing skin cancer and SCC, in particular, has also been reported in patients with chronic lymphocytic leukemia (CLL) where individuals lack competent innate and humoral immunity [16,17]. Finally, individuals with the autosomal-dominant disorder nevoid basal cell © 2009 Bentham Science Publishers Ltd.
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Fig. (1). UV-induced pathways associated with SCC formation in skin epithelium. Schematic of adult skin showing the multiple epidermal layers, pilosebaceous unit and dermis. SCC lesion is designated within the epidermis as mutant (red) cells. UVA and UVB have differential effects on the skin due in part to their penetration into various layers of the skin (depth indicated by red arrows). (A) UVA induced cellular changes arises from increases in ROS production. (B) UVB induced cellular changes implicated in SCC pathogenesis include: increases in COX-2 activity, production of inflammatory cytokines, mutations in mtDNA and signature mutations in the tumor suppressor TP53. Abbreviations: DP, dermal papilla; SG, sebaceous gland; HF, hair follicle, GL, granular layer; SC, spinous layer; BL, basal layer.
carcinoma syndrome (Gorlin-Goltz syndrome) present with large numbers of BCC that recur throughout life [18]. While typical surgical intervention may be effective and in some cases curative for these patients, these procedures are highly distressful when performed frequently. Recent interest in targeted-based therapy for the treatment of skin cancer has shed light on numerous signaling pathways that might provide new therapeutic targets. Cell signaling pathways involved in proliferation and viability are discussed in a recent review which highlights potential molecular targets specific to SCC and BCC or that are common in both tumor types [19]. The focus of this review will be to highlight UV-mediated cellular changes, particularly events mediated by reactive oxygen species (ROS) and proinflammatory molecules that could potentially be exploited therapeutically through a targeted approach to ameliorate the deleterious effects of UV exposure. We will also highlight recent work regarding epithelial stem cells in the skin and discuss their potential role as chemopreventive and therapeutic targets. CURRENT NMSC
THERAPEUTIC
APPROACHES
FOR
Surgery is the most common form of treatment for both BCC and SCC. The goal of surgical treatment is complete removal of the tumor while preserving normal surrounding
tissue [20]. Nonsurgical options are available and include radiation therapy, topical chemotherapeutics (5-fluorouracil), retinoids and photodynamic therapy. While nonsurgical options are used less frequently, they may be beneficial when surgery would produce disfiguring results or for patients unable to tolerate surgery [21,22]. Several agents have been used in chemoprevention clinical trials including retinoids (retinol or Vitamin A, isotretinoin or 13-cis-retinoic acid), -carotene and selenium. Retinoids are derivatives of Vitamin A and affect keratinocyte growth, differentiation, adhesion and immune modulation mediated through the binding of the nuclear retinoic acid receptors and retinoid C receptors [23]. -carotene and selenium both act as antioxidants to reduce UV-induced reactive oxygen species [24,25]. Chemoprevention trials involving selenium, -carotene, and isotretinoin showed that these compounds have no effect on decreasing NMSC incidence in patients with a history of SCC or BCC whereas patients receiving retinol had a 26% less risk of being diagnosed with SCC [26-28]. Treatment of moderate risk patients, having a history of more than 10 actinic keratoses (the premalignant precursor to SCC), but no more than two preexisting SCC or BCC, with retinol was effective in preventing new SCC but not BCC [28]. However, retinol and isotretinoin were ineffective in blocking the development of new NMSC in highrisk patients (history of four or more SCC or BCC) [29]. The therapeutic effects of retinoids in reducing NMSC in patients
Squamous Cell Carcinoma of the Skin
with actinic keratoses but not in patients with more progressed skin cancer suggest that these agents are more effective in the earlier stages of UV-induced skin carcinogenesis. Consistent with this idea, retinoids have proven effective in reducing the number of new actinic keratoses or NMSC in patients known to be at very high risk for developing skin cancer. Individuals with defects in DNA repair (Xeroderma Pigmentosa or XP patients) and organ transplant recipients receiving immunosuppressive had a 63% and 42% reduction in incidence, respectively [30,31]. While retinoids may be effective at treating these early lesions induced by UV light, no single agent has been identified that has had significant impact on more progressed stages of skin cancer. Definition of Ultraviolet Spectrum The UV component of sunlight is strongly implicated with both BCC and SCC pathogenesis and is capable of inducing pleiotropic cellular changes that can alter tissue homeostasis by impacting proliferation, differentiation, apoptosis and inflammation. The UV spectrum is divided into three regions based on wavelength, UVA (320-400nm), UVB (280-320nm) and UVC (200-280nm). Wavelengths up to 310nm are filtered by the ozone layer thereby preventing UVC and most of the UVB spectrum (~95%) from reaching the surface of the Earth. Therefore, 90-99% of the biologically relevant solar radiation reaching the surface of the Earth is comprised of UVA, with the remaining 1-10% composed of UVB [32]. When applied separately, both UVA and UVB can act as complete carcinogens in experimental mouse skin models; however, UVB is 1000-10,000 times more potent in inducing skin tumorigenesis than UVA [33,34]. Importantly, UVA and UVB induce different cellular changes including erythema, inflammation, signal transduction, and ROS production as a consequence of the ability of different energy wavelengths to penetrate distinct layers of the skin. While both UVA and UVB play a role in SCC formation, these unique properties may have clinical significance, as distinct chemopreventive strategies may be required for each component of UV light. At the tissue level, long-wave UVA penetrates deep into the dermis of the skin whereas short wave UVB only penetrates through the epidermis (Fig. 1). At the cellular level, UVB causes direct excitation of genomic DNA, inducing DNA adducts, strand breaks, DNA crosslinks and DNAprotein crosslinks [35]. If left unrepaired, these DNA adducts can lead to mutations upon DNA replication, the most common of which are detected in the tumor suppressor gene TP53 in sun-damaged skin [36-41]. In contrast, the deleterious effects due to chronic UVA exposure are thought to be a result of increased ROS [42]. UVA produces oxidative DNA damage more efficiently than UVB as ROS is generated upon absorption through cellular photosensitizers such as porphyrins and NADH [43-45]. Interestingly, it has recently been reported that UVA causes photoproducts similar to those induced by UVB albeit at much lower frequencies [46]. These adducts occur at unique sequences from that observed with UVB irradiation and suggest UVA-induced genetic damage may act independently from direct excitation of DNA.
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UV-Induced Signature Mutations Characteristic UV-induced genetic mutations have been detected in the TP53 tumor suppressor from chronically sundamaged skin, actinic keratoses and SCC, indicating that mutations in TP53 gene are an early event in UV-induced skin carcinogenesis [37,39,40,47]. TP53 is the most commonly mutated gene in human SCC with reports ranging from 35%-58% [36,48]. TP53 plays an important role in maintaining the genomic integrity of the cell by blocking DNA replication in response to UV-induced DNA damage. In both cultured cells and the epidermis of skin, exposure to UVB induces the expression of wild-type TP53. If the cell has sustained irreparable DNA damage due to UV radiation, the induction of p53 blocks cell cycle progression and causes cells to undergo programmed cell death, or apoptosis [49]. In response to high doses of UVB, sunburn cells can also be observed in the epidermis, initiated via a TP53-dependent pathway and resulting in the removal of damaged cells, thereby minimizing the risk of tumor development. This concept is illustrated in Trp53 -/- mice where increased sensitivity to UVB and fewer apoptotic cells are observed in the epidermis [47]. In addition, Trp53 -/- mice develop more aggressive tumors when compared to mice expressing functional p53 indicating the necessity for removal of damaged cells for the prevention of tumor development [50]. The use of sunscreens can be highly effective in the prevention of UV-induced TP53 mutations. A sunscreen absorbing in the UVB only or UVA and UVB spectra showed an 88% and 92% reduction, respectively in Trp53 mutation frequency [51]. The findings of this study were reproduced in a follow-up report using solar-simulated radiation (SSR), a combination of UVA and UVB mimicking normal environmental exposure, where an 80-100% decrease in the mutation frequency of Trp53 was reported using sunscreens that absorbed only UVB or both UVA and UVB [52]. More importantly, the reduction of Trp53 mutations correlated with a reduction in skin tumor indicating that p53 is a useful biological endpoint to indicate risk of UV-induced skin tumor formation. When non-sun exposed human skin was grafted onto immunocompromised mice and exposed to solarsimulated UV radiation, UV-induced TP53 mutations could be detected in all grafts while grafts treated with SPF-15 sunscreen blocked the induction of mutations in TP53 [49]. From a therapeutic perspective, these data underscore the value of primary prevention of UV-induced genetic damage in reducing the incidence of skin tumor development. Despite the ability of sunscreens in reducing TP53 mutations and decreasing SCC formation under experimental conditions, their effect in decreasing overall NMSC incidence has been limited. In fact, several clinical trials have reported no decrease in NMSC incidence with the use of sunscreens [53,54] and a substantial portion of diagnosed SCC do not contain TP53 mutations [36]. Therefore, it seems likely that UV radiation is producing additional responses in the cell that might serve as potential chemopreventive targets. Although sunscreens have not proven successful in significantly impacting NMSC incidence, new chemopreventive strategies in addition to sunscreen application may be a more effective means of preventing skin cancer.
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UV MEDIATED INFLAMMATION AND IMMUNE SUPPRESSION
to protect the body from tumor development both in the skin and in distant organs.
Inflammation is a key factor for tumorigenesis in numerous tissues. In UV-irradiated skin, inflammation is observed as a result of increased blood flow and infiltration by macrophages and neutrophils [55]. In addition, epithelial keratinocytes exposed to UV release pro-inflammatory cytokines, such as TNF-, IL-1, IL-1 and IL-6 [55]. TNF- secretion plays an important role in SCC development in mouse epidermis [56]. TNF- knockout (KO) mice treated in a twostage chemical carcinogenesis protocol were resistant to keratinocyte hyperproliferation, inflammation and the development of benign and malignant skin tumors, suggesting a role for inflammation in SCC development that can be mediated by UV-induced cytokines [56]. While TNF- therapies have not been tested as targets for the treatment of NMSC, they have been met with some success in other epithelial cancers. Patients with recurrent ovarian cancer receiving etanercept, a TNF- inhibitor, achieved prolonged disease stabilization although no tumor regression was observed [57]. In metastatic breast cancer patients, however etanercept did not achieve significant disease stabilization or tumor regression [58]. It is well established that keratinocytes secrete IL-1 and IL-1 in response to UV radiation; however, a direct link between induction of either IL-1 and IL-1 in skin tumor development in vivo remains to be determined. UVB irradiated IL-6 KO mice display delayed epidermal hyperplasia and greater overall damage to the epidermis suggesting IL-6 plays an important role in the repair of UVdamaged skin [59]; however, IL-6 protection from UV damage may not be critical in preventing tumor formation [60]. Recently, an IL-6 monoclonal antibody, tocilizumab was tested for its use in the treatment of rheumatoid arthritis; however its efficacy in the treatment of epithelial cancers has not yet been tested [61]. Interestingly, IL-6 may play a larger role in outside of inflammation by regulating levels of IL-10, a critical mediator of UV-induced immunosuppression. In addition to pro-inflammatory molecules produced by epidermal cells, the importance of the inflammatory response derived from the underlying dermis has recently been illustrated. Mice deficient in the D6 chemokine receptor demonstrate that sequestration of the inflammatory D6 chemokine, produced by dermal mast cells, is essential to suppress epidermal carcinogenesis by altering keratinocyte proliferation [62]. Whether UV can impact the induction of dermal chemokines and influence cutaneous tumor development is an interesting concept that requires further investigation.
UV induces lipid peroxidation in the cutaneous epithelium, which results in damage to the cell membrane and increases in ROS [65]. In response to lipid peroxidation caused by UVA and UVB irradiation, cells also increase production of prostaglandins as a consequence of arachidonic release from membrane-bound phospholipase A2. The production of nitric oxide and prostaglandins further contribute to the prooxidant state of UV-irradiated cells through production of additional ROS. PGE2, a product of cyclooxygenase-2 (COX-2) enzyme catalysis of arachidonic acid, contributes to UV-induced inflammation, edema, keratinocyte proliferation and inhibits UV-induced apoptosis [66]. Increases in PGE2 levels resulting from UV-induced oxidative damage also contribute to immunosuppression [67]. Basal COX-2 expression is maintained at low levels in the skin and is induced in response to UVB [68]. Increased COX enzymatic activity produces ROS, which in combination with ROS produced by inflammatory cells may contribute to oxidative DNA damage [35,65,69]. COX-2 gene expression is critical for UVinduced skin tumorigenesis as evidenced by reduction in the number of tumors and increase in latency in COX-2 KO mice. COX-2 +/- mice also exhibit reduced levels of PGE2 following UVB irradiation [70]. Transgenic mice overexpressing COX-2 in the basal layer of the epidermis had dramatically increased levels of PGE2 in the epidermis and displayed greater sensitivity to tumor formation [71]. In addition to altering levels of prostaglandins in the skin, it has been suggested that by inhibiting COX-2 activity and subsequent PGE2 levels in the skin, levels of apoptosis are enhanced, as skin tumors are dependent on PGE2 signaling for growth [72].
In conjunction with the pro-inflammatory response of the skin, UV radiation also has local and systemic immunosuppressive effects. Local immunosuppression primarily involves depletion in the number of Langerhans cells, the major antigen-presenting cell within the skin, which could be advantageous for the survival of mutant epidermal cells. Langerhans cells are also compromised in their ability to present antigens due to a reduction in major histocompatibility complex II surface molecules following UV exposure [63]. Keratinocytes produce IL-10 in response to UV radiation, which appears to be the critical mediator of UVinduced immunosuppression by inhibiting the function of antigen presenting cells [64]. Overall, UV exposure in the skin can impact normal immune surveillance that functions
Traditional non-steroidal anti-inflammatory drugs (NSAIDs), inhibitors of both COX-1 and COX-2 isoforms, as well as selective COX-2 inhibitors, such as celecoxib and indomethacin, can inhibit UV-induced tumorigenesis in mice [73,74]. The inhibition of COX-2 appears to be more effective as a chemoprevention strategy, as treatment with celecoxib had little or no therapeutic impact on regression of existing tumors [69,73,74]. The development of inhibitors of PGE2 signaling may prove more clinically viable in human trials as the side effects of COX inhibitors preclude them for use as chemopreventive agents. NSAIDs, such as aspirin, have harmful gastrointestinal complications resulting from the inhibition of both COX-1 and COX-2 isoforms. These side effects were the initial impetus for the development of selective COX-2 inhibitors, although these have also recently been shown to be associated with increased risk for cardiovascular complications [75]. Further studies will be necessary to develop a strategy to target PGE2 signaling as a chemopreventive strategy while minimizing side effects. ROS-INDUCED EFFECTS ON SKIN ROS are generated by epithelial keratinocytes and dermal fibroblasts in skin at low levels during the course of aerobic metabolism but are normally removed by nonenzymatic and enzymatic processes to prevent oxidative cellular damage. UVA and UVB irradiation also induce specific ROS species production. When the antioxidant defense mechanisms be-
Squamous Cell Carcinoma of the Skin
come overwhelmed in favor of pro-oxidants, the cell enters a state of oxidative stress, as occurs in UV-irradiated skin [76,77]. In addition, the effects of UV irradiation are compounded by the inability of cells to sufficiently scavenge ROS, it remains of interest to explore the effects of altering enzymatic oxidants within the cell. One of the primary enzymatic antioxidant defenses that metabolize ROS within the cell is catalase [78]. The importance of catalase in the breakdown of ROS has been demonstrated using malignant mouse keratinocytes [79]. In these malignant cells, overexpression of catalase can inhibit ROS and concomitantly establish a functional role for decreased catalase levels in augmenting the malignant phenotype [78,80-82]. The overexpression of catalase also reduces UVB-induced apoptosis in normal human keratinocytes preventing DNA damage induced by increases in ROS [83,84]. Collectively, these studies highlight catalase as a key player in mediating the effects of ROS on tumor progression. Manipulating catalase levels has also been utilized to alter levels of apoptosis in reconstructed epidermis from Xeroderma pigmentosum type C (XPC) patients. XPC is an autosomal recessive disease, resulting from inactivation of the XPC protein involved in DNA damage recognition and resulting in defective nucleotide excision repair. UVB irradiation of reconstructed XPC human epidermis overexpressing catalase resulted in a significant decrease in sunburn cell formation, caspase-3 activation and p53 accumulation [83]. These studies highlight the clinical impact that enzyme replacement or manipulation can have in ameliorating a genetic defect. In addition to targeting the XPC genetic defect itself, manipulation of enzymatic pathways may be a viable alternative in limiting photosensitivity in XPC patients. Resveratrol is a polyphenolic compound found in grapes and red wine, having anti-carcinogenic effects associated with its antioxidant activity. Resveratrol can inhibit a number of cellular proteins, including COX enzymes, hydroperoxidase, protein kinase C, Bcl-2 phosphorylation, AKT, focal adhesion kinase, NF-B, matrix metalloproteinase-9, and cell cycle regulators [85]. Resveratrol treatment decreases tumor incidence and delays the onset of tumor formation in mice exposed to UVB irradiation or treated with a two-stage chemical carcinogenesis protocol [86,87]. Pretreatment of UVB-irradiated mice mouse skin with resveratrol decreased bifold skin thickness, edema and reduced the number of infiltrating leukocytes [88]. In addition, resveratrol treatment reduced UVB-induced COX-2 expression and H2O2 production in the skin, confirming previous reports of resveratrol as a strong antioxidant [89,90]. The putative ability of resveratrol to target multiple UV-induced cellular changes makes it a strong candidate for use as a chemopreventive agent in NMSC. While nuclear DNA is the usually the focus of UVinduced damage, mitochondrial DNA (mtDNA) can also be damaged by oxidative stress [91]. Mutations in nuclear DNA are more likely to be repaired whereas mutations occurring in mtDNA are apt to accumulate, as repair mechanisms are less efficient [92]. Accordingly, sun-exposed samples of human skin contained mutated mtDNA in 27% of samples analyzed, compared to 1.1% in non sun-exposed areas [93]. Interestingly, 93% of the mutations were contained in the
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dermal compartment. Further studies demonstrated that the deletions in dermal cell mtDNA were attributable to singlet oxygen, the principle ROS species induced by UVA irradiation [94]. Reciprocal exchange experiments, which swap the mtDNA between tumor cells with low metastatic potential and cells with high metastatic potential, demonstrate that mutations in mtDNA can critically impact metastasis [95]. Defects in complex I mtDNA were shown to be responsible for acquiring metastatic potential and were due to a mutant NADH dehydrogenase subunit 6 gene which also leads to the overproduction of ROS [95-97]. The concomitant increases in ROS can stimulate metastasis through the upregulation of genes associated with tumor progression, including HIF-1, MCL-1 and VEGF. Interestingly, inappropriate expression of these genes was reversed when the mutated mtDNA was replaced with wild-type mtNDA [95]. Therapies targeting mtDNA defects remain largely unexplored and may offer significant clinical impact for UV-induced NMSC. EPIDERMAL STEM CELLS AS TARGETS FOR CHEMOPREVENTION Despite the vulnerability of epidermal keratinocytes to UV-induced genetic mutations, relatively few cancers develop as most of these mutant cells are shed from the skin during the normal process of terminal differentiation [6]. Given that the development of skin cancer takes 20-30 years to develop in humans, only cells that are long-term residents in the skin, i.e. stem cells, are capable of acquiring the necessary number of genetic hits necessary to result in tumor formation. The cutaneous epithelium is now thought to be composed of multiple progenitor cell niches. Epithelial progenitor cell populations have been identified in both the sebaceous gland (SG) and interfollicular epidermis (IFE) [98101]. In addition to the stem cell reservoir in the bulge region [102], additional progenitor cell populations have also recently been identified in the hair follicle [103-105]. These progenitor populations are long-term residents in the skin and can maintain distinct epithelial lineages including the epidermis, hair follicles and sebaceous glands [103-105]. Importantly, expression array studies identified genes that are unique or enriched in these progenitor populations and in some cases are responsible for maintaining the integrity of certain epithelial lineages [102,105]. These aforementioned characteristics of epithelial progenitor populations suggests that these cells may serve as target cells for UV-induced NMSC and raises the possibility that genes unique to these cells may play a role in propagating epithelial tumor stem cells. A better understanding of these regionalized compartments of phenotypically distinct epithelial progenitor cells may aid in the design for more effective strategies in preventing and treating UV-induced NMSC. Multiple lines of experimental evidence have provided support for the idea that genes responsible for the maintenance of stem cells may also be altered in skin cancer. Overexpression of genes involved in epidermal stem cell maintenance including c-myc or Lef1 lead to de novo skin tumor formation [106,107] whereas ablation of the small Rho GTPase, Rac1, leads to depletion of the entire epidermis and hair follicles [108]. Sox9, a gene essential for maintenance of the hair follicle bulge stem cell compartment is also constitutively expressed in BCC through upregulation of upstream
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signaling pathways [109,110]. Manipulation of stem cell niches in other tissue types including Wnt and Hedgehog signaling in the hematopoietic system and Bmi1 in brain indicate they have a role in both development and oncogenesis [111,112]. Observations made by augmenting expression of genes within the stem cell compartment underscore their capacity to regulate homeostasis and tumorigenesis and indicate that genes unique to epidermal stem cells are potentially lucrative targets for SCC treatment and prevention. A prime example supporting this concept has been observed in CD34+ cancer stem cells recently identified in chemically induced SCC [113]. This subpopulation of CD34+ keratinocytes, but not CD34- SCC cells, recapitulate the original tumor phenotype indicating that the CD34+ population of cells contains putative cancer stem cells [113]. Interestingly, CD34+ SCC cells were found to be dependent on -catenin signaling, a pathway required for normal hair follicle development, as deletion of -catenin in these tumors led to a reduced population of CD34+ cells, along with increased terminal differentiation. These findings were also applicable to human SCC development as knockdown of -catenin signaling in a human SCC cell line led to decreases in xenograft tumors. This study illustrates how a signaling pathway modulating stem cell maintenance under normal conditions affect tumorigenesis and offers an avenue for therapeutic intervention.
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ACKNOWLEDGEMENTS This work was supported by National Institutes of Health F32AR055007 (M.A.B.), RO3AR054071 (D.M.O.) and R01CA114014 (D.M.O.) research grants. REFERENCES [1]
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Revised: December 11, 2008
Accepted: December 13, 2008
