Carol Mikulec

Cyclooxygenase-2 expression is critical for chronic UV-induced murine skin carcinogenesis.

While it has been established that both the constitutive and inducible forms of cyclooxygenase (COX‐1 and COX‐2, respectively) play important roles in chemical initiation‐promotion protocols with phorbol ester tumor promoters, the contribution of these two enzymes to ultraviolet (UV) light‐induced skin tumors has not been fully assessed. To better understand the contribution of COX‐1 and COX‐2 to UV carcinogenesis, we transferred the null allele for each isoform onto the SKH‐1 hairless strain of mouse. Due to low viability on this background with complete knockout of COX‐2, heterozygous mice were used in UV carcinogenesis experiments. While the lack of one allele of COX‐1 had no effect on tumor outcome, the lack of one allele of COX‐2 resulted in a 50–65% reduction in tumor multiplicity and a marked decrease in tumor size. Additionally, transgenic SKH‐1 mice that overexpress COX‐2 under the control of a keratin 14 promoter developed 70% more tumors than wild‐type SKH‐1 mice. The lack of one allele of either COX‐1 or COX‐2 reduced prostaglandin (PG) E2 levels in response to a single UV treatment. The proliferative response to UV was significantly reduced in COX‐2, but not COX‐1, heterozygous mice. UV‐induced apoptosis, however, was greater in COX‐2 heterozygous mice. Collectively, these results clearly establish the requirement for COX‐2 in the development of skin tumors.

A role for cyclooxygenase-2 in ultraviolet light-induced skin carcinogenesis

Nonmelanoma skin cancer is the most prevalent cancer in the United States and its incidence is on the rise. These cancers generally arise on sun‐exposed areas of the body and the ultraviolet (UV) B spectrum of sunlight has been clearly identified as the major carcinogen responsible for skin cancer development. Besides inducing DNA damage directly, UV exposure of the skin induces the expression of the enzyme cyclooxygenase‐2 (COX‐2), which catalyzes the first step in the conversion of arachidonic acid to prostaglandins, the primary product in skin being prostaglandin E2 (PGE2). COX‐2 has been shown to be overexpressed in premalignant lesions as well as in nonmelanoma skin cancers in both humans and mice chronically exposed to UV. Through the use of COX‐2‐selective inhibitors and COX‐2 knockout mice, it has been shown that UV‐induced COX‐2 expression plays a major role in UV‐induced PGE2 production, inflammation, edema, keratinocyte proliferation, epidermal hyperplasia, and generation of a pro‐oxidant state leading to oxidative DNA damage. Chronic exposure to UV leads to chronic up‐regulation of COX‐2 expression and chronic inflammation along with the accumulation of DNA damage and mutations, all of which combine to induce malignant changes in epidermal keratinocytes and skin cancers. Both inhibition of COX‐2 activity and reduction in COX‐2 expression by genetic manipulations significantly reduce, while overexpression of COX‐2 in transgenic mice significantly increases UV‐induced skin carcinogenesis. Together these studies demonstrate that COX‐2 expression/activity is critical to the development of UV‐related nonmelanoma skin cancers.

Targeted disruption of glutathione peroxidase 4 in mouse skin epithelial cells impairs postnatal hair follicle morphogenesis that is partially rescued through inhibition of COX-2.

Selenoproteins are essential molecules for the mammalian antioxidant network. We previously demonstrated that targeted loss of all selenoproteins in mouse epidermis disrupted skin and hair development and caused premature death. In the current study we targeted specific selenoproteins for epidermal deletion to determine whether similar phenotypes developed. Keratinocyte-specific knockout mice lacking either the glutathione peroxidase 4 (GPx4) or thioredoxin reductase 1 (TR1) gene were generated by cre-lox technology using K14-cre. TR1 knockout mice had a normal phenotype in resting skin while GPx4 loss in epidermis caused epidermal hyperplasia, dermal inflammatory infiltrate, dysmorphic hair follicles and alopecia in perinatal mice. Unlike epidermal ablation of all selenoproteins, mice ablated for GPx4 recovered after 5 weeks and had a normal lifespan. GPx1 and TR1 were upregulated in the skin and keratinocytes of GPx4 knockout mice. GPx4 deletion reduces keratinocyte adhesion in culture and increases lipid peroxidation and COX-2 levels in cultured keratinocytes and whole skin. Feeding a COX-2 inhibitor to nursing mothers partially prevents development of the abnormal skin phenotype in knockout pups. These data link the activity of cutaneous GPx4 to the regulation of COX-2 and hair follicle morphogenesis and provide insight into the function of individual selenoprotein activity in maintaining cutaneous homeostasis.

Cell-type-specific roles for COX-2 in UVB-induced skin cancer.

In human tumors, and in mouse models, cyclooxygenase-2 (COX-2) levels are frequently correlated with tumor development/burden. In addition to intrinsic tumor cell expression, COX-2 is often present in fibroblasts, myofibroblasts and endothelial cells of the tumor microenvironment, and in infiltrating immune cells. Intrinsic cancer cell COX-2 expression is postulated as only one of many sources for prostanoids required for tumor promotion/progression. Although both COX-2 inhibition and global Cox-2 gene deletion ameliorate ultraviolet B (UVB)-induced SKH-1 mouse skin tumorigenesis, neither manipulation can elucidate the cell type(s) in which COX-2 expression is required for tumorigenesis; both eliminate COX-2 activity in all cells. To address this question, we created Cox-2(flox/flox) mice, in which the Cox-2 gene can be eliminated in a cell-type-specific fashion by targeted Cre recombinase expression. Cox-2 deletion in skin epithelial cells of SKH-1 Cox-2(flox/flox);K14Cre(+) mice resulted, following UVB irradiation, in reduced skin hyperplasia and increased apoptosis. Targeted epithelial cell Cox-2 deletion also resulted in reduced tumor incidence, frequency, size and proliferation rate, altered tumor cell differentiation and reduced tumor vascularization. Moreover, Cox-2(flox/flox);K14Cre(+) papillomas did not progress to squamous cell carcinomas. In contrast, Cox-2 deletion in SKH-1 Cox-2(flox/flox); LysMCre(+) myeloid cells had no effect on UVB tumor induction. We conclude that (i) intrinsic epithelial COX-2 activity plays a major role in UVB-induced skin cancer, (ii) macrophage/myeloid COX-2 plays no role in UVB-induced skin cancer and (iii) either there may be another COX-2-dependent prostanoid source(s) that drives UVB skin tumor induction or there may exist a COX-2-independent pathway(s) to UVB-induced skin cancer.

UVB Radiation-Induced β-catenin Signaling is Enhanced by COX-2 Expression in Keratinocytes

UVB radiation is the major carcinogen responsible for skin carcinogenesis, thus elucidation of the molecular pathways altered in skin in response to UVB would reveal novel targets for therapeutic intervention. It is well established that UVB leads to upregulation of cyclooxygenase 2 (COX‐2) in the skin which contributes to skin carcinogenesis. Overexpression of COX‐2 has been shown to promote colon cancer cell growth through β‐catenin signaling, however, little is known about the connection between UVB, COX‐2, and β‐catenin in the skin. In the present study, we have identified a novel pathway in which UVB induces β‐catenin signaling in keratinocytes, which is modulated by COX‐2 expression. Exposure of the mouse 308 keratinocyte cell line (308 cells) and primary normal human epidermal keratinocytes (NHEKs) to UVB resulted in increased protein levels of both N‐terminally unphosphorylated and total β‐catenin. In addition, we found that UVB‐enhanced β‐catenin‐dependent TOPflash reporter activity and expression of a downstream β‐catenin target gene. We demonstrated that UVB‐induced β‐catenin signaling is modulated by COX‐2, as treatment of keratinocytes with the specific COX‐2 inhibitor NS398 blocked UVB induction of β‐catenin. Additionally, β‐catenin target gene expression was reduced in UVB‐treated COX‐2 knockout (KO) MEFs compared to wild‐type (WT) MEFs. Furthermore, epidermis from UVB‐exposed SKH‐1 mice exhibited increased N‐terminally unphosphorylated and total β‐catenin protein levels and increased staining for total β‐catenin, and both responses were reduced in COX‐2 heterozygous mice. Taken together, these results suggest a novel pathway in which UVB induces β‐catenin signaling in keratinocytes which is enhanced by COX‐2 expression.

Upregulation of the EP1 receptor for prostaglandin E2 promotes skin tumor progression

Prostaglandin E2 (PGE2) has been shown to promote the development of murine skin tumors. EP1 is 1 of the 4 PGE2 G‐protein‐coupled membrane receptors expressed by murine keratinocytes. EP1 mRNA levels were increased ∼2‐fold after topical treatment with 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) or exposure to ultraviolet (UV) light, as well as increased ∼3‐ to 12‐fold in tumors induced by 7,12‐dimethyl‐benz[a]anthracene (DMBA) initiation/TPA promotion or by UV exposure. To determine the effect of EP1 levels on tumor development, we generated BK5.EP1 transgenic mice that overexpress EP1 in the basal layer of the epidermis. Skins of these mice were histologically indistinguishable from wild type (WT) mice and had similar levels of proliferation after TPA treatment. Using a DMBA/TPA carcinogenesis protocol, BK5.EP1 mice had a reduced tumor multiplicity compared to WT mice, likely due to the observed down‐regulation of protein kinase C (PKC). However, the BK5.EP1 mice had an ∼8‐fold higher papilloma to carcinoma conversion rate. When DMBA/anthralin was used, BK5.EP1 mice produced more tumors than WT mice, as well as a ninefold increase in carcinomas, indicating that the tumor response is dependent on the type of tumor promoter agent used. Additionally, although almost undetectable in WT mice, cyclooxygenase‐2 (COX‐2) was expressed in the untreated epidermis of BK5.EP1 mice. While TPA highly induced COX‐2 in WT mice, COX‐2 expression in the BK5.EP1 mice did not change after TPA treatment; PGE2 levels were likewise affected. These data indicate that EP1 is more important in tumor progression than in tumor promotion and that it indirectly regulates COX‐2 expression.

Targeted Deletion and Lipidomic Analysis Identify Epithelial Cell COX-2 as a Major Driver of Chemically Induced Skin Cancer

Pharmacologic and global gene deletion studies demonstrate that cyclooxygenase-2 (PTGS2/COX-2) plays a critical role in DMBA/TPA–induced skin tumor induction. Although many cell types in the tumor microenvironment express COX-2, the cell types in which COX-2 expression is required for tumor promotion are not clearly established. Here, cell type–specific Cox-2 gene deletion reveals a vital role for skin epithelial cell COX-2 expression in DMBA/TPA tumor induction. In contrast, myeloid Cox-2 gene deletion has no effect on DMBA/TPA tumorigenesis. The infrequent, small tumors that develop on mice with an epithelial cell–specific Cox-2 gene deletion have decreased proliferation and increased cell differentiation properties. Blood vessel density is reduced in tumors with an epithelial cell–specific Cox-2 gene deletion, compared with littermate control tumors, suggesting a reciprocal relationship in tumor progression between COX-2–expressing tumor epithelial cells and microenvironment endothelial cells. Lipidomics analysis of skin and tumors from DMBA/TPA–treated mice suggests that the prostaglandins PGE2 and PGF2α are likely candidates for the epithelial cell COX-2–dependent eicosanoids that mediate tumor progression. This study both illustrates the value of cell type–specific gene deletions in understanding the cellular roles of signal-generating pathways in complex microenvironments and emphasizes the benefit of a systems-based lipidomic analysis approach to identify candidate lipid mediators of biologic responses. Implications: Cox-2 gene deletion demonstrates that intrinsic COX-2 expression in initiated keratinocytes is a principal driver of skin carcinogenesis; lipidomic analysis identifies likely prostanoid effectors. Mol Cancer Res; 12(11); 1677–88. ©2014 AACR.

The EP1 receptor for prostaglandin E2 promotes the development and progression of malignant murine skin tumors

High levels of prostaglandin E2 (PGE2) synthesis resulting from the up‐regulation of cyclooxygenase (COX)‐2 has been shown to be critical for the development of non‐melanoma skin tumors. This effect of PGE2 is likely mediated by one or more of its 4 G‐protein coupled membrane receptors, EP1‐4. A previous study showed that BK5.EP1 transgenic mice produced more carcinomas than wild type (WT) mice using initiation/promotion protocols, although the tumor response was dependent on the type of tumor promoter used. In this study, a single topical application of either 7,12‐dimethylbenz[a]anthracene (DMBA) or benzo[a]pyrene (B[a]P), alone, was found to elicit squamous cell carcinomas (SCCs) in the BK5.EP1 transgenic mice, but not in WT mice. While the epidermis of both WT and transgenic mice was hyperplastic several days after DMBA, this effect regressed in the WT mice while proliferation continued in the transgenic mice. Several parameters associated with carcinogen initiation were measured and were found to be similar between genotypes, including CYP1B1 and aromatase expression, B[a]P adduct formation, Ras activity, and keratinocyte stem cell numbers. However, EP1 transgene expression elevated COX‐2 levels in the epidermis and SCC could be completely prevented in DMBA‐treated BK5.EP1 mice either by feeding the selective COX‐2 inhibitor celecoxib in their diet or by crossing them onto a COX‐2 null background. These data suggest that the tumor promoting/progressing effects of EP1 require the PGE2 synthesized by COX‐2.

The tumor promoting activity of the EP4 receptor for prostaglandin E2 in murine skin

To determine whether the EP4 receptor for prostaglandin E2 (PGE2) contributes to the tumor promoting activity of PGs in murine skin, EP4 over‐expressing mice (BK5.EP4) were generated and subjected carcinogenesis protocols. An initiation/promotion protocol resulted in 25‐fold more squamous cell carcinomas (SCCs) in the BK5.EP4 mice than wild type (WT) mice. An increase in SCCs also occurred following treatment with initiator alone or UV irradiation. The initiator dimethylbenz[a]anthracene caused cytotoxicity in BK5.EP4, but not WT mice, characterized by sloughing of the interfollicular epidermis, regeneration and subsequent SCC development. A comparison of transcriptomes between BK5.EP4 and WT mice treated with PGE2 showed a significant upregulation of a number of genes known to be associated with tumor development, supporting a pro‐tumorigenic role for the EP4 receptor.

SKHIN/Sprd, a new genetically defined inbred hairless mouse strain for UV-induced skin carcinogenesis studies

Abstract:  Strains of mice vary in their susceptibility to ultra‐violet (UV) radiation‐induced skin tumors. Some strains of hairless mice (homozygous for the spontaneous Hrhr mutation) are particularly susceptible to these tumors. The skin tumors that develop in hairless mice resemble, both at the morphologic and molecular levels, UV‐induced squamous cell carcinomas (SCC) and their precursors in human. The most commonly employed hairless mice belong to the SKH1 stock. However, these mice are outbred and their genetic background is not characterized, which makes them a poor model for genetic studies. We have developed a new inbred strain from outbred SKH1 mice that we named SKHIN/Sprd (now at generation F31). In order to characterize the genetic background of this new strain, we genotyped a cohort of mice at F30 with 92 microsatellites and 140 single nucleotide polymorphisms (SNP) evenly distributed throughout the mouse genome. We also exposed SKHIN/Sprd mice to chronic UV irradiation and showed that they are as susceptible to UV‐induced skin carcinogenesis as outbred SKH1 mice. In addition, we proved that, albeit with low efficiency, inbred SKHIN/Sprd mice are suitable for transgenic production by classical pronuclear microinjection. This new inbred strain will be useful for the development of transgenic and congenic strains on a hairless inbred background as well as the establishment of syngeneic tumor cell lines. These new tools can potentially help elucidate a number of features of the cutaneous response to UV irradiation in humans, including the effect of genetic background and modifier genes.