Karen A. Blohm-Mangone

Role of polyamines in arginine-dependent colon carcinogenesis in ApcMin/+ mice

We evaluated the role of polyamines in arginine‐dependent intestinal tumorigenesis in ApcMin/+ mice. Arginine is a substrate for ornithine synthesis and thus can influence polyamine production. Supplementing the diet with arginine increased intestinal and colonic polyamine levels and colonic carcinogenesis. Inhibiting polyamine synthesis with D,L‐α‐diflouromethylornithine (DFMO) decreased small intestinal and colonic polyamine pools. In mice provided basal diet, but not when supplemented with arginine, DFMO decreased small intestinal tumor number and burden, and increased intestinal apoptosis. In mice provided supplemental arginine in the diet, DFMO induced late apoptosis and decreased tumorigenesis in the colon. DFMO slightly reduced tumor incidence, number, and size while significantly decreasing tumor burden and grade. These changes in colon tumorigenesis did not occur in mice not provided supplemental arginine. Our study indicates that polyamines play unique roles in intestinal and colonic carcinogenesis in ApcMin/+ mice. Inhibition of polyamine synthesis suppresses the arginine‐dependent risk of colon tumorigenesis, resulting in apoptosis induction and decreased tumorigenesis, in this murine model.

Role of c-Myc in intestinal tumorigenesis of the ApcMin/+ mouse

The c-MYC oncogene plays an important role in tumorigenesis and is commonly highly expressed in gastrointestinal cancers. In colon cells, c-MYC is regulated by the adenomatous polyposis coli (Apc) tumor suppressor gene. Multiple intestinal neoplasia (ApcMin/+ or Min) mice are heterozygous for a truncating Apc mutation and serve as a model of familial adenomatous polyposis (FAP) disease. To study the role of c-Myc in the mutant Apc-mediated colon tumorigenesis, we have developed a transgenic mouse with the conditional deletion of the floxed c-Myc alleles in the intestinal crypts of ApcMin/+ mice (ApcMin/+; c-Mycfl/fl). The floxed c-Myc deletion was initiated via a Cre recombinase controlled by the intestine-specific transcriptional regulatory elements of the liver fatty acid-binding protein gene (Fabpl4×at-132). Fabpl4×at-132-mediated Cre expression and recombination resulted in a two-fold decrease in c-MYC protein expression with no effect on intestinal tract morphology. Small intestinal tumorigenesis was significantly suppressed throughout the small intestinal tract of ApcMin/+; c-Mycfl/fl mice compared to c-Myc wild type littermates. In ApcMin/+; c-Mycfl/fl mice, the intestinal apoptosis was higher in the areas of the small intestine with the decreased c-Myc protein expression (P= 0.0016, compared to their littermates with the wild type c-Myc). Thus, conditional inactivation of c-Myc, mediated by Fabpl4×at-132-driven Cre-recombinase, suppresses Apc-dependent intestinal tumorigenesis in adult ApcMin/+) mice, without apparent effect on normal intestinal mucosa.

Unique dietary‐related mouse model of colitis

Background: A high‐fat diet is a risk factor for the development of inflammatory bowel disease (IBD) in humans. Deoxycholate (DOC) is increased in the colonic contents in response to a high‐fat diet. Thus, an elevated level of DOC in the colonic lumen may play a role in the natural course of development of IBD. Methods: Wild‐type B6.129 mice were fed an AIN‐93G diet, either supplemented with 0.2% DOC or unsupplemented and sacrificed at 1 week, 1 month, 3 months, 4 months, and 8 months. Colon samples were assessed by histopathological, immunohistochemical, and cDNA microarray analyses. Results: Mice fed the DOC‐supplemented diet developed focal areas of colonic inflammation associated with increases in angiogenesis, nitrosative stress, DNA/RNA damage, and proliferation. Genes that play a central role in inflammation and angiogenesis and other related processes such as epithelial barrier function, oxidative stress, apoptosis, cell proliferation/cell cycle/DNA repair, membrane transport, and the ubiquitin‐proteasome pathway showed altered expression in the DOC‐fed mice compared with the control mice. Changes in expression of individual genes (increases or reductions) correlated over time. These changes were greatest 1 month after the start of DOC feeding. Conclusions: The results suggest that exposure of the colonic mucosa to DOC may be a key etiologic factor in IBD. The DOC‐fed mouse model may reflect the natural course of development of colitis/IBD in humans, and thus may be useful for determining new preventive strategies and lifestyle changes in affected individuals.

Polyamine transport is mediated by both endocytic and solute carrier transport mechanisms in the gastrointestinal tract

The polyamines spermidine and spermine, and their precursor putrescine, are required for cell growth and cellular functions. The high levels of tissue polyamines are implicated in carcinogenesis. The major sources of exogenous polyamines are diet and intestinal luminal bacteria in gastrointestinal (GI) tissues. Both endocytic and solute carrier-dependent mechanisms have been described for polyamine uptake. Knocking down of caveolin-1 protein increased polyamine uptake in colon cancer-derived HCT116 cells. Dietary supplied putrescine was accumulated in GI tissues and liver in caveolin-1 knockout mice more than wild-type mice. Knocking out of nitric oxide synthase (NOS2), which has been implicated in the release of exogenous polyamines from internalized vesicles, abolished the accumulation of dietary putrescine in GI tissues. Under conditions of reduced endogenous tissue putrescine contents, caused by treatment with the polyamine synthesis inhibitor difluoromethylornithine (DFMO), small intestinal and colonic mucosal polyamine contents increased with dietary putrescine levels, even in mice lacking NOS2. Knocking down the solute carrier transporter SLC3A2 in HCT116-derived Hkh2 cells reduced the accumulation of exogenous putrescine and total polyamine contents in DFMO treated cells, relative to non-DFMO-treated cells. These data demonstrate that exogenous putrescine is transported into GI tissues by caveolin-1- and NOS2-dependent mechanisms, but that the solute carrier transporter SLC3A2 can function bidirectionally to import putrescine under conditions of low tissue polyamines.

Combination Chemoprevention of Intestinal Carcinogenesis in a Murine Model of Familial Adenomatous Polyposis

Familial adenomatous polyposis (FAP) is an autosomal dominantly inherited syndrome in humans. The Apc Min/+ mouse, which expresses a mutant homolog of the adenomatous polyposis coli gene, is a model of FAP in humans. Treatment with the nonsteroidal anti-inflammatory drugs (NSAIDS) sulindac or celecoxib can suppress polyp development in FAP patients, but responses are generally transient and incomplete. Combination chemoprevention with the ornithine decarboxylase inhibitor difluoromethylornithine (DFMO) and either celecoxib or sulindac was evaluated in the Apc Min/+ mouse. Combinations of DFMO and either NSAID reduced intestinal tumor number by more than 80% (P < 0.0001) compared to untreated controls. In addition to the dramatic reduction in tumor number, the combination of DFMO and sulindac reduced the development of high-grade intestinal adenomas compared to sulindac alone (P = 0.003). The fraction of high-grade intestinal adenomas remaining after treatment was similar for the combination of DFMO and celecoxib and celecoxib alone. Only combinations of DFMO plus sulindac reduced total intestinal polyamine contents compared to untreated mice. These data support the rationale for treatment of FAP patients postcolectomy with DFMO combined with either celecoxib or sulindac but indicate that sulindac may be more effective than celecoxib in reducing intestinal polyamine contents and the incidence of high-grade intestinal adenomas when combined with DFMO.

The role of NO synthases in arginine-dependent small intestinal and colonic carcinogenesis

Arginine is catabolized by NOS2 and other nitric oxide synthases to form nitric oxide. We evaluated the roles of dietary arginine and Nos2 in Apc‐dependent intestinal tumorigenesis in Min mice with and without a functional Nos2 gene. NOS2 protein was expressed only in intestinal tissues of ApcMin/+ Nos2+/+ mice. NOS3 expression was higher in intestinal tissues of mice lacking Nos2, mainly in the small intestine. When diet was supplemented with arginine (0.2% and 2% in drinking water), lack of Nos2 results in decreased tumorigenesis in both small intestine and colon. In Nos2 knockout mice, supplemental arginine (up to 2%) caused a decrease in small intestinal tumor number and size. The arginine‐dependent decrease was associated with an increase in nitrotyrosine formation and apoptosis in the region of intestinal stem cells. Mice expressing Nos2 did not show these changes. These mice did, however, show an arginine‐dependent increase in colon tumor number and incidence, while no effect on apoptosis was seen. These changes were associated with increased nitrotyrosine formation in epithelial cells. Mice lacking Nos2 did not show changes in tumorigenesis or nitrotyrosine formation, while demonstrating an arginine‐dependent increase in apoptosis. These data suggest that Nos2 and dietary arginine have significant effects on intestinal and colonic tumorigenesis in Min mice. In both tissues, loss of Nos2 is associated with decreased tumorigenesis when mice are supplemented with dietary arginine. In the small intestine, Nos2 prevents the arginine‐induced decrease in tumor number and size, which is associated with NOS3 expression and increased apoptosis. In the colon, Nos2 is required for the arginine‐induced increase in tumor number and incidence.

Dietary putrescine reduces the intestinal anticarcinogenic activity of sulindac in a murine model of familial adenomatous polyposis.

Abstract: The nonsteroidal antiinflammatory drug sulindac displays chemopreventive activity in patients with familial adenomatous polyposis (FAP). Sulindac metabolites induce apoptosis in colon tumor cells, in part, by a polyamine-dependent mechanism that can be suppressed with exogenous putrescine. To determine the relevance of this mechanism in animals, we treated ApcMin/+ mice, a model of human FAP, with sulindac alone or in combination with dietary putrescine. Sulindac increased steady-state RNA levels and enzymatic activity of the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase and intestinal levels of monoacetylspermidine, spermidine, and spermine in the small intestine of mice. Sulindac also decreased the activity of the biosynthetic enzyme ornithine decarboxylase but not adenosylmethionine decarboxylase (AMD). Dietary putrescine increased intestinal putrescine contents, whereas the combination of dietary putrescine and sulindac yielded the highest levels of intestinal putrescine and correlated with a statistically significant reduction in AMD enzyme activity. Dietary putrescine did not statistically significantly increase tumorigenesis, although it significantly increased the grade of adenoma dysplasia (P < 0.05). The effectiveness of sulindac to suppress intestinal carcinogenesis was partially abrogated by dietary putrescine. These data suggest that sulindac exerts at least some of its anticarcinogenic effects in mice via a polyamine-dependent mechanism. Because high concentrations of putrescine can be found in certain dietary components, it may be advantageous to restrict dietary putrescine consumption in patients undergoing treatment with sulindac.

Deoxycholate-Induced Colitis is Markedly Attenuated in Nos2 Knockout Mice in Association with Modulation of Gene Expression Profiles

Nos2 knockout mice were compared to wild-type mice for susceptibility to colitis in response to a diet supplemented with deoxycholate, a bile acid increased in the colon of individuals on a high-fat diet. Wild-type mice fed a fat-related diet, supplemented with 0.2% DOC, develop colonic inflammation associated with increases in nitrosative stress, proliferation, oxidative DNA/RNA damage, and angiogenesis, as well as altered expression of numerous genes. However, Nos2 knockout mice fed a diet supplemented with deoxycholate were resistant to these alterations. In particular, 35 genes were identified whose expression was significantly altered at the mRNA level in deoxycholate-fed Nos2(+/+) mice but not in deoxycholate-fed Nos2(–/–) mice. Some of these alterations in NOS2-dependent gene expression correspond to those reported in human inflammatory bowel disease. Overall, our results indicate that NOS2 expression is necessary for the development of deoxycholate-induced colitis in mice, a unique dietary-related model of colitis.

Inhibition of Akt enhances the chemopreventive effects of topical rapamycin in mouse skin

The PI3Kinase/Akt/mTOR pathway has important roles in cancer development for multiple tumor types, including UV-induced nonmelanoma skin cancer. Immunosuppressed populations are at increased risk of aggressive cutaneous squamous cell carcinoma (SCC). Individuals who are treated with rapamycin (sirolimus, a classical mTOR inhibitor) have significantly decreased rates of developing new cutaneous SCCs compared with those that receive traditional immunosuppression. However, systemic rapamycin use can lead to significant adverse events. Here, we explored the use of topical rapamycin as a chemopreventive agent in the context of solar-simulated light (SSL)-induced skin carcinogenesis. In SKH-1 mice, topical rapamycin treatment decreased tumor yields when applied after completion of 15 weeks of SSL exposure compared with controls. However, applying rapamycin during SSL exposure for 15 weeks, and continuing for 10 weeks after UV treatment, increased tumor yields. We also examined whether a combinatorial approach might result in more significant tumor suppression by rapamycin. We validated that rapamycin causes increased Akt (S473) phosphorylation in the epidermis after SSL, and show for the first time that this dysregulation can be inhibited in vivo by a selective PDK1/Akt inhibitor, PHT-427. Combining rapamycin with PHT-427 on tumor prone skin additively caused a significant reduction of tumor multiplicity compared with vehicle controls. Our findings indicate that patients taking rapamycin should avoid sun exposure, and that combining topical mTOR inhibitors and Akt inhibitors may be a viable chemoprevention option for individuals at high risk for cutaneous SCC. Cancer Prev Res; 9(3); 215–24. ©2016 AACR.

Resatorvid-based Pharmacological Antagonism of Cutaneous TLR4 Blocks UV-induced NF-κB and AP-1 Signaling in Keratinocytes and Mouse Skin

Cutaneous exposure to solar ultraviolet (UV) radiation is a major causative factor in skin carcinogenesis, and improved molecular strategies for efficacious chemoprevention of nonmelanoma skin cancer (NMSC) are urgently needed. Toll‐like receptor 4 (TLR4) signaling has been shown to drive skin inflammation, photoimmunosuppression, and chemical carcinogenesis. Here we have examined the feasibility of genetic and pharmacological antagonism targeting cutaneous TLR4 for the suppression of UV‐induced NF‐κB and AP‐1 signaling in keratinocytes and mouse skin. Using immunohistochemical and proteomic microarray analysis of human skin, we demonstrate for the first time that a significant increase in expression of TLR4 occurs in keratinocytes during the progression from normal skin to actinic keratosis, also detectible during further progression to squamous cell carcinoma. Next, we demonstrate that siRNA‐based genetic TLR4 inhibition blocks UV‐induced stress signaling in cultured keratinocytes. Importantly, we observed that resatorvid (TAK‐242), a molecularly targeted clinical TLR4 antagonist, blocks UV‐induced NF‐κB and MAP kinase/AP‐1 activity and cytokine expression (Il‐6, Il‐8, and Il‐10) in cultured keratinocytes and in topically treated murine skin. Taken together, our data reveal that pharmacological TLR4 antagonism can suppress UV‐induced cutaneous signaling, and future experiments will explore the potential of TLR4‐directed strategies for prevention of NMSC.