Coagulation proteases and the generation of thrombin are increased in breast tumor epithelial and stromal cells. Since thrombin can modify tumor cell behavior directly through the activation of protease-activated receptors (PARs) or indirectly by generating fibrin matrices, the effect of dabigatran etexilate, a direct thrombin inhibitor, on breast cancer development was evaluated. Dabigatran inhibited invasiveness of MDA-MB-231 breast carcinoma cells across Matrigel-coated membranes at concentrations that had no effect on the proliferation index of cultured tumor cells. In vivo evaluation of invasiveness of MDA-MB-231 cells in tracheal xenotransplants in nude mice orally administered dabigatran etexilate twice daily at a dose of 45 mg/kg over 4 weeks demonstrated less invasion of tumor cells through the tracheal wall compared to vehicle-treated mice. To evaluate the effect of dabigatran on the development of metastatic foci, 4T1 tumor cells were injected orthotopically in the mammary fat pads of syngeneic Balb/c mice. Dabigatran etexilate treatment exhibited evidence of antitumor activity with a 50% reduction in tumor volume at 4 weeks following orthotopic injection of 4T1 cells in syngeneic Balb/c mice with no weight loss in treated mice. Dabigatran etexilate reduced both 4T1 tumor cells in the blood and liver micrometastases by 50-60%. These results suggest that oral administration of the direct thrombin inhibitor, dabigatran etexilate, inhibits both invasion and metastasis of malignant breast tumors, suggesting that it may be beneficial in not only preventing thrombotic events in cancer patients, but also as adjunct therapy to treat malignant tumors.

Elevated expression of ornithine decarboxylase (ODC) and increased synthesis of polyamines are hallmarks of epithelial tumorigenesis. The skin and tumors of K6/ODC and ODC/ Ras transgenic mice, in which overexpression of ODC has been targeted to hair follicles, were found to exhibit intrinsically high histone acetyltransferase (HAT) activity. We identified Tip60 as a candidate enzyme for contributing significantly to this abnormally high HAT activity. Compared with normal littermate controls, the levels of Tip60 protein and an alternative splice variant Tip53 were found to be greater in K6/ODC mouse skin. Furthermore, skin tumors that spontaneously develop in ODC/Ras bigenic mice typically have substantially more Tip60 protein than adjacent non-tumorbearing skin and exhibit a unique pattern of Tip60 size variants and chemically modified protein isoforms. Steadystate Tip60 and Tip53 mRNA levels were not affected in ODC-overexpressing skin and tumors, implying novel posttranscriptional regulation by polyamines. Given the diverse roles of Tip60, the overabundance of Tip60 protein is predicted to have biological consequences. Compared with normal littermate skin, we detected altered association of Tip60 with E2F1 and a subset of newly identified Tip60-interacting transcription factors in ODC transgenic mouse skin and tumors. E2F1 was shown to be bound in greater amounts to up-regulated target genes in ODC-overexpressing skin. Thus, up-regulation of Tip60 protein, influencing the expression of Tip60-regulated genes, could play a contributing role in polyaminemediated tumor promotion. (Cancer Res 2006; 66(16): 8116-22)

Overexpression of ornithine decarboxylase (ODC), resulting in increased polyamine metabolism, is a common feature of epithelial tumors. Polyamines play a complex role in promoting tumor development, affecting diverse cellular processes, including gene expression. One way polyamines may affect gene expression is to modulate the multiprotein complexes comprised of transcription factors and coregulatory factors that alter chromatin structure by acetylating/deacetylating nearby histones. We have capitalized on ODC-overexpressing cultured cells and K6/ODC and ODC/Ras transgenic mouse models, in which ODC overexpression is targeted to hair follicles, to evaluate the influence of polyamines on the acetylation of histones and other proteins. ODC overexpression was found to alter intrinsic histone acetyltransferase (HAT) and deacetylase activities and histone acetylation patterns. The high HAT activity exhibited by ODC transgenic mouse skin and tumors might be partly attributed to enhanced p300/creb-binding protein (CBP)-associated HAT activity and increased levels of Tat interactive protein, 60 kDa (Tip60) HAT protein isoforms. Altered association of Tip60 with E2F1 and a subset of newly identified Tip60-interacting transcription factors was detected in ODC mouse skin and tumors, implying novel polyamine modulation of Tip60-regulated gene expression. Polyamine effects on HAT enzymes also influence the acetylation status of nonhistone proteins. Overexpression of ODC in skin serves as a novel stimulus for acetylation of the tumor suppressor protein, p53--a target of both p300/CBP and Tip60--with concomitant increased binding to, and increased transcription of, a downstream target gene. The future challenge will be to elucidate the multiple mechanisms by which polyamines influence enzymes that regulate protein acetylation and gene transcription to promote cancer.

Ornithine decarboxylase (ODC) overexpression coupled with activated Ras is fully sufficient to oncogenically transform primary keratinocytes. To determine the Ras effector pathways that represent the minimal essential contribution to full oncogenic transformation in this context, we evaluated the cooperativity of different Ras effector mutants with overexpressed ODC in an in vivo tracheal xenotransplantation assay for epithelial cell invasiveness. Primary keratinocytes, isolated from either K6/ODC transgenic mouse skin (expressing increased ODC) or from normal littermate skin were infected with retrovirus producing an activated RasV12 or partial loss-of-function effector mutants of RasV12 that selectively induce only the Raf/ERK, RalGDS, or the PI3-kinase signaling pathway. Whereas keratinocytes expressing a fully activated RasV12 are not invasive in tracheal xenotransplants, ODC-overexpressing keratinocytes acquire an invasive phenotype with additional expression of either RasV12 or activation of the Raf/ERK pathway. Independent of a mutated ras, elevated levels of ODC activate the Akt/mTOR signaling pathway as well as the Rho/Rac pathway in primary keratinocytes. Thus, Raf/ERK signaling is sufficient to cooperate with increased ODC activity in the conversion of normal keratinocytes to invasive cells. In order to promote invasiveness in keratinocytes, elevated levels of ODC may cooperate with Raf/ERK via activation of the Akt and Rho/Rac signaling pathway.

Induction of ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis, in ODC transgenic skin stimulates epidermal proliferation but not hyperplasia, activates underlying stromal cells and promotes skin tumorigenesis following a single subthreshold dose of a carcinogen. Because chronic wounds are a well-recognized risk factor for skin cancer, we investigated the response to a tissue remodeling event in normal skin that is abraded to remove only the epidermal layer in K6/ODC transgenic (follicular ODC expression) and in inducible ODCER transgenic mice (suprabasal ODC expression). When regenerative epidermal hyperplasia was resolved in normal littermates following abrasion, ODC transgenic mice exhibited progressive epidermal hyperplasia with formation of benign tumor growths and maintained an increased epidermal proliferation index and activation of translation-associated proteins at abrasion sites. The epidermal hyperplasia and tumor-like growth was accompanied by activation of underlying stromal cells and prolonged infiltration of inflammatory cells. Treatment with the anti-inflammatory agent dexamethasone did not reduce the high proliferative index in the regenerated epidermis but dramatically reduced the epidermal hyperplasia and prevented the wound-induced tumor growths in abraded ODCER skin. Treatment with α-difluoromethylornithine, a specific inhibitor of ODC activity, normalized the wound response in transgenic mice and decreased wound-induced inflammation if administered from the time of abrasion but not if initiated 4 days following abrasion. These results suggest a role for polyamines in prolonging wound-associated inflammation in addition to stimulating proliferation both of which are sufficient to sustain epidermal hyperplasia and benign tumor growth even in the absence of genetic damage.