Building on the foundation of daring hypotheses: using the MKK4 metastasis suppressor to develop models of dormancy and metastatic colonization.

The identification of a novel metastasis suppressor function for the MAP Kinase Kinase 4 protein established a role for the stress-activated kinases in regulating the growth of disseminated cancer cells. In this review, we describe MKK4's biological mechanism of action and how this information is being used to guide the development of new models to study cancer cell dormancy and metastatic colonization. Specifically, we describe the novel application of microvolume structures, which can be modified to represent characteristics similar to those that cancer cells experience at metastatic sites. Although MKK4 is currently one of many known metastasis suppressors, this field of research started with a single daring hypothesis, which revolutionized our understanding of metastasis, and opened up new areas of exploration for basic research. The combination of our increasing knowledge of metastasis suppressors and such novel technologies provide hope for possible clinical interventions to prevent suffering from the burden of metastatic disease.

From prostate to bone: key players in prostate cancer bone metastasis.

Bone is the most common site for metastasis in human prostate cancer patients. Skeletal metastases are a significant cause of morbidity and mortality and overall greatly affect the quality of life of prostate cancer patients. Despite advances in our understanding of the biology of primary prostate tumors, our knowledge of how and why secondary tumors derived from prostate cancer cells preferentially localize bone remains limited. The physiochemical properties of bone, and signaling molecules including specific chemokines and their receptors, are distinct in nature and function, yet play intricate and significant roles in prostate cancer bone metastasis. Examining the impact of these facets of bone metastasis in vivo remains a significant challenge, as animal models that mimic the natural history and malignant progression clinical prostate cancer are rare. The goals of this article are to discuss (1) characteristics of bone that most likely render it a favorable environment for prostate tumor cell growth, (2) chemokine signaling that is critical in the recruitment and migration of prostate cancer cells to the bone, and (3) current animal models utilized in studying prostate cancer bone metastasis. Further research is necessary to elucidate the mechanisms underlying the extravasation of disseminated prostate cancer cells into the bone and to provide a better understanding of the basis of cancer cell survival within the bone microenvironment. The development of animal models that recapitulate more closely the human clinical scenario of prostate cancer will greatly benefit the generation of better therapies.

Ron-receptor tyrosine kinase in tumorigenesis and metastasis.

The Ron-receptor tyrosine kinase has been increasingly recognized for its tumorigenic potential in the last decade. Ron-receptor activation leads to the activation of common receptor tyrosine kinase downstream-signaling pathways, and most prominently in tumor models, activation of MAPK, PI3K and beta-catenin. Numerous experimental models of mammalian tumorigenesis have demonstrated that increased Ron-receptor activity correlates with increased tumorigenesis in a variety of organs of epithelial origin. The evidence for Ron as an oncogene in human tumor biology is growing. The Ron receptor is overexpressed and over activated in a large number of human tumors, and overexpression of Ron correlates with a worse clinical outcome for patients in at least two human cancer states, namely breast and bladder cancer. Several experimental approaches have been demonstrated to successfully block Ron activity and function, and given these convincing data, approaches to block Ron-receptor activity in targeted human cancers should prove to be fruitful in the setting of future clinical research trials.

Mammary-specific Ron receptor overexpression induces highly metastatic mammary tumors associated with beta-catenin activation.

Activated growth factor receptor tyrosine kinases (RTK) play pivotal roles in a variety of human cancers, including breast cancer. Ron, a member of the Met RTK proto-oncogene family, is overexpressed or constitutively active in 50% of human breast cancers. To define the significance of Ron overexpression and activation in vivo, we generated transgenic mice that overexpress a wild-type or constitutively active Ron receptor in the mammary epithelium. In these animals, Ron expression is significantly elevated in mammary glands and leads to a hyperplastic phenotype by 12 weeks of age. Ron overexpression is sufficient to induce mammary transformation in all transgenic animals and is associated with a high degree of metastasis, with metastatic foci detected in liver and lungs of >86% of all transgenic animals. Furthermore, we show that Ron overexpression leads to receptor phosphorylation and is associated with elevated levels of tyrosine phosphorylated beta-catenin and the up-regulation of genes, including cyclin D1 and c-myc, which are associated with poor prognosis in patients with human breast cancers. These studies suggest that Ron overexpression may be a causative factor in breast tumorigenesis and provides a model to dissect the mechanism by which the Ron induces transformation and metastasis.

Therapeutic implications of a human neutralizing antibody to the macrophage-stimulating protein receptor tyrosine kinase (RON), a c-MET family member.

RON is a member of the c-MET receptor tyrosine kinase family. Like c-MET, RON is expressed by a variety of epithelial-derived tumors and cancer cell lines and it is thought to play a functional role in tumorigenesis. To date, antagonists of RON activity have not been tested in vivo to validate RON as a potential cancer target. In this report, we used an antibody phage display library to generate IMC-41A10, a human immunoglobulin G1 (IgG1) antibody that binds with high affinity (ED50 = 0.15 nmol/L) to RON and effectively blocks interaction with its ligand, macrophage-stimulating protein (MSP; IC50 = 2 nmol/L). We found IMC-41A10 to be a potent inhibitor of receptor and downstream signaling, cell migration, and tumorigenesis. It antagonized MSP-induced phosphorylation of RON, mitogen-activated protein kinase (MAPK), and AKT in several cancer cell lines. In HT-29 colon, NCI-H292 lung, and BXPC-3 pancreatic cancer xenograft tumor models, IMC-41A10 inhibited tumor growth by 50% to 60% as a single agent, and in BXPC-3 xenografts, it led to tumor regressions when combined with Erbitux. Western blot analyses of HT-29 and NCI-H292 xenograft tumors treated with IMC-41A10 revealed a decrease in MAPK phosphorylation compared with control IgG-treated tumors, suggesting that inhibition of MAPK activity may be required for the antitumor activity of IMC-41A10. To our knowledge, this is the first demonstration that a RON antagonist and specifically an inhibitory antibody of RON negatively affects tumorigenesis. Another major contribution of this report is an extensive analysis of RON expression in approximately 100 cancer cell lines and approximately 300 patient tumor samples representing 10 major cancer types. Taken together, our results highlight the potential therapeutic usefulness of RON activity inhibition in human cancers.

The human DEK proto-oncogene is a senescence inhibitor and an upregulated target of high-risk human papillomavirus E7.

The human DEK proto-oncogene is a nucleic acid binding protein with suspected roles in human carcinogenesis, autoimmune disease, and viral infection. Intracellular DEK functions, however, are poorly understood. In papillomavirus-positive cervical cancer cells, downregulation of viral E6/E7 oncogene expression results in cellular senescence. We report here the specific repression of DEK message and protein levels in senescing human papillomavirus type 16- (HPV16-) and HPV18-positive cancer cell lines as well as in primary cells undergoing replicative senescence. Cervical cancer cell senescence was partially overcome by DEK overexpression, and DEK overexpression was sufficient for extending the life span of primary keratinocytes, supporting critical roles for this molecule as a senescence regulator. In order to determine whether DEK is a bona fide HPV oncogene target in primary cells, DEK expression was monitored in human keratinocytes transduced with HPV E6 and/or E7. The results identify high-risk HPV E7 as a positive DEK regulator, an activity that is not shared by low-risk HPV E7 protein. Experiments in mouse embryo fibroblasts recapitulated the observed E7-mediated DEK induction and demonstrated that both basal and E7-induced regulation of DEK expression are controlled by the retinoblastoma protein family. Taken together, our results suggest that DEK upregulation may be a common event in human carcinogenesis and may reflect its senescence inhibitory function.