Research Grants Awarded
The Role of Six1, and its target TGF beta R-I, in mediating breast cancer metastasis
Tumor Cell Biology II
a) Background- During development, genes are activated to stimulate proliferation, migration, invasion, neovascularization, and survival. These genes are often downregulated once organ development is complete. In cancer, the same genes are often re-activated, stimulating the same properties out of context. The homeobox gene and transcription factor Six1 is one such gene. Six1 is expressed during embryogenesis, lost in most differentiated tissues, and re-expressed in a number of different cancers including breast cancer, where the gene is amplified. During normal development and in cancer, Six1 stimulates cellular proliferation and survival. Interestingly, Six1 also plays a role in the epithelial to mesenchymal transition (EMT) during normal development. As Six1 is overexpressed in 90% of metastatic lesions compared to 50% of primary breast cancers, and as genes that cause EMT are often associated with metastasis, we asked whether Six1 could induce aspects of EMT and/or metastasis in mammary cells. Indeed, preliminary data within this proposal demonstrate that Six1 overexpression leads to changes characteristic of EMT and to in vivo metastasis, as well as to the upregulation of the transforming growth factor-beta receptor type I (T?R-1), a known regulator of EMT and metastasis. b) Hypothesis: Six1 overexpression leads to metastatic breast cancer through upregulation of T?R-I and activation of the TGF-? signaling pathway. Inhibition of Six1 in breast cancer cells will inhibit metastatic dissemination. c and d) Specific Aims and Study Design: In the 1st Aim, we will use Six1 overexpression and knockdown in two different mouse models of breast cancer to determine whether Six1 is sufficient, and necessary, for breast cancer metastasis, and whether its removal can inhibit metastatic disease. Aim 2. In the second aim, we will examine whether the TGF-? signaling pathway is activated in response to Six1 overexpression and whether Six1 is dependent on T?R-I signaling to mediate metastasis. e) Potential Outcomes and Benefits of Research: Preliminary data from our laboratory demonstrate that Six1 influences both tumor initiation and metastasis. Thus, targeting it therapeutically has the potential to inhibit breast cancer both at early and later stages of disease progression. In addition, as Six1 is embryonic specific and not expressed in most adult normal tissues, it is an ideal drug target whose inactivation will inhibit tumor cell proliferation, survival, and metastasis with limited side effects to the patient. In this grant, we will specifically examine whether Six1 inhibition is a viable strategy for inhibiting breast cancer metastasis, and we will further dissect the pathway through which it causes metastatic disease in an effort to identify potential downstream-mediators of Six1-induced metastasis that can also be targeted therapeutically.
In 2005, approximately 210,000 women were diagnosed with invasive breast cancer, and greater than 40,000 died of the disease. Women do not die due to the cancer that is localized in the breast, but rather because the cancer cells metastasize, spreading to vital organs and ultimately resulting in their failure. Even though it is common knowledge that metastatic disease kills breast cancer patients, research to understand the molecular mechanism(s) of metastasis lags far behind that in other areas of cancer biology. If we hope to diminish death associated with breast cancer, we need not only to improve early detection, but also to devise strategies to target metastatic cells, or to inhibit metastasis from occurring altogether. We have previously demonstrated that the developmental regulator, Six1, is overexpressed in a much higher percentage of metastastic lesions (90%) compared to primary (50%) breast cancers, and that its overexpression in primary breast cancer correlates with lymph node involvement and not with any other clinical parameters examined. Our preliminary data demonstrate that Six1 overexpression in tumorigenic, but nonmetastatic mammary carcinoma cells results in metastases to the lymph nodes, and in rare cases, to the bone, organs in which metastases are typically found in breast cancer. In this proposal, we test the hypothesis that misexpression of Six1 in breast cancer leads to the induction of metastasis, and that inhibition of Six1 can eliminate, or dramatically reduce, metastatic disease. We further examine the molecular mechanism by which Six1 confers metastasis, focusing on the transforming growth factor-beta pathway, with the hope of identifying multiple molecules within the Six1 pathway that can be targeted therapeutically to inhibit metastatic disease. Six1 has previously been implicated in tumor formation through its ability to increase cell growth and cell survival, and in this grant we examine whether Six1 plays an additional role in breast cancer metastasis. This is important because if Six1 influences multiple stages of the tumorigenic process, targeting it therapeutically has the potential to inhibit breast cancer both at early and later stages of disease progression. In addition, as Six1 is embryonic specific and not expressed in most adult normal tissues, it is an ideal drug target whose inactivation will inhibit tumor cell proliferation, survival, and metastasis with limited side effects to the patient. In this grant, we begin to dissect how Six1 influences breast cancer metastasis, and to examine whether inhibition of Six1 may indeed be an effective anti-breast cancer therapy.