Transforming growth factor-bs (TGF-bs) are potent inhibitors of epithelial cell growth. Recently components of the TGF-b response path have been shown to be diminished or absent in a number of human malignancies, implicating loss of TGF-b function as one mechanism contributing to tumor development. However, TGF-b expression is often upregulated in advanced human cancers suggesting that the role played by the TGF-b system may be complex. We propose that TGF-bs function as """"""""conditional tumor suppressors"""""""", with suppressor activity dependent on (i) the levels of the TGF-bs and their receptors, (ii) the stage of tumorigenesis, and (iii) the nature of cooperating oncogenic events. Since the effects of TGF-b are highly context-dependent, we have chosen to study these questions in vivo in the intact organism where all the complex contextual cues such as cell-cell interaction, hormonal milieu and appropriate extracellular matrix are maintained. Our approach has been to generate genetically engineered mice in which TGF-b function is experimentally compromised in target organs, and we are focussing particularly on the mammary gland/breast. To address the role of TGF-b in breast cancer, we have generated transgenic mice overexpressing antagonists of TGF-b action in the mammary gland. To date we have used both a dominant negative mutant form of the type II TGF-b receptor (DNR), which functions as a cell-autonomous antagonist, and a soluble TGF-b antagonist consisting of the extracellular domain of the type II TGF-b receptor fused to an immunoglobulin Fc domain. Work is ongoing to generate improved forms of both type of antagonist. Using the dominant negative receptor approach, we have shown that loss of TGF-b response causes abnormal mammary gland development and an increased susceptibility to tumorigenesis induced by chemical carcinogens. This proves that TGF-b can have tumor suppressor activity in the mammary gland. Since TGF-b is proposed to have different roles at different stages of tumorigenesis, we are developing regulatable transgenic mouse models that will allow us to control the stage at which we inactivate the TGF-b system. In addition, we are working with mice in which the various Smad components of the TGF-b signal transduction pathway have been genetically knocked out. We are using these to begin to identify which TGF-b signal transduction components are involved in which TGF-b responses. This will allow us to determine whether tumor suppressor and oncogenic activities are mediated by distinct signal transduction pathways. Finally, we complement the transgenic work with experiments in which we genetically modify breast-derived cell lines representing different stages of the tumorigenic process, and assess their tumorigenicity in a nude mouse xenograft system. A major focus will be the elucidation of molecular mechanisms underlying the observed changes in tumorigenicity, both by ad hoc analysis of candidate genes and pathways, and by global cDNA expression analysis. Results from all these experiments should give clinically useful insights into the functions of TGF-bs during tumor initiation , promotion and progression, and illuminate how the system could be most effectively used in novel chemopreventive strategies.
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