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The Role of Twist in Mammary Tumor Metastasis
Tumor Cell Biology I
BACKGROUND: The transcription factor Twist has been implicated as playing a role in breast cancer metastasis, however the mechanisms underlying its actions are still unclear. Twist is a basic-Helix-Loop-Helix (bHLH) transcription factor involved in the specification and differentiation of mesenchymal tissues. We have recently found that the activity of Twist depends on its dimer partner. Unlike most other bHLH proteins, we find that Twist can form functional homodimers (T/T) as well as heterodimers with ubiquitously expressed bHLH E proteins (T/E). We find that the T/T and T/E dimers have distinct activities, regulating the expression of different sets of genes and have opposing effects on cell proliferation and migration. Consequently, Twist may positively or negatively affect tumor cell behavior dependent upon which Twist dimer is prevalent. The ratio of T/T to T/E within a cell is regulated by the HLH protein Id, which dimerizes with E proteins and prevents functional bHLH heterodimers from forming. Increasing levels of Id inhibit T/E formation and promote T/T formation. Interestingly, increased expression of Twist and Id are found in several different tumor types, including breast cancer, and are associated with more aggressive tumor phenotypes, suggesting that T/T dimers may promote tumor progression. Consistent with this, we find that T/T dimers promote tumor cell migration and invasion while T/E dimers inhibit these processes. HYPOTHESIS: This proposal seeks to follow up on these findings to test the hypothesis that T/T homodimers promote tumor progression while T/E heterodimers inhibit this process. SPECIFIC AIMS: 1) Characterize the effects of the Twist dimers on metastatic growth of tumor cell lines. 2) Perform gene expression analysis to identify genes that are differentially regulated by the Twist dimers and that correlate with metastatic potential. 3) Characterize the effects of Twist dimer expression on de novo tumor growth and metastasis. STUDY DESIGN: All three Aims utilize the MMTV-PyVT mammary tumor model, which is one of the best murine models to mimic the human disease. The first two Aims will compare the effects of the expression of the Twist dimers on the cell behavior, gene expression, and metastatic ability of tumor cell lines derived from these mice. In Aim 3 we will utilize transgenic mice to express the Twist dimers to characterize the effects of Twist on de novo tumor formation and metastasis in the MMTV-PyVT mice. POTENTIAL OUTCOMES: These studies will determine whether T/T dimers promote tumor progression and if T/E dimers are inhibitory, and therefore will establish whether targeting Twist dimerization may be a fruitful therapeutic target to regulate metastatic growth. The identification of genes that are differentially regulated by T/T and T/E may also be useful diagnostic indicators of metastatic potential.
Breast cancer is among the most common human cancers, affecting up to 1 in every 8 women. Tumor metastasis is the major cause of death from breast cancers and while there have been improvements in diagnosis and treatment it is still unclear which molecular changes in breast tumors are likely to lead to invasion and metastasis. The transcription factor Twist is over-expressed in many cancers, including breast cancers, and its expression has generally been correlated with more aggressive tumor phenotypes. Twist is a basic-Helix-Loop-Helix (bHLH) transcription factor that plays both positive and negative roles in the regulation of early specification and differentiation of mesenchymal tissues. The acquisition of mesenchymal characteristics by tumor cells enables them to invade into host tissues and metastasize and Twist has been implicated as mediating this process although the mechanisms underlying its actions are still unclear. In our efforts to better characterize the molecular basis of Twist function we have recently found that the activity of Twist depends on whether it interacts with itself to form homodimers (T/T) or with bHLH E proteins to form heterodimers (T/E). These two dimers have opposing activities on gene expression and cell behavior. Preliminary experiments suggest that Twist homodimers promote tumor cell migration and invasion while T/E heterodimers inhibit these processes. This proposal seeks to follow up on these findings to test the hypothesis that T/T homodimers promote tumor progression while T/E heterodimers inhibit this process. Our objective is to determine the role that Twist plays in breast tumor progression and determine whether Twist dimerization may be a fruitful therapeutic target to regulate metastatic growth. We will also identify genes that are differentially regulated by the T/T and T/E dimers. There is a dearth of diagnostic indicators of the metastatic potential of primary tumors, which are critical for treatment decisions, and genes that are specifically regulated by the T/T homodimers should provide a genetic profile for the metastatic disposition of a tumor.