Trask, a Candidate Breast Cancer Metastasis Protein

Institution: University of California, San Francisco
Investigator(s): Ching Hang Wong, Ph.D. -
Award Cycle: 2007 (Cycle 13) Grant #: 13FB-0146 Award: $90,000
Award Type: Postdoctoral Fellowship
Research Priorities
Biology of the Breast Cell>Pathogenesis: understanding the disease

Initial Award Abstract (2007)
Src is a tyrosine kinase oncogene and is found to be activated in most breast cancers. We have been studying the functions of src in breast cancers and have cloned a new src target protein, called Trask that is usually phosphorylated during mitosis in normal cells, but is phosphorylated continuously in many breast cancer cells. When we increase the amount of Trask in breast cancer cells, the cells detach and separate, similar to what happens in metastasis. In a survey of normal human tissues, we found that Trask is expressed in most epithelial tissues, including breast epithelium, but is never phosphorylated in any of these normal tissues. In a survey of breast cancer cell lines, we have found that Trask is abnormally phosphorylated in many breast cancer cell lines but not in a normal breast epithelial cell line. This has led us to believe that Trask may be a key mediator of tumor metastasis in breast cancers.

We believe that the physiologic function of Trask is to allow cells to temporarily detach during cell division. However in breast cancers, this mechanism is prematurely activated by hyperactive src kinases leading to detachment of cancer cells to promote metastasis. In cell culture systems we will explore the molecular mechanism by which we think Trask regulates cell adhesion. ▀-catenin was first identified as a structural protein that binds to E-cadherin on the cell membrane to confer cell adhesion. During tumorigenesis, the E-cadherin/▀-catenin association is usually lost, leading to the loss of adhesion function. The loss of this complex is mediated by src-phosphorylation of ▀-catenin. It is possible that phospho-Trask brings src kinases in proximity to the E-cadherin/▀-catenin complex, where src can phosphorylate ▀-catenin more readily. In mouse models we will determine whether overexpression and phosphorylation of Trask induces MCF-7 breast cancer cell metastasis. In parallel studies using human tumor samples, we will determine by immunohistochemistry (IHC) whether there is increased Trask phosphorylation in primary tumor and metastasis specimens. We have already developed antibodies to differentiate Trask and its phosphorylated forms.

If our hypothesis is proven, then Trask will be a unique protein because it is specifically phosphorylated in breast cancers, but not in normal breast cells. If we find that it is phosphorylated specifically in cancer metastasis, it will be the first such protein described. In either case, it is potentially a novel biomarker that may identify which patients could benefit by being treated with src inhibitors. If we show in our mouse models that Trask promotes the development of metastases, than it will identify a whole new pathway for follow-up studies. This may lead to the successful use of new targeted therapies that may have fewer side effects than chemotherapy.

Final Report (2009)
Src is a tyrosine kinase oncogene that is activated in most breast cancers. Trask is its substrate that usually only gets phosphorylated during mitosis in normal cells but is phosphorylated in many breast cancer cell lines. Our hypothesis is that Trask is a cell-adhesion modulator that is constitutively phosporylated by hyperactive src kinases in breast cancers, leading to the detachment of cancer cells and therefore metastasis. In this project, we proposed to investigate if the over-expression and phosphorylation of Trask promote tumor cell metastasis in experimental models, to study the phosphorylation status of Trask in human breast cancer specimens and the mechanism by which Trask mediates cell adhesion function.

In the past two years, we have been able to establish two MCF-7 breast cancer cell lines that express either a full length wild-type Trask or a phosphorylation defective mutant of Trask under tetracycline induction to study the ôgain-of-functionö effect of Trask in cancer progression and metastasis. Using these cells, we established an in vivo breast cancer metastasis model in which metastasis was detected different organs after cells were orthotopically implanted in mammary fat pads of mice. The experimental results showed that MCF-7 cells are strikingly different than other breast cell types in that the overexpression of Trask shows no biologic phenotype in MCF-7 cells. This in vivo model is being expanded now to include additional breast cancer cell types.

In addition, we performed immunohistochemistry staining of Trask and phospho-Trask in a panel of human breast cancer tissues at different stages of disease. We found that Trask is expressed in both normal breast epithelium and breast cancers. There is no phosphorylation of Trask in the normal epithelium. However, we see Trask phosphorylation in a subset of breast cancer tissues including in situ and invasive cancers. This suggests that in many breast cancers, Trask phosphorylation is an early event. This is consistent with our experimental models which suggest that Trask phosphorylation promotes the anchorage-independent state of epithelial cells, enabling ductal epithelium to start growing within the lumen of the ducts or lobules without adhesion to the basal layer, and eventually leading to metastasis.

Lastly, we studied the interaction between Trask and beta-catenin, an important protein that is known to regulate cell-cell adhesion, in a cell culture model. We found that these two molecules existed in the same protein complex, and their interaction was significantly enhanced in breast cancer cells that were grown without adhesion. The interaction between beta-catenin and its usual binding partner E-cadherin was not affected by the presence of Trask, therefore the anti-adhesive effects of Trask phosphorylation are not mediated through disruption of beta-catenin /E-cadherin complex. The mechanisms that mediate the anti-adhesive effects of phosphorylated Trask in conjunction with beta-catenin are now under investigation.

Phosphorylation of the SRC epithelial substrate Trask is tightly regulated in normal epithelia but widespread in many human epithelial cancers.
Periodical:Clinical Cancer Research
Index Medicus: Clin Cancer Res
Authors: Wong CH, Baehner FL, et al, and Moasser MM
Yr: 2009 Vol: 15 Nbr: 7 Abs: Pg:2311-22