Treatment of the immortalized mouse mammary epithelial cell line Scp2 in vitro with exogenous recombinant Str1 rapidly induced an epithelial-to-mesenchymal cell transition (EMT). EMT is characterized by loss of cell-cell interactions, acquisition of a scattered morphology, downregulation of epithelial cytokeratins, and upregulation of the mesenchymal marker vimentin.
Using the second approach, it was found that Scp2 cells transfected with Str1 controlled by an inducible promoter undergo EMT when induced and can also form anchorage-independent colonies in agar. In the absence of Str1 expression, Scp2 cells transplanted into cleared mammary fat pads form duct-like structures. When Str1 gene expression is induced, EMT is triggered and cells become tumorigenic and invasive in vivo. Once triggered, repression of Str1 expression could not reverse their tumorigenicity.
To examine the long-term effects of Str1 on normal mammary epithelial cells in vivo, WAP-Str1 transgenic mice were used. At age 6-24 months, such mice develop mammary abnormalities, premalignant lesions and tumors. Less than one in ten mice examined had a mammary tumor and the average latency was 18 months. Hyperplastic lesions were apparent in half of the animals at six months and in three-quarters at two years, consistent with a multistage neoplastic progression induced by Str1 expression. In total, twelve tumors were seen, of which nine were well differentiated and three were undifferentiated with evidence of EMT.
Microdissection and comparative genome hybridization (CGH) revealed that tumors exhibiting EMT contained DNA copy number gains on chromosome 15, specifically in fibroblast-like areas of the tumor.