Transforming growth factor-β enhances invasion and metastasis in Ras-transfected human malignant epidermal keratinocytes.

Transforming growth factor-β (TGF-β) is known to act as a tumour suppressor early in carcinogenesis, but then switches to a pro-metastatic factor in some late stage cancers. However, the actions of TGF-β are context dependent, and it is currently unclear how TGF-β influences the progression of human squamous cell carcinoma (SCC). This study examined the effect of overexpression of TGF-β1 or TGF-β2 in Ras-transfected human malignant epidermal keratinocytes that represent the early stages of human SCC. In vitro, the proliferation of cells overexpressing TGF-β1 or TGF-β2 was inhibited by exogenous TGF-β1; cells overexpressing TGF-β1 also grew more slowly than controls, but the growth rate of TGF-β2 overexpressing cells was unaltered. However, cells that overexpressed either TGF-β1 or TGF-β2 were markedly more invasive than controls in an organotypic model of SCC. The proliferation of the invading TGF-β1 overexpressing cells in the organotypic assays was higher than controls. Similarly, tumours formed by the TGF-β1 overexpressing cells following transplantation to athymic mice were larger than tumours formed by control cells and proliferated at a higher rate. Our results demonstrate that elevated expression of either TGF-β1 or TGF-β2 in cells that represent the early stages in the development of human SCC results in a more aggressive phenotype.