Transforming Growth Factor-β Signaling Strength Determines Target Gene Expression Profile in Human Keratinocytes

Abstract

Transforming Growth Factor-(TGF ) maintains keratinocyte homeostasis, and induces epithelial-tomesenchymal transition (EMT) in response to tissue injury. To investigate how these two TGF responses might become uncoupled during malignant transformation, we examined the TGF -regulated gene expression programs and cellular responses in human keratinocytes as a function of TGF type I receptor (T R-I) kinase activity and TGF level. The TGFmediated homeostatic gene response program and cellular growth arrest were extremely sensitive to a reduction in receptor kinase activity, while much stronger inhibition of TGF receptor activity was needed to inhibit the tissue injury response gene expression program and EMT. Both endogenous TGF and high exogenous levels of TGF induced the homeostatic response, while only high levels of TGF induced EMT. These results suggest that a reduction in receptor signaling activity may be sufficient for keratinocytes to escape from TGF 's tumor suppressor function, while higher levels of TGF associated with tumor progression are pro-invasive/metastatic. These findings have important implications for the optimal use of TGF /Smad signaling antagonists as anti-cancer therapeutics.

Transforming growth factor-β signaling: emerging stem cell target in metastatic breast cancer?

“…A key point is to distinguish between alterations in the level of TGFβ ligand and of the cellular components of the signaling separately. We have recently shown that these two variables have very different effects on the transcriptional profile of epithelial cells [67]. Our findings indicated that a quantitative reduction in the level of receptor expression or activity eliminates a set of genes from the TGFβ-regulated transcriptional program that are primarily involved in maintaining homeostasis and development.…”

“…In the same set of MDA-MB-231-derived bone and soft tissue metastases, the expression of VEGF-A, -B, and -C was strongly up-regulated in the bone/bone marrow metastases compared to those in brain or lung. Importantly, VEGF is also a direct TGFβ target gene in epithelial cells [67]. Consistent with these observations, treatment of tumor bearing mice with the 2G7 anti-TGFβ neutralizing antibody significantly reduced circulating VEGF levels [112] (Genentech, US Patent Application 2005/0276802 A1).…”

“…Thus, in this context, TGFβ signaling acquires the properties of an oncogene. This shift appears to be brought about by attenuation of receptor signaling [67] resulting in the clonal expansion of tumor cells that have escaped from TGFβ-mediated growth inhibition on one hand, combined with an increasing constitutive production and release of TGFβ by the tumor cells and activation of latent TGFβ within the microenvironment, which result in a constitutive EMT/mesenchymal phenotype (reviewed in [50, 51]).…”

“…Our working model [67], which is entirely consistent with the animal and in vitro models summarized above, is that two major changes in TGFβ signaling occur during breast cancer development: the first is associated with a global reduction in receptor (or, perhaps, Smad) signaling, which results in loss of homeostatic control and of TGFβ's tumor suppressive activity, while the second is associated with overproduction of bioactive TGFβ, resulting in activation of a pro-invasive, -angiogenic, and -metastatic TGFβ-regulated gene expression program, which induces a highly mesenchymal, motile, invasive and metastatic phenotype.…”

“…Functional validation studies showed that four of these genes ( IL-11, CTGF, CXCR4, MMP-1 ) acted cooperatively to cause osteolytic metastases. Importantly, each of these four genes are TGFβ targets [67]. In addition, some animals developed skeletal metastases following a prolonged period of dormancy.…”