Stromal fibroblasts impaired in TGF-beta signaling prime the microenvironment with extracellular hydrogen peroxide and mitogenic factors to support the oncogenic transformation of adjacent epithelia
Tan, Ming Jie
Date of Issue2014
School of Biological Sciences
Epithelial cancers arise when the homeostatic interaction between epithelial cells and stromal fibroblasts is perturbed, and the fibroblasts can support the development of a tumor microenvironment by providing the necessary oncogenic signals. Transforming growth factor p (TGFp), which signals through the canonical Smad cascade and several non-canonical TGFP activated kinase-1 (TAK1)-dependent cascades, is an important cytokine that regulates tumorstroma interactions. However, the effects of TAK1 and Smad signaling in stromal fibroblasts on the oncogenic potential of adjacent epithelia remain unclear. We observed that TGFP Type II receptor (TGFPRII), Smad3 and TAK1 are frequently downregulated in human squamous cell carcinomas (SCC). Organotypic cocultures (OTCs) constructed using normal keratinocytes and fibroblasts- that are deficient in TGFPRII, Smad3 or TAK1 (FrGFpRih Fsmad3, FrAKI) revealed that the attenuation of TGFP signaling in fibroblasts led to increased proliferation of the overlying epithelia. Interestingly, we observed that intracellular reactive oxygen species (ROSs) were increased in FrGFPRih Fsmad3 and FrAKI fibroblasts concomitant with increased extracellular hydrogen peroxide (H20 2). The prolonged exposure of keratinocytes to H202 was sufficient to induce colony formation in soft agar, and this was in part due to the oxidative modification of phosphatase and tensin homolog (PTEN) and proto-oncogene tyrosine-protein kinase (Src) and the subsequent augmentation of phosphatidylinositol-3-kinase (PBK)/protein kinase Ba (PKBa) signaling. Furthermore, conditioned media from the various OTCs contained elevated levels of mitogenic factors that were differentially regulated by Smad3 and TAKl. Collectively, these mitogenic factors primed the gene expression and signaling activity of the epithelia towards tumorigenesis. Our study revealed that the tumor-suppressive function of stromal TGFP signaling includes modulating the microenvironmental redox state, and this work may aid in the identification of novel therapeutic targets to reverse or delay cancer progression.