A Raf-independent Epidermal Growth Factor Receptor Autocrine Loop Is Necessary for Ras Transformation of Rat Intestinal Epithelial Cells

We recently have shown that activated Ras, but not Raf, causes transformation of intestinal (RIE-1, IEC-6) epithelial cells, whereas both activated Ras and Raf transform NIH 3T3 fibroblasts (Oldham, S. M., Clark, G. J., Gangarosa, L. M., Coffey, R. J., and Der, C. J. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 6924-6928). The observations that conditioned medium from Ras-, but not Raf-, transfected RIE-1 cells, as well as exogenous transforming growth factor alpha (TGFalpha), promoted morphological transformation of parental RIE-1 cells prompted us to identify epidermal growth factor (EGF) receptor (EGFR) ligands produced by Ras-transformed RIE-1 cells responsible for this autocrine effect. Since studies in fibroblasts have shown that v-Src is transforming, we also determined if v-Src could transform RIE-1 cells. H- or K-Ras-transformed cells secreted significant amounts of TGFalpha protein, and mRNA transcripts for TGFalpha, amphiregulin (AR), and heparin-binding EGF-like growth factor (HB-EGF) were induced. Like Ras, v-Src caused morphological and growth transformation of parental RIE-1 cells. However, TGFalpha protein was not secreted by RIE-1 cells stably expressing v-Src or activated Raf, and only minor increases in EGFR ligand mRNA expression were detected in these cells. A selective EGFR tyrosine kinase inhibitor PD153035 attenuated the Ras-, but not Src-, transformed phenotype. Taken together, these observations provide a mechanistic and biochemical basis for the ability of activated Ras, but not activated Raf, to cause transformation of RIE-1 cells. Finally, we suggest that an EGFR-dependent mechanism is necessary for Ras, but not Src, transformation of these intestinal epithelial cells

Excess PLAC8 promotes an unconventional ERK2-dependent EMT in colon cancer

The epithelial-to-mesenchymal transition (EMT) transcriptional program is characterized by repression of E-cadherin (CDH1) and induction of N-cadherin (CDH2), and mesenchymal genes like vimentin (VIM). Placenta-specific 8 (PLAC8) has been implicated in colon cancer; however, how PLAC8 contributes to disease is unknown, and endogenous PLAC8 protein has not been studied. We analyzed zebrafish and human tissues and found that endogenous PLAC8 localizes to the apical domain of differentiated intestinal epithelium. Colon cancer cells with elevated PLAC8 levels exhibited EMT features, including increased expression of VIM and zinc finger E-box binding homeobox 1 (ZEB1), aberrant cell motility, and increased invasiveness. In contrast to classical EMT, PLAC8 overexpression reduced cell surface CDH1 and upregulated P-cadherin (CDH3) without affecting CDH2 expression. PLAC8-induced EMT was linked to increased phosphorylated ERK2 (p-ERK2), and ERK2 knockdown restored cell surface CDH1 and suppressed CDH3, VIM, and ZEB1 upregulation. In vitro, PLAC8 directly bound and inactivated the ERK2 phosphatase DUSP6, thereby increasing p-ERK2. In a murine xenograft model, knockdown of endogenous PLAC8 in colon cancer cells resulted in smaller tumors, reduced local invasion, and decreased p-ERK2. Using MultiOmyx, a multiplex immunofluorescence-based methodology, we observed coexpression of cytosolic PLAC8, CDH3, and VIM at the leading edge of a human colorectal tumor, supporting a role for PLAC8 in cancer invasion in vivo

Cell surface integrin α5ß1 clustering negatively regulates receptor tyrosine kinase signaling in colorectal cancer cells via glycogen synthase kinase 3

Abstract As a key process within the tissue microenvironment, integrin signaling can influence cell functional responses to growth factor stimuli. We show here that clustering of integrin α5ß1 at the plasma membrane of colorectal cancer-derived epithelial cells modulates their ability to respond to stimulation by receptor tyrosine kinase (RTK)-activating growth factors EGF, NRG and HGF, through GSK3-mediated suppression of Akt pathway. We observed that integrin α5ß1 is lost from the membrane of poorly organized human colorectal tumors and that treatment with the integrin-clustering antibody P4G11 is sufficient to induce polarity in a mouse tumor xenograft model. While adding RTK growth factors (EGF, NRG and HGF) to polarized colorectal cancer cells induced invasion and loss of monolayer formation in 2D and 3D, this pathological behavior could be blocked by P4G11. Phosphorylation of ErbB family members as well as MET following EGF, NRG and HGF treatment was diminished in cells pretreated with P4G11. Focusing on EGFR, we found that blockade of integrin α5ß1 increased EGFR phosphorylation. Since activity of multiple downstream kinase pathways were altered by these various treatments, we employed computational machine learning techniques to ascertain the most important effects. Partial least-squares discriminant analysis identified GSK3 as a major regulator of EGFR pathway activities influenced by integrin α5ß1. Moreover, we used partial correlation analysis to examine signaling pathway crosstalk downstream of EGF stimulation and found that integrin α5ß1 acts as a negative regulator of the AKT signaling cascade downstream of EGFR, with GSK3 acting as a key mediator. We experimentally validated these computational inferences by confirming that blockade of GSK3 activity is sufficient to induce loss of polarity and increase of oncogenic signaling in the colonic epithelial cells.</jats:p

TACE/ADAM-17: a component of the epidermal growth factor receptor axis and a promising therapeutic target in colorectal cancer

Activation of the epidermal growth factor receptor (EGFR) requires cell surface cleavage of EGFR ligands, uptake of soluble ligand by the receptor, and initiation of EGFR tyrosine kinase activity. We define these collective events as the EGFR axis. Transforming growth factor-alpha (TGF-alpha) and amphiregulin are two EGFR ligands that are delivered preferentially to the basolateral surface of polarized epithelial cells where the EGFR resides. TACE/ADAM-17 (tumor necrosis factor-alpha converting enzyme/a disintegrin and metalloprotease) has been implicated in ectodomain cleavage of TGF-alpha and amphiregulin. Using a human polarizing colorectal cancer (CRC) cell line, HCA-7, and a tissue array of normal colonic mucosa and primary and metastatic CRC, we determined the intracellular localization of TACE and the effects of EGFR axis inhibition in CRC. Herein, we show that TACE is localized to the basolateral plasma membrane of polarized HCA-7 cells. TACE is overexpressed in primary and metastatic CRC tumors compared with normal colonic mucosa; the intensity of its immunoreactivity is inversely correlated with that of TGF-alpha and amphiregulin. Pharmacologic blockade of HCA-7 cells with an EGFR monoclonal antibody, a selective EGFR tyrosine kinase inhibitor, and a selective TACE inhibitor results in concentration-dependent decreases in cell proliferation and active, phosphorylated mitogen-activated protein kinase. Combining suboptimal concentrations of these agents results in cooperative growth inhibition, increased apoptosis, and reduced mitogen-activated protein kinase pathway activation. Furthermore, an EGFR tyrosine kinase-resistant clone of HCA-7 cells is growth-inhibited by combined monoclonal antibody and TACE inhibition. These results implicate TACE as a promising target of EGFR axis inhibition in CRC