Inhibition of autocrine HGF maturation overcomes cetuximab resistance in colorectal cancer.

Although amplifications and mutations in receptor tyrosine kinases (RTKs) act as bona fide oncogenes, in most cancers, RTKs maintain moderate expression and remain wild-type. Consequently, cognate ligands control many facets of tumorigenesis, including resistance to anti-RTK therapies. Herein, we show that the ligands for the RTKs MET and RON, HGF and HGFL, respectively, are synthesized as inactive precursors that are activated by cellular proteases. Our newly generated HGF/HGFL protease inhibitors could overcome both de novo and acquired cetuximab resistance in colorectal cancer (CRC). Conversely, HGF overexpression was necessary and sufficient to induce cetuximab resistance and loss of polarity. Moreover, HGF-induced cetuximab resistance could be overcome by the downstream MET inhibitor, crizotinib, and upstream protease inhibitors. Additionally, HAI-1, an endogenous inhibitor of HGF proteases, (i) was downregulated in CRC, (ii) exhibited increased genomic methylation that correlated with poor prognosis, (iii) HAI-1 expression correlated with cetuximab response in a panel of cancer cell lines, and (iv) exogenous addition of recombinant HAI-1 overcame cetuximab resistance in CC-HGF cells. Thus, we describe a targetable, autocrine HAI-1/Protease/HGF/MET axis in cetuximab resistance in CRC.

Interaction of lncRNA MIR100HG with hnRNPA2B1 facilitates m6A-dependent stabilization of TCF7L2 mRNA and colorectal cancer progression.

BACKGROUND: Epithelial-to-mesenchymal transition (EMT) is a process linked to metastasis and drug resistance with non-coding RNAs (ncRNAs) playing pivotal roles. We previously showed that miR-100 and miR-125b, embedded within the third intron of the ncRNA host gene MIR100HG, confer resistance to cetuximab, an anti-epidermal growth factor receptor (EGFR) monoclonal antibody, in colorectal cancer (CRC). However, whether the MIR100HG transcript itself has a role in cetuximab resistance or EMT is unknown. METHODS: The correlation between MIR100HG and EMT was analyzed by curating public CRC data repositories. The biological roles of MIR100HG in EMT, metastasis and cetuximab resistance in CRC were determined both in vitro and in vivo. The expression patterns of MIR100HG, hnRNPA2B1 and TCF7L2 in CRC specimens from patients who progressed on cetuximab and patients with metastatic disease were analyzed by RNAscope and immunohistochemical staining. RESULTS: The expression of MIR100HG was strongly correlated with EMT markers and acted as a positive regulator of EMT. MIR100HG sustained cetuximab resistance and facilitated invasion and metastasis in CRC cells both in vitro and in vivo. hnRNPA2B1 was identified as a binding partner of MIR100HG. Mechanistically, MIR100HG maintained mRNA stability of TCF7L2, a major transcriptional coactivator of the Wnt/ß-catenin signaling, by interacting with hnRNPA2B1. hnRNPA2B1 recognized the N6-methyladenosine (m6A) site of TCF7L2 mRNA in the presence of MIR100HG. TCF7L2, in turn, activated MIR100HG transcription, forming a feed forward regulatory loop. The MIR100HG/hnRNPA2B1/TCF7L2 axis was augmented in specimens from CRC patients who either developed local or distant metastasis or had disease progression that was associated with cetuximab resistance. CONCLUSIONS: MIR100HG and hnRNPA2B1 interact to control the transcriptional activity of Wnt signaling in CRC via regulation of TCF7L2 mRNA stability. Our findings identified MIR100HG as a potent EMT inducer in CRC that may contribute to cetuximab resistance and metastasis by activation of a MIR100HG/hnRNPA2B1/TCF7L2 feedback loop.

Supermeres are functional extracellular nanoparticles replete with disease biomarkers and therapeutic targets.

Extracellular vesicles and exomere nanoparticles are under intense investigation as sources of clinically relevant cargo. Here we report the discovery of a distinct extracellular nanoparticle, termed supermere. Supermeres are morphologically distinct from exomeres and display a markedly greater uptake in vivo compared with small extracellular vesicles and exomeres. The protein and RNA composition of supermeres differs from small extracellular vesicles and exomeres. Supermeres are highly enriched with cargo involved in multiple cancers (glycolytic enzymes, TGFBI, miR-1246, MET, GPC1 and AGO2), Alzheimer's disease (APP) and cardiovascular disease (ACE2, ACE and PCSK9). The majority of extracellular RNA is associated with supermeres rather than small extracellular vesicles and exomeres. Cancer-derived supermeres increase lactate secretion, transfer cetuximab resistance and decrease hepatic lipids and glycogen in vivo. This study identifies a distinct functional nanoparticle replete with potential circulating biomarkers and therapeutic targets for a host of human diseases.

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

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.

KRAS Mutation-Responsive miR-139-5p inhibits Colorectal Cancer Progression and is repressed by Wnt Signaling.

Introduction: Colorectal cancer (CRC) frequently harbors KRAS mutations that result in chemoresistance and metastasis. MicroRNAs (miRNAs) are usually dysregulated and play important regulatory roles in tumor progression. However, the KRAS mutation-responsive miRNA profile in CRC remains uninvestigated. Methods: miR-139-5p was identified and evaluated by small RNA sequencing, qRT-PCR and in situ hybridization. The roles of miR-139-5p in CRC cells with and without KRAS mutation were determined by Cell Counting Kit-8 (CCK-8), colony formation, flow cytometry and transwell assays in vitro and by tumorigenesis and metastasis assays in vivo. Microarrays followed by bioinformatic analyses, luciferase reporter assays and Western blotting were applied for mechanistic studies. Results: miR-139-5p was significantly downregulated in KRAS-mutated CRC cells and tissues compared with their wild-type counterparts. Low miR-139-5p expression was associated with aggressive phenotypes and poor prognosis in CRC patients. miR-139-5p overexpression inhibited CRC cell proliferation, migration and invasion in vitro, sensitized tumors to chemotherapy, and impaired tumor growth and metastasis in vivo. Transcriptomic profiling identified multiple modulators in the Ras (JUN and FOS) and Wnt (CTNNB1 and DVL1) signaling pathways and the epithelial-to-mesenchymal transition (EMT) process (ZEB1) as direct targets of miR-139-5p, and inverse correlations were confirmed in CRC clinical tissues. Aberrantly activated Wnt signaling in KRAS-mutant cells was demonstrated to transcriptionally repress miR-139-5p through TCF4, forming a miR-139-5p/Wnt signaling double-negative feedback loop. Conclusions: We identified miR-139-5p as a KRAS-responsive miRNA and demonstrated its involvement in CRC progression. KRAS mutation disrupted the miR-139-5p/Wnt signaling reciprocal negative feedback mechanism, which might cause miR-139-5p downregulation and derepression of oncogenic signaling pathways and EMT. These results reveal a transcriptional regulatory mode of KRAS-driven malignant transformation and highlight miR-139-5p as a novel regulator of crosstalk between the Ras and Wnt signaling pathways in CRC.

Combined blockade of EGFR and glutamine metabolism in preclinical models of colorectal cancer.

Improving response to epidermal growth factor receptor (EGFR)-targeted therapies in patients with advanced wild-type (WT) RAS colorectal cancer (CRC) remains an unmet need. In this preclinical work, we evaluated a new therapeutic combination aimed at enhancing efficacy by targeting cancer cell metabolism in concert with EGFR. We hypothesized that combined blockade of glutamine metabolism and EGFR represents a promising treatment approach by targeting both the "fuel" and "signaling" components that these tumors need to survive. To explore this hypothesis, we combined CB-839, an inhibitor of glutaminase 1 (GLS1), the mitochondrial enzyme responsible for catalyzing conversion of glutamine to glutamate, with cetuximab, an EGFR-targeted monoclonal antibody in preclinical models of CRC. 2D and 3D in vitro assays were executed following treatment with either single agent or combination therapy. The combination of cetuximab with CB-839 resulted in reduced cell viability and demonstrated synergism in several cell lines. In vivo efficacy experiments were performed in cell-line xenograft models propagated in athymic nude mice. Tumor volumes were measured followed by immunohistochemical (IHC) analysis of proliferation (Ki67), mechanistic target of rapamycin (mTOR) signaling (pS6), and multiple mechanisms of cell death to annotate molecular determinants of response. In vivo, a significant reduction in tumor growth and reduced Ki67 and pS6 IHC staining were observed with combination therapy, which was accompanied by increased apoptosis and/or necrosis. The combination showed efficacy in cetuximab-sensitive as well as resistant models. In conclusion, this therapeutic combination represents a promising new precision medicine approach for patients with refractory metastatic WT RAS CRC.

Transfer of Functional Cargo in Exomeres.

Exomeres are a recently discovered type of extracellular nanoparticle with no known biological function. Herein, we describe a simple ultracentrifugation-based method for separation of exomeres from exosomes. Exomeres are enriched in Argonaute 1-3 and amyloid precursor protein. We identify distinct functions of exomeres mediated by two of their cargo, the ß-galactoside α2,6-sialyltransferase 1 (ST6Gal-I) that α2,6- sialylates N-glycans, and the EGFR ligand, amphiregulin (AREG). Functional ST6Gal-I in exomeres can be transferred to cells, resulting in hypersialylation of recipient cell-surface proteins including ß1-integrin. AREG-containing exomeres elicit prolonged EGFR and downstream signaling in recipient cells, modulate EGFR trafficking in normal intestinal organoids, and dramatically enhance the growth of colonic tumor organoids. This study provides a simplified method of exomere isolation and demonstrates that exomeres contain and can transfer functional cargo. These findings underscore the heterogeneity of nanoparticles and should accelerate advances in determining the composition and biological functions of exomeres.

Protein kinase A-mediated phosphorylation of naked cuticle homolog 2 stimulates cell-surface delivery of transforming growth factor-α for epidermal growth factor receptor transactivation.

The classic mode of G protein-coupled receptor (GPCR)-mediated transactivation of the receptor tyrosine kinase epidermal growth factor receptor (EGFR) transactivation occurs via matrix metalloprotease (MMP)-mediated cleavage of plasma membrane-anchored EGFR ligands. Herein, we show that the Gαs-activating GPCR ligands vasoactive intestinal peptide (VIP) and prostaglandin E2 (PGE2 ) transactivate EGFR through increased cell-surface delivery of the EGFR ligand transforming growth factor-α (TGFα) in polarizing madin-darby canine kidney (MDCK) and Caco-2 cells. This is achieved by PKA-mediated phosphorylation of naked cuticle homolog 2 (NKD2), previously shown to bind TGFα and direct delivery of TGFα-containing vesicles to the basolateral surface of polarized epithelial cells. VIP and PGE2 rapidly activate protein kinase A (PKA) that then phosphorylates NKD2 at Ser-223, a process that is facilitated by the molecular scaffold A-kinase anchoring protein 12 (AKAP12). This phosphorylation stabilized NKD2, ensuring efficient cell-surface delivery of TGFα and increased EGFR activation. Thus, GPCR-triggered, PKA/AKAP12/NKD2-regulated targeting of TGFα to the cell surface represents a new mode of EGFR transactivation that occurs proximal to ligand cleavage by MMPs.

Broad-spectrum receptor tyrosine kinase inhibitors overcome de novo and acquired modes of resistance to EGFR-targeted therapies in colorectal cancer.

It is increasingly appreciated that 3D cultures are more predictive of in vivo therapeutic efficacy than 2D cultures. Using in vitro 3D type I collagen cultures of human colorectal cancer (CRC) cell line HCA-7 derivatives CC, SC, and CC-CR, we previously identified that activation of receptor tyrosine kinases (RTKs) MET and RON contributed to resistance to the EGF receptor (EGFR)-directed therapeutic antibody cetuximab. The de novo mode of cetuximab resistance in SC cells could be overcome by crizotinib, a multi-RTK inhibitor that also targets MET and RON. We now show that crizotinib also overcomes acquired cetuximab resistance in CC-CR cells. Phospho-RTK array analysis showed increased phosphorylation of several RTKs, including MET and RON, in SC and CC-CR cells compared to cetuximab-sensitive CC counterparts. Furthermore, other multi-RTK inhibitors cabozantinib and BMS-777607 helped overcome cetuximab resistance, as measured by 3D colony growth and activation state of key signaling molecules. Conversely, addition of RTK ligands HGF and NRG1 induced cetuximab resistance in CC cells, which could be blocked by addition of crizotinib. We further determined the mechanism of the cooperative effect of cetuximab and crizotinib by FACS analysis and observed increased cell cycle arrest in G1 phase in cetuximab-resistant CRC 3D cultures. Finally, we show that crizotinib overcomes cetuximab resistance in vivo in SC nude mice xenografts. Thus, our work shows that multi-RTK inhibition strategy is a potent, broadly applicable strategy to overcome resistance to EGFR-targeted therapeutics in CRC and highlights the relevance of 3D cultures in these studies. Statement of implication: Using in vitro 3D CRC cultures and in vivo CRC xenografts, we show that parallel inhibition of multiple RTKs with small molecule inhibitors overcomes de novo and acquired resistance to EGFR-directed therapies in CRC.

lncRNA MIR100HG-derived miR-100 and miR-125b mediate cetuximab resistance via Wnt/ß-catenin signaling.

De novo and acquired resistance, which are largely attributed to genetic alterations, are barriers to effective anti-epidermal-growth-factor-receptor (EGFR) therapy. To generate cetuximab-resistant cells, we exposed cetuximab-sensitive colorectal cancer cells to cetuximab in three-dimensional culture. Using whole-exome sequencing and transcriptional profiling, we found that the long non-coding RNA MIR100HG and two embedded microRNAs, miR-100 and miR-125b, were overexpressed in the absence of known genetic events linked to cetuximab resistance. MIR100HG, miR-100 and miR-125b overexpression was also observed in cetuximab-resistant colorectal cancer and head and neck squamous cell cancer cell lines and in tumors from colorectal cancer patients that progressed on cetuximab. miR-100 and miR-125b coordinately repressed five Wnt/ß-catenin negative regulators, resulting in increased Wnt signaling, and Wnt inhibition in cetuximab-resistant cells restored cetuximab responsiveness. Our results describe a double-negative feedback loop between MIR100HG and the transcription factor GATA6, whereby GATA6 represses MIR100HG, but this repression is relieved by miR-125b targeting of GATA6. These findings identify a clinically actionable, epigenetic cause of cetuximab resistance.

Clustering of integrin α5 at the lateral membrane restores epithelial polarity in invasive colorectal cancer cells.

Apicobasolateral polarity is a fundamental property of epithelial cells, and its loss is a hallmark of cancer. Integrin-mediated contact with the extracellular matrix defines the basal surface, setting in motion E-cadherin-mediated cell-cell contact, which establishes apicobasolateral polarity. Role(s) for lateral integrins in this polarization process and the consequences of their disruption are incompletely understood. We show that addition of an integrin ß1-activating monoclonal antibody, P4G11, to invasive colorectal cancer cells in three-dimensional type 1 collagen reverts the invasive phenotype and restores apicobasolateral polarity. P4G11 induces clustering of integrin α5ß1 at lateral, intercellular surfaces. This leads to deposition and polymerization of fibronectin and recruitment of paxillin to sites of lateral integrin α5ß1 clustering and is followed by tight junction formation, as determined by ZO-1 localization. Inducible elimination of integrin α5 abrogates the epithelial-organizing effects of P4G11. In addition, polymerization of fibronectin is required for the effects of P4G11, and addition of polymerized superfibronectin is sufficient to induce tight junction formation and apicobasolateral polarization. In the normal human colon, we show that integrin α5 localizes to the lateral membrane of terminally differentiated colonocytes and that integrin α5 staining may be reduced in colorectal cancer. Thus we propose a novel role for integrin α5ß1 in regulating epithelial morphogenesis.

Three-dimensional culture system identifies a new mode of cetuximab resistance and disease-relevant genes in colorectal cancer.

We previously reported that single cells from a human colorectal cancer (CRC) cell line (HCA-7) formed either hollow single-layered polarized cysts or solid spiky masses when plated in 3D in type-I collagen. To begin in-depth analyses into whether clonal cysts and spiky masses possessed divergent properties, individual colonies of each morphology were isolated and expanded. The lines thus derived faithfully retained their parental cystic and spiky morphologies and were termed CC (cystic) and SC (spiky), respectively. Although both CC and SC expressed EGF receptor (EGFR), the EGFR-neutralizing monoclonal antibody, cetuximab, strongly inhibited growth of CC, whereas SC was resistant to growth inhibition, and this was coupled to increased tyrosine phosphorylation of MET and RON. Addition of the dual MET/RON tyrosine kinase inhibitor, crizotinib, restored cetuximab sensitivity in SC. To further characterize these two lines, we performed comprehensive genomic and transcriptomic analysis of CC and SC in 3D. One of the most up-regulated genes in CC was the tumor suppressor 15-PGDH/HPGD, and the most up-regulated gene in SC was versican (VCAN) in 3D and xenografts. Analysis of a CRC tissue microarray showed that epithelial, but not stromal, VCAN staining strongly correlated with reduced survival, and combined epithelial VCAN and absent HPGD staining portended a poorer prognosis. Thus, with this 3D system, we have identified a mode of cetuximab resistance and a potential prognostic marker in CRC. As such, this represents a potentially powerful system to identify additional therapeutic strategies and disease-relevant genes in CRC and possibly other solid tumors.

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.

EGF receptor-independent action of TGF-alpha protects Naked2 from AO7-mediated ubiquitylation and proteasomal degradation.

Naked family members (Drosophila Naked Cuticle and mammalian Naked1 and Naked2) have been identified as inducible antagonists of canonical Wnt signaling. We recently reported that Naked2, but not Naked1, interacts with the cytoplasmic tail of TGF-alpha, thereby coating TGF-alpha-containing exocytic vesicles and directing these vesicles to the basolateral corner of polarized epithelial cells. Here, we show that Naked2 is a short-lived protein with a half-life of 60 min caused by its rapid ubiquitin-mediated proteasomal degradation. Overexpression of TGF-alpha stabilizes Naked2 protein in an EGF receptor (EGFR)-independent manner; a physical interaction between the cytoplasmic tail of TGF-alpha and Naked2 is necessary and sufficient for this protection. We have identified a RING finger protein, AO7/RNF25, as a ubiquitin ligase for Naked2, and we have shown that overexpression of TGF-alpha reduces binding of AO7 to Naked2. These results identify an EGFR-independent action of TGF-alpha, in which it protects Naked2 from proteasomal degradation, thus ensuring its delivery to the basolateral surface of polarized epithelial cells.

Use of fluorescence-activated vesicle sorting for isolation of Naked2-associated, basolaterally targeted exocytic vesicles for proteomics analysis.

By interacting with the cytoplasmic tail of a Golgi-processed form of transforming growth factor-alpha (TGFalpha), Naked2 coats TGFalpha-containing exocytic vesicles and directs them to the basolateral corner of polarized epithelial cells where the vesicles dock and fuse in a Naked2 myristoylation-dependent manner. These TGFalpha-containing Naked2-associated vesicles are not directed to the subapical Sec6/8 exocyst complex as has been reported for other basolateral cargo, and thus they appear to represent a distinct set of basolaterally targeted vesicles. To identify constituents of these vesicles, we exploited our finding that myristoylation-deficient Naked2 G2A vesicles are unable to fuse at the plasma membrane. Isolation of a population of myristoylation-deficient, green fluorescent protein-tagged G2A Naked2-associated vesicles was achieved by biochemical enrichment followed by flow cytometric fluorescence-activated vesicle sorting. The protein content of these plasma membrane de-enriched, flow-sorted fluorescent G2A Naked2 vesicles was determined by LC/LC-MS/MS analysis. Three independent isolations were performed, and 389 proteins were found in all three sets of G2A Naked2 vesicles. Rab10 and myosin IIA were identified as core machinery, and Na(+)/K(+)-ATPase alpha1 was identified as an additional cargo within these vesicles. As an initial validation step, we confirmed their presence and that of three additional proteins tested (annexin A1, annexin A2, and IQGAP1) in wild-type Naked2 vesicles. To our knowledge, this is the first large scale protein characterization of a population of basolaterally targeted exocytic vesicles and supports the use of fluorescence-activated vesicle sorting as a useful tool for isolation of cellular organelles for comprehensive proteomics analysis.

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

PURPOSE: 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. EXPERIMENTAL DESIGN: 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. RESULTS: 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. CONCLUSION: These results implicate TACE as a promising target of EGFR axis inhibition in CRC.

Naked2 acts as a cargo recognition and targeting protein to ensure proper delivery and fusion of TGF-alpha containing exocytic vesicles at the lower lateral membrane of polarized MDCK cells.

Transforming growth factor-alpha (TGF-alpha) is the major autocrine EGF receptor ligand in vivo. In polarized epithelial cells, proTGF-alpha is synthesized and then delivered to the basolateral cell surface. We previously reported that Naked2 interacts with basolateral sorting determinants in the cytoplasmic tail of a Golgi-processed form of TGF-alpha and that TGF-alpha is not detected at the basolateral surface of Madin-Darby canine kidney (MDCK) cells expressing myristoylation-deficient (G2A) Naked2. By high-resolution microscopy, we now show that wild-type, but not G2A, Naked2-associated vesicles fuse at the plasma membrane. We further demonstrate that Naked2-associated vesicles are delivered to the lower lateral membrane of polarized MDCK cells independent of mu1B adaptin. We identify a basolateral targeting segment within Naked2; residues 1-173 redirect NHERF-1 from the apical cytoplasm to the basolateral membrane, and internal deletion of residues 37-104 results in apical mislocalization of Naked2 and TGF-alpha. Short hairpin RNA knockdown of Naked2 leads to a dramatic reduction in the 16-kDa cell surface isoform of TGF-alpha and increased cytosolic TGF-alpha immunoreactivity. We propose that Naked2 acts as a cargo recognition and targeting (CaRT) protein to ensure proper delivery, tethering, and fusion of TGF-alpha-containing vesicles to a distinct region at the basolateral surface of polarized epithelial cells.

Oncogenic KRAS provides a uniquely powerful and variable oncogenic contribution among RAS family members in the colonic epithelium.

Activating mutations of the RAS family of small GTPases are among the most common genetic events in human tumorigenesis. Constitutive activation of the three canonical family members, KRAS, NRAS, and HRAS segregate strongly by tissue type. Of these, KRAS mutations predominate in human tumors, including those arising from the colon and lung. We sought to compare the oncogenic contributions of different RAS isoforms in a comparable genetic setting and to explore downstream molecular changes that may explain the apparent differential oncogenic effects of the various RAS family members. We utilized colorectal cancer cell lines characterized by oncogenic KRAS in parallel with isogenically derived lines in which the mutant allele has been disrupted. We additionally attempted to reconstitute the isogenic derivatives with oncogenic forms of other RAS family members and analyze them in parallel. Pairwise analysis of HCT 116 and DLD-1 cell lines as well as their isogenic derivatives reveals distinct K-RAS(G13D) signatures despite the genetic similarities of these cell lines. In DLD-1, for example, oncogenic K-RAS enhances the motility of these cells by downregulation of Rap1 activity, yet is not associated with increased ERK1/2 phosphorylation. In HCT 116, however, ERK1/2 phosphorylation is elevated relative to the isogenic derivative, but Rap1 activity is unchanged. K-RAS is uniquely oncogenic in the colonic epithelium, though the molecular aspects of its oncogenic contribution are not necessarily conserved across cell lines. We therefore conclude that the oncogenic contribution of K-RAS is a function of its multifaceted functionality and is highly context-dependent.

Differential effects of amphiregulin and TGF-alpha on the morphology of MDCK cells.

Although both amphiregulin and TGF-alpha are known to exert their effects through the EGF receptor, we find that concentrations of recombinant human amphiregulin and TGF-alpha that are equipotent in EGF receptor activation and mitogenesis exhibit markedly different effects on MDCK cell morphology. Amphiregulin induces a spindle-like morphology that is associated with a redistribution of E-cadherin from a Triton-insoluble to Triton-soluble pool. TGF-alpha does not affect epithelial morphology nor does it affect the distribution of the Triton-soluble or -insoluble pool of E-cadherin. The effects of amphiregulin on E-cadherin are associated with actin rearrangement. The morphological and biochemical effects of amphiregulin are prevented by EGF receptor blockade but require Src-family kinase activity and MAPK signaling. These results identify an action of amphiregulin that is distinct from TGF-alpha that may contribute to amphiregulin's participation in the pathogenesis of inflammatory disorders like psoriasis and cancer.

Myristoylated Naked2 escorts transforming growth factor alpha to the basolateral plasma membrane of polarized epithelial cells.

The epidermal growth factor receptor ligands transforming growth factor alpha (TGF alpha) and amphiregulin are delivered to the basolateral surface of polarized epithelial cells where they are cleaved by TACE/ADAM17. Basolateral sorting information resides in their cytoplasmic tail domains, but tail-interacting proteins required for basolateral trafficking have not been identified. Naked (NKD)1 and NKD2 are mammalian homologs of Drosophila Naked Cuticle, which negatively regulates canonical Wnt signaling by binding Dishevelled. We present evidence that NKD2, but not NKD1, binds to basolateral sorting motifs in the cytoplasmic tail of TGF alpha. Processing and cell-surface delivery of TGF alpha are accelerated in NKD2-overexpressing Madin-Darby canine kidney cells. NKD2 is myristoylated on glycine, the second residue. On expression of myristoylation-defective (G2A) NKD2, neither NKD2 nor TGF alpha appears at the basolateral plasma membrane of polarized Madin-Darby canine kidney cells; however, membrane staining for TGF alpha is restored on silencing expression of this mutant NKD2. Amphiregulin does not interact with NKD2 and retains its basolateral localization in G2A-NKD2-expressing cells, as do Na(+), K(+) ATPase alpha 1 and E-cadherin. These data identify an unexpected function for NKD2, i.e., myristoylation-dependent escort of TGF alpha to the basolateral plasma membrane of polarized epithelial cells.