Fusobacterium nucleatum, a Gram‐negative oral anaerobe, is a significant contributor to colorectal cancer. Using an in vitro cancer progression model, we discover that F. nucleatum stimulates the growth of colorectal cancer cells without affecting the pre‐cancerous adenoma cells. Annexin A1, a previously unrecognized modulator of Wnt/β‐catenin signaling, is a key component through which F. nucleatum exerts its stimulatory effect. Annexin A1 is specifically expressed in proliferating colorectal cancer cells and involved in activation of Cyclin D1. Its expression level in colon cancer is a predictor of poor prognosis independent of cancer stage, grade, age, and sex. The FadA adhesin from F. nucleatum up‐regulates Annexin A1 expression through E‐cadherin. A positive feedback loop between FadA and Annexin A1 is identified in the cancerous cells, absent in the non‐cancerous cells. We therefore propose a “two‐hit” model in colorectal carcinogenesis, with somatic mutation(s) serving as the first hit, and F. nucleatum as the second hit exacerbating cancer progression after benign cells become cancerous. This model extends the “adenoma‐carcinoma” model and identifies microbes such as F. nucleatum as cancer “facilitators”.

A variety of G-protein-coupled receptor (GPCR) screening technologies have successfully partnered a number of GPCRs with their cognate ligands. GPCR-mediated β-arrestin recruitment is now recognized as a distinct intracellular signaling pathway, and ligand-receptor interactions may show a bias toward β-arrestin over classical GPCR signaling pathways. We hypothesized that the failure to identify native ligands for the remaining orphan GPCRs may be a consequence of biased β-arrestin signaling. To investigate this, we assembled 10 500 candidate ligands and screened 82 GPCRs using PathHunter β-arrestin recruitment technology. High-quality screening assays were validated by the inclusion of liganded receptors and the detection and confirmation of these established ligand-receptor pairings. We describe a candidate endogenous orphan GPCR ligand and a number of novel surrogate ligands. However, for the majority of orphan receptors studied, measurement of β-arrestin recruitment did not lead to the identification of cognate ligands from our screening sets. β-Arrestin recruitment represents a robust GPCR screening technology, and ligand-biased signaling is emerging as a therapeutically exploitable feature of GPCR biology. The identification of cognate ligands for the orphan GPCRs and the extent to which receptors may exist to preferentially signal through β-arrestin in response to their native ligand remain to be determined.

The orphan receptor tyrosine kinase ErbB2 is activated by each of the EGFR family members upon ligand binding. However, difficulties monitoring the dynamic interactions of the membrane receptors have hindered the elucidation of the mechanism of ErbB2 activation. We have engineered a system to monitor proteinprotein interactions in intact mammalian cells such that different sets of protein interactions can be quantitatively compared. Application of this system to the interactions of the EGFR family showed that ErbB2 interacts stably with the EGFR and ErbB3, but fails to spontaneously homooligomerize. The widely used anticancer antibody Herceptin was found to effectively inhibit the interaction of the EGFR and ErbB2 but not to interfere with the interaction of ErbB2-ErbB3. Treatment of cells expressing EGFR and ErbB2 with Herceptin results in increased EGFR homooligomerization in the presence of EGF and a subsequent rapid internalization and down-regulation of the EGFR. In summary, the protein interaction system described here enabled the characterization of ErbB2 interactions within the biological context of the plasma membrane and provides insight into the mechanism of Herceptin action on cells overexpressing ErbB2.anti-cancer ͉ EGF receptor T he EGF family of receptor tyrosine kinases consists of four members, EGFR, ErbB2, ErbB3, and ErbB4, that become activated in response to ligand-induced dimerization. ErbB2 (HER2͞Neu) does not itself bind any known ligand, and activation of this receptor is believed to be mediated through heterodimerization with any of the other EGF family members. Physical characterization of this process has proven difficult using conventional biochemical methods, but it is of considerable interest because of the role of ErbB2 in breast cancer pathogenesis.ErbB2 is overexpressed in 30% of breast cancers and most clearly associated with a malignant phenotype and poor prognosis, especially if coexpressed with the EGF receptor (EGFR) (1-3). For a subset of breast cancer patients whose tumors overexpress ErbB2, the monoclonal antibody Herceptin has revolutionized treatment by extending lifespan and decreasing recurrence rate in an unprecedented manner (4-6). Although there is evidence that Herceptin targets tumor cells for destruction by the immune system (7), the antibody was originally selected as an inhibitor of tumor cell growth in vitro independent of an immune response (8). Herceptin is not known to block the formation of heterodimers of ErbB2, yet its inhibitory effects on cell proliferation suggest that it interferes with signal transduction by the ErbB family of tyrosine kinases. One reason that the mechanism of action of Herceptin has remained elusive is the difficulty in monitoring the interactions of the ErbB receptors in a quantitative manner using available biochemical methods, including purified or coimmunoprecipitated receptors (9-11).We postulated that the ␤-gal system we recently developed for assays of protein translocation (12) could enable a comparative analysis of the co...

The recruitment of arrestins to activated 7TMRs results in the activation of alternative signaling pathways, quenching of G-protein activation, and coupling to clathrin-mediated endocytosis. The nearly ubiquitous involvement of arrestin in 7TMR signaling has spurred the development of several methods for monitoring this interaction in mammalian cells. Nonetheless, few maintain the reproducibility and precision necessary for drug discovery applications. Enzyme fragment complementation technology (EFC) is an emerging protein-protein interaction technology based on the forced complementation of a split enzyme that has proven to be highly effective in monitoring the formation of GPCR-arrestin complexes. In these systems, the target proteins are fused to two fragments of an enzyme that show little or no spontaneous complementation. Interaction of the two proteins forces the complementation of the enzyme, resulting in an enzymatic measure of the protein interaction. This chapter discusses the utility and methods involved in using the PathHunter β-galactosidase complementation system to monitor arrestin recruitment and the advantages of exploiting this pathway in the characterization of 7TMR function.