Grb7, a Critical Mediator of EGFR/ErbB Signaling, in Cancer Development and as a Potential Therapeutic Target.

The partner of activated epidermal growth factor receptor (EGFR), growth factor receptor bound protein-7 (Grb7), a functionally multidomain adaptor protein, has been demonstrated to be a pivotal regulator for varied physiological and pathological processes by interacting with phospho-tyrosine-related signaling molecules to affect the transmission through a number of signaling pathways. In particular, critical roles of Grb7 in erythroblastic leukemia viral oncogene homolog (ERBB) family-mediated cancer development and malignancy have been intensively evaluated. The overexpression of Grb7 or the coamplification/cooverexpression of Grb7 and members of the ERBB family play essential roles in advanced human cancers and are associated with decreased survival and recurrence of cancers, emphasizing Grb7's value as a prognostic marker and a therapeutic target. Peptide inhibitors of Grb7 are being tested in preclinical trials for their possible therapeutic effects. Here, we review the molecular, functional, and clinical aspects of Grb7 in ERBB family-mediated cancer development and malignancy with the aim to reveal alternative and effective therapeutic strategies.

GRB7 is required for triple-negative breast cancer cell invasion and survival.

Triple-negative breast cancer (TNBC) is a heterogeneous disease that is usually associated with poor prognosis, and frequently associated with the basal-like breast cancer gene expression profile. There are no targeted therapeutic modalities for this disease, and no useful biomarkers. High GRB7 RNA expression levels are associated with an elevated risk of recurrence in patients with operable TNBC treated with standard adjuvant anthracycline and taxane therapy. To determine whether GRB7 is involved in the pathobiology of TNBC, we evaluated the biological effects of GRB7 inhibition in a panel of triple-negative cell lines-MDA-MB-468, MDA-MB-231, HCC70, and T4-2. We found GRB7 inhibition reduced cell motility and invasion of these cell lines and promoted cell death by apoptosis in 3D culture. These data suggest that GRB7 itself, or GRB7-dependent pathways, may prove to be important therapeutic targets in this disease.

Differential functions of growth factor receptor-bound protein 7 (GRB7) and its variant GRB7v in ovarian carcinogenesis.

PURPOSE: Aberrant overexpression of growth factor receptor-bound protein 7 (GRB7) and its variant GRB7v has been found in numerous human cancers. The goal of this study was to characterize the functions of GRB7 and GRB7v in the ovarian carcinogenesis and to investigate the differential roles of GRB7 and GRB7v in the modulation of signaling pathways. EXPERIMENTAL DESIGN: Quantitative reverse transcription-PCR, Western blot, and immunohistochemical analyses were used to evaluate the levels of GRB7 and GRB7v. The cellular localization, functions, and signaling pathways regulated by GRB7 and GRB7v were investigated by enforced expression of GRB7 and GRB7v. RESULTS: Quantitative reverse transcription-PCR and Western blot analyses showed that GRB7 and GRB7v were frequently upregulated in ovarian cancer samples. The overexpressed GRB7 (P = 0.009) and GRB7v (P = 0.017) were significantly correlated with high-grade ovarian cancer. Immunohistochemical analysis on ovarian cancer tissue array confirmed that the upregulated GRB7 was significantly correlated with high-grade ovarian cancer (P = 0.001). Confocal microscopy analysis showed that GRB7 and GRB7v predominately localized in cytoplasm of ovarian cancer cells, consistent with their roles as signaling adaptors. Enforced expression of GRB7 promoted cell proliferation, migration, and invasion, whereas GRB7v only increased cell proliferation and anchorage-independent growth ability. With the treatment of specific kinase inhibitors, we showed that both GRB7 and GRB7v promoted cell proliferation through activating extracellular signal-regulated kinase signaling, whereas GRB7 enhanced cell migration/invasion by activating c-Jun NH(2) terminal kinase signaling. CONCLUSIONS: Our studies implicate that the overexpressed GRB7 and GRB7v are associated with high-grade tumors and exert distinct tumorigenic functions through regulating different signaling pathways in ovarian cancer cells.

17q12-21 - the pursuit of targeted therapy in breast cancer.

PURPOSE: Identification of HER2/neu, and the subsequent development of targeted therapy for patients who over-express it, has revolutionized their management. Research has since focused on the area of chromosome 17 in which HER2/neu is located in order to identify other genes in the vicinity. The aims of this review are, firstly, to discuss current thinking in relation to the role of these genes in the pathogenesis of breast cancer and, secondly, to examine how this evidence may be assimilated such that new forms of targeted therapy can be developed. EXPERIMENTAL DESIGN: This review discusses the evidence in relation to 4 genes located at the HER2/neu amplicon, namely TOP2A, GRB7, STARD3 and RARA. RESULTS: TOP2A has aroused particular interest as over-expression of its protein has been shown to correlate, both with amplification of HER2/neu, and with response to anthracycline-based chemotherapeutic agents in breast cancer. GRB7 is included on Oncotype DXtm, and has recently been implicated in gastric and oesophageal cancer. STARD3 and RARA also hold clinical relevance, the former having been shown to function in steroidogenesis and therefore implicated in hormone-receptor-positive breast cancer. Finally, RARA may be the key to unlocking the problem of resistance to all-trans retinoic acid (ATRA) in breast cancer sufferers; this treatment has previously been demonstrated to induce remission in over 80% of patients with acute promyelocytic leukaemia (APML). CONCLUSION: These genes hold potential as therapeutic targets, and warrant further investigation as we move towards our goal of individually tailored therapeutic strategies in breast cancer.

GRB-7 facilitates HER-2/Neu-mediated signal transduction and tumor formation.

Growth factor receptor-bound protein-7 (GRB-7), an adaptor molecule, can interact with multiple signal transduction molecules. GRB-7 is amplified concurrently with HER-2/Neu in most, if not all, of breast cancer with chromosome 17q11-21 amplification. GRB-7 gene amplification is associated with RNA over-expression. We show GRB-7 protein is over-expressed by immunoblotting in breast cancer cell lines and primary breast tumors with HER-2/Neu protein over-expression. Over-expression of GRB-7 in MCF-7 breast cancer cells that over-express HER-2/Neu leads to activation of tyrosine phosphorylation of HER-2/Neu. Knockdown of GRB-7 expression in SKBR-3 breast cancer cells with naturally occurring HER-2/Neu gene amplification decreases tyrosine phosphorylation of HER-2/Neu. Activation of HER-2/Neu phosphorylation is associated with increase in tyrosine phosphorylation of phosphoinositide-specific lipase C-gamma-1 (PLC-gamma-1) and recruitment of PLC-gamma-1 to HER-2/Neu protein molecule. Activation of downstream protein kinase C (PKC) pathway is evidenced by increase in the phosphorylation of a common PKC substrate-myristoylated alanine-rich protein kinase C substrate (MARCKS). In addition, over-expression of GRB-7 in MCF-7 breast cancer cells that over-express HER-2/Neu leads to activation of AKT phosphorylation. Knockdown of GRB-7 expression in MB-453 and SKBR-3 breast cancer cells results in decrease in AKT phosphorylation. GRB-7 over-expression therefore facilitates activation of phosphorylation of HER-2/Neu and AKT in breast cancer cells with HER-2/Neu over-expression. GRB-7 over-expression in MCF-7 cells over-expressing HER-2/Neu leads to morphologic change of cells and promotes tumor xenograft growth in nude mice. GRB-7 over-expression therefore plays pivotal roles in activating signal transduction and promoting tumor growth in breast cancer cells with chromosome 17q11-21 amplification.

Grb7-based molecular therapeutics in cancer.

Traditional anti-cancer drugs preferentially kill rapidly growing tumour cells rather than normal cells. However, most of these drugs have no preferential selection towards cancer cells and are taken up by the whole body, resulting in significant adverse side effects. Therapeutic molecules that could specifically inhibit undesirable phenotypes are an attractive way of eliminating cancer cells. There is a widespread effort to develop inhibitors against signal transduction molecules that play a key role in the proliferative, migratory and invasive properties of a cancer cell. Grb7 is an adaptor-type signalling protein that is recruited via its Src-homology 2 (SH2) domain to a variety of tyrosine kinases. Grb7 is overexpressed in breast, oesophageal and gastric cancers, and may contribute to the invasive potential of cancer cells. Molecular interactions involving Grb7 therefore provide attractive targets for therapeutic intervention.