Calmodulin regulates the translocation of Grb7 into the nucleus

Calmodulin influences MAPK signaling by binding KSR1

The mitogen-activated protein kinase (MAPK) cascade is a fundamental signaling pathway that regulates cell fate decisions in response to external stimuli. Several scaffold proteins bind directly to kinase components of this pathway and regulate their activation by growth factors. One of the best studied MAPK scaffolds is kinase suppressor of Ras1 (KSR1), which is induced by epidermal growth factor (EGF) to translocate to the plasma membrane where it activates extracellular signal-regulated kinase (ERK). While Ca²⁺ has been shown to modulate MAPK signaling, the molecular mechanisms by which this occurs are incompletely understood. Here we tested the hypothesis that Ca²⁺ alters MAPK activity at least in part via KSR1. Using several approaches, including fusion proteins, immunoprecipitation, confocal microscopy, and a cell-permeable chemical inhibitor, we investigated the functional interaction between KSR1 and calmodulin. In vitro analysis with pure proteins reveals that calmodulin binds directly to KSR1. Moreover, endogenous calmodulin and KSR1 co-immunoprecipitate from mammalian cell lysates. Importantly, Ca²⁺ is required for the association between calmodulin and KSR1, both in vitro and in cells. The cell-permeable calmodulin antagonist CGS9343B significantly reduced activation of ERK by EGF in mouse embryo fibroblasts that overexpress KSR1, but not in control cells. Moreover, CGS9343B impaired the ability of EGF to induce KSR1 translocation to the plasma membrane and to stimulate formation of KSR1-ERK and KSR1-pERK (phosphorylated ERK) complexes in cells. Collectively, our data identify a previously unrecognized mechanism by which the scaffold protein KSR1 couples Ca²⁺ and calmodulin signaling to the MAPK cascade.

Partners of wild type Grb7 and a mutant lacking its calmodulin-binding domain

Growth factor receptor bound protein 7 (Grb7) is a mammalian adaptor protein participating in signaling pathways implicated in cell migration, metastatic invasion, cell proliferation and tumor-associated angiogenesis. We expressed tagged versions of wild type Grb7 and the mutant Grb7Δ, lacking its calmodulin-binding domain (CaM-BD), in human embryonic kidney (HEK) 293 cells and rat glioma C6 cells to identify novel binding partners using shot-gun proteomics. Among the new identified proteins, we validated the ubiquitin-ligase Nedd4 (neural precursor cell expressed developmentally down-regulated protein 4), the heat-shock protein Hsc70/HSPA8 (heat shock cognate protein 70) and the cell cycle regulatory protein caprin-1 (cytoplasmic activation/proliferation-associated protein 1) in rat glioma C6 cells. Our results suggest a role of Grb7 in pathways where these proteins are implicated. These include protein trafficking and degradation, stress-response, chaperone-mediated autophagy, apoptosis and cell proliferation.

The prognostic significance of GRB7 protein expression and localization in human breast and ovarian cancers

Objective: To study GRB7 protein expression in normal human tissues and breast and ovarian cancers, and determine its clinical significance. Results: GRB7 protein was expressed in multiple tissues, including myoepithelial cells of normal breast and fibroadenoma. Cytoplasmic GRB7 expression was seen predominantly in HER-2 positive and, to a lesser extent, triple negative breast cancer. Membrane localization of GRB7 was present in a subset of breast cancers with high cytoplasmic GRB7 expression. Univariate and multivariate analysis found that cytoplasmic GRB7 expression was associated with a negative progesterone receptor status, while membrane GRB7 expression was associated with a negative axillary nodal status. Membrane associated GRB7 expression was present in a subset of ovarian cancers with high cytoplasmic GRB7 expression. Membrane GRB7 expression displayed a trend towards improved recurrence free survival (RFS). Landmark analysis suggested an RFS advantage for ovarian cancers that had GRB7 membrane expression and survived beyond 27 months; GRB7 membrane expression in two or more cores (out of three) predicted an improved RFS. Membrane expression of GRB7 protein was observed in breast cancer cell lines with high GRB7 protein expression in vitro. Conclusion: GRB7 protein membrane expression may be associated with a better prognosis in breast and ovarian cancers. The favorable prognostic value of GRB7 protein membrane expression and its underlying mechanism is worthy of further investigation. Methods: Immunohistochemistry of normal human tissues, breast tissues of various pathologies, and clinically annotated ovarian cancers was performed to correlate the patterns of GRB7 expression with biomarkers or clinical outcome.

Grb7-derived calmodulin-binding peptides inhibit proliferation, migration and invasiveness of tumor cells while they enhance attachment to the substrate

The growth factor receptor bound protein 7 (Grb7) is a Ca²⁺-dependent calmodulin (CaM)-binding adaptor protein implicated, among other functions, in cell proliferation, migration and tumor-associated angiogenesis. The goal of this study was to determine whether a peptide based on the CaM binding site of Grb7 disrupts cellular processes, relevant for the malignancy of tumor cells, in which this adaptor protein is implicated. We designed synthetic myristoylated and non-myristoylated peptides corresponding to the CaM-binding domain of human Grb7 with the sequence ²⁴³RKLWKRFFCFLRRS²⁵⁶ and a variant peptide with the mutated sequence RKLERFFCFLRRE (W246E-ΔK247-S256E). The two non-myristoylated peptides bind dansyl-CaM with higher efficiency in the presence than in the absence of Ca²⁺ and they enter into the cell, as tested with 5(6)-carboxytetramethylrhodamine (TAMRA)-labeled peptides. The myristoylated and non-myristoylated peptides inhibit the proliferation, migration and invasiveness of A431 tumor cells while they enhance their adhesion to the substrate. The myristoylated peptides have stronger inhibitory effect than the non-myristoylated counterparts, in agreement with their expected higher cell-permeant capacity. The myristoylated and non-myristoylated W246E-ΔK247-S256E mutant peptide has a lesser inhibitory effect on cell proliferation as compared to the wild-type peptide. We also demonstrated that the myristoylated peptides were more efficient than the CaM antagonist N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) inhibiting cell migration and equally efficient inhibiting cell proliferation.

Direct Interaction between Calmodulin and the Grb7 RA-PH Domain

Grb7 is a signalling adapter protein that engages activated receptor tyrosine kinases at cellular membranes to effect downstream pathways of cell migration, proliferation and survival. Grb7’s cellular location was shown to be regulated by the small calcium binding protein calmodulin (CaM). While evidence for a Grb7/CaM interaction is compelling, a direct interaction between CaM and purified Grb7 has not been demonstrated and quantitated. In this study we sought to determine this, and prepared pure full-length Grb7, as well as its RA-PH and SH2 subdomains, and tested for CaM binding using surface plasmon resonance. We report a direct interaction between full-length Grb7 and CaM that occurs in a calcium dependent manner. While no binding was observed to the SH2 domain alone, we observed a high micromolar affinity interaction between the Grb7 RA-PH domain and CaM, suggesting that the Grb7/CaM interaction is mediated through this region of Grb7. Together, our data support the model of a CaM interaction with Grb7 via its RA-PH domain.

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.

Calmodulin as a protein linker and a regulator of adaptor/scaffold proteins

Calmodulin (CaM) is a universal regulator for a huge number of proteins in all eukaryotic cells. Best known is its function as a calcium-dependent modulator of the activity of enzymes, such as protein kinases and phosphatases, as well as other signaling proteins including membrane receptors, channels and structural proteins. However, less well known is the fact that CaM can also function as a Ca²⁺-dependent adaptor protein, either by bridging between different domains of the same protein or by linking two identical or different target proteins together. These activities are possible due to the fact that CaM contains two independently-folded Ca²⁺ binding lobes that are able to interact differentially and to some degree separately with targets proteins. In addition, CaM can interact with and regulates several proteins that function exclusively as adaptors. This review provides an overview over our present knowledge concerning the structural and functional aspects of the role of CaM as an adaptor protein and as a regulator of known adaptor/scaffold proteins.

The adaptors Grb10 and Grb14 are calmodulin-binding proteins

We identified the Grb7 family members, Grb10 and Grb14, as Ca²⁺ -dependent CaM-binding proteins using Ca²⁺ -dependent CaM-affinity chromatography as we previously did with Grb7. The potential CaM-binding sites were identified and experimentally tested using fluorescent-labeled peptides corresponding to these sites. The apparent affinity constant of these peptides for CaM, and the minimum number of calcium ions bound to CaM that are required for effective binding to these peptides were also determined. We prepared deletion mutants of the three adaptor proteins lacking the identified sites and determined that they lost or strongly diminished their CaM-binding capacity following the sequence Grb7 > > Grb14 > Grb10. More than one CaM-binding site and/or accessory CaM-binding sites appear to exist in Grb10 and Grb14, as compared to a single one present in Grb7.

Nesprin-2 mediated nuclear trafficking and its clinical implications

Nuclear translocation of proteins has a crucial role in the pathogenesis of cancer, Alzheimer disease and viral infections. A complete understanding of nuclear trafficking mechanisms is therefore necessary in order to establish effective intervention strategies. Here we elucidate the role of Nesprin-2 in Ca²⁺/Calmodulin mediated nuclear transport. Nesprin-2 is an actin-binding nuclear envelope (NE) protein with roles in maintaining nuclear structure and location, regulation of transcription and mechanotransduction. Upon depletion of Nesprin-2 using shRNA, HaCaT cells show abnormal localization of the shuttling proteins BRCA1 and NF-κB. We show that their nuclear transport is unlikely due to the canonical RAN mediated nuclear import, but rather to a RAN independent Ca²⁺/Calmodulin driven mechanism involving Nesprin-2. We report novel interactions between the actin-binding domain of Nesprin-2 and Calmodulin and between the NLS containing region of BRCA1 and Calmodulin. Strikingly, displacing Nesprins from the NE resulted in increased steady state Ca²⁺ concentrations in the cytoplasm suggesting a previously unidentified role of Nesprins in Ca²⁺ regulation. On comparing Nesprin-2 and BRCA1 localization in the ovarian cancer cell lines SKOV-3 and Caov-3, Nesprin-2 and BRCA1 were localized to the NE envelope and the nucleus in SKOV-3, respectively, and to the cytoplasm in Caov-3 cells. Fibroblasts obtained from EDMD5 (Emery Dreifuss muscular dystrophy) patients showed loss of Nesprin-2 from the nuclear envelope, corresponding reduced nuclear localization of BRCA1 and enhanced cytoplasmic Ca²⁺. Taken together, the data suggests a novel role of Nesprin-2 in Ca²⁺/Calmodulin mediated nuclear trafficking and provides new insights which can guide future therapies.

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

Nuclear magnetic resonance imaging of tumour growth and neovasculature performance in vivo reveals Grb7 as a novel antiangiogenic target

Development of neovasculature is a necessary requirement for tumour growth and it provides additional opportunities for therapeutic intervention. However, current antiangiogenic therapies have limited efficacy, mostly because of the development of resistance. Hence, characterization of new antiangiogenic molecular targets is of considerable clinical interest. We report that a calmodulin-binding domain (CaM-BD) deletion mutant of the growth factor receptor bound protein 7 (Grb7) (denoted Grb7Δ) impairs tumour growth and associated angiogenesis in vivo. We implanted glioma C6 cells in rat brains transfected with an enhanced yellow fluorescent protein (EYFP) chimera of Grb7∆, its EYFP-Grb7 wild type counterpart, and EYFP alone. We systematically followed intracerebral growth of the tumours, their cellularity and the functional performance of tumour-associated microvasculature using magnetic resonance imaging, including anatomical T1W and T2W images and functional diffusion and perfusion parameters. Tumours grown from implanted C6 cells expressing EYFP-Grb7Δ developed slower, became smaller and presented lower apparent cellularity than those derived from cells expressing EYFP-Grb7 and EYFP. Vascular perfusion measurements within tumours derived from EYFP-Grb7∆-expressing cells showed significantly lower cerebral blood flow (CBF), cerebral blood volume (CBV) and mean transit time (MTT) values. These findings were independently validated by histological and immunohistochemical techniques. Taken together, these findings confirm that the CaM-BD of Grb7 plays an important role in tumour growth and associated angiogenesis in vivo, thus identifying this region of the protein as a novel target for antiangiogenic treatment.