Signaling via Shc family adapter proteins

Biological aspects of signal transduction by cell adhesion receptors

Cell adhesion receptors such as integrins, cadherins, selectins, and immunoglobulin family receptors profoundly modulate many signal transduction cascades. In this review we examine aspects of adhesion receptor signaling and how this impinges on key biological processes. We have chosen to focus on cell migration and on programmed cell death. We examine many of the cytoplasmic signaling molecules that interface with adhesion receptors, including focal adhesion kinase (FAK), phosphatidylinositol-3-kinase (PI3K), and elements of the Erk/MAP kinase pathway. In many cases these molecules impinge on both the regulation of cell movement and on control of apoptosis.

The "Gab" in signal transduction

Tyrosine phosphorylation plays an important role in controlling cellular growth, differentiation and function. Abnormal regulation of tyrosine phosphorylation can result in human diseases such as cancer. A major challenge of signal transduction research is to determine how the initial activation of protein-tyrosine kinases (PTKs) by extracellular stimuli triggers multiple downstream signaling cascades, which ultimately elicit diverse cellular responses. Recent studies reveal that members of the Gab/Dos subfamily of scaffolding adaptor proteins (hereafter, "Gab proteins") play a crucial role in transmitting key signals that control cell growth, differentiation and function from multiple receptors. Here, we review the structure, mechanism of action and function of these interesting molecules in normal biology and disease.

Positional mapping for amplified DNA sequences on 1q21-q22 in hepatocellular carcinoma indicates candidate genes over-expression

BACKGROUND/AIMS

Comparative genomic hybridization analysis on hepatocellular carcinoma (HCC) indicated frequent gains of 1q and an amplicon at 1q21-q22. Current cytogenetic evidences confer much importance on 1q21-q22, where a role in drug resistance, tumor metastasis and shorter patient survival had been implicated.

METHODS

Using positional mapping by interphase cytogenetics, we investigated the amplicon 1q21-q22 in five HCC cases. Three amplification maxima represented by yeast artificial chromosomes (YACs) 955E11, 876B11 and 945D5 that mapped to regions 1q21.1, 1q21.2 and 1q22, respectively, were indicated. We further investigated candidate genes expression in the mapped YACs by quantitative reverse-transcription-polymerase chain reaction. A panel of genes encoding protein transcripts involved in apoptosis, cell cycle progression, calcium binding and jumping translocation was studied.

RESULTS

Among ten HCC cases with the amplicon 1q21-q22 examined, we found a significant gene expression level of JTB, SHC1, CCT3 and COPA in the tumors than the paired adjacent non-malignant liver tissues (P< or =0.04).

CONCLUSIONS

Our interphase findings on 1q21-q22 pinpointed three affected loci between D1S305 and D1S2369. Up-regulation of candidate genes identified within these over-represented regions may represent targets in the progression of HCC and may carry prognostic significance.

Role of Shc in T-cell development and function

Shc is a prototype adapter protein that is expressed from the earliest stages of T-cell development. Shc becomes rapidly tyrosine phosphorylated after T-cell receptor (TCR) engagement. Expression of dominant negative forms of Shc in T-cell lines had also suggested a role for this adapter downstream of the TCR. However, until recently, the relative significance of Shc compared to several other adapters in T cells was unclear. Mice lacking Shc expression specifically in the T-cell lineage together with inducible expression of dominant negative Shc in transgenic mice have revealed an essential and nonredundant role for Shc in thymic T-cell development. Functional defects in a Jurkat T-cell line lacking Shc expression also suggest a role for Shc in mature T-cell functions. While the requirement of Shc in T-cell signaling is now established, precisely what signaling pathways downstream of Shc make this adapter unique are less clear. Although the Shc-mediated activation of the extracellular signal regulated kinase (Erk)/mitogen-activated protein kinase (MAPK) pathway could be one component, Shc likely signals to other pathways in T cells that are not yet discovered. A better molecular understanding of Shc function in the future could provide insights into how multiple adapters coordinate the various outcomes downstream of the TCR.

Kinetic perspectives of T cell antigen receptor signaling. A two-tier model for T cell full activation

T-cell activation consists of multiple layers of signaling events. Interleukin-2 production is of interest for many, since its expression determines a critical difference between partial and full T-cell activation. To achieve full activation of T cells, it is necessary for the T-cell antigen receptor (TCR) to be engaged for an extended period of time. However, why extended stimulation is required for full T-cell activation is not understood at the molecular level. In this review, orchestrated events of TCR signal transduction will be analyzed in a kinetic manner and connected toward the understanding of the mechanism of T-cell activation. Based on recent results, a model of the mechanism that dictates the threshold between partial and full T-cell activation is proposed.

Differential regulation of Shc adaptor proteins in skeletal muscle, spinal cord and forebrain of aged rats with sensorimotor impairment

The Shc family of proteins participates in mitogenic and survival signalling through binding to receptor tyrosine kinases. We report here on the expression of Shc in forebrain, spinal cord and hind limb muscles from 30-month-old rats with different degrees of sensorimotor impairment. ShcA (mRNA and protein) is up-regulated in skeletal muscles and spinal cord of aged rats, and this change relates to biological age, i.e. degree of behavioural incapacitation, rather than to chronological age. Western blot and RT-PCR revealed that the increase in ShcA selectively affected the p46 isoform in the spinal cord, whereas in muscle tissue a robust increase of p66(ShcA) was also evident. Furthermore, in parallel with the up-regulation of ShcA, an increase of p75(NTR) mRNA in the aged animals was observed. ShcB mRNA showed a tendency for down-regulation in both spinal cord and skeletal muscles, whereas the expression of ShcC was unaltered. Our data show that the regulation of Shc mRNAs in senescence is region as well as isoform specific. The regulatory changes may reflect changes in mitogenic/survival signalling induced by age-related cell and tissue damage. The coup-regulation of p66(ShcA) and p75(NTR) is interesting since both molecules have been associated with apoptosis.

Grit, a GTPase-activating protein for the Rho family, regulates neurite extension through association with the TrkA receptor and N-Shc and CrkL/Crk adapter molecules

Neurotrophins are key regulators of the fate and shape of neuronal cells and act as guidance cues for growth cones by remodeling the actin cytoskeleton. Actin dynamics is controlled by Rho GTPases. We identified a novel Rho GTPase-activating protein (Grit) for Rho/Rac/Cdc42 small GTPases. Grit was abundant in neuronal cells and directly interacted with TrkA, a high-affinity receptor for nerve growth factor (NGF). Another pool of Grit was recruited to the activated receptor tyrosine kinase through its binding to N-Shc and CrkL/Crk, adapter molecules downstream of activated receptor tyrosine kinases. Overexpression of the TrkA-binding region of Grit inhibited NGF-induced neurite elongation. Further, we found some tendency for neurite promotion in full-length Grit-overexpressing PC12 cells upon NGF stimulation. These results suggest that Grit, a novel TrkA-interacting protein, regulates neurite outgrowth by modulating the Rho family of small GTPases.

Bovine leukemia virus gp30 transmembrane (TM) protein is not tyrosine phosphorylated: examining potential interactions with host tyrosine-mediated signaling

Bovine leukemia virus (BLV) causes persistent lymphocytosis, a preneoplastic, polyclonal expansion of B lymphocytes. The expansion increases viral transmission to new hosts, but the mechanisms of this expansion have not been determined. We hypothesized that BLV infection contributes to B-cell expansion by signaling initiated via viral transmembrane protein motifs undergoing tyrosine phosphorylation. Viral mimicry of host cell proteins is a well-demonstrated mechanism by which viruses may increase propagation or decrease recognition by the host immune system. The cytoplasmic tail of BLV transmembrane protein gp30 (TM) has multiple areas of homology to motifs of host cell signaling proteins, including two immunoreceptor tyrosine-based activation motifs (ITAMs) and two immunoreceptor tyrosine-based inhibition motifs (ITIMs), which are homologous to B-cell receptor and inhibitory co-receptor motifs. Signaling by these motifs in B cells typically relies on tyrosine phosphorylation, followed by interactions with Src-homology-2 (SH2) domains of nonreceptor protein tyrosine kinases or phosphatases. Phosphorylation of tyrosine residues in the cytoplasmic tail of TM was tested in four systems including ex vivo cultured peripheral blood mononuclear cells from BLV infected cows, BLV-expressing fetal lamb kidney cell and bat lung cell lines, and DT40 B cells transfected with a fusion of mouse extracellular CD8alpha and cytoplasmic TM. No phosphorylation of TM was detected in our experiments in any of the cell types utilized, or with various stimulation methods. Detection was attempted by immunoblotting for phosphotyrosines, or by metabolic labeling of cells. Thus BLV TM is not likely to modify host signal pathways through interactions between phosphorylated tyrosines of the ITAM or ITIM motifs and host-cell tyrosine kinases or phosphatases.

A nonredundant role for the adapter protein Shc in thymic T cell development

Signaling via the pre-T cell receptor (pre-TCR) regulates survival, proliferation, allelic exclusion and differentiation of thymocytes. The role played by the adapter protein Shc in T cells has remained controversial, and its role in pre-TCR signaling has not been addressed. We examined Shc function in thymic T cell development using two genetic approaches. Cre-loxP-mediated inducible expression in transgenic mice of a phosphorylation-defective mutant of Shc impaired signaling through the pre-TCR as well as subsequent proliferation and differentiation. Conditional deletion of the Shc locus in thymocytes also affected thymic maturation at the same pre-TCR developmental stage. Thus, both Shc expression and its tyrosine phosphorylation play an essential and nonredundant role in thymic T cell development.

Signal transduction pathways: the molecular basis for targeted therapies

The elucidation of the signal transduction pathways that regulate cell growth and differentiation has led to a number of exciting opportunities for novel cancer therapies. It is now well known that growth factors and cell matrix molecules activate cognate growth factor receptors and integrins, respectively, to initiate a complex signaling cascade that ultimately targets the nucleus, cell surface, and mitochondria. Signaling to these target molecules results in the regulation of gene transcription, cell adhesion and motility, and cell survival, all of which are integral parts of cellular growth control mechanisms. As a result of increased understanding of cell growth regulation mechanisms, a number of novel therapeutic agents have been developed and tested in preclinical models and, to some extent, in clinical trials. Based on our current understanding of growth regulation in normal and cancer cells, one would predict that these new agents could influence proliferation and survival of cancer cells, as well as their response to traditional cytotoxic therapies. In this overview, the mechanistic basis for the use of signal transduction-targeted novel therapeutics is presented, along with predictions regarding how they may interact with ionizing radiation in different subgroups of patients.

Comparative expression profiles of Trk receptors and Shc-related phosphotyrosine adapters during retinal development: potential roles of N-Shc/ShcC in brain-derived neurotrophic factor signal transduction and modulation

Neurotrophins (NTs) have multiple roles in retinal development and survival, which are mediated through their specific receptors and signaling molecules. An emerging family of adapter protein, Shc (Src homology and collagen)-related molecules, i.e., Shc/ShcA, Sck/ShcB, and N-Shc/ShcC, has been implicated in various phosphotyrosine signal transduction mechanisms, including that for NTs. To explore the potential role(s) of Shc-related adapters in NT signaling in the retina, we compared the developmental changes of the mRNA expression of TrkA -B, and -C in the rat retina, on one hand and, on the other hand, studied which members of the Shc family were activated after brain-derived neurotrophic factor (BDNF) application in axotomized rat retinas. Early in development, both TrkA and ShcA were highly expressed, whereas, in late development to adulthood, TrkB/C and ShcB/C were highly expressed. In the mature retinal ganglion cell layer, the expression of ShcB/C and TrkB/C was evident. Immunoreactivity of ShcC was located in the retinal ganglion cells, amacrine cells, and inner plexiform layer. The response of ShcC following retinal axotomy was most profound with the administration of BDNF, and there was some response with neurotrophin-3. These results indicate that ShcC could be a potential phosphotyrosine adapter among the Shc family members for BDNF signaling and function during retinal development and regeneration in vivo.

Herstatin, an autoinhibitor of the human epidermal growth factor receptor 2 tyrosine kinase, modulates epidermal growth factor signaling pathways resulting in growth arrest

Herstatin is an autoinhibitor of the ErbB family consisting of subdomains I and II of the human epidermal growth factor receptor 2 (ErbB-2) extracellular domain and a novel C-terminal domain encoded by an intron. Herstatin binds to human epidermal growth factor receptor 2 and to the epidermal growth factor receptor (EGFR), blocking receptor oligomerization and tyrosine phosphorylation. In this study, we characterized several early steps in EGFR activation and investigated downstream signaling events induced by epidermal growth factor (EGF) and by transforming growth factor alpha (TGF-alpha) in NIH3T3 cell lines expressing EGFR with and without herstatin. Herstatin expression decreased EGF-induced EGFR tyrosine phosphorylation and delayed receptor down-regulation despite receptor occupancy by ligand with normal binding affinity. Akt stimulation by EGF and TGF-alpha, but not by fibroblast growth factor 2, was almost completely blocked in the presence of herstatin. Surprisingly, EGF and TGF-alpha induced full activation of MAPK in duration and intensity and stimulated association of the EGFR with Shc and Grb2. Although MAPK was fully stimulated, herstatin expression prevented TGF-alpha-induced DNA synthesis and EGF-induced proliferation. The herstatin-mediated uncoupling of MAPK from Akt activation was also observed in Chinese hamster ovary cells co-transfected with EGFR and herstatin. These findings show that herstatin expression alters EGF and TGF-alpha signaling profiles, culminating in inhibition of proliferation.

Phosphotyrosine-binding domains in signal transduction

Protein phosphorylation provides molecular control of complex physiological events within cells. In many cases, phosphorylation on specific amino acids directly controls the assembly of multi-protein complexes by recruiting phospho-specific binding modules. Here, the function, structure, and cell biology of phosphotyrosine-binding domains is discussed.