Cell signaling by receptor-tyrosine kinases

When epidermal growth factor and its relatives bind the ErbB family of receptors, they trigger a rich network of signalling pathways, culminating in responses ranging from cell division to death, motility to adhesion. The network is often dysregulated in cancer and lends credence to the mantra that molecular understanding yields clinical benefit: over 25,000 women with breast cancer have now been treated with trastuzumab (Herceptin), a recombinant antibody designed to block the receptor ErbB2. Likewise, small-molecule enzyme inhibitors and monoclonal antibodies to ErbB1 are in advanced phases of clinical testing. What can this pathway teach us about translating basic science into clinical use?

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Untangling the ErbB signalling network

The classical pathway of interferon-gamma-dependent activation of macrophages by T helper 1 (T(H)1)-type responses is a well-established feature of cellular immunity to infection with intracellular pathogens, such as Mycobacterium tuberculosis and HIV. The concept of an alternative pathway of macrophage activation by the T(H)2-type cytokines interleukin-4 (IL-4) and IL-13 has gained credence in the past decade, to account for a distinctive macrophage phenotype that is consistent with a different role in humoral immunity and repair. In this review, I assess the evidence in favour of alternative macrophage activation in the light of macrophage heterogeneity, and define its limits and relevance to a range of immune and inflammatory conditions.

Alternative activation of macrophages

Through the study of transcriptional activation in response to interferon alpha (IFN-alpha) and interferon gamma (IFN-gamma), a previously unrecognized direct signal transduction pathway to the nucleus has been uncovered: IFN-receptor interaction at the cell surface leads to the activation of kinases of the Jak family that then phosphorylate substrate proteins called STATs (signal transducers and activators of transcription). The phosphorylated STAT proteins move to the nucleus, bind specific DNA elements, and direct transcription. Recognition of the molecules involved in the IFN-alpha and IFN-gamma pathway has led to discoveries that a number of STAT family members exist and that other polypeptide ligands also use the Jak-STAT molecules in signal transduction.

https://science.sciencemag.org/content/sci/264/5164/1415.full.pdf

Jak-STAT pathways and transcriptional activation in response to IFNs and other extracellular signaling proteins

The concept that the immune system can recognize and destroy nascent transformed cells was originally embodied in the cancer immunosurveillance hypothesis of Burnet and Thomas. This hypothesis was abandoned shortly afterwards because of the absence of strong experimental evidence supporting the concept. New data, however, clearly show the existence of cancer immunosurveillance and also indicate that it may function as a component of a more general process of cancer immunoediting. This process is responsible for both eliminating tumors and sculpting the immunogenic phenotypes of tumors that eventually form in immunocompetent hosts. In this review, we will summarize the historical and experimental basis of cancer immunoediting and discuss its dual roles in promoting host protection against cancer and facilitating tumor escape from immune destruction.

Cancer immunoediting: from immunosurveillance to tumor escape.

IgG antibodies are secreted from B cells and bind to a variety of pathogens to control infections as well as contribute to inflammatory diseases. Many of the functions of IgGs are mediated through Fcγ receptors (FcγRs), which transduce interactions with immune complexes, leading to a variety of cellular outcomes depending on the FcγRs and cell types engaged. Which FcγRs and cell types will be engaged during an immune response depends on the structure of Fc domains within immune complexes that are formed when IgGs bind to cognate antigen(s). Recent studies have revealed an unexpected degree of structural variability in IgG Fc domains among people, driven primarily by differences in IgG subclasses and N-linked glycosylation of the CH2 domain. This translates, in turn, to functional immune diversification through type I and type II FcγR-mediated cellular functions. For example, Fc domain sialylation triggers conformational changes of IgG1 that enable interactions with type II FcγRs; these receptors mediate cellular functions including antiinflammatory activity or definition of thresholds for B cell selection based on B cell receptor affinity. Similarly, presence or absence of a core fucose alters type I FcγR binding of IgG1 by modulating the Fc's affinity for FcγRIIIa, thereby altering its proinflammatory activity. How heterogeneity in IgG Fc domains contributes to human immune diversity is now being elucidated, including impacts on vaccine responses and susceptibility to disease and its sequelae during infections. Here, we discuss how Fc structures arising from sialylation and fucosylation impact immunity, focusing on responses to vaccination and infection. We also review work defining individual differences in Fc glycosylation, regulation of Fc glycosylation, and clinical implications of these pathways.

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Functional diversification of IgGs through Fc glycosylation

The human low affinity receptors for the Fc domain of immunoglobulin G, Fc gamma RIII, are encoded by two genes (IIIA and IIIB) which share >95% sequence identity in both coding and flanking sequences. Despite this extraordinary sequence conservation, IIIA is expressed in natural killer (NK) cells and macrophages and is absent in neutrophils, whereas IIIB is expressed only in neutrophils. To determine the molecular basis for this differential expression, we have generated transgenic mice using the genomic sequences of IIIA and IIIB. IIIA and IIIB transgenic mice show faithful reconstitution of this human pattern of cell type specificity. To determine the cis acting sequence elements that confer this specificity, we constructed chimeric genes in which 5.8 kb of 5' sequences of the IIIB gene has been replaced with a homologous region from the IIIA gene, and conversely, IIIA 5' sequences have been substituted for the analogous region of the IIIB gene. Promoter swap transgenic mice that carry IIIA 5' flanking sequences express Fc gamma RIII in macrophages and NK cells. In contrast, promoter swap transgenic mice that contain IIIB 5' sequences express Fc gamma RIII in neutrophils only. These studies define the elements conferring the cell type-specific expression of the human Fc gamma RIII genes within the 5' flanking sequences and first intron of the human Fc gamma RIIIA and Fc gamma RIIIB genes.

/pdf/reconstitution-of-human-fc-gamma-riii-cell-type-specificity-4lzd7egwu6.pdf

Reconstitution of human Fc gamma RIII cell type specificity in transgenic mice.

Intravenous immune globulin prevents venular vaso-occlusion in sickle cell mice by inhibiting leukocyte adhesion and the interactions between sickle erythrocytes and adherent leukocytes.

An interferon-inducible cytokine, IP-10, containing homology to a family of proteins having chemotactic (platelet factor 4, beta-thromboglobulin) and mitogenic (connective tissue-activating peptide III) activities has been mapped to chromosome 4 at band q21, a locus associated with an acute monocytic/B-lymphocyte lineage leukemia that exhibits the nonrandom translocation t(4;11)(q21;q23). In situ hybridization of t(4;11)(q21;q23)-carrying leukemic cells revealed that the IP-10 gene is proximal to the breakpoint of this translocation. No DNA rearrangement was evident when the IP-10 gene was hybridized to genomic DNA isolated from two patients' leukemic cells that contain t(4;11)(q21;q23). However, restriction fragment length polymorphism in the 5' region of the IP-10 gene was detected. The ETS1 protooncogene is located at 11q23 and is known to translocate to chromosome 4 in t(4;11) (q21;q23) and into the interferon gene cluster in (9;11) (p22;q23). Both translocations are associated with acute monocytic leukemia. These results suggest a model in which juxtaposition of genetic loci regulated by antiproliferative signals, such as interferon, next to an oncogene, like ETS1, could effectively short circuit homeostatic control circuits and contribute to the neoplastic state.

Interferon-inducible gene maps to a chromosomal band associated with a (4;11) translocation in acute leukemia cells.

Src homology-2 domain-containing inositol polyphosphate 5'-phosphatase (SHIP) is a recently identified protein that has been implicated as an important signaling molecule. Although SHIP has been shown to participate in the FcgammaRIIB-mediated inhibitory signal, the functional role of SHIP in activation responses by immunoreceptor tyrosine-based activation motif-bearing receptors such as B cell receptor (BCR) remains unclear. Indeed, it has been proposed that SHIP serves as a linking molecule for the regulation of the extracellular signal-regulated kinase pathway in BCR signaling, because SHIP associates with Shc. We now report that SHIP-deficient DT40 B cells display enhanced Ca2+ mobilization in response to BCR ligation, whereas extracellular signal-regulated kinase activation is unaffected. This Ca2+ enhancement is due to a sustained intracellular Ca2+ increase or to long-lasting Ca2+ oscillations by loss of SHIP, as revealed by single-cell Ca2+ imaging analysis. These results demonstrate the importance of SHIP in B cell activation by the modulation of Ca2+ mobilization.

Jeffrey V. Ravetch

Papers

Functional diversification of IgGs through Fc glycosylation

Reconstitution of human Fc gamma RIII cell type specificity in transgenic mice.

Intravenous immune globulin prevents venular vaso-occlusion in sickle cell mice by inhibiting leukocyte adhesion and the interactions between sickle erythrocytes and adherent leukocytes.

Interferon-inducible gene maps to a chromosomal band associated with a (4;11) translocation in acute leukemia cells.

Cutting Edge: Role of the Inositol Phosphatase SHIP in B Cell Receptor-Induced Ca2+ Oscillatory Response