Review

Src Family Tyrosine Kinases and Growth Factor Signaling

This article introduces the Src family of protein-tyrosine kinases. Src is the prototypical oncogene and protein tyrosine kinase. The Src family is the subgroup of kinases that share common regulatory mechanisms and possesses overlapping substrate specificity with Src. Src family kinases transduce signals from adhesion receptors, growth factor receptors, and antigen receptors, and integrate many cell behaviors including proliferation, differentiation, and migration. Their deregulation can lead to malignant transformation.

When properly employed, targeted therapies are effective cancer treatments. However, the development of such therapies requires the identification of targetable drivers of cancer development and metastasis. The expression and nuclear localization of the transcriptional coactivators Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are increased in many human cancers, and experimental evidence indicates that aberrant YAP or TAZ activation drives tumor formation and metastasis. Although these findings make YAP and TAZ appealing therapeutic targets, both have important functions in adult tissues, so directly targeting them could cause adverse effects. The identification of pathways active in cancer cells and required for YAP/TAZ activity could provide a way to inhibit YAP and TAZ. Here, we show that SRC proto-oncogene, nonreceptor tyrosine kinase (SRC) is an important driver of YAP/TAZ activity in human breast cancer and melanoma cells. SRC activation increased YAP/TAZ activity and the expression of YAP/TAZ-regulated genes. In contrast, SRC inhibition or knockdown repressed both YAP/TAZ activity and the expression of YAP/TAZ-regulated genes. We also show that SRC increases the activity of YAP and TAZ by repressing large tumor suppressor homolog (LATS), and we identify the GTPase-activating protein GIT ArfGAP 1 (GIT1) as an SRC effector that regulates both YAP and TAZ. Importantly, we demonstrate that SRC-mediated YAP/TAZ activity promotes tumor growth and enhances metastasis and that SRC-dependent tumor progression depends, at least in part, on YAP and TAZ. Our findings suggest that therapies targeting SRC could help manage some YAP/TAZ-dependent cancers.

Bombyx mori cytoplasmic polyhedrosis virus (BmCPV) is a non-enveloped dsRNA virus, which specifically infect the midgut epithelium of B. mori. BmCPV enters permissive cells via clathrin-dependent endocytosis employing β1 integrin mediated internalization. Until now, the cell entry mechanism of BmCPV has not been known clearly. Here, we investigated whether tyrosine-protein kinase Src64B-like is involved in the cell entry of BmCPV. The Src64B-like gene was cloned and expressed in Escherichia coli (E. coli), and the recombinant protein Src64B-like was used to immunize mouse for preparation of anti-Src64B-like polyclonal antibody (pAb). After Src64B-like gene was silenced by RNAi, the infection of BmCPV was reduced by 59.48% ± 2.18% and 92.22% ± 1.12% in vitro and in vivo autonomously. Contrary to it, BmCPV infection could be enhanced by increasing the expression of Src64B-like. In addition, immunofluorescence assay showed that Src64B-like protein did not co-localize with BmCPV in the cultured BmN cells during viral infection. These results indicate that Src64B-like protein participates and plays an important role in the cell entry of BmCPV, but not contacting directly with BmCPV.

The balance between pro-inflammatory and anti-inflammatory macrophage generation, a process known as polarization, is critical for immune homoeostasis. Identifying the molecular mechanisms underlying polarization and the generation of anti-inflammatory macrophages is an area of high current interest. Our previous work has demonstrated that integrin CD11b promotes IL-10 production in macrophages by activating the Sarcoma gene (Src), yet whether and how Src modulates anti-inflammatory macrophages is not known. Here we show that Src inhibitor (Dasatinib)-treated mice were highly susceptible to dextran sulfate sodium (DSS)-induced colitis, with a much more sever inflammation response, accompanying by high TNF-α, and low IL-10 expression. Inhibition of Src enhanced the expression of pro-inflammatory macrophage marker inducible nitric oxide synthase (iNOS), but reduced the expression of anti-inflammatory macrophage marker arginase1 (Arg1) in both intestinal macrophages and bone marrow-derived macrophages (BMDMs). Overexpression of constitutively activated Src promoted IL-4-induced expression of Arg1 through STAT6 phosphorylation, and Jak1 was also involved in this process. Our results reveal that the IL-4-Src-STAT6 pathway plays a major role in polarizing anti-inflammatory macrophage generation and suggest a potential anti-inflammatory role for Src in inflammatory diseases.

Reversible phosphorylation of proteins on the amino acids serine/threonine and/or tyrosine is a very effective way for endothelial cells to respond to environmental challenges. Phosphorylation on serine and threonine residues modifies the conformation of signaling proteins and their associations with specific substrates. Phosphorylation on tyrosine residues promotes the protein-protein interactions in signal transduction pathways. Target proteins are phosphorylated by protein kinases and these phosphate groups are removed by specific phosphatases, maintaining the system homeostasis. Protein kinases and phosphatases are major players in angiogenesis. Angiogenesis involves proliferation, migration, and differentiation of endothelial cells. In addition to protein phosphorylation, accumulating experimental evidence points to the participation of redox-based posttranslational modifications in angiogenesis. S-nitrosylation of cysteine residues and nitration of tyrosine residues are redox-based posttranslational modifications operating in angiogenesis. In this chapter we provide an overview on the participation of nonredox and redox-based posttranslational modifications and associated signaling pathways in angiogenesis.