Transcriptional Regulation of Signal Regulatory Protein α1 Inhibitory Receptors by Epidermal Growth Factor Receptor Signaling

SIRPγ-expressing cancer stem-like cells promote immune escape of lung cancer via Hippo signaling

Cancer stem-like cells (CSLCs) acquire enhanced immune checkpoint responses to evade immune cell killing and promote tumor progression. Here we showed that signal regulatory protein γ (SIRPγ) determined CSLC properties and immune evasiveness in a small population of lung adenocarcinoma (LUAD) cancer cells. A SIRPγ^hi population displayed CSLC properties and transmitted the immune escape signal through sustaining CD47 expression in both SIRPγ^hi and SIRPγ^lo/– tumor cells. SIRPγ bridged MST1 and PP2A to facilitate MST1 dephosphorylation, resulting in Hippo/YAP activation and leading to cytokine release by CSLCs, which stimulated CD47 expression in LUAD cells and consequently inhibited tumor cell phagocytosis. SIRPγ promoted tumor growth and metastasis in vivo through YAP signaling. Notably, SIRPγ targeting with genetic SIRPγ knockdown or a SIRPγ-neutralizing antibody inhibited CSLC phenotypes and elicited phagocytosis that suppressed tumor growth in vivo. SIRPG was upregulated in human LUAD and its overexpression predicted poor survival outcome. Thus, SIRPγ^hi cells serve as CSLCs and tumor immune checkpoint–initiating cells, propagating the immune escape signal to the entire cancer cell population. Our study identifies Hippo/YAP signaling as the first mechanism by which SIRPγ is engaged and reveals that targeting SIRPγ represents an immune- and CSLC-targeting strategy for lung cancer therapy.

SIRPalpha1 receptors interfere with the EGFRvIII signalosome to inhibit glioblastoma cell transformation and migration

EGFRvIII, a frequent genetic alteration of the epidermal growth factor receptor (EGFR), has been shown to increase the migratory potential of tumor cells and normal fibroblasts. Previously, we showed that signal regulatory protein alpha1 (SIRPalpha1) receptors interact with SHP-2 to inhibit wild-type (wt) EGFR-mediated tumor migration, survival and cell transformation. However, the effects of SIRPalpha1 inhibitory receptors on EGFRvIII-mediated phenotypes are unclear. The aim of this study was to investigate the effect of SIRPalpha1 receptor on the EGFRvIII signalosome and phenotypes. Overexpression of SIRPalpha1 in U87MG.EGFRvIII cells inhibited transformation and migration in a MAPK-dependent manner, and is independent of the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway. We observed reduced EGFRvIII/SHP-2/Gab1/Grb2/Sos-1 interaction and enhanced SIRP/SHP-2 association in U87MG.EGFRvIII/SIRPalpha1 cells when compared with empty vector control cells. Interestingly, SIRPalpha1 overexpression differentially modulated SHP-2 phosphorylation at tyrosyl 542 and 580 residues, which may regulate Erk1/2 activity and the EGFRvIII phenotype. In addition, SIRPalpha1-expressing cells exhibited reduced focal adhesion kinase (FAK) phosphorylation and its recruitment to the EGFRvIII/Grb2/Sos-1/Gab1/SHP-2 complex. Collectively, our data indicate that SIRPalpha1 specifically affects the SHP-2/FAK/Grb2/Sos-1/MAPK activation loop to downmodulate EGFRvIII-mediated migration and transformation. Further understanding of the molecular interactions between the SIRPalpha1 inhibitory receptor and the EGFRvIII signalosome may facilitate the identification of novel targets to inhibit the EGFRvIII glioblastoma phenotype.

Altered expression of prolactin receptor-associated signaling proteins in human breast carcinoma

Prolactin receptor signaling can modulate proliferation, survival, motility, angiogenesis, and differentiation in breast cancer. Increased serum prolactin is associated with a significantly increased risk of breast cancer in post-menopausal women. The purpose of this study was to examine the expression of prolactin receptor-associated signaling proteins in breast cancer vs benign breast tissue. Breast tissue microarrays representing 40 cases of benign and malignant pathologies were obtained from the Cooperative Human Tissue Network. Standard immunohistochemistry for prolactin and prolactin receptor-associated proteins was performed. Both positive regulators (c-Myb, Nek3, Vav2) and negative regulators (PIAS3, SIRP) of prolactin receptor signaling were examined. Virtual slides were created from the stained breast tissue microarrays. Labels were scored by region of interest and labeling indices incorporating percent target labeled and label intensity were created. Quantitative determinations of labels were made using the Clarient image system. The unpaired t-test was used to compare labels from benign and malignant tissues. Visual scoring data showed upregulation of Nek3 (P=0.000377), PIAS3 (P=0.000257), and prolactin (P=0.002576) in breast cancer vs normal/hyperplastic epithelium. c-Myb showed a trend toward upregulation, but this did not achieve statistical significance (P=0.107374). SIRP (P=0.002060) was downregulated. Vav2 showed a trend toward downregulation (P=0.107456), but this did not achieve statistical significance. Clarient analysis corroborated upregulation in cancer of Nek3 (P=0.000013), PIAS3 (P=0.000067), and prolactin (P=0.017569). In conclusion, regulators of prolactin receptor signaling show heterogeneity in their expression in benign vs malignant breast tissue. Since these species are known to regulate prolactin-mediated actions, these results suggest multiple targets for modulating prolactin receptor-mediated growth and differentiation in breast cancer.

Functional elements on SIRPalpha IgV domain mediate cell surface binding to CD47

SIRPalpha and SIRPbeta1, the two major isoforms of the signal regulatory protein (SIRP) family, are co-expressed in human leukocytes but mediate distinct extracellular binding interactions and divergent cell signaling responses. Previous studies have demonstrated that binding of SIRPalpha with CD47, another important cell surface molecule, through the extracellular IgV domain regulates important leukocyte functions including macrophage recognition, leukocyte adhesion and transmigration. Although SIRPbeta1 shares highly homologous extracellular IgV structure with SIRPalpha, it does not bind to CD47. Here, we defined key amino acid residues exclusively expressing in the IgV domain of SIRPalpha, but not SIRPbeta1, which determine the extracellular binding interaction of SIRPalpha to CD47. These key residues include Gln67, a small hydrophobic amino acid (Ala or Val) at the 57th position and Met102. We found that Gln67 and Ala/Val57 are critical. Mutation of either of these residues abates SIRPalpha directly binding to CD47. Functional cell adhesion and leukocyte transmigration assays further demonstrated central roles of Gln67 and Ala/Val57 in SIRPalpha extracellular binding mediated cell interactions and cell migration. Another SIRPalpha-specific residue, Met102, appears to assist SIRPalpha IgV binding through Gln67 and Ala/Val57. An essential role of these amino acid residues in SIRPalpha binding to CD47 was further confirmed by introducing these residues into the SIRPbeta1 IgV domain, which dramatically converts SIRPbeta1 into a CD47-binding molecule. Our results thus revealed the molecular basis by which SIRPalpha binds to CD47 and shed new light into the structural mechanisms of SIRP isoform mediated distinctive extracellular interactions and cellular responses.

SIRPbeta1 is expressed as a disulfide-linked homodimer in leukocytes and positively regulates neutrophil transepithelial migration

Signal regulatory proteins (SIRPs) comprise a family of cell surface signaling receptors differentially expressed in leukocytes and the central nervous system. Although the extracellular domains of SIRPs are highly similar, classical motifs in the cytoplasmic or transmembrane domains distinguish them as either activating (beta) or inhibitory (alpha) isoforms. We reported previously that human neutrophils (polymorphonuclear leukocytes (PMN)) express multiple SIRP isoforms and that SIRPalpha binding to its ligand CD47 regulates PMN transmigration. Here we further characterized the expression of PMN SIRPs, and we reported that the major SIRPalpha and SIRPbeta isoforms expressed in PMN include Bit/PTPNS-1 and SIRPbeta1, respectively. Furthermore, although SIRPalpha (Bit/PTPNS-1) is expressed as a monomer, we showed that SIRPbeta1 is expressed on the cell surface as a disulfide-linked homodimer with bond formation mediated by Cys-320 in the membrane-proximal Ig loop. Subcellular fractionation studies revealed a major pool of SIRPbeta1 within the plasma membrane fractions of PMN. In contrast, the majority of SIRPalpha (Bit/PTPNS-1) is present in fractions enriched in secondary granules and is translocated to the cell surface after chemoattractant (formylmethionylleucylphenylalanine) stimulation. Functional studies revealed that antibody-mediated ligation of SIRPbeta1 enhanced formylmethionylleucylphenylalanine-driven PMN transepithelial migration. Co-immunoprecipitation experiments to identify associated adaptor proteins revealed a 10-12-kDa protein associated with SIRPbeta1 that was tyrosine-phosphorylated after PMN stimulation and is not DAP10/12 or Fc receptor gamma chain. These results provide new insights into the structure and function of SIRPs in leukocytes and their potential role(s) in fine-tuning responses to inflammatory stimuli.