с-Met receptor can be activated by extracellular alkaline medium.

Receptor tyrosine kinase (RTK) Met or c-Met is a target of hepatocyte growth factor (HGF) and it plays an important role under normal and pathological conditions. Activation of Met signaling pathway is associated with several cellular processes, such as proliferation, survival, motility, angiogenesis, invasion, and metastasis. In this article, we describe the ability of Met to activate upon a mild alkali treatment. To identify potential alkali-regulated proteins, CAKI-1 cells were treated with alkaline media and further tested for protein phosphorylation changes. By anti-phosphotyrosine antibody precipitation and lectin chromatography, we identified Met as a major cytoplasmic membrane protein that responded to pH changes by its phosphorylation. The activation of Met by alkali occurred at pH >8.0 and was dose-dependent. Specificity of the Met response to alkali was confirmed by the treatment with Met kinase inhibitor SU11274 and also by Met receptor knockout using CRISPR/CAS9 genome editing system. Both approaches completely blocked the Met phosphorylation response in CAKI-1 cells. Similar pH-dependent Met activation was observed in the HeLa cell line. Our data suggest existence of ligand-independent mechanism of Met receptor activation.

Autophosphorylation of Orphan Receptor ERBB2 Can Be Induced by Extracellular Treatment with Mildly Alkaline Media.

ErbB2 is an oncogene receptor tyrosine kinase linked to breast cancer. It is a member of the epidermal growth factor receptor (EGFR) minifamily. ErbB2 is currently viewed as an orphan receptor since, by itself, it does not bind EGF-like ligands and can be activated only when overexpressed in malignant cells or complexed with ErbB3, another member of the EGFR minifamily. Here, we report that ErbB2 can be activated by extracellular application of mildly alkaline (pH 8⁻9) media to ErbB2-transfected cells. We also show that the activation of the ErbB2 receptor by alkali is dose-dependent and buffer-independent. The endogenous ErbB2 receptor of A431 cell line can also undergo alkali-dependent autophosphorylation. Thus, we describe a novel ligand-independent mechanism of ErbB2 receptor activation.

Site-Directed Mutagenesis of the Fibronectin Domains in Insulin Receptor-Related Receptor.

The orphan insulin receptor-related receptor (IRR), in contrast to its close homologs, the insulin receptor (IR) and insulin-like growth factor receptor (IGF-IR) can be activated by mildly alkaline extracellular medium. We have previously demonstrated that IRR activation is defined by its extracellular region, involves multiple domains, and shows positive cooperativity with two synergistic sites. By the analyses of point mutants and chimeras of IRR with IR in, we now address the role of the fibronectin type III (FnIII) repeats in the IRR pH-sensing. The first activation site includes the intrinsically disordered subdomain ID (646-716) within the FnIII-2 domain at the C-terminus of IRR alpha subunit together with closely located residues L135, G188, R244, H318, and K319 of L1 and C domains of the second subunit. The second site involves residue T582 of FnIII-1 domain at the top of IRR lambda-shape pyramid together with M406, V407, and D408 from L2 domain within the second subunit. A possible importance of the IRR carbohydrate moiety for its activation was also assessed. IRR is normally less glycosylated than IR and IGF-IR. Swapping both FnIII-2 and FnIII-3 IRR domains with those of IR shifted beta-subunit mass from 68 kDa for IRR to about 100 kDa due to increased glycosylation and abolished the IRR pH response. However, mutations of four asparagine residues, potential glycosylation sites in chimera IRR with swapped FnIII-2/3 domains of IR, decreased the chimera glycosylation and resulted in a partial restoration of IRR pH-sensing activity, suggesting that the extensive glycosylation of FnIII-2/3 provides steric hindrance for the alkali-induced rearrangement of the IRR ectodomain.

Mapping of alkali-sensing sites of the insulin receptor-related receptor. The role of L2 and fibronectin domains.

Insulin receptor-related receptor (IRR) is a member of the insulin receptor (IR) family that works as an extracellular alkali sensor with positive cooperativity. The pH sensing property of IRR is defined by its extracellular region and involves multiple domains. We have previously demonstrated the primary role of L1C domains and identified potentially important amino acid residues within these domains. In this study, we addressed the roles of L2 and FnIII domains. Within the L2 domain, five amino acid residues (M406, V407, D408, P436 and V437) were identified as IRR-specific by performing a species conservation analysis of the IR family. Single-point mutations of these five residues to alanine produced either little or no negative effect on IRR pH-sensing activity. However, the triple mutation of M406, V407 and D408 (MVD) showed a strong negative effect, with a 4 fold decrease in IRR activity as estimated by in vitro autophosphorylation assay of solubilized receptors. The analysis of this mutant in intact cells revealed the absence of positive cooperativity. Unexpectedly, the double mutation of vicinal P436 and V437 (PV) exhibited a significant positive effect in the in vitro assay and partial positive cooperativity in the whole-cell assay. The role of FnIII domains was addressed by analyzing chimeras of IRR and IR. When the IRR FnIII domains were swapped with those of IR in different combinations, the activity was significantly reduced and positive cooperativity eliminated. However, two mutants with the targeted C-terminal part of IRR alpha subunit that lies within FnIII-2 domain and have been shown to be important for insulin binding by IR, appeared to be as active as wild-type IRR. On the basis of available data, we propose that IRR activation involves two separate centers of pH-dependent rearrangements that act synergistically to induce a major conformational change in the IRR molecule, resulting in internal kinase domains rapprochement and autophosphorylation.