title: Kinetic analysis of high affinity forms of interleukin (IL)-13 receptors: suppression of IL-13 binding by IL-2 receptor gamma chain.
abstract: Interleukin-13 (IL-13) is a pleiotropic cytokine that controls growth, differentiation, and apoptosis of immune and tumor cells. To understand the mechanisms of interaction between IL-13 and IL-13 receptors (IL-13R), and the role of the IL-2 receptor common gamma chain (gammac) in IL-13 binding and processing, we have examined IL-13 binding kinetics, dissociation/shedding, and internalization in renal cell carcinoma (RCC) cell lines. We observed a new phenomena in that the apparent rate of association, but not the dissociation, was strongly related to IL-13 concentration. We also observed cooperativity phenomena in IL-13 and IL-13R interaction in control RCC (MLneo) cells, but not in cells transfected with gammac chain (MLgammac). The number of IL-13 binding sites, the effective rate of ligand association, and the dissociation rate constants were reduced in gammac-transfected cells compared to control RCC cells. Two forms of IL-13R were detected in these cell lines, which differed in the kinetics of endocytosis and dissociation/exocytosis. Only a small fraction of bound receptors (14-24%) was rapidly internalized and the same fraction of the ligand-receptor complexes was shed and/or dissociated. The expression of gammac chain did not change any of these processes. A two independent high-affinity and moderate-affinity receptor model fit the kinetic observations in gammac-transfected cells. However, in control cells, the binding kinetics were more complicated. A mathematical model that fit a set of kinetic and steady state data in control cells was selected from a set of possible models. This best-fit model predicts that 1) two different IL-13R are expressed on the cell membrane, 2) a minor fraction of IL-13R exist as microclusters (homodimers and/or heterodimers) without exogenous IL-13, 3) high morphological complexity of the gammac-negative control cell membrane affects the cooperativity phenomena of IL-13 binding, and 4) a large number of co-receptor molecules is present, which helps keep the ligand on the cell surface for a long period of time after fast IL-13 binding and provides a negative control for ligand binding via production of the high affinity inhibitor bound to IL-13. Our data demonstrate that gammac exerts dramatic changes in the kinetic mechanisms of IL-13 binding.
authors.affiliation: Laboratory of Mathematical ImmunoBiophysics, Institute of Biochemical Physics of the Russian Academy of Sciences, Kosygin Str. 4/8, Moscow 117977, Russia.
authors.forename: V A
authors.initials: VA
authors.lastname: Kuznetsov
authors.forename: R K
authors.initials: RK
authors.lastname: Puri
mesh_terms: D001667:Binding, Competitive; D002292:Carcinoma, Renal Cell; D002455:Cell Division / Q000235:genetics; D019281:Dimerization; D004705:Endocytosis; D006801:Humans; D018793:Interleukin-13 / Q000037:antagonists & inhibitors*; D053661:Interleukin-13 Receptor alpha1 Subunit; D007457:Iodine Radioisotopes; D007680:Kidney Neoplasms; D007700:Kinetics; D008962:Models, Theoretical; D011485:Protein Binding; D018123:Receptors, Interleukin / Q000737:chemistry*; D053660:Receptors, Interleukin-13; D015375:Receptors, Interleukin-2 / Q000378:metabolism*; D014162:Transfection; D014407:Tumor Cells, Cultured
chemical_list: C508181:IL13RA1 protein, human; D018793:Interleukin-13; D053661:Interleukin-13 Receptor alpha1 Subunit; D007457:Iodine Radioisotopes; D018123:Receptors, Interleukin; D053660:Receptors, Interleukin-13; D015375:Receptors, Interleukin-2