Biophys J, July 1999, p. 154-172, Vol. 77, No. 1

Kinetic Analysis of High Affinity Forms of Interleukin (IL)-13 Receptors: Suppression of IL-13 Binding by IL-2 Receptor  Chain

 ^*Laboratory of Molecular Tumor Biology, Division of Cellular and Gene Therapies, Center for Biologics Evaluation and Research, FDA, Bethesda, Maryland 20892 and  ^#Laboratory of Mathematical ImmunoBiophysics, Institute of Biochemical Physics of the Russian Academy of Sciences, Kosygin Str. 4/8, Moscow 117977, Russia

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  chain (_c) 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 (ML_neo) cells, but not in cells transfected with _c chain (ML_c). The number of IL-13 binding sites, the effective rate of ligand association, and the dissociation rate constants were reduced in _c-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 _c chain did not change any of these processes. A two independent high-affinity and moderate-affinity receptor model fit the kinetic observations in _c-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 _c-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 _c exerts dramatic changes in the kinetic mechanisms of IL-13 binding.

Biophys J, July 1999, p. 154-172, Vol. 77, No. 1
© 1999 by the Biophysical Society   0006-3495/99/07/154/19  $2.00

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