Interleukin 1 receptors on rabbit articular chondrocytes: relationship between biological activity and receptor binding kinetics

This study gives an account of the biologic and kinetic binding properties of interleukin 1α (IL 1α), interleukin 1β (IL 1β), and Glu‐4 (an NH 2 ‐terminal mutant of IL 1β) to interleukin 1 (IL 1) receptors in rabbit articular chondrocytes. All three IL 1's demonstrated full agonist properties in their ability to stimulate prostaglandin E 2 (PGE 2 ) synthesis. IL 1α was 23‐fold more biologically active than IL 1β, which was around 110‐fold more active than Glu‐4 based on the concentration of IL 1 required for half‐maximal stimulation of PGE 2 . The binding of all three ligands was concentration‐dependent and saturable at 4°C. Scatchard analysis of receptor binding data showed that the dissociation constant ( K D ) of IL 1α was 46 ± 12 pM, and the receptor density was 3120 sites/cell. The association of IL 1α at 4°C did not attain equilibrium until after 10 h at 100 pM of 125 I‐labeled IL 1α. The dissociation of bound IL 1α was very slow, t 1/2 of 21 h, although only one class of high‐affinity receptors was detected. The K D of IL 1β binding was 72 ± 3 pM with a receptor density of 800 ± 40 sites/cell. Dissociation of bound 125 I‐labeled IL 1β at 4°C appeared to indicate the presence of two receptor subsets, a fast and a slower component with a t 1/2 of 2 min and 5 h, respectively. The receptor binding affinity of Glu‐4 was 324 ± 3 pM, in line with its reduced biologic activity. Both IL la and IL 1β are rapidly internalized in chondrocytes in a time‐ and temperature‐dependent manner.— C hin , J. E.; H oruk , R. Interleukin 1 receptors on rabbit articular chondrocytes: relationship between biological activity and receptor binding kinetics. FASEB J. 4: 1481‐1487; 1990.