Promiscuity Of Ligand Binding In The Chemokine Beta Receptor Family
Ligand receptor interactions by chemokine beta cytokines stimulate a variety of cellular and subcellular events including monocyte and T-cell chemotaxis, adhesion, calcium mobilization and histamine release. To investigate the molecular nature of these events we have identified and analyzed the ligand binding characteristics of receptors for MIP-lα, MIP-1β, RANTES, and MCAF. Scatchard analysis of binding studies on THP-1 cells using 1251 labeled ligands revealed specific receptors for MIP- lα, MIP-1β, RANTES, and MCAF with affinities of 500pM, 600pM, 400pM, and 5nM respectively. Competitive inhibition studies showed MIP- la and MIP-lß to inhibit 100% of MIP- la, MIP-1β, and RANTES binding to the THP-1 cell line. MCAF could only partially inhibit MIP- lα and MIP-1β binding, however, MCAF like MIP-lα,β, could compete for 100% RANTES binding on THP-1 cells. Ligand desensitization studies for Ca2+ mobilization and chemotaxis using the above ligands yielded similar results in that MIP-la and MIP-113 desensitized MIP- lα, MIP-1β and RANTES stimulation, and MCAF partially desensitized MIP-lα and MIP-1β stimulation, but completely desensitized RANTES induction. We hypothesize from these results that at least three distinct receptors are expressed on THP-1 cells, the MIP- 1 α,β, receptor that binds MIP- lα and β, the MCAF receptor that binds MCAF, and the RANTES receptor which binds RANTES, MIP-lα, MIP-1β, and MCAF. Therefore, a single chemokine β ligand can bind to different receptors and a given receptor can bind different chemokine β ligands.