Up-regulation of the α2-macroglobulin signaling receptor on rheumatoid synovial fibroblasts

Abstract

In the present study, we demonstrate that the α2-macroglobulin (α2M) signaling receptor is up-regulated on rheumatoid synovial fibroblasts. In rheumatoid cells, 125I-α2M-methylamine bound to two sites; namely, one of high affinity (K(d) ~52 pM) and the second of lower affinity (K(d) ~9.7 nM). In normal synovial fibroblasts only one site for 125I-α2M- methylamine (K(d) ~5.36 nM) was present. Receptor-associated protein did not inhibit the binding of α2M-methylamine to the high affinity binding sites, but it caused a 70-80% reduction in its binding to low affinity binding sites establishing its identity as the low density lipoprotein receptor-related protein/α2M receptor. Binding of α2M-methylamine to rheumatoid but not normal synovial fibroblasts caused a rapid rise in inositol 1,4,5- trisphosphate synthesis with a peak reached within 10 s of ligand exposure. Concomitantly, rheumatoid but not normal cells showed a rise in intracellular Ca2+. Pretreatment of rheumatoid cells with Receptor-associated protein or pertussis toxin did not affect the α2M-methylamine-induced increase in intracellular Ca2+. These are characteristic properties of ligation by α2M-methylamine of the α2M signaling receptor but not the lipoprotein receptor-related protein/α2M receptor. Binding of α2M-methylamine to rheumatoid synovial fibroblasts significantly increased the synthesis of DNA compared with normal synovial fibroblasts treated similarly.