THE ROLE OF CALCITONIN-GENE-RELATED PEPTIDE (CGRP) IN MACROPHAGES - THE PRESENCE OF FUNCTIONAL RECEPTORS AND EFFECTS ON PROLIFERATION AND DIFFERENTIATION INTO OSTEOCLAST-LIKE CELLS

It has been shown that both calcitonin gene-related peptide (CGRP) and amylin bind weakly to calcitonin (CT) receptors in osteoclast-like ce lls formed in vitro and inhibit bone resorption by a cAMP-dependent me chanism. Osteoclasts are thought to be derived from cells of the monoc yte macrophage lineage, in which CGRP, but not CT, induces cAMP produc tion. In this study, we determined the presence of functional receptor s for CGRP in mouse alveolar macrophages and the effects of this pepti de on proliferation and osteoclastic differentiation in mouse alveolar and bone marrow-derived macrophages. Human CT did not stimulate cAMP production in macrophages. Human CGRP stimulated cAMP production in mo use alveolar macrophages and bone marrow-derived macrophages dose-depe ndently. Human amylin, which has 43% homology with human CGRP, also st imulated these macrophages to produce cAMP, but only at a 100-fold hig her concentration. The increment in cAMP production induced by human C GRP and amylin was abolished by the addition of human CGRP(8-37), a se lective antagonist for CGRP receptors. Specific binding of [I-125]huma n CGRP to alveolar macrophages was detected (dissociation constant, 2. 5 x 10(-8) M; binding sites, 1.4 x 10(4)/cell). Amylin, but not CT, di splaced the bound [I-125]human CGRP from alveolar macrophages, but at a 100-fold higher concentration. No specific binding of [I-125]human C T and [I-125]human amylin to alveolar macrophages could be detected. P retreatment with human CGRP for 24 h dose-dependently suppressed DNA s ynthesis in alveolar macrophages induced by granulocyte-macrophage col ony-stimulating factor (GM-CSF). CGRP also suppressed the number of ma crophage colonies formed from bone marrow cells induced by macrophage colony-stimulating factor (M-CSF). Pre-treatment of alveolar macrophag es with CGRP inhibited differentiation into osteoclast-like cells in c o-cultures with primary osteoblastic cells in the presence of 1alpha,2 5-dihydroxyvitamin D3. These results indicate that specific receptors for CGRP are present in macrophages and that CGRP modulates proliferat ion and differentiation of macrophages into osteoclast-like cells by a receptor-mediated mechanism involving cAMP.