The neuropeptide calcitonin gene-related peptide inhibits TNF-α but poorly induces IL-6 production by fetal rat osteoblasts

Tumour necrosis factor alpha (TNF-alpha) and interleukin 6 (IL-6) are potent inflammatory cytokines produced by osteoblasts and whose contribution to bone loss occurring in oestrogen deficiency is well documented, Calcitonin gene-related peptide (CC;RP) is a neuropeptide abundantly concentrated in sensory nerve endings innervating bone metaphyses and periosteum suggesting that it controls bone homeostasis locally, Since CGRP was shown to inhibit TNF-alpha production by T cells and stimulate IL-6 expression by fibroblasts, this study was designed to investigate whether CGRP regulated TNF-alpha and IL-6 production by osteoblasts. We show that CGRP inhibits the production of TNF-alpha by both lipopolysaccharide (LPS)-and LL-l-stimulated fetal rat osteoblasts. Like CGRP, the cAMP agonists prostaglandin E-2 (PGE(2)), dibutyryl cAMP (Bt(2)cAMP) and forskolin inhibit TNF-alpha production by osteoblasts. Exposure of osteoblasts to a high dose of phorbol myristoyl acetate (PMA) to deplete PKC activity abolished CGRP-mediated TNF-alpha suppression, In contrast with its potent inhibition of TNP-alpha production, we show that CGRP is a weak inducer of IL-6 when compared to PGE(2), Bt(2)cAMP and forskolin, However, in presence of isobutylmethylxanthine (IBMX) CGRP stimulates the production of IL-6, Collectively, these data suggest that the inhibition of TNF-alpha by CGRP is cAMP dependent and PMA sensitive and that the concentration of intracellular cAMP may be a regulatory mechanism for IL-6 gene expression in osteoblasts. (C) 1997 Academic Press Limited.