Calcitonin gene-related peptide inhibits interleukin-1β-induced endogenous monocyte chemoattractant protein-1 secretion in type II alveolar epithelial cells

As important multifunctional cells in the lung, alveolar epithelial type II (AEII) cells secrete numerous chemokines on various stimuli. Our previous data showed that AEII cells also express the neuropeptide calcitonin gene-related peptide (CGRP) and the proinflammatory factor interleukin (IL)-1β induces CGRP secretion in the A549 human AEII cell line. In the present study, the CGRP-1 receptor antagonist human (h)CGRP8–37(0.1–1 nM) greatly amplified the production of IL-1β-induced monocyte chemoattractant protein (MCP)-1. The inhibition of CGRP expression by small interfering RNA significantly increased MCP-1 secretion on IL-1β stimulation. However, exogenous hCGRP (10–100 nM) suppressed IL-1β-evoked MCP-1 secretion in MCP-1 promoter activity, and CGRP gene stably transfected cell clones significantly inhibited both the mRNA and protein levels of MCP-1 induced by IL-1β. These data imply that AEII-derived CGRP suppressed IL-1β-induced MCP-1 secretion in an autocrine/paracrine mode. Subsequent investigation revealed that CGRP inhibited IL-1β-evoked NF-κB activity by suppressing IκBα phosphorylation and degradation. Moreover, CGRP attenuated IL-1β-induced reactive oxygen species (ROS) formation, the early event in proinflammatory factor signaling. We previously showed that the CGRP inhibitory effect was mediated by elevated intracellular cAMP and show here that analogs of cAMP, 8-bromoadenosine 3′,5′-cyclic monophosphothioate and the Sp isomer of adenosine 3′,5′-cyclic monophosphothioate, mimicked the CGRP suppressive effect on IL-1β-induced ROS formation, NF-κB activation, and MCP-1 secretion. Thus increased endogenous CGRP secretion in lung inflammatory disease might eliminate the excessive response by elevating the cAMP level through inhibiting the ROS-NF-κB-MCP-1 pathway.