The extracellular CGRP immunoreactivity eluted in the same fraction as rat CGRP- on high performance liquid chromatography. II cells was reversibly blocked by monensin, an inhibitor of secretory protein transport. CGRP secretion was stimulated in a concentration-dependent fashion by phorbol myristate acetate, but it was not affected by forskolin, capsaicin, bradykinin or nicotine. CGRP was not detected in culture media from alveolar macrophages or fibroblasts, potential contaminants of primary type II cell cultures. Calcitonin is expressed by neuroendocrine cells but it was not detected in conditioned media from type II cell cultures.
Secretion occurs constitutively and is regulated by a protein kinase C-dependent pathway.
Secretion is unaffected by increases in cAMP or by treatments that induce release of CGRP from sensory afferent nerve terminals in the airways. PTHrP receptor expression in A549 cells was shown by detection of a 0.3 kb reverse transcriptase PCR product formed by primers specific for PTHrP receptor. In situ hybridization studies localized the site of production of PTHrP and PTHrP receptor mRNA in rat lung to cells with morphology and location typical of type II cells. Thus, PTHrP 1-34 promoted a differentiated type II cell phenotype. Since cultured type II cells, alveolar epithelial cells in vivo , and A549 cells express PTHrP and the PTHrP receptor, PTHrP 1-34 may be an autocrine regulatory factor in type II cells and lung cancer cells.