Leukotriene D-4-induced activation of smooth-muscle cells from human bronchi is partly Ca2+-independent

Cysteine-containing leukotrienes (cysteinyl-LTs) are potent bronchoconstric tors and play a key role in asthma. We found that histamine and LTD, marked ly constrict strips of human bronchi (HB) with similar efficacy. However, i n human airway smooth-muscle (HASM) cells, LTD4, at variance with histamine , elicited only a small, transient change in intracellular calcium ion conc entration. HASM cells express both Ca2+-dependent and -independent isoforms of protein kinase C (PKC) (i.e., PKC-alpha and PKC-epsilon). Western blot analysis showed that PKC-alpha is activated by histamine and, to a lesser e xtent, by LTD4, whereas only LTD4 translocates PKC-epsilon. This translocat ion was specifically inhibited by the LTD4 antagonist pobilukast. Phorbol-d ibutyrate ester (PDBu) (a PKC activator) contracted HE strips to the same e xtent in the presence as in the absence of extra- and intracellular Ca2+. I n the absence of Ca2+, LTD4 contracted HB strips to the same extent as did PDBu, suggesting the involvement of a Ca2+-independent PKC in LTD4-mediated signal transduction. PDBu-induced desensitization and the PKC inhibitor H7 abolished the slow and sustained LTD4-triggered contraction of HE strips i n the absence of Ca2+, although H7 did not greatly affect the response in t he presence of the ion. Thus, in human airways, we identified a novel LTD4 transduction mechanism linked to bronchial smooth-muscle contraction, which is partly independent of Ca2+ and involves the activation of PKC-epsilon.