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Molecular mechanisms of the synergy between cysteinyl-leukotrienes and receptor tyrosine kinase growth factors on human bronchial fibroblast proliferation

Division of Infection, Inflammation and Repair, Southampton University School of Medicine, Southampton, UK

CORRESPONDENCE: Hajime Yoshisue, Novartis Pharma KK, Tsukuba Research Institute, Tsukuba, Ibaraki, Japan

We have reported that cysteinyl-leukotrienes (cys-LTs) synergise not only with epidermal growth factor (EGF) but also with platelet-derived growth factor (PDGF) and fibroblast growth factor (FGF) to induce mitogenesis in human bronchial fibroblasts. We now describe the molecular mechanisms underlying this synergism. Mitogenesis was assessed by incorporation of [³H]thymidine into DNA and changes in protein phosphorylation by Western blotting. Surprisingly, no CysLT receptor antagonists (MK-571, montelukast, BAY u9773) prevented the synergistic mitogenesis. LTD₄ did not cause phosphorylation of EGFR nor did it augment EGF-induced phosphorylation of EGFR, and the synergy between LTD₄ and EGF was not blocked by the metalloproteinase inhibitor GM6001 or by an HB-EGF neutralising antibody. The EGFR-selective kinase inhibitor, AG1478, suppressed the synergy by LTD₄ and EGF, but had no effect on the synergy with PDGF and FGF. While inhibitors of mitogen-activated protein kinase, phosphatidylinositol 3-kinase and protein kinase C (PKC) prevented the synergy, these drugs also inhibited mitogenesis elicited by EGF alone. In contrast, pertussis toxin (PTX) efficiently inhibited the potentiating effect of LTD₄ on EGF-induced mitogenesis, as well as that provoked by PDGF or FGF, but had no effect on mitogenesis elicited by the growth factors alone. Whereas LTD₄ alone did not augment phosphorylation of extracellular signal-regulated kinase (Erk)-1/2 and Akt, it increased phosphorylation of PKC in a Gi-dependent manner. Addition of LTD₄ prolonged the duration of EGF-induced phosphorylation of Erk-1/2 and Akt, both of which were sensitive to PTX. The effect of cys-LTs involves a PTX-sensitive and PKC-mediated intracellular pathway leading to sustained growth factor-dependent phosphorylation of Erk-1/2 and Akt.