ET-1 cooperates with EGF to induce mitogenesis via a PTX-sensitive pathway in airway smooth muscle cells

We have examined the mitogenic effect of endothelin-1 (ET-1) alone or in combination with epidermal growth factor (EGF) in cultured airway smooth muscle cells (ASM) from guinea pig. ET-1 showed a weak mitogenic activity compared with the effect of EGF. However, when ET-1 and EGF were applied simultaneously, ET-1 synergistically enhanced the mitogenic activity of EGF. Neither inhibition of phospholipase C-beta nor depletion of protein kinase C affected this synergism. On the other hand, pertussis toxin (PTX), a Gi protein inhibitor, abolished the synergistic effect of ET-1 on EGF-induced mitogenesis. ET-1 induced a transient mitogen-activated protein (MAP) kinase activation peaking at 5 min. In contrast, EGF induced a stronger signal that was maintained for up to 20 min. However, concomitant stimulation of ASM with ET-1 and EGF caused an enhanced MAP kinase activation compared with EGF alone. Moreover, PTX abolished the enhanced MAP kinase activation observed in this condition. These results indicate that ET-1 can interact with an EGF-induced mitogenic axis through the Gi protein-dependent pathway, which is distinct from its direct mitogenic pathway.