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-l (ET-1) alone or in combination with epidermal growth factor (EGF) in cultured airway s mooth 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 mitogen ic activity of EGF. Neither inhibition of phospholipase C-beta nor dep letion of protein kinase C affected this synergism. On the other hand, pertussis toxin (PTX), a G(i) protein inhibitor, abolished the synerg istic effect of ET-1 on EGF-induced mitogenesis. ET-1 induced a transi ent mitogen-activated protein (MAP) kinase activation peaking at 5 min . In contrast, EGF induced a stronger signal that was maintained for u p to 20 min. However, concomitant stimulation of ASM with ET-1 and EGF caused an enhanced MAP kinase activation compared with EGF alone. Mor eover, PTX abolished the enhanced MAP kinase activation observed in th is condition. These results indicate that ET-1 can interact with an EG F-induced mitogenic axis through the G(i) protein-dependent pathway, w hich is distinct from its direct mitogenic pathway.