ENDOTHELIN-1 PROMOTES MITOGENESIS IN AIRWAY SMOOTH-MUSCLE CELLS

Endothelin exists as three isoforms (ET-1, ET-2, and ET-3) and exhibit s vasoconstricting, bronchoconstricting, and growth-promoting properti es in vascular smooth muscle. In the airways, ET-1 immunoreactivity an d mRNA have been detected and localized to the epithelium, smooth musc le, and endothelium in different species, including humans. It has bee n suggested that ET-1 may have a role in the airway smooth muscle hype rplasia and hypertrophy seen in patients with bronchial asthma. We stu died ovine airway smooth muscle cells (SMC) in vitro and showed satura ble binding of [I-125]ET-1 with a dissociation constant (K-d) of 0.4 n M and high affinity binding sites (B-max) for ET-1 (104 fmol/10(6) cel ls). This binding was functional as ET-1 promoted mitogenesis of these muscle cells as measured by increased cell number in the absence of s erum. Twenty-four hours after exposing the cells to graded doses of ET -1 from 1 pM to 1 mu M, cell number increased significantly over contr ol in a dose-dependent manner. ET-1 also enhanced the transient expres sion of c-fos mRNA by 2.5-fold over control, with maximal expression o ccurring at 30 min. These observations provide evidence that: (1) airw ay SMC possess high affinity binding sites for ET-1, and (2) ET-1 is m itogenic for airway SMC as determined by increased cell number and amp lification of c-fos mRNA expression. ET-1 may have a fundamental role in influencing the growth of smooth muscle in the airways.