ENDOTHELIN-1-INDUCED POTENTIATION OF HUMAN AIRWAY SMOOTH-MUSCLE PROLIFERATION - AN ETA-RECEPTOR-MEDIATED PHENOMENON

1 In this study the mitogenic effects in human cultured tracheal smoot h muscle cells of endothelin-l (ET-I), ET-3, and sarafotoxin S6c (S6c) , the ET(B) receptor-selective agonist, were explored either alone or in combination with the potent mitogen, epidermal growth factor(EGF). 2 In confluent, growth-arrested human airway smooth, neither ET-1 (0.0 1 nM - 1 mu M) nor ET-3 (0.001 nM - 1 mu M) Or S6c (0.01 nM - 1 mu M) induced cell proliferation, as assessed by [H-3]-thymidine incorporati on. In contrast, EGF (1.6 pM - 16 nM) produced concentration-dependent stimulation of DNA synthesis (EC(50) of about 0.06 nM). The maximum i ncrease of about 60 fold above control, elicited by 16 nM EGF, was sim ilar to that obtained with 10% foetal bovine serum (FBS). EGF (0.16 - 16 nM) also produced a concentration-dependent increase in cell counts , whereas ET-1 (1 - 100 nM) was without effect on this index of mitoge nesis. 3 ET-1 (1 - 100 nM) potentiated EGF-induced proliferation of hu man tracheal smooth muscle cells. For example, ET-I (100 nM), which al one was without significant effect, increased by 3.0 to 3.5 fold the m itogenic influence of EGF (0.16 nM). The potentiating effect of ET-1 o n EGF-induced proliferation was antagonized by BQ-123 (3 mu M), the ET (A) receptor antagonist, but was unaffected by the ET(B) receptor anta gonist BQ-788 (10 mu M). 4 Neither ET-3 (1 - 100 nM) nor S6c (1 - 100 nM) influenced the mitogenic effects of EGF (0.16 - 1.6 nM). 5 [I-125] -ET-1 binding studies revealed that on average the ratio of ET(A) to E T(B) receptors in human cultured tracheal smooth muscle cells was 35:6 5 (+/- 3; n = 4), confirming the predominance of the ET(B) receptor su btype in human airway smooth muscle. 6 These data indicate that ET-1 a lone does not induce significant human airway smooth muscle cell proli feration. However, it potently potentiated mitogenesis induced by EGF, apparently via an ET(A) receptor-mediated mechanism. These findings s uggest that ET-l, a mediator detected in increased amounts in patients with acute asthma, may potentiate the proliferative effects of mitoge ns and contribute to the airway smooth muscle hyperplasia associated w ith chronic severe asthma.