Thromboxane prostanoid receptor in human airway smooth muscle cells : a relevant role in proliferation

Thromboxane A2 has been implicated as a mediator of bronchial hyperresponsiveness in asthma. Modulating agents are currently marketed in Japan and under clinical evaluation in the US, but full characterization of the thromboxane A2 receptor and the signaling pathways that link it to the proliferative events taking place during airways structural remodeling has not been achieved. Here, we report that the presence of mRNA for both α and β isoforms of the thromboxane A2 receptor in smooth muscle cells from human bronchi correlates with protein expression evaluated by radioligand binding of the antagonist, SQ29,548 ([1S-[1α,2α(Z),3α,4α]]-7-[3-[[2-[(phenylamino)carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2.1]hept-2-yl]-5-heptenoic-acid) (Kd= 3.4 nM ± 44%CV, coefficient of variation, Bmax= 41 fmol/mg prot ± 38%CV). The receptor is functional, as the agonist, U46619 (9,11-dideoxy-9α,11α-methanoepoxy-prosta-5Z,13E-dien-1-oic-acid), induced a concentration-dependent Ca2+ transient (EC50= 0.12 µM ± 27%CV). Furthermore, U46619 concentration dependently increased DNA synthesis and markedly potentiated the epidermal growth factor mitogenic effect. Both events were specifically inhibited by SQ29,548, independently from transactivation of the epidermal growth factor receptor and partially sensitive to pertussis toxin.