Contribution of SRF, Elk-1, and myocardin to airway smooth muscle remodeling in heaves, an asthma-like disease of horses

Myocyte hyperplasia and hypertrophy contribute to the increased mass of airway smooth muscle (ASM) in asthma. Serum-response factor (SRF) is a transcription factor that regulates myocyte differentiation in vitro in vascular and intestinal smooth muscles. When SRF is associated with phosphorylated (p) Elk-1, it promotes ASM proliferation while binding to myocardin (MYOCD) leading to the expression of contractile elements in these tissues. The objective of this study was therefore to characterize the expression of SRF, pElk-1, and MYOCD in ASM cells from central and peripheral airways in heaves, a spontaneously occurring asthma-like disease of horses, and in controls. Six horses with heaves and five aged-matched controls kept in the same environment were studied. Nuclear protein expression of SRF, pElk-1, and MYOCD was evaluated in peripheral airways and endobronchial biopsies obtained during disease remission and after 1 and 30 days of naturally occurring antigenic exposure using immunohistochemistry and immunofluorescence techniques. Nuclear expression of SRF (P = 0.03, remission vs. 30 days) and MYOCD (P = 0.05, controls vs. heaves at 30 days) increased in the peripheral airways of horses with heaves during disease exacerbation, while MYOCD (P = 0.04, remission vs. 30 days) decreased in the central airways of control horses. No changes were observed in the expression of pElk-1 protein in either tissue. In conclusion, SRF and its cofactor MYOCD likely contribute to the hypertrophy of peripheral ASM observed in equine asthmatic airways, while the remodeling of the central airways is more static or involves different transcription factors.