The importance of ERK activity in the regulation of cyclin D1 levels and DNA synthesis in human cultured airway smooth muscle

The relationship between persistent ERK (extracellular signal‐regulated kinase) activity, cyclin D1 protein and mRNA levels and cell cycle progression in human cultured airway smooth muscle was examined in response to stimulation by ET‐1 (endothelin‐1), thrombin and bFGF (basic fibroblast growth factor). Thrombin (0.3 and 3 u ml −1 ) and bFGF (0.3 and 3 n M ) increased ERK activity for more than 2 h and increased cell number, whereas ET‐1 (100 n M ) transiently stimulated ERK activity and was non‐mitogenic. The MEK1 (mitogen‐activated ERK kinase) inhibitor, PD 98059 (30 μ M ), inhibited both ERK phosphorylation and activity, and either prevented (thrombin 0.3 and 3 u ml −1 , bFGF 300 p M ) or attenuated (bFGF 3 n M ) DNA synthesis. Thrombin and bFGF increased both cyclin D1 mRNA and protein levels. PD 98059 decreased cyclin D1 protein levels stimulated by the lower but not higher thrombin concentrations. Moreover, increases in cyclin D1 mRNA levels were unaffected by PD 98059 pretreatment, irrespective of the mitogen or its concentration, suggesting that inhibition of cyclin D1 protein levels occurred by a post‐transcriptional mechanism. These findings indicate that the control of cyclin D1 protein levels may occur independently of the MEK1/ERK signalling pathways. The inhibition of S phase entry by PD 98059 at higher thrombin concentrations appears to result from effects on pathways downstream or parallel to those regulating cyclin D1 protein levels. These findings suggest heterogeneity in the signalling of DNA synthesis in human cultured airway smooth muscle.British Journal of Pharmacology (2000) 131 , 17–28; doi: 10.1038/sj.bjp.0703454