Airway smooth muscle hyperplasia and hypertrophy correlate with glycogen synthase kinase-3β phosphorylation in a mouse model of asthma

Abstract

Increased airway smooth muscle (ASM) mass, a characteristic finding in asthma, may be caused by hyperplasia or hypertrophy. Cell growth requires increased translation of contractile apparatus mRNA, which is controlled, in part, by glycogen synthase kinase (GSK)-3β, a constitutively active kinase that inhibits eukaryotic initiation factor-2 activity and binding of methionyl tRNA to the ribosome. Phosphorylation of GSK-3β inactivates it, enhancing translation. We sought to quantify the contributions of hyperplasia and hypertrophy to increased ASM mass in ovalbumin (OVA)-sensitized and -challenged BALB/c mice and the role of GSK-3β in this process. Immunofluorescent probes, confocal microscopy, and stereological methods were used to analyze the number and volume of cells expressing α-smooth muscle actin and phospho-Ser9 GSK-3β (pGSK). OVA treatment caused a 3-fold increase in ASM fractional unit volume or volume density (Vv) (PBS, 0.006 ± 0.0003; OVA, 0.014 ± 0.001), a 1.5-fold increase in ASM number per unit volume (Nv), and a 59% increase in volume per cell (Vv/Nv) (PBS, 824 ± 76 μm3; OVA, 1,310 ± 183 μm3). In OVA-treated mice, there was a 12-fold increase in the Vv of pGSK (+) ASM, a 5-fold increase in the Nv of pGSK (+) ASM, and a 1.6-fold increase in Vv/Nv. Lung homogenates from OVA-treated mice showed increased GSK-3β phosphorylation and lower GSK-3β activity. Both hyperplasia and hypertrophy are responsible for increased ASM mass in OVA-treated mice. Phosphorylation and inactivation of GSK-3β are associated with ASM hypertrophy, suggesting that this kinase may play a role in asthmatic airway remodeling.