HYPOXIA-INDUCED INHIBITION OF TROPOELASTIN SYNTHESIS BY NEONATAL CALF PULMONARY-ARTERY SMOOTH-MUSCLE CELLS

Animals chronically exposed to hypoxia develop characteristic structural changes in the pulmonary arterial vasculature including cell hypertrophy, hyperplasia, and increased deposition of extracellular matrix proteins. The medial smooth muscle cells' (SMC) increase in tropoelastin mRNA expression and elastin deposition as determined by in situ hybridization and histologic examination appears to contribute significantly to this increase in matrix protein accumulation. The primary stimulus for the increased tropoelastin production, which persists in vitro, is unknown but mechanical forces and hypoxia seem to play a role. In order to determine the direct effects of hypoxia on tropoelastin production by pulmonary artery SMC, cultured neonatal bovine pulmonary artery SMC were exposed to 3%, 10%, and 21% O2 concentrations for 48, 72, and 120 h and soluble tropoelastin was measured by direct immunoassay. Tropoelastin mRNA levels were also determined by Northern and slot blot analysis after 48 h of incubation under hypoxic conditions. SMC cultured in 3% and 10% O2 for 120 h showed dose-dependent decreases (11-fold and 2-fold, respectively) in measured tropoelastin levels compared with SMC cultured in 21% O2 conditions. This decrease was not due to cell damage or accumulation of toxic metabolites while under hypoxic conditions nor to a change in tropoelastin partitioning between the cell and media. Tropoelastin mRNA levels were also decreased under hypoxic conditions. Secreted, cell layer, and total protein synthesis determined by L-[H-3]leucine incorporation again showed a dose-dependent decrease under hypoxic conditions but not to the same extent as tropoelastin production. Total protein synthesis after 120 h in hypoxia was decreased by 2-fold in 3% conditions and by 22% in 10% O2 conditions compared with control SMC in 21% O2 conditions. These data suggest that the direct effects of hypoxia alone are not responsible for the increased tropoelastin production seen by SMC in the pulmonary vasculature upon chronic exposure to hypoxia.