Hypoxia inhibits amino acid uptake in human lung fibroblasts

Hypoxia and amino acid deprivation downregulate expression of extracellular matrix genes in lung fibroblasts. We examined the effect of hypoxia on ami no acid uptake and protein formation in human lung fibroblasts. Low O-2 ten sion (0% O-2) suppressed incorporation of [H-3]proline into type I collagen without affecting [S-35]methionine labeling of other proteins. Initial dec reases in intracellular [3H]proline incorporation occurred after 2 h of exp osure to 0% O-2, with maximal suppression of intracellular [3H]proline leve ls at 6 h of treatment. Hypoxia significantly inhibited the uptake of radio -labeled proline, 2-aminoisobutyric acid (AIB), and 2-(methyl-amino)isobuty ric acid (methyl-AIB) while inducing minor decreases in leucine transport. Neither cycloheximide nor indomethacin abrogated hypoxia-related suppressio n of methyl-AIB uptake. Efflux studies demonstrated that hypoxia inhibited methyl-AIB transport in a bidirectional fashion. The downregulation of amin o acid transport was not due to a toxic effect; function recovered on retur n to standard O-2 conditions. Kinetic analysis of AIB transport revealed a 10-fold increase in K-m accompanied by a small increase in maximal transpor t velocity among cells exposed to 0% O-2. These data indicate that low O-2 tension regulates the system A transporter by decreasing transporter substr ate affinity.