Amino acid availability regulates type I procollagen accumulation in human lung fibroblasts

Fibrotic lung diseases are characterized by excessive deposition of type I collagen. Amino acid availability regulates type I collagen mRNA levels in quiescent human lung fibroblasts. In these studies, the effect of amino acid availability on type I collagen protein accumulation in quiescent human lung fibroblasts was examined. Following amino acid deprivation, alpha1(I) procollagen protein levels were not detected by Western blot analysis in either the intracellular or the extracellular compartments. Fibronectin levels and total protein levels were not affected. Amino acid deprivation resulted in a more pronounced decrease in alpha1(I) procollagen protein levels than in alpha1(I) procollagen mRNA levels, suggesting that post-transcriptional events were responsible for the further decrease inalpha1(I) procollagen protein levels. The addition of transforming growth factor-beta to amino acid deprived fibroblasts increased alpha1(I) procollagen mRNA levels without affecting alpha1(I) procollagen protein levels, confirming a post-transcriptional site for regulatory control by amino acid deprivation. In the absence of ascorbic acid, alpha1(I) procollagen protein levels increased in amino acid deprived fibroblasts, but alpha1(I) procollagen mRNA levels were not affected. The absence of ascorbic acid likely resulted in the accumulation of nonhelical procollagen in the endoplasmic reticulum, indicating that translational mechanisms for alpha1(I) procollagen were intact. The addition of chloroquine, an inhibitor of lysosomal degradation of proteins, increased alpha1(I) procollagen protein levels in amino acid deprived fibroblasts. These data suggest that following amino acid deprivation of quiescent fibroblasts, newly synthesized type I collagen was degraded intracellularly, primarily by a process that involved lysosomal proteinases.