Regulation of the production of secretory proteins: Intracellular degradation of newly synthesized “defective” collagen

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Abstract
Confluent cultures of human fetal lung fibroblasts degrade .apprx. 10% of their newly synthesized collagen within the cell prior to secretion. This basal level of intracellular degradation could not be inhibited by colchicine or cytochalasin B, inhibitors of microtubular and microfilament function, respectively, or by N.alpha.-p-tosyl-L-lysine chloromethyl ketone, chloroquine or NH4Cl, inhibitors of lysosomal enzymes. Cells in early log growth degrade .apprx. 30% of their newly synthesized collagen. This enhanced degradation of collagen in rapidly growing cells could be suppressed by inhibitors of lysosomal proteases and partially inhibited by disrupters of microtubular and microfilament function. A significant proportion of the collagen synthesized by these cultures contained prolyl residues that were incompletely hydroxylated. Because such collagen is defective (i.e., not capable of assuming a triple helical conformation), enhanced intracellular degradation may be a mechanism by which cells control the quality of collagen they produce. To test this hypothesis, confluent cells were incubated with the proline analog cis-4-hydroxyproline; such cells demonstrated enhanced collagen degradation that could be inhibited by agents that interfere with lysosomal, microtubular or microfilament function. Because collagen containing cis-4-hydroxyproline cannot form a perfect triple helix, defective collagen may be recognized by cells and degraded prior to secretion. Proportion of newly synthesized collagen that undergoes intracellular degradation seems to be partially modulated by the collagen molecule conformation. Intracellular proteolysis may represent a means by which collagen-producing cells regulate the quality and quantity of collagen available for extracellular function. Although the exact mechanism of intracellular collagen degradation is unknown, lysosomal proteases may be included in this process.