Intracellular degradation of newly synthesized collagen

This assay reviews current work on intracellular degradation of newly synthesized collagen in cell culture systems. Various methodological and conceptual problems are discussed and areas of disagreement are highlighted, and a model that accounts for much of the experimental data is proposed. All degradation studies are based on the premise that free hydroxyproline is a marker for collagen breakdown. A recent hypothesis that some hydroxyproline might be produced independently of collagen synthesis has been subjected to various tests, and the evidence strongly suggests that the hypothesized mechanism is not operative in cell culture systems. Approximately 15% of the collagen synthesized by human fibroblasts maintained under normal culture conditions is broken down rapidly. This process, termed basal degradation, functions continuously and independently of collagen synthesis; it is posttranslational rather than cotranslational; and it is not inhibited by lysosomotropic agents or colchicine. Degradation is enhanced when culture conditions are manipulated so that structurally abnormal collagen is synthesized; the increase above the basal level can be suppressed by lysosomotropic agents and colchicine. Degradation is also enhanced when cells are exposed to agents that elevate the intracellular level of cAMP. The major feature of the proposed model is that there are two distinct pathways for degradation. Basal degradation is viewed as a stochastic, or random, process. Collagen molecules that enter this pathway are not distinguishable from molecules that escape breakdown, and the probability of being degraded is ca. 1/6. The model predicts that the basal degradation mechanism is located in the distal region of the endoplasmic reticulum (smooth ER) or the cis-region of the Golgi complex, and that enzymes capable of attacking collagen or collagenous peptides are located in one of these organelles. Enhanced degradation is depicted as a deterministic process that results from the interaction between newly synthesized collagen molecules and an apparatus that recognizes abnormal structures, responds to external signals, and directs molecules either toward sites for packaging into secretory vesicles or toward the site of degradation. This apparatus is probably located in the Golgi complex, but the actual breakdown of the molecules occurs in lysosomes. Transport from the recognition and sorting mechanism to the site of degradation can be blocked by colchicine. A specific prediction of the model is that all enhanced degradation is mediated by lysosomal proteases and occurs in lysosomes.