Remodeling of the rough endoplasmic reticulum during stimulation of procollagen secretion by ascorbic acid in cultured chondrocytes. A biochemical and morphological study.

The cellular and molecular mechanisms regulating the reversible accumulation of nonhelical, underhydroxylated procollagen in the rough endoplasmic reticulum (RER) remain obscure. To clarify these mechanisms, we isolated chondrocytes from chick vertebral cartilage and kept them in scorbutic monolayer cultures. By Day 9 of culture, the chondrocytes had accumulated a large amount of underhydroxylated Type II procollagen in their RER. Within 1 h of ascorbate treatment, the accumulated procollagen was hydroxylated; this was accompanied by a slight stimulation of procollagen secretion and was followed by a marked stimulation starting between 2 and 3 h of treatment. Secretion of the accumulated procollagen was completed by about 24 h of treatment. Strikingly, the marked stimulation of procollagen secretion at 2-3 h of treatment was associated with marked remodeling of the RER. This organelle came to consist of a few, unusually large cisternae ("sacs") and many flat cisternae while the RER in untreated cells consisted of uniform, oval cisternae. The RER remodeling was accompanied by a comparable redistribution of the accumulated Type II procollagen stored in it. The RER sacs and flat cisternae invariably communicated directly and were still detectable by 8 h but not by 24 h of treatment. RER remodeling and procollagen redistribution also occurred in untreated chondrocytes that had been shifted to 23 degrees C for 2-3 h. Together, the data indicate that folding of the accumulated procollagen molecules into their normal helical configuration is followed by procollagen redistribution within, and remodeling of, the RER. These processes may have a role in stimulating procollagen export from the RER and secretion.