The production of procollagen molecules by human diploid fetal lung fibroblasts (HFL-1 cells) remains constant in both rapid and stationary growth phases. However, log phase cells degrade 3-fold more newly synthesized collagen inside the cell prior to secretion than do stationary phase cells. Procollagen mRNA levels, measured by hybridization with a type I procollagen mRNA-specific complementary DNA, are approximately 2-fold higher in confluent cells than in log phase cells. There are no

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... cant differences in the ability of either log phase or confluent HFL-1 cell procollagen mRNA to be translated in an in vitro cell-free translation system. Therefore, the ability of HFL-1 cells to maintain constant collagen production irrespective of the growth status of the cells results from the combined action of a number of regulatory mechanisms, including changes in procollagen mRNA levels, the utilization of procollagen mRNA, and intracellular procollagen degradation.