High serum levels interfere with the normal differentiated state of avian tendon cells by altering translational regulation

In low serum (0.2%) medium, ascorbate stimulates primary avian tendon cells to increase procollagen synthesis from 12 to 50% of total protein synthesis. This is reversibly blocked by an increase of serum levels from 0.2 to 3%. Ascorbate in low serum medium has been shown previously to stimulate the procollagen pathway by sequentially increasing by sixfold the secretion rate constant, then translation rates, and finally mRNA levels. We now show that addition of ascorbate to cultures containing 3% serum induces a sixfold increase in the secretion rate constant but translation rates and mRNA levels remain unchanged. In fully induced cells, an increase in serum levels causes a down-regulation of procollagen synthesis. In this case, the translational products of the induced cell are rapidly altered (less than 1 h), with noncollagen protein synthesis being stimulated preferentially over procollagen synthesis. This change is not reflected in procollagen mRNA levels since they remain constant for at least 6 h following addition of high serum. After 48 h in high serum, the induction of procollagen synthesis by ascorbate is reversed and the level of procollagen mRNA drops to that of uninduced cells. The data are consistent with the model that serum acts primarily at the translational level. High serum levels break the coupling in the ascorbate induction process that ties the stimulation of procollagen secretion rates to the increase in procollagen translation rates, and this prevents the maintenance of the induced state.