Transformation-induced changes in transferrin and iron metabolism in myogenic cells.

The uptake of transferrin and iron by cultured myogenic cells transformed with a temperature-sensitive strain of the Rous sarcoma virus (tsLA24) was compared with that of normal developing myogenic cells which were proliferating at the same rate as the transformed cells. The mechanism of transferrin and iron uptake was the same in the transformed cells as in normal myogenic cells and involved receptor-mediated endocytosis of transferrin. However, there were differences in transferrin receptor numbers and receptor function. The number of receptors in transformed cells was more than twice as great as in the normal cells largely due to increased surface receptor numbers. Despite this, the rate of iron uptake increased by only 20% in the transformed cells due to less efficient cycling of the transferrin receptors and less efficient release of iron from transferrin to intracellular sites. Some internalized iron was released from the transformed cells still bound to transferrin. A fast and a slow rate of transferrin exocytosis were identified in transformed cells, as in normal cells, indicating that there were at least two intracellular pathways for transferrin. The fast pathway predominated in the transformed cells, compared with an equal importance of the two pathways in the normal cells.