Abstract
Surface transferrin (Tf) receptors are displayed by cultured human hematopoietic cells and provide Fe required for cell growth. Cell cycle status, cell density in culture, exposure to Fe, and differentiation alter Tf receptor display by myeloid leukemia cells. To investigate mechanisms controlling Tf receptor expression, rates of receptor synthesis and steady-state mRNA levels were measured in HL60 promyelocytic cells grown in serum and serum-free media or after differentiation in response to dimethylsulfoxide (DMSO). Although surface binding sites were unchanged during the first three days in culture with serum or in serum-free media containing Tf, by the third day, rates of receptor biosynthesis and steady-state mRNA levels declined, consistent with cell density-dependent, receptor regulation. Cells grown with soluble Fe instead of Tf showed reduced Tf binding sites, rates of receptor synthesis, and Tf receptor mRNA. When cells grown with Fe were subcultured, Tf receptor mRNA levels increased within 15 minutes and peaked by 24 hours. This was followed by a decline in receptor and gene expression so that by three days cells grown in the presence of Fe expressed approximately four times fewer receptors and had half the rates of Tf receptor synthesis and mRNA levels of cells grown in serum or Tf. Cells treated with DMSO showed a rapid decline in surface receptors, receptor synthesis, and steady- state mRNA levels. Modulation of Tf receptor expression was not due to redistribution between the cell surface and an internal receptor pool. In each instance, concurrent assessment of N-ras transcripts showed that changes in Tf receptor mRNA levels were not due to generalized alterations in protein synthesis. Exposure of cells grown in Fe or treated with DMSO to cycloheximide did not alter Tf receptor mRNA levels, thereby suggesting that receptor expression was not regulated by posttranscriptional processes dependent on protein synthesis. Actinomycin D inhibition of Tf receptor mRNA was compatible with a transcript half-life of approximately 2.2 hours. Nuclear transcription studies showed reduced rates of Tf receptor transcription after culture with Fe or exposure to DMSO. The present studies show complex patterns of Tf receptor gene regulation in cultured myeloid leukemia cells and demonstrate that transcriptional regulation is a major mechanism controlling Tf receptor gene expression in response to Fe and differentiation.
