Transferrin receptor 1 (TFRC1) is a major mediator of iron uptake in mammalian cells and thus plays an important role in normal cellular metabolism. Moreover, overexpression of TFRC1 is a common feature of many human malignancies. Therapeutic strategies designed to interfere with tumor iron metabolism have targeted TFRC1. The MYC proto-oncogene encodes a transcription factor that stimulates cellular proliferation and growth through the activation of thousands of target genes. Here we demonstrate that TFRC1 is directly activated by c-Myc and is necessary for cell cycle progression. TFRC1 expression was found to be c-Myc-responsive in a human B-cell line harboring a tetracycline-repressible MYC transgene. TFRC expression is also induced by c-Myc in an in vivo mouse model of B-cell lymphoma. Additionally, TFRC expression is greatly diminished in MYC null rat fibroblasts, demonstrating that MYC is required for physiologic TFRC1 expression. Using chromatin immunoprecipitation, c-Myc was shown to bind directly to an evolutionarily conserved E-box motif (CACGTG) in intron 1 of the TFRC gene. Since TFRC expression is elevated in lymphomas and is regulated by MYC, we sought to determine whether TFRC is required for MYC-mediated cell cycle proliferation and cell size control. Abrogation of TFRC expression using siRNA results in a cell proliferation defect, G1 arrest, and activation of cell cycle checkpoint genes p53 and p21. These effects are also induced by iron chelation with desferrioxamine (DFX). In contrast, neither knock-down of TFRC using siRNA nor DFX treatment affected the MYC-mediated increase in cell size. To globally assess expression of genes involved in cell cycle regulation and cell size control, we examined expression profiles in B-cells treated with TFRC1 siRNA. Downregulated transcripts included cell division cycle (cdc) genes, cyclins, and other factors required for DNA replication. Upregulated transcripts include p53-responsive and apoptosis-promoting genes. Consistent with our observation that TFRC1 knockdown did not influence cell size, expression of genes involved in ribosome biogenesis and cell size regulation was unaffected in siRNA-treated cells. In order to explore the physiologic consequences of TFRC1 overexpression, we examined the proliferative capacity of rat fibroblasts growing in limiting serum. We provide evidence that overexpression of TFRC1 confers a significant growth advantage under these conditions. These data suggest that upregulation of TFRC1 by c-Myc may be advantageous to cells growing in limiting nutrients, the typical environment of tumor cells. Our findings provide a molecular basis for increased TFRC1 expression in human tumors, pinpoint the role of TFRC1 in the c-Myc target gene network, and support the idea that TFRC1 may be an attractive target for cancer therapy.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]