The transferrin receptor part II: Targeted delivery of therapeutic agents into cancer cells

Abstract

Traditional anti-cancer treatments consist of chemotherapeutic drugs that effectively eliminate rapidly dividing tumor cells. However, in many cases chemotherapy fails to eliminate the tumor and even when chemotherapy is successful, its systemic cytotoxicity often results in detrimental side effects. To overcome these problems, many laboratories have focused on the design of novel therapies that exhibit tumor specific toxicity. The transferrin receptor (TfR), a cell membrane-associated glycoprotein involved in iron homeostasis and cell growth, has been explored as a target to deliver therapeutics into cancer cells due to its increased expression on malignant cells, accessibility on the cell surface, and constitutive endocytosis. The TfR can be targeted by direct interaction with conjugates of its ligand transferrin (Tf) or by monoclonal antibodies specific for the TfR. In this review we summarize the strategies of targeting the TfR in order to deliver therapeutic agents into tumor cells by receptor-mediated endocytosis.

Introduction

The TfR (also known as CD71), a type II transmembrane glycoprotein found as a homodimer (180 kDa) on the surface of cells, is a vital protein involved in iron homeostasis and the regulation of cell growth (recently reviewed in [1]). The TfR monomer contains a large extracellular C-terminal domain, a single-pass transmembrane domain, and a short intracellular N-terminal domain. The TfR is ubiquitously expressed on normal cells and expression is increased on cells with a high proliferation rate or on cells that require large amounts of iron [1]. Little or no TfR expression has been detected on pluripotent hematopoietic stem cells, while late erythroid and myeloid progenitor cells demonstrate TfR expression. Expression of the TfR is significantly upregulated in a variety of malignant cells and in many cases, increased expression correlates with tumor stage and is associated with poor prognosis [1].

Iron is involved in a variety of cellular processes and is a required co-factor for many enzymatic reactions including those involved in metabolism, respiration, and DNA synthesis [2], [3]. Delivery and cellular uptake of iron occurs through the interaction and internalization of iron-loaded Tf mediated by the TfR [1] (Fig. 1). Tf is a monomeric glycoprotein (apo-Tf) that can transport one (monoferric Tf) or two (diferric Tf) iron atoms. Diferric Tf has the highest affinity for the TfR and is 10- to 100-fold greater than that of apo-Tf at physiological pH [3]. Upon binding the TfR, the Tf/TfR complex is internalized in clathrin-coated pits through receptor-mediated endocytosis. Due to the decrease in pH, iron is released from transferrin in the endosome. Tf remains bound to the receptor at this pH and the apo-Tf/TfR complex is recycled back to the cell surface where apo-Tf is then released. The TfR is constitutively recycled independently of Tf binding.

A second transferrin receptor (TfR2) was identified and has a 25-fold lower affinity for Tf than TfR1 [1], [4], [5], [6]. The human TfR2 α and β transcripts are produced by alternative splicing [6]. The TfR2 α and TfR1 only show similarity in their extracellular domains. In contrast to TfR1, TfR2α expression appears to be limited to hepatocytes and enterocytes of the small intestine and is not regulated by intracellular iron levels. High surface expression of TfR2α was detected in many solid and hematopoietic malignant human cell lines. The intracellular TfR2 β (lacks the transmembrane and cytoplasmic domains) is ubiquitously expressed at low levels and its function remains unclear.

Traditional cancer therapy consists of chemotherapeutic drugs that can be successful in irradicating the tumor, but are often toxic to normal cells as well. Targeting the TfR is a promising strategy actively being explored as a drug alternative to offset these dangerous side effects. The high levels of expression of TfR in cancer cells, which may be up to 100-fold higher than the average expression of normal cells [7], [8], [9], its extracellular accessibility, its ability to internalize, and its central role in the cellular pathology of human cancer, make this receptor an attractive target for cancer therapy. In fact, the TfR can be successfully used to deliver cytotoxic agents into malignant cells including chemotherapeutic drugs, cytotoxic proteins, or high molecular weight compounds including liposomes, viruses, or nanoparticles (Fig. 2).