Iron is indispensible for life and essential for such processes as oxygen transport, electron transfer and DNA synthesis. Transferrin (Tf) is a ubiquitous protein with a central role in iron transport and metabolism. There is evidence, however, that Tf has many other biological roles in addition to its primary function of facilitating iron transport and metabolism, such as its profound effect on mammalian cell growth and productivity. The multiple functions of Tf can be exploited to develop many novel applications. Indeed, over the past several years, considerable efforts have been directed towards exploring human serum Tf (hTf), especially the use of recombinant native hTf and recombinant Tf fusion proteins, for various applications within biotechnology and medicine. Here, we review some of the remarkable progress that has been made towards the application of hTf in these diverse areas and discuss some of the exciting future prospects for hTf.
"Our in vitro cellular assay data showed that the antidiabetic activity of prEx-4-Tf was improved compared with commercial Ex-4. This is highly likely to have resulted from increased stability of Ex-4 acquired through its fusion to Tf. Human Tf is very stable, possessing a t1/2 in excess of 14–17 days (Brandsma et al., 2011; Li and Qian, 2002). Therefore, if a fusion molecule between Ex-4 or any peptide and Tf can be created, the stabilizing properties of Tf will likely be imbued to the peptide. "
[Show abstract][Hide abstract] ABSTRACT: The incretin hormone glucagon-like peptide-1 (GLP-1) is recognized as a promising candidate for the treatment of type 2 diabetes (T2D), with one of its mimetics, exenatide (synthetic exendin-4) having already been licensed for clinical use. We seek to further improve the therapeutic efficacy of exendin-4 (Ex-4) using innovative fusion protein technology. Here, we report the production in plants a fusion protein containing Ex-4 coupled with human transferrin (Ex-4-Tf) and its characterization. We demonstrated that plant-made Ex-4-Tf retained the activity of both proteins. In particular, the fusion protein stimulated insulin release from pancreatic β-cells, promoted β-cell proliferation, stimulated differentiation of pancreatic precursor cells into insulin-producing cells, retained the ability to internalize into human intestinal cells and resisted stomach acid and proteolytic enzymes. Importantly, oral administration of partially purified Ex-4-Tf significantly improved glucose tolerance, whereas commercial Ex-4 administered by the same oral route failed to show any significant improvement in glucose tolerance in mice. Furthermore, intraperitoneal (IP) injection of Ex-4-Tf showed a beneficial effect in mice similar to IP-injected Ex-4. We also showed that plants provide a robust system for the expression of Ex-4-Tf, producing up to 37 μg prEx-4-Tf/g fresh leaf weight in transgenic tobacco and 137 μg prEx-4-Tf/g freshweight in transiently transformed leaves of N. benthamiana. These results indicate that Ex-4-Tf holds substantial promise as a new oral therapy for type 2 diabetes. The production of prEx-4-Tf in plants may offer a convenient and cost-effective method to deliver the antidiabetic medicine in partially processed plant food products.
"TF has been widely used as an important supplement in culture medium for various mammalian cells and stem cells because of the absolute requirement of iron for cellular growth and proliferation [4,5]. TF has also been actively pursued as a drug delivery vehicle due to its unique receptor-mediated endocytosis pathway as well as its added advantages of being biodegradable, nontoxic, and nonimmunogenic [6-9]. Moreover, TF is also exploited for oral delivery of protein-based therapeutics [10,11], as TFR is abundantly expressed in human gastrointestinal (GI) epithelium and TF is resistant to proteolytic degradation [10,12]. "
[Show abstract][Hide abstract] ABSTRACT: Background
Transferrin (TF) plays a critical physiological role in cellular iron delivery via the transferrin receptor (TFR)-mediated endocytosis pathway in nearly all eukaryotic organisms. Human serum TF (hTF) is extensively used as an iron-delivery vehicle in various mammalian cell cultures for production of therapeutic proteins, and is also being explored for use as a drug carrier to treat a number of diseases by employing its unique TFR-mediated endocytosis pathway. With the increasing concerns over the risk of transmission of infectious pathogenic agents of human plasma-derived TF, recombinant hTF is preferred to use for these applications. Here, we carry out comparative studies of the TFR binding, TFR-mediated endocytosis and cellular iron delivery of recombinant hTF from rice (rhTF), and evaluate its suitability for biopharmaceutical applications.
Through a TFR competition binding affinity assay with HeLa human cervic carcinoma cells (CCL-2) and Caco-2 human colon carcinoma cells (HTB-37), we show that rhTF competes similarly as hTF to bind TFR, and both the TFR binding capacity and dissociation constant of rhTF are comparable to that of hTF. The endocytosis assay confirms that rhTF behaves similarly as hTF in the slow accumulation in enterocyte-like Caco-2 cells and the rapid recycling pathway in HeLa cells. The pulse-chase assay of rhTF in Caco-2 and HeLa cells further illustrates that rice-derived rhTF possesses the similar endocytosis and intracellular processing compared to hTF. The cell culture assays show that rhTF is functionally similar to hTF in the delivery of iron to two diverse mammalian cell lines, HL-60 human promyelocytic leukemia cells (CCL-240) and murine hybridoma cells derived from a Sp2/0-Ag14 myeloma fusion partner (HB-72), for supporting their proliferation, differentiation, and physiological function of antibody production.
The functional similarity between rice derived rhTF and native hTF in their cellular iron delivery, TFR binding, and TFR-mediated endocytosis and intracellular processing support that rice-derived rhTF can be used as a safe and animal-free alternative to serum hTF for bioprocessing and biopharmaceutical applications.
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