Antisense suppression of transferrin receptor gene expression in a human hepatoma cell (HuH-7) line

Characterization of the interaction between diferric transferrin and transferrin receptor 2 by functional assays and atomic force microscopy

Transferrin receptor 2 (TfR2), a homologue of the classical transferrin receptor 1 (TfR1), is found in two isoforms, alpha and beta. Like TfR1, TfR2alpha is a type II membrane protein, but the beta form lacks transmembrane portions and therefore is likely to be an intracellular protein. To investigate the functional properties of TfR2alpha, we expressed the protein with FLAG tagging in transferrin-receptor-deficient Chinese hamster ovary cells. The association constant for the binding of diferric transferrin (Tf) to TfR2alpha is 5.6x10(6) M(-)(1), which is about 50 times lower than that for the binding of Tf to TfR1, with correspondingly reduced rates of iron uptake. Evidence for Tf internalization and recycling via TfR2alpha without degradation, as in the TfR1 pathway, was also found. The interaction of TfR2alpha with Tf was further investigated using atomic force microscopy, a powerful tool used for investigating the interaction between a ligand and its receptor at the single-molecule level on the living cell surface. Dynamic force microscopy reveals a difference in the interactions of Tf with TfR2alpha and TfR1, with Tf-TfR1 unbinding characterized by two energy barriers, while only one is present for Tf-TfR2. We speculate that this difference may reflect Tf binding to TfR2alpha by a single lobe, whereas two lobes of Tf participate in binding to TfR1. The difference in the binding properties of Tf to TfR1 and TfR2alpha may help account for the different physiological roles of the two receptors.

Tachpyridine, a metal chelator, induces G2 cell-cycle arrest, activates checkpoint kinases, and sensitizes cells to ionizing radiation

Iron is critical for cell growth and proliferation. Iron chelators are being explored for a number of clinical applications, including the treatment of neurodegenerative disorders, heart disease, and cancer. To uncover mechanisms of action of tachpyridine, a chelator currently undergoing preclinical evaluation as an anticancer agent, cell-cycle analysis was performed. Tachpyridine arrested cells at G2, a radiosensitive phase of the cell cycle, and enhanced the sensitivity of cancer cells but not nontransformed cells to ionizing radiation. G2 arrest was p53 independent and was accompanied by activation of the checkpoint kinases CHK1 and CHK2. G2 arrest was blocked by UCN-01, a CHK1 inhibitor, but proceeded in CHK2 knock-out cells, indicating a critical role for CHK1 in G2 arrest. Tachpyridine-induced cell-cycle arrest was abrogated in cells treated with caffeine, an inhibitor of the ataxia-telangiectasia mutated/ataxia-telangiectasia-mutated and Rad3-related (ATM/ATR) kinases. Further, G2 arrest proceeded in ATM-deficient cells but was blocked in ATR-deficient cells, implicating ATR as the proximal kinase in tachpyridine-mediated G2 arrest. Collectively, our results suggest that iron chelators may function as antitumor and radioenhancing agents and uncover a previously unexplored activity of iron chelators in activation of ATR and checkpoint kinases.

Recycling, degradation and sensitivity to the synergistic anion of transferrin in the receptor-independent route of iron uptake by human hepatoma (HuH-7) cells

To secure iron from transferrin, hepatocytes use two pathways, one dependent on transferrin receptor (TfR 1) and the other, of greater capacity but lower affinity, independent of TfR 1. To clarify further similarities and differences of the two pathways, we have suppressed TfR 1 by 75-80% in human hepatoma-derived HuH-7 cells co-transfected with vectors bearing full-length TfR 1 cDNA or its first 100 bases in antisense orientation. Suppression of TfR 1 does not lead to down regulation of TfR 2, a recently described second transferrin receptor of as yet uncertain function. Both pathways depend on acidification of the compartments in which iron release from transferrin takes place. Recycling of transferrin is a feature of both pathways, but is substantially more efficient in the receptor-dependent route. Degradation of transferrin occurs only in the receptor-independent route, in the first example of a specific catabolic pathway of transferrin. Linkage of cellular iron uptake to release of the synergistic anion (without which iron is not bound by transferrin) is particularly evident in the receptor-independent pathway. Although the relative importance of the two pathways in normal and deranged hepatic iron metabolism remains to be determined, the receptor-independent route is a substantial accessory for iron uptake to the better-known receptor-dependent track.

Effects of cell proliferation on the uptake of transferrin-bound iron by human hepatoma cells

The effects of cellular proliferation on the uptake of transferrin-bound iron (Tf-Fe) and expression of transferrin receptor-1 (TfR1) and transferrin receptor-2 (TfR2) were investigated using a human hepatoma (HuH7) cell line stably transfected with TfR1 antisense RNA expression vector to suppress TfR1 expression. At transferrin (Tf) concentrations of 50 nmol/L and 5 micromol/L, when Tf-Fe uptake occurs by the TfR1- and TfR1-independent (NTfR1)-mediated process, respectively, the rate of Fe uptake by proliferating cells was approximately 250% that of stationary cells. The maximum rate of Fe uptake by the TfR1- and NTfR1-mediated process by proliferating cells was increased to 200% and 300% that of stationary cells, respectively. The maximum binding of Tf by both TfR1- and NTfR1-mediated processes by proliferating cells was increased significantly to 160% that of stationary cells. TfR1 and TfR2-alpha protein levels expressed by proliferating cells was observed to be approximately 300% and 200% greater than the stationary cells, respectively. During the proliferating growth phase, expression of TfR1 messenger RNA (mRNA) increased to 300% whereas TfR2-alpha mRNA decreased to 50% that of stationary cells. In conclusion, an increase in Tf-Fe uptake by TfR1-mediated pathway by proliferating cells was associated with increased TfR1 mRNA and protein expression. An increase in Tf-Fe uptake by NTfR1-mediated pathway was correlated with an increase in TfR2-alpha protein expression but not TfR2-alpha mRNA. In conclusion, TfR2-alpha protein is likely to have a role in the mediation of Tf-Fe uptake by the NTfR1 process by HuH7 hepatoma cell in proliferating and stationary stages of growth.

Iron transport

Iron homeostasis is maintained by regulating its absorption: Under conditions of deficiency, assimilation is enhanced but iron uptake is otherwise limited to prevent toxicity due to overload. Iron deficiency remains the most important micronutrient deficiency worldwide, but increasing awareness of the genetic basis for iron-loading diseases points to iron overload as a major public health issue as well. Recent identification of mutant alleles causing iron uptake disorders in mice and humans provides new insights into the mechanisms involved in iron transport and its regulation. This article summarizes these discoveries and discusses their impact on our current understanding of iron transport and its regulation.

Transferrin receptor overexpression enhances transferrin responsiveness and the metastatic growth of a rat mammary adenocarcinoma cell line

We previously found that breast cancer cell transferrin receptor expression and proliferative response to transferrin often correlated with metastatic capability. To further explore this, we transfected mammary tumor cells with a cDNA coding for the transferrin receptor and examined the effects of its overexpression on various cellular properties. A human transferrin receptor expression plasmid was made by excising the cDNA for the receptor from pcDTR1 and ligating it into the multiple cloning site of pcDNAINeo. The resulting construct was transfected into the poorly metastatic rat MTLn2 line that expresses low endogenous levels of rat transferrin receptor, and transfection-induced receptor expression was ascertained using antibodies specific for the human protein. Approximately 50% of the initial geneticin-resistant transfected MTLn2 cells overexpressed human transferrin receptor protein. High expressors were further isolated by four sequential FACS sorts. The final cell population expressed approximately 3-7 times more cell surface transferrin receptor than did vector transfected controls. Both lines proliferated at the same rate in normal (medium plus 5% FBS) culture conditions. However, in serum-free conditions, the transferrin receptor overexpressor cells displayed a pronounced proliferative response to transferrin whereas the control line did not. When injected into the mammary fat pads of female nude mice, cells from both lines formed micrometastases to the lung that were specifically visualized by immunohistochemical staining of rat cytokeratin 17. This revealed that the transferrin receptor transfected line formed larger lesions of this nature than did cells from the vector transfected controls. When injected into the tail vein of female nude mice, the transferrin receptor overexpressors likewise formed gross lung metastases of remarkably greater size than did the vector only transfectants. Overexpression of cell surface human transferrin receptor on MTLn2 cells appeared to affect their in vitro growth response to transferrin and their ability to grow at a secondary site in vivo.

Mechanisms of ferric citrate uptake by human hepatoma cells

The mechanisms of uptake of non-transferrin-bound iron by human hepatoma cells (HuH7) were investigated using 59Fe-citrate and [14C]citrate. The amount of iron associated with the cells increased linearly with time, whereas citrate uptake reached a plateau after 45 min, resulting in a cellular accumulation of iron over citrate. The cells displayed high-affinity membrane binding sites for citrate with maximum binding of 118 +/- 17 pmol citrate/mg protein and a dissociation constant of 21 +/- 2 microM (n = 3). Iron uptake was saturable with a maximum uptake rate of 1.95 +/- 0.43 pmol . mg protein-1 . min-1 and an apparent Michaelis constant of 1.1 +/- 0.1 microM. Nonradioactive ferric citrate and citrate inhibited 59Fe uptake to a similar degree. This suggests that the uptake of citrate-bound iron is dependent on either binding to specific citrate binding sites or the concentration of unbound iron. The uptake of iron was inhibited by ferricyanide (>100 microM) and ferrous iron chelators but stimulated by ferrocyanide and ascorbate, suggesting that the iron is reduced from Fe3+ to Fe2+ and transported into the cell by an iron carrier-mediated step.

Recent status of the antisense oligonucleotide approaches in oncology

Antisense oligonucleotides designed to complement a region of a particular messenger RNA may inhibit gene expression potentially through sequence-specific hybridization. Their inhibiting effect has been shown in a variety of in vitro and in vivo models in oncology, whereas much rarer clinical trials have been carried out. Rigorous demonstration of in vitro and in vivo specific effects upon their targets is mandatory before their use as drugs in cancer therapy.

Loss of E-cadherin-dependent cell-cell adhesion due to mutation of the beta-catenin gene in a human cancer cell line, HSC-39

Detachment of cell-cell adhesion is indispensable for the first step of invasion and metastasis of cancer. This mechanism is frequently associated with the impairment of either E-cadherin expression or function. However, mechanisms of such abnormalities have not been fully elucidated. In this study, we demonstrated that the function of E-cadherin was completely abolished in the human gastric cancer cell line HSC-39, despite the high expression of E-cadherin, because of mutations in one of the E-cadherin-associated cytoplasmic proteins, beta-catenin. Although immunofluorescence staining of HSC-39 cells by using an anti-E-cadherin antibody (HECD-1) revealed the strong and uniform expression of E-cadherin on the cell surface, cell compaction and cell aggregation were not observed in this cell. Western blotting (immunoblotting) using HECD-1 exhibited a 120-kDa band which is equivalent to normal E-cadherin. Northern (RNA) blotting demonstrated a 4.7-kb band, the same as mature E-cadherin mRNA. Immunoprecipitation of metabolically labeled proteins with HECD-1 revealed three bands corresponding to E-cadherin, alpha-catenin, and gamma-catenin and a 79-kDa band which was apparently smaller than that of normal beta-catenin, indicating truncated beta-catenin. The 79-kDa band was immunologically identified as beta-catenin by using immunoblotting with anti-beta-catenin antibodies. Examination of beta-catenin mRNA by the reverse transcriptase-PCR method revealed a transcript which was shorter than that of normal beta-catenin. The sequencing of PCR product for beta-catenin confirmed deletion in 321 bases from nucleotides +82 to +402. Southern blotting of beta-catenin DNA disclosed mutation at the genomic level. Expression vectors of Beta-catenin were introduced into HSC-39 cells by transfection. In the obtained transfectants, E-cadherin-dependent cell-cell adhesiveness was recovered, as revealed by cell compaction, cell aggregation, and immunoflourescence staining. From these results, it was concluded that in HSC-39 cells, impaired cell-cell adhesion is due to mutations in beta-catenin which results in the dysfunction of E-cadherin.