Reduction of transferrin receptor expression by interferon gamma in a human cell line sensitive to its antiproliferative effect

Mechanism of Decrease in Transferrin Receptor Synthesis by Interferon-α Treated Human Lymphoblastoid Cells

To clarify the mechanism of antiproliferative action of interferon-α (IFN-α) in hematological malignancy, we examined the transferrin receptor system in the lymphoblastoid cell line, Daudi cells treated with IFN-α. When cells were cultured with 10(4)U/ml of IFN-α, the number of surface transferrin receptors was decreased to 60% of that seen in the control culture. This decrease was not neutralized by co-incubation with the iron chelator, desferrioxamine (10-200 μM), suggesting that the change in the level of chelatable iron did not account for the decrease in transferrin receptor numbers. When determined by metabolic labeling using (35)S-methionine, IFN-α markedly decreased the rate of transferrin receptor biosynthesis. Uptake of iron and the cellular ferritin content also decreased by 50% when incubated with 10(4)U/ml of IFN-α. These data indicate that IFN-α inhibits transferrin receptor biosynthesis in an iron-independent fashion and the subsequent cellular iron-deficiency state may play a role in the antiproliferative action of IFN-α.

Role of plasma-membrane-bound sialidase NEU3 in clathrin-mediated endocytosis

Gangliosides are sialic acid-containing glycosphingolipids mainly expressed at the outer leaflet of the plasma membrane. Sialidase NEU3 is a key enzyme in the catabolism of gangliosides with its up-regulation having been observed in human cancer cells. In the case of CME (clathrin-mediated endocytosis), although this has been widely studied, the role of NEU3 and gangliosides in this cellular process has not yet been established. In the present study, we found an increased internalization of Tf (transferrin), the archetypical cargo for CME, in cells expressing complex gangliosides with high levels of sialylation. The ectopic expression of NEU3 led to a drastic decrease in Tf endocytosis, suggesting the participation of gangliosides in this process. However, the reduction in Tf endocytosis caused by NEU3 was still observed in glycosphingolipid-depleted cells, indicating that NEU3 could operate in a way that is independent of its action on gangliosides. Additionally, internalization of α2-macroglobulin and low-density lipoprotein, other typical ligands in CME, was also decreased in NEU3-expressing cells. In contrast, internalization of cholera toxin β-subunit, which is endocytosed by both clathrin-dependent and clathrin-independent mechanisms, remained unaltered. Kinetic assays revealed that NEU3 caused a reduction in the sorting of endocytosed Tf to early and recycling endosomes, with the Tf binding at the cell surface being also reduced. NEU3-expressing cells showed an altered subcellular distribution of clathrin adaptor AP-2 (adaptor protein 2), but did not reveal any changes in the membrane distribution of clathrin, PtdIns(4,5)P2 or caveolin-1. Overall, these results suggest a specific and novel role of NEU3 in CME.

The Role of the Immune Response in Chlamydia trachomatis Infection of the Male Genital Tract: A Double-Edged Sword

Chlamydia trachomatis (CT) is the most prevalent bacterial sexually transmitted infection in the world, with more than 100 million cases reported annually. While there have been extensive studies into the adverse effects that CT infection has on the female genital tract, and on the subsequent ability of these women to conceive, studies into the consequences on male fertility have been limited and controversial. This is in part due to the asymptomatic nature of the infection, where it is estimated that 50% of men with Chlamydia fail to show any symptoms. It is accepted, however, that acute and/or persistent CT infection is the causative agent for conditions such as urethritis, epididymitis, epididymo-orchitis, and potentially prostatitis. As with most infections, the immune system plays a fundamental role in the body’s attempts to eradicate the infection. The first and most important immune response to Chlamydia infection is a local one, whereby immune cells such as leukocytes are recruited to the site of infections, and subsequently secrete pro-inflammatory cytokines and chemokines such as interferon gamma. Immune cells also work to initiate and potentiate chronic inflammation through the production of reactive oxygen species (ROS), and the release of molecules with degradative properties including defensins, elastase, collagenase, cathespins, and lysozyme. This long-term inflammation can lead to cell proliferation (a possible precursor to cancer), tissue remodeling, and scarring, as well as being linked to the onset of autoimmune responses in genetically disposed individuals. This review will focus on the ability of the immune system to recognize and clear acute and persistent chlamydial infections in the male genital tract, and on the paradoxical damage that chronic inflammation resulting from the infection can cause on the reproductive health of the individual.

Involvement of proteases in the action of IFN-gamma on WISH cells

This study investigates the possible involvement of serine proteases in interferon-gamma (IFN-gamma) activity on WISH cells. It was observed that inhibition of (3)H-thymidine incorporation induced by IFN-gamma was abrogated by the serine protease inhibitors Nalpha-tosyl-L-lysyl-chloromethane and soybean trypsin inhibitor, both of which act mainly on trypsin. Phenylmethyl sulfonyl fluoride also had a partial inhibitory effect. Other protease inhibitors specific to the cysteine, the aspartic, and the metalloprotease families were not effective. Kinetic analysis revealed that a trypsin-like protease is involved in IFN-gamma activity for up to 7 h. Trypsin-like activity induced by IFN-gamma was detected in the particulate fraction but not in the cytosolic fraction, whereas chymotrypsin activity was not enhanced in either the cytosolic or particulate fractions under similar conditions. Following separation on a gelatin substrate gel, two trypsin-like protease activities located in the particulate fraction were found to increase in response to IFN-gamma treatment. Hence, it seems that a specific membrane-associated trypsin-like protease activity induced by IFN-gamma may play a role in the action of the cytokine on thymidine incorporation in WISH cells.

Post-transcriptional regulation of transferrin receptor mRNA by IFN gamma

IFN gamma inhibits the rise in transferrin receptor mRNA level which is normally observed when stationary WISH cells are stimulated to proliferate. This effect is not attributable to a change in the transcription rate of the transferrin receptor gene or in the cytoplasmic stability of the mRNA. The IFN gamma-induced reduction of the transferrin receptor mRNA content is already present at the nuclear level to an extent comparable to that observed in whole cells. Thus, IFN gamma does not impair the passage of this mRNA from the nuclear to the cytoplasmic compartment but probably interferes with a nuclear post-transcriptional event during the processing of the immature transferrin receptor mRNA. Two different levels of regulation of transferrin receptor mRNA have been previously reported. Iron modulates the cytoplasmic stability of this mRNA through the binding of a specific cytoplasmic factor, whereas cell growth variation influences the transcription of this gene. Our results suggest the existence of another mechanism of regulation for transferrin receptor gene expression not so far considered. Furthermore, the distinction between the mechanism of regulation exerted by IFN gamma and that exerted by cell proliferation on transferrin receptor gene expression suggests that, in WISH cells, the IFN-induced transferrin receptor decay is not a consequence of cell growth arrest but rather one of the causes of the antiproliferative effect of IFN through iron deprivation.

A sensitive two-site enzyme immunoassay for the detection of rat interferon-gamma in biological fluids

Two noncompeting monoclonal antibodies (mAbs) directed to rat interferon-gamma (IFN-gamma) were used in a sensitive two-site enzyme-linked immunosorbent assay (ELISA). This permitted quantitation of rat IFN-gamma in culture medium, and serum and was quicker and more sensitive than the conventional antiviral bioassay. A standard curve was linear between 0.1 (15 pg) and 10 (1,500 pg) U/ml. There was no reaction with rat IFN-alpha or -beta or human IFN-gamma, but mouse IFN-gamma was detected with a lower limit of sensitivity of 300 U/ml. Complement-dependent inactivation of rat IFN-gamma reduced the sensitivity of the ELISA approximately seven-fold but this could be overcome by adding ethylenediamine tetraacetic acid (EDTA, 10 mM) or other anticomplementary substances to the serum samples. IFN-gamma activity determined by ELISA and bioassay decreased in parallel upon heat denaturation and pH 2.5 treatment, but proteases caused a relatively greater reduction in biological activity.