Analysis of transferrin in urine in patients with bladder cancer using nanobodies

Abstract

It is known that the saturation ratio of transferrin (Tf) with iron in human blood is an important clinical parameter. Specific antibodies can be used to analyze subtle changes in the relative abundance of different forms of transferrin potentially associated with a pathological process. Recently, the authors of this study were able to obtain and characterize highly specific single-domain antibodies (nanobodies) that predominantly recognize the iron-saturated (holo-Tf) or iron-unsaturated (apo-Tf) form of transferrin. In this work, under conditions closer to physiological than in previous experiments, we further demonstrated that these unique nanobodies have extremely high differential binding specificity for different forms of Tf in different human biological fluids. Using these nanobodies, we were able to analyze for the first time the relative abundance of transferrin forms in urine samples from patients with bladder cancer (BC). We have shown that an increase in the concentration of total Tf in urine samples normalized for creatinine is associated with the degree of progress and growth of malignancy of BC. In the samples of healthy donors and in the early stages of BC (G1), Tf is detected in much smaller amounts (compared to later stages) and only with additional concentration of the studied samples. For most of the studied urine samples from BC patients, it is expected (as previously shown in the case of Tf in the blood of terminal ovarian cancer patients) that the concentration of apo-Tf is clearly higher than holo-Tf, especially in the case of the most advanced muscle-invasive BC. It was a surprise for us that approximately equal amounts of apo-Tf and “holo-Tf” were found in the urine samples of some patients with BC. We hypothesized that the “holo-Tf” fraction in this case can be largely represented by “secondary complexes” formed by apo-Tf in combination with ions other than Fe3+, which accumulate in the urine of some cancer patients and are able to bind to apo-Tf, changing its conformation towards holo-Tf. Using inductively coupled plasma ionization mass spectroscopy (ICP-MS), we obtained the first results confirming our hypothesis. The “holo-Tf” preparation in these urine samples was found to be highly enriched in zinc and nickel. Also in this preparation, a relative enrichment in cadmium is observed, but it is present in much lower concentrations. The data obtained indicate that the nanobody used, recognizing predominantly the iron-saturated form of transferrin (holo-Tf), is also capable of binding transferrin in association with other metal ions that are different from iron. This ability could potentially open up new possibilities in studies of the relative abundance of various metal ions in association with transferrin in human biological fluids in normal and pathological conditions.