Physiological implications of NTBI uptake by T lymphocytes

Jorge P Pinto,Yuri Rikers,Vera Dias,Susana Oliveira,Inês Vieira,Matthijn Vos,João Arezes,Maria Rangel,Anna Carlsson,Mónica Costa,Graça Porto

doi:10.3389/fphar.2014.00024

Abstract

In iron overload disorders a significant fraction of the total iron circulates in the plasma as low molecular weight complexes not bound to transferrin, known as non-transferrin-bound iron (NTBI). By catalyzing the formation of free radicals, NTBI accumulation results in oxidative stress and cellular damage, being a major cause of organ toxicity. NTBI is rapidly and preferentially cleared from circulation by the liver and the myocardium, the main disease targets in iron overload conditions. We have recently demonstrated that human peripheral blood T lymphocytes take up NTBI in vitro, with a pattern that resembles that of hepatocytes. Since T lymphocytes constitute a numerically important component of the circulating cell pool, these findings support a putative role for this cell type in the systemic protection against iron toxicity. Here we tested the hypothesis that the circulating peripheral blood T lymphocyte pool constitutes an important storage compartment for NTBI and is thus a modifier of NTBI deposition in target organs. First we show that NTBI uptake by human T lymphocytes increases the expression of the iron-storage protein ferritin and of the iron exporter ferroportin via an IRE-dependent mechanism. NTBI retention by T lymphocytes is shown to be critically controlled by the hepcidin-mediated modulation of ferroportin both in vitro and in vivo. Finally, the protective effect of T lymphocytes was tested by analyzing the patterns of iron accumulation in the T lymphocyte-deficient mouse model Foxn1nu before and after reconstitution with T lymphocytes by adoptive transfer. The results confirmed a significant increase of liver and pancreas iron accumulation in T lymphocyte-deficient mice. NTBI accumulation in the liver and spleen was prevented by reconstitution with syngeneic T lymphocytes. Altogether, our results demonstrate that T lymphocytes are important components of a circulating “NTBI storage compartment” and show its physiological relevance as a modifier of tissue iron overload.

Highlights

Iron, the most abundant transition metal in mammalian systems, is essential for various vital metabolic processes
5 μM of Fe-citrate for 0–12 h and the expression of the iron storage protein Ferritin H (FTH) quantified by Western blot analysis. We observed that both T lymphocyte populations increase FTH levels in response to Fe-citrate, an effect abrogated by the supplementation of the culture medium with 5 μM desferrioxamine (DFO) (Figure 1A)
The changes in FTH expression are in agreement with our previous results showing that a plateau in intracellular iron is reached earlier than 6 h in the presence of a constant non-transferrin-bound iron (NTBI) concentration (Arezes et al, 2013) and suggest the capacity of the cells to store a fraction of the intracellular iron in the Ferritin core

Summary

Introduction

The most abundant transition metal in mammalian systems, is essential for various vital metabolic processes. NTBI is the main source of iron for storage in the liver (Zimelman et al, 1977; Brissot et al, 1985). In contrast to transferrin-bound-iron, NTBI is potentially toxic, causing cellular damage, of the plasma membrane and of various intracellular organelles, due to its involvement in the formation of reactive oxygen species (reviewed in Brissot et al, 2012). The uptake of NTBI by hepatocytes is viewed as a clearance mechanism of potentially toxic circulating iron that could otherwise cause damage to other cell types. When the clearance capacity of liver is exceeded, other organs, namely pancreas, heart or hypophysis, are affected by NTBI uptake and accumulation, leading to the fullblown clinical picture of severe iron overload with liver cirrhosis, diabetes, cardiomyopathy, and hypogonadotrophic hypogonadism (Pietrangelo, 2004). An effective removal of circulating NTBI is a major goal in the management of iron overload disorders

Methods

Results

Conclusion