Iron Homeostasis and Methionine-Centered Redox Cycle in Aged Rat Spleen.

Abstract 4049

Poster Board III-984

Progressive oxidation of cellular components is a major aspect of most mechanisms of aging. Commonly, this is associated with a marked decrease in the tissue levels of anti-oxidant proteins and low molecular weight molecules. Splenic lymphocytes play important roles in the aging process and in age-related pathologies, including the progressive decline of the immune system and a concomitant increased frequency of infectious diseases. Labile and redox-active cellular iron (LIP) is known to be involved in redox cycling and in the formation of reactive oxygen-derived species (ROS), leading to oxidative stress. Neutralizing the hazardous potential of ROS, by blocking its formation with Iron-selective chelation is one of most important mechanisms of protection. Scavenging of LIP by ferritin (Ft) protein, is especially relevant for the spleen, where iron accumulation is higher than in other organs. Another protective mechanism is the Methionine-Centered Redox-Cycle (MCRC) proteins, which responsible for the reducing of oxidized methionine (Met(O)) and damaged proteins. We studied age-related changes in these two major antioxidant systems in rat spleen. In aged rats (24-25 months old) the splenic Ft level was 2.2-fold of that in young ones (2 months old). Relative iron saturation of Ft was elevated by a factor of 1.3, and the total Ft-bound-iron was 2.8-fold in the old spleen than the corresponding young one (p<0.05), indicating respective elevated iron accumulation with age. The experimental ratios for “old/young” MCRC proteins levels, in the spleen, were: methionine sulfoxide reductase A and B (MsrA and MsrB) – 0.5 and 0.7 respectively, thioredoxin1 (Trx) –1.9 and Trx Reductase (TrxR) –1.5 (all p<0.05). The maximal Msr enzymatic activity decreased with age by 40%, demonstrating a decline in the ability to reduce oxidized methionine. An age-dependant increase in the expression of the genes (as indicated by an elevation in the level of the corresponding mRNAs) for the above discussed proteins were observed. This was particularly prominent for the Ft-subunits - Ft-H and Ft-L, and for the MsrA mRNA. When livers from old and young rats were studied, comparable results were found. The liver is the major iron storage site in the body and is rich in RE (reticulo-endothelial) cells. A 2.9-fold Ft accumulation and 1.3 fold increase in Ft-iron were found in livers of old rats, than in young ones (p<0.05). Also, the total Ft-bound-iron increased 3.6-fold in the old livers. Partially similar trend was observed in the heart (Bulvik et-al, 2009, Mech Ageing Dev 130:139-44). Heart of aged rat contained double the amount of Ft than those of young ones, but the iron concentration in the Ft molecule was halved, with no accumulation of total iron Ft. Thus, iron depended Ft accumulation were found in the aged spleen and liver, but not in heart, i.e., only in organs specializing in iron storage. MsrA and MsrB level decreased drastically with age, more in the liver then in the spleen, reaching 30 and 60% of the values of young livers, respectively (p<0.05). In the heart these levels increased (1.7- and 1.3-fold for MsrA and MsrB, respectively; p<0.05). The hearts Msr activity decreased by 20% with age, however, in aged rat liver Msr activity increased by 50%. The Trx and TrxR levels in the liver and the heart were unchanged by aging, in contrast to the spleen where an increase in those proteins level was observed. These observations suggest that the age–dependent increase in Ft levels is dependent on the accumulation of Ft-bound iron, which takes place in iron storage organs only. Msr proteins level decreases with age only in iron storage organs and its activity is organ dependent. Administration of selective iron chelators along aging might limit the age-related accumulation of iron in its storage organs.