Iron Response ElementS

STATE OF ART

INTRODUCTION

Many genes in the organism encode proteins to provide efficient and safe use elements or molecules for crucial reactions in cell division, respiration, metabolic regulation and detoxification. Genes most of all related to iron, oxygen, NO (nitric oxide), hydrogen peroxide and grown factors. They develop an important role in iron cell trafficking, heme group synthesis, ATP production and a great deal of oxidation-reduction cell reactions. Due to its importance a subset of such genes contains non-coding sequences that are transcribed to the mature mRNA to regulate protein synthesis in response to environmental changes like iron concentration, free radicals and other metabolic signals.

There are plenty pathways to regulate the concentration of a protein in the cell. Basically they can be grouped into:

- Transcriptional level: allowing or not the function of DNA polymerase and transcriptional factors over the DNA according to the stimules that surround the cell.

- Translational level: translational factors, rRNA, tRNA and the enzymes that synthetize them affect in the same way the mRNA translation rate.

- Post-translational level: there are a lot of modifications that a protein can suffer after its translation but the only ones that affects directly to the steady state concentration of it are the proteolitic reactions that are taken part by endopeptidases or the proteasome system.

- Post-transcriptional level: different elements and processes such as capping, splicing, addition of polyA tail and cis elements and trans acting factors are involved in mRNA stability, which is determinant for the protein synthesis and then proportional to the amount of protein synthetized. Just remember that the stability of an mRNA determines how rapidly the synthesis of the encoded protein can be shut down.

From all of these levels, those focused on mRNA are the most suitable for a strict and precise control. Targeting mRNA to regulate gene expression provides us an advantage because of the cell specificity and the low cell copy numbers of this molecule. As the table bellow shows, high sensitivity to small changes in cellular concentrations of regulatory signals is possible with DNA and RNA because of the low copy number present in the cell. An high selectivity of regulatory signals is possible with RNA or protein targets because of the cell specificity of them. Just RNA combines both features, being for this reason the most efficient target for regulation.
 

In this project we are going to focus on iron metabolism and its regulation. The ability to obtain and safely use this element is a central requirement for nearly all forms of live. Iron is directly associated with proteins, just think about hemoglobin where it is an essencial cofactor, being required for crucial cellular processes including oxygen transport (respiration), mitochondrial energy metabolism, electron transport, deoxynucleotide synthesis, detoxification, synthesis of many proteins such as citocroms and ribonucleoproteins reductases, and cell division. However, the biological use of iron is limited by its low solubility and the propensity of non-protein to bound free iron and make it to participate in the formation of potentially lethal oxidizing agents that damage membranes and DNA. Given then, the important biological roles of iron it is not surprising that variations in body iron status influence human and animal health. Insufficient iron uptake determines in culture the cell growth, and nutritional deprivation or malabsorption in whole organisms generates anemia. In contrast an increase of iron uptake leads to permanent cell and tissue damage. The need to mantain the iron homeostasis arises for, manteining the base levels needed to suply the main functions in which iron takes part but as well, to prevent accumulation of excess iron.

The major processes responsible for modulating mammalian iron homeostasis are intestinal absoption, interorgan transport and uptake, and cellular utilization. Mammals have several proteins that facilitate the safe and efficient transport, uptake, use and storage of iron. Interorgan transport and uptake of nonheme iron (iron that is not binded to the hemo group) is controlled by the transferrin (Tf)/transferrin receptor (TfR) system. Transferrin is a serum glycoprotein that binds at least two iron atoms. The transferrin receptor is a disulfide-linked homodimer present in the plasma membrane that binds one transferrin molecule per each monomer. Once the transferrine is attached to its receptor the hole complex is internalized by endocytosis and directed to the endosomal compartment of the cytoplasm. Endosomal acidification, to a pH around 5.5, is required for release of iron from transferrin. The way that iron transfers the endosomal membrane for getting into the cytosol is not well understood, but two proteins identified recently seem to play an important role in this process. These proteins have been called natural resistance associated macrophage protein 2 (nRAMP2), also know as divalent cation transporter 1 (DCT1), and stimulator of iron transport.

Although iron plays an important role in cell metabolism, as we have told before a high concentration of it can cause membrane and DNA damage thanks to its nature to form free reactive radicals. This is the reason why most organisms have developed a system to accumulate this metal inside the cell after its detoxification and another one to export it to the surrounding environment. The first is mediated by ferritin, an intracellular protein integrated by two chains (the light -L and the heavy one, -H) that binds preventing it to react with non-protein molecules that contribute to the free radicals formation. It generates then an intracellular store of non-toxic iron. The second system is formed by ferroportin (Fpn1, IREG1 or MTP1=metal transport protein), an iron efflux transporter.

These proteins share with those related to oxygen metabolism -concreately eALAS=aminolevulinate synthase, the enzyme that takes part in heme synthesis, and mt-aconitase the one does the same but in trichloroacetic acid cycle- a high degree of secuence homology at their 5' or 3' UTR (untranslated regions) and a regulation pathway very similar. This is because their messengers contain particular structures called IREs (Iron Response Elements). Thanks to this structures mRNAs can interact with the IRPs (Iron Regulatory Proteins) which have a determinant role in translation or mRNA stability. The two members identified till now, IRP-1 and IRP-2, show a high degree of sequence homolgy with c-aconitase and this is the reason why they are know as well as c-aconitase-like proteins.