Transferrin receptor mRNA proteins

Under low-iron conditions, IRE-BP binds to these IREs. However, given the location of these binding sites, the transferrin-receptor mRNA can still be translated. What happens when the iron level increases and the IRE-BP no longer binds transferrin-receptor mRNA Freed from the IRE-BP, transferrin-receptor mRNA is rapidly degraded. Thus, an increase in the cellular iron level leads to the destruction of transferrin-receptor mRNA and, hence, a reduction in the production of transferrin-receptor protein. 

Figure 31.39. Transferrin-receptor mRNA. This mRNA has a set of ironresponse elements (IREs) in its 3 untranslated region. The binding of the IRE-binding protein to these elements stabilizes the mRNA but does not interfere with translation.

Coordinate translational regulation of ferritin mRNA and transferrin receptor mRNA in nonerythroid cells. Iron regulatory proteins (IRP) are RNA-binding proteins that bind to iron regulatory elements (IREs). IREs are hairpin structures with loops consisting of CAGUGN sequences and are located at the 5 -untranslated region (UTR) and 3 -UTR for ferritin mRNA and transferrin mRNA, respectively. 

Cytoplasmic aconitase binding results in an increase in the steady-state level of transferrin receptor mRNA, and subsequently TfR protein. Once the intracellular levels of iron return to normal, IRE binding activity of cytoplasmic aconitase is inactivated and TfR mRNA is rapidly degraded. 

Fig. 16.24. Regulation of degradation of the mRNA for the transferrin receptor. Degradation of the mRNA is prevented by binding of the iron response element binding protein (IRE-BP) to iron response elements (IRE), which are hairpin loops located at the 3 -end of the transferrin receptor mRNA. When iron levels are high, IRE-BP binds iron and is not bound to the mRNA. The mRNA is rapidly degraded, preventing synthesis of the transferrin receptor.

Regulation of transcription by iron. A cell s ability to acquire and store iron is a carefully controlled process. Iron obtained from the diet is absorbed in the intestine and released into the circulation, where it is bound by transferrin, the iron transport protein in plasma. When a cell requires iron, the plasma iron-transferrin complex binds to the transferrin receptor in the cell membrane and is internalized into the cell. Once the iron is freed from transferrin, it then binds to ferritin, which is the cellular storage protein for iron. Ferritin has the capacity to store up to 4,000 molecules of iron per ferritin molecule. Both transcriptional and translational controls work to maintain intracellular levels of iron (see Figs. 16.23 and 16.24). When iron levels are low, the iron response element binding protein (IRE-BP) binds to specific looped structures on both the ferritin and transferrin receptor mRNAs. This binding event stabilizes the transferrin receptor mRNA so that it can be translated and the number of transferrin receptors in the cell membrane increased. Consequently, cells will take up more iron, even when plasma transferrin/iron levels are low. The binding of IRE-BP to the ferritin mRNA, however, blocks translation of the mRNA. With low levels of intracellular iron, there is little iron to store and less need for intracellular ferritin. Thus, the IRE-BP can stabilize one mRNA, and block translation from a different mRNA. 

Two well characterized internal loops that serve as protein recognition elements in an RNA mediated regulation are the Iron Responsive Element, IRE, found in the 5 -untranslated region (UTR) of ferritin mRNA and the 3 UTR of transferrin receptor mRNA and the Rev Response Element, RRE, found in the env gene of the HIV-1 retrovirus. These RNA elements are the binding sites of the IRP and Rev proteins respectively. 

Formation of a site-specific mRNA-protein complex is involved in the translational control of the biosynthesis of ferritin, an iron storage protein, which is stimulated in response to the presence of iron. In this instance, a cytoplasmic repressor protein of 85 kDa binds to a highly conserved 28-nucleotide stem-loop structure in the 5 untranslated region of ferritin mRNAs in the absence of iron. In the presence of iron, the protein dissociates from the mRNA, which is then available for translation. A similar loop motif occurs in the 3 untranslated region of transferrin receptor mRNA, which is also subject to translational control by an iron-responsive repressor. 

Synthesis of the transferrin receptor (TfR) and that of ferritin are reciprocally linked to cellular iron content. Specific untranslated sequences of the mRNAs for both proteins (named iron response elements) interact with a cytosolic protein sensitive to variations in levels of cellular iron (iron-responsive element-binding protein). When iron levels are high, cells use stored ferritin mRNA to synthesize ferritin, and the TfR mRNA is degraded. In contrast, when iron levels are low, the TfR mRNA is stabilized and increased synthesis of receptors occurs, while ferritin mRNA is apparently stored in an inactive form. This is an important example of control of expression of proteins at the translational level. 

Finally, we remind our reader that in its apoform, cytoplasmic aconitase is active as an ion regulatory protein (see Chapter 8), binding to iron regulatory elements in the mRNAs of ferritin and transferrin receptor and regulating their translation. 

Figure 15.18 The role of the iron-regubting protein on translation of mRNA for the transferrin receptor number in the membrane and the concentration of apoferritin. IRP decreases the rate of degradation of mRNA for translation of the transferrin receptor which increases the number of the receptor molecules. IRP decreases directly the rate of translation of mRNA for apoferritin.

Ferritin and transferrin receptor expression levels are reciprocally related in their responses to changes in iron levels. When iron is scarce, the amount of transferrin receptor increases and little or no new ferritin is synthesized. Interestingly, the extent of mRNA synthesis for these proteins does not change correspondingly. Instead, regulation takes place at the level of translation. 

The levels of transferrin receptor and ferritin are regulated in a coordinated manner. When a cell needs more iron, the transferrin receptor increases in number in the plasma membrane of the cell, thus promoting uptake of Fe. At the same time, ferritin synthesis decreases, promoting the use of iron by Fe-requiring proteins in the cell. The levels of transferritin and ferritin are controlled by changes in the mRNA coding for these proteins, as detailed later in this section. 

When iron is low in the cell, the mRNA for ferritin is translated at a lesser rate (resulting in lesser amoimts of this protein in the cell). The goal here is to cut down on the excessive synthesis of our major iron storage protein, if no excess iron is available for storing. When iron is low in the cell, the mRNA for the transferrin receptor is translated more (creating more transferrin). When iron is low in the cell, the mRNA for 5-aminolevulinate synthase, an enz)mie in the heme biosynthetic pathway, is translated less. The overall goal here is to cut down on the wasteful synthesis of heme, if no iron is available for completing this cofactor. 

Heme synthesis is controlled by a regulatory negative feedback loop in which heme inhibits the activity of fer-rochelatase and acquisition of iron fi om the transport protein transferrin. The decrease in iron acquisition leads to a decrease in iron uptake into the cell with subsequent decrease in 8-aminolevulinic acid and heme production. Iron deficiency and increased erythropoietin synthesis lead to the combination of the iron regulatory proteins with the iron-responsive elements in the transferrin receptor protein messenger ribonucleic acid (mRNA). This combination in turn leads to protection of the mRNA from degradation with subsequent increased uptake of iron into erythroid cells because of the increased expression of transferrin receptors on the cell membrane. 

---