Background
Discovered in 1937 by French scientist Laufberger, ferritin was first detected in horse spleen and a few years later in humans [1]. Ferritin is a protein that is responsible for iron storage and iron homeostasis, as well as various physiologic and pathological processes in prokaryotes and eukaryotes. Iron homeostasis is essential to maintaining life because iron can be toxic to DNA and proteins if not properly regulated. If there is an overload of iron, reactive oxygen species can be produced, lipid peroxidation can occur, and there can be damage to DNA. Typically, ferritin presents as a cytosolic protein, but there are also mitochondrial and nuclear forms that have recently been discovered. The most common measurement of ferritin is collected from serum ferritin, which is ferritin stored in red blood cells. Many variations of ferritin exist, as it is presumed that evolutionary adaptations were made in order to allow certain organisms to survive.
Structure
The structure of ferritin consists of a spherical apoferritin shell that has 24 subunits to form a cage that contains two types of subunits: H and L [2]. The ratio of H to L subunits is dependent upon inflammation and tissue type, and varies greatly. H-subunits are mostly found in the kidneys and heart while the L-subunits are mostly found in the liver and spleen. The genes that encode for these H and L subunits are found on chromosomes 11q and 19q [1]. For L subunits, there is only one region where at least four residues are identical throughout sequences. Each subunit is constructed from four α-helices, A, B, C, and D, which combine to form helix E. This can be seen in the tertiary structure of ferritin. In the quaternary structure, eukaryotic ferritin presents as spherical with 4-3-2 symmetry. Within the apoferritin shell is where sequestered iron is kept. It contains insoluble iron (III) oxide hydroxide and iron (III) phosphate. Ferritin is able to be degraded through lysosomal or proteasomal mechanisms depending on if degradation is needed.
Function
Ferritin’s main function is to convert Fe(II) to Fe(III) by acting as a ferroxidase. In a clinical setting, ferritin is used as an indicator for an iron deficiency [3]. Any extracellular ferritin can act as a carrier for iron in order to transport iron to cells. This is because each ferritin molecule can sequester a maximum of 4500 iron atoms [1]. There has also been research to show that ferritin H can suppress immune activity by inducing IL-10 in lymphocytes, and can inhibit delayed-type hypersensitivity (DTH) without having any effect of antibody mediated inflammatory responses. Ferritin has protective cages that are very large and stable, but if all of the iron is released from ferritin, then ferritin cages are degraded within the cytoplasm. If there is too much iron for ferritin to store, the iron could be stored as hemosiderin, which is a mixture of lipids and denatured proteins. The mechanism representing ferritin function is summarized in six steps: assembly of subunits, entry of Fe (II) to ferritin, binding to catalytic centers, oxidation of Fe (II), storage of Fe (III), and release of Fe (III) from the core of ferritin. High iron organs, like the heart, participate more in ferroxidase activity versus organs, like the liver, which are meant more for iron storage in the core of ferritin.
Bacterioferritin
is another ferritin molecule found in bacteria. The structure remains relatively similar with the 24 subunits that form a sphere and consist of four helix bundles surrounding a ferroxidase center. Bacterioferritin also consists of 12 hemes that are bound at 2-fold intersubunit sites [4]. B-pores, which are formed as asymmetric sites between three subunits, are lined with negatively charged residues that are also hydrophilic and are found in bacterioferritin. In studies using P. aeruginosa, it was found that there are two distinct genes that code for bacterioferritin (bfr): bfrA and bfrB. The research showed that bfrB levels were increased in response to high iron conditions and bfrA had no response to changed iron concentrations. This is due to the difference in binding sites for heme in bfrA and bfrB. BfrA has a binding site at M48, but it is too far to bind heme iron. BfrB has a binding site at M52, which is located at the center of helix B and can bind heme. Both bacterioferritin also have different ferroxidase center structures, which could have an effect on binding. In fact, there has been research to show that bfrA is a bacterial ferritin that is now referred to as ftnA. The protein that is created from bfrB still remains a true bacterioferritin [4].
Clinical Uses
Ferritin is a valuable tool in the clinical setting for evaluating iron levels and diagnosing iron deficiencies. Some of the diseases and conditions ferritin levels suggest are iron deficiency anemia, hereditary hemochromatosis, and chronic transfusion therapy. With serum ferritin being the most useful marker, it is commonly included in blood panels to diagnose these conditions. Normal serum levels for men are 30-300 ng/mL and are 10-200 ng/mL for women [1]. Anything lower than these levels is indicative of iron deficiency anemia, hypothyroidism, or ascorbate deficiency, all of which are vastly different from one another. Any serum ferritin levels that are higher than 1000 ng/mL, in male or female, are indicative of infections or cancers. Pulling from research, conditions that have been linked to increased serum ferritin levels include liver disease, renal disease, HIV, systemic infections, chronic transfusion, reactive hemophagocytic syndrome, Still’s disease and sickle cell [1]. If elevated ferritin levels are found in a critically ill patient, sepsis or multiorgan dysfunction should be considered.
Nanoparticles are essential for drug delivery and ferritin aids in the production of nanoparticles. Nanoparticles are narrow in size and have low toxicity levels in the blood. Within ferritin, nanoparticles have been synthesized through reduction of metal ions from ferritin or assembly of subunits around the nanoparticles or drugs being used. Ferritin is also useful for synthesizing other materials. Amino acids that line ferritin are able to be modified in order to develop new technological applications. An example of this is from a study surrounding the influenza virus. Hemagglutinin, the virus surface protein, had eight trimeric spikes on ferritin at four 3-fold symmetry axes. This was able to be used as a vaccine that had a stronger immune response than normal influenza vaccines [5].
Relevance to Disease/Illness
There are a few links between various cancers and ferritin. In neuroblastomas and malignancies, there are elevated ferritin levels and secretion; however, there is no research to show that the ferritin secretions from neuroblastomas change the overall serum ferritin levels. It is known that the increase in iron can be damaging to the body, so in more recent studies, it has been hypothesized that increased iron levels can increase the chances of developing breast cancer. Other cancers that have increased ferritin levels include Hodgkin’s lymphoma, cervical cancer, oral squamous cell cancer, renal cell cancer, T cell lymphoma, and CRC [6]. In all metastatic cancers, iron plays a large role in the growth stage. If there is iron induced oxidative stress in the cell environment, then there is a rise in tumor heterogeneity, which is when cells from the same tumor have diverse phenotypes. This gives rise to metastatic potential and increases the risk and/or severity of cancer. MicroRNAs control gene expression after transcription, including regulating genes involved in iron metabolism even though these microRNAs are regulated by the levels of iron. Because iron levels regulate microRNA function, and some microRNA act as tumor suppressors, a change in iron could allow oncogenes to transform and allow tumors to grow. The major source of increased ferritin levels in cancer patients is from tumor associated macrophages. These tumor associated macrophages secrete iron that metabolically reprograms the cancer cells. To try and prevent increased iron levels from leading to cancer, iron chelation therapy is done most often. In iron chelation therapy, the patient is treated with different drugs to try and remove excess iron from the body.
In recent studies, related to the COVID-19 pandemic, ferritin levels have been hypothesized as playing a crucial role in the severity of certain cases. In severe cases of COVID-19, ferritin levels were considerably higher than normal and continued to remain high as the patient battled the virus. These high levels of ferritin contributed to the initiation of a cytokine storm, which is a contributor for fatal cases of COVID-19 [7]. In patients with diabetes, serum ferritin levels are already elevated, which makes those individuals more at risk for severe complications if they fall victim to COVID-19. There is still a lot of research being done surrounding the relationship of ferritin levels and COVID-19. As of right now, there are a few treatment options to help decrease ferritin levels. One option includes deferoxamine, which is typically used for long term iron chelation therapy and has been approved by the FDA. Another option would be to alter the iron intake in diets, so that the serum ferritin levels would be regulated.
