Notably, β-thalassemia is highly reported in the Mediterranean and Southeast Asian countries as one of the most common genetic disorders
[5]. Nevertheless, a lack of information on knowledge, attitudes, and practices towards β-thalassemia poses a challenge in many countries, including Malaysia
[5]. This disease can be categorized into β-thalassemia major, intermedia, or minor
[6]. β-thalassemia major and intermedia are genetically homozygous or heterozygous (β
0 and β
+), whereas β-thalassemia minor is usually heterozygous
[6].
The β globin chain is encoded by the β globin gene
(HBB), located on chromosome 11 at the short arm position 15.4
[7,8][7][8]. Individuals with β-thalassemia major and intermedia inherit the mutation in both copies of the
HBB gene, affecting normal β globin chain production
[7]. The clinical features of β-thalassemia major are revealed as early as the first two years of the patient’s life and are usually connected to transfusional iron overload, whereas the clinical presentation for β-thalassemia intermedia occurs later in life
[2]. In contrast, β-thalassemia minor or trait has one mutation in the HBB gene and is described as a carrier
[6]. They are usually symptomless, with a hypochromic microcytic blood picture and mild anemia, and can potentially increase in severity with malnutrition
[6].
Beta-thalassemia can be present alongside other diseases associated with an abnormal β globin chain, such as the hemoglobin E (HbE) disease, exhibiting severe anemia
[2]. HbE is a hemoglobin (Hb) variant caused by a single base substitution of glutamic acid to lysine at position 26 of the globin chain, commonly found in Southeast Asia
[9]. It can be classified into three types: heterozygous, homozygous, or compound heterozygous
[9]. When the HbE trait is coinherited with β-thalassemia, it is called compound heterozygous, a condition known as HbE/β-thalassemia, which resembles homozygous β
0-thalassemia clinically and hematologically
[10].
3. TGF-β/SMAD Signaling
SMADs are proteins that are activated by the transforming growth factor β (TGF-β), BMP signaling, to mediate cell proliferation and differentiation
[132,133][56][57]. Endosome-associated Fab1 (yeast orthologue of PIKfyve, YOTB, vesicle transport protein (Vac1), and EEA1 (FYVE zinc finger domain)-domain protein (endofin)) influences hepcidin expression by regulating SMAD1/5/8 phosphorylation
[134][58]. STAT and SMAD signaling regulate
hepcidin expression
[135][59]. Ablation of SMAD4, specifically in the liver, triggers an iron overload in multiple organs due to decreased levels of liver hepcidin
[136][60]. However, SMAD7 acts as an effective inhibitor of
hepcidin mRNA expression through a negative regulation effect on TGF-β and BMP/SMAD signaling
[137,138][61][62]. TGF-β is the prototypical ligand of the TGF-β superfamily, which signals during activation of serine/threonine receptor kinases. This superfamily is subdivided into the TGFβ/activin branch and BMP/growth and differentiation factor (GDF) branch.
TGF-β is expressed in most cell types and translated into a proprotein that is proteolytically cleaved into a noncovalently linked mature TGF-β and latency-associated protein (LAP)
[139,140][63][64]. The active TGF-β ligand is a 25 kDa dimer, covalently linked by bisulfide bonds between cysteine residues of each monomeric peptide
[139,140][63][64].
Various mechanisms are used to regulate the bioavailability of TGF-β in vivo. Once the bioavailable TGF-β reaches the target cell’s surface, it bonds with the homodimer of TGF-β type II receptor (TβRII)
[141][65]. The TGF-β–TβRII complex provides a structural interface that forms a stable complex with the homodimer of the TGF-β type I receptor (TβRI)
[132][56]. Subsequently, the active receptor–ligand complex is a heterotetrametric complex composed of TGFβ dimer and homodimer of TβRII and TβRI. In the active receptor complex, TβRII is constitutively activated and stimulates the transphosphorylation of TβRI
[132,142][56][66]. In the TGF-β pathway, SMAD2 and SMAD3 are receptor-regulated effector proteins (R-SMADs) phosphorylated by activated TβRI on the C-terminal SSXS motif, leading to the nuclear accumulation of R-SMAD
[132][56].
The activated receptor complex bound to the ligand is internalized by endocytosis
[143][67]. Internalization of cell surface receptors can occur through clathrin-mediated or caveolae-mediated endocytosis
[144][68].
Upon ligand stimulation, the SMADs accumulate in the nucleus as R-SMAD/CO-SMAD complex, leading to a decrease in their nuclear export rate
[145,146][69][70]. The SMAD complex binds to DNA with other transcription factors and interacts with the general transcription machinery to regulate the expression of target genes (
Figure 51)
[147][71].
Figure 51. Signal transduction of TGF-β and BMP. The binding of TGF-β to the TβRII dimer allows the ligand to bind to the TβRI dimer and stimulate TβRI kinase activity. In SMAD-mediated TGF-β signal transduction, TβRI phosphorylates cytoplasmic SMAD2 and SMAD3, which interact with SMAD4 after dissociating from TβRI. The two receptors activate the trimeric complex of SMAD2 and SMAD3, and a SMAD4 then enters the nucleus, where it interacts with the DNA-binding transcription factor (TF) and coregulators of the target gene. Similarly, the BMP signals run parallel to the TGF-β signals. In response to the binding of the BMP ligand to the BMPRII heteromeric receptor complex and BMPRI transmembrane kinase, receptor-activated SMAD1 and SMAD5 bind to SMAD4 and are transported to the nucleus to activate or inhibit transcription of hepcidin. ➔: Activation and ⊥: Inhibit.
Signaling pathways such as STAT and SMAD regulate the expression of
hepcidin. Therefore, it is hypothesized that STAT and SMAD could be dephosphorylated in thalassemia cases, including HbE/β-thalassemia patients, resulting in
hepcidin downregulation with the presence of iron accumulation.
4. Conclusions
In summary, iron homeostasis dysregulation has a dominant physiological effect on transfusion-dependent and transfusion-independent β-thalassemia patients. Thus, understanding the expression of
hepcidin and its regulation in β-thalassemia patients is vital in developing rational therapeutic interventions to provide safe, effective, and lifelong treatment options for their management. Therefore, recovery of
hepcidin in β-thalassemia patients through the activation of STAT 3, STAT 5, SMAD 1/5/8, and SMAD 4 signaling could be a potential therapeutic target for managing iron overload (
Figure 62). Therefore, it is highly recommended for future preclinical and clinical studies to evaluate the related risks and benefits of
hepcidin-targeted treatment approaches.
Figure 62. Regulation of STAT and SMAD signaling pathway on hepcidin expression.