The PDGFRB gene provides instructions for creating a protein known as platelet-derived growth factor receptor beta (PDGFRβ). This protein belongs to a family of proteins called receptor tyrosine kinases, which are located on the surface of certain cells. PDGFRβ acts as a receiver, binding to specific growth factors like platelet-derived growth factor (PDGF) outside the cell. This binding initiates a chain of signals inside the cell, influencing many cellular activities.
Cellular Communication and Growth
The PDGFRβ protein is embedded in the cell membrane. Its normal function begins when a platelet-derived growth factor (PDGF) molecule binds to it. This binding causes the PDGFRβ proteins to come together and activate each other through autophosphorylation, a process where they add phosphate groups to specific positions on themselves.
This activation triggers a cascade of internal signals within the cell. These signaling pathways, including the Ras-MAPK, PI3K, and PLC pathways, control fundamental cellular processes such as cell growth, division, movement, and survival.
PDGFRβ signaling is also involved in the development of various cell types throughout the body. For example, it plays a role in the formation of blood vessels, a process called angiogenesis, by promoting the proliferation, migration, and recruitment of pericytes and smooth muscle cells to endothelial cells. This signaling pathway also contributes to the maintenance of connective tissues in adults and is involved in wound healing.
PDGFRB’s Role in Health Conditions
When the PDGFRB gene or its protein malfunctions, it can contribute to several health conditions. Dysregulation, such as overactivity or mutations, can lead to uncontrolled cell behavior and tissue abnormalities. This can occur through various mechanisms, including genetic rearrangements that lead to continuously active signaling or mutations that alter protein function.
In fibrotic diseases, PDGFRB plays a role in the excessive formation of scar tissue. For example, in pulmonary fibrosis, there is an increased expression and activation of PDGFRβ in fibrotic lung tissue. Similarly, in kidney fibrosis, increased expression and phosphorylation of PDGFRβ are hallmarks, particularly in renal mesangial cells, interstitial fibroblasts, and pericytes. This overactivity drives the proliferation and activation of mesenchymal cells, which are responsible for producing the excess extracellular matrix that characterizes fibrosis.
In the context of cancer, PDGFRB dysregulation can fuel tumor growth, angiogenesis, and metastasis. Some cancers, such as gastrointestinal stromal tumors (GISTs), can have mutations in the related PDGFRA gene, which, like PDGFRB, encodes a receptor tyrosine kinase. PDGFRB mutations can promote cell proliferation and tumor development. Another example is dermatofibrosarcoma protuberans (DFSP), a rare skin cancer, where a common genetic rearrangement fuses the COL1A1 gene with the PDGFB gene. This fusion leads to excessive production of a protein that continuously activates the PDGFRB receptor, driving uncontrolled cell proliferation and tumor formation.
PDGFRB is implicated in vascular disorders, conditions affecting blood vessels. Abnormal PDGFRB function can lead to issues such as abnormal vessel formation or narrowing. For instance, the PDGFRB gene is involved in the normal recruitment of pericytes to blood vessels. A deficiency in these cells due to impaired PDGFRB signaling can lead to increased blood-brain barrier permeability or abnormal vessel structures. Overexpression of PDGF-BB, a ligand for PDGFRβ, has also been linked to vascular and skeletal disorders, including arterial stiffening and osteoarthritis, by enhancing abnormal blood vessel formation.
Therapeutic Approaches Targeting PDGFRB
Understanding the role of PDGFRB in various diseases has opened avenues for targeted therapies. These treatments aim to specifically interfere with the abnormal signaling pathways driven by PDGFRB. The goal is to halt or slow disease progression by correcting the cellular malfunctions.
A prominent class of drugs used in this approach are Tyrosine Kinase Inhibitors (TKIs). These small molecules are designed to block the signaling activity of PDGFRβ. They achieve this by binding to the ATP-binding site within the kinase domain of the receptor, which prevents the receptor from activating other proteins inside the cell. This interruption stops aberrant cell growth and proliferation signals.
TKIs are employed in treating specific cancers and fibrotic conditions where PDGFRB is implicated. For instance, imatinib and sunitinib are examples of TKIs that have shown promise in treating conditions like infantile myofibromatosis, which is linked to gain-of-function mutations in PDGFRB. These drugs effectively inhibit cell proliferation and block the activation of downstream signaling molecules. Similarly, in fibrotic diseases like idiopathic pulmonary fibrosis, PDGFRβ inhibition, either through specific antibodies or TKIs like nintedanib, has been explored to reduce fibroblast proliferation and collagen deposition.