PDGFR

Platelet-derived growth factor was discovered in 1974 (Ross et al. 1974) and was identified in the late 1970s as a platelet-derived factor that promotes the proliferation of mesenchymal cells.

PDGFR is the acronym for Platelet-Derived Growth Factor Receptor (see also PDGFRA gene). Five different PDGF isoforms are known: PDGF AA, PDGF AB, PDGF BB, PDGF CC and PDGF DD. The long known growth factors PDGF AA, AB and BB are composed of homo- and heterodimers of the polypeptide A and B chains. The respective chains are connected by disulfide bridges. Both A and B chains show approximately 60% amino acid sequence similarity. PDGF shows great functional and structural similarities to VEGF.

Much later, two other proteins of the PDGF family were discovered: PDGF CC and PDGF DD. They are secreted as inactive forms and can bind to the receptor only when the CUB domain is cleaved by extracellular proteases. The CUB domain is an evolutionarily new structural module of the PDGF/VEGF family and is located at the N-termini of the PDGF C-/D-chains (Fredriksson et al. 2004).

PDGF receptors are widespread, so it can be assumed that their functionality is not yet fully known. In the cell systems listed below, PDGF receptors have been detected in varying densities:

PDGF alpha receptors:

• Haemato-poetic and vascular cells: smooth vascular muscle cells, thrombocytes
• nervous system: astrocytes, neurons, Schwann cells
• Lungs: Mesenchymal cells
• Kidney: mesangial cells
• reproductive organs: Leydig cells, intestinal cells, gonads
• Other: fibroblasts, epithelial cells
• Liver: Sinusoidal, liver cells

PDGF beta receptors:

• vascular smooth muscle cells, vascular endothelial cells, pericytes, T cells, macrophages, myeloid hematopoietic cells
• nervous system: astrocytes, neurons, Schwann cells, postnatal neurons
• Kidney: mesangial cells
• Organs of procreation: Leydig cells,
• Intestinal cells, gonads
• Other: fibroblasts, myoblasts
• Liver: Ito cells

There are 2 known receptor types: alpha-PDGFR and beta-PDGFR. Both are transmembrane receptors and have a molecular size of about 170 and 180 kDa. The coding alpha-receptor gene is located on chromosome 4q12 near the VEGF and SCF receptor genes. The beta-receptor gene is located on chromosome 5 (Yarden et al. 1986). Each receptor forms five extracellular immunoglobulin-like domains. Intracellularly, the tyrosine kinase domain transmits the extracellularly received signals. Morphologically, the PDGFs have a dimer structure. They bind simultaneously to two receptors, change the arrangement of the two receptor parts in relation to each other through this binding and thus activate the receptor tyrosine kinase.

The PDGF receptor dimerization and activation induced by its ligands leads to autophosphorylation of the receptor and suppression of phosphatase activity. This biochemical reaction causes an increased kinase activity and expression of a formation of docking sites for signal transduction molecules. Within minutes, several signalling systems are activated (Ras signalling pathway). While phosphorylation within the kinase domain is essential for increasing the catalytic effectiveness of the kinases, phosphorylation of the PDGF receptor outside the kinase domain plays an important role in the activation of the signaling enzymes PI3K, PLCγ and Src (PI3K - signaling pathway). These bind to the receptor with high affinity.

Besides these signaling enzymes, a number of other enzymes can bind to the PDGF receptor via their SH2 domain (SHP-2, GAP, Shc, Grb2/7, Crk etc. - Tallquist et al. 2004). When the Ras cascade is activated, the adapter proteins Grb2 and Shc are first bound to the plasma membrane. The activation of Ras influences cell growth, migration and cell differentiation. There is a "cross-talk" between the two signalling pathways PI3K and Ras, which can influence the signalling cascades in each case.

Physiological functions of PDGF and PDGFR

Major effects of PDGF are the promotion of migration, proliferation and survival of cells. PDGF unfolds its effects both paracrine and autocrine. The growth factor PDGF B and the associated PDGF beta-receptor are more active in the vascular system, while PDGF A and the PDGF alpha-receptor are involved in a broader spectrum of embryogenesis, CNS and organ development. PDGF and PDGF receptors play an important role in embryonic development.

Experimental results: The inactivation of the genes for the A and B PDGF isofactors and for the alpha and beta receptors allows (in mice) insights into their vivo functions. For example, if the genes for the B-chain and the β receptor are inactivated, this leads to a lack of mesangium cells of the kidneys, the defective development of blood vessels and characteristic bleeding during birth (Leveen et al. 1994). The bleedings are due to an incorrect recruitment of pericytes to the blood vessels, the vessel wall of which loses stability as a result.

Knock-out of the A-chain leads to defective development of the lung with an emphysema-like phenotype. The inactivation of the alpha-receptor gene causes cranial malformations (Soriano et al. 1997).

CNS: PDGF and its receptors also play a role in the CNS. The expression of the A-chain in neurons and astrocytes influences for example the development of oligodendrocytes (Yeh et al. 1993). In the olfactory system of the CNS the expression of the B-chain is very high both during embryonic development and in adulthood. Since the sensory neurons in the olfactory system have the ability to regenerate throughout life, PDGF appears to be a neurotrophic factor.

Vascular system: In the vascular system PDGF receptors are found on capillary endothelial cells and in pericytes. PDGF also has an angiogenetic effect, but weaker than VEGF and FGF (fibroblast growth factor) (Battegay et al. 1994). The PDGF B-chain formed by endothelial cells and the PDGF beta-receptor formed on pericytes are crucial for pericyte recruitment and thus influence the structural integrity of the vessels (Lindahl et al. 1997). PDGF also plays a role in the regulation of vascular tone and feedback control of platelet aggregation. Interstitial tissue pressure is also regulated by PDGF (Heuchel et al. 1999). This is made possible by the ability of PDGF to stimulate interactions between cells and molecules of the extracellular matrix.

Wound healing: PDGF is significantly involved in wound healing. Paracrine PDGF secreted by macrophages and platelets stimulates cell division, chemotaxis of fibroblasts and vascular smooth muscle cells and chemotaxis of neutrophil granulocytes and macrophages.

PDGF as mediator of autocrine tumor growth: First evidence that autocrine activation of PDGF receptors may play a role in tumor development was provided by the discovery of homology between the Simian Sarcoma Viral Oncogene product v-Sis and the PDGF B-chain. Both v-Sis and PDGF isoforms can exert transformative effects on cells (Doolittle et al. 1983).

Furthermore, it has been shown that some malignant tumors exhibit mutagenic activation of PDGF or PDGF receptors. Thus the dermatofibrosarcoma protuberans (DFSP) and the juvenile large cell fibrosarcoma (GCF), both dermal neoplasias of medium malignancy. Both tumors have chromosomal 17/22 translocations, which leads to an overexpression of PDGF-B (Simon et al. 1997).

Further examples of oncogenic receptor mutations are Bcr-Abl-positive chronic myeloid leukemia, in which the PDGF beta receptor is activated due to a translocation. In the hypereosinophilic syndrome a deletion on chromosome 4 is present, which leads to an activating PDGF alpha-receptor fusion protein (Gotlib et al. 2004).

Neurological tumors: Other studies have shown that co-expression of PDGF and PDGF receptors in malignant tumors originating from PDGF-responsive cells is involved in tumor development. An example is gliomas, which are the most common primary brain tumors in the CNS. Glioblastoma multiforme (GBM) is the most aggressive variant which occurs in two variants: variant 1 is characterized by an amplification of the EGF receptor and by a PTEN mutation. Variant 2 is characterized by an autocrine receptor activation of the PDGF alpha-receptor caused by a mutation of the tumor suppressor p53 (Maher E et al. 2001).

Tumor angiogenesis: PDGF is also involved in tumor angiogenesis. PDGF mainly influences pericytes. The formation of the PDGF beta-receptor on pericytes is very constant, whereas this receptor is only sporadically present on endothelial cells. Pericytes are pathologically distributed and aligned in tumor vessels (Baluk et al. 2003). Furthermore, the PDGFD beta-receptor is clearly overexpressed in the stroma of numerous tumors (Skobe et al. 1998). The PDGF beta-receptor is also involved in the regulation of interstitial tissue pressure, which co-regulates the transcapillary mass exchange.

PDGF receptors as targets of tumor treatment: For about 30 years, a number of specific PDGF receptor-kinase inhibitors (quinoxalines; indolonones) etc. have been developed, which are specifically designed to interrupt the signaling cascade of the PDGF receptor. Imatinib (Buchdunger et al. 2000) has been successfully introduced into the therapy of dermatofibrosarcoma. The interstitial tissue pressure could also be reduced by imatinib and thus an improved drug absorption could be ensured (Levitzki et al. 2004). Thus Gleevec seems to show a promising perspective for the specific treatment of malignant diseases, especially in combination with other drugs.

Other PDGFR inhibitors that block the receptor tyrosine kinase (both also inhibit c-Kit and some serine threonine kinases) are:

• sunitinib and
• Sorafenib.

The PDGF receptor also appears to be important in the anti-angiogenetic therapy of tumors. So far, therapeutic strategies have mainly targeted VEGF inhibitors, which are, however, mainly effective in immature vessels. The combination of VEGF and PDGF inhibitors therefore seems to be particularly beneficial, as PDGF inhibitors specifically inhibit the organisation of pericytes, whereas VEGF inhibitors act on endothelial cells (Bergers et al. 2003).

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