Activation and function of PDGF/PDGFR pathway
The platelet-derived growth factor (PDGF) signaling network consists of four ligands, PDGF-A, PDGF-B, PDGF-C, and PDGF-D, and two receptors, PDGFR-α and PDGFR-β. All PDGFs function as disulfide-linked homodimers, but only PDGF-A and -B can form functional heterodimers. Before binding to the protein tyrosine kinase receptors PDGFR-α and PDGFR-β, the four PDGF ligands are inactive in their monomeric forms. The receptor isoforms dimerize upon binding, leading to three possible receptor combinations, namely -AA, -BB, and -AB, causing the subsequent activation of kinase. Kinase activation is visualized as tyrosine phosphorylation of the receptor molecules, which occurs between the dimerized receptor molecules (transphosphorylation). The receptor molecules then undergo conformational changes that allow a basal kinase activity to phosphorylate a critical tyrosine residue, thereby “unlocking” the kinase, leading to full enzymatic activity directed toward other tyrosine residues in the receptor molecules, as well as other substrates for the kinase [
12,
13].
More than 10 different molecules bind selectively to phosphorylated residues in the PDGF receptors, including the Src family, SHP-2 tyrosine phosphatase, phospholipase C-γ (PLC-γ), and the GTPase activating protein (GAP) for Ras. Furthermore, the receptors bind and activate signal transducers and activators of transcription (STATs). Finally, some of the receptors binding molecules lack intrinsic enzymatic activities, but can form complexes with other signaling molecules. For example, the regulatory subunit p85 of the PI3K forms a complex with the p110 catalytic subunit, and Grb2 activates Ras and the Erk MAP-kinase pathway by binding with SOS1. In addition, the PDGF receptors bind other adaptors (e.g., Shc, Nck, Crk, and GAB) and mediate interactions with numerous different downstream signaling molecules [
14,
15]. The activation of these signaling pathways leads to cell survival, proliferation, angiogenesis, and cell migration.
PDGFs and PDGFRs are expressed by a large variety of normal human tissues and organs. PDGFs are major mitogens for many cell types of mesenchymal or neuro-ectodermal in origin. PDGFs have chemo-attractant properties and are involved in erythropoiesis, wound healing, bone formation, and angiogenesis. Much evidence suggests involvement in the normal development of important organs such as kidney, brain, and cardiovascular and respiratory systems [
16]. During normal development, cell proliferation significantly increases as a consequence of PDGF overexpression and decreases in PDGF null mutants.
PDGF/PDGFR pathway in tumor development and metastasis
A complex interplay between cancer cells, endothelial cells, and other stromal cells occurs in tumor angiogenesis. The PDGF/PDGFR pathway plays an important role in the development and metastasis of tumors in at least three different ways: (1) direct autocrine stimulation of tumor cells [
17]; (2) paracrine stimulation of tumor stromal cells [
12] and promotion of angiogenesis to overcome hypoxia in the tumor microenvironment [
18]; (3) modulation of tumor interstitial fluid pressure (IFP) to control the influx and efflux of agents [
19].
The mode of action of PDGF and PDGFR involvement in tumor development and progression is mainly through direct autocrine stimulation of tumor cell growth and cell autonomy, whereas in normal tissues paracrine stimulation is predominant [
20]. Direct blockade of autocrine stimulation using inhibitors of PDGFR in cell lines and xenograft models show positive results in tumors such as lung cancer [
21], hepatic cancer [
22], gastrointestinal stromal tumor (GIST) [
23], dermatofibrosarcoma protuberans [
24], and osteosarcoma, which expressed PDGFR-α on the cell surface [
25]. The levels of phosphorylation of both PDGFRs and the downstream signaling proteins such as protein kinase B (AKT) and extracellular regulated kinase (ERK) are downregulated by PDGFR antibodies or small molecular tyrosine kinase inhibitors (TKIs) [
25].
In addition to direct autocrine stimulation, the PDGF/PDGFR pathway exerts paracrine stimulation on tumor stromal cells. PDGF promotes the formation of a rich stromal compartment, characterized by deposition of extracellular matrix components and blood vessel formation [
26]. Experimental models showed that VEGF-null cells require PDGFR to recruit fibroblasts in tumor stroma [
27]. Cancer-associated fibroblasts (CAFs), which secrete several active factors to stimulate angiogenesis and tumor invasion, constitute a functionally important component of tumor stroma in many types of cancer [
28].
Clinical data indicate that carcinomas with desmoplastic stroma, consisting of fibroblast cells and extracellular matrix, are associated with a poor prognosis [
29]. An experimental model of glioma revealed that PDGF-B enhances angiogenesis by stimulating VEGF expression in tumor-associated endothelial cells and by recruiting pericytes [
30]. Several other preclinical models demonstrated that using antibodies or TKIs to disrupt the paracrine signaling with PDGFR can significantly reduce tumor growth by inhibiting tumor cell growth, recruitment of fibroblasts, and angiogenesis in xenograft tumor models or genetic mouse models of cancer [
31], such as melanoma [
26], cervical cancer [
32], and colon cancer [
33].
Targeting the PDGF/PDGFR pathway can regulate the influx and efflux of agents and thus modify the uptake of drugs. Previous data showed that connective tissue cells control interstitial fluid pressure (IFP) by exerting tension on the collagen/microfibrillar network via collagen binding integrin a
2b
1 [
34,
35]. Experiments using transgenic mice carrying PDGFR-β receptor mutations suggest a function for PDGF signaling through PI3K in IFP homeostasis by modulating the tension between cells and extracellular matrix structures [
35]. Previous studies reported that the inhibition of PDGFR reduced interstitial hypertension and increased transcapillary transport in tumors. An increased tumor uptake of the tracer compound
51Cr-EDTA and paclitaxel, after inhibition of PDGF receptor using TKIs in tumor stroma, was previously demonstrated [
18,
19,
36].
An increased tumor uptake of cytotoxic drugs has therefore been proposed as the mechanism and further promotes combination treatment with PDGF antagonists and chemotherapy, which is called an “add-on” strategy. “Add-on” strategy has been used in a variety of tumors with different partners, such as fludarabine phosphate (F-AMP) in GIST [
37], doxorubicin in breast cancer [
38], and fucoxanthin in chronic myeloid leukemia [
39]. Because of the mild adverse effect of PDGF antagonists, most combinations are safe and well-tolerated.