Pigment Epithelium-Derived Factor (PEDF) is a natural factor with surprisingly diverse therapeutic functions. Since its identification more than 20 years ago, PEDF has been recognized as a neurotrophic factor, a stem cell niche factor, an anti-inflammatory factor, an anti-angiogenic factor, a tumor inhibitor, and a protein with declined expression in aging. This secreted factor has been demonstrated to have diverse therapeutic value in inhibiting the pathogenesis of several major diseases such as diverse cancer types including melanoma, neuroblastoma, osteosarcoma, hepatoblastoma, Lewis lung carcinoma, chondrosarcoma, gastric carcinoma, glioma, Wilm's tumor, prostate cancer, and pancreatic cancer and several major blinding diseases including ischemia-induced retinopathy, diabetic retinopathy, retinitis pigmentosa, and age-related macular degeneration. Although this multifunctional factor has drawn increasing attention, its therapeutic potential is greatly hampered by the lack of knowledge on PEDF’s signaling mechanism. Despite considerable efforts in academia and industry, the cell-surface receptors that transduce PEDF signal eluded identification since the 1990s.
To transform the high therapeutic value of PEDF to potent and efficient therapeutic agents to treat human diseases, we aimed to achieve three general goals. The first is to identify and characterize the cell-surface receptors for PEDF. The second is to investigate the mechanism of PEDF signaling through these receptors. The third is to screen small molecules that mimic PEDF functions by targeting its receptors. Receptors are ideal therapeutic targets due to their specificity in changing cell behaviors and small molecule drugs targeting receptors account for the largest fraction of drugs used clinically in treating human diseases.
After many years of effort and trying many strategies, we identified the long-sought PEDF cell-surface receptors as PLXDC1 and PLXDC2, two single transmembrane domain proteins that do not belong to well-known receptor families. We found that these two homologous transmembrane receptors not only confer cell-surface binding to PEDF but also mediate diverse PEDF functions such as promoting IL-10 secretion in macrophage, inducing endothelial cell death and protecting neuronal cell in a cell-type specific manner. We also found unique mechanisms of interaction between PEDF and its receptors, such as receptor-induced PEDF dimerization through a disulfide bond. To identify compounds that specifically target these receptors, we have developed a novel cell-based, fluorescence-based and high-throughput screening strategy that color-codes receptor-expressing cells. Using this strategy, we have identified potent lead compounds that mimic PEDF actions such as causing cell death in a receptor-specific manner. These compounds are potential first-in-class drugs in treating diseases.
In summary, identification of PEDF receptors allows further investigation of the molecular mechanism of PEDF signaling and provides novel therapeutic targets. By developing a novel cell-based screening technique, we have identified chemical compounds that specifically and potently target the PEDF receptors and mimic PEDF actions.