Recombinant Human Serpin F1/PEDF Protein, CF

Serpin F1 (SERine Proteinase Inhibitor‑clade F1; also PEDF/Pigment Epithelium‑Derived Factor, EPC‑1 and PIG35) is a monomeric, 52‑55 kDa secreted phosphoglycoprotein that belongs to the clade F subfamily, serpin superfamily of proteinase inhibitors (1‑4).  Serpins in general form two groups: one that demonstrates protease inhibition, and another (containing Serpin F1) whose members show no protease inhibition, but which act as chaperones and circulating transporters (2)   Serpin F1 is synthesized as a 418 amino acid (aa) precursor that contains a 19 aa signal sequence plus a 399 aa mature region that shows a pyroglutamate at Gln20 (5, 6, 7).  Like other serpins, it contains three beta‑sheets, 8‑10 alpha‑helices, and a C‑terminal RCL (reactive center loop).  Unlike other serpins with Ser protease inhibiting activity, the RCL of Serpin F1 does not undergo a conformational change upon target protease cleavage, a prerequisite for anti‑protease activity.  Such cleavage does, however, generate a 46 kDa fragment that possesses nonprotease‑associated (i.e.‑neurotrophic) bioactivity (8).  Phosphorylation occurs both intracellularly and extracellularly. Intracellularly, PKA phosphorylates Ser227, promoting anti‑angiogenic activity.  Extracellularly, CK2 phosphorylates Ser24 and Ser114, promoting neurotrophic activity (9).  Multiple sites have been identified and associated with distinct biological activities.  Amino acids 63‑70 constitute a functional NLS, and a 36 kDa Serpin F1 isoform has been reported in the nucleus (10).  In addition, lipase activity (perhaps a consequence of receptor binding) has been mapped to aa 78‑141, anti‑angiogenic activity to aa 20‑70 plus Ser114 and Ser227, neurotrophic activity to aa 32‑141, and heparin binding to aa 121‑149 (4).  Mature human Serpin F1 shares 86% aa sequence identity with mouse Serpin F1.

Serpin F1 is known to be synthesized by multiple cell types, including retinal pigment epithelium (RPE), fibroblasts, mammary epithelium, preadipocytes and adipocytes, hepatocytes, osteoblasts and osteoclasts (1, 2, 11‑15).  This diversity of cell types reflects its many activities, among which are neuroprotection, lipolysis, antiangiogenesis, and anticarcinogenesis (2‑4).  Much work has been done with RPE, a cell type that is known to both provide neuroprotection and block angiogenesis.  On the neuroprotection side, Serpin F1 binding to an 83 kDa, 4‑transmembrane PEDF receptor (also known as PNPLP2 and TTS‑2.2) results in the activation of receptor‑associated PLA2 activity.  This activity is directed towards RPE membrane triglycerides that contain a dietary omega‑3 fatty acid called DHA.  Once released via lipase activity, DHA is converted into NDP1, a neuroprotective lipid that acts on the surrounding neural complex (2, 16‑18).  RPE‑derived Serpin F1 also minimizes retinal vascularization.  This apparently occurs through a variety of mechanisms.  First, Serpin F1 binds to PEDFR on endothelial cells (EC), generating PPAR gamma ligands via EC CYP450.  These ligands activate PPAR gamma induces p53 and subsequent EC apoptosis (19, 20).  Second, Serpin F1 apparently binds the 67 kDa laminin receptor (LR) on EC.  This activates caspase‑3 with the initiation of an apoptotic program (21).  Finally, Serpin F1 interfers with VEGF signaling.  This appears to happen through at least two mechanisms.  First, there is a direct competitive binding of Serpin F1 to the EC VEGFR2, blocking VEGF signaling (22).  Second, Serpin F1 binding to EC activates membrane gamma‑secretase, resulting in the proteolytic cleavage of both VEGFR1 and R2, which abrogates signaling and creates a soluble receptor for VEGF (22, 23, 24).