PEDF: Raising both hopes and questions in controlling angiogenesis.

D on timing and venue, new blood vessel growth can be either beneficial or pathological. Vigorous vessel growth is absolutely necessary for tissue, organ, and limb development as was graphically demonstrated a few decades ago with thalidomide, now known to be a compound with antiangiogenic properties. As one ages though, the need for angiogenesis decreases or stops entirely except for specialized cases such as wound repair. In fact, neovascularization in the adult is usually associated with some form of pathology—tumor growth, for example, or diabetes. New work by Bouck and coworkers (1), as reported in PNAS, now gives evidence that a natural protein, pigment epithelium-derived factor (PEDF), is a potent inhibitor of abnormal blood vessel growth in a murine model of ocular neovascularization. This, together with previous data from the same group (2), gives compelling evidence that PEDF may be pivotal in controlling both normal and abnormal blood vessel growth. Although this gives hope for ultimately controlling neovascularization, it also raises interesting questions about PEDF’s biological activities. In 1989, a novel neurotrophic activity was first described by Tombran-Tink and Johnson (3) in conditioned medium from cultured fetal retinal pigment epithelial (RPE) cells. In a test system using cultured, primitive retinoblastoma cells, extensive neuronal-like processes were induced after exposure to the RPE conditioned medium (Fig. 1). This factor, PEDF, has now been purified (4) and cloned from both human (5) and mouse (6). The gene is active at 17 weeks in human fetal RPE cells (7), making it a candidate as a factor involved in early neuronal development. Subsequent work has demonstrated that PEDF is mainly a secreted glycoprotein of about 50 kDa that is not uniquely synthesized by fetal RPE cells. PEDF mRNA is found in most tissues, normal cell types (neuronal and nonneuronal), and tumors, as surveyed by Western and Northern blot analysis (8). DNA sequence analysis shows that PEDF belongs to the serine protease inhibitor (SERPIN) gene family (5, 8) which includes a large number of proteins of widely divergent function, such as ovalbumin, alpha 1–antitrypsin, angiotensinogen, and GDNyPN-1, another serpin with neurotrophic properties. Interestingly, PEDF does not seem to exhibit antiprotease activity with the neurotrophic active site residing at the other end of the peptide chain from the antiprotease active center (9, 10) seen in the classical serpins. The noninhibitory nature of the PEDF molecule has been well studied by Becerra and coworkers (11). During the time when the PEDF protein and gene were being isolated, Cristofalo and coworkers (12) described a protein in WI-38 fibroblast cells, a prime model system for studying aging and senescence, whose expression was (i) restricted to the Go stage of the cell cycle in young cells and (ii) not expressed in older, senescent cells. This protein, called EPC-1, was found to be identical to PEDF. Subsequent work (13, 14) reinforces the notion that PEDF is intimately associated with both the cell cycle and senescence. The expression of PEDF mRNA and the secreted protein are much more highly (100-fold) expressed in Go (growth arrest) of early-passaged cells than in rapidly proliferating young cells or in later-passaged (senescent) cells. Palmieri and coworkers (15) have found similar results in PEDF down-regulation in proliferating and in senescent human endometrial stromal fibroblasts. With the finding that PEDF was neurotrophic in cultured retinoblastoma tumor cells, it was of interest to determine whether PEDF had an effect on normal neurons. For this purpose, the effect(s) of both native and recombinant PEDF on normal cerebellar granule cells (CGCs) in primary culture was examined. Tanawaki