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Clermont, Allen

Prostacyclin-stimulating factor (PSF) acts on vascular endothelial cells to stimulate the synthesis of the vasodilatory molecule prostacyclin (PGI2). We have examined the expression, regulation, and hemodynamic bioactivity of PSF both in whole retina and in cultured cells derived from this tissue. PSF was expressed in all retinal cell types examined in vitro, but immunohistochemical analysis revealed PSF mainly associated with retinal vessels. PSF expression was constitutive in retinal pericytes (RPCs) but could be modulated in bovine retinal capillary endothelial cells (RECs) by cell confluency, hypoxia, serum starvation, high glucose concentrations, or inversely by soluble factors present in early vs. late retinopathy, such as TGF-β, VEGF, or bFGF. In addition, RPC-conditioned media dramatically increased REC PGI2 production, a response inhibited by blocking PSF with a specific antisense oligodeoxynucleotide (ODN). In vivo, PGI2 increased retinal blood flow (RBF) in control and diabetic animals. Furthermore, the early drop in RBF during the initial weeks after inducing diabetes in rats, as well as the later increase in RBF, both correlated with levels of retinal PSF. RBF also responded to treatment with RPC-conditioned media, and this effect could be partially blocked using the antisense PSF ODN. We conclude that PSF expressed by ocular cells can induce PGI2, retinal vascular dilation, and increased retinal blood flow, and that alterations in retinal PSF expression may explain the biphasic changes in RBF observed in diabetes.

Purpose. Originally identified as a lipopolysaccharide binding protein with Gram-negative bactericidal activity in the leukocytes, bactericidal/permeability-increasing protein (BPI) has been shown to induce various effects in retinal cells in vivo and in vitro. Methods. The authors recently reported that BPI can induce ERK1/2 and Akt activity and that it increases DNA synthesis in the bovine retinal pigment epithelial (RPE) and pericyte cells. The authors have extended the characterization of BPI interaction with membrane proteins from bovine RPE. Crude membrane pools from RPE were isolated, solubilized, and bound to rBPI21 affinity column. Bound proteins were separated by SDS-PAGE and stained with Coomassie blue, which showed an intense band at 36 kDa consistently displaced by rBPI21. Results. Tandem mass spectrometry of the 36-kDa band suggested that cell surface protein glypican 4 (GPC4) serves as a putative BPI-binding protein. Heparitinase, phosphatidylinositol-specific phospholipase C, and anti–GPC4 antibody suppressed BPI-induced ERK and Akt phosphorylation in bovine RPE. Moreover, heparitinase also inhibited BPI actions on VEGF and PDGF-B mRNA expression induced by H2O2. Conclusions. These new findings suggest that GPC4 is a specific binding protein for BPI on RPE to mediate the activation of ERK1/2, Akt, and the mRNA expressions of PDGF-B and VEGF.

Purpose. Although vascular endothelial growth factor (VEGF) is a key mediator of retinal vascular permeability (RVP), there may be additional humoral contributors. Hepatocyte growth factor (HGF) induces endothelial cell separation, regulates expression of cell adhesion molecules and is increased in the vitreous fluid of patients with proliferative diabetic retinopathy. The purpose of this study was to evaluate the in vivo effects of HGF on RVP and retinal hemodynamics and delineate the signaling pathways. Methods. RVP was assessed by vitreous fluorescein fluorophotometry in rats. Time course and dose-response were determined after intravitreal HGF injection. MAP kinase (MAPK), phosphatidylinositol 3-kinase (PI-3 kinase), and protein kinase C (PKC) involvement were examined by using selective inhibitors. Retinal blood flow (RBF) and mean circulation time (MCT) were evaluated by video fluorescein angiography. Results. HGF increased RVP in a time- and dose-dependent manner. HGF-induced RVP was evident 5 minutes after injection, and reached maximal levels after 25 minutes (+107% versus vehicle, P = 0.002). This effect was comparable to that of maximum VEGF stimulation (134% ± 128% at 25 ng/mL). Selective inhibitors of MAPK (PD98059) and PI-3 kinase (LY294002) suppressed HGF-induced RVP by 86% ± 44% (P = 0.015) and 97% ± 59% (P = 0.021), respectively. Non-isoform-selective inhibition of PKC did not significantly decrease HGF-induced RVP. Although VEGF increases RBF and reduces MCT, HGF did not affect either. Conclusions. HGF increases RVP in a time- and dose-dependent manner at physiologically relevant concentrations with a magnitude and profile similar to that of VEGF, without affecting retinal hemodynamics. Thus, HGF may represent another clinically significant contributor to retinal edema distinct from the actions of VEGF.

Cellular apoptosis induced by hyperglycemia occurs in many vascular cells and is crucial for the initiation of diabetic pathologies. In the retina, pericyte apoptosis and the formation of acellular capillaries, the most specific vascular pathologies attributed to hyperglycemia, is linked to the loss of platelet-derived growth factor (PDGF)-mediated survival actions owing to unknown mechanisms. Here we show that hyperglycemia persistently activates protein kinase C-delta (PKC-delta, encoded by Prkcd) and p38alpha mitogen-activated protein kinase (MAPK) to increase the expression of a previously unknown target of PKC-delta signaling, Src homology-2 domain-containing phosphatase-1 (SHP-1), a protein tyrosine phosphatase. This signaling cascade leads to PDGF receptor-beta dephosphorylation and a reduction in downstream signaling from this receptor, resulting in pericyte apoptosis independently of nuclear factor-kappaB (NF-kappaB) signaling. We observed increased PKC-delta activity and an increase in the number of acellular capillaries in diabetic mouse retinas, which were not reversible with insulin treatment that achieved normoglycemia. Unlike diabetic age-matched wild-type mice, diabetic Prkcd(-/-) mice did not show activation of p38alpha MAPK or SHP-1, inhibition of PDGF signaling in vascular cells or the presence of acellular capillaries. We also observed PKC-delta, p38alpha MAPK and SHP-1 activation in brain pericytes and in the renal cortex of diabetic mice. These findings elucidate a new signaling pathway by which hyperglycemia can induce PDGF resistance and increase vascular cell apoptosis to cause diabetic vascular complications.