Octreotide Inhibits Platelet–Derived Growth Factor (PDGF) Simulated Signaling Pathways in Human Retinal Pigment Epithelial (hRPE) Cells

Abstract

Purpose: : PDGF is a growth factor for hRPE that acts via multiple signaling pathways. Previously, we have shown that octreotide inhibits the PDGF–stimulated STAT3 pathway. Recently, PDGF has been shown to stimulate the Extra–cellular signal regulated protein kinase (ERK) pathway. Since PDGF and the ERK pathway have been implicated in pathological epiretinal membrane formation, we investigated the effect of octreotide on the intracellular signaling molecule phosphoylated ERK1–2 and on PDGF–induced hRPE cell proliferation.

Methods: : hRPE cultures were established from human eyes obtained from the Michigan eye bank. Proliferation of hRPE cells in the presence of increasing concentrations of PDGF in the presence and absence of octreotide were measured by the trypan blue exclusion method and 3H–thymidine incorporation (³H–thy). Phosphorylated ERK1–2 (¹⁴C–pERK) protein synthesis was measured by immunoprecipitating ¹⁴C–Methionine–labeled pERK in the presence of increasing concentrations of PDGF (1–100 pg/ml) in the presence and absence of octreotide. Immunohistochemistry was also performed to determine the effect of octreotide on PDGF stimulated pERK immunoreactivity in hRPE cells. Data were analyzed by Student 't' test. Value of p<0.05 was considered to be significantly different.

Results: : PDGF stimulated hRPE cell number in a dose–dependent manner. Octreotide (100 nM) inhibited PDGF (100 pg/ml) stimulated hRPE cell proliferation(17.00±1.78 vs. 23.25±1.58, cell number x10⁴±SEM, p <0.05, n=5) as well as ³H–thy incorporation (4360±377 vs. 7449±771, CPM±SEM, p<0.05, n=5). PDGF also stimulated ¹⁴C–pERK synthesis in a dose–dependent manner. Octreotide (100 nM) inhibited PDGF (100 pg/ml) stimulated ¹⁴C–pERK synthesis (455±100 vs. 1462±363, CPM±SEM, p<0.05, n=4). Immunohistochemical studies showed an increase in pERK immunoreactivity in presence of PDGF which was not seen in presence of octreotide+PDGF.

Conclusions: : PDGF stimulates hRPE cell proliferation through signal transduction involving pERK activation. Octreotide inhibits hRPE cell proliferation as well as pERK synthesis. Octretide has a broad spectrum of inhibitory effects on signal transduction and may provide a novel therapeutic tool in the treatment of patients with proliferative eye diseases.

Keywords: retinal pigment epithelium • proliferative vitreoretinopathy • signal transduction: pharmacology/physiology