Osteopontin Is Induced by TGF-beta2 and Regulates Metabolic Cell Activity in Cultured Human Optic Nerve Head Astrocytes

The aqueous humor (AH) component transforming growth factor (TGF)-beta2 is strongly correlated to primary open-angle glaucoma (POAG), and was shown to up-regulate glaucoma-associated extracellular matrix (ECM) components, members of the ECM degradation system and heat shock proteins (HSP) in primary ocular cells. Here we present osteopontin (OPN) as a new TGF-beta2 responsive factor in cultured human optic nerve head (ONH) astrocytes. Activation was initially demonstrated by Oligo GEArray microarray and confirmed by semiquantitative (sq) RT-PCR, realtime RT-PCR and western blot. Expressions of most prevalent OPN receptors CD44 and integrin receptor subunits alphaV, alpha4, alpha5, alpha6, alpha9, beta1, beta3 and beta5 by ONH astrocytes were shown by sqRT-PCR and immunofluorescence labeling. TGF-beta2 treatment did not affect their expression levels. OPN did not regulate gene expression of described TGF-beta2 targets shown by sqRT-PCR. In MTS-assays, OPN had a time- and dose-dependent stimulating effect on the metabolic activity of ONH astrocytes, whereas TGF-beta2 significantly reduced metabolism. OPN signaling via CD44 mediated a repressive outcome on metabolic activity, whereas signaling via integrin receptors resulted in a pro-metabolic effect. In summary, our findings characterize OPN as a TGF-beta2 responsive factor that is not involved in TGF-beta2 mediated ECM and HSP modulation, but affects the metabolic activity of astrocytes. A potential involvement in a protective response to TGF-beta2 triggered damage is indicated, but requires further investigation