Transforming growth factor-beta induces extracellular matrix protein cross-linking lysyl oxidase (LOX) genes in human trabecular meshwork cells

The role of TGF-β in the pathogenesis of primary open-angle glaucoma

Transforming growth factor-β2 (TGF-β2) is found in increasing amounts in aqueous humor and reactive optic nerve astrocytes of patients with primary open-angle glaucoma (POAG), a major cause of blindness worldwide. The available data strongly indicate that TGF-β2 is a key player contributing to the structural changes in the extracellular matrix (ECM) of the trabecular meshwork and optic nerve head as characteristically seen in POAG. The changes involve an induction in the expression of various ECM molecules and are remarkably similar in trabecular meshwork cells and optic nerve head astrocytes. The ECM changes in the trabecular meshwork most probably play a role in the increase of aqueous humor outflow resistance causing higher intraocular pressure (IOP). In the optic nerve head, TGF-β2-induced changes might contribute to deformation of the optic nerve axons causing impairment of axonal transport and neurotrophic supply and leading to their continuous degeneration. The increase in IOP further adds mechanical stress and strain to optic nerve axons and accelerates degenerative changes. In addition, high IOP might induce the expression of activated TGF-β1 in trabecular meshwork cells and optic nerve head astrocytes; this again might significantly lead to the progress of axonal degeneration. The action of TGF-β2 in POAG is largely mediated through the connective tissue growth factor, whereas the activities of TGF-β1 and -β2 are modulated by the blocking effects of bone morphogenetic protein-4 (BMP-4) and BMP-7, by gremlin that inhibits BMP signaling and by several species of microRNAs.

Quantitative proteomics: TGFβ₂ signaling in trabecular meshwork cells

PURPOSE

Transforming growth factor beta 2 (TGFβ₂) is often elevated in the aqueous humor (AH) and trabecular meshwork (TM) of patients with primary open-angle glaucoma (POAG) and appears to contribute to POAG pathogenesis. To better understand TGFβ₂ signaling in the eye, TGFβ₂-induced proteomic changes were identified in cells cultured from the TM, a tissue involved in intraocular pressure (IOP) elevation in glaucoma.

METHODS

Primary cultures of human TM cells from four donors were treated with or without TGFβ₂ (5 ng/mL) for 48 hours; then cellular protein was analyzed by liquid chromatography-mass spectrometry iTRAQ (isobaric tags for relative and absolute quantitation) technology.

RESULTS

A total of 853 proteins were quantified. TGFβ₂ treatment significantly altered the abundance of 47 proteins, 40 of which have not previously been associated with TGFβ₂ signaling in the eye. More than half the 30 elevated proteins support growing evidence that TGFβ₂ induces extracellular matrix remodeling and abnormal cytoskeletal interactions in the TM. The levels of 17 proteins were reduced, including four cytoskeletal and six regulatory proteins. Both elevated and decreased regulatory proteins implicate TGFβ₂-altered processes involving transcription, translation, and the glutamate/glutamine cycle. Altered levels of eight mitochondrial proteins support TGFβ₂-induced mitochondrial dysfunction in the TM that in POAG could contribute to oxidative damage in the AH outflow pathway, TM senescence, and elevated IOP.

CONCLUSIONS

The results expand the repertoire of proteins known to participate in TGFβ₂ signaling, provide new molecular insight into POAG, and establish a quantitative proteomics database for the TM that includes candidate glaucoma biomarkers for future validation studies.

Role of TGFbeta/Smad signaling in gremlin induction of human trabecular meshwork extracellular matrix proteins

PURPOSE

The bone morphogenic protein (BMP) antagonist gremlin is elevated in glaucomatous trabecular meshwork (TM) cells and tissues and elevates intraocular pressure (IOP). Gremlin also blocks BMP4 inhibition of transforming growth factor (TGF)-β2 induction of TM extracellular matrix (ECM) proteins. The purpose of this study was to determine whether Gremlin regulates ECM proteins in cultured human TM cells.

METHODS

Human TM cells were treated with recombinant gremlin to determine the effects on ECM gene and protein expression. Expression of the ECM genes FN, COL1, PAI1, and ELN was examined in cultured human TM cells by quantitative RT-PCR and Western immunoblot analysis. TM cells were pretreated with TGFBR inhibitors (LY364947, SB431542 or TGFBR1/TGFB2 siRNAs), inhibitors of the Smad signaling pathway (SIS3 or Smad2/3/4 siRNAs), or CTGF siRNA to identify the signaling pathway(s) involved in gremlin induction of ECM gene and protein expression.

RESULTS

All ECM genes analyzed (FN, COL1, PAI1, and ELN) were induced by gremlin. This gremlin induction of ECM genes and protein expression was blocked by inhibitors of TGFBR and the canonical Smad2/3/4 and CTGF signaling pathways.

CONCLUSIONS

Gremlin employs canonical TGFβ2/Smad signaling to induce ECM genes and proteins in cultured human TM cells. Gremlin also induces both TGFβ2 and CTGF, which can act downstream to mediate some of these ECM changes in TM cells.