Regulation of SPARC by transforming growth factor β2 in human trabecular meshwork

An increased aqueous level of TGF-β2 has been found in many primary open-angle glaucoma patients. Secreted Protein, Acidic, and Rich in Cysteine (SPARC)-null mice have a lower intraocular pressure. The mechanistic relationship between SPARC and TGF-β2 in trabecular meshwork (TM) is unknown. We hypothesized that TGF-β2 upregulates SPARC expression in TM.

Cultured TM cells were incubated with selective inhibitors for p38 MAP kinase (p38), Smad3, p42, JNK, RhoA, PI3K, or TGF-β2 receptor for 2 hours, and then TGF-β2 was added for 24 hours in serum-free media. Quantitative polymerase chain reaction (qPCR) and immunoblot analysis were performed. Immunofluorescent microscopy was used to determine nuclear translocation of signaling proteins. Ad5.hSPARC and Lentiviral shRNA for p38 and Smad3 were constructed, and infected human TM cells.

SPARC was upregulated by TGF-β2 in the human TM cells (3.8 ± 1.7-fold, n = 6, P = 0.01 for protein and 7.1 ± 3.7-fold, n = 6, P = 0.01 for mRNA), while upregulation of SPARC had no effect on TGF-β2. TGF-β2-induced SPARC expression was suppressed by inhibitors against p38 (-40.3 ± 20.9%, n = 10, P = 0.0001), Smad3 (-56.2 ± 18.9%, n = 10, P = 0.0001), JNK (-49.1 ± 24.6%, n = 10, P = 0.0001), and TGF-β2 receptor (-83.6 ± 14.4%, n = 6, P = 0.003). Phosphorylation and translocation of Smad3, p38, and MAPKAPK2 were detected at 30 minutes and 1 hour, respectively, following TGF-β2 treatment. Phosphorylation of JNK and c-jun was detected before TGF-β2 treatment. SPARC was suppressed 31 ± 13% (n = 5, P < 0.0001) by shRNA-p38 and 41 ± 3% (n = 5, P < 0.0001) by shRNA-Smad3.

TGF-β2 upregulates SPARC expression in human TM through Smad-dependent (Smad2/3) or -independent (p38) signaling pathways. SPARC may be a downstream regulatory node of TGF-β2-mediated IOP elevation.