Integrin-Linked Kinase Suppression Reduces SPARC's Effect on Intraocular Pressure and Extracellular Matrix

The molecular pathway for increased outflow resistance in the trabecular meshwork (TM) remains unknown. Secreted protein, acidic and rich in cysteine (SPARC) has been shown to regulate intraocular pressure (IOP) correlating to extracellular matrix (ECM) alterations. In other tissues, SPARC binds integrin-linked kinase (ILK) and regulates ECM organization through signaling cascades. We hypothesized SPARC regulates ECM proteins partly through ILK in the TM.

Adenovirus carrying cDNA of human SPARC (Ad.SPARC) was used to overexpress SPARC and a lentivirus carrying short-hairpin RNA (shRNA) targeting human ILK (shILK) was used to inhibit ILK in live mice, primary human TM cells, and perfused human cadaveric anterior segments. IOP was measured in mice and human anterior segments. Selected ECM proteins were analyzed by immunoblotting and immunostaining.

SPARC overexpression elevated IOP 1.87 ± 0.50 millimeters of mercury (mm Hg; P = 0.009, n = 15) in mice. Coinfection with ILK inhibition reduced IOP by 3.64 ± 0.57 mm Hg (P < 0.001, n = 15) and histologically reduced ECM proteins compared to SPARC overexpression. In human anterior segments, SPARC overexpression elevated IOP 2.10 ± 0.25-fold (P = 0.021, n = 4), whereas addition of shILK attenuated SPARC's effect and decreased IOP 0.61 ± 0.31-fold (P = 0.015, n = 4). In human TM cells, Ad.SPARC infection elevated levels of SPARC and laminin. Coinfection with shILK reduced levels of ILK, collagen I, collagen VI, and laminin.

ILK inhibition attenuated the effects of SPARC overexpression on IOP and ECM proteins, suggesting ILK signaling may contribute to the pathway of SPARC-mediated regulation of ECM homeostasis in TM. Future studies evaluating the downstream transcription factors may provide an elucidated pathway for POAG pathogenesis.