Abstract
Purpose: :
SPARC is a matricellular protein associated with profibrotic states that is located within a known primary open-angle glaucoma (POAG) locus, 5q22.1-q32 (GLC1M). SPARC-null mice exhibit a 15-20% lower intraocular pressure (IOP). Steroid-induced glaucoma and POAG both result from increased outflow resistance. SPARC may be part of the pathophysiology of steroid-induced glaucoma as well. We studied the effect of dexamethasone (DEX) on the IOP of SPARC-null (KO) and wild-type (WT) mice as well as on SPARC protein and mRNA levels in vitro.
Methods: :
Five to six week-old WT and KO mice were fed ad lib and given drinking water containing 10 mg/L of dexamethasone sodium phosphate. IOPs were measured with the TonoLab tonometer prior to starting treatment, weekly during treatment, and 4 days after stopping treatment. We also incubated trabecular meshwork (TM) and ciliary body smooth muscle (CBSM) cells with DEX for 1, 3, and 7 days. Expression of SPARC mRNA, cell/extracellular matrix (ECM)-bound protein, and soluble protein secreted into the culture media were measured by real-time RT-PCR, western blotting, and ELISA, respectively.
Results: :
Average IOP measurements of WT mice treated with DEX and control were 21.3, 22.4, 24.5, 19.7, 20.7, 19.5 mm Hg; and 21.2, 20.5, 22.7, 19.6, 19.4, 18.9 mm Hg at 0, 1, 2, 3, 4, and 4.5 weeks, respectively (n=22, 20; p>0.05 at all time points). Average IOP measurements of SPARC-null mice treated with DEX and control were 17.7, 19.6, 20.8, 20.5, 19.2, 18.5 mm Hg; and 16.9, 16.4, 17.8, 17.9, 16.9, 17.9 mm Hg at 0, 1, 2, 3, 4, and 4.5 weeks, respectively (n=20, 18; p<0.05 at 1, 2, 3 and 4 weeks). KO mice exhibited proportionally greater peak elevations in IOP at 1 week (19.2% versus 9.7%). Compared to control-treated TM cells, cells incubated with DEX showed a significant decrease in SPARC mRNA levels at 3- and 7-days, by 0.323- and 0.372-fold, respectively (p <0.05). Cell/ECM-bound SPARC protein in TM cells was decreased at 1-, 3-, and 7-days, by 30.8, 20.1, and 17.6% (p <0.05). Soluble, secreted SPARC protein was unchanged at all time points. CBSM cells exhibited no significant changes in SPARC mRNA, or protein levels.
Conclusions: :
Administration of DEX resulted in statistically significant IOP elevations in SPARC-null mice but not in WT mice. Incubation with DEX lowered cell/ECM-bound SPARC protein and mRNA levels in TM but not CBSM cells. SPARC may play a counter-regulatory role in steroid-induced glaucoma.