Abstract
Purpose: :
Hypertension has been identified as an independent risk factor for the progression of diabetic retinopathy (DR). However, the molecular mechanisms by which hypertension contributes to DR remains largely unknown. Elevated blood pressure imposes increased mechanical stress on the vascular wall. which results in the increased accumulation of rigid ECM proteins which ultimately results in decreased blood flow to the retina, an important factor in vascular cell apoptosis. MMPs, a family of extracellular matrix–degrading endopeptidases, are produced by vascular cells and play an important role in ECM turnover and modification. The aims of this study are to: i) study the combined effects of hyperglcemia and cyclic strain on matrix turnover in human retinal smooth muscle cells (HRSMC) and ii) ascertain if these conditions contribute to HRSMC apoptosis, a process indicative of DR in vivo.
Methods: :
The FlexercellTM system was used to expose HRSMC to physiological levels of cyclic strain; (0–10% strain, 60 cycles/min, 24–72h) in the presence or absence of 30mM glucose. MMP–2 activity was determined by gelatin zymography. Nitric oxide (NO) release was assessed in the conditioned media by the DAN assay. Expression of matrix proteins was determined by real–time PCR. Apoptosis was assessed by annexin v/propidium iodide FACS analysis.
Results: :
Following exposure to 5% or 10% cyclic strain resulted in a significant 2.1 or 2.4 fold (n=3 p<0.05) increase in MMP–2 activity. Exposure to 30mM glucose significantly attenuated strain–induced increases in MMP–2 activity but had no effect on basal MMP–2 activity. Nitric oxide release was increased following exposure to either 5% or 10% cyclic strain (1.6 or 1.3 fold n=3 p<0.05). Hyperglycaemic conditions significantly decreased NO levels in both strained and control cells. These data may suggest that NO may at least in part regulate glucose–mediated inhibition of MMP–2 activity. Real–time PCR analysis demonstrated that neither strain nor 30mM glucose had a significant effect on the expression of collagen type I or fibronectin. Moreover, these conditions did not significantly contribute to HRSMC apoptosis.
Conclusions: :
These data suggest that hyperglycaemic conditions may contribute to ECM accumulation by inhibiting MMP–2 activity possibly via a NO dependent pathway. These conditions did not result in increased HRSMC apoptosis, however apoptotic events may be associated with a more chronic exposure to these factors.
Keywords: nitric oxide • vascular cells • diabetes