Effects of mechanical stress and high glucose on pericyte proliferation, apoptosis and contractile phenotype

Pericyte loss is an early step of diabetic retinopathy. High glucose induces apoptosis in retinal pericytes, but systemic and capillary hypertension are also believed to be important in the onset and progression of diabetic retinopathy. The haemodynamic insult of retinal capillary hypertension can be mimicked by exposing pericytes to mechanical stretch. We investigated the effect of stretch combined with high glucose on pericyte proliferation/apoptosis and morphology. Bovine retinal pericytes, cultured in either normal or high glucose concentrations in flexible-base plates, were exposed to mechanical stretch for 48/72 h. Cell replication was determined by both cell counting and DNA synthesis, apoptosis by ELISA, cell morphology and actin cytoskeleton distribution by immunofluorescence. Both reduction in cell proliferation and increase in apoptosis were confirmed in high glucose alone. When cells were subjected to stretch, proliferation was reduced and apoptosis increased in both normal and high glucose in comparison with unstretched controls. In both cases, a synergistic effect of hyperglycaemia combined with stretch was shown. Cell morphology showed modifications of cytoskeleton in all experimental conditions; in particular, cells subjected to stretch showed a clear elongation and translocation of actin fibres. In conclusion, our results show that stretch, alone or combined with high glucose, reduces cell proliferation, increases apoptosis and induces morphological changes in pericyte cytoskeleton. Further elucidations of the mechanisms on the basis of reduced proliferation of pericytes subjected to high glucose and stretch could help to clarify the effects of combined hyperglycaemia and hypertension in the pathogenesis of diabetic retinopathy.