Insulin increases deformation-induced lung cell death

Introduction

With insulin use increasing to treat ICU hyperglycemia, we wished to determine whether insulin is cytoprotective in the presence of deformation-induced lung cell injury.

Methods

A549 epithelial cells were grown in full growth media for 24 hours on deformable six-well culture plates coated with type 1 collagen. Cells were then grown in glucose concentrations of 1.26 g/l, 4.26 g/l and 7.26 g/l for an additional 24 hours. After 2 hours of serum starvation, cells were exposed to 100 nM insulin for 30 minutes, followed by stretching for 2 minutes at 8 Hz, 30% strain amplitude and 140%/s strain velocity in the presence of 1% FITC solution, a marker of plasma membrane injury and reseal. Following stretching, the cells were incubated with 1% PI, a marker of cell death, with quantification of death performed by confocal microscopy.

Results

In undeformed cells the rate of cell death was 0.8%, 0.5% and 1.2% at 1.26 g/l, 4.26 g/l and 7.26 g/l glucose, respectively. In undeformed cells treated with 100 nM insulin, cell death was 0.2%, 1.3% and 2.1%. Deformation increased the percentage of cellular death to 3%, 5% and 2% compared with undeformed cells. Deformation increased the percentage of death in cells treated with 100 nM insulin to 15%, 15%, and 8% as compared with undeformed cells treated with insulin and deformed cells grown in glucose alone. Cell death did not differ from control at insulin concentrations ranging up to 100 nM but doubled and plateaued, with concentrations of 100–300 nM.

Conclusion

Insulin increases deformation-induced death in lung cells. This is consistent with a previously published multivariate analysis of patients with respiratory failure showing that the amount of infused insulin and the mean glucose level were independent risk factors for increased mortality. Our findings may have important implications for the use of insulin therapy in critically ill and ventilated patients. The mechanism by which insulin influences lung cell death after deformation is not yet known.