Renal proximal tubular cell fibronectin accumulation in response to glucose is polyol pathway dependent

Thickening and reduplication of the tubular basement membrane have been reported as early events in diabetic nephropathy. In this study, we have examined the polar requirements of proximal tubular cells for the d-glucose-stimulated accumulation of fibronectin and the mechanism by which this occurred, with particular emphasis on the polyol pathway.

To determine the polarity of fibronectin generation in response to glucose, LLC-PK1 cells were grown on porous tissue culture inserts. Monolayer confluence was determined by serial measurement of transepithelial resistance. Confluent cells were growth arrested by serum deprivation, and all experiments were performed under serum-free conditions.

Application of 25 mm d-glucose to either the apical or basolateral aspect of LLC-PK1 cells led to fibronectin accumulation in the basolateral compartment. This reached statistical significance 24 hours following apical addition of glucose (2.6-fold increase compared with 5 mm d-glucose, P = 0.0025, N = 6 vs. N = 4 controls) and 12 hours after the basolateral addition of glucose (2.5-fold increase compared with 5 mm d-glucose, P = 0.03, N = 6 vs. N = 4 controls). Exposure of cells to glucose at either their apical or basolateral aspect leads to accumulation of intracellular glucose and polyol pathway activation, as assessed by sorbitol accumulation. The increase in fibronectin concentration in response to glucose was inhibited by the aldose reductase inhibitor sorbinil. At a dose of 100 micron sorbinil, there was a 59% inhibition of fibronectin accumulation in response to apical glucose (P = 0.004, N = 3 sorbinil vs. N = 4 controls) and a 66% inhibition in response to basolateral glucose (P = 0.008, N = 3 sorbinil vs. N = 4 controls) 48 hours after the addition of the inhibitor. Furthermore, fibronectin accumulation was also demonstrated following both the apical and basolateral addition of 1 mm sorbitol, but not following the addition of 25 mm galactose to either aspect of the cells. Following the addition of sorbitol, there was a 2. 8-fold increase in fibronectin 48 hours after apical stimulation (P = 0.01, N = 3 treated vs. N = 4 control) and a 2.27-fold increase following basolateral stimulation (P = 0.04, N = 3 treated vs. N = 4 control) at 24 hours.

In summary, these data demonstrate that fibronectin generation in response to glucose was nonpolar in terms of application of glucose but was polar in terms of fibronectin accumulation. The mechanisms of glucose-induced modulation of fibronectin were mediated by polyol pathway activation and were more specifically related to the metabolism of sorbitol to fructose.