Decreased degradation of collagen and fibronectin following exposure of proximal cells to glucose

Background/Aims: Thickening and reduplication of the tubular basement membr ane has been reported as an early event in diabetic nephropathy. The aim of the work outlined here was to examine the effects and mechanisms involved in the modulation of renal proximal tubular type-IV collagen and fibronecti n turnover by glucose. Methods: The effect of glucose on type-IV collagen a nd fibronectin generation was studied by exposure of primary cultures of hu man renal proximal tubular cells (HPTC) to elevated D-glucose concentration s. Subsequently the mechanism of modulation of fibronectin generation was e xamined in a polarised system utilising the porcine proximal tubular cell l ine LLC-PK1 grown on porous tissue culture inserts. Results: Incubation of confluent growth-arrested HPTC with 25 mM D-glucose led to the accumulation of both type-IV collagen;and fibronectin. This increase was not dependent on new gene transcription for either protein. Exposure of HPTC to 25 mM D-g lucose also led to the induction of tissue inhibitor of metalloproteinases (TIMP-1 and TIMP-2) and also gelatinase A. There was, however, a net decrea se in overall gelatinolytic activity. Incubation of confluent monolayers of LLC-PK1 cells grown on tissue culture inserts with 25 mM D-glucose on eith er their apical or basolateral aspect led to fibronectin accumulation seen only in the basolateral compartment. Under these experimental conditions, w e can demonstrate polyol pathway activation, and furthermore the increase i n fibronectin concentration in response to glucose was inhibited by the ald ose reductase inhibitor sorbinil. Fibronectin accumulation was also demonst rated following both apical and basolateral addition of 1 mM sorbitol, but not following the addition of 25 mM galactose to either aspect of the cells . Conclusions: These data demonstrate that the glucose-induced accumulation of type-IV collagen and fibronectin was associated with alterations in the degradative pathway of these matrix components. In addition fibronectin ge neration in response to glucose was non-polar in terms of application of gl ucose, but polar in terms of fibronectin accumulation. The mechanisms of gl ucose-induced modulation of fibronectin were mediated by polyol pathway act ivation, and more specifically related to the metabolism of sorbitol to fru ctose. Copyright (C) 1999 S. Karger AG, Basel.