Background: Tubulointerstitial fibrosis is an important component of diabetic nephropathy, which is characterized by increased expression of interstitial extracellular matrix components and aberrant expression of the basement membrane component collagen type IV. The present study examined the effect of high ambient glucose and transforming growth factor-beta1 (TGF-beta1) on collagen secretion by human renal fibroblasts and proximal tubular epithelial cells (PTECs).
Methods: Human renal fibroblasts (TK173) and PTECs (HK2) were used to examine the effects of high glucose (25 mM d-glucose) and TGF-beta1 (1 ng/ml) on collagen type I, III and IV secretion compared with control medium (5.5 mM glucose). Matrix components were measured by enzyme-linked immunosorbent assay (ELISA) and reverse transcription-polymerase chain reaction (RT-PCR).
Results: Renal fibroblasts are the main producers of the interstitial components collagen type I and type III, while collagen type IV was secreted predominantly by PTECs. However, renal fibroblasts were also able to secrete collagen type IV. Secretion of collagen type IV by fibroblasts was increased upon stimulation with TGF-beta1, reaching levels comparable with those secreted by TGF-beta1-induced PTECs. Moreover, high glucose stimulated increased collagen type IV secretion. Importantly, this could not be attenuated by neutralizing pan-specific anti-TGF-beta antibodies.
Conclusions: These data show that renal fibroblasts secrete collagen type IV, which can be increased by high glucose independent of endogenous TGF-beta. This suggests that as well as the increased expression of interstitial components, renal fibroblasts can contribute to the increased expression of the basement membrane component collagen type IV in tubulointerstitial fibrosis observed during diabetic nephropathy.