High-Glucose Media Enhance the Responsiveness of Tubular Cells to Growth Promoters: Effect on Lysosomal Cathepsins and Protein Degradation

Abstract

Nephromegaly is a prominent feature of diabetic nephropathy and predominantly reflects increased renal tubule mass, mostly due to hypertrophy. To elucidate pathogenetic factors involved, we studied the effects of high glucose (HG) alone, and in combination with hormones/growth promoters: angiotensin II (10^–7 M); parathyroid hormone (10^–7 M); insulin-like growth factor-1 (10^–7 M), or transforming growth factor-β₁ (TGF-β₁, 10 ng/ml) in a renal cell line (LLC-PK1) with many characteristics of the proximal tubule. Activities of lysosomal cathepsins (B, L+B and H) and the protein turnover were investigated. Exposure to HG (25 mM) for up to 48 h increased cellular protein content, due to enhanced protein synthesis, while protein degradation rate and cathepsin activities tended to lower values. Hyperosmotic mechanisms of glucose action were excluded, since these effects were not induced by mannitol. In normoglycemic conditions only TGF-β₁ decreased cathepsin activities and protein degradation rate significantly. However, in HG media all applied hormones/growth factors significantly lowered the protein degradation rate, as well as lysosomal cathepsin activities. The enhanced responsiveness could contribute to the impaired protein turnover, with consequent hypertrophy of the tubulointerstitium in diabetic nephropathy.