Background: Transforming growth factor beta is recognized as a major cytokine in extracellular matrix (ECM) pathobiology as occurs in diabetic nephropathy. While experimental studies have advanced a protective role of carnosine for diabetic complications, a link between carnosine, TGF-β and matrix accumulation remains to be elucidated. In the present study, we tested the hypothesis that L-carnosine inhibits TGF-β production and signalling, thereby reducing hyperglycaemia-associated ECM accumulation.
Methods: Human mesangial cells (MC) were cultured in high-glucose (HG, 25 mM D-glucose) medium alone or in HG medium to which 20 mM L-carnosine was added. Collagen VI (Col6) and fibronectin (FN) deposition and messenger RNA expression were studied. In addition, TGF-β production and activation of Smad1/5/8 (ALK1) and Smad2/3 (ALK5) pathways were assessed.
Results: Under HG conditions, deposition of Col6 and FN were increased 1.4- and 1.6-fold. This was significantly inhibited on the protein and messenger RNA level by L-carnosine. TGF-β production increased under HG conditions but was completely normalized by addition of L-carnosine. Addition of exogenous TGF-β could not overcome the effect of L-carnosine on Col6 and FN expression, indicating additionally interference with TGF-β downstream signalling. Along the same line, L-carnosine reduced TGF-β-mediated Smad2 phosphorylation, suggesting an inhibitory effect on ALK5 signalling. ALK1 signalling remained unchanged. Under HG conditions, pharmacologic inhibition of ALK5 prevented Col6 accumulation but did not change FN deposition.
Conclusions: L-carnosine can modulate matrix accumulation in two ways. Firstly, inhibition of TGF-β production might result in an overall inhibition of matrix accumulation and secondly, L-carnosine inhibits TGF-β-induced matrix accumulation, most likely via inhibition of the ALK5 pathway.