Angiotensin II blockade and low-protein diet produce additive therapeutic effects in experimental glomerulonephritis

Abstract

Background: Transforming growth factor-beta (TGF-beta) overexpression plays a key role in the accumulation of extracellular matrix in acute and chronic renal diseases. Recent studies have suggested that the degree of reduction in pathological TGF-beta overexpression can be used as a therapeutic index to evaluate the antifibrotic potential of pharmacological angiotensin II (Ang II) blockade in renal disease. Using this target, we found that treatment with the angiotensin I-converting enzyme inhibitor enalapril or the Ang II type 1 receptor antagonist losartan reduced TGF-beta overexpression more effectively at doses clearly higher than those required to control blood pressure. However, both forms of Ang II blockade were only partially effective in normalizing TGF-beta expression. This study investigated whether a greater antifibrotic, TGF-beta-reducing benefit can be achieved when Ang II blockade is combined with dietary protein restriction.

Methods: Mesangioproliferative glomerulonephritis was induced in male Sprague-Dawley rats on a normal-protein diet. Treatment with a low-protein diet and/or maximally effective doses of enalapril or losartan was started one day after disease induction. On the fifth day, 24-hour urine protein excretion was measured. On the sixth day, cortical kidney tissue was taken for periodic acid-Schiff staining. Isolated glomeruli were used for mRNA extraction or were placed in culture for determination of production of TGF-beta1, the matrix protein fibronectin, and the protease inhibitor plasmin activator inhibitor type 1 (PAI-1) by enzyme-linked immunosorbent assay.

Results: Compared with untreated nephritic animals on a normal-protein diet, a single treatment with enalapril, losartan, or low-protein diet significantly reduced glomerular TGF-beta production, albeit to a similar degree of approximately 45%. A moderate, but significant further reduction in pathological TGF-beta expression of a total of 65% for enalapril and 60% for losartan was achieved when these drugs were combined with low-protein feeding. This reduction in TGF-beta overexpression paralleled decreased proteinuria, glomerular matrix accumulation, and overproduction of fibronectin and PAI-1.

Conclusions: Ang II blockade and low-protein diet have additive effects on disease reduction, suggesting that disease progression in humans with chronic renal failure may be slowed more effectively when Ang II blockade and low-protein diet are combined. Since maximal pharmacological Ang II inhibition was used, it is likely that dietary protein restriction further reduces pathological TGF-beta overexpression by mechanisms different from those of enalapril or losartan.