Potential mechanism of fibronectin deposits in acute renal failure induced by mercuric chloride

Many glomerular diseases are associated with changes in the expression and distribution in the components of extracellular matrix. A remarkable feature in acute renal failure induced by mercuric chloride in rats was large fibronectin (Fn) deposits in kidneys 1 h post-HgCl2 injection (5 mg/kg body wt., s.c.). Our study examined some mechanisms as potential explanation of the early Fn deposits in mercuric chloride induced acute renal failure. Total tissue mRNA of livers and kidneys of control and treated rats were used in Northern blot to determine whether accumulation of Fn in kidney is associated with increases in the expression of this protein in the kidney and/or in the liver. Analysis of Fn levels by Western blot were also performed. Northern blot did not show significant difference between control and treated rats, while the abundance of polymerized-Fn in kidney tissue was increased 1 h and 5 h post HgCl2 injection.HgCl2 influence on Fn folding was studied in vitro to detect possible conformational changes that could altered its normal pattern of matrix assembly and/or binding to different ligands. In this context HgCl2 binding to Fn was measured following native thryptophan fluorescence of Fn in the presence of HgCl2 (0.5–250 mM). Binding parameters for the HgCl2–Fn complex formation were Kd = (1.6 ± 0.2) 10–4 M; n = 1 ± 0.3, indicating a low apparent affinity and one type binding site. Thermal denaturation of Fn showed, between 30–60°C, a soft reversible conformational change, while between 75–80°C a highly and irreversible transition is produced suggesting a modification of the tertiary structure. HgCl2 abolished this transition. The kinetic of thermal unfolding of Fn was also measured and the effects observed due to HgCl2 presence reinforced the previous data. Finally, the effect of HgCl2 on Fn binding to denatured collagen (gelatin) was also measured as an index of the effect of this cation on biological properties of Fn. Fn binds gelatin strongest in the presence of HgCl2.Our results suggest that higher Fn deposits in kidney treated rats seems not to be associated to augmented mRNA-Fn neither in kidney nor in liver. On the other hand, increased levels of polymerized Fn abundance was observed in kidney tissue from mercury-treated rats. We also describe that HgCl2 promotes, in vitro, conformational changes on Fn structure, inducing its denaturation and increasing its binding to gelatin, all events that could be related to the Fn deposits in renal tissues of HgCl2-treated rats, and could be expected in other situations that promoted interstitial fibrosis, not associated to overexpression of matrix-proteins.