Consequences of haemolysis without haptoglobin

Abstract

Haptoglobin (Hp), a conserved plasma glycoprotein, forms very stable soluble complexes with free plasma haemoglobin. Haemoglobin binding by haptoglobin is thought to be important in the rapid hepatic clearance of haemoglobin from the plasma and in the inhibition of glomerular filtration of haemoglobin. It is thought to reduce haemoglobin-induced renal damage during haemolysis. To evaluate these functions, Hp knockout (Hp^-/-) mice were created. The Hp^-/- mouse was generated by a standard gene replacement technique in mouse embryonic stem cells. These mice were evaluated with and without haemolysis using several parameters: mortality, haemoglobin clearance, renal tissue damage and function.

Hp^-/- mice were viable but had a small, significant reduction in postnatal viability. The lack of Hp did not impair clearance of free plasma haemoglobin. Induction of severe haemolysis by phenylhydrazine caused extensive haemoglobin precipitation in the renal tubular cells. However, haemoglobin precipitation in the kidney was not increased in Hp^-/- mice. Nevertheless, Hp^-/- mice were more susceptible to phenylhydrazine with a mortality rate of 55% in Hp^-/- mice versus 18% in Hp^+/+ mice. In general, phenylhydrazine-treated Hp^-/- mice suffered greater tissue damage, as evidenced by the induction of a hepatic acute phase response, resulting in increased plasma₁-acidic glycoprotein (AGP) levels and higher plasma malonaldehyde (MDA) and 4-hydroxy-2(E)-nonenal (HNE) levels. Gross pathological analysis indicated that the kidney was the most affected tissue in phenylhydrazine-treated Hp^-/- and Hp^+/+ mice, and Hp^-/- mice were more severely affected. They had lower mitotic indices in their kidneys, higher basal levels of renal lipid peroxidation, as evidenced by levels of malonaldehyde and 4-hydroxy-2(E)-nonenal (MDA/HNE) and elevated levels of 8-hydroxyguanine (but not other products of oxidative DNA damage). There also was increased induction of haem oxygenase-1. The more severe renal damage in Hp^-/- mice was also evident in the delayed erythropoietin gene expression and poorer renal clearance of ³H]-inulin. The reduction in glomerular filtration function in Hp ^+/+ and Hp^-/- mice could be restored to baseline by vasodilators (prazosin or diazoxide), implicating renal vasoconstriction as a major mechanism of acute renal failure during induced haemolysis.

These data suggest that Hp plays a pivotal role in reducing renal oxidative damage during haemolysis.