In vivo and in vitro studies establishing haptoglobin as a major susceptibility gene for diabetic vascular disease

Abstract

Hemoglobin (Hb) released during hemolysis is a potent oxidant. Extracorpuscular Hb may enter the vessel wall and mediate low-density lipoprotein oxidation, thereby promoting the development and progression of atherosclerosis. Haptoglobin (Hp) is an antioxidant protein as a result of its ability to bind Hb and block Hb-induced oxidative damage. Hp also facilitates the removal of Hb from the extravascular compartment via the CD163 macrophage scavenger receptor. In man, there are two common alleles for Hp denoted 1 and 2, and correspondingly, three different possible genotypes: Hp1-1, Hp2-1, and Hp2-2. We have recently demonstrated in several longitudinal studies that Hp genotype is an independent risk factor for diabetic vascular complications. Specifically, we have shown that diabetic individuals with Hp2-2 are more likely to develop nephropathy, retinopathy, and cardiovascular disease as compared with those with Hp2-1 or Hp1-1. Mechanistically, we have found significant Hp type differences in the antioxidant and CD163-mediated scavenging and activation functions of the different Hp protein types. Furthermore, we have demonstrated that these functions are modified in the diabetic state. In this review, we focus on the clinical studies associating the Hp polymorphism and diabetic vascular complications, and the molecular basis behind this interaction.

Figure 1

Subunit organization of Hp. The Hp1 monomer can bind just one other monomer to form a homodimer in Hp1-1 individuals or as one of the different possible complexes in Hp2-1 individuals. Hp2 monomer can bind two monomers to form a variety of cyclic multimers in Hp2-2 or a variety of linear complexes in Hp2-1 individuals. In the Hp-Hb complex, each β subunit of the Hp monomer binds an αβ-dimer of Hb.