Comment on Nielsen et al, page 2846
The findings of Nielsen and colleagues that haptoglobin-related protein (Hpr) binds to free hemoglobin as avidly as haptoglobin emphasize the critical redundancy in hemoglobin clearance mechanisms and suggest the existence of intravascular cross talk between pathways often considered distinct: vasomotor, hemostatic, immunologic, metabolic, and oxidative.
The release of hemoglobin from lysed red cells into blood plasma unleashes its vasculotoxic potential, by directly impairing endothelial function and generating inflammatory and oxidative stress.1 It appears that the basic biologic necessity to clear hemoglobin efficiently from blood plasma has generated enough evolutionary pressure to produce redundant mechanisms to do the job, as Nielsen and colleagues from Denmark and London show in this issue of Blood.
Previous work by this group and others has shown that the plasma protein haptoglobin and cell-free plasma hemoglobin form a high-affinity complex, which displays a neoepitope recognized by CD163, the hemoglobin-scavenger protein expressed on reticuloendothelial cells.2 Receptor-ligand endocytosis of the complex rapidly clears it from blood plasma and drives expression of an antioxidant pathway that includes heme-oxygenase 1 and biliverdin reductase.1 Based on affinity chromatography and surface plasmon resonance analysis, Nielsen and colleagues now show that haptoglobin-related protein (Hpr) also binds to free hemoglobin as avidly as does haptoglobin. Of interest, the circulating Hpr-hemoglobin complexes are not directed to CD163, but rather into specialized high-density lipoprotein (HDL3) particles, where they are sequestered with apolipoproteins apoA-I, apoL-I, and others. The apoL family is a recently identified class of apolipoproteins that constitute part of the innate immune system in humans.3 These interesting pore-forming proteins carry a motif similar to the BH3 domain of the Bcl-2 family of proteins.
The vascular system of complex organisms is normally protected from the toxic, oxidative, and nitric oxide (NO)-scavenging properties of hemoglobin by its encapsulation within the red blood cell membrane. When hemoglobin escapes into blood plasma, it can scavenge NO, the signaling molecule that orchestrates a protective program that guards vascular health.4 Extracellular hemoglobin promotes highly oxidative reactions involving iron, the heme porphyrin ring, and oxygen radicals that produce endothelial dysfunction. The resulting reactive oxygen species can then react with NO, further reducing its bioavailability and producing the highly oxidative peroxynitrite.
Decreased NO bioavailability results in chronic vasoconstriction, hemostatic activation, and vascular smooth muscle proliferation. These pathways are associated with pulmonary hypertension, priapism, and cutaneous leg ulceration in sickle cell disease and possibly in other disorders of intravascular hemolysis severe enough to overwhelm the hemoglobin-scavenging system.5 These epidemiologic associations implicate chronic hemoglobinemia as a pathophysiologic mechanism in vasculopathy.
The current findings of Nielsen et al now link hemoglobin scavenging to the HDL particle and suggest an intravascular cross-talk between pathways often considered distinct: vasomotor, hemostatic, immunologic, metabolic, and oxidative. Many fascinating questions arise concerning the role of Hpr-bound hemoglobin in HDL3 particles. This HDL particle subtype is characterized by the presence of Hpr, apoL-I, and apoA-I and is also called trypanosome lytic factor-1 (TLF-1), due to its potent antimicrobial activity on Trypanosoma brucei brucei.3 Nielsen and colleagues find that hemoglobin-bound Hpr copurifies with apoL-I and apoA-I. This colocalization with the physiologic antioxidant apoA-I might neutralize the oxidant effects of Hpr-bound hemoglobin. However, it remains unknown whether this complex can still scavenge NO. If this NO scavenging and reactive oxygen species generation is limited by such sequestration into the HDL3 particle, this mechanism might account for part of the protective effects of HDL against oxidant stress and vasculopathy in atherosclerosis. This hemoglobin-scavenging effect might be particularly important for intraplaque hemorrhage, which might lead to release of free hemoglobin and local NO scavenging, potentially contributing to progression of the coronary artery occlusion. Furthermore, one might speculate that the release of heme iron into microorganisms or infected cells might play a role in oxidative killing by TLF-1, although published data have been conflicting. All of these questions merit deeper investigation. ▪