Hemopexin and haptoglobin: allies against heme toxicity from hemoglobin not contenders

Abstract

The goal here is to describe our current understanding of heme metabolism and the deleterious effects of "free" heme on immunological processes, endothelial function, systemic inflammation, and various end-organ tissues (e.g., kidney, lung, liver, etc.), with particular attention paid to the role of hemopexin (HPX). Because heme toxicity is the impetus for much of the pathology in sepsis, sickle cell disease (SCD), and other hemolytic conditions, the biological importance and clinical relevance of HPX, the predominant heme binding protein, is reinforced. A perspective on the function of HPX and haptoglobin (Hp) is presented, updating how these two proteins and their respective receptors act simultaneously to protect the body in clinical conditions that entail hemolysis and/or systemic intravascular (IVH) inflammation. Evidence from longitudinal studies in patients supports that HPX plays a Hp-independent role in genetic and non-genetic hemolytic diseases without the need for global Hp depletion. Evidence also supports that HPX has an important role in the prognosis of complex illnesses characterized predominantly by the presence of hemolysis, such as SCD, sepsis, hemolytic-uremic syndrome, and conditions involving IVH and extravascular hemolysis (EVH), such as that generated by extracorporeal circulation during cardiopulmonary bypass (CPB) and from blood transfusions. We propose that quantitating the amounts of plasma heme, HPX, Hb-Hp, heme-HPX, and heme-albumin levels in various disease states may aid in the diagnosis and treatment of the above-mentioned conditions, which is crucial to developing targeted plasma protein supplementation (i.e., "replenishment") therapies for patients with heme toxicity due to HPX depletion.

Keywords: erythrophagocytosis; haptoglobin; heme; hemolysis; hemolytic index; hemopexin; iron; plasma protein therapeutics.

Figure 1

Model for the development of HPX depletion states. This model is based on data from rhesus monkeys given heme i.v. (Foidart et al., 1982), HPX metabolism studies in humans (Foidart et al., 1983), known induction of HPX by heme (Smith, ; He et al., 2010), normal recycling of HPX after heme delivery (Smith and Morgan, 1978, 1979), and HPX catabolism after i.v. administration of heme several fold higher than the binding capacity of HPX (Sears, ; Lane et al., 1972). As heme-HPX forms in plasma, uptake of this complex by receptor-mediated endocytosis into liver parenchymal cells raises heme levels. In HPX deficiency states, heme will traffic unregulated into cells. HOs initially degrade this heme releasing redox active ferrous iron, CO, and biliverdin, which is converted to bilirubin by cytosolic biliverdin reductase. Heme also travels to the nucleus to de-repress Bach 1 target genes including HO-1 and also to induce other genes including the HPX gene. As intracellular levels of heme rise in liver cells in response to increases in plasma heme, changes in HPX synthesis and catabolism are reflected in plasma HPX levels as indicated. Heme is a normal component of bile and as hemolysis progresses, unregulated heme diffusion into cells raises heme levels such that any unmetabolized heme can potentially be exported into the bile (Petryka et al., ; McCormack et al., 1982).

Figure 2

Kicking heme toxicity. As originally suggested by Delanghe et al. (2010) and Drieghe et al. (2013), there is at the bedside a currently unrecognized and potentially quite frequent need to distinguish impaired heme clearance from ongoing hemolysis. Several major organs may be involved in the pathology of heme toxicity, including the liver, lungs, heart, spleen, intestine, and kidney. Certain immune-privileged sites such as the brain and gonads are initially protected by strong endothelial barriers with tight junctions, and certain immune-privileged cells exist in an immunosuppressive environment. Thus, endothelial cells provide a physical barrier and epithelial cells provide a selective cell gate. Several markers in blood and urine samples are indicated here, in addition to the standard liver and kidney function tests and parameters used for the hemolytic index. If routinely measured, e.g., free heme, free HPX and free Hp in plasma, and heme in urine should prove useful in diagnosis and help discern when there is impaired heme clearance due to a HPX deficiency state. As endothelial cells die and lyse they may be the principal source of plasma HO1 (Ghosh et al., 2011) and also raise LDH levels Drieghe et al., . The soluble ectodomain (sed) of scavenger receptors are released into the circulation by proteolysis and it is expected that sedCD163 will bind Hb and/or Hp-Hb, and sedLRP1/CD91 may bind heme-HPX.