Enhanced Renal Clearance of Heme Triggers Acute Kidney Injury in Sickle Cell Disease

Circulating heme is scavenged primarily by hemopexin (Hx) for degradation in the liver, while alpha-1-microglobulin (A1M) binds heme for urinary clearance. Individuals with sickle cell disease (SCD) have acquired Hx deficiency. Hitherto, it has not been determined whether the low Hx in SCD results in a disproportionately large amount of heme exposure in the kidneys sufficient to promote acute tissue injury. Acute kidney injury (AKI) is a common downstream complication of acute chest syndrome (ACS) and vaso-occlusive pain crisis (VOC). Emerging evidence indicates acute elevations in circulating heme contributes to both ACS and VOC. Therefore, we tested the hypothesis that acute hemolytic crisis triggers AKI in SCD. We found that steady-state plasma A1M is raised significantly in SCD patients compared to controls (SS: 0.27±0.03, AA: 0.18±0.02 mg/ml p<0.05; n=12), and we determined that the molar A1M/Hx ratio is 14-fold higher in transgenic homozygous SCD (SS) mice compared to control (AA) littermates (n=6; p<0.05). Infusion of a non-lethal dose of purified heme (20 μmoles/kg) raised the concentration of heme in the liver but not in the kidneys of AA mice in 48 hours. On the contrary, the heme infusion over the same duration raised the total heme concentration in the kidneys but not in the liver of the SS mice (p<0.05). The SS mice but not the control AA mice developed severe renal tissue damage, defined by tubular atrophy and cast formation, loss of epithelial brush borders and apoptosis of the proximal tubular epithelium, within 48 hours of the heme infusion. Clinical detection of AKI is typically defined by elevation in serum creatinine and urinary albumin creatinine ratio (uACR). The acute elevation in circulating heme increased the plasma creatinine by 4-fold (p<0.001) with a 2-fold elevation in uACR (p<0.01) exclusively in the SS mice but not in control AA mice. Urinary kidney injury molecule-1 (KIM-1), which is a reliable predictor of AKI increased ~3-fold in the SS mice following the heme infusion, while no alteration of KIM-1 was evident in AA mice treated with the same challenge (n=6). To define the relationship between AKI in SCD and the hepatic and renal mechanisms of heme clearance, we measured plasma A1M and Hx, and urinary KIM-1 in a cohort of Ghanaian SCD patients aged 1-14 years (n=40). The molar ratio of A1M/Hx correlated positively with KIM-1 in this patient population (Pearson r=0.54; p=0.0003). Together, our data supports the conclusion that acute elevation of circulating heme triggers AKI in SCD. Clinically, the relative molar concentrations of plasma A1M and Hx can be developed as a biomarker to assess AKI risk in SCD patients. Finally, our data suggest acute infusions of Hx may protect SCD patients from AKI during episodes of hemolytic crisis.