Effects of 5‐hydroxymethyl‐2‐furfural on the volume and membrane permeability of red blood cells from patients with sickle cell disease

Abstract

Key points We addressed the hypothesis that the heterocyclic aldehyde 5‐hydroxymethyl‐2‐furfural (5HMF) may act synergistically to ameliorate the complications of sickle cell disease through effects on red blood cell (RBC) membrane transport, in addition to its well‐known action of increasing the oxygen affinity of the abnormal form of haemoglobin, HbS. 5HMF was found to reduce deoxygenation‐induced dehydration of RBCs, whether in response to maintained deoxygenation or cyclical deoxygenation/re‐oxygenation. Acting at low millimolar concentrations, 5HMF reduced the activity of deoxygenation‐induced cation conductance (sometimes termed P sickle ), an effect which correlated with reduction in sickling. 5HMF similarly inhibited deoxygenation‐induced activation of the Ca 2+ ‐activated K + channel (or Gardos channel), an effect not seen following pharmacologically mediated increases in intracellular Ca 2+ via the ionophore A23187. Deoxygenation‐induced phosphatidylserine exposure, which is associated with Ca 2+ entry via P sickle , was also inhibited by 5HMF. By contrast, effects of 5HMF on the K + –Cl − cotransporter (KCC) were modest, with slight inhibition following treatment with N ‐ethylmaleimide (NEM) to abolish activity of its regulatory protein kinases, but stimulation in RBCs untreated with NEM. It would therefore appear that an important beneficial action of 5HMF, in addition to effects on HbS oxygen affinity, is reduction in P sickle ‐mediated Ca 2+ entry following RBC sickling, thereby inhibiting the deleterious sequelae of Gardos channel activation, RBC dehydration and also lipid scrambling. Abstract The heterocyclic aldehyde 5‐hydroxymethyl‐2‐furfural (5HMF) interacts allosterically with the abnormal form of haemoglobin (Hb), HbS, in red blood cells (RBCs) from patients with sickle cell disease (SCD), thereby increasing oxygen affinity and decreasing HbS polymerization and RBC sickling during hypoxia. We hypothesized that should 5HMF also inhibit the main cation pathways implicated in the dehydration of RBCs from SCD patients – the deoxygenation‐induced cation pathway (P sickle ), the Ca 2+ ‐activated K + channel (the Gardos channel) and the K + –Cl − cotransporter (KCC) – it would have a synergistic effect in protection against sickling, directly through interacting with HbS, and indirectly through maintaining hydration and reducing [HbS]. This study was therefore designed to investigate the effects of 5HMF on RBC volume and K + permeability in vitro . 5HMF markedly reduced the deoxygenation‐induced dehydration of RBCs whether in response to maintained deoxygenation or to cyclical deoxygenation/re‐oxygenation. 5HMF was found to inhibit P sickle , an effect which correlated with its effects on sickling. Deoxygenation‐induced activation of the Gardos channel and exposure of phosphatidylserine were also inhibited, probably indirectly via reduced entry of Ca 2+ through the P sickle pathway. Effects of 5HMF on KCC were more modest with a slight inhibition in N ‐ethylmaleimide (NEM, 1 m m )‐treated RBCs and stimulation in RBCs untreated with NEM. These findings support the hypothesis that 5HMF may also be beneficial through effects on RBC ion and water homeostasis.