Hemolysis and free heme signaling in pulmonary hypertension

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Pulmonary arterial hypertension (PAH) is a fatal disease with progressive lung vascular remodeling leading to right heart hypertrophy and failure. There is an extreme disparity between the prevalence of PAH in the general population (0.001%) and patients with hemolytic anemias (10-80%). Our previously published data indicate that idiopathic PAH patients, which have never experienced hemolytic conditions, exhibited 5-fold increase in free hemoglobin (Hb) levels in plasma, and free Hb levels significantly correlated with PAH progression. Increased hemolysis has also been observed in pre-clinical PAH models and correlated with a disease progression. While the pathogenic properties of free Hb have been previously reported, these studies were mainly focused on free Hb and free heme extracellular functions related to redox and nitric oxide scavenging activities. Therefore, there is a critical gap in understanding the mechanisms of hemolysis-mediated intracellular signaling in PAH. Indeed, our preliminary data demonstrated that free heme induces activation of phosphofructokinase (PFK) via p38/MK2 axis in pulmonary artery endothelial cells and smooth muscle cells resulting in a metabolic switch toward glycolysis. Another important question is why PAH patients have increased hemolysis? We recently discovered that the dysfunction of pyruvate dehydrogenase (PDH), a critical enzyme in metabolic homeostasis, impairs reticulocyte maturation, and responsible for the accumulation of immature reticulocytes that are susceptible to rupture. PDH insufficiency is known to be directly involved in PAH pathogenesis by compromising normal mitochondrial function. We propose that PDH inactivation is also responsible for increased hemolysis in PAH patients that have no hemolytic disorders otherwise. Based on our preliminary data, we hypothesize that PDH insufficiency increases intravascular hemolysis and activates free heme-mediated metabolic switch through stimulation of PFK. This metabolic shift occurred on the background of PDH dysfunction results in reprogramming of pulmonary vascular cells toward highly proliferative phenotype and promoted pulmonary vascular remodeling. We will delineate the importance of heme-mediated pathways in PAH in the following aims: 1) To elucidate the role of heme-mediated signaling in the activation of a glycolytic switch; 2) To determine the role of pyruvate dehydrogenase (PDH) in hemolysis; and 3) To examine the crosstalk between heme-mediated signaling and PDH deficiency in EC/SMC reprogramming. If successful, this proposal will uncover the mechanisms responsible for a strong association between hemolytic conditions and PAH, provide the potential treatments for attenuating heme-mediated pathways in PAH with hemolytic and non-hemolytic background. New targets will be tested in the proposed studies and will advance therapeutic approaches that are needed due to the lack of adequate PAH treatment options.