Smooth Muscle Cytochrome b5 Reductase 3 Deficiency Accelerates Pulmonary Hypertension Development in Sickle Cell Mice

Pulmonary and systemic vasculopathy are significant risk factors for early morbidity and death in patients with sickle cell disease (SCD). An underlying mechanism of SCD vasculopathy is vascular smooth muscle (VSM) nitric oxide (NO) resistance, which is mediated by NO scavenging reactions with plasma hemoglobin and reactive oxygen species that can oxidize soluble guanylyl cyclase (sGC), the NO receptor. Prior studies show that cytochrome b5 reductase 3 (CYB5R3), known as methemoglobin reductase in erythrocytes, functions in VSM as an sGC heme iron reductase critical for reducing and sensitizing sGC to NO and generating cGMP for vasodilation. Therefore, we hypothesized that VSM CYB5R3 deficiency accelerates development of pulmonary hypertension (PH) in SCD. Bone marrow transplant was used to create SCD chimeric mice with background smooth muscle cell (SMC) specific tamoxifen-inducible Cyb5r3 knockout (SMC R3 KO) and wild-type (WT) control.Three weeks after completing tamoxifen treatment, we observed 60% knockdown of pulmonary arterial SMC CYB5R3, 5-6 mmHg elevated right ventricular maximum systolic pressure (RVmaxSP) and biventricular hypertrophy in SS chimeras with SMC R3 KO (SS/R3^KD) relative to wild-type (SS/R3^WT). Right ventricular (RV) contractility, heart rate, hematological parameters and cell-free hemoglobin were similar between groups. When identically generated SS/R3 chimeras were studied 12-weeks after completing tamoxifen treatment, RVmaxSP in SS/R3^KD had not increased further, but RV hypertrophy relative to SS/R3^WT persisted. These are the first studies to establish involvement of SMC CYB5R3 in SCD-associated development of PH, which can exist in mice by 5 weeks of SMC CYB5R3 protein deficiency.