Aldosterone Stimulates Neutrophils Leading To Increased β-Glucuronidase, Protein Disulfide Isomerase and Myeloperoxidase Secretion

Aldosterone (ALDO) has been shown to play an important role in inflammatory responses in addition to its well described effects on sodium homeostasis via activation of the mineralocorticoid receptor (MR). However, its effects on polymorphonuclear leukocytes (PMNC) are not well described. We isolated untouched circulating human PMNC by immunomagnetic isolation following density gradient sedimentation with PolymorphPrep from otherwise healthy subjects. Flow cytometric analyses showed greater than 97% of PMNC were positive for the myeloid-neutrophil markers, CD45, CD16 and CD66b. We show that PMNC express MR by western blot and RT-PCR analyses. We incubated PMNC with ALDO (10^–9–10^–7M) for 30 min and observed a dose-dependent rise in β–glucuronidase release with an EC₅₀ of 6.11 nM (P<0.001, n=3), an event that was blocked by pre-incubation of cells with 1μM canrenoic acid (CA), an MR antagonist (P<0.04, n=3). In addition, our results show that incubation of human PMNC with 10^-8M ALDO likewise led to increases in myeloperoxidase ([MPO], P<0.05, n=3) and protein disulfide isomerase ([PDI], P<0.01, n=4), a multifunctional enzyme of the thioredoxin superfamily that mediates redox modifications, regulates KCNN4 channel and erythrocyte volume and is up-regulated under hypoxic conditions (Prado, 2013 FASEB J). We then studied the effects of ALDO on HL-60, a human promyelocytic cell line, induced to differentiate into neutrophil-like cells by incubation for 5 days with 1.3% DMSO. Our results likewise show an increase in MPO responses upon 10^–8M ALDO stimulation as compared to vehicle (AUC: 1090±147 to 505±48, P<0.02, n=3). We have recently reported that aldosterone stimulates increases of striatin, a scaffolding protein that interacts with caveolin-1, and co-precipitates with striatin and as such may facilitate cross talk of signaling complexes. As there are no pharmacological inhibitors of striatin we used a molecular approach to reduce striatin levels. In differentiated HL-60 cells, siRNA against striatin led to reduced MPO responses (AUC: 590±14 to 528±13, P<0.05, n=3) that were associated with significantly reduced striatin mRNA levels but not when cells were transfected with scrambled siRNA as determined by quantitative RT-PCR with ABI TaqMan detection probes and β-microglobulin used as an endogenous control (P<0.01, n=3). These results suggest that striatin plays an important role in ALDO-stimulated degranulation responses. Of importance we also observed that incubation with ALDO (10^–9–10^–7M) in differentiated HL60 cells led to increases in the oxidative-respiratory burst [superoxide production] in a dose- and time-dependent manner (P<0.01, n=4). Consistent with these results, we observed that ALDO likewise led to significant increases in the oxidative-respiratory burst in human PMNC (P<0.01, n=3). As there is evidence that activated neutrophils, MPO and PDI are elevated in Sickle Cell Disease, we studied the in vivo effects of MR blockade in BERK sickle transgenic mice, a model of increased oxidative stress. Sickle mice were randomized to receive either normal rodent chow or chow containing eplerenone (156 mg/kg per day), an MR receptor antagonist, and tap water ad libitum for 14 days at which time the mice were sacrificed and blood collected. We observed that mice on eplerenone had significantly lower plasma PDI activity than mice on regular chow (63.7 ± 8.7 control diet to 47.9 ± 2.4 eplerenone, Relative Fluorescence Units [RFU]; P<0.005, n=6 and 9) and lower MPO levels (AUC: 214±11 to 73±20, P<0.03, n=3); events that were associated with increases in both erythrocyte MCV (41.3±2.5 vs 47.4±1.1 fL, P<0.03, n=7) and reticulocyte MCV (53.6.3±2.8 vs 60.1±0.6 fL, P<0.02, n=7). Thus, our results suggest that MR activation by ALDO is a novel mechanism for neutrophil stimulation and as such represents a novel therapeutic target aimed at ameliorating the vascular complications of Sickle Cell Disease. Supported by NIH R01HL090632 (AR) and R01HL096518 (JRR).