Cholesterol, a repressor of natriuretic proteins

Abstract

Salt‐sensitivity in Dahl rats is due, in part, to the reduction of hemoxygenase‐1 (HO1), cyclooxygenase 2 (COX2), and nitric oxide synthase‐2 (NOS2) in the renal medulla. Renal overexpression of HIF1‐α, a regulator of HO1, COX2, and NOS2, enhances salt and water excretion and reduces blood pressure. Cholesterol (chol) incorporation into collecting duct (CD) cells represses COX2 activity in response to tubular flow or fluid shear stress (FSS). We hypothesized that dietary chol ingestion represses flow responsive a genes necessary to effectuate salt and water excretion.To this end, mice were fed either a 0% or 1% chol diet for up to 12 weeks, injected with SQ isotonic saline at 5% of body weight, and urine collected at 2, 4, and 6 hrs. Kidneys were extracted to measure medullary expression of HO1, COX1, and NOS2. Mice were divided into three dietary time points: (1) 3–5 (2) 6–8 and (3) >9 weeks of diet.Urine volume was less in the chol group at the 6 hr collection for both 6–8 (n=8 each group; p<0.05) and >9 week (n=8 each group, p<0.05) groups. Urinary [Na], [K] and osmolality were higher while Na excretion was lower at the 6 hr urine collection time point in the chol (n=8; p<0.05) vs. control (n=8) fed mice at 6–8 week group.. Renal medullary HO1 (0.76±0.09; p<0.05), and NOS2 (0.76±0.1; p<0.05) mRNA expression were down regulated compared to controls while COX2 and HIF1‐α were unaffected. ATP‐binding cassette transporter (ABCA1), a tissue chol efflux transporter, was increased in the medulla (2.0±0.3; p<0.05) of chol fed mice. To test the flow responsiveness of HO1, COX2, NOS2 and HIf1‐α, IMCD3 cells were exposed to 0.4 dynes/cm2 of FSS for up to 6 hours. HO1 mRNA increased by 350X fold, COX2 by 25X fold, and NOS2 by 8X fold at 6 hrs in sheared (n=4–6; p<0.01) vs. static (n=3–6) cells. HIF1‐α mRNA increased by ~50% (p<0.05) at 4 and 6 hrs. Western blotting of FSS exposed cells increased the steady state protein abundance of NOS2 at 2, 4, and 6 hrs vs. static controls, while HO1 and COX2 protein abundance increased at 4 and 6 hrs vs. static controls. In contradistinction, HIF1‐a protein expression was reduced at 2, 4, and 6 hrs in FSS exposed cells vs static controls. Next, shear exposed, chol loaded IMCD3 cells were compared to shear exposed, untreated IMCD3 cells to test whether cholloading represses FSS sensitive natriuretic protein expression. At 2 hrs of FSS, COX2 (−29%), NOS2 (−23%) and HO1 (−18%) protein expression was significantly (p<0.05) but modestly reduced in chol loaded cells vs. untreated controls. Similar to the mouse, chol incorporation into cells raised ABCA1 mRNA levels 3.7±0.2 fold vs. untreated cells.Dietary chol is associated reduced urine volume and Na excretion at 6 hrs after saline injection and suppressed flow induced HO1 and NOS2 gene expression. In vitro studies of cultured IMCD3 cells illustrate that FSS induce COX2, NOS2 and HO1 expression, and that chol loading of cells can suppress this effect. We speculate that chol incorporation into tubular epithelia represses of natriuretic factors necessary excrete Na.Support or Funding InformationVAThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.