Tannic Acid Downregulates Angiotensin Type 1 Receptor in Aortic Smooth Muscle Cells Through EGFR Dependent Phosphoinositide 3‐kinase Pathway

Abstract

Tannic acid (TA), a hydrolysable plant polyphenol commonly found in food and beverages, possesses several cardioprotective effects through its anti‐inflammatory and anti‐oxidant activities. In this study, we aimed to investigate the role of TA on the angiotensin type 1 receptor (AT1R) a major hypertension target and its associated signaling pathways in primary rat aortic smooth muscle cells (ASMCs). In the presence of TA (10 μg/ml for 20 h), cells exhibited down‐regulation of AT1R specific [3H] angiotensin II (AngII) binding compared to untreated control (3628.32±289.42 vs. 7244.66 ± 40.26 DPM/mg protein, P < 0.001). TA mediated AT1R receptor downregulation in a dose dependent (0–40 μg/ml) and a time‐dependent (0–24 hours) manner, and was reversible. The reduction in AngII binding in TA‐treated cells was due to decreased receptor density rather than a change in receptor affinity as demonstrated by competition binding assay (Kd 1.302±0.152nM for TA treated vs. Kd 1.391±0.048 nM for untreated). Consistent with downregulation of AngII binding, AT1R mRNA was inhibited upon TA treatment (49.63% compared to control). TA mediated AT1R downregulation was associated with phosphorylation of extracellular signal regulated kinases (ERKs) p42/44. Pretreatment with MEK inhibitor PD98059 (20 mM) blocked TA induced ERKs p42/44 phosphorylation and AT1R downregulation. Moreover, AG1478 (5μm), an EGFR specific inhibitor, blocked TA mediated ERKs activation and down‐regulation of AT1R. Under similar conditions, treating cells with EGF (0.5 mg/ml) alone mediated ERKs activation and AT1R downregulation independent of TA (EGF; 3788.04±206.51 vs. Control; 6549.22±115.48 DPM/mg protein, P < 0.001). LY294002 (10 mM), an inhibitor of phosphoinositide 3‐kinase (PI3K), inhibited TA and EGF mediated EGFR induced ERKs phosphorylation and reversed AT1R downregulation suggesting that TA mediated AT1R downregulation is through EGFR induced PI3K dependent ERK p42/44 pathway. Consistent with the receptor downregulation, quantitative calcium analysis revealed significant inhibition of AngII induced intracellular calcium release in TA treated cells, which was reversed when cells were pretreated with either MEK or EGFR inhibitor. Collectively, our study demonstrates for the first time that TA induced EGFR, PI3K and ERK p42/44 signaling axis is essential for downregulation of AT1R expression in ASMCs. Furthermore, the study highlights the significance of TA rich diet for the prevention of hypertension and associated cardiovascular disorders.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.