Endoplasmic Reticulum Stress Inhibition Blunts the Development of Essential Hypertension in the Spontaneously Hypertensive Rat

Endoplasmic reticulum stress inhibition blunts the development of essential hypertension in the spontaneously hypertensive rat.

Safaa Naiel1, Chao Lu1, Victor Tat1, Jeffrey G. Dickhout1

1. Department of Medicine, Hamilton Centre for Kidney Research, McMaster University and St. Joseph’s Healthcare Hamilton, Hamilton, ON, Canada

ABSTRACT

Essential hypertension is the leading contributor to premature death worldwide. However, its cause remains uncertain. Endoplasmic reticulum (ER) stress has recently been associated with essential hypertension. However, it is unclear whether ER stress causes hypertension. To clarify this question, we determined if ER stress occurs in blood vessels before the development of hypertension, and if ER stress inhibition would prevent hypertension development. We used the spontaneously hypertensive rat (SHR) as a model of human essential hypertension and the Wistar Kyoto (WKY) rat as its normotensive control. SHR and WKY rats were sacrificed before the development of hypertension to collect resistance arteries and assess levels of ER stress markers. We found ER stress to be present in the SHR before hypertension development. To assess the effect of ER stress inhibition on hypertension development, another subset of SHR were treated with 4-phenylbutyric acid (1g/kg/day) for 8 weeks from 5 weeks of age. Blood pressure was measured via radiotelemetry and compared to vehicle treated SHR and WKY rats. Mesenteric arteries were collected at sacrifice and assessed for structural and functional changes associated with hypertension. Systolic and diastolic blood pressure was significantly lower in the 4-PBA-treated SHR groups than in vehicle treated SHRs. In addition, mesenteric arteries from the 4-PBA treated SHRs had a significant decrease in media-to-lumen ratio, ER stress marker expressions, as well as improved vasodilatory response to carbachol and reduced contractile responses to phenylephrine. Overall, ER stress inhibition blunted the development of hypertension in the SHR. Our data suggest that a component of the increase in blood pressure found in the SHR is caused by ER stress.

INTRODUCTION

Hypertension is responsible for approximately 7.6 million deaths each year, which accounts for 13.5% of the total deaths worldwide1. Despite its high prevalence, there is much controversy regarding the biological cause of hypertension.

Blood pressure (BP) is the product of cardiac output and total peripheral resistance (TPR). The structure of small resistance arteries and their resultant influence on blood vessel function is an important regulator of long-term BP2. Small resistance arteries, such as muscular arteries, predominantly regulate the resistance to blood flow found in the vasculature2. Studies have shown that arteries with a diameter size between 100 to 300 micrometers are considered resistance vessels2, 3. Schiffrin and colleagues determined if there were differences in the resistance vessels of hypertensive and normotensive patients4. Subcutaneous resistance arteries of lumen diameter less than 300 micrometers were isolated from both groups. Analyzing the vessels through a pressurized myograph, and quantifying the media-to-lumen ratios of hypertensives' arteries, it was shown that theses arteries had a higher media-to-lumen ratio and greater medial width versus the non-hypertensive patients4.    

The SHR is the most commonly used animal model to study human essential hypertension. Blood pressure begins to diverge between SHR and Wistar Kyoto (WKY) rat, its normotensive control, at 4 to 6 weeks of age5, 6. Blood vessels of the SHR at this pre-hypertensive age have been shown to have impaired endothelium-dependant relaxation, as well as exaggerated contractile responses 2, 5, 7-9.

Emerging evidence from numerous studies has indicated the potential role of endoplasmic reticulum (ER) stress in diseased blood vessels and hypertension10-17. Different animal models have been used to study the association between endoplasmic reticulum (ER) stress and hypertension including AngII infusion13, 18, high salt diet feeding14 and the SHR11. A recent study revealed that the altered contractile responses seen in the aorta of the SHR may be attributed to ER stress9. The ER manufactures secretory and membrane proteins19. It also provides the optimal biochemical environment for protein folding and assembly, producing functional and mature proteins16, 20, 21. Certain protein mutations prevent proper protein folding which consequently causes an accumulation of unfolded proteins in the lumen of the ER22. This excess of misfolded proteins and other disruptions in ER homeostasis cause ER stress and can be observed by increases in ER stress markers expression11, 19. The accumulation of misfolded proteins that accumulate in the ER can occur in diseased blood vessels and may be contributing to hypertension11. A study has shown that acute infusion of tunicamycin in mice caused an increase in systolic and diastolic blood pressure and treatment with a low molecular chaperone reduced overall blood pressure23. ER stress inhibition with a molecular chaperone, has been shown to correct resistant arteries structure and function and reduce overall hypertension in adult SHRs11.  

An underlying question that is yet to be answered is if ER stress causes hypertension. In this study, we hypothesized that if ER stress causes hypertension, then ER stress markers expression should occur in resistance vessels at the initiation of hypertension development. Additionally, we hypothesized that inhibiting ER stress in the blood vessels would prevent the development of essential hypertension in the young SHR by correcting for resistant arteries structure and function.

In this study, we used the ER stress inhibitor 4-phenylbutyric acid (4-PBA). 4-PBA is a low molecular weight chemical chaperone that has been demonstrated to alleviate the accumulation of misfolded proteins in the ER24. We expect 4-PBA to reduce the accumulation of unfolded proteins in the ER lumen of hypertensive blood vessels. We hypothesize that preventing this unfolded protein accumulation will prevent ER stress, increase TPR, and hypertension in the SHR.