Vascular Stress Signaling in Hypertension

Abstract

Cells respond to stress by activating a variety of defense signaling pathways, including cell survival and cell death pathways. Although cell survival signaling helps the cell to recover from acute insults, cell death or senescence pathways induced by chronic insults can lead to unresolved pathologies. Arterial hypertension results from chronic physiological maladaptation against various stressors represented by abnormal circulating or local neurohormonal factors, mechanical stress, intracellular accumulation of toxic molecules, and dysfunctional organelles. Hypertension and aging share common mechanisms that mediate or prolong chronic cell stress, such as endoplasmic reticulum stress and accumulation of protein aggregates, oxidative stress, metabolic mitochondrial stress, DNA damage, stress-induced senescence, and proinflammatory processes. This review discusses common adaptive signaling mechanisms against these stresses including unfolded protein responses, antioxidant response element signaling, autophagy, mitophagy, and mitochondrial fission/fusion, STING (signaling effector stimulator of interferon genes)-mediated responses, and activation of pattern recognition receptors. The main molecular mechanisms by which the vasculature copes with hypertensive and aging stressors are presented and recent advancements in stress-adaptive signaling mechanisms as well as potential therapeutic targets are discussed.

Keywords: cell survival; endoplasmic reticulum; hypertension; inflammation; mitochondria.

Figure 1.. Pharmacological targets of ER stress signaling in hypertension.

AT1R signaling disrupts ER homeostasis via increased protein synthesis demand and oxidative stress. Adaptive stress responses including the UPR, ER mediated autophagy (ER-phagy), and ER associated degradation are induced. These responses are initiated by chaperone GRP78 detachment from three transmembrane molecules in order to aid in refolding of misfolded proteins. IRE1α oligomerizes and autophosphorylates to induce RNAse activity to alternatively splice XBP1 mRNA to XBP1s. XBP1s serves as a transcription factor binding to ER stress response elements (ERSE) to induce transcription of cytokines, lipid biogenesis, chaperones, and ERAD. IRE1α also acts as a scaffold for TRAF2 docking and initiation of proinflammatory JNK and NF-κB signaling. PERK dimerizes and autophosphorylates leading to phosphorylation of eIF2α and translation attenuation in an attempt to reduce overwhelming demand of protein folding on the ER. There is an increase translation in ATF4 expression which acts as a transcription factor for amino acid metabolism, autophagy elements, and an antioxidant response. However, CHOP expression is also induced which leads to apoptosis. GRP78 detachment from ATF6 results in its translocation to the Golgi where it is cleaved by S1 and S2 proteases resulting in ATF6 cleaved for which transcribes chaperones, ERAD elements, and XBP1. ERAD induction is mediated by VCP/p97, a Ca²⁺-associated ATPase which participates in ubiquitin-proteasome system to degraded misfolded proteins by interacting with E3 ubiquitin ligases such as HRD1. ER-phagy involves PI3K complexes which form at ER tubules to recruit autophagic initiation complexes, leading to the lipidation of LC3 and formation of autophagosome. Created with BioRender.com

Figure 2.. Pharmacological targets of mitochondrial stress signaling in hypertension.

AT1R signaling induces oxidative stress in the mitochondria. In the vasculature, hypertensive stimuli lead to decreased antioxidant mitochondrial components SIRT3 and SOD2, increased ATP production via enhanced FAM3A and ETC complexes. Increased mitochondrial ROS and damage induces adaptative stress responses. Mitochondrial-associated membranes (MAMs) link mitochondria to the ER. Cellular stress causes a surge of Ca²⁺ via VDAC1 and IP3R which contributes to apoptosis initiation. Mitochondrial damage also leads to the release of mtDNA which can be sensed by cyclic GMP-AMP synthase (cGAS) and its downstream signaling effector stimulator of interferon genes (STING). This leads to NF-κB and IRF3 transcription of interferons and cytokines to contribute to inflammation. Mitophagy via PINK1 depolarization in the inner mitochondrial space leads to Parkin ubiquitin ligase activity on the outer mitochondria membrane which recruits autophagy machinery for phagosome engulfment of damaged mitochondria. Mitochondrial damage also heightens DRP1 GTPase activity to induce fission and tilt the balance away from fusion (mediated via MFN2). Excessive mitophagy and mitochondrial fission has been found in hypertensive models and contributes to vascular pathology. Created with BioRender.com.

Figure 3.. NRF2 and oxidative stress.

Impaired NRF2 activation reduces the expression of antioxidant proteins, leading to exacerbation of cellular oxidative damage. In endothelial cells, ROS overproduction reduces NO bioavailability, impairs vasodilation, causes inflammatory endothelial activation, ER stress and mitochondrial dysfunction. In VSMCs, oxidative damage also activates proliferative pathways and increases vasoconstrictor responses. NADPH Oxidase (NOX), Toll-like receptors (TLR), angiotensin II (Ang II), nitric oxide (NO), nuclear factor (erythroid-derived 2)-like 2 (NRF2), Kelch-like ECH-associated protein 1 (KEAP1), antioxidant response element (ARE), Small Maf proteins (sMAF), damage-associated molecular patterns (DAMPs), pathogen-associated molecular patterns (PAMPs). Created with BioRender.com.

Figure 4.. Ligands for TLRs and NLRP3 inflammasome.

TLRs and NLRP3 inflammasome are involved in a growing number of infectious, autoimmune, and metabolic diseases. Nearly every class of microbe and many cell-derived as well as synthetic compounds activate or modulate TLRs and NLRP3 inflammasome. The Hemozoin, disposal product formed from the digestion of blood by some blood-feeding parasites such as Plasmodium. Nigericin, microbial toxin derived from Streptomyces hygroscopicus. Flagellin, structural component of the bacterial flagellum. PFT, pore-forming toxins. ATP, adenosine 5’-triphosphate. Drusen, aging- and macular degeneration-associated accumulations of extracellular material that build up between Bruch’s membrane and the retinal pigment epithelium of the eye. Nano-SiO₂, silica dioxide nanoparticles.

Figure 5.. Activation of TLRs pathway and their downstream targets.

In hypertension, TLRs expressed in the cell membrane or in the endosomes are activated by damage-associated molecular patterns (DAMPs) derived from the cellular damage and it triggers the downstream signaling pathway through the activation of adaptor molecules myeloid differentiation protein (MyD88) and TIR domain-containing adaptor protein inducing interferon-β (TRIF) to induce the activation of NF-κB, which will, ultimately, lead to the formation of proinflammatory cytokines and interferons.​

Figure 6.. Two step model for NLRP3 inflammasome activation.

In hypertension and aging, the priming signal (first signal) is provided by proinflammatory cytokines, such as IL-1β and TNF, or molecules released from damaged cells (the so called damage-associated molecular patterns – DAMPs) that interact with specify membrane receptors (IL-1R, TNFR, TLR) leading to activation of transcription factor nuclear factor-kappa B signaling. NF-κB upregulates gene expression of NLRP3 components (NLRP3, ASC adaptor molecule), pro-caspase 1, pro-IL-1β, pro-IL-18 and other pro-inflammatory mediators. The complete NLRP3 assembly is induced by a second signal, that can be represented by a plethora of factors - K+ efflux, increased intracellular Ca²⁺, lysosomal leakage, mitochondrial damage and ROS mainly derived from ER stress, damaged mitochondria [(mt)ROS and mtDNA], and NADPH oxidase (NOX). ER stress also activates NF-κB, TXNIP, and SREBP signaling, further contributing to NLRP3 activation. Activated NLRP3 recruits and activates caspase-1 that cleavages pro-IL-1β, pro-IL-18 and FL-GSDMD (Gasdermin D) to their respective active forms. GSDMD induces pyroptotic cell death and IL-1β and IL-18 contribute to hypertension- and aging-associated vascular damage. Created with BioRender.com.