Endothelial KCa channels: Novel targets to reduce atherosclerosis-driven vascular dysfunction

Abstract

Elevated levels of cholesterol in the blood can induce endothelial dysfunction, a condition characterized by impaired nitric oxide production and decreased vasodilatory capacity. Endothelial dysfunction can promote vascular disease, such as atherosclerosis, where macrophages accumulate in the vascular intima and fatty plaques form that impair normal blood flow in conduit arteries. Current pharmacological strategies to treat atherosclerosis mostly focus on lipid lowering to prevent high levels of plasma cholesterol that induce endothelial dysfunction and atherosclerosis. While this approach is effective for most patients with atherosclerosis, for some, lipid lowering is not enough to reduce their cardiovascular risk factors associated with atherosclerosis (e.g., hypertension, cardiac dysfunction, stroke, etc.). For such patients, additional strategies targeted at reducing endothelial dysfunction may be beneficial. One novel strategy to restore endothelial function and mitigate atherosclerosis risk is to enhance the activity of Ca²⁺-activated K⁺ (KCa) channels in the endothelium with positive gating modulator drugs. Here, we review the mechanism of action of these small molecules and discuss their ability to improve endothelial function. We then explore how this strategy could mitigate endothelial dysfunction in the context of atherosclerosis by examining how KCa modulators can improve cardiovascular function in other settings, such as aging and type 2 diabetes. Finally, we consider questions that will need to be addressed to determine whether KCa channel activation could be used as a long-term add-on to lipid lowering to augment atherosclerosis treatment, particularly in patients where lipid-lowering is not adequate to improve their cardiovascular health.

Keywords: KCa channel; SKA-31; atherosclerosis; endothelium; vascular function.

FIGURE 1

Initiation and progression stages of atherosclerosis, and the effects of KCa channel facilitation on these processes. 1) High levels of circulating low-density lipoprotein cholesterol (LDL-C) in the blood pass through the endothelium and enter the arterial intima. There, the LDL-C undergoes oxidation by reactive oxygen species (ROS), leading to oxidized LDL-C (oxLDL-C). The oxLDL-C can induce dysfunction of the endothelium, which manifests as structural and functional changes to the endothelial cells, such as an increased expression of adhesion molecules like ICAM-1 and VCAM-1. 2) Endothelial dysfunction promotes the recruitment of platelets and monocytes to the endothelium, and the monocytes then extravasate through the endothelium to enter the intima. 3) There, cytokines secreted by the dysfunctional endothelium (e.g., M-CSF, GM-CSF) induce the differentiation of monocytes into macrophages. Macrophages in the intima secrete cytokines such as MCP-1 that recruit more monocytes to the endothelium and thus promote a cyclical process that increases the number of macrophages in the intima. 4) Macrophages take up oxLDL-C via scavenger receptors and become foam cells. Concurrently, contractile smooth muscle cells (SMCs) dedifferentiate and become proliferative SMCs and SMC-derived foam cells. The continuing proliferation of foam cells and other SMC-derived cell types contributes to atherosclerotic fatty plaque formation. The red minus signs indicate processes that would be inhibited by KCa channel facilitation and oppose atherosclerosis, including decreasing ROS levels in the intima, inhibiting adhesion molecule expression by the endothelium, inhibiting platelet/monocyte recruitment to the endothelium, and inhibiting SMC phenotype change and proliferation. The green plus sign indicates that healthy endothelial function would be promoted by KCa channel facilitation, and the molecular mechanisms behind this effect are highlighted in Figure 2. This figure was created with www.BioRender.com.

FIGURE 2

KCa channel positive modulators promote endothelium-dependent vasodilatory signaling. Vasodilatory agonists in the vascular lumen, such as acetylcholine (ACh) bind to their cognate endothelial G-protein coupled receptor (GPCR), which initiates a series of cascading events that results in Ca²⁺ release from the endoplasmic reticulum (ER). Ca²⁺ released from the ER can activate endothelial nitric oxide synthase (eNOS) to produce the potent vasodilator nitric oxide (NO). Additionally, the released Ca²⁺ can activate Ca²⁺-activated K⁺ (KCa) channels that induce endothelium-derived hyperpolarization (EDH). 1) This electrical signal transfers to the smooth muscle cells via myoendothelial gap junctions (MEGJs) located within a myoendothelial projection (MEP) to inhibit Ca²⁺ entry via voltage-gated calcium channels (VGCCs). Reduced Ca²⁺ entry into the smooth muscle cell limits myosin phosphorylation by Ca²⁺-dependent myosin light chain kinase (MLCK), and the subsequent level of muscle contraction. 2) In addition to EDH-induced vasorelaxation, EDH increases the electrical driving force to promote external Ca²⁺ entry into the endothelial cell via Ca²⁺ permeable channels activated in response to a vasodilatory stimulus (e.g., Ca²⁺-release activated Ca²⁺ (CRAC) channels, TRPC3, TRPC4, TRPC6, TRPV1 and TRPV4 channels present in peripheral arteries, such as the aorta). This elevation of Ca²⁺ in the cytosol and beneath the plasma membrane can promote Ca²⁺-dependent vasodilatory signaling, such as NO production by eNOS and KCa channel activity. NO readily diffuses to the adjacent smooth muscle to induce relaxation via the cGMP/Protein Kinase G (PKG) pathway. A selective KCa2.X/KCa3.1 positive modulator (e.g., SKA-31) can facilitate the activity of endothelial KCa2.X/KCa3.1 channels, which would enhance EDH and NO production to promote vascular relaxation and healthy endothelial function. The green plus signs highlight mechanisms contributing to endothelium-dependent vasodilation that would be enhanced by KCa channel facilitation. This figure was adapted from CM John et al. (2018) and created with www.BioRender.com.