Convergence of endothelial dysfunction, inflammation and glucocorticoid resistance in depression-related cardiovascular diseases

Abstract

Major Depressive Disorder, or depression, has been extensively linked to dysregulated HPA axis function, chronic inflammation and cardiovascular diseases. While the former two have been studied in depth, the mechanistic connection between depression and cardiovascular disease is unclear. As major mediators of vascular homeostasis, vascular pathology and immune activity, endothelial cells represent an important player connecting the diseases. Exaggerated inflammation and glucocorticoid function are important topics to explore in the endothelial response to MDD. Glucocorticoid resistance in several cell types strongly promotes inflammatory signaling and results in worsened severity in many diseases. However, endothelial health and inflammation in chronic stress and depression are rarely considered from the perspective of glucocorticoid signaling and resistance. In this review, we aim to discuss (1) endothelial dysfunction in depression, (2) inflammation in depression, (3) general glucocorticoid resistance in depression and (4) endothelial glucocorticoid resistance in depression co-morbid inflammatory diseases. We will first describe vascular pathology, inflammation and glucocorticoid resistance separately in depression and then describe their potential interactions with one another in depression-relevant diseases. Lastly, we will hypothesize potential mechanisms by which glucocorticoid resistance in endothelial cells may contribute to vascular disease states in depressed people. Overall, endothelial-glucocorticoid signaling may play an important role in connecting depression and vascular pathology and warrants further study.

Introduction

Major Depressive Disorder (MDD), or “depression” is a widespread, multifaceted mental disorder which affects nearly 400 million worldwide every year and is a leading cause of disability [1]. Depressed patients develop a heterogenous set of symptoms including, but not limited to, disturbances in sleep and appetite, a unhappy mood, loss of motivation and reward (anhedonia) and in the most severe cases, suicidal ideations and/or actions [2]. While genetics may predispose certain people to depression, it is thought that environmental factors are mostly responsible for its development, with psychosocial stress and chronic anxiety being the primary factors [2].

Mechanistically, depression has long been considered in the context of aberrant serotonergic and dopaminergic signaling, resulting in the development and widespread use of pharmacological agents (such as selective serotonin reuptake inhibitors and monoamine oxidase inhibitors) intended to bolster these pathways [3]. However, these drugs may take weeks or months to have clinical effect, despite their rapid effects on their respective protein targets; moreover, a large portion of depressed patients never see benefit after taking these drugs, indicating the need to consider alternative mechanisms and pathways in the pathology of depression [3].

The HPA axis is the key modulator of blood cortisol levels, ensuring homeostatic functioning of various physiological responses [13]. The cascade is initiated in the hypothalamus by releasing corticotropin releasing hormone (CRH) in response to stress [9]. CRH acts on the anterior pituitary to stimulate the release of adrenocorticotropin hormone (ACTH), which acts on the adrenal cortex to produce glucocorticoids (GCs) including cortisol and its rodent analog, corticosterone [9]. Cortisol activity is highly self-regulated owing to its negative-feedback activity and ACTH production [9] (Fig. 1, below). Acute upregulation of GCs induces the “fight or flight” response to allow the body to adapt to stressors. Normal action of GCs allows the body to respond to various circumstances and produce physiological responses such as vasoconstriction [14]. However, when environmental stressors are being experienced over a prolonged period of time, dysfunction of the negative feedback loop controlling the HPA axis may occur where cortisol levels fail to inhibit production of CRH and ACTH. Loss of negative feedback would logically result in excess circulating cortisol levels, and these prolonged levels of GCs may also induce the development of GC resistance [9]. Indeed, elevated cortisol levels are well documented in treatment-resistant depressed patients, with some studies finding twice as much cortisol output in depressed patients compared to age/sex matched non-depressed controls [8]. Additionally, depressed patients show impaired suppression of cortisol secretion following dexamethasone administration compared to non-depressed controls, indicating altered negative feedback function [15]. GC resistance in depressed patients is often described in the context of glucocorticoid receptor (GR) function, as GR is responsive to high levels of endogenous GCs and less responsive to basal levels of endogenous GCs [16]. Post-mortem mRNA analysis reveals alterations in GR expression in the PFC and the HPC in multiple mood disorders, including depression [17].

There is a high degree of coordination between glucocorticoid (GC) signaling and suppression of inflammation in immune response [10]. Dexamethasone and other glucocorticoids are often therapeutically used in chronic inflammatory conditions such as autoimmune disorders. As previously mentioned, excessive cortisol and glucocorticoid resistance are well-documented findings in depressed patients [9]. Importantly, inability to propagate glucocorticoid signaling is associated with excessive immune activity and disease [11, 12].

Recent research reports the presence a chronic low-grade inflammatory state in the central nervous system (CNS) as well as in the serum of depressed patients which likely contributes to the development of depression [4]. Microglia, CNS resident innate immune cells, are involved in response to psychosocial stressors and are considered to be primary drivers of CNS inflammation. Clinical and preclinical studies reveal changes in microglial response and serum inflammatory cytokines in depressed patients and relevant rodent models [5,6,7].

Fig. 1

Schematic of loss of negative feedback cycle following repeated stress exposure. Persistent release of CRH and ACTH results in persistent secretion of cortisol from the adrenal gland. This dysregulated secretion of cortisol fails to inhibit CRH and ACTH action (depicted by red “X” marks), thus failing to self-control its release. This results in excessive cortisol levels which can be seen in chronic stress and depression. Created in Biorender.com

There is a significant bidirectional relationship between depression and most forms of cardio- and cerebrovascular disease (CVD). After adjusting for lifestyle differences (diet, exercise, smoking habits, etc.), patients who are diagnosed with depression have an increased likelihood of suffering from diseases such as hypertension, atherosclerosis, myocardial infarction and stroke [18,19,20]. Depressive patients are also more likely to suffer a worse prognosis compared to non-depressed patients in outcomes such as post-stroke disability and stroke fatality [19]. Clinical studies also suggest dysfunction in endothelia, cells which line the lumen of blood vessels and form a barrier between blood and tissue parenchyma [21, 22]. A clear mechanism which links depression to vascular pathologies has yet to be elucidated, though much of the current literature suggests inflammatory molecules are present in both disease states.

Glucocorticoid signaling is conventionally thought to have strong immunosuppressive and anti-inflammatory effects. Indeed, treatment for many chronic inflammatory diseases, such as arthritis and multiple sclerosis, includes use of exogenous glucocorticoids which provide temporary symptom relief. Dexamethasone, a widely used GC, has been experimentally shown to prevent microglial activation in vivo and in vitro [23,24,25]. However, prolonged exposure to endogenous and exogenous glucocorticoids decreases their biological activity and induces a resistant phenotype [26]. GC resistance can exacerbate cardiovascular disease and disrupt immune homeostasis towards an inflammatory phenotype [27, 28].

In this review, we aim to dissect how changes in the HPA axis may influence inflammation and vascular health in depressive patients. We will describe the findings from clinical research as well as preclinical models of stress. While a multitude of previous literature explores the HPA axis and inflammation, to our knowledge no prior work discusses how these interactions may affect vascular health in the context of Major Depressive Disorder.

Chronic inflammatory phenotype of depression

The low rates of response to first-line therapeutics for MDD are evidence that the wrong systems are potentially being targeted, at least in a subset of patients. Supporting this idea, recent meta-reviews have found weak evidence for the mechanisms in which these therapeutics are based [29].

Around the time of fluoxetine’s approval (1987), a clinical trial looking at the potential therapeutic benefit of interferon-α (IFNα) for chronic viral hepatitis was being conducted [30]. Interestingly, the IFNα treatment led to the development of psychiatric side effects such as emotional lability, irritability, depression, and even suicidal thoughts. This study marked the first association between inflammatory processes and MDD. A few years later, in 1992, a study investigating the associations of the peripheral immune system and depression found elevated numbers of Ly6C-positive monocytes in the blood of patients diagnosed with MDD, giving substance to the initial association [31]. Maes et al. furthered their investigations in 1995 when they published on increases in soluble cytokines found in MDD patients [32]. Together, these studies led to the proposal of the neuroinflammatory hypothesis of depression which states that psychosocial stressors activate inflammatory responses causing release of pro-inflammatory cytokines and chemokines that contribute to monoaminergic and glutamatergic dysfunction and cognitive deficits [33].

Additional clinical and pre-clinical explorations of the neuroinflammatory hypothesis have solidified its place among the highly regarded theories of depression. In the clinic, PET tracers for mitochondrial translocator and cyclooxygenase-1 have been used to successfully show increased levels of inflammation in the brain of MDD patients [5]. Moreover, post-mortem analyses of the PFC of suicide victims have shown an increase in microglial density compared to natural death controls [5]. Interestingly, microglia are prominent expressors of both mitochondrial translocator and cyclooxygenase-1 in the CNS [34, 35]. In pre-clinical models, we and others have shown that microglia are significant contributors to stress susceptibility [7, 36].

Microglia originate from primitive macrophage progenitors (PMPs) that arise from the yolk sac during primitive hematopoiesis. PMPs migrate into the CNS and colonize the developing neural tissue before the blood-brain barrier is fully formed [37]. Once within the CNS, microglia establish themselves through interactions with developing neurons and other glial cells, undergoing morphological changes to assume their ramified, surveillant state. During development, microglia play crucial roles in shaping neuronal circuits, by pruning synapses, and maintaining homeostasis within the CNS microenvironment [38,39,40].

In adulthood, microglia exist in a surveillant state and in response to insult or injury undergo rapid activation [38]. The activation is accompanied by clonal expansion, where activated microglia proliferate to increase their numbers at the site of injury [41]. This clonal expansion serves to amplify the inflammatory response, facilitate the clearance of debris, and ultimately aid in recovery of damaged tissue. The dysregulated, prolonged, microglial activation can lead to chronic neuroinflammation [41]. This activation is also present in the aging population, who also experience depression at increased rates [42, 43].

In fact, the presence of active microglia has been noted (at times) upstream of the monoaminergic and glutamatergic changes seen in MDD. The increased production of NOS by microglia can lead to increases in the expression of the tryptophan catabolizing enzyme, indoleamine-2,3-dioxygenase [44]. The breakdown of tryptophan, as well as tyrosine, leads to lower production of serotonin, dopamine, and norepinephrine – causing the depletions observed in the synaptic cleft [44]. Additionally, the secretion of IFNs, TNF, and IL-1β by microglia can influence P38/MAPK signaling that will over activate the SERT – shuttling even more serotonin away from the post-synaptic neuron [45]. On the glutamatergic end, the secretion of pro-inflammatory cytokines as well as increased production of ROS and RNS by microglia can directly influence astrocytes to decrease their glutamate reuptake, leaving more in the synaptic cleft. Excess glutamate can then spill over and activate extrasynaptic NMDA receptors which would shut down CREB signaling essential to BDNF synthesis [46, 47]. Additionally, quinolinic acid secreted by microglia can directly bind to these extrasynaptic receptors and decrease BDNF [48]. Though these facets of inflammation in MDD have been explored and well-defined, the full extent of inflammation’s actions under chronic stress are not known. Further exploration can elucidate exactly how inflammatory processes contribute to the development and persistence of depressive symptoms and aid in targeting those mechanisms.

GC resistance can result in hyperactivity in immune cells affecting the brain and periphery [49]. Thus, it is possible that the dysregulated HPA axis may be partially responsible for the chronic low grade inflammatory phenotype that is present in some depressed patients. However, the reverse may also be true, in that circulating inflammatory cytokines are capable of reducing GR expression and function [11, 12]. This is further supported by the fact that biomarkers of both inflammation and GC resistance are associated with antidepressant resistance [50]. Successful antidepressant treatment is also associated with normalized GR sensitivity to dexamethasone suppression [51]. Additionally, GR in the hippocampus and hypothalamus is restored following successful antidepressant treatment [50]. Finally, glucocorticoid resistance may play a mediating role in comorbid MDD and CVD. Nikkheslat et al. found that peripheral blood mononuclear cells isolated from depressed coronary heart disease (CHD) patients were less sensitive to dexamethasone treatment and were more prone to inflammatory stimuli compared to non-depressed CHD patients [52]. Large population-based studies reveal that certain genetic GR variants are associated with two-to-three-fold risk of myocardial infarction and CHD, respectively [27].

Preclinical models of chronic stress allow us to explore the mechanistic interplay between GC and GR response and inflammation. Changes in GC resistance and inflammatory phenotype in stress and depression are very similar between humans and rodent models. Various studies have implicated prefrontal cortical GR in rodent stress response [53, 54]. Interestingly, genetically downregulating GR expression (i.e. more GC resistant) increased learned helplessness response after stress exposure [55]. Conversely, overexpression of GR (i.e. less GC resistant) decreased learned helplessness [55]. Studies by Jung et al. suggest that GC resistance following social defeat stress may be due to decreased mRNA expression of GR in macrophages and miRNA molecules predicted to bind GR mRNA may be responsible [56]. Engler et al. found that IL-1R deficiency attenuated stress-induced GC resistance in LPS-stimulated splenocytes, further supporting a role for inflammation in GC resistance development [57]. In mice, social stress causes decreased GC sensitivity in both splenocytes and spleen-derived monocytes as well as increased serum corticosterone [58,59,60]. Splenic monocytes also demonstrated an increased in inflammatory response [59]. When surgical adrenalectomy is utilized to attenuate corticosterone signaling, bone marrow-derived monocytes were prevented from entering circulation even following social defeat stress, whereas sham mice showed increased monocyte release [61]. This same study also demonstrate bone-marrow retention of the chemokine CXCL12 following surgical adrenalectomy [61]. Stress-induced microglial remodeling as well as expression of various inflammatory biomarkers and GC resistance were also decreased in adrenalectomized mice [61]. Overall, these studies suggest that changes in GR expression and GC resistance can promote inflammation, thus worsening depression severity.

Endothelial dysfunction in depression

Aside from nutrient and oxygen delivery, the endothelium plays a vital role in immune regulation and maintenance of homeostasis [62]. Endothelial cells are ‘bound to’ adjacent endothelial cells via paracellular junctional adhesion molecules, which form a tight barrier to prevent blood contents from leaking into tissue parenchyma [63]. The apical membrane of endothelial cells is also lined with adhesion molecules which bind circulating immune cells and platelets [64, 65]. The regulation of these adhesion molecules has major implications for immune cell trafficking, inflammation, and vascular disease progression [66]. The endothelial phenotype varies widely in different tissue types because of expression of junction adhesion molecules, cellular adhesion proteins and the influence of underlying basal lamina and surrounding cell types [67, 68]. Brain endothelial cells, for example, are affected by cells exclusive to the CNS such as astrocytes and microglia [69].

Loss of blood-brain barrier function

The function of the Blood-Brain Barrier (BBB) is to regulate cerebral blood flow and maintain CNS homeostasis by preventing infiltration of pathogens, peripheral blood proteins and peripheral immune cells [70]. The BBB is uniquely impermeable compared to peripheral vasculature due to enrichment of tight junction (TJ) proteins, such as zonula-occludens 1 (ZO1), Claudin 5 and Occludin, which link adjacent endothelial cells [63]. Under normal function, the BBB successfully blocks large ( > ~ 500 Da) and charged molecules from entering CNS parenchyma [63, 71]. Pericytes, astrocytes, microglia and neurons interact with endothelial cells of the BBB to provide trophic support as well as an enhancement of barrier function and regulation of blood flow, forming what is referred to as the neurovascular unit (NVU) [69]. Despite its strong barrier properties, the BBB is sensitive to inflammatory stimuli in a multitude of clinical and preclinical contexts [72,73,74,75].

There is extensive clinical evidence of BBB dysfunction in depressive patients. fMRI analysis studies have found increased white matter hyperintensities (indicative of BBB dysfunction) in depressed patients compared to non-depressed controls [42, 76]. Post-mortem analysis of suicide victims reveal loss of TJ protein expression in regions of the brain affected by depression [77]. Additionally, proteins that are normally unique to the CNS, including neurotrophic factor S100B, are found in the serum of depressed patients [78]. Conversely, proteins normally unique to the periphery, including albumin, has been detected in the CSF of depressed patients [79].

Multiple models of chronic stress largely suggest similar BBB changes in rodents as in humans. However, certain details in rodent models are left unclear, as there exist conflicting conclusions about region specific BBB dysfunction, as well as sex differences in NVU response to chronic stress. These discrepancies may be linked to differences in the model used. Lehmann et al. found that social defeat stress resulted in the presence of extravascular IgG, fibrinogen and erythrocyte deposition in male mice, though the exact brain region studied was unclear [80]. Their previous work indicated important microglial changes relating to barrier deficits [36, 81]. Despite this, they did not report changes in TJ protein expression in socially stressed mice. However, Menard et al. reported decreases in Cldn5 expression uniquely in the nucleus accumbens (NAc) of stress-susceptible male mice; moreover, loss of TJ expression was associated with depression-relevant behavioral changes, even outside the context of social defeat [77]. Rescuing Cldn5 expression promoted resilience to social defeat stress [82]. They later found that stress-susceptible female mice did not experience decreased TJ protein transcription in the NAc, though there were significant BBB alterations in the prefrontal cortex (PFC) [73]. Finally, hippocampal Cldn5 expression as well as that of the astrocyte end feet water channel Aquaporin 4 (considered to be a marker for BBB function) were both decreased in rats undergoing chronic unpredictable mild stress [83].

Endothelial activation

Endothelial activation describes a graded proinflammatory state of endothelial cells defined by increased expression of inflammatory cytokines and chemokines, loss of barrier functionality, increased platelet binding and coagulation and increased adherence of immune cells and their transmigration into tissue parenchyma. It is often triggered by cytokines released from stimulated immune cells including TNFα (Tumor Necrosis Factor α) and IL-1β (Interleukin 1 β) and is associated with many inflammatory disease states such as atherosclerosis and acute coronary syndrome [84, 85]. Common markers of endothelial activation include ICAM-1 (Intercellular Adhesion Molecule 1), VCAM-1 (Vascular Cell Adhesion Molecule 1) and E-selectin [65]. The solubilized form of these proteins, found within serum, is often used as a measure of endothelial damage [22].

Clinical studies exploring endothelial damage and activation have primarily focused on solubilized biomarkers of endothelial damage. Across multiple studies, depressed patients consistently exhibited significantly increased serum sICAM-1 [22, 85]. This was true in depressive populations that did not suffer from cardiovascular disease as well as patients recovering from acute coronary syndrome and other forms of heart disease, indicating a potentially important link between depression and CVD [85, 86]. Indeed, depressive patients that show increased sICAM-1 but no obvious cardiovascular disease may represent an “at-risk” population, as sICAM-1 is associated with increased subclinical atherosclerosis and risk of disease [87, 88]. Lastly, bioinformatic studies used Disease Ontology analysis to explore common upregulated genes and pathways associated with depression and acute myocardial infarction, and found ICAM-1 as a hub gene linking the two diseases [66].

We and others have also found evidence of endothelial activation in rodent models of psychosocial defeat stress [7, 89]. Our group has described increased endothelial ICAM-1 and VCAM-1 in the PFC of stress-susceptible male mice compared to stress-naïve mice through immunofluorescence and immunoblotting analysis [7]. Yin et al. performed transcriptomic analysis on the PFC of socially stressed male mice and found similar results [64]. Studies performed by Sawicki et al. revealed increased transcription of endothelial adhesion markers as well as chemokines CXCL1 and CXCL2 following social defeat stress [89]. To our knowledge, preclinical studies primarily focus on endothelial expression of activation markers and are mostly performed in male rodents. Thus, data on serum levels of endothelial damage markers as well as expression in female rodents is still scarce and should be further explored. Future studies may explore the source of serum sICAM-1 (endothelial-derived or immune-derived) as well as the mechanism by which stress leads to increased sICAM-1 levels.

Matrix metalloproteinase activity

Matrix metalloproteinases (MMPs) are a family of 28 different zinc-dependent proteases which play a major role in regulating blood vessel remodeling and function [90]. We summarize differential MMP activities in stress and depression in Table 1. Due to the available literature and its pathogenic role in vascular diseases, we will focus our discussion on MMP-9. MMP-9 is a gelatinase commonly referenced for its roles in the pathogenesis of multiple disease states. MMP-9 is upregulated by inflammatory cytokines and NFκB activation, and actively degrades ECM and TJ proteins thereby reducing barrier integrity and stimulating angiogenesis through release of VEGF [91,92,93,94]. Excessive MMP-9 activity promotes intimal thickening (heightened cell and ECM deposition within the inner vessel wall) and atherosclerotic plaque instability, thus inducing thrombosis [95,96,97]. Higher levels of MMP-9 in patients with cardiovascular disease were associated with poor prognosis, and had stronger predictive value than commonly used biomarkers including C-reactive protein (CRP) [98, 99]. Studies using preclinical models of cerebral ischemia using fibrin-based Middle Cerebral Artery occlusion (MCAo) have found that selective inhibition of MMP-2 and − 9 using SB-3CT alleviated behavioral, neuronal and vascular deficits caused by occlusion [100, 101]. Inhibition of MMP-2/-9 using this same molecule also attenuated the neurodegenerative BBB-altering effects in a rotenone-induced rodent model of Parkinson’s Disease. The findings of this study also support a role for microglia in activation of MMP-2/-9 [72]. While these disease contexts are separate from MDD, they are still important to consider as they all exist and are exacerbated by a neuroinflammatory milieu. Additionally, both stroke and Parkinson’s disease have significant comorbidity with depression [102].

Table 1 Summary table of existing literature relevant to MMP and TIMP proteins in human MDD and preclinical models of chronic stress

While the pathogenic role of MMP-9 has been studied extensively in a variety of CNS and peripheral vascular diseases, its connection to stress and depression are less clear. Some studies find increased MMP-9 activity in sera of patients with psychological disorders as well as decreases in endogenous MMP inhibitors (Tissue Inhibitors of Metalloproteinases, TIMPs), while others find either no change or decreased MMP-9 levels in depressed patients [103,104,105,106]. Additionally, certain polymorphisms of MMP-9 were found to influence the likelihood of suffering from recurrent depressive disorder [103]. Electroconvulsive therapy reduces serum MMP-9 levels in those who respond, but does not in non-responsive patients [107]. Data on CNS levels of MMP-9 in depression are scarce, though one study found increased MMP-9 in depressed suicide victims [108]. Mice undergoing chronic corticosterone treatment saw increased MMP-9 activity in the CA1 and CA3 of the hippocampus (HPC) as well as in the medial prefrontal cortex (mPFC), both areas of the brain commonly associated with depression, and these increases in proteolytic activity correlated with increased behavioral despair [109]. Rodent models of chronic mild stress show similar increases [110]. However, Cathomas et al. found no difference in circulating proMMP-9 serum levels in mice undergoing social defeat stress [111]. This discrepancy may be due to differences in the models used, or it may be that MMP-9 expression in stress/depression is tissue-dependent. The exact mechanism by which stress may increase MMP-9 expression is not known, though it may be related to the increased levels of different inflammatory cytokines and NFκB activation [91, 93].

Atherosclerosis

Atherosclerosis, the buildup of plaque primarily composed of fats and low-density lipoprotein (LDL) cholesterol, is a primary underlying cause of many cardio- and cerebrovascular diseases including stroke. First, circulating monocytes infiltrate the intima and differentiate into macrophages. These macrophages consume oxidized LDL to become foam cells, which are deposited on arterial walls within the intima. Foam cells promote smooth muscle proliferation and synthesis of ECM components to form a fibrous cap [112]. Endothelial damage and activation are important early mediators in the development of atherosclerotic plaques [112]. Presence of the plaque can impede blood flow through the artery, and rupture of the plaque can ultimately lead to vessel occlusion. Multiple risk factors for atherosclerosis contribute to disease including hyperlipidemia, hypertension, obesity and inflammation [113].

Given the link between depression/stress and the risk factors for atherosclerosis, it is perhaps unsurprising that depression increases the likelihood of developing plaques. Indeed, numerous studies have reported on chronic stress being a risk factor for atherosclerosis independent of confounding variables such as obesity and hypertension [114,115,116]. There are multiple biomarkers of atherosclerotic severity which are also upregulated in depressed patients including IL-6 and CRP [117, 118]. Since adhesion molecules are necessary for the onset of atherosclerosis and are upregulated in MDD, they may play a role in connecting the two disease states. Lastly, multiple mouse models of chronic stress, including social defeat stress, have been combined with the apolipoprotein E deficient mouse + high-fat diet mouse model of atherogenesis and have consistently shown increased plaque burden in stressed mice compared to unstressed mice [119,120,121]. Though there is variability in stress and atherogenic mouse models, their use allows for the contribution of stress in plaque development to be mechanistically studied.

Endothelial glucocorticoid resistance in disease

Similarly to immune cells, short term GC treatment typically ameliorates inflammatory responses in endothelial cells. As such, GCs often alleviate pathological phenomena such as endothelial activation, that are exacerbated by inflammation. For example, in vitro studies on mouse and human endothelial cells revealed a number of physiological benefits following GC treatment including increased expression of TIMP1, ZO-1, claudin-5 and occludin as well as decreased expression of ICAM-1 and VCAM-1 [74, 122,123,124,125,126,127]. These effects were consistent when endothelial cells were given inflammatory challenges including LPS treatment and administration of serum from MS patients [75, 128]. In vivo and clinical studies also found that GCs provided benefits in diseases strongly defined by endothelial activation, including human endotoxemia and during cardiac surgery [129, 130]. Zielinska et al. thoroughly reviewed GC effects on endothelial cells in a multitude of inflammatory diseases, often outside of the context of GC resistance [131]. However, GC resistance remains an obstacle in endothelial cells as they do in immune cells. As such, anti-inflammatory effects of GC begin to wain and inflammatory markers become more abundant (Fig. 2). In this section we will describe endothelial glucocorticoid insensitivity in inflammatory diseases that are associated with depression as well as cellular mechanisms by which this phenomenon may occur.

Fig. 2

Effects of GC stimulation on endothelial cells and immune cells. Normal stimulation inhibits production of common markers of endothelial activation/inflammation including MMPs and adhesion molecules, while promoting junction molecule expression. GC resistance (by either downregulation of GR or inability of GR to translocate to the nucleus [132]) lowers normal stimulation, thus contributing to a pro-inflammatory state. Black arrows indicate processes following normal stimulation. Red arrows indicate changes to normal processes which occur in resistance phenotype. Created in Biorender.com

Modulation of glucocorticoid sensitivity in endothelial cells

Endothelial sensitivity to GC is dependent on a few important factors: GR proteins levels and ability of the GR to translocate to the nucleus following GC exposure. Mata-Greenwood et al. found that in primary human umbilical vein endothelial cells (HUVECs), there was a positive correlation between nuclear GR and dexamethasone sensitivity [132]. Additionally, dexamethasone-resistant HUVECs had shorter GR half-lives compared to dexamethasone-sensitive HUVECs [132]. This is likely due to increased interaction between dexamethasone-resistant GR and the proteasomal recruiting protein, BCL1-associated athanogene 1 (BAG1) [133]. Indeed, some studies have shown that inhibition of the proteasome at multiple steps were capable of restoring GC sensitivity in endothelial cells, indicating that it may constitute a potential therapeutic target to address GC resistance [132, 134]. It was also shown that dexamethasone-resistant HUVECs demonstrated stronger interactions between their GR and BAG1 [132]. Aside from promoting degradation of GR, BAG1 may bind to DNA to prevent GR-mediated transcription or affect GR protein folding in an HSP70-mediated manner [135, 136]. Epigenetic regulation of GR expression may also play a role in GC sensitivity, as GC-resistant endothelia show distinct patterns of GR promoter methylation compared to GC-sensitive endothelia [137]. As the effects of GC are typically seen from the context of immune cells, studies exploring GC resistance in endothelial cells are limited. Given the unique role of endothelial cells in regulating immune response and homeostasis in nearly all tissues, it is important that further studies be conducted in the future.

Kidney disease

Kidney disease has a bidirectional association with MDD: Chronic kidney disease patients are more likely to suffer from depression and depressed patients are more likely to develop CKD and have worse prognosis [138, 139]. Like in depressed patients, kidney disease is associated with excess cortisol production [140]. It is likely, though not yet known, that the cardiovascular changes associated with depression are likely contributors to this relationship [141]. Endothelial cells play a major role in kidney homeostasis. Mice lacking endothelial GR suffered from accelerated diabetic renal fibrosis compared to control mice, as well as aberrant cytokine and chemokine production, both of which were likely mediated by Wnt signaling [142,143,144,145,146]. While this study demonstrates that endothelial GR may affect disease outcome, it does not necessarily suggest that the reverse is occurring: that kidney disease is affecting endothelial GR. As changes in GC resistance are associated with more severe outcome in both MDD and kidney disease, exploring GC signaling may yield important findings.

Atherosclerosis

As previously described, MDD patients have an increased risk of developing atherosclerotic plaques [18]. Though glucocorticoids may promote hypertension, a risk factor for atherosclerosis, they also have strongly alleviated inflammation and immune-endothelial interactions, another major risk factor for atherosclerosis [147]. Interestingly, excess GC is a risk factor for cardiovascular disease, though excess GC is often associated with GC-resistance and other confounding factors such as depression [148]. The limited studies investigating endothelial GR in atherosclerosis seem to demonstrate that loss of endothelial GR exacerbates atherosclerosis [149]. Some researchers have hypothesized that Wnt signaling in endothelial cells may be involved in GC-mediated regulation of atherogenesis [143]. These studies explore the loss of endothelial GR, and do not specifically test for GR resistance. However, GR expression and function are often used as a proxy for GC sensitivity, since GR mediates signaling at high GC levels. Additionally, the complex interactions between immune cells and endothelia in atherogenesis as well as the ubiquitous nature of GC binding make it difficult to determine precisely how GR signaling in each cell type may be involved in disease progression.

Stroke

MDD patients have worsened risk and prognosis of ischemic stroke, as well as multiple risk factors for stroke onset including hypertension and atherosclerosis [18,19,20]. As such, effects of GR signaling on these risk factors may indirectly affect risk and outcome of stroke. However, GR signaling in stroke is more complicated than in other disease contexts. GR antagonism by mifepristone injection was found to ameliorate the negative effects of chronic stress on stroke outcome. However, injections were performed prior to chronic stress and it was unclear how often mice were injected in relationship to the stress paradigm, so GC resistance may not have been established [150]. Other studies have found that GC administration had a neuroprotective effect in stroke by promoting endothelial Nitric Oxide Synthase (eNOS)-mediated cerebral blood flow. The neuroprotective effects were blocked by the GR antagonist RU486 [151]. For both studies, it is important to note that systemic administration of GC will activate GR in many tissue types, so the observed effects may be due to nonendothelial GR signaling. Furthermore, some studies have found neither a harmful nor a beneficial effect of GR signaling in stroke [152]. To our knowledge, two studies have specifically explored endothelial GR signaling. Kleinschintz et al. found that ischemic endothelial cells are resistant to GC and fail to react to dexamethasone treatment. However, sensitization of GR to GC ligands via proteasome inhibition restored BBB stabilization after dexamethasone treatment [134]. Subsequent mouse studies revealed that dexamethasone treatment combined with proteasome inhibition reduced brain edema and functional deficits following MCAo [134]. On the other hand, Sorrells et al. observed that endothelial GR KO mice expressed lower TJ proteins, which was especially surprising given that GC have been found numerous times to stabilize the BBB by upregulating TJ proteins [74, 124, 126, 153].

Multiple factors may explain the discrepancies between studies. One technical factor may be differences in stroke surgeries. The effectiveness and the duration of ischemia are major players which can drastically affect the extent of hypoxia in endothelial cells. Therefore, a more effective or longer MCAo may render endothelial cells more GC resistant, which can easily affect efficacy of GC treatment [134]. Counteracting biological processes may also explain these discrepancies. While GC ameliorate inflammatory signaling (thus providing a neuroprotective effect in stroke), their vasoconstrictive affects may also lessen blood flow (thus providing a neurotoxic effect in stroke). These factors emphasize the need for more precise, cell-type-specific studies on GR signaling in the many cell types that affect the dynamic environment in ischemic stroke as well as stronger consistency in stroke models.

Concluding remarks

Endothelial cells are critical for maintaining homeostasis and regulating immune response in all tissues. They are also major mediators for cardiovascular diseases that affect a huge proportion of the population in the world [154]. MDD also affects a large proportion of people and is highly comorbid with most forms of cardiovascular disease. Therefore, it is important to understand the vascular changes that occur in depression.

GC resistance has been extensively described in preclinical models and MDD patients in many tissue types including neurons in multiple brain regions as well as several peripheral and CNS immune cells [17, 50, 55]. Despite this, we know of no studies which specifically explore endothelial GR function in MDD. Given the disrupted HPA axis function in depression, the comorbidity of depression with most vascular diseases, and the pivotal role that GR signaling has in numerous physiological and disease contexts, endothelial GC resistance in depression represents a major direction of future research. Other psychiatric illnesses like anxiety, bipolar disorder and schizophrenia, though less researched, have been linked to inflammatory response and cardiovascular changes [105, 155, 156].

Since GC resistance is highly prevalent in multiple cell types in depression, we hypothesize that GC resistance also is evident in endothelial cells during depression. Links between inflammation and GC resistance have been drawn in multiple diseases, including MDD, where both processes may contribute to one another. Therefore, cells which play major roles in regulating inflammation are also affected by GR dysregulation. As described above, MDD patients express many markers of vascular damage and endothelial inflammation. As such, it is reasonable to assume that changes in GC signaling/GR function occur in endothelial cells in MDD. The disease implications of this hypothetical GC resistance are less clear. The clinical effects of endothelial GC resistance appear to depend on the disease context, though GC resistance tends to be associated with stronger disease severity and worse outcomes. Since MDD is often comorbid with many vascular pathologies, it is difficult to assign a single role of endothelial GC resistance in depression-related cardiovascular disease.

Chronic inflammation and dysregulated HPA axis activity and GC resistance are known hallmarks of depression. However, to our knowledge, these phenomena have yet to be discussed in relation to one another and in relation to vascular health in MDD. In this review, we summarized changes in endothelial function, immune activity, and GC signaling in MDD and in multiple cardiovascular disease states often comorbid with MDD. An understanding of endothelial changes in GC signaling in depression may reveal important drivers of depression-CVD interactions.