The Physiopathological Crossroads of Aging

Stress, inflammation and Plasminogen activator inhibitor 1 (PAI-1) are key mechanisms throughout the development of aging, constituting a crossroad in the most frequent pathologies that accompany it. Among metabolic processes, obesity, metabolic syndrome and type 2 diabetes mellitus are included and Alzheimer’s disease among the neurodegenerative processes. Stress is a mechanism of defense of the organism against exogenous and endogenous actions called stressors. In the case of low intensity stimuli, the organism responds with actions aimed at a physiological adaptation (Homeostasis). On the other hand, when a high intensity (experimental level) or chronic stimu-lus (oxidative stress) is repeated, structural and functional changes are observed in different organs with activation of the hypothalamus-pituitary-adrenal axis, the renin angiotensin system and the sympathetic nervous system, sti-mulating the production of hormones that release cytokines with proinflam-matory/antiinflammatory properties that play an important role in the pre-viously mentioned pathologies, as well as a marked increase in PAI-1, a gene regulated by stress and by cytokines, with manifest action at the origin of thromboembolic disease, so frequent in aging. The objective of this review is to highlight the importance of the binomial stress and PAI-1 in aging and in the pathologies that accompany it. Because PAI-1 is part of the pathology and complications in aging, some authors suggest the study of PAI-1 inhibitors to achieve its physiological levels, as part of the treatment of these diseases.


Introduction
The concept of stress was defined decades ago by Hans Hugo Bruno Selye as a physiopathological syndrome generated by various endogenous/exogenous harmful agents. Afterward, it has been considered as the "General Adaptation Syndrome", an implicit concept of a complex process of damage and specific or nonspecific defense that accompanies us throughout our lives, especially in the aging process and in the illnesses that accompany it, whose symptoms are independent from the nature of the harmful agent as such, representing a response to the damage [1] [2] [3]. All agents can act as stressing agents (physical or psychological damage) always carrying stress and specific actions. These actions affect the organs differently and stress "per se" acts only through the general syndrome of adaptation causing defense by means of the mobilization of agonists or antagonists, in order to stabilize or adjust the response to damage. However, the general adaptation syndrome can create an organic alteration (damage) through non-specific defense [3]. In relation to adaptation to endogenous/exogenous damage, in which the brain is considered the main organ of stress reactivity [4] [5], there are different interpretations to define the cellular or organic response (allostasis/hormesis) to adapt and maintain homeostasis [6] [7] [8].
The development of the individual after its conception, development and subsequent decline, a phase that accompanies processes such as stress of lower intensity [9] [10] [11] [12], constantly produces homeostatic adaptations [13] [ 14], through exhibitions of sublethal stressing factors [15]. At the experimental level, tensions of greater intensity, which are associated with the administration of diphtheria toxin, produce changes in anatomical structure and function, as well as vascular infarcts by intravascular coagulation in different organs and in the hypothalamus-pituitary-adrenal (HPA) axis [4]. There is a marked increase in the expression of PAI-1 after the administration of endotoxin [16]- [21] or in the face of any inductor in acute/chronic phase, which would explain the frequency of cardiovascular-thrombotic complication in the different pathologies that accompany aging [22]. In summary, repeated episodes of chronic or acute stress generate an inflammatory process through the activation of the autonomic nervous system and the HPA axis, which causes the release of stress hormones, proinflammatory cytokines and a series of reactions in cascade, forming a pathogenic crossroad that leads to atherosclerotic, cardiovascular, metabolic and neurodegenerative injuries, leading to the presence of multiple diseases [23] [24] [25].
The crossroads of physiopathology: stress, aging and PAI-1.
The organic response to stress includes the activation of organs and systems: the HPA axis, the renin-angiotensin system and the (TGF-β1), interleukin 6 (IL-6), increase in the tumor necrosis factor alpha (TNF-α), accompanied by a significant increase in PAI-1, induced by stress.
PAI-1 is considered a star gene for its pathophysiological importance and pleiotropy, finding a large increase in this protein in senescence and aging ( Figure 1) [21] [22].
A myriad of theories or hypotheses has been exposed since 1990 to explain the process of human aging [36]. [43].
The aging process involves a numerous complex of pathophysiological actions of repair and defense as a strategy, not only for its maintenance but also to avoid diseases, and is considered as a biological process outside the field of pathology [12] [44] [45] [46]. The most significant physiopathological characteristic that reveals aging is a gradual loss of function or degeneration that takes place at the molecular and cellular level, as well as at the tissue and organic level, giving rise to cellular senescence, finding chronic inflammation as a fundamental substrate [47] [48]. The gerontological effort of the research has been concentrated in achieving a healthy aging and an extension of the longevity through the hormesis induced by a mild stress with physical, biological and alimentary means [8] [10] [44] [49]- [56].
Different contributions indicate how the resistance to different acute stress stimuli is lower with age, probably due to the inability to respond to stress, as evidenced in experimental studies in rats, in which there was a marked decrease in the expression of heat shock proteins HSP, specific HSP70 [26] [27] and in those carried out in nematodes (C. elegans) subjected to different stimuli (cold, heat, oxidative stress etc.) [57].
A great variety of physiopathological changes are carried out in aging due to a harmful effect of chronic stress [29], of extrinsic or intrinsic origin, which causes an imbalance in the oxidation/oxidation defense mechanism, which leads to a decrease in reduced GSH levels, both at the tissue level and at the cellular level [58], a secondary process to the release of adrenocorticotropic hormone or cortisone, accompanied by an increase in oxidized GSH [59] [60].
GSH is the most important antioxidant agent used by each of our cells and, among other actions, regulates the functions of the immune system, protects the DNA from mutations and helps inhibit viral replication, being the most important component of the antioxidant mechanism of the brain [61] [62]. However, throughout the development of aging, GSH levels tend to decrease [58] [63]- [68] in the pathogenic entities that accompany aging [62]. This favors the action of cell damage in neurodegenerative diseases and cerebrovascular accidents [69] [70].
The progressive decrease of antioxidant GSH, which is recognized in the development of aging as a consequence of oxidative stress and the inflammatory process that accompanies it, leads to the release of different stress hormones, acute phase proteins and pro-inflammatory cytokines that are going to be part of the most frequent pathogenic entities in the development of aging. [23] [29] [30] [71].
The human being is constantly under the influence of different alarms or stimuli that generate stress. The most recognized mechanism to explain the development of aging is the "oxidative stress" caused by the decline of antioxidant defense mechanisms, which entails to the formation of harmful substances that cause damage to subcellular units, protein and nucleus acid, which can explain the most frequent pathologies in animal aging, a direct consequence of the metabolism of aerobic cells [39] [42] [72] [73] [74].
Oxidative stress has been recognized as a factor in the development of aging and a process involved in many of the pathologies associated with aging. In the field of chemistry, oxidative stress refers to the reduction of cellular potential of the cellular redox pair as the main cellular antioxidant GSH [67] [75] [76] [77] [78]. GSH constitutes a cellular defense system at different levels of stress: in the face of mild stress, an increase in GSH levels has been observed, being considered as a critical feature of preconditioning and tolerance.
A characteristic of the brain in Alzheimer's disease is oxidative stress, as evidenced by markers of protein oxidation, lipid peroxidation, generation of oxygen free radicals by beta-amyloid peptide (β-amyloid), the oxidation of DNA and the formation of the 3-nitrotyrosine marker among others [79] [80] [81] [82].
Stress and inflammation, as an integral process inherent to stress, are physiopathological mechanisms closely linked to aging and the most frequent pathology that sustains it [30]. Both processes form a unique path in the different chronic pathologies that develop throughout aging [83] [84] [85], leading to a significant increase, in epidemic type, of pathogenic entities associated with metabolic disorders: obesity, MS, DM2, Alzheimer's disease, Parkinson's disease, atherosclerosis and in several types of cancer (Breast, Colorectal, Liver, Pancreas). They are characterized by the presence of the binomial insulin resistance/PAI-1, the first link in the relationship between the different pathologies for the development of cardiovascular and thrombotic diseases, entities considered as the leading cause of morbidity/mortality [22]

Cytokines and PAI-1 at the Physiopathological Crossroads
Inflammation, as a physiological response to stress, plays a key role in the onset and progression of the different chronic pathologies that often accompany the development of physiological aging [98].
The prevalence of obesity increases in the most developed countries in all age groups, including the elderly group [99] [100] [101], correlating with stress through the activation of the HPA axis, favoring the accumulation of fat (obesity). Thus a chronic and inflammatory stress action is exerted with the release of adipose tissue from substances (cytokines, hormones), among which are of interest: adiponectin, leptin, TNF-α, monocytic chemoattractant protein-1 (MCP-1), different interleukins (IL-1 beta, IL-6, IL-8), TGF-β1, PAI-1, and is accompanied by oxidative stress. All this forms a crossroads (or physiopathological template) [102] [103] [104] between aging and the most frequent pathologies that support it, which are MS, DM2, neurodegenerative pathology, cancer, cardiovascular complications and thromboembolic disease [105]- [123], which increase the risk of morbidity/mortality [124]- [129].
Obesity is defined as an excessive accumulation of fatty acids in adipose tissue [130], considered as an endocrine organ [131], and as an accumulation of other cell types (immune cells, fibroblasts, endothelial cells) [132] [133] [134]. Obesity, especially of the endothelial type, is associated with inflammation and this was shown in experimental studies in rodents with obesity and diabetes, as it found an increase in the expression of TNF-α in adipose tissue and its ability to induce resistance to insulin [135]. Insulin resistance by glucose stimulation is a very widespread process found in patients with alteration in glucose tolerance, in patients with DM2 and up to 25 percent in non-obese patients with normal tolerance to glucose orally [136].
Visceral (android) obesity is the main risk factor or central and causal component of MS, also known as insulin resistance syndrome. The MS is defined as the grouping of physiopathologically interconnected processes; biochemical, clinical and metabolic factors, constituting a complex combination of central obesity (abdominal), insulin resistance/hyperinsulinemia, dyslipidemia and hypertension, chronic stress and a hypercoagulable state, due, among other factors, to the increase of the PAI-1 gene, a characteristic of the pathology that accompanies aging [135] [137] [138] [139] [140] and that increases the risk of cardiovascular complications and mortality [141].
Excessive accumulation of visceral fat is associated with high levels of adipokine and especially with high levels of PAI-1, a factor very linked to the MS and which carries with it insulin resistance [115] [160], which allows us to consider the PAI-1 as a therapeutic target in the field of these pathologies.
After the discovery of insulin by FG Bantin and Ch Best, 1921, the results found with the implementation of the technique of glucose tolerance in normal subjects by Himsworth [161], lead to the conclusion that there were two different entities of diabetes, one of them sensitive to insulin and the other insensitive to it. Years later, these results are interpreted as a defect of assimilation by the different tissues of the patients [162], or as a peripheral resistance of the insulin due to a defect of the postreceptor in the place of the action of the insulin [163].
It is stated that insulin resistance stimulated by insulin is found in the majority of patients with glucose intolerance, non-insulin-dependent diabetes mellitus and in approximately 25% of non-obese individuals with normal oral tolerance of glucose [136]. In the same line, patients with hypertension with or without treatment, are insulin resistant, hyperglycemic and hyperinsulinemic, and are associated with a plasma increase in the concentration of triglycerides and a decrease in high density lipoprotein cholesterol, defining this clinical-analytical conglomerate as syndrome X [136] being known as MS later or also defined as Insulin resistance syndrome, forming part of the beginning and evolution of processes or entities such as hypertension, DM2, atherothrombotic and atherosclerotic development, cardiovascular entities, Alzheimer's disease and cancer [118] [122] [164] [165] [166].
The oxidative stress associated with the obesity, resistance to the insulin metabolic, provides a high risk of suffering diabetes type 2, like shows for the high concentration of a isoprotane, the marker 8-isoprostaglandin F(2 alpha) that those patients present, significant of a bigger lipid peroxidation in plasma [167] [168] [169]. In this line, repeated episodes of acute or chronic stress cause an induction of answer of acute phase inside an inflammatory process as integral part of the answer to the stress, associating with the cytokines presence, mainly the IL-6 and the reactants of sharp phase and C Reactive Protein (CRP) (30), considered as a factor of risk for the development of DM2 [170]. DM2 is associated with a sanguine increment of markers of answer of acute phase, CPR, serum amyloid A and cortisol, and the main interleukin of the inflammatory answer IL-6 [171]. Nevertheless, debate exists on the origin or the circulating markers in DM2 based on the evidence that the body fat is the main origin of the basal level and the increase of IL-6 and CPR is a consequence of the resistance to the insulin [172]. DM2 patients have a high incidence of morbidity/mortality attributed to premature atherosclerosis or to the high incidence of cardiovascular processes, thrombosis and cerebrovascular diseases, due to their complex pathogenic arsenal associated in greater or lesser proportion with oxidative stress, obesity, hyperglycemia, dyslipidemia and hypertension, in an inflammatory setting with its courtship of proinflammatory cytokines and acute phase proteins, processes associated with the most frequent pathology in aging, and, above all, the marked increase in PAI-1, the common denominator of these pathologies and main factor of atherothrombotic complications [88] [91] [173]- [179].

Metabolic Diseases and Their Affinity for Alzheimer's Disease
Stress has a great impact on neurodegenerative diseases, having a great role in its development, progress and evolution [180]. There is evidence of the effect of stress on the brain compartment during the development of aging, mainly showing a functional decrease in cognitive activity [181]. An exposure to stress at the beginning of life or in adolescent age can program the HPA axis towards the field of pathology and increase its susceptibility in the future, to the development of metabolic, neuropsychiatric and neurodegenerative diseases [59] [182] [183] [184]. Acute/chronic stress responds to the secretion of cortisol, the most important glucocorticoid in humans due to its regulatory action on the HPA axis [185]. Glucocorticoids at the level of the brain compartment play a fundamental role in microglial proinflammatory potentiation by means of IL-1β, IL-6, Nuclear Factor kappa B1α through the N-methyl-d-aspartate receptor [186] [187]. Epidemiological studies show us that part of the pathogenic coincidences found in the complex metabolic processes that support aging (obesity, MS and DM2) accompany neurodegenerative entities, and especially Alzheimer's Disease (AD) [188]. The Rotterdam study was the first to suggest that individuals with DM2 had a higher risk of developing AD [189], and that diabetes could contribute a significant proportion in the process of AD in patients with cognitive disorders [190]. An increase in the prevalence of DM2 and Impaired Fasting Glycemia (prediabetes) has also been observed in a group of patients diagnosed with AD [191].
From the last decade of the last century the researchers focused their attention on the inflammatory mechanisms and their implication in the neurodegenerative processes, with special attention to the AD, finding acute phase proteins such as alpha-1 antichymiotrypsin and CRP, as well as microglia and astroglia activated in the brain with AD, presence of cytokines and chemokines, and activation of the classic and alternative pathways of the complement system [192] [193] [194] [195], finding high concentrations of IL-6, IL-1β, IL-12, IL18 and high levels of TGF-β in the cerebrospinal fluid (CSF) and in the serum before and after death [196] [197].
Scientific studies show the presence of IL-1β and IL-6 in senile plaques and in cortical neurons [198], as well as their ability to stimulate the synthesis of the β-amyloid precursor protein in AD [199], linking these cytokines with the deposit of β-amyloid in AD [200]. Also, there are significantly elevated concentrations of these cytokines in the cerebrospinal fluid of neurodegenerative processes in AD and Parkinson's disease [201] intervening in the transformation of diffuse plaques [202] or pre-amyloid deposits [203], very extended in the brain, in AD neuritic plaques [204], requiring the activation of the microglia [205] [206].
The microglia is cells with phagocytic capacity that are a main part of the mechanism immune system of the central nervous system. Microglia present two functional activation pathways: classical pathway induced by Th1 cytokines (interferon gamma, IL-6, IL-12) giving expression to proinflammatory cytokines TNF , IL-1β, IL-6, MCP-1 and to reactive oxygen species and nitric oxide and, secondly, to the alternative pathway, induced by the Th2 cytokines (IL-4, IL-13 ) giving expression to anti-inflammatory cytokines, IL-4, IL-10, TGF β [207] [208], leading to TGF-β contributing to the survival of phagocytic microglia after autocrine suppression of TNF-α production and oxidative stress [209]. Briefly, TGF-β, during the alternative activation induced by IL-4 of microglia, acquires a very important role as antiinflammatory molecule and as immunoregulatory factor for microglia [210].
The scientific evidence recognizes the brain as an organ sensitive to insulin and responsible for the physiopathological changes of metabolic processes due to insulin receptors (IR) on the cell surface that are widely expressed in different cell groups, finding among the main ones those corresponding to the hypothalamus, hippocampus and cerebral cortex [211]- [216].
As stated, insulin resistance is a process that plays an important role in AD. There are scientific criteria to consider the binomial insulin resistance/PAI-1 as a characteristic of AD, a pathogenic aspect that aims to support the association between metabolic diseases, the development of AD and related dementias [166] [217]- [225].
It is known that there are no available means for a premature diagnosis or for the application of an effective treatment for AD [226]. Since the last century there has been an interest in studying biomarkers, among which the PAI-1 assessment is included. A study in neurological patients versus patients without neurological disease showed that the level of PAI-1 in CSF in neurological patients was significantly increased in the following processes: AD, cerebral infarction, infection of the Central Nervous System (CNS], seizures due to alcohol withdrawal and CNS neoplasms, reaching the conclusion that the PAI-1 assessment could represent a non-specific marker of CNS disease [227]. Another study in patients with mild cognitive impairment or with AD process versus healthy controls showed that PAI-1 levels gradually increased in patients as dementia progressed, concluding that the level of plasma PAI-1 is a biomarker potential for early detection and diagnosis of AD [228].
Decades have been passed since the proposal of the amyloid hypothesis; hundreds of experimental and clinical trials have been carried out with little expressive results in the preventive and therapeutic field. However, the study of the PAI-1 and its main physiological inductor TGF-β1 is taken into account [229] [230] [231], as a basis for therapeutic projects based on substances that can decrease the activity of PAI-1 at physiological levels to obtain a functionalism with stability in the field of prevention and treatment of the pathogenic entities present in the aging development [61] [232]- [245].

Conclusion
The concept of stress implicitly involves a complex process that accompanies us throughout our lives. After the stressful stimuli, the stress hormones are released, modifying the physiological functions of the endocrine, immunological and cardiovascular systems, causing metabolic, molecular alterations and pro-inflammatory/anti-inflammatory reactions (hormones, cytokines), which will configure in a great way aging as a support for the pathology that accompanies it, finding among the most frequent diseases: obesity, thromboembolic diseases, arthrosclerosis, hypertension, cardiovascular illnesses, MS, DM2, neurodegenerative diseases, mainly Alzheimer's and Parkinson's diseases, cancer. These pathologies have a common biochemical phenotype like nexus pathogenic of union, highlighting a significant increase in the activity of PAI-1. Highlighted PAI-1 is considered a star gene due to its path physiological importance and pleiotropy, and the basis of therapeutic projects with drugs or substances that can decrease the activity of PAI-1 at physiological levels, in order to achieve effectiveness in the field of prevention and treatment of the most frequent thromboembolic complications in the pathogenic processes that accompany the development of aging ( Figure 2). For this, more scientific studies are necessarily required, whose results can serve to correct these pathologies in some way and provide greater satisfaction in this period of life.