*2.4. Dyslipidemia, Insulin Resistance, Hypertension and Cluster of CV Risk Factors*

Dyslipidemias are disorders of lipoprotein metabolism that can result in the following abnormalities: high total cholesterol (TC), high low-density lipoprotein cholesterol (LDL-C), high non-high-density lipoprotein cholesterol (non-HDL-C), high triglycerides (TGs), and low HDL-C [52]. Lipid levels vary by age and sex and reference lipid and lipoprotein values have been derived from the population-based Lipid Research Clinical Prevalence Study, which obtained between 1972 and 1976 fasting lipoprotein profiles in more than 15,000 children and adolescents (age range 0 to 19 years), and from the United States National Health and Nutrition Examination Surveys (NHANES), which analyzed lipid levels in 7000 children between 1988 and 1994 [53–56]. In most patients with hyperlipidemia this condition is caused by some underlying "non-lipid" etiology rather than by a primary disorder of lipoprotein metabolism. Among the CV risk factors, lipids and lipoproteins are of special importance and in many studies, childhood obesity has been shown to be associated which increased levels of TC, LDL-C and TGs and decreased level of HDL-C [57,58]. CVD is the number one cause of death in the United States and 38% of adults affected by CVD have risk factors such as elevated serum lipid levels, diabetes, and high blood pressure. Many studies have confirmed an additional role of body fat distribution and in particular of excess visceral fat even in the adolescent age group [59,60].

Atherosclerosis can start at young age, and the number of young individuals developing atherosclerosis is on the rise, especially in children with risk factors such as familial hypercholesterolemia (FH), type 1 diabetes mellitus, chronic kidney disease and HTN. Furthermore, many studies have identified dyslipidemia as a risk for premature atherosclerosis, even in children and adolescents. In the Bogalusa Heart Study, autopsy studies performed in 204 young subjects demonstrated fatty streaks in 50 percent of cases between 2 and 15 years of age and in 85 percent of older subjects between 21 and 39 years of age [61]. The prevalence of raised fibrous plaques in the aorta and coronary arteries also

increased with age from approximately 20 percent in subjects between 2 and 15 years of age to 70 percent in those between 26 and 39 years of age. The prevalence and the extent of atherosclerosis found in the aorta and coronary arteries were greater with increasing BMI, BP, and levels of serum TC and LDL-C. The degree of atherosclerotic changes increased with worsening severity and greater numbers of risk factors [62].

Similarly, the Pathobiological Determinants of Atherosclerosis in Youth (PDAY) study [63] reported raised fatty streaks in 10 percent of coronary arteries and 30 percent of aortas in subjects aged 15 to 19. The extent of fatty streaks increased with increasing age, elevated BP, higher serum LDL-C, and lower serum HDL-C. Female patients lagged by five years behind male patients in the progression of the extent of raised lesions in the right coronary arteries. In a subsequent report, individuals with early and more severe atherosclerotic changes were more likely to have had one or more CVD risk factor (including dyslipidemia, obesity, hyperglycemia, HTN, or smoking) [64].

Several longitudinal studies reported tracking of adverse lipid levels from childhood to adulthood. In a cohort of 725 adults (age range 33 to 42 years) in the Muscatine Study, childhood TC levels positively predicted adult carotid intima-media thickness (cIMT). In women, childhood BMI was also a significant predictor of cIMT [65,66]. In the Cardiovascular Risk in Young Finns study, elevated childhood levels of LDL-C and insulin, as well as obesity, were predictive of increased cIMT (observation period 27 years). In subsequent studies, carotid artery elasticity decreased as the number of childhood CVD risk factors increased and flow-mediated dilation was lower in male patients who had elevated BP during adolescence. In this cohort, exposure to CVD risk factors over time correlated with the extent of coronary artery calcification by computed tomography [67]. Similarly, in a cohort of patients involved in the CARDIA study, initially recruited at age 18 to 30 years and followed for 15 years, baseline CVD risk factors (smoking and higher LDL-C, glucose, and systolic blood pressure levels) were associated with increased risk of coronary artery calcium later in life [68]. The International Childhood Cardiovascular Cohort Consortium has performed a meta-analysis that combined data from the four prospective studies mentioned above. In this analysis, the number of childhood CVD risk factors (e.g., increased cholesterol, TGs, BP, and BMI), even in children as young as nine years of age, was predictive of elevated adult cIMT with progressive strengthening of the association through adolescence [69]. Further studies have shown that dyslipidemia in adolescence predicts increased adult cIMT, even after accounting for sex, obesity, and HTN. Screening of lipid levels in children may reveal both genetic lipid abnormalities (e.g., including familial hypercholesterolemia, which affects 1 in 250 people), and dyslipidemia, which responds favorably to lifestyle changes [70]. Even a small weight loss is associated with a significant decrease in the concentration of TG and an increase in the concentration of HDL-C. In addition to the recommended diet, physical activity (PA) is strongly recommended. According to the National Heart, Lung and Blood Institute (NHLBI) recommendations, in patients in whom non-pharmacological management has no effect, the use of lipid-lowering drugs should be considered [71].

The relationship between obesity and its complications, in particular insulin resistance, and arterial hypertension was first noticed in the 1950s [72]. Many studies over the years have confirmed that excess of fat tissue regardless of age and gender is associated with an increase of blood pressure [73–75]. Among children with obesity, the prevalence of arterial HTN is up to 30%, in contrast to less than 3 (5) % in the normal weight pediatric population [76–79] and weight gain is accounted for up to 75% of the risk for primary HTN [80].

The pathogenesis of HTN in obese individuals is complex and still not fully understood. Currently, it is believed that the abnormally increased activity of adipose tissue in the production of hormones and adipokines is of key importance. Pro-inflammatory substances as tumor necrosis factor-α, interleukin-6, C-reactive protein may lead to macrophage recruitment, which could increase pathological lipolysis. Subsequently excess lipid delivery could promote ectopic lipid accumulation leading to the associated impairments in insulin

signaling, mainly in the liver and skeletal muscles, that in turn may also contribute to development of insulin resistance [81]. Insulin resistance and hyperinsulinemia are independent activators of the sympathetic nervous system. Other factors with a documented independent role in activating sympathetic nervous system in obese individuals are: leptin excess and intermittent hypoxia caused by sleep-disordered breathing [82–84]. The enhanced activity of sympathetic nervous system causes vasoconstriction and reduced renal blood flow, which is a trigger for renin release, and subsequent activation of the renin–angiotensin– aldosterone system (RAAS) results in sodium and water retention [82]. Additionally, it causes β2-adrenergic receptors dependent activation of the NaCl-cotransporter in the distal tubule, that is considered to be one of the most important mechanisms for the development of salt-sensitive HTN [85]. Although the activation of the RAAS has been well documented in adults and in experimental models, data regarding the role of this mechanism in the development of HTN are contradictory [86]. Interestingly, not only does the classic way of activating the RAAS play an important role in the development of obesity-related arterial HTN, but also plasma aldosterone concentration seems to be positively correlated with the amount of visceral adipose tissue, independent of plasma renin activity [87,88]. Contrary to lean hypertensive subjects, patients with obesity show a positive paradoxical correlation between sodium intake and aldosterone levels. It has been suggested, that some adipokines, as yet unidentified, may directly stimulate aldosterone release from adrenals in angiotensin II-independent manner [87,88]. It has also been proven that adipose tissue can produce angiotensinogen, angiotensin, and angiotensin II itself, stimulating aldosterone secretion by adipocytes in a paracrine/autocrine way independently from the inhibitory effect of high salt consumption [88,89]. Moreover, the results of genetic studies of humans suggest the association of obesity-related HTN with the variants of several genes involved in aldosterone secretion and metabolism, such as glucocorticoid receptor, aldosterone synthase (CYP11B2), and serum and glucocorticoid-regulated kinase 1 [90–92]. Variants of the latter are described to be associated with predisposition to HTN, hyperinsulinism and high salt intake [92]. An additional element may be the excessive stimulation of renin production in vitamin D deficiency, a condition often found in obese people [93,94]. Finally, cortisol production by adipose tissue may stimulate renin production exerting aldosterone-like effects through its mineralocorticoid activity and, moreover, may increase insulin resistance [95]. Undoubtedly, aldosterone is not only a hormone that regulates electrolytes and fluid volume, but can be an important mediator of obesity development independently of calorie intake and target-organ damage. Excess of aldosterone contributes to insulin resistance, and leptin resistance [96]. In the kidney, aldosterone causes podocyte injury, which leads to proteinuria and glomerulosclerosis, and proinflammatory responses, mediating perivascular and interstitial fibrosis [97,98].

Hyperinsulinemia has a similar effect leading to direct kidney damage by impairment in insulin metabolic signaling resulting in reduced NO production, associated impairment of tubuloglomerular feedback, and subsequently hyperfiltration and sodium retention [99]. Hyperinsulinemia is also directly related to the reduction in uric acid excretion. In a number of clinical trials, such as NHANES I, the Framingham Study and the Bogalusa Heart Study (including the pediatric population), it was shown that serum uric acid concentration is an independent prognostic factor for the development of arterial HTN [34,80,100]. Hyperuricemia causes renal vasculitis by the stimulation of nuclear transcription factors, release of pro-inflammatory cytokines, and pre-glomerular arteriolopathy due to e.g., increasing the proliferation of vascular smooth cells and causing inflammation and tubulointerstitial fibrosis. These changes further activate the RAAS and may additionally favor the adverse effect of urates on the glomerular vessels [82]. Additionally, direct compression of the renal parenchyma by perinephric fat may reduce intrarenal blood flow and increase sodium reabsorption, leading to volume expansion, increase of cardiac output and decrease in blood flow reserve [82]. This phenomenon occurs even in the absence of signs of glomerular sclerosis or chronic kidney disease [82]. While in the early stage of obesity induced HTN, the increased glomerular filtration rate and renal blood flow induce an

increase in renal sodium absorption, with prolonged HTN, renal vasodilation, glomerular hyperfiltration, and neurohumoral activation led to further increase of BP, glomerular injury, and an impaired renal capacity for sodium excretion, resulting in the gradual loss of nephron and kidney function. In this way, obese subjects require a higher BP than lean subjects to maintain the sodium balance, indicating impaired renal-pressure natriuresis ('salt-sensitive' HTN) [86]. Also, pro-inflammatory substances formed in adipose tissue play a direct role in endothelial damage and increased vascular stiffness. Since HTN itself is also a factor leading to endothelial pathology a "vicious circle" phenomenon should be taken into account. With respect to the above-mentioned mechanisms of the development of HTN in obese children, it seems obvious that the basis of treatment and prevention of complications should be effective reduction of adipose tissue [101,102]. Limiting salt intake is a crucial element in the treatment of obesity-associated HTN. If pharmacotherapy has to be introduced, first-line drugs are antagonists of the RAAS (angiotensin-converting enzyme inhibitors, angiotensin receptor blockers [101,102], but also mineralocorticoid receptor antagonists may be considered in selected cases [103,104]).
