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(Hypertension. 2001;37:1060.)
© 2001 American Heart Association, Inc.
Hypothesis |
From the Hypertension Research Center, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, NJ.
Correspondence to Dr Abraham Aviv, Room F-464, MSB, Hypertension Research Center, 185 S Orange Ave, Newark, NJ 07103-2714. E-mail avivab{at}umdnj.edu
| Abstract |
|---|
Key Words: aging hypertension, essential telomere reactive oxygen species menopause evolution genetics
| Introduction |
|---|
| Essential Hypertension and Human Aging |
|---|
Two clear conclusions can be drawn from these findings: arterial aging has a major role in age-dependent rise in pulse pressure and the biology of aging differs between men and women. The observed gender effect suggests, in addition, that chronological age, as determined by calendar time, is distinct from biologic age, a progressive process of deterioration in the vitality of organ systems, which at the present is irreversible. Accordingly, pulse pressure or systolic blood pressure, which are primarily related to biologic aging, should be relatively refractory to treatment19 20 and associated with excess risk of death from all causes.21
The age-dependent rise in pulse pressure is, no doubt, the price of human longevity, but what does age-adjusted, high pulse pressure mean after the exclusion of secondary causes of hypertension? A possibility that merits exploration is that in a subset of the human population, a high systolic (pulse) pressure is the outcome of a process whereby biologic age has surpassed chronological age. Interactions between the genetic script of biologic aging and environmental factors are likely to play a pivotal role in such a putative process. These interactions almost certainly commence in utero.
| The Fetal Origin Hypothesis |
|---|
The potential link of adult cardiovascular diseases to intrauterine growth prompted exploration of the relation between anthropometric parameters at birth and blood pressure in adulthood and during childhood. Results of numerous studies throughout the world, including longitudinal studies, suggest that low birth weight is associated with elevated blood pressure in all age groups.31 32 33 Most studies reported an association between birth weight and systolic blood pressure, presumably because no significant relation was found between birth weight and diastolic blood pressure. Studies that did report results of both systolic and diastolic pressures found either an inverse relation with birth weight for systolic but not diastolic blood pressure34 35 or an inverse relation with birth weight that was greater for systolic than for diastolic blood pressure.36 37 These findings indicate, therefore, that suboptimal intrauterine growth gives rise to an increase in pulse pressure during the postnatal period.
A number of studies have failed to confirm the association between low birth weight and either cardiovascular risk or hypertension.38 39 40 41 42 43 44 45 In addition, the majority of supportive reports have relied on prospective data that, as noted, might have been subject to ascertainment bias.46 47 Another unresolved matter is the mechanistic link between intrauterine growth retardation and cardiovascular disease. Barker and colleagues48 proposed that inadequate nutrition in utero, including poor placental transfer of nutrients, permanently alters the structure and physiology of the body, thereby predisposing a person to cardiovascular disease. Adequate nutrition is, no doubt, essential for normal fetal development, but the precise mechanisms by which suboptimal intrauterine growth alters cardiovascular parameters are poorly understood. Thus, although highly provocative, the fetal origin hypothesis has not been fully tested and offers few clues to the underlying predilection of adults with low birth weight to essential hypertension and other diseases of aging. What the fetal origin hypothesis does seem to suggest is that intrauterine growth retardation results in a forward resetting of arterial age during extrauterine life. In line with this concept is the finding of an inverse relation between birth weight and aortic pulse wave velocity in adults.36
| Human Longevity and Limitations of Animal Models of Genetic Hypertension |
|---|
The short life-span of animal models of hypertension precludes the reliable simulation of aging of organ systems, particularly the cardiovascular system, which is centrally involved in the pathobiology of essential hypertension in humans.7 Thus, the premise that hypertension in the spontaneously hypertensive rat or the Dahl salt-sensitive rat, for instance, mirrors essential hypertension as it exists in humans may not withstand close scrutiny. Although deciphering variant genes that cause hypertension in experiments based on animal models is a valuable exercise in blood pressure physiology, the hypertensive genes uncovered by these models might have little in common with the genes that harbor susceptibility to essential hypertension in humans.
The limitations imposed by the short life-spans of most
laboratory animals on models of vascular aging in humans and
misconceptions perpetuated by these models are further illustrated by
the notion that vascular endothelial cells are
quiescent in vivo. This concept is based on observations that the
mitotic index of the vascular endothelium in rodents is
0.1%/d.49 Rat vascular
endothelial cells would, therefore, undergo
1 round
of replication during the extrauterine life-span of the animals,
assuming that all vascular endothelial cells are
subject to an equal likelihood of cell divisions. Cellular replication
may then be a minor factor in the behavior of the rat vascular
endothelium during the life-span of the rat. However, a
0.1% mitotic index extrapolated to 70 to 80 years of human life yields
a considerable number of replications (
20 to 30) per each vascular
endothelial cell, which amounts to each cell giving
rise to
106 to
109 cells, a substantial cellular turnover
within a human life-span. Recent research suggests that at least in
segments of the vasculature, the replication of human vascular
endothelial cells in vivo is
substantial,50 51
a finding with significant implications for age-dependent disorders of
the cardiovascular system, including essential
hypertension in human beings.
| Antagonistic Pleiotropy, Human Aging, and the Menopause Enigma |
|---|
The exceptionally long postreproductive life of women is a fairly recent phenomenon on the evolutionary time-scale of humans and, like aging itself, is the ultimate outcome of the weakening force of natural selection.52 53 54 55 The tradeoff concept was recently explored in epidemiological studies that sought links between menopause and human longevity. These studies showed that women with fewer children, and particularly women who bear children late in life, as late as in the fifth decade of life, exhibit increased longevity.56 57 58 A potential mediator of this tradeoff is estrogen. An increase in the lifetime effect of estrogen through menstrual activity (and, for that matter, estrogen supplementation) or an augmented sensitivity of target cells to estrogen action appears to increase the risk of cancer of the breast and endometrium.59 60 61 62 63 64 65 Although breast and endometrial cancers would be expected to affect overall longevity by causing premature death in a subset of women, the lasting effect of estrogen would offset the increased risk for cancer by lowering cardiovascular risk.
If menopause and estrogen are determinants of human aging, can they also explain the sexual dimorphism of blood pressure in humans66 ? Not only are women different than men in age-dependent profiles of blood pressure, but they also exhibit a curious difference from men in that a number of physiological parameters that are correlated with blood pressure in men (eg, cellular ion transport systems and the response of the renal vasculature and the adrenal glands to altered sodium intake) are poorly correlated with blood pressure in women.67 68 69 A possible explanation of this gender effect may be that estrogen exerts a vasoactive effect, confounding the relation between physiological parameters and blood pressure, which are readily observed in men. Indeed, estrogen replacement therapy may lower blood pressure in postmenopausal women.70 71 72 73 74 75 This effect appears to be prompt and modest and has been attributed to the modified behavior of a host of homeostatic mechanisms, including the renin-angiotensin system.76 77 78 However, a vasoactive effect can hardly explain the relation between estrogen and blood pressure with regard to menopause. The postmenopausal period is marked by an abrupt decline in circulating estrogen, whereas the catch-up in systolic blood pressure of postmenopausal women with men can take as long as 20 years.4 79 A more satisfactory explanation for the effect of estrogen on blood pressure is that estrogen (during the premenopausal period) retards and its absence (during the postmenopausal period) accelerates biologic aging. Such a concept can be tested by monitoring biologic indicators of human aging. It turns out that one of these indicators may be telomere length.
| The Telomere Hypothesis of Cellular Aging |
|---|
A reverse transcriptase termed telomerase elongates telomeres through the de novo synthesis of TTAGGG repeats, thereby counteracting telomere attrition.86 87 Cultured somatic cells from humans exhibit rudimentary or no telomerase activity. In contrast, most cancer cells or immortalized cell lines show robust telomerase activity and almost limitless replicative capacity. Recent research has proved that the telomeric clock serves not only as a biomarker of cellular replication but also as a central determinant of cellular senescence. This was shown by ectopically expressing the catalytic component of telomerase in cultured somatic cells that lack telomerase activity and by inhibiting telomerase in cancer cells. Although the forced expression of telomerase in somatic cells promotes an unabated or a substantially extended replicative capacity in concert with curtailment of telomere attrition,88 89 90 inhibition of telomerase in cancer cells results in rapid telomeric attrition and cell death.91
A well-established hypothesis proposes that aging results from cumulative cellular damage over a lifetime and that reactive oxygen species are the main source of this damage.92 Reactive oxygen species are important determinants in cardiovascular biology and pathobiology93 and were recently found to increase the rate of telomere attrition per each cycle of cellular replication.94 95 96 Of interest is the fact that homocysteine, a known risk factor for human atherosclerosis,97 98 enhances the rate of telomere attrition per replicative cycle in cultured human vascular endothelial cells.95 To a large extent, this effect appears to be mediated by reactive oxygen species. Based on these experiments, a new factor, namely, telomere attrition, appears to emerge as a link between cardiovascular risk factors and biologic aging of the vasculature in humans.
Overall, empiric and experimental observations of telomere biology fully support the telomere hypothesis of cellular aging, originally proposed by Harley and colleagues.99 The hypothesis simply posits that telomeres serve as a mitotic clock. The young field of telomere biology, therefore, casts a new perspective on age in vitro, distinguishing biologic age, paced by somatic cell division, from chronological age. Remaining to be resolved is the issue of whether human telomeres serve as a biologic clock in vivo.
| Telomeres and Biologic Aging In Vivo |
|---|
Several features render human telomeres suitable for the tall task of biologic timekeeping in vivo: (1) telomere length is highly variable among humans; this is observed at birth103 104 and thereafter51 105 106 107 108 109 ; (2) telomere length is highly heritable105 108 ; (3) telomere length is inversely related to chronological age50 104 105 106 107 108 ; and (4) telomere length is longer in women than in men.109 110 The heritability and variability of telomere length among humans are supportive of the role of genetic factors in the biology of human aging. Longer telomere length in women than in men is consistent with the notion that for a given chronological age, women are biologically younger than men, which is in line with the greater longevity of women.
The gender-related difference in telomere length is probably the lasting signature of estrogen, because estrogen stimulates telomerase and an estrogen response element exists on the catalytic subunit of the enzyme.111 Because estrogen receptors are ubiquitous and present in vascular cells,112 it is only reasonable to deduce that different cell types are targets of this hormone. It follows then that estrogen-mediated surges in telomerase activity during the menstrual cycle may attenuate telomere attrition rates in multiple tissues including blood vessels. In the vasculature, this "genomic" effect of estrogen should be distinguished from its "nongenomic" effect, which appears to cause vasodilation.112 Of note, however, is the likelihood that not only estrogen but also other steroid hormones, which are involved in cell growth, affect telomerase activity in vivo. In the final analysis, the balance among these hormones may influence the activity of the enzyme and the rate of telomere attrition at any given time.
| Telomere Length and Pulse Pressure in Humans |
|---|
| Conclusions |
|---|
Herein lies a clear shortcoming of animal research. It is not that the study of animal models with genetic hypertension is unrevealing or less relevant than clinical research. On the contrary, animal research has thrust the field of essential hypertension into previously uncharted territories, generating new and exciting appreciation of hypertension at the cellular and molecular levels. What is in question is the premise that the variant genes that cause hypertension in inbred, short-lived animals also cause essential hypertension in humans. No doubt, variant genes account to a great extent for differences in blood pressure levels among both animals and humans. However, these variant genes and their modes of interactions with environmental factors are likely to differ between humans and other animals. Moreover, the nature of genes that cause hypertension is not the only factor distinguishing hypertension in humans from, for instance, hypertension in the spontaneously hypertensive rat. As perplexing are differences in circumstances and particularly in the timing of hypertensive gene expression between rats and humans.
| Acknowledgments |
|---|
Received August 15, 2000; first decision October 20, 2000; accepted October 23, 2000.
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