Atherosclerosis 152 (2000) 527 – 528
Letter to the editors
www.elsevier.com/locate/atherosclerosis
Cellular resistance to homocysteine: a key for longevity?
Recent works have identified moderately elevated plasma levels of homocysteine as a risk factor for atherosclerosis [1]. Indeed, the mechanism by which homocysteine induces atherosclerosis is still not com-pletely understood. Several hypotheses have been sug-gested for explaining the negative impact of homocysteine on endothelium; (i) homocytseine might lead to an increase in oxidative stress but future studies in humans are needed to confirm such a possibility; (ii) studies in hyperhomocysteinemic vascular patients have shown that endothelial antithrombotic properties ap-pear to be more severely impaired than in patients with normohomocysteinemia; (iii) a negative impact of ho-mocysteine on NO-production also cannot be ruled out [2,3]. We have recently found in vitro an inhibitory action of homocysteine on platelet NO production both in healthy and — to a larger extent — in diabetic subjects [4] and we hypothesise that one of the mecha-nisms causing the homocysteine-linked vascular damage might be the platelet activation caused by a reduction in NO release [4].
Healthy centenarians (HC) have recently gained the attention of researchers looking for the biological mechanisms at the basis of longevity and successful ageing. HC have been protected during the course of their life from the development of atherosclerotic dam-age, but the defending mechanisms have not been clarified. Indeed, it has been recently demonstrated that HC have a less degree of oxidative stress [5] and a preserved response to NO production at vascular level [6] compared with healthy aged subjects. Whether cellu-lar production of NO in response to a proxidant factor is also different between HC and healthy aged subjects might be a further piece of the puzzle which could help us to understand the reason for extreme longevity. Thus, the aim of the present work was to study in vitro the effect of the incubation with homocysteine on the platelet production of NO in HC.
The study was performed on ten healthy centenarians (age\100 years, three men, seven women) and 20 middle aged healthy subjects (15 men, five women, age=48914 years). Both the centenarians and control subjects were submitted to the following inclusion
crite-ria; liver, kidney, and thyroid function tests within the normal range; absence of history of diabetes, hyperten-sion or coronary heart disease; no drug or vitamin supplement in the 4 weeks before the study; absence of Alzheimer’s disease or secondary dementia. Blood was drawn in the fasting state for the isolation of platelet rich plasma (PRP), which was divided into three aliquots, used, respectively, for an immediate determi-nation of platelet NO production and for a 3-h incuba-tion with or without 100-mM homocysteine and
subsequent measurement of NO production. NO re-leased by the platelets was directly measured in the PRP using an isolated NO meter and its associated probe (IsoNO Mk-11, World Precision Instruments, Sarasota, FL) equipped with the Duo. 18 Data Acquisi-tion System, as recently described by Chakravarthy et al. [7].
All results were mean9S.D. Changes (%) with the basal value equal to 100%. Difference between values at baseline and those found after adding homocysteine in the medium were evaluated by paired Student’s t-test. No significant differences in basal NO-formation in HC versus middle aged control group (HC, 2.8990.53 nmol NO/min per l06 cells; controls, 3.0590.52 nmol
NO/min per l06 cells P=NS) were found. Incubation
of freshly isolated platelets with 100 mM HCys for 3 h,
significantly decreased the rate of NO-formation (
27%) in the control subjects (2.1390.31 nmol NO/min per l06cells,PB0.01 vs. basal levels), but only slightly
(12%) in HC (2.6190.28 nmol NO/min per l06cells,
P=NS vs. basal levels).
The data obtained in vitro in middle-aged healthy subjects confirm the hypothesis that high homocysteine plasma levels cause an increase platelet aggregation by inhibiting the NO production. Several possible mecha-nisms by which HCys can inhibit NO-production in platelets have been hypothesised; among them the inhi-bition ofL-Arg transport across the plasma membrane or the direct blockage of nitric oxide synthase (NOS) [3] are the most likely ones. Interestingly enough, platelets from HC displayed resistance to the homocysteine-in-duced inhibition of NO production, so that the NO-for-mation in platelets from HC after incubation with homocysteine was not significantly different from basal NO production and strongly different from the result
Letter to the Editors
528
found in the middle aged group. To the best of our knowledge, this the first report showing a cellular (platelet) resistance to a NO-inhibition due to homocys-teine. Such data is in agreement with the earlier findings showing HC to have a lower degree of oxidative stress [5] and more preserved endothelium function [6] than the aged subjects. It should also be pointed out that due to the low prevalence of atherosclerosis in HC [8], one might speculate that a cellular resistance to homocys-teine might be viewed, as one of the mechanisms at the basis of successful ageing in HC. Nevertheless, it should underlined that the molecular mechanisms at the basis of the resistance need further investigations.
References
[1] Boushey CJ, Beresford SAA, Omenn GS, Motulsky AG. A quantitative assessment of plasma homocysteine as a risk factor for vascular disease. J Am Med Assoc 1995;274:1049 – 57. [2] de Jong SC, van den Berg M, Rauwerda JA, Stehouwer CD.
Hyperhomocysteinemia and atherothrombotic diseases. Semin Thromb Hemost 1998;24:381 – 5.
[3] Radomsky MW, Palmer RMJ, Moncada S. AnL-arginine/nitric oxide pathway present in human platelets regulates aggregation. Proc Natl Acad Sci USA 1990;87:5193 – 7.
[4] Mutus B, Rabini RA, Mazzanti L. Diabetic subjects show creased sensitivity to homocysteine-dependent inhibition of in-traplatelet nitric oxide production, Circulation, (1999), in press. [5] Paolisso G, Tagliamonte MR, Rizzo MR, Manzella D,
Gam-bardella A, Varricchio M. Oxidative stress and advancing age: results in healthy centenarians. J Am Geriatr Soc 1998;46:833 – 8. [6] Paolisso G, Tagliamonte MR, Rizzo MR, et al. Mean arterial blood pressure and serum levels of the molar ratio of IGF-1 WF-BP3 in healthy centenarians. J Hypertens 1999;17:67 – 73. [7] Chakravarthy U, Hayes RG, Stitt AW, McAuley E, Archer DB.
Constitutive nitric oxide synthase expression in retinal vascular endothelial cells is suppressed by high glucose and advanced glycation end products. Diabetes 1998;47:945 – 52.
[8] The Italian Multicenter Study on Centenarians. I centenari in
Italia: aspetti epidemiologici e clinico biologici. In: 96th Meeting of the Italian Society of Internal Medicine, Pozzi (Rome, Italy). Rome: Italian Society of Internal Medicine, 1995, 1995:23 – 109.
B. Mutus
Department of Chemistry and Biochemistry Windsor Uni6ersity Windsor Canada
R.A. Rabini, C. Franceschi
Department of Geriatrics INRCA Ancona Italy
G. Paolisso, M.R. Rizzo, E. Ragno
Department of Geriatric Medicine and Metabolic Diseases IV Medicina, Interna e Malattie dell’ In6ecchiamento Piazza Miraglia 2, 1-80138 Napoli Italy
E-mail: gpaoliss@tin.it
A. Rappelli, M. Braconi
Department of Internal Medicine and Geriatrics Uni6ersity of Ancona Ancona Italy
L. Mazzanti
Institute of Biochemistry School of Medicine Uni6ersity of Ancona Ancona Italy
