The major observation of the present study is that levels of circulating progenitor cells and endothelial progenitor cells are decreased in healthy young women with chronic nicotine abuse. In addition, the number of CD34 + progenitor cells positively correlates with endothelial function assessed as flow-mediated dilation of the brachial artery.
Previous studies have shown that chronic nicotine abuse leads to a decrease in circulating progenitor cell levels in men . Two hypotheses have been discussed to explain underlying mechanisms: First, smoking-related ROS production may decrease the bioavailability of nitric oxide (NO), thereby reducing mobilization of PCs from the bone marrow [26, 27]. Second, smoking leads to endothelial dysfunction, and injured vessels may use PCs to maintain endothelial function . An imbalance of injury and repair of vasculature promotes the progression of atherosclerosis, and reduced PC levels may be a marker of such an imbalance caused by smoking .
PCs are known to be upregulated by estrogens through inhibition of apoptosis, stimulation of telomerase, and bone marrow mobilization [11, 28, 29]. Fertile women have higher levels of PCs than do age-matched men; after menopause, levels of PCs no longer evidence sex-specific differences . Higher PC levels in fertile women have been suggested to contribute to the lower cardiovascular risk compared to men. In order to explain the effect of smoking and the estrogen status on progenitor cell and EPC-CFU content, we first examined women at menstruation, the cycle phase with lowest concentrations of sexual steroids. We established a significant decrease of CD34 + progenitor cells in smoking women. Importantly - in a cross-analysis of six different progenitor cell subtypes characterized by different combinations of the surface markers CD34, CD133, and VEGFR2 - it had been previously shown that the abundance of the progenitor cell population with CD34 + alone revealed the best correlation with cardiovascular parameters and risk estimates . Concordantly, we found a positive correlation of levels of CD34 + cells with endothelial function in young healthy women. These data concur with other studies describing a positive correlation between FMD and PCs: which emphasizes the close relationship of these two markers in reflection of vascular homeostasis [6, 14].
It is well established that cigarette smoking is sufficient to impair endothelial function in healthy adults [18, 30]. Although hormonal cycle-dependent changes of endothelial function are relevant in premenopausal women, most clinical studies of the effects of smoking on endothelial-dependent vasodilation have not considered menstrual cycle phases in women. We have recently shown that there are no significant differences in endothelial function between smoking and non-smoking women at menstruation . Consequently, the results of the present study support the conclusion that chronic smoking affects the level of CD34 + progenitor cells before manifestation of endothelial dysfunction in healthy young women. It has been suggested that oxidative stress and decreased availability of NO are causative for endothelial dysfunction induced by smoking [32, 33]. Since there is evidence that NO is necessary for PC mobilization and function [26, 27, 34], NO deficiency may be a key factor resulting in the PC decrease in young smoking women. We suggest, that deficient mobilization of PCs due to limited NO availability caused by smoking precedes detectable alterations of endothelial function and in this sense may represent a sensitive, early parameter of cardiovascular risk. Clearly, this should be an important consideration for future studies.
Investigations of the influence of nicotine abuse on PC levels in premenopausal women should in fact consider hormonal fluctuations during the menstrual cycle. In our study, however, we found no significant differences in the amount of progenitor cells between follicular and luteal phases of the menstrual cycle, despite an increase in estrogen levels. These results concur with previous studies showing significant periovulatory increase of progenitor cells, but no differences between follicular and luteal phases [13–15]. It is interesting to note that the differences in levels of CD34 + cells between smoking and non-smoking women were maintained in all three phases, independently of the estrogen status.
There are some limitations in the present study. In conjunction with the small sample size, smoking women were somewhat older than non-smoking. It cannot be ruled out that older age in smoking women contributes to decreased numbers of progenitor cells. However, adjustment of progenitor cell number and FMD values in a linear regression analysis did not show an influence of age differences on our study results.