Better Late Than Early? The Role of the Vascular Endothelium in Late-Onset Menopause

The menopausal transition is a pivotal phase in a woman’s life, effectively marking the end of the reproductive lifespan. While the median age of natural menopause in the United States is 51 years, timing of menopause can vary considerably, with approximately 20% of women experiencing menopause either before age 45 years or after age 55¹. There is growing recognition that timing of menopause has long-term implications on cardiovascular health. For example, previous studies have shown an inverse dose-response relationship between age at menopause and cardiovascular disease (CVD) risk², with premature menopause (before age 40) associated with greater risk of coronary heart disease and overall cardiovascular mortality³. These data have been incorporated into recent American Heart Association/American College of Cardiology guidelines that now classify premature menopause as a risk enhancing factor for atherosclerotic CVD⁴. Less is known about late onset of menopause and CVD risk although emerging data show that later age of menopause may be associated with lower risk of CVD. Specifically, in a population-based cohort study from the Netherlands, women with late onset menopause (defined as age >51 years at menopause) had an 18% lower risk of cardiovascular mortality compared with women who underwent menopause prior to 45 years⁵.

Observational studies have provided robust evidence that menopause is accompanied by adverse changes in cardiometabolic health, including a shift to a more atherogenic lipid profile and increase in visceral adipose tissue⁶. Women also experience adverse changes in vascular health and function, including increase in arterial stiffness and decline in endothelial function. A prior analysis from the Study of Women’s Health across the Nation (SWAN), a longitudinal cohort study of the menopause transition, found that the arterial stiffness as measured by carotid femoral pulse wave velocity markedly increased during the perimenopause period, independent of chronological age⁷. SWAN also showed a stepwise decline in endothelial function ascertained by brachial artery flow mediated dilation (FMD) across the menopause stages from pre-menopause to late post-menopause. These data have implicated impaired vascular and endothelial function in the pathogenesis of menopause-related CVD risk⁸. Oxidative stress pathways involving excess reactive oxygen species (ROS) including production by mitochondria (mitoROS) is thought to be a key mechanism of endothelial dysfunction associated with menopause and aging, but precise pathophysiological mechanisms are not fully elucidated^9,10. Less is known about the influence of menopause timing and endothelial function, which may provide additional insights into the mechanistic pathways driving menopause-related CVD.

In this issue of Circulation Research, Darvish et al¹¹ investigate whether endothelial function is preserved in postmenopausal women (PMW) with late onset menopause (defined as age of menopause ≥ 55 years) and explore mitoROS and circulating metabolites as potential mediators of the link between menopause age and endothelial function (Figure). The analysis included 71 PMW, including 49 women with normal-onset menopause (mean age 68 ± 8 years, mean age of menopause 51 ± 2 years) and 22 with late-onset menopause (mean age 68 ± 6 years, age of menopause 57 ± 2 years). Women with history of oophorectomy or hysterectomy and/or receiving hormonal therapies were excluded. Finally, 21 premenopausal women were included as controls. Traditional CVD risk factors including blood pressure, low-density lipoprotein cholesterol, body mass index, and physical activity were similar between PMW with both normal- and late-onset menopause. The PMW groups also had similar sex hormones levels, age at menarche, prior hormonal therapy use, female-specific conditions (including endometriosis, polycystic ovary syndrome, and fibroids), and pregnancy-related variables (including gravidity and adverse pregnancy conditions).

Figure:

Compared with PMW with normal-onset menopause, PMW with late-onset menopause exhibit lower levels of triglyceride metabolites, decreased mitoROS bioactivity, greater endothelial function (as measured by flow-mediated dilation), and lower risk of cardiovascular disease.

In primary analyses, Darvish et al., compared endothelial function between age-matched late-onset and normal-onset PMW and a young premenopausal reference group. In vivo assessment of endothelial function was ascertained using noninvasive measurement of brachial artery flow-mediated dilation (FMD_BA). As hypothesized, endothelial function was higher in PMW with late-onset vs normal-onset menopause, with 54% higher FMD_BA in late-onset PMW compared with normal-onset PMW (FMD_BA 6.3 ± 0.6% in late-onset PMW vs 4.1 +/− 0.4% in normal onset PMW) and FMD_BA levels only 24% lower than premenopausal women (premenopausal women: FMD 8.3 +/− 0.7%). Age at menopause was significantly associated with FMD_BA even after adjustment for traditional CV risk factors and time since menopause, suggesting that observed differences in endothelial dysfunction between PMW with late- vs normal-onset menopause were not attributable to differences in CVD risk profile or changes related to the menopause transition itself. Finally, authors found no differences in brachial artery FMD in response to nitroglycerin, confirming true differences in endothelial-specific function and not simply differences related to smooth muscle sensitivity to nitric oxide.

To further probe the mechanisms underlying their primary findings, authors examined the effect of in vivo mitoROS suppression via supratherapeutic dose of mitochondria-targeted antioxidant MitoQ on endothelial function in a subgroup of 27 PMW¹⁰. MitoROS suppression increased FMD_BA in both the late-onset and normal-onset PMW but the absolute change in FMD_BA was less pronounced in the late-onset menopause group. These findings suggest PMW with late-onset menopause exhibit lower ROS-associated oxidative stress, which may explain why endothelial function was higher in late-onset vs normal-onset menopause PMW. This observation was further supported by in vitro experiments that showed that mitoROS bioactivity was lower in human aortic endothelial cells (HAEC) harvested from the late- vs normal-onset PMW and negatively correlated with FMD_BA and age at menopause.

Finally, investigators leveraged mass spectrometry metabolite profiling to identify potential circulating factors that may contribute to differences in mitoROS observed between late-onset vs normal-onset PMW. Using polar non-lipid metabolomics, untargeted lipidomics, and semi-targeted oxylipins analyses in serum samples from 21 PMW (8 late-onset, 13 normal-onset PMW), the authors found distinct circulating lipidome profiles between late- and normal-onset PMW, with triglyceride (TG)-related lipid metabolites displaying most significant differences between groups. Among TG metabolites that significantly differed between late- and normal-onset PMW, TG(16:0) demonstrated the strongest relationship with mitoROS bioactivity. Further, normalizing serum concentrations of TG(16:0) between late- and normal-onset PMW and premenopausal women attenuated differences in mitoROS bioactivity, establishing a potential mechanistic role of TG(16:0) in modulating ROS bioactivity in PMW.

This study is among the first to demonstrate that endothelial dysfunction is attenuated in PMW with late vs normal-onset menopause. The findings are compelling. Using complementary and innovative deep phenotyping approaches including brachial artery FMD, the most widely used validated method to assess endothelial function¹², in vitro targeted studies of mitoROS production, and metabolomic profiling, the authors show that the late-onset PMW exhibited a 2.2%-unit higher FMD_BA than normal-onset PMW, a clinically significant finding as every 1% increase in FMD has been shown to reflect 17% lower cardiovascular risk¹³. Subsequent functional experiments in HAEC and metabolomic profiling provide additional insights into the mechanistic roles of mitoROS and circulating lipid metabolites in altering vascular aging and dysfunction in PMW. However, it’s worth noting a few important limitations. First, targeted mitoROS studies were performed on cultured cells exposed to serum which prevents direct assessment of the vasculature and raises uncertainty about whether mitoROS measurements from HAECs reflect mitoROS bioactivity in the vasculature. Second, while circulating lipid metabolites were strongly associated with mitoROS, this study does not provide definitive support that TG(16:0) mediates the preservation of endothelial function among PMW with late-onset menopause as investigators did not evaluate the effect of mitoROS suppression on lipidomic profiles. Finally, the cross-sectional nature of the study provides only one snapshot in time. Whether vascular trajectories are distinct between PMW with normal- vs late-onset menopause remains an unanswered question.

While prior studies investigating menopause timing and cardiovascular risk have predominately focused on early-onset menopause^3,14, this study shifts the focus to the other end of the spectrum: women with delayed reproductive aging. In premature menopause, it is hypothesized that early loss of estrogen may lead to heightened risk for CVD, though further studies unraveling the underlying pathways are still needed¹⁵. Darvish et al., now show that in the case of menopause, it may be better to be late than early, with late-onset menopause associated with preservation of vascular endothelial function, potentially mediated by lower mitoROS. Overall, this important and elegant work underscores that menopause is not simply a singular event in a woman’s life driven exclusively by decline in estrogen, but a heterogeneous experience marked by complex changes in sex hormone levels and vascular health. Further mechanistic understanding of how menopause timing impacts vascular endothelial function may ultimately help to identify potential molecular targets to mitigate risk and preserve optimal vascular health among menopausal women.