Regulatory Role of Sex Hormones in Cardiovascular Calcification

[madman]

Abstract: Sex differences in cardiovascular disease (CVD), including aortic stenosis, atherosclerosis, and cardiovascular calcification, are well documented. High levels of testosterone, the primary male sex hormone, are associated with an increased risk of cardiovascular calcification, whilst estrogen, the primary female sex hormone, is considered cardioprotective. The current understanding of sexual dimorphism in cardiovascular calcification is still very limited. This review assesses the evidence that the actions of sex hormones influence the development of cardiovascular calcification. We address the current question of whether sex hormones could play a role in the sexual dimorphism seen in cardiovascular calcification, by discussing potential mechanisms of actions of sex hormones and evidence in pre-clinical research. More advanced investigations and understanding of sex hormones in calcification could provide a better translational outcome for those suffering from cardiovascular calcification.




1. Clinical Consequences of Cardiovascular Calcification

Cardiovascular calcification describes the regulated deposition of minerals in the blood vessels (vascular calcification) and heart valves (valvular calcification). Calcification is considered a predictor of risk associated with vascular disease [1], with more than 60% of people over 65 years of age displaying calcification in their cardiovascular system [2]. If left untreated, calcification can lead to a number of significant clinical consequences, including coronary insufficiency, aortic stenosis, and, in severe cases, heart failure. Vascular calcification was previously considered the consequence of passive precipitation of calcium and phosphate in the vascular system due to aging. However, over the past decades, studies have revealed that cardiovascular calcification is indeed an actively regulated process that shares many similarities with physiological bone formation [3]. Despite extensive characterization of cardiovascular calcification in patients, the precise mechanisms that initiate and regulate calcification are still unclear. There is also a distinct sex difference in patients, with males having a tendency to acquire calcification earlier in life, and females developing calcification post-menopause [4]. Although the mechanism(s) behind this sex difference remain(s) to be fully elucidated, current evidence suggests a strong link between the specific actions of individual sex hormones and cardiovascular calcification. In this review, we seek to complement the recent excellent publication by Zhang et al. [5] by discussing the current understanding of the underlying mechanisms through which estrogens and androgens regulate calcification in blood vessels and valves, including a focus on the role of preclinical models in this research.




2. Types of Cardiovascular Calcification

According to its location, cardiovascular calcification can be divided into three major types: atherosclerotic intimal vascular calcification, medial vascular calcification, and aortic valve calcification. Within the scientific literature, cardiovascular and vascular calcification terms are frequently used interchangeably, however, it is important to recognize that these are separate processes. The following section addresses these different types of calcification in more detail (Figure 1).

2.1 Atherosclerotic Intimal Calcification

2.2 Medial Calcification

2.3 Valvular Calcification

2.4. Pharmaceutical Strategies




3. Current Understanding of Cardiovascular Calcification

3.1 Calcification Is Similar To Physiological Bone Formation

3.2. Loss of Endogenous Inhibitors Induces Vascular Calcification

3.3. Matrix Vesicles and Apoptotic Bodies Promote Cardiovascular Calcification




4. The Role of Sex and Sex Hormones in Cardiovascular Calcification

4.1. Sex difference Exists in Cardiovascular Calcification

4.2. Estrogen and Activation of the Estrogen Receptor Prevents Calcification

4.3 Testosterone Is a Risk Factor for Cardiovascular Calcification




5. Sex Hormones Mediate Cellular Signalling Pathways in the Cardiovascular System

5.1. Estrogen Signalling and Cardiovascular Function

5.2. Testosterone Signalling and Cardiovascular Function




6. Animal Models Offer Insights into Sex Differences in Cardiovascular Calcification

6.1. Rodent Models

6.2. Large Animal Models




7. Future Perspectives

Despite testosterone being an established risk factor for cardiovascular calcification (and many other vascular diseases), its clinical impact is unclear and its mechanism of action in cardiovascular calcification remains to be fully understood. Whilst females are believed to be ‘cardio-protected’, pre-menopausal patients are severely under-represented in preclinical studies, which may contribute to our lack of understanding of the mechanisms underpinning the cardioprotective role of estrogens. Whether these sex hormones directly contribute to the increased vascular calcification observed in postmenopausal women remains to be investigated. Furthermore, elucidating these mechanisms is hampered not only by the limitations of preclinical models but also by the severe under-representation of the female sex in preclinical research. Further research is essential to bridge the knowledge gap between the cellular mechanisms of calcification and the clinical sex risk factors, in order to ensure equitable treatment approaches for patients of both sexes.

 

[madman]

Figure 1. Comparison of types of cardiovascular calcification. The most common types of cardiovascular calcification are medial, intimal, and valve. This figure displays both the normal and disease state for each of these diseases. Medial calcification occurs in the tunica media of the aorta, around the smooth muscle cells. Intimal calcification occurs within an atherosclerotic plaque in the intima of the artery. Valve interstitial cells, which could have a role in valve calcification, mostly accumulate in the aortic side of the valve.

 

[madman]

Figure 2. Vascular calcification mechanisms. In individuals with chronic kidney disease (CKD) high serum phosphate induces osteogenic transition and calcification of vascular smooth muscle cells (VSMCs) through the sodium-dependent phosphate cotransporter, Pit-1. In addition, normal blood vessels express a number of calcification inhibitors including ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and matrix Gla protein (MGP). Loss of these inhibitors promotes osteogenic transition and calcification of VSMCs. RUNX2, ALP, and BMP2 become elevated which promotes osteoblast-like redifferentiation in SMCs. Matrix vesicles and apoptotic bodies also play a role in vascular calcification. Pi-phosphate RUNX2- runt-related transcription factor 2, ALP-alkaline phosphatase Bmp2-bone morphogenic protein 2.

 

[madman]

Figure 3. Estrogen binds to estrogen receptors (ERs), which act as transcription factors to regulate the expression of genes containing estrogen response elements (ERE). In addition, estrogen also rapidly induces phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signaling through the orphan G-protein coupled receptor (GPR30). Through genomic and non-genomic pathways, estrogen regulates a wide range of vascular cell functions, including proliferation, migration, and senescence. E2- Estradiol. Testosterone acts in a similar mechanism, binding to the androgen receptor (AR), which acts as transcription factors to regulate the expression of genes containing the androgen response elements (ARE). Testosterone is also believed to have non-genomic effects, although these pathways are less clear than estrogens.

 

[madman]

Table 1. Sex differences in rodent cardiovascular calcification studies. This table summarises key findings of cardiovascular calcification studies that have investigated both sexes, including the model and sex difference. In mice, increased calcification has typically been reported in females compared to males whereas in rats lower calcification has been observed in females. ApoE—Apolipoprotein E, DHT—Dihydrotestosterone, ↑—increased.