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. Author manuscript; available in PMC: 2024 Sep 1.
Published in final edited form as: Arterioscler Thromb Vasc Biol. 2023 Jul 6;43(9):1617–1625. doi: 10.1161/ATVBAHA.122.318247

Sex-specific differences in lipoprotein production and clearance

Donna M Conlon 1, Francine K Welty 2, Gissette Reyes-Soffer 3,*, Jaume Amengual 4,*
PMCID: PMC10527393  NIHMSID: NIHMS1912862  PMID: 37409532

Abstract

Therapeutic approaches to reduce atherogenic lipid and lipoprotein levels remain the most effective and assessable strategies to prevent and treat cardiovascular disease (CVD). The discovery of novel research targets linked to pathways associated with CVD development has enhanced our ability to decrease disease burden, however, residual CVD risks remain. Advancements in genetics and personalized medicine are essential to understand some of the factors driving residual risk. Biological sex is among the most relevant factors affecting plasma lipid and lipoprotein profiles, playing a pivotal role in the development of CVD. This minireview summarizes the most recent preclinical and clinical studies covering the effect of sex on plasma lipid and lipoprotein levels. We highlight the recent advances in the mechanisms regulating hepatic lipoprotein production and clearance as potential drivers of disease presentation. We focus on using sex as a biological variable in studying circulating lipid and lipoprotein levels.

Graphical Abstract

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1. Introduction

Biomedical research is shifting away from the “one fits all” approach towards more personalized strategies. This is highlighted by new policies of funding agencies that require the inclusion of sex as a biological variable to enhance our knowledge of biological processes implicated in human health and disease that differ by sex [1]. A prime example of sex disparities is the doubling of risk in presentation of adverse drug reactions in women when compared to men [2]. Importantly, sex biases in biomedical research are rooted on the premise that the fluctuations in hormone levels during the female reproductive cycle can negatively impact reproducibility in scientific studies, an obsolete notion that has been debunked over the past years [3, 4]. In addition to sex hormones, there are genetic, behavioral, and socioeconomic factors associated with sex that influence health disparities among sexes [5].

Seminal scientific advancements in the last century achieved a profound reduction in the incidence of cardiovascular disease (CVD), however, there is an increased awareness that addressing remaining residual risk will require the use of personalized strategies of which sex as a biological variable in both basic and human studies will have to be considered [6, 7]. Studies highlight sex differences in many factors implicated in the development of atherosclerosis, the main underlying cause of CVD. Among these factors, sex differences in plasma lipid and lipoprotein profile appeared as one of them [8, 9]. This minireview focuses on the recent advances regarding sex differences and mechanisms that regulate plasma lipid and lipoprotein profiles.

2. Hepatic Lipoprotein Metabolism of Apolipoprotein B100 and Apolipoprotein AI

Apolipoprotein (apo) B hepatic lipoproteins:

The liver plays a key role in the distribution of fats throughout the body. To this end, hepatocytes rely in the synthesis of very low-density lipoprotein (VLDL) particles to transport fats to extrahepatic tissues. Human hepatocytes produce VLDL containing a single apoB100, while murine hepatocytes produce both apoB100 and apoB48-containing VLDLs. Other proteins on isolated VLDL particles have suggested its link to coagulation pathways [10]. The major lipid components of VLDL are triglycerides (TG) (~55%), followed by total cholesterol (TC) (~25%) and phospholipids (~20%). Upon VLDL secretion into the circulation, it interacts with lipoprotein lipase and endothelial lipase to either become a cholesterol enriched low-density lipoprotein (LDL) or return to the liver as VLDL remnant. Importantly, there is evidence of direct hepatic secretion of LDL-size particles [11].

Lipoprotein (a) (Lp(a)) is also an apoB100 containing lipoprotein that is synthesized in the liver. This lipoprotein is characterized by the binding of apo(a) to apoB100 particles. The size of apo(a) is mostly genetically determined by the LPA gene, and has a large variation due to the varying number of kringle 4 type 2 repeats which range from 3 to greater than 40 repeats. Smaller apo(a) size particles are associated with higher levels of Lp(a) and vice versa. Although there have been important advancements in understanding its biology and pathophysiology, the complete synthetic pathway, and regulators of Lp(a) production and clearance have not been fully elucidated [12, 13].

Apolipoprotein A1 (apoAI) and high-density lipoprotein (HDL):

Hepatocytes are also the main cells implicated in the production of apolipoprotein A1 (apoAI) and apoAII, the main structural components of HDL. HDL can deliver cholesterol and other lipids to extrahepatic tissues, although its main role is to remove free cholesterol accumulated in cells such as those present in atherosclerotic lesions by cholesterol efflux, which is a key component of cholesterol elimination by reverse cholesterol transport [14]. However, epidemiological studies indicate that therapies aiming to increase the cholesterol present in the HDL fraction (HDL-C) are rather ineffective [15]. Failure to improve outcomes were countered by the proposal that the activity of the HDL particle (i.e. reverse cholesterol transport), and not HDL-C levels, are driving clinical benefit [14]. Recent studies in mice suggest that the anti-atherogenic properties of HDL could depend on the particle content in free cholesterol, adding novel insights into HDL functionality [16]. HDL particles are highly heterogeneous in both composition and size. Recent proteomic assays have identified hundreds of proteins associated to HDL, highlighting the heterogeneous nature of HDL particles and its association to several metabolic pathways [17, 18].

Once in lipoproteins are released to the circulation, these particles are susceptible to modification by the incorporation or proteins and lipids found in cells and other lipoprotein particles [19].

3. Determinants in Hepatic Lipoprotein Production as a Function of Sex

Regulation of circulating lipoprotein levels by sex hormones:

Hepatic VLDL production is regulated at the transcriptional, co-, and post-translational levels, including the lipidation of nascent apoB [20] (Figure 1). Administration of estrogen to both fetal and adult hepatocytes increase both apoB and apoAI transcription in different experimental models [21–24]. Estrogen, the main steroid hormone in females, regulates gene expression via the steroid nuclear hormone receptors estrogen receptor alpha (ERα) and beta (ERβ) [25]. ERα regulates hepatic lipid metabolism in mice [26], although the exact mechanism of action remained elusive until recently. Yang and colleagues showed that the estrogen related receptor alpha (ERRα), an ERα direct target gene, is enriched in the liver of female mice in comparison to male counterparts [27]. Knockout of ERRα in hepatocytes resulted in a decrease in VLDL-TG production, which was accompanied by lower circulating TG, TC, and apoB100 levels in both male and female mice. These effects were accompanied by an increase in hepatic fat content that was worse in the female mice. In comparison to control mice, ovariectomized mice displayed a greater hepatic TG production, while tamoxifen-treated mice developed non-alcoholic fatty liver disease (NAFLD) accompanied by the reduction of hepatic TG production. Both phenotypes were reversed with the overexpression of ERRα using an adenoviral vector, highlighting the role of this receptor in hepatic lipid metabolism [27]. At the molecular level, ERRα increases the expression of hepatic apoB and the microsomal-triglyceride transfer protein (MTP), which play a pivotal role in VLDL lipidation at the level of the endoplasmic reticulum. ERRα also stimulates phospholipase A2 G12B (Pla2g12b) expression, a relatively uncharacterized phospholipase lacking catalytical activity. In 2019, Faber’s group identified Pla2g12b as regulator of lipoprotein size [28], to later show that it interacts with apoB and MTP at the level of the endoplasmic reticulum membrane to promote lipoprotein expansion [29] (Figure 1A).

Figure 1. Factors affecting hepatic lipoprotein metabolism as a function of sex.

Figure 1.

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(A) Sex hormones and sex chromosomes influence hepatic apolipoprotein synthesis and lipidation. The picture displays the factors discussed in this minireview. (B) Summary of the main sex differences on lipoprotein parameters. This table compares the levels of the indicated lipoproteins of either postmenopausal women (PostM) or men relative to premenopausal women. Abbreviations: ApoAI-ApolipoproteinAI; ApoB100-ApolipoproteinB100; ApoB48-ApolipoproteinB48; AR-Androgen receptor; ER-Endoplasmic reticulum; ERα-Estrogen receptor alpha; ERRα-Estrogen related receptor alpha; GPER-G protein-coupled estrogen receptor; HDL-High-density lipoprotein; LDLR-Low-density lipoprotein receptor; Lp(a)-Lipoprotein(a); MTP-Microsomal triglyceride transfer protein; PCSK9-Proprotein convertase subtilisin/kexin type 9; PDI-Protein disulfide isomerase; Pla2g12b-Phospholipase A2 group XIIB; Tm6sf2-Transmembrane 6 superfamily member; TRL-Triglyceride rich lipoprotein; VLDL-Very low-density lipoprotein

Testosterone is the primary sex hormone in males, and together with its 5α-dehydrotestosterone metabolite, it binds and activates the androgen receptor to regulate gene expression. Clinical data show that low levels of endogenous testosterone are associated with increased TC [30]. Studies in orchiectomized pigs to deplete testosterone results in an increase in plasma TC that can be reversed by testosterone replacement therapy [31]. Similar results were observed in orchiectomized rats, where the authors describe the upregulation of the proprotein convertase subtilisin kexin type 9 (PCSK9) as the molecular mediator of these effects. The absence of testosterone upregulates PCSK9 in hepatocytes that targets the LDL receptor (LDLR) to lysosomal degradation, resulting in an increase circulating LDL levels [32].

Over the past years, the contribution of sex hormones via non-canonical signaling pathways has emerged. One example is the role of estrogens on G protein-coupled estrogen receptor (GPER), a plasma membrane receptor that mediates a rapid response to estrogens [33]. The ablation of GPER results in the increase of adiposity in both male and female mice [34]. GPER loss of function in a cohort of Northern European descents results in increased TC, LDL-C, and apoB levels in female but not male homozygous carriers [35]. Studies in HepG2 cells exposed to a GPER agonist resulted in a decrease in PCSK9 levels accompanied by an increase of LDLR protein levels accompanied by an increase in LDL uptake [35]. These findings were corroborated by another group showing that the activation of GPER by 17β-estradiol in a PCSK9-dependent manner by blocking the accumulation of clathrin in the cytosol, which is required for the internalization of the LDLR [36]. Similarly, the use of an agonist for GPER lowered plasma TC in ovariectomized mice and diet-induced obese male mice [37].

Another non-canonical action of estradiol on plasma lipid levels results from its role on VLDL hepatic lipidation. Estradiol binds the protein disulfide isomerase (PDI), which is one of the two subunits that form MTP [38]. Computational modeling shows that the binding of estradiol to PDI results in the destabilization of the MTP complex, suggesting that sex hormones can also affect VLDL lipidation in a non-canonical pathway [39] (Figure 1A).

Insulin resistance and liver fat as regulators of hepatic lipoprotein lipidation:

Factors such as insulin sensitivity, hepatic TG synthesis, and NAFLD affect VLDL in rodents and humans in a sex-dependent manner (recently reviewed in [40]). Human studies show that premenopausal women are protected from development of insulin resistance and NAFLD as compared to males due to the protective role of estrogens [41], while estrogen deficiency exacerbates the risk of fibrosis in women diagnosed with NAFLD [42]. In mouse models of insulin resistance and metabolic syndrome, females are also protected from suffering NAFLD when compared to male mice [43]. Estrogen supplementation in ovariectomized mice prevents the development of NAFLD, improves insulin resistance, and normalizes VLDL production, directly pinpointing sex hormones as the main drivers of these effects [44, 45]. Studies in liver-specific ERα and ERRα-deficient mice revealed both receptors are critical for estrogen regulation of insulin sensitivity and secretion [27, 45].

Novel molecular mediators are being characterized as key players in VLDL lipidation in preclinical models, some of which are of clinical relevance [46, 47]. Among these, the transmembrane 6 superfamily member 2 (TM6SF2) has been recently identified to play role in NAFLD, fasting TG levels, and type 2 diabetes in people. Subjects carrying the rs58542926 variant in the TM6SF2 gene, which results in lower protein content in hepatocytes, present a greater liver TG content and lower circulating TG levels [48, 49]. Later, Boren and colleagues showed that rs58542926 carriers showed a reduction in large TG-rich VLDL particles and a reduced VLDL production rate [50]. This study was performed in 10 rs58542926 carriers and 10 matched controls, although the authors failed to report the sex of the study participants [50]. A recent study addressed sex-specific effects of the rs58542926 variant in multiple cohorts [51]. Circulating TG in subjects carrying rs58542926 decreased in both sexes, however, the authors observed increased plasma glucose and HbA1c in male, but not in female carriers. Mechanistic studies revealed that mutations in TM6SF2 affects glucose intolerance primarily in both human and male mice [51].

Sex chromosomes as key determinants in plasma lipid profile:

Both sex hormones and sex chromosomes are determinants of biological sex, and until recently, their individual contribution remained undefined. AlSiraj and colleagues took advantage of the Four Core Genotypes (FCG) mouse model [52]. FCG mice are characterized by the absence of the Sry gene in the Y chromosome, which is required for the formation of the testis. As a result, XY mice lacking Sry (XY-) will be phenotypically females. Testis formation in XY- mice can be rescued by the insertion of the Sry transgene in an autosomal chromosome (XY-Sry), which will become functional male. XY-Sry males combined with wild-type females (XX) will produce the FCG mouse progeny: (1) XX: Phenotype female, (2) XY-: Phenotype female (3) XY-Sry: Phenotype male (4) XXSry: Phenotype males. The authors backcrossed FCG mice with the atheroprone Ldlr−/− mice and observed that the mice with two X chromosomes (1 & 4) presented greater plasma TC than those with a single X copy. These results were comparable in both intact and gonadectomized mice. Similar results were observed in FCG crossed with apolipoprotein E deficient gonadectomized mice, implicating the X chromosome in at least some of the sex differences attributed to plasma lipid and lipoprotein profiles [52]. The authors also measured TG secretion over time, observing that mice with a male phenotype (3 & 4) secrete more TG than phenotypically female mice. ApoB100 levels measured at the 3h timepoint showed an increase apoB100 production in those mice with two X chromosomes, which would agree with the greater atherogenesis in these mice. While the authors did not study the kinetics of apoB100 production, their results suggest that mice with two X chromosomes secrete more VLDL particles, but these particles would contain more TG in those mice presenting a male phenotype [52].

4. Clinical Studies on Lipoprotein Kinetics and Function: Effects of Sex, Weight, and Age

The use of stable isotopes in the late 1990’s and early 2000’s revolutionized the field of lipid metabolism and provided valuable insight into mechanisms of lipid metabolism in humans [53]. Plasma proteins concentrations, linked to lipid metabolic pathways and disease, are regulated by their production and clearance from the circulation. Various protocols are currently used to study these pathways, employing labeled amino acids such as leucine or phenylalanine or labeled glycerol that serves as a TG precursor and sodium acetate for de novo lipogenesis. Studies using bolus and primed-constant infusion of deuterated leucine have investigated TG-rich lipoprotein (TRL), which includes apoB100 (VLDL) and apoB48 (chylomicrons), the main carrier of TG, and apoAI and apoAII, the main proteins in HDL. These labeling studies are performed in fasting and non-fasting states, allowing for isolation of liver and intestine-synthesized lipoproteins in humans undergoing lipid altering treatment or with and without diseases that drive lipid changes. Using advanced mass spectrometry techniques, the stable isotope enrichment (amount of labeled within a protein at a specific time) of a protein of interest can be obtained. The results from various studies have shown that lipoprotein metabolism is affected by age, sex and hormonal levels and are summarized below [54–56]. Figure 1B shows the main outcomes described below regarding lipoprotein levels between premenopausal and postmenopausal women, and men.

Kinetic studies focused on VLDL/LDL particles:

In eumenorrheic women, endogenous plasma estradiol does not correlate with basal TG or apoB100 levels in the VLDL fraction, suggesting that the declining plasma estradiol concentration that occurs with age are not key determinants of VLDL production [57]. However, it is well established that cholesterol present in the LDL fraction (LDL-C) levels increase as women transition into menopause [58, 59]. TC, LDL-C, cholesterol present in TRL, and TG levels were higher (50%, 55%, 130%, and 232%, respectively) in postmenopausal compared with premenopausal women, whereas HDL-C levels were similar [54]. The higher LDL-C and TG levels in postmenopausal women were due to lower fractional catabolic rates of TRL and LDL-apoB100 (P<0.05) with no difference in production rates. The latter was hypothesized to occur due to LDLR expression in the liver. There was no significant difference between groups in HDL-C levels or apoAI kinetic parameters. The latter results show that menopausal status adversely affects LDL-C levels and kinetics [54, 60, 61] unlike the findings from eumenorrheic women. These results raise questions about the reliability of preclinical studies to infer results in human populations. One might take into consideration the technical approaches utilized in preclinical studies (e.g. ovariectomy) and the differences between estrous and menstrual cycles.

Using similar experimental approaches, Mittendorfer’s group examined sex differences in VLDL-apoB100/TG secretion and clearance, both in lean and obese states (recently reviewed in [62]). In lean young individuals, women secreted a lower number of VLDL particles, although these particles were enriched with TG in comparison to men [63]. In a second study with a larger cohort, Mittendorfer and colleagues observed a positive association between VLDL-TG levels and obesity in both sexes. VLDL-TG levels were associated with obesity in both males and females. While VLDL-TG levels were directly related to their production rate, this factor was primarily determined by VLDL-TG clearance rate. In men, however, the VLDL-TG production rate was the main determinant of VLDL-TG circulating levels [64].

Lp(a) production studies:

In the last 10 years, association between Lp(a) levels and the development of atherosclerotic CVD has been highlighted by genome-wide association, epidemiological, and clinical studies [13]. Lp(a) levels are highly regulated by the LPA gene [13]. As mentioned previously, small isoform sizes are linked to high levels of Lp(a). Both apo(a) clearance and production have been associated with regulating small isoforms, however larger isoforms have only been linked to apo(a) production. [12, 65, 66]. Importantly, Lp(a) levels are higher in black individuals and women, but the mechanisms behind these differences are not well understood. Changes in the expression of the LPA gene have been proposed as regulators of the production of Lp(a) and these transcriptional gene changes could be responsible for the higher Lp(a) levels in women when compared to men [67, 68]. Additionally, various proteins on Lp(a) particles have been identified as potential mediators of Lp(a) production and clearance [69, 70]. Accurate and detailed proteomic analyses to compare Lp(a) composition between sexes and age (pre vs postmenopausal women) have not been studied.

Studies performed in post-menopausal women taking hormone replacement therapy showed Lp(a) reductions of up to 44%, although evidence indicating a concomitant reduction in CVD risk associated with Lp(a) is lacking. A recent meta-analysis showed that Lp(a) concentrations were lower in premenopausal than in postmenopausal women. However, four of the studies analyzed included pre- and postmenopausal women, matched for age, and these found no difference in Lp(a) concentrations between groups. Three of these studies provided data for Lp(a) in women before and after bilateral oophorectomy, and these found no difference between them [12, 65, 66]. The authors conclude that transition to menopause may increase Lp(a) concentrations, but the process of aging could not be excluded [71].

Studies performed in post-menopausal women taking hormone replacement therapy (mainly oral estrogens) and the synthetic steroid tibolone showed Lp(a) reductions of up to 44%, although evidence indicating a concomitant reduction in CVD risk associated with Lp(a) is lacking. Our incomplete understanding of the mechanisms regulation the synthesis, production, and clearance of Lp(a) in women vs. men hinders the development of adequate therapies, ongoing studies of targeted apo(a) lowering therapies should provide additional insights into this important issue.

HDL kinetics and function:

Circulating HDL particles are heterogeneous in size, charge, density, and composition [18]. HDL biogenesis is a complex process that initiates by the lipidation of apoAI at the level of the plasma membrane where it interacts with the ATP-binding cassette ab cassette transporter A1. Regarding HDL, obese women had a 52% higher HDL-C than obese men (50 vs 33 mg/dL, respectively; P = .012) whereas HDL-C levels were similar in obese and nonobese women [56]. Compared with nonobese men, nonobese women had a higher level of HDL-C and apoAI due to a 48% higher apoAI production rate (P = .05) whereas there was no difference in HDL-C levels or apoAI kinetics in nonobese and obese women [56]. In contrast, compared with nonobese men, obese men had a 9% lower apoAI level due to a 64% higher fractional catabolic rate partially offset by a 47% higher PR. a finding related to the faster apoAI clearance rate in obese men. Body mass index (BMI) was directly correlated with apoAI clearance (r = 0.84, P < .001) and production (r = 0.79, P < .001) in men but not in women. Sixty-two percent of the variability in production and 71% of the variability in clearance were due to BMI in men and only 3% and 23%, respectively, in women [56]. In conclusion, BMI has a significant effect on apoAI production and clearance in men but not in women. The lower HDL-C in men could increase the risk for development of coronary artery disease.

In general, clinical studies indicate women have higher levels of HDL-C and lower levels of TG than men. In a group of 32 normolipidemic older men and postmenopausal women (aged 41 to 79 years), mean HDL-C level was 23% higher and TG level was 27% lower in women compared to men (2). ApoAI levels were also 10% greater in women, as suggested by the higher HDL-C levels, than in men. The higher apoAI in women was due to an increase of large HDL particles and, lower fractional catabolic rate (0.199+/−0.037 versus 0. 225+/−0.062 pools per day, P = .11) when compared to men. ApoAI production rate was similar in men and women (12.28+/−3.64 versus 11.96+/−2.92 mg/kg per day) [55].

It is important to consider that the conventional HDL-C measurements do not reflect the cholesterol efflux capacity of HDL. Recent discoveries highlight the importance of particle size and composition as a function of cholesterol efflux, but to date, the most accurate measure of HDL function is its direct measurement utilizing cells loaded with radiolabeled cholesterol [72, 73]. Studies comparing age-matched men and women typically show that HDL in women has a greater efflux capacity than men [74, 75]. However, the effect of HDL function during the development of menopause was not systematically evaluated until recently. In the SWAN-HDL study, Rader’s group characterized HDL composition and function in 471 women experiencing menopause transition. HDL-C and particle number increased during the onset of menopause. However, the authors observed an overall reduction in HDL function when normalized by particle number because of alterations in HDL composition [76].

Impact of female reproductive cycle and pregnancy on lipoprotein metabolism:

Oscillations in hormones during the lifespan of women are key determinants of their metabolic health. Higher estrogen levels are associated with decreased TC and LDL-C as well as increased HDL-C. During the menstrual cycle, estrogen levels change dramatically in a relatively short span of time, however, these variations do not affect VLDL-C and TG levels, nor hepatic lipid production and clearance [77, 78]. With the transition to menopause, the decrease of circulating estrogens is associated with the dysregulation of lipid metabolism [79]. Loss of sex hormones after menopause are associated with a worsening in the lipoprotein profile. Kinetic studies performed on postmenopausal showed that the supplementation with estrogens results in increased VLDL-apoB100 and LDL-apoB production. However, due to an increase in LDL-apoB clearance rate, overall LDL levels were increased in the participants despite the change in production. The addition of progesterone prevented the increase in these parameters [80]. Endogenous estradiol levels in premenopausal women, however, failed to associate with VLDL-apoB100 or VLDL-TG production rates [57].

During pregnancy, the increase of steroid hormones released by the placenta results in drastic alterations in the circulating lipid profile [81]. During early stages of pregnancy, the mother undergoes an anabolic phase in preparation for the increased energy demands by the developing embryo. During late pregnancy stages, increases in TC, LDL-C, and TG occur [82], which can result in maternal dyslipidemia. However, both an increase and a decrease of circulating lipids during pregnancy result in adverse health outcomes for the mother and the newborn [83, 84], highlighting the importance of maintaining plasma lipid homeostasis during pregnancy.

5. Effect of sex hormone therapies on plasma lipid profile

Sex hormone treatments can be administered to complement the loss of sex hormones with age, serve as sex reassignment strategy in transgender individuals, treat some cancers, among others. This section focuses on the effects of sex hormone therapies in cis and trans individuals.

In cis men, low levels of endogenous testosterone associates with increased incidence of CVD, while the risk of CVD risk and atherosclerosis in subjects exposed to exogenous testosterone remains elusive [30]. On one hand, the Cardiovascular Testosterone Trial failed to observe alterations in the in intima-media thickness in cis men 60 years or older with low or low-normal serum testosterone, while a second study with comparable demographics observed an increase in plaque volume [85, 86]. On the other hand, a study carried out in trans men exposed to gender affirming therapy suggest testosterone could promote shear stress on the endothelial wall vessel [87].

The effect of sex hormones in transgender individuals was recently explored by Robinson and colleagues. They evaluated the number and size of lipoprotein particles in cis pre-pubertal children and young post-pubertal cis and trans individuals utilizing an NMR spectroscopy platform. The authors did not observe sex differences between pre-pubertal children; however, post-pubertal cis women displayed a more favorable lipid profile (lower LDL and greater HDL levels) in comparison to cis men. Results in trans individuals suggested that sex hormones are key determinants of plasma lipid profile. This observation was especially obvious in the association between increased estradiol concentration and greater HDL/apoAI levels [88].

6. Conclusions and final remarks

Clinical and preclinical studies suggest that sex hormones are the main drivers of the alterations in plasma lipid and lipoprotein profiles, however, novel experimental approaches in rodents suggest that these parameters could also be affected by sex chromosomes (Figure 1). The need of a better understanding of the exact molecular determinants that control lipoprotein production and clearance will allow us to develop targeted therapeutic strategies tackling not only biological sex, but also gender and reproductive status. Study design focused on sex differences must consider the differences between biological sex and gender, and whether these parameters are considered adequately. The increasing use of gender affirming hormone therapies, the time and combination of hormones utilized, and the use of adequate control individuals is an important matter of consideration [89].

Lastly, most preclinical studies have consistently focused on performing experiments using one sex, typically male mice. Therefore, some of the findings reported in the literature may have overlooked important differences in regulation of lipoprotein metabolism between sexes. Recent guidelines from the NIH requiring sex to be used as a biological variable in all human and vertebrate studies could lead new pathways to be uncovered in lipoprotein production and clearance and introduce new therapeutic targets for both females and males [90].

Highlights Section.

  • Biological sex plays a major role in regulation of plasma lipid and lipoprotein profiles and hence the development and outcomes of CVD.

  • Preclinical studies show that estrogen administration to both fetal and adult hepatocytes increase both apoB and apoAI. These actions are meant to occur through nuclear hormone receptors named estrogen receptors.

  • Human studies show that premenopausal women are protected from development of insulin resistance and NAFLD as compared to males due to the protective role of estrogens. Results in trans individuals suggest that sex hormones are key determinants of plasma lipid profile.

  • The absence of testosterone upregulates PCSK9 in hepatocytes that targets the LDLR to lysosomal degradation, resulting in an increase circulating LDL levels.

  • During pregnancy, the increase of steroid hormones released by the placenta results in drastic alterations in the circulating lipid profile.

Sources of Funding

This work was supported by the National Institutes of Health (HL139759 to G.R-S and HL147252 to JA) and the USDA NIFA program (W5002) to JA.

Non-standard abbreviations and acronyms

CVD

Cardiovascular disease

TG

Triglyceride

TC

Total cholesterol

VLDL

Very low-density lipoprotein

HDL-C

Cholesterol in the high-density lipoprotein fraction

ERα

Estrogen receptor alpha

ERRα

Estrogen related receptor alpha

PCSK9

Proprotein convertase subtilisin/kexin type 9

LDLR

Low-density lipoprotein receptor

Lp(a)

Lipoprotein (a)

Footnotes

Disclosures

None

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