Sex and Liver Disease: The Necessity of an Overarching Theory to Explain the Effect of Sex on Nonreproductive Functions

Abstract

The number of studies illuminating major sex differences in liver metabolic activities is growing, but we still lack a theory to explain the origin of the functional differences we are identifying. In the animal kingdom, energy metabolism is tightly associated with reproduction; conceivably, the major evolutionary step that occurred about 200 million years ago with placentation determined a significant change in female physiology, as females had to create new energy strategies to allow the growth of the embryo in the womb and the lactation of the newborn. In vertebrates the liver is the metabolic organ most tuned to gonadal functions because the liver synthesizes and transports of all the components necessary for the maturation of the egg upon estrogenic stimulation. Thus, in mammals, evolution must have worked on the already strict gonad-liver relationship fostering these novel reproductive needs. As a consequence, the functions of mammalian liver in females diverged from that in males to acquire the flexibility necessary to tailor metabolism according to reproductive status and to ensure the parsimonious exploitation and storage of energy for the continuation of gestation in case of food scarcity. Indeed, several studies show that male and female livers adopt very different strategies when confronted with nutritional stress of varied origins. Considering the role of liver and energy metabolism in most pathologies, a better focus on liver functions in the 2 sexes might be of considerable help in personalizing medicine and pharmacology for male and female needs.

In the context of personalized therapies, the study of sex differences has gained momentum because of the accumulation of evidence pointing to a major sex specificity of the response to drug treatment and in disease susceptibility. Indeed, drug side effects are 1.6 times more frequent in women; this phenomenon was initially explained as a mere problem of dosage associated with women’s lower body weight and known physiological factors contributing to a sex-dependent drug pharmacokinetics. First among those, the differences in drug absorption (e.g., due to higher gastric flow and pH, intestinal motility, lung capacity, and body surface in males), catabolism of both phase I (higher content of CYP1A and CYP2E1 in male liver and CYP2D6 and CYP3A in female liver) and phase II (the content of UDP-glucuronosyl transferase, sulfotransferase, and methyltransferase are higher in male than in female liver), and excretion as generally females have a lower rate of glomerular filtration and sex hormones may influence renal activity (1-3).

While these differences may explain a higher probability for women to manifest a given side effect to a drug for which dosage has been perfected in studies conducted in men, less understood is the fact that for selected groups of drugs the side effects registered for women are qualitatively different than in men: for instance, in response to antidepressants such as selective inhibitors of serotonin uptake, side effects are very different between men and women, with men showing mainly aggression, suicidal thoughts and attempts, and sexual dysfunctions while women have hematomas, reduction of visual acuity, palpitations, tremor, dry mouth, alopecia, nausea, and weight increase (4). Such a qualitative difference in the adverse effects suggests that the sex-specific responses are not simply ascribable to a dosage difference, but to a variety of physiological differences. A similar conclusion may be drawn considering the sex-based differences in the susceptibility, clinical manifestations, and course of a wide variety of pathologies in humans. As an example, women have higher susceptibility to autoimmune diseases (Graves disease, Hashimoto thyroiditis, multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, type 1 diabetes) while in men there is a prevalence in cancers of the nonreproductive organs (bladder, bowel, kidney, leukemia, liver, lung, skin [malignant melanoma], esophagus, stomach); in terms of infectious diseases women are more prone to be infected with HIV, influenza, toxoplasmosis, malaria, and Zika, while males appear to be more susceptible to contract Ebola, MERS, COVID-19, hepatitis B, tuberculosis, leptospirosis, schistosomiasis, amebiasis, and aspergillosis (5, 6).

The understanding of the cellular and molecular bases of these dissimilarities is fundamental for the personalization of future medical practice and pharmacology. Therefore, in the last decade several authors have attempted to better define sex biological differences in humans. Current belief is that in mammals there are 3 major sex determinants: (a) heteromorphy of sex chromosomes; (b) circulating sex steroids; and (c) sexual differentiation that occurs in the final phases of embryo development and at puberty. In spite of the increased insights that we are gaining on the mechanisms driving the functional and phenotypic sexual differentiation, such knowledge is far from sufficient to fully understand the origin of the sex-based differences in the epidemiology, clinical manifestations, and response to therapy, particularly for diseases not directly associated with reproductive functions. In the past 30 years, investigations in our group have been specifically focused on the role of sex in the physiology of nonreproductive organs. Our experimental results led us to formulate a theory, here summarized, that might be of help to understand of the genesis of sex specificity in mammalian physiopathology.

Sexual Differentiation in Mammals

Sex Chromosomes

As anticipated, sex chromosomes have the key role in mammalian sexual differentiation: the female karyotype is characterized by 2 identical sex chromosomes (XX), while the male karyotype has 1 X and 1 Y chromosome (7). Females, therefore, are totally deprived of the elements present on the Y chromosome (a few dozen genes) but have the potential of higher expression of all genes and regulatory elements contained on the X chromosomes, which represent 5% of the entire genome in humans (more than 1000 genes). In reality, this latter possibility is partially reduced by the mechanism of random methylation that inactivates several genes in the X chromosome; even if about 15% of the genes present in the X chromosomes escape the inactivation and others are only partially inactivated; the extent of this phenomenon is quite variable and it may change even within a single individual (for instance in relation to age) (8, 9). In Theria (placental mammals and marsupials), the Y chromosome is very small, and contains long stretches of heterochromatin and non-coding DNA. Of the about 80 genes present in the Y chromosome in total, some are housekeeping, but the large part are unique to the Y chromosome and are relevant for testicular functions (10, 11); particularly important is the sex-determining region Y (sry) that encodes the testis determining factor (TDF), the transcription factor essential for male sex determination. In the course of embryogenesis, TDF targets the bipotential cells of the primordial gonad, that lie along the urogenital ridge, and induces their differentiation into Sertoli cells by eliciting the synthesis of SOX9. Therefore, TDF is essential for the masculinization process and the lack of sry in the embryo genome leads to the development of ovaries and production of ovarian hormones. Thus, the female sex represents the default pathway in the sexual differentiation of the gonads (the main source of sex hormones) and genital organs. More recent studies on the activity of the Y chromosome in different tissues revealed that the expression of some of the transcription factors encoded by this chromosome may be very highly expressed in several tissues, with a consequent sex-dependent imbalance. Likely, this element may also play some role in sexual dimorphism that remains to be better elucidated (12).

Sex Steroids

The sex steroids—testosterone and estrogens in particular—are the effectors of the major influence that the gonads exert on reproductive and nonreproductive tissues. Specifically, the 2 most relevant time windows in which testosterone and estradiol are determinant for phenotypic sexual development are: the end of fetal development and puberty.

Morpho-functional Sexual Differentiation

At the end of fetal development (or in the first days of life, depending on the species), male gonads become very active and synthesize testosterone for a short period of time, after which they return quiescent until puberty. At this early time, the steroids act in reproductive and nonreproductive organs (e.g., the brain) to induce their sexual differentiation (13, 14). While the effects of testosterone and its metabolite estradiol are well-studied in the reproductive organs, which become primed to respond to sex steroids at puberty when the secondary sex characteristics develop, the effects of this initial spur of male gonadal activity in nonreproductive organs, where testosterone may also be converted into estradiol by the enzyme aromatase (CYP19), are less known and understood. For instance, highly reproduced studies in mammals demonstrated that at perinatal time testicular testosterone reaches the brain where is converted into estradiol due to the presence of high concentrations of CYP19 in neural cells. Estradiol, possibly together with testosterone or its dihydro-derivative (dihydrotestosterone; DHT), primes the differentiation of several of the brain areas that in the adult animals will be indispensable for behaviors directly or indirectly involved in reproduction (such as copulatory behavior, aggression, parental care, and others) (13). Still little studied are effects of the perinatal surge of testosterone in organs other than in mammalian brain; a sequela of studies in our group showed that the liver may represent an additional target for the priming effects induced by the perinatal activity of male gonads (15-17), but other targets remain unraveled.

Together with these 3 major and essential elements, others may contribute to the sex-specific physiopathology, including genomic imprinting, that is, the developmental process leading to the exclusive expression of alleles from a specific parent that highlights maternal or paternal features randomly; differential interactions between nuclear genome and the maternally-derived mitochondria genome; epigenetic changes due to social, environmental, and in humans, economic factors; and fetal cells that persist in the mother’s circulation for years inducing the described higher immune reactivity in females (18).

Reaching a Comprehensive View of the Impact of Genetic, Epigenetic, and Hormonal Factors on Sex-Specific Functions Across Reproductive and Nonreproductive Tissues

The amount, degree of interconnections, and biological variability of most of the elements that come into play to define sex-specific physiopathology impede making a systematic list of all sex-biased biological processes. To tackle this overwhelming complexity, the study of sex-specific physiopathology has been so far approached by means of comparative descriptions leading to long lists of sex-biased functions/pathologies, for example, male and female gene transcription in selected tissue, cellular functions, changes in activities after cell trans-sex transplantations (19-22), effect of endocrine manipulations at all developmental ages, and in adults, epidemiological investigations on human pathologies (23). The major merit of the results obtained so far is that all these studies have highlighted the breadth and depth of the divergences characterizing the physiology of the 2 sexes.

Curiously, in the context of the studies of sex-differential physiopathology, little attention has been given to the role played by evolution; this calls for a reflection on this crucial biological aspect.

Mammalian Evolution and Sex-Biased Physiopathology

Reproduction and nutrition are both essential to the continuation of life on earth and indissolubly entangled; the bond between these 2 functions is a biological requirement because famine, when available food is insufficient to feed both parents and offspring, could cause the death for both, with extinction of the species as a consequence. Indeed, food deprivation generally leads to infertility. Being essential to the continuation of life, therefore, the mechanisms preventing reproduction in case of food shortage should be very well conserved. Ideally, such biological mechanism should involve a reproductive and a metabolic organ: with the metabolic organ gauging the nutritional level of the whole organism and blocking reproduction in case of prolonged shortage of food supply. We now know that this is the case: all through the phyla, the major metabolic organ is responsible for the synthesis of the egg components, which occurs upon gonadal stimulation in the presence of a permissive supply of circulating essential amino acids (AA) only. Because circulating AA are provided by the diet, this elementary mechanism ensures the blockade of ovulation in case of major limitations in food supply. This mechanism has been very well preserved in the course of speciation and can be traced back to insects or nematodes (24). In mammals (e.g., mice) the liver synthesizes the structural and transport proteins indispensable to convey to the gonads all the elements essential for the maturation of the egg. This synthesis occurs upon estrogen stimulation, via the hepatic estrogen receptor alpha (ERα). In the case of food deprivation, the hepatic ERα cannot reach the extent of transcriptional activation necessary to trigger the synthesis of the egg proteins and ovulation is prevented. This process is reversible, because the administration of AA reinstates ERα ability to regulate all the pathways necessary for the maturation of the egg (25). Such a control mechanism is extremely well conserved in the animal kingdom in spite of the major evolution that occurred with speciation in the metabolic organs (e.g., in nematodes the major metabolic organ is the intestine or the hepatopancreas in arthropods) and gonadal hormones (e.g., ecdysone in insects, estradiol in mammals) (24).

A major evolutionary step forward with regard to reproductive functions has been placentation, which enabled females to develop the fertilized egg into complete animals within their womb. This biological revolution must have inordinately affected female metabolism, which had to acquire completely novel flexibility to allow the timed progression of the ovulatory cycle, the progressive growth and maturation of the embryo, and the lactation of the newborn. Considering that in oviparous (egg-laying animals) the liver is the metabolic organ controlled by and controlling reproductive functions via the synthesis of the egg proteins and the transport of the necessary nutrients to the egg, it could be inferred that in mammals this same organ was the most subjected to the selective pressure aimed to adapt it to the large variety of the novel reproductive requirements. If this were the case, with the appearance of Theria on Earth (about 200 million years ago) the functions of female liver started to diverge significantly from those characterizing the liver of oviparous animals; on the other hand, males were not subjected to any adaptive pressure because they maintained same reproductive functions of egg-laying animals.

Considering that the liver is the major metabolic organ in mammals, we believe that the adaptive pressure that reproduction imparted on this organ created a major physiological divide between female and males with obvious repercussions for pathology susceptibility. On this line of thought, in the past years we designed a series of experiments to support the theory that in mammals the major perpetrator of sex-biased functions is the change in liver metabolism associated with female reproductive functions and that the hepatic ERα has a major role in maintaining metabolic differences that support a large number of sex-dependent physiopathological activities in humans.

The Liver, a Reproductive Organ in Female Mouse?

Strong evidence for the mechanism of integration of liver activities with reproductive functions in mammals was provided by studying the reporter mouse ERE-Luc where the ubiquitous luciferase expression is directly proportional to the transcriptional activity of estrogen receptors (ER) (26). The quantitative analysis of luciferase activity in the course of the estrous cycle showed, for the first time, that in a large number of nonreproductive estrogen target tissues (thymus, intestine, bone, and several others) the ER transcriptional activity was not directly proportional to the concentration of the circulating estrogens. This phenomenon is likely associated with the complexity of intracellular ER activities and the fact that their transcriptional activation may be triggered by posttranslational modifications also in the absence of the designated ligand (25). Differently from the nonreproductive organs, in all the organs involved in reproductive functions, there was a direct relationship between ER activity and the amount of circulating estrogen; the liver was included in this group (15, 26). This strict association between hepatic ERα and gonadal synthesis of estrogens suggested an involvement of this organ in reproductive functions. Most interestingly, this relationship is lost with starvation, in fact the liver isoform of ER (ERα) becomes estrogen-insensitive in case of prolonged dietary restrictions, while it is transcriptionally activated by dietary AA (25). This is in line with the hypothesis that the liver is the metabolic organ responsible for the assessment of circulating levels of AA and gonadal hormones prior to the synthesis of the proteins relevant for the maturation of the egg and the appropriate continuation of the reproductive cycle.

Further studies showed the relevance in mice of the tetradian oscillation of the hepatic ERα for the metabolism and transport of lipids and cholesterol (27); in addition, it was demonstrated that alterations of ovarian ability to synthesize sex steroids (e.g., with pregnancy) or absence of ovarian activity (in case of ovariectomy or in aging) are associated with dysregulation of lipid metabolism and pathological deposition of fat in the hepatic parenchyma (16). Most interestingly, when we tested the metabolic effects of a diet enriched in specific AA in ovariectomized (OVX) mice, the diet was able to block the OVX-induced weight gain and fat deposition in the liver. The use of liver-specific ERα knockout mice demonstrated that the hepatic ERα has a key role in the systemic response to OVX through the control of liver lipid metabolism (28).

More in-depth investigations of the effect of the reproductive cycle on cholesterol transport showed that the production of high-density lipoprotein (HDL), low-density lipoprotein (LDL), and very-low-density lipoprotein (VLDL) changes significantly in the course of the reproductive cycle, with the highest HDL/LDL ratio at proestrus. This highlighted an interesting mechanism adopted by mouse female liver to maximize cholesterol exploitation that consists in the production at proestrus of HDL of molecular weight and dimensions smaller than the HDL synthesized in the other phases of the cycle. These HDL have an enhanced avidity for cholesterol; this is a simple expedient to maximize the inverse transport of cholesterol that has a double advantage: from one end it avoids undesired accumulation of cholesterol in sites other than its store deposits and on the other it saves synthetic energy. The synthesis of these specific HDL is strictly dependent from the full activation of the hepatic ER and ceases with the cessation of ovarian functions (29).

All together these observations provide good evidence of the strict interdependence of liver and gonadal activities in females and may explain the lower susceptibility of fertile women to cardiovascular disorders and why after menopause the incidence of metabolic disorders and atherosclerosis in women is comparable to men. Indeed, the cessation of ovarian functions is associated with lipid deposits in the liver parenchyma (with pathological consequences for female metabolism) and cholesterol starts to accumulate in the blood vessels.

Hepatic Sex Differences and the Role of ERα

The hypothesis of a functional deviation of mammalian female liver from the male counterpart is supported by experimental evidence provided by the direct comparison of the hepatic metabolism in males and females, particularly in response to dietary challenges such as fasting or exposure to unbalanced diets.

High Fat Diet

When siblings of both sexes were exposed for 16 weeks to a diet enriched in fat, females compensated for the excess of dietary lipids by limiting lipid import in the liver, by avoiding de novo lipid synthesis, and efficiently promoting mitochondrial fatty acid oxidation; all these effects are mediated by the hepatic ERα (because abolished in LERKO mice with liver-specific ablation of Esr1). Males appeared to be unable to adopt homeostatic measures aimed at limiting the damages of a prolonged exposure to high fat diet; conversely, in males there was increased liver lipid uptake, augmented synthesis of lipids, and impaired fatty acid oxidation. All this leads to a degenerated hepatic metabolism determining fatty liver and a significant increase of circulating cholesterol (17).

Fasting

In an opposite experiment where both sexes were subjected to short-term fasting, female liver quickly adapted to the alimentary shortage: to ensure that all the energy components of the diet could be conveniently maintained and stored, there was an increased production of AA catabolic enzymes and a metabolic shift in which the available AA were utilized as substrate for lipid synthesis. This parsimonious attitude was not observed in male liver, which limited lipid anabolism to cope with fasting, regardless of the good supply of AA (30).

These observations point to energy-saving strategies and ability to deal with dietary imbalances specific in female and not present in male liver. Such a clear functional difference and ability to manage dietary stress are compatible with the acquisition, by female liver, of the metabolic flexibility necessary and sufficient to satisfy the energy needs of reproduction in terms of quantity and variability. In this context, the importance of gonadal hormones and hepatic ERα is emphasized by the loss of such flexibility observed in liver-specific ERα knock out mice and by the major metabolic disruption ensuing ovariectomy.

Interestingly, our studies did not highlight any major sexual differences in the regulation of glucose metabolism by the liver. This was in line with other studies on the mechanisms aimed at maintaining/restoring euglycemia after fasting, exercise, or in case of hypoglycemia. Most studies showed that the homeostatic mechanisms involved lipid metabolism in females and carbohydrate metabolism in males (31). A potential explanation for these findings is that, in a world where food is not always available, female metabolism must be geared toward a prudent usage of energy assets and this can be achieved by transforming all the components present in food in a form that is apt to long term deposits like lipids.

The precise mechanisms selected by evolution to provide females with enhanced metabolic flexibility remain to be clarified as a whole. In all metabolic studies it is difficulty to dissociate primary from secondary responses and to define the extent to which the homeostatic responses observed are associated with organ-intrinsic or extrinsic influences. Because of that, several authors suggested a role for the nervous system in the control of the peripheral metabolic sex differences reported: suggested mechanisms include estrogen effects on circulating catecholamines, gender-based differences in epinephrine sensitivity, differences in the central nervous system triggering gender-specific responses. The involvement of the somatotropic axis on the sex-specific metabolic responses of liver is likely due to the strong endocrine communications among liver, central nervous system, and gonads (32) and only an extensive use of a variety of ER selective knockout mice will enable us to shed some light on liver-specific functions in the homeostatic responses described above. However, this is not relevant to our aim, which is to stress how much in the course of evolution the transition from egg-laying to live birth differentiated female from male metabolism and how this should be taken into consideration for our further understanding of sex-specific physiopathology.

Human Pathologies and Sex

Several of the sex differences in human pathologies have a direct association with genetics: because of the double X complement, women are generally protected from X-linked recessive disorders commonly afflicting men, such as hemophilia, Duchenne muscular dystrophy, and color blindness. The genetic origin is more difficult to be demonstrated for others, such as the higher incidence in females of autoimmune diseases (e.g., lupus erythematosus, Sjogren’s syndrome, scleroderma), osteoporosis, Alzheimer disease, and depression (5, 6, 33-38), or higher male prevalence of neuropsychiatric and neurological diseases like autism, Parkinson disease, and schizophrenia (39, 40). In addition, there are diseases, such as cardiovascular disorders and stroke, where the differences between the 2 sexes are in the age of onset, manifestation of the pathology, and mortality (41-43). For these latter pathologies, genome-wide association studies (GWAS) in large human cohorts are attempting to identify a genetic contribution to the sex differences with some success: for obesity, as an example, a meta-analysis of studies made on more than 200 000 individuals identified 49 loci affecting adiposity; 20 of them showed sex-specific effects, with 19 having higher effects on women (44). These findings are important for further identification of sex-biased genetic factors in the case of body fat distribution, for other pathologies still much remains to be done to understand the cause-effect relations and why the organisms of the 2 sexes diverge in their response.

In addition to genetics, sex hormones were shown to be involved in the etiology and progression of several diseases such as cancer, osteoporosis, neurodegeneration, immune disorders, metabolic pathologies including diabetes, and related cardiovascular diseases. Both androgen and estrogen receptors are widely distributed in the tissues affected by the pathologies above mentioned and the onset or progression of the pathology may occur as a result of a direct action in the target cells due to excessive or deficient hormonal stimulation. For many pathologies, the role of the sex hormone has been long studied and is better defined, for example, in the case of osteoporosis occurring in females with ovarian failure—the deficiency of circulating estrogens causes an imbalance between bone formation and resorption with osteoclast activity becoming predominant. Osteoblasts, osteocytes, and osteoclasts all express ERs and the sex steroids actively participate in the metabolism of all of these cells: a diminished production of estrogens is associated with increased osteoclast numbers (due to enhanced osteoclast proliferation and reduced apoptosis) and activity. This would occur via paracrine effects with the de-repression of the synthesis of inflammatory cytokines (interleukin-1, interleukin-6, tumor necrosis factor [TNF]-α, granulocyte macrophage colony-stimulating factor, macrophage colony-stimulating factor [M-CSF], and prostaglandin-E2 [PGE2]) responsible for the increased pool size of osteoclast precursors in bone marrow. However, the effect of estrogens does not appear to be strictly limited to bone cells as in the absence of the hormone, T cells produce TNF-α which, acting through the TNF-α receptor p55, augments macrophage colony-stimulating factor–induced and RANKL-induced osteoclastogenesis (45). These studies show the difficulty in getting a clear picture of the hormonal influence in a given pathology. Indeed, estrogens act and regulate the activities of virtually all mammalian cells, thus impeding the definition of the exact group of cells that primarily/predominantly respond to the change of hormonal activity. Such difficulty increases exponentially with the complexity of the organ affected by the pathology: in neurodegenerative diseases with sex prevalence, sex steroids were reported to be neuroprotective through complex and integrated processes that depend on: (i) the type and amount of the cognate steroid receptor; (ii) the amount of steroids synthesized locally; and (iii) the target cell type (either neurons, glia, or microglia).

Is Liver Sexual Dimorphism Relevant for Sex Differences in Disease Predisposition?

Considering the significant role played by liver in mammals, its profound sexual dimorphism may be considered a major player in determining the sex differences in the predisposition and manifestation of pathologies. Liver metabolism is essential for the metabolism, storage, and body distribution of carbohydrates, fat, proteins, vitamins, and minerals; in addition, the liver synthesizes factors of the immune system to fight against infection or the proteins responsible for blood clotting, and exerts a key detoxifying role clearing the blood of damaged circulating cells, debris, and toxic catabolites and xenobiotics. As earlier illustrated, in mammalian female liver, the evolutionary pressure must have carved substantially on the original functions to adapt to the novel necessities of reproduction, to ensure a parsimonious use of all nutrients, and possibly to protect from damages due to the continuous metabolic fluctuations. All of these adaptive changes might explain the resilience of fertile females against cardiometabolic disorders and the loss of such protection with the cessation of ovarian functions. In fact, lipid metabolism is highly affected by the cessation of ovarian functions, with major alterations in their body distribution; after the menopause, the accumulation of fat in the liver may possibly trigger a chain of events leading to generalized inflammation that is at the basis of most of the pathologies associated with aging in women. Furthermore, the lack of estrogenic stimulation in the liver may alter the transport of minerals, vitamins, and other elements, with consequences for the metabolism of other organs (Fig. 1).

Figure 1.

Placentation and its role in initiating liver sexual differentiation.

Therefore, our studies should be more focused toward the understanding of these functional modifications and their impact on the regulation of the overall physiology and potential influence on women’s susceptibility to disease. This might reveal new pathways for a pharmacology personalized on sex. For instance, the finding of the development of a specific strategy for cholesterol distribution and reverse transport that is associated with the reproductive cycle could be relevant to understanding why fertile women are protected against atherosclerosis prior to menopause and may enable the creation of new therapies based on lipoprotein substitution; similar considerations could be made for the finding of a sex-specific ability to catabolize proteins that may result in different responses of females and males to diets based on high protein intake for weight loss. Finally, the key role of the hepatic estrogen receptor in the control of female metabolism should be better investigated. Our initial studies have shown that the content of ER in the adult male liver is much lower than in females and its functional role appears to be quite different because it does not seem to be indispensable for the maintenance of the metabolic equilibrium as in females. On the other hand, the liver also expresses the androgen receptor, which certainly plays a role in liver functions that at the moment are largely unknown.

In conclusion, a better knowledge of liver functional differences in males and females might have a significant impact for gender-based medicine and provide novel insights for innovative drugs for both sexes.

The transition from egg-laying to live birth had a major toll on females, demanding a major reshaping of metabolic functions. The liver being instrumental for female reproductive functions, it is conceivable that this was the organ where the evolutionary selective pressure was particularly strong. This determined a progressive deviation from the original functions for female liver, but not for males as their reproductive functions were unaffected by the novel reproductive strategy.

Glossary

Abbreviations

AA

amino acid

ER

estrogen receptor

ERα

estrogen receptor α

HDL

high-density lipoprotein

LDL

low-density lipoprotein

OVX

ovariectomized

TDF

testis determining factor

TNF-α

tumor necrosis factor-α

Additional Information

Disclosures: The author does not have any conflict of interest.

Data Availability

Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.