Gender-based research underscores sex differences in biological processes, clinical disorders and pharmacological interventions

Abstract

Earlier research has presumed that the male and female biology is similar in most organs except the reproductive system, leading to major misconceptions in research interpretations and clinical implications, with serious disorders being overlooked or misdiagnosed. Careful research has now identified sex differences in the cardiovascular, renal, endocrine, gastrointestinal, immune, nervous, and musculoskeletal systems. Also, several cardiovascular, immunological, and neurological disorders have shown differences in prevalence and severity between males and females. Genetic variations in the sex chromosomes have been implicated in several disorders at young age and before puberty. The levels of the gonadal hormones estrogen, progesterone and testosterone and their receptors play a role in the sex differences between adult males and premenopausal women. Hormonal deficiencies and cell senescence have been implicated in differences between postmenopausal and premenopausal women. Specifically, cardiovascular disorders are more common in adult men vs premenopausal women, but the trend is reversed with age with the incidence being greater in postmenopausal women than age-matched men. Gender-specific disorders in females such as polycystic ovary syndrome, hypertension-in-pregnancy and gestational diabetes have attained further research recognition. Other gender-related research areas include menopausal hormone therapy, the "Estrogen Paradox" in pulmonary arterial hypertension being more predominant but less severe in young females, and how testosterone may cause deleterious effects in the kidney while having vasodilator effects in the coronary circulation. This has prompted the National Institutes of Health (NIH) initiative to consider sex as a biological variable in research. The NIH and other funding agencies have provided resources to establish state-of-the-art centers for women health and sex differences in biology and disease in several academic institutions. Scientific societies and journals have taken similar steps to organize specialized conferences and publish special issues on gender-based research. These combined efforts should promote research to enhance our understanding of the sex differences in biological systems beyond just the reproductive system, and provide better guidance and pharmacological tools for the management of various clinical disorders in a gender-specific manner.

Keywords: Endothelium; Estrogen; Hypertension; Testosterone; Vascular smooth muscle.

Fig. 1.

Genomic and nongenomic estrogen (E2)/estrogen receptor (ER) mediated pathways in endothelial cells (ECs) and vascular smooth muscle cells (VSMC). In ECs, E2 binds to cytosolic/nuclear ERα and ERβ and stimulates genomic pathways involving MAPK activation, gene transcription, EC growth and eNOS expression. E2 also binds plasmalemmal ERα, ERβ and GPER in ECs and activates nongenomic pathways and phospholipase C (PLC) to produce inositol 1,4,5-triphosphate (IP₃) and diacylglycerol (DAG). IP₃ releases Ca²⁺ from the endoplasmic reticulum (ER) to form a complex with calmodulin (CAM) and initiate eNOS activation. E2 also activates phosphatidylinositol 3-kinase (PI₃K) to transform phosphatidylinositol-4,5-bisphosphate (PIP₂) into phosphatidylinositol 3,4,5-trisphosphate (PIP3) and activate Akt. ER-mediated activation of MAPK/Akt causes phosphorylation and full activation of eNOS, which transforms L-arginine to L-citrulline and produces NO. NO diffuses to VSMC, activates guanylate cyclase (GC) to form cyclic guanosine monophosphate (cGMP), which decreases [Ca²⁺]_c by stimulating plasmalemmal Ca²⁺ extrusion pump (PMCA) and sarcoplasmic reticulum (SR) Ca²⁺ uptake pump (SERCA), and decreases the actin-myosin myofilaments force sensitivity to [Ca²⁺]_c leading to VSM relaxation. E2 also inhibits NADPH and peroxynitrite (ONOO⁻) formation thus promoting antioxidant effects that prevent NO inactivation. E2 also activates cyclooxygenases (COX) to produce prostacyclin (PGI₂) which binds PGI₂ receptor in VSMC, activates adenylate cyclase (AC) to form cyclic adenosine monophosphate (cAMP), which similar to cGMP decreases [Ca²⁺]_c and myofilament force sensitivity to Ca²⁺ leading to VSM relaxation. E2 also increases endothelium-derived hyperpolarizing factor (EDHF), causing activation of VSM K⁺ channels, hyperpolarization, inhibition of Ca²⁺ influx through Ca²⁺ channels and VSM relaxation. In VSM, E2 binds cytosolic/nuclear ERα and ERβ to activate genomic pathway, and inhibit MAPK, gene transcription and VSMC proliferation. Also, in VSM, a vasoconstrictor agonist such as ET-1, TXA₂ or Ang II activates its specific receptor (R) to stimulate PLC and generate IP₃ and DAG. IP₃ stimulates Ca²⁺ release from SR. Agonists also stimulate Ca²⁺ entry through Ca²⁺ channels. Ca²⁺ binds CAM to activate myosin light chain kinase (MLCK), causing MLC phosphorylation, actin-myosin interaction and VSM contraction. DAG activates PKC to phosphorylate calponin (CaP) or activate MAPK kinase (MEK) and MAPK cascade, leading to phosphorylation of caldesmon (CaD) and increased myofilament sensitivity to [Ca²⁺]_c. E2 binds plasmalemmal ERα, ERβ and GPER to activate non-genomic pathways and inhibit agonist-activated mechanisms of VSM contraction including PKC, MAPK and ROCK [Ca²⁺]_c sensitization pathways. Dashed lines indicate inhibition.

Fig. 2.

Sex differences in representative biological processes in adult males and females. Sex differences have been observed in the central nervous system (CNS), cardiovascular system (CVS), the immune response in the spleen, lymph nodes and T-helper cells, the kidneys and various ion transport mechanisms, and the metabolic system including the gastrointestinal tract, liver and pancreas. The sex differences have been attributed to genetic differences in the XX and XY chromosomes, and the levels of the sex hormones E2, P4 and T3. FFA, free fatty acids; Na/Cl, sodium chloride cotransporter, Na/H3 sodium/hydrogen exchanger 3; Na/Pi, sodium/phosphate cotransporter, SGL2, sodium-glucose cotransporter 2; NEFAs, non-esterified fatty acids

Fig. 3.

Sex differences in representative clinical disorders. Sex differences are observed in the prevalence of anxiety [237], depression (lifetime, U.S., 1990–1992) [236], substance use disorders (adults, Sweden, 2003–2004) [282], asthma (adults ~40, worldwide, 2018) [227], hypertension (adults ~40, U.S., 2007–2012) [283, 284], myocardial infarction (U.S., 1993), stroke (lifetime, worldwide, 2015) [285, 286], dyspepsia (adults, U.S., Canada, United Kingdom 2018) [197], irritable bowel syndrome (householders, U.S., 1990) [200], diabetes (adults 20–79, U.S., 2013–2016) [213], chronic kidney disease (U.S., 2015–2016) [287], and arthritis (U.S., 2013–2015) [288].

Fig. 4.

Changes in the sex differences in the prevalence of representative clinical disorders with age. Sex differences are reversed with age in the prevalence of hypertension (U.S., 2007–2012) [283, 284] (A) and asthma (worldwide, 2018) [227] (B), and further exaggerated with age in the prevalence of Alzheimer’s disease and other dementias (worldwide, 1990–2016) [289] (C).