Sex steroid hormones: an overlooked yet fundamental factor in oral homeostasis in humans

Abstract

Sex steroid hormones (SSH) are extremely versatile molecules with a myriad of physiological functions. Next to their well-known role in sexual development and reproduction, SSH play active roles in practically every tissue in the human body, including the oral cavity. It has long been demonstrated that periodontal tissues express SSH receptors and therefore are responsive to the presence of SSH. Interestingly, SSH not only interact with the periodontal tissues but also with other tissues in the oral cavity such as dental enamel, pulp, cementum, oral mucosa, and salivary glands. Questions concerning the possible physiological functions of these receptors and their role in maintenance of oral health, remain unanswered. The purpose of this scoping review was to gather and summarize all the available evidence on the role of SSH in physiological processes in the oral cavity in humans. Two comprehensive literature searches were performed. References were screened and selected based on title, abstract and full text according to our inclusion criteria. Both searches yielded 18,992 results of which 73 were included. Results were divided into four categories: (1) Periodontium; (2) Dental structure; (3) Mucosa; and (4) Salivary glands. The interaction of these tissues with progestagens, androgens and estrogens are summarized. Sex steroid hormones are an overlooked yet fundamental factor in oral homeostasis. They play important roles in the development and function of the periodontium, dental structure, mucosa and salivary glands. Dentists and healthcare providers should consider these hormonal factors when assessing and treating oral health conditions.

1 Introduction

When asked about the role of sex steroid hormones (SSH) on the human body, the first thought that comes to mind is their essential and primary function on the sexual and reproductive system. Although this holds true and is indeed the most studied and well-known role of SSH, it is by no means their only known function. These hormones are incredibly versatile and capable of inducing various physiological responses (1). Alongside their primary and well-known role in the sexual and reproductive system, SSH are involved in a variety of biological processes that bear no clear relationship with their main function. SSH –namely progestagens, androgens and estrogens– are known to act as appetite modulators (2), to influence skeletal muscle strength and power (3), to play a role in adipose tissue regulation (4, 5), bone mineral density (6) and regulation of the immune response (7), to name a few. Considering SSH’s versatility and ubiquity, it is not surprising that these molecules also play a role in the oral cavity (8).

Synthesis of SSH begins with the enzymatic process known as steroidogenesis (Figure 1), where their precursor –cholesterol– is converted into biologically active SSH (10). Cholesterol is transported to the inner membrane of the mitochondria by specific proteins. The steroidogenic acute regulatory protein (StAR), and translocator protein (TSPO), in a complex with proteins such as voltage-dependent anion channel (VDAC), ATPase family AAA-domain containing protein 3A (ATAD3A), amongst others, are involved in this process. Once cholesterol has reached the inner membrane of the mitochondria, it is further converted into pregnenolone by the cholesterol side-chain cleavage enzyme (P450_SCC; CYP11A1) (10, 11). Pregnenolone will then passively diffuse from the mitochondria to the smooth endoplasmic reticulum for further conversion into either progesterone by 3-beta-hydroxysteroid dehydrogenase (3β-HSD) or 17-alpha-hydroxypregnenolone by 17-alpha-hydroxylase/17,20-lyase (CYP17) (12). CYP17 and 3β-HSD can respectively convert progesterone and 17-alpha-hydroxypregnenolone into 17-alpha-hydroxyprogesterone. Pregnenolone, progesterone, 17-alpha-hydroxypregnenolone and 17-alpha-hydroxyprogesterone compose the progestagens class. CYP17 can go on to further convert 17-alpha-hydroxypregnenolone into dehydroepiandrosterone (DHEA) and 17-alpha-hydroxyprogesterone into androstenedione. DHEA will then be the precursor of androstenedione, androstenediol, testosterone, and dihydrotestosterone (DHT). These four SSH, together with DHEA compose the androgens class. Aromatization of androstenedione and testosterone will originate estrone and estradiol, respectively. Together with estriol, these three SSH compose the estrogens class (10).

Figure 1

In mammals, these molecules are mainly secreted by the testis, ovaries, adrenal cortex and during pregnancy, by the placenta (Figure 1) (13). Interestingly, the biosynthesis of sex steroid hormones has also been observed in the central nervous system, skin, and adipose tissue, albeit at a lower rate than in the reproductive organs (14–18).

Classical delivery modes of SSH are endocrine, paracrine, and autocrine. The endocrine mode involves hormone secretion and transport via blood vessels to reach distant target tissues. Paracrine delivery involves local synthesis of SSH and diffusion through extracellular fluid, covering smaller distances, usually within the same organ. In the case of the autocrine mode of delivery and action, a cell is activated by its own hormonal signals, becoming thus both hormone source and target (19).

Once SSH are synthesized and delivered, a high percentage binds to plasma proteins (such as SHBG). The remaining non-bound fraction of the hormones can bind to specific intracellular, membrane-associated, or transmembrane receptors, activating a signaling cascade that results in biological effects (20, 21).

It has long been demonstrated that periodontal tissues exhibit SSH receptors and therefore are susceptible to the presence of SSH (22–24). However, questions such as the possible physiological functions of these receptors or how they engage in the maintenance of oral health, remain unanswered.

The purpose of this review is to gather and summarize all the available evidence on the role of SSH in normal physiological processes in the oral cavity in humans. This includes how SSH influence oral homeostasis and what is known about their role on the development and function of different oral tissues. Clinical implications and future challenges will be discussed.

2 Materials and methods

2.1 Search and protocol

This scoping review was conducted in accordance with the Preferred for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-Scr) checklist explanation (25). In collaboration with a medical librarian (LS), two comprehensive searches were carried out on different dates. The first search, from inception to April 6^th, 2020 was carried out in the bibliographic database PubMed. The second search –aimed as an update from the first one– was run on PubMed and Embase.com from inception to October 4^th, 2023. Search terms varied slightly between searches and included controlled terms (MeSH-terms) as well as free text terms. The following terms were used (including synonyms and closely related words) as index terms or free-text words: ‘oral tissues’ and ‘sex hormones’. On the first search, a filter was used to exclude animal studies. On the second search, additional filters were used to further limit the results. The search was performed without date or language restrictions. The full search strategy can be found on Appendix 1.

2.2 Screening process and eligibility criteria

References resulting from the search strategy were imported in Rayyan (26), an online application used for screening articles simultaneously and independently by the reviewers. Screening of the search results was done independently by three reviewers (P.C., R.N. and J.B). Articles were initially screened based on title and later based on abstract. On each step, discrepancies were discussed until reaching an agreement. The same process was followed for full-text review. References were managed using Endnote 20 (27). Studies were ultimately verified for their eligibility for inclusion based on full text.

Studies that met the following criteria were included in this scoping review: (1) in vitro or in vivo studies; (2) information about the physiological role of sex steroid hormones progesterone, estrogen and testosterone and their precursors in periodontium, dental structure, mucosa, salivary glands, local immune response and orofacial perception. Studies reporting on: (1) puberty; (2) menstrual cycle; (3) pregnancy; (4) menopause; (5) andropause; (6) pathological hormonal imbalances; (7) hormonal therapy; (8) other non-hormonal therapy; (9) cancer; (10) microbiome; (11) biofilm; (12) animal studies; (13) discussion papers; (14) letters to the editor; (15) reviews that did not specify the search methodology and; (16) case-control studies, were excluded.

2.3 Data extraction and summary

In this review, the evidence was divided into four distinct categories: (1) Periodontium; (2) Dental structure; (3) Mucosa; and (4) Salivary glands. Their interaction with progestagens, androgens and estrogens were summarized.

3 Results

3.1 Selection of evidence

The screening process and the number of selected articles are summarized in Figure 2. The first search yielded 10,962 records in PubMed. The second search yielded 22,722 results in total (9,941 in PubMed and 12,781 in EMBASE). After comparing both searches from 2020 and 2023 and removing the duplicates using the computer software DedupEndNote (28), our combined searches yielded 18,992 results. From these results, 162 articles were selected by title (from the first search) and 62 (from the second search). During the next screening phase, 92 articles were selected by abstract. Out of 92 records, 19 records were excluded for the following reasons: (1) no full text available; (2) no direct relation to oral health and; (3) main focus did not match the objectives of this scoping review; (4) not in English, Dutch, Spanish, Italian or Portuguese language. The remaining 73 records were considered included in this review.

Figure 2

4 The oral cavity as a target organ

Periodontal tissues and their response to changes in SSH –such as during pregnancy– have for long been the main focus of oral endocrinology research (29). This special interest on periodontal tissues can be attributed to the clinical changes and associated detrimental consequences for oral and general health (30). The understanding of the physiology and physiopathology of these phenomena becomes fundamental in disease treatment and prevention. However, and as it has already been exposed above, SSH are extremely versatile molecules with a great variety of target organs, which is also reflected in the oral cavity. In addition to the effects on periodontal tissues during periods of fluctuating hormonal levels, SSH have been reported to interact with practically every other tissue type present in the oral cavity. There is evidence of direct effects of SSH on teeth, periodontium, oral mucosa and salivary glands. Also, SSH’s role has been described in amelogenesis, odontogenesis, bone metabolism and local immune response.

In this review, the evidence is divided into four categories: (1) Periodontium; (2) Dental structure; (3) Mucosa; and (4) Salivary glands. Their interaction with progestagens, androgens and estrogens will be summarized. Human and in vitro studies were included where only physiologic effects were demonstrated. Studies addressing pathological effects or treatment are beyond the scope of this review.

4.1 Periodontium

The periodontium includes the tissues supporting and surrounding the teeth. These tissues are the gingiva, periodontal ligament (PDL), cementum and the alveolar bone.

The gingiva is mainly composed of both epithelial cells and fibroblasts (31). PDL is a connective tissue structure surrounding the root of the tooth, connecting it to the alveolar bone. Cells present in the PDL include periodontal ligament stem cells (PDLSCs), fibroblasts, endothelial cells, cementoblasts, osteoblasts, osteoclasts, tissue macrophages, and stratified epithelial cells. PDLSc have differentiation potential, which has proved to be essential in periodontal tissue repair and regeneration (32). Cementum is the calcified mesenchymal tissue surrounding the root of the tooth which is secreted by cementoblasts. This tissue allows the insertion of the PDL fibers to the tooth and the alveolar bone (33) which is the osseous tissue of the maxilla and the mandible, forming the tooth socket that surrounds the tooth (34). The existing studies reporting interactions between SSH and the periodontium are listed in Table 1.

4.1.1 Receptors

In order for the periodontium to communicate with molecular messengers such as SSH, specific receptors should be present in the target tissues. Several studies have demonstrated the presence of SSH receptors in gingiva, PDL, cementum and alveolar bone. In gingival tissue, receptors for progesterone (35), androgens (22, 24, 36, 37) and estrogens (23, 38, 39) have been described in both epithelial cells and fibroblasts.

PDL expresses receptors for progesterone (57), androgens (24, 37, 53) and estrogens (53, 56, 58–66).

Cementoblasts are known to express estrogen receptors α (ERα) and β (ERβ) (75).

Osteoblast-like cells –responsible for synthetizing bone tissue– express progesterone (77), estrogen (78) and androgen receptors (79). This also applies to alveolar bone. Previous publications have extensively reviewed the expression of sex steroid receptors and the physiology of the interaction between bone tissue and SSH (80, 81). Addressing this topic falls outside the scope of this review. Therefore, only papers specifically focusing on alveolar bone will be discussed.

All four tissues of the periodontium express receptors for SSH, confirming their susceptibility to sex steroid hormones.

4.1.2 Metabolism

Periodontal cells present enzymes capable of metabolizing SSH, resulting in the conversion of sex hormones into different by-products. The first in vitro evidence of the presence of enzymes in gingiva able to metabolize progesterone was reported in 1971 (82). El Attar, demonstrated the presence of 5α-reductase, 3β-hydroxysteroid-dehydrogenase, and 20α-hydroxysteroid-dehydrogenase in gingival tissue from patients with periodontitis, concluding that the metabolism and degradation of progesterone could contribute to the state of health of the gingiva. The capacity of healthy gingiva to metabolize progesterone (40, 41), androgens (44–46) and estrogens (47) has also been reported. Interestingly, inflamed gingiva has proved to be significantly more active than healthy gingiva at metabolizing SSH.

Notably, the rate of metabolic conversion of certain SSH can in turn be influenced by the presence of other SSH (42). In cultured inflamed human gingival fibroblasts (hGF) from primary cells, supplementation of different concentrations of either estradiol or progesterone have shown antagonistic effects. While estradiol stimulated androgen conversion, progesterone inhibited it. When combined, an initial increase in androgen conversion was followed by an inhibitory effect, directly related to the increasing concentration of both estradiol and progesterone combined (43).

Other compounds such as cytokines, growth factors, prostaglandins and medication are capable of influencing the synthesis of SSH by gingival cells by either promoting (83–87) or inhibiting (87) the synthesis of SSH by gingival cells.

PDL cells are known to metabolize testosterone and androstenedione in vitro, indicating the presence of reductase enzymes (67).

Bone metabolism and SSH have been previously reviewed (88) and will not be addressed. Shortly, SSH metabolism by osteoblast-like cells has been reported (89, 90) and there is enough evidence that confirms a central role of SSH and bone resorption and apposition (91).

Based on the existing evidence, gingival cells and PDL are capable of metabolizing SSH into different by-products. Also, gingival cells are susceptible to both the presence of SSH and diverse external factors, which can modulate SSH metabolism. Research on PDL and hormone metabolism is still very limited, but the existing evidence is in line with what has been reported for gingival tissue. There is to our knowledge no available evidence on metabolic activity of SSH by cementoblasts.

4.1.3 Cellular growth, differentiation, proliferation and migration

Evidence about the expression of SSH receptors and metabolic enzymes in the periodontium supported the hypothesis that SSH were capable of inducing physiological changes in periodontal tissues. This was eventually tested in the early 2000s by different research groups that assessed the response of human gingival fibroblasts (hGF) to different SSH during different cellular processes (36, 48). Effects such as changes in cellular growth, differentiation, proliferation and migration were found for all three types of sex steroids. In gingival tissue, direct effects of testosterone and estrogen have been described. Testosterone can stimulate cellular proliferation and migration (36) and estrogen has been described to induce hGF proliferation and decrease protein production (48).

PDL cells have been the focus of broader research. As previously mentioned, periodontal cells have differentiation potential and have thus the ability to form bone, cementum and collagen fibers (32). PDL cells bridge the interaction between tooth and alveolar bone and by this, they play an active role in bone-remodeling and the physiology of the periodontium. SSH progesterone and estrogen can modulate this process. When exposed to progesterone in vitro, PDL cells have shown enhanced proliferation and osteogenic differentiation (70). Different responses of PDL cells to estradiol have been described. Estradiol has been found to either inhibit the growth of PDL cells in a dose-dependent manner (56) or not exerting any measurable effect on proliferation and collagen formation (68). Contrary to these results, estrogen, like progesterone, has been reported to enhance the proliferation and osteogenic differentiation of PDL cells (62, 73) and also mediate osteogenic differentiation in PDL stem cells (65, 66). Also, estrogen is capable of inhibiting the formation of osteoclast-like cells, which are responsible for bone resorption in cocultures of PDL fibroblasts and peripheral blood mononuclear cells (69). In cementoblasts, estradiol has been reported to enhance their proliferation (76).

Additional effects of SSH on PDL cells have also been described. Estrogen has been reported to promote osteocalcin production (70, 71) and mineralized nodule formation (72). Additionally, estrogen has been described to enhance alkaline phosphatase activity (61) and increase the expression of osteoprotegerin while decreasing the expression of RANKL in PDL cells via ERβ (61). These findings indicate an active role of estrogen in bone-remodeling and the physiology of the periodontium.

The described effects support the hypothesis of a relevant physiological role in homeostasis of SSH in the oral cavity.

4.1.4 Sex steroid hormones and cytokine production in the periodontium – a bidirectional interaction

Several studies have reported immunomodulating effects of SSH via different mechanisms. Also, different oral cells are capable of producing different cytokines when exposed to sex steroids. A number of studies have reported the up and downregulation of cytokine production by SSH in gingival fibroblasts with contradictory results. Progesterone has been described to inhibit the production of IL-6 in a dose-dependent manner (37, 92) but also –together with estradiol– increase the production of IL-6 and IL-8 together with an increased secretion of vascular endothelial growth factor (VEGF) (50). This has also been reported in non-oral cell types (93, 94). Also, in immune challenged gingival fibroblasts, progestin and estradiol are capable of downregulating various inflammatory cytokines (95).

Androgens are also capable of modulating the production of certain cytokines and prostaglandins by gingival fibroblasts. Testosterone (36, 37) and DHT (37, 52) downregulate the production of IL-6. Testosterone has also been described to downregulate the synthesis of prostaglandins (54), suggesting an anti-inflammatory effect in gingival tissue. In PDL cells, testosterone and estradiol seem capable of downregulating the production of IL-17 (53).

Cytokine expression and its modulation by SSH in LPS-stimulated PDL cells has also been studied. Shu et al. tested the effects of estradiol on both spontaneous and LPS-stimulated cytokine expression (55). Results showed that estradiol did not have a big effect on spontaneous cytokine production, but it did actively regulate cytokine expression when co-cultured with LPS. TNF-α, IL-1β, IL-6 and RANKL which are normally upregulated by LPS, were significantly suppressed in the presence of estradiol. Osteoprotegerin (OPG) was upregulated. This suggests a modulation of the stimulatory effects of LPS on these cytokines. This phenomenon has also been observed in the expression of certain chemokines, which are either up or downregulated by estradiol when PDL cells are challenged by exposure to LPS (96).

Just like SSH are capable of regulating cytokine production, certain cytokines have been described to modulate the conversion rate of SSH in periodontal inflamed tissue. In particular, IL-1 has been described to enhance the conversion of androstenedione and testosterone to dihydrotestosterone (DHT) –a potent androgen– in gingival tissue and PDL cells (83, 97). Although research regarding this phenomenon in healthy tissues is not available, it is important to acknowledge the possibility that healthy tissues may respond in a similar manner.

An increase in androgen conversion (DHT and 4-androstenedione) by healthy gingival tissue in the presence of prostaglandins was reported in one study (86). Prostaglandins are lipid-derived molecules involved in the regulation of inflammation. Conversely, another study reported that exposure of gingival fibroblast to either progesterone, estradiol or both, upregulated the expression of COX-2, an enzyme involved in the synthesis of prostaglandins (51). These responses by gingival cells provide further insight in the effects of SSH and adaptative changes in hormone metabolism during inflammation.

4.2 Dental structure

The tooth is composed of four distinct tissues, enamel –the outer layer of the tooth– and the underlying layers dentin, pulp and cementum.

Each of these tissues is composed of distinct cell types. Enamel is the mineralized tissue secreted by ameloblasts. Dentine is a matrix secreted by odontoblasts. Pulp is composed by a complex multicellular organization including fibroblasts as well as odontoblasts, immune cells, neural fibers, amongst others. Cementum is secreted by cementoblasts (33). In all these tissues, direct or indirect evidence has been found on the presence of SSH receptors for one or more sex steroids. The existing evidence is listed in Table 2.

4.2.1 Receptors

Evidence of SSH receptors on enamel development and maturation is scarce. The presence of androgen (113) and estrogen receptors (114) in ameloblasts and their involvement in the developmental process of enamel has been reported in rats. In humans, only indirect evidence has been described, linking the presence of estrogen receptor’s polymorphisms to the incidence of clinical changes in the enamel. Developmental defects of the enamel (DDE) (98), a higher caries incidence (99), a higher incidence of fluorosis in general (100) and in high-fluoride-exposure areas (101) indicate an important role of estrogen and estrogen receptor in amelogenesis.

In pulp tissue, progesterone (102), androgen (103, 104) and estrogen receptors (103–107) have been identified.

The expression of androgen receptors in the pulp can apparently change when exposed to certain SSH. In vitro, the addition of androstenedione or estradiol increased the expression of androgen receptors (AR). On the contrary, the addition of testosterone reduced its expression (104). This indicates an active role of SSH in the pulp responsiveness to androgens, not only by direct interaction with sex steroid receptors but also by manipulating the expression of AR. Pulp estrogen receptors can also be up or downregulated during osteogenic differentiation (105).

4.2.2 Cellular growth, differentiation, proliferation and migration

As previously mentioned, pulp tissue includes different cell types including stem cells. It has been reported that the differentiation potential of these pulp stem cells can be enhanced by estradiol. An increased alkaline phosphatase activity (ALP), mineralization capacity and upregulation of odonto/osteogenic differentiation markers have been described (74, 108). This has also been observed in stem cells from the apical papilla (portion at the apex of the root) of pulp tissue from immature teeth (74, 110, 111) and to a lower degree in stem cells from the dental follicle (tissue surrounding an unerupted tooth) (74). In human dental pulp cells, estradiol has been also reported to increase proliferation (109) as well as odontoblastic differentiation (103, 109). Genes directly involved in odontogenesis and osteoblast differentiation such as AMBN, IFT88, TP63 are upregulated by estradiol and the androgen DHT (103). An increase of osteoprotegerin (OPT) –an essential protein in bone remodeling and homeostasis– has also been reported to be susceptible to estradiol, which enhances its expression (112).

Most studies addressing the effects of SSH on pulp tissue have focused on the effects of estradiol, possibly motivated by the onset of bone resorption as estrogen decreases (115). Estradiol enhances odonto/osteogenic differentiation in different cell types of the pulp and surrounding tissues, evidencing its role in tissue formation, repair and adaptation to changing external factors, all important aspects in homeostasis.

4.3 Oral mucosa

The oral mucosa is an important protection barrier against microbes, toxins and mechanical and chemical damage through its physical and immunological functions (116). Like gingival tissue, oral mucosa stems from two distinct embryonic layers. The epithelial layer originates from the ectoderm and the deeper layers stem from the neural crest ectomesenchyme (endoderm) (117). The interaction of these layers is fundamental for the proper development and function of the oral mucosa, and sex steroid hormones also play a role in these processes. The existing evidence is listed in Table 3.

4.3.1 Receptors

Traditionally, oral mucosa has not been regarded as a target tissue for SSH in healthy individuals. The existing studies have focused on complaints manifesting around and after the onset of menopause, where women experience discomfort in different oral mucosal tissues, and not on the physiological role of SSH in oral mucosa (123). Limited research has been done on the expression of SSH receptors in oral mucosa in humans with some studies reporting on the presence of androgen and estrogen receptors. Androgen receptors have been described in buccal mucosa of healthy individuals from both sexes (118). Estrogen receptors have been described in buccal mucosa of young and post-menopausal women (119) and in buccal mucosa of men and women (39). The limited available evidence supports the notion that oral mucosa is also a target tissue for SSH.

4.3.2 Wound repair

The process of wound repair has been previously studied, and histatins have been found to play a fundamental role in restoring the integrity of damaged mucosa (124). Nonetheless, some studies have addressed the role of SSH and mucosal wound healing. This has been tested in vivo by inflicting a small wound on the hard palate of 212 individuals from both sexes ranging from 18–35 years old and 55–88 years old (120). As expected, age negatively influenced wound healing regardless of sex. However, wound healing in females was significantly slower regardless of age. A follow-up to this study used the same methodology in a larger cohort (121). A group of 329 individuals (age range 18–43) and a smaller group of 93 individuals (age range 55–88) were inflicted a small wound on the hard palate and videographed every 24 hours for 1 week. Hormone levels in blood were also measured. Results showed that lower testosterone levels in the younger group related to faster wound healing whereas higher testosterone levels in post-menopausal women related to faster healing times.

In cutaneous wound healing, women have a significant advantage compared to males (125). In other mucosal surfaces, estrogen has been put forward as a protective factor in mucosal injury (126). This data suggests that sex hormones and wound repair could be tissue specific. When contrasting the existing in vivo research with studies assessing wound healing in other tissues it is reasonable to suggest that wound healing is a complicated process that not only involves migration of cells but also the host’s immune response, which –as it will be soon discussed– can be modulated by SSH.

4.3.3 Sex steroid hormones and cytokine production in oral mucosa

As previously mentioned, the oral mucosa has a gatekeeper function which is extremely important for the host. Moreover, the oral mucosa also exerts a regulatory control over the local immune response (127). This modulation of the local immune response has been reported during the interaction of the commensal oral flora with the mucosal surfaces (128), during recurrent mechanical damage (129) and when exposed to mucosal sensitizers (130), to name some. However, little is known about the potential role of hormones during this process.

What we know so far is that estradiol might have a regulatory effect on the expression of certain cytokines in oral epithelial cells (122). When these cells were exposed to IL-1β, the mRNA and protein expression of Human β-Defensin 2 (hBD-2)– an antimicrobial peptide–, IL-6 and IL-8 were upregulated. In the presence of estradiol, a downregulation of these molecules was observed, suggesting and anti-inflammatory effect of estradiol in oral mucosal epithelial cells.

4.4 Salivary glands

In humans, saliva is produced by major and minor salivary glands, which originate from the oral epithelium. Major salivary glands consist of three pairs of glands, namely the parotid, submandibular and sublingual glands. Minor salivary glands can be found throughout the oral cavity (131). The effect of sex steroids on the structure, salivary production amongst others have been widely studied using animal models (132–135). In humans, some studies have been published on the effects of different SSH on salivary glands. These are summarized in Table 4.

4.4.1 Receptors

Androgen (136, 137) and estrogen receptors (39, 119, 138) have been reported in major and minor salivary glands.

The existing literature confirms the susceptibility of salivary glands to androgens and estrogens.

4.4.2 Metabolism

Only one study has addressed the capacity of salivary glands to metabolize SSH. Blom et al. measured several sex-steroid by-products after incubating salivary gland samples with progesterone and testosterone in vitro (139). Based on the detected metabolites, it was concluded that these cells presented steroid-reductase, dehydrogenase and hydroxylase enzymes.

4.4.3 Sex steroid hormones and cytokine production in salivary glands

Cytokine production and modulation by SSH is salivary glands is an extremely understudied topic. To our knowledge, only inhibition of IFN-γ-induced ICAM-1 expression by estrogen was reported in one study (138).

5 SSH-mediated tissue interactions and maintenance of oral health

In this review we have provided a comprehensive overview of the scientific literature concerning the interactions between sex steroid hormones and oral tissues. By this, we aimed to clarify the physiological roles of sex steroids within the oral cavity, their role in oral homeostasis and in the development and function of different oral tissues. Our analysis of the existing body of evidence confirms the enormous versatility of SSH in contributing to various aspects of oral health maintenance (Figure 3). Their contribution can be summarized as follows: (1) Sex steroids exert a significant influence on periodontal health by playing an active role in bone remodeling and its immune response; (2) Several sex-hormone-mediated inflammatory mediators are up or downregulated by SSH, providing protection to external disturbances; (3) Sex steroids significantly contribute to the proper development of enamel, a fundamental process that ensures adequate protection of the dental structure; and (4) SSH assume a vital role in preserving mucosal health and integrity by promoting vascularization and regulation of inflammation, thus facilitating optimal tissue turnover and wound healing. These processes and balance could not be attained without the finely orchestrated interplay between SSH and their biosynthesis, conversion, transport, and consequent signaling to achieve the desired biological effects.

Figure 3

All things considered, it is evident that sex steroid hormones assume a pivotal role in the physiological maturation and maintenance of oral tissues in the absence of physiological disbalances.

But what happens when the balance is disrupted?

In the event of abrupt hormonal fluctuations, the systemic repercussions are notable. Temporary hormonal perturbations can be assimilated by the body: its inherent resilience and immune fitness allow a swift return to homeostasis without significant consequences (e.g. pregnancy). However, the persistence of such hormonal alterations can hinder the body’s adaptive capacity, progressively weakening physiological functions and eventually resulting in further instability of the system and the onset of disease. Moreover, disease does not affect males and females in the same way.

5.1 Sexual dimorphism in oral disease

Numerous diseases exhibit variations in prevalence, progression, and manifestation between males and females. These and other biological differences between males and females are referred to as sexual dimorphism. Regarding oral diseases, males are more prone to severe periodontal disease than females (140, 141). This may be attributed –in part– to differences in hormonal levels and their impact on immune modulation. Several examples for different pathologies can be found in the literature.

A recent systematic review investigated the expression of sex steroid hormones (SSH) in oral squamous cell carcinoma (OSCC), a condition observed to be twice as prevalent in males than females (142). This review concluded that sex hormones and sex steroid receptors play a significant role in influencing the physiopathology of OSCC.

Other pathologies such as Sjögren syndrome (SS), burning mouth syndrome (BMS) and temporomandibular disorders (TMD) are more prevalent in females than in males (143–145). SS is an autoimmune condition that affects the lacrimal and salivary glands, negatively affecting saliva production, which results in severe dry-mouth symptoms (146). Low levels of dehydroepiandrosterone (DHEA) have been associated with SS manifestation and hormonal supplementation has proved to improve dry mouth symptoms (147). BMS has also been associated with lower levels of DHEA (148) and TMD has been associated with low estrogen levels (149) and estrogen receptor gene polymorphism (150).

The role of SSH on the incidence and progression of conditions in the oral cavity is not completely understood, but sexual dimorphism in disease has been observed in several conditions (151).

Aside from genetical and behavioral aspects that also influence the differential response to disease (151), factors such as SSH receptors, the interplay and balance between different sex steroids and their conversion in biological tissues play an active role on disease (152, 153). In endometrial cancer, the loss of progesterone and estrogen receptors is associated with an advanced disease stage (154) and hormonal imbalances can result in changes in gene expression, further influencing disease progression (155). Also, changes in SSH signaling can contribute to lung disease (156) and changes in the enzymatic expression of aromatase in tumor-associated macrophages have been found to correlate with the severity of breast cancer (157). This also exemplifies the influence and relevance steroidogenesis, and the subsequent cascade of events can have in the maintenance or loss of homeostasis.

Changing hormone levels are closely related to aging (158). Aging increases the risk and severity of various diseases, and the oral cavity is no exception (159). This is particularly evident as SSH levels sharply decrease with the onset of menopause in women and gradually decline due to reduced androgen levels in men (160, 161). These decrease in SSH significantly impact the immune system, among other effects (162). Several studies have been conducted on the changes of SSH, sex, aging and oral diseases (163–167), confirming the significant influence of these factors on oral homeostasis.

Sexual dimorphism is present in several diseases and therefore sex differences should be included as a variable in research. Recognizing and understanding the different roles of SSH in oral physiology are fundamental for preventing and treating oral diseases.

5.2 Clinical implications and future challenges

This review has focused on the role of SSH in the maintenance of oral health and their involvement in different physiological processes in the oral cavity. Nevertheless, the microbiome and its influence on the maintenance of homeostasis in the oral cavity has not been mentioned. It is beyond the scope of this review to discuss this; however, it is important to stress that both homeostasis and dysbiosis are multifactorial biological processes. Therefore, future research should also consider the interplay between SSH, the oral microbiome and the host, and how they influence homeostasis and dysbiosis in the oral cavity in humans.

It is important to emphasize that hundreds of studies on this topic have been conducted using animal models. These studies have significantly contributed to our understanding of how sex steroid hormones interact with complex biological systems. However, results from animal studies are not always easily translatable to human physiology. We therefore focused this scoping review only on studies related to humans. Nonetheless, animal studies have served as a crucial steppingstone, allowing several hypotheses to be tested in humans, as presented in this paper.

6 Conclusion

Sex steroid hormones are an overlooked yet fundamental factor in oral homeostasis. They play important roles in the development and function of the periodontium, mucosa, dental structure and salivary glands. Nevertheless, it is important to acknowledge that the onset, progression and ultimate resolution of disease are multifactorial processes, and the presence of hormonal imbalances alone is insufficient to account for their etiology and eventual reinstatement of homeostasis. However, it is important to consider sex steroid hormones as a relevant factor in oral health maintenance, which is complex and can vary from person to person. Hormone levels can fluctuate due to factors like age, hormonal treatments and medical conditions. Research on this topic is ongoing, and the precise mechanisms and clinical implications are areas of active study in the field of dentistry and oral health. Dentists and healthcare providers should consider these hormonal factors when assessing and treating oral health conditions.

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