The Relationship between Cysteine, Homocysteine, and Osteoporosis

Abstract

Both cysteine and homocysteine are sulfur-containing amino acids that play distinct roles in the body. Cysteine is an amino acid that contributes to the synthesis of collagen, a crucial protein for bone structure. Collagen provides the structural framework for bones, contributing to their strength and flexibility. Adequate collagen formation is vital for maintaining bone integrity, and cysteine’s role in collagen synthesis suggests a potential indirect impact on bone health. Elevated levels of homocysteine have been associated with an increased risk of osteoporosis and bone fractures. The exact mechanisms through which homocysteine affects bone metabolism are not fully understood, but it is suggested to involve interference with collagen cross-linking, increased oxidative stress, and altered bone remodeling. The relationship between cysteine, homocysteine, and osteoporosis is intertwined within complex biochemical pathways, constituting a continually evolving area of research.

1. Introduction

Both cysteine (Cys) and homocysteine (Hcy) are amino acids that contain a sulfur atom. The amino acid cysteine can be obtained from both the diet and produced from the degradation of methionine through the transsulfuration pathway [1, 2]. Methionine, another sulfur-containing essential amino acid, is found in a variety of protein-rich foods, including meat, eggs, dairy products, and legumes [3]. Homocysteine is a sulfur-containing amino acid that is not directly obtained from dietary protein or utilized for its endogenous synthesis. Instead, it functions as an intermediate in the metabolic pathway of methionine. Elevated plasma levels of homocysteine, known as hyperhomocysteinemia, are a risk factor for various pathological disorders. Homocysteine metabolism is influenced by B vitamins, and there have been suggestions for vitamin treatments to lower Hcy concentration [4, 5]. However, clinical trials have not yet established a consensus on the effectiveness of vitamin supplements in reducing homocysteine levels and improving pathological conditions [6]. This is particularly evident in elderly patients with associated pathologies, indicating that these dietary, as well as non-dietary factors may contribute to elevated homocysteine levels [6].

A frequently encountered condition, especially in the elderly, is osteoporosis with serious health consequences due to its association with fragility fractures. As a result of the increase in life expectancy, the total number of fragility fractures will increase and, therefore, the cost to society will increase significantly in the coming years. Identifying risk factors or risk indicators to prevent osteoporosis and develop new treatment strategies are real concerns [7].

The link between homocysteine and the bone system has been observed in studies of hyperhomocysteinuria. People with hyperhomocysteinuria have numerous skeletal defects, including reduced BMD and osteopenia [8]. Elevated levels of homocysteine have been correlated with osteoporosis and an increased incidence of hip fractures in postmenopausal women. The increased bone fragility in this situation has been explained as a result of the change in the bone matrix when homocysteine interferes with collagen cross-linking [9]. Many studies report that Hcy and vitamins involved in its metabolism, such as folic acid and vitamins B6 and B12, affect bone metabolism, bone quality, and fracture risk, especially in the elderly. Considering the previously mentioned Hcy could be seen as a risk indicator for osteoporosis related to micronutrient deficiency [10, 11, 12, 13, 14, 15]. Experimental results indicate that cysteine (Cys) is involved in bone metabolism by incorporation into collagen and cysteine protease enzymes [16]. The association between osteoporosis and the decrease in cysteine was explained either by the reduced availability of cysteine for collagen formation or due to the increased use of cysteine by proteases in the osteoclastic hyperactivity that underlies the osteoporotic process [17]. Fractures in osteoporosis represent a severe manifestation of the condition, often accompanied by visible clinical symptoms. Collagen stands out as a vital element in the bone matrix, and any disruption of its cross-linking formation increases susceptibility to fracture. Vitamin B6 serves as a key regulator in cross-linked collagen formation. Vitamin B6 deficiency correlates with reduced bone strength due to its impact on collagen reticular disruption, both directly and via homocysteine-related pathways.

2. Overview of cysteine and homocysteine metabolism

Cysteine is primarily synthesized through the transsulfuration pathway, which involves the conversion of homocysteine to cysteine. Cysteine biosynthesis uses the sulfur group of methionine with the carbon backbone of serine. This pathway intersects with the methionine cycle, which supports transmethylation reactions and forms homocysteine as a key intermediate. This process occurs primarily in the liver and involves several enzymatic steps, including the action of cystathionine beta-synthase (CBS) and cystathionine gamma-lyase (CGL) (Figure 1) [18].

Figure 1.

Schematic cysteine and homocysteine pathways. Methionine adenosyltransferase (MAT); methyltransferase (MT); S-adenosylhomocysteinase hydrolase (SAHH); betaine-homocysteine methyltransferase (BHMT); cystathionine β-synthase (CBS); cystathionine gamma-lyase (CGL); glutathione (GSH); hydrogen sulfide (H2S). Adapted from Al-Sadeq et al. [18].

The transsulfuration pathway is regulated by various factors, including enzyme activity, substrate availability, and the presence of cofactors, such as vitamin B6 (pyridoxal phosphate). In addition to de novo synthesis, cysteine can also be obtained from dietary sources rich in sulfur-containing amino acids, such as methionine.

The first reaction of methionine metabolism is its transformation into S-adenosylmethionine (SAM), a process catalyzed by the enzyme methionine adenosyltransferase (MAT). SAM is transformed into S-adenosylhomocysteine (SAH). The product of this reaction, S-adenosylhomocysteine, is hydrolyzed by S-adenosylhomocysteine hydrolase to yield adenosine and homocysteine [19]. Homocysteine is combined with serine through the action of cystathionine β-synthase (CBS), resulting in the formation of cystathionine. Subsequently, cystathionine gamma-lyase catalyzes the cleavage of cystathionine, yielding cysteine. The primary role of CGL is to catalyze the cleavage of cystathionine, resulting in the production of cysteine, α-ketobutyrate, and ammonia, which occurs in the second step of the transsulfuration pathway. Additionally, CGL can catalyze side reactions with either cysteine, homocysteine, or both as substrates, potentially leading to the production of hydrogen sulfide [20]. In the process of transsulfuration, serine contributes its carbon chain to cysteine, while the sulfur atom required for cysteine synthesis originates from methionine [21]. Cysteine can be further converted to glutathione, ammonia, pyruvate, and hydrogen sulfide [18, 22]. The synthesis of cysteine through the transsulfuration pathway can be seen as a component of the degradation process of methionine and homocysteine. Cysteine acts as a carrier for the conversion of sulfur from methionine and homocysteine into end products that can be eliminated through urine. Enzymes involved in these pathways are subject to regulation by cofactors such as vitamin B6, vitamin B12, and folate, among others. Deficiencies in these cofactors can lead to disruptions in cysteine and homocysteine metabolism, potentially resulting in health issues, such as cardiovascular diseases and neural tube defects [23, 24, 25]. Cysteine and homocysteine metabolism intersect with several other metabolic pathways. For instance, cysteine serves as a precursor for the synthesis of glutathione, an important antioxidant [26]. Additionally, homocysteine metabolism is interconnected with the folate and methionine cycles, which are essential for DNA synthesis and methylation reactions [27].

3. Osteoporosis

Osteoporosis is a condition characterized by reduced bone mass and microarchitectural deterioration of bone tissue. The consequence is increased bone fragility and increased susceptibility to fractures [28]. Although this condition is commonly associated with the elderly, it can also affect young people (juvenile osteoporosis) or adults who are undergoing treatment with drugs such as steroids, chemotherapy, anticonvulsants, and others [29]. Changes in the mechanical properties of bones due to aging are noteworthy. Comparing individuals at an advanced age with those at 30 years of age, there is an 8% decrease in elastic modulus in cortical bone, a 64% decrease in trabecular bone, an 11% reduction in peak strength in cortical bone, and a substantial decrease with 68% in trabecular bone. In addition, hardness decreases by 34% in cortical bone and by 70% in trabecular bone [30].

3.1 Bone remodeling

Bone, a fundamental element of the human skeleton, is characterized by its robust, solid, and durable composition. They play crucial physiological roles, providing mechanical support, protecting vital organs, facilitating hematopoiesis, and contributing to mineral homeostasis. Bones can be classified according to position, shape, size, or structure. Bones have an outer layer called cortex, smooth, compact, continuous, and of variable thickness, and inside the bone tissue is arranged in a network of intersecting plates and spicules called trabeculae [31, 32].

In developed bone, the trabeculae are organized systematically, forming uninterrupted sections of bone tissue running parallel to the directions of compressive or tensile stress. These trabeculae create a sophisticated network of internal cross-shaped rods designed to enhance the bone’s stiffness [33]. Bones exposed primarily to compressive or tensile forces typically possess thin cortices and achieve structural strength via trabeculae. In contrast, bones. Such as the femur, which endure bending, shearing, or torsional forces, often feature thick cortices, a tubular structure, and a continuous central cavity known as the medullary cavity. Bone tissue is made up of an organic matrix called osteoid, made up of collagen fibers and proteoglycans, which are impregnated with salts, mainly made up of calcium and phosphate; the main crystallized salt is hydroxyapatite [34]. Bone tissue undergoes periodic remodeling; under physiological conditions, the rate of bone tissue formation is in balance with the rate of bone resorption. Bone diseases in the elderly are associated with increased morbidity and mortality. Osteoporosis is a disease that affects the quality of life in the elderly [35]. Although bone growth stops after adolescence, bone is a very dynamic tissue. Bone tissue is continuously resorbed and regenerated through constant structural change. One of the important characteristics of bones is the remodeling capacity. This process of bone remodeling occurs during growth and continues throughout life [36]. Bone remodeling occurs continuously, resulting in the arrangement of bone structure into regular units, enhancing its ability to withstand mechanical forces effectively. Osteoclasts, responsible for bone resorption, remove older bone, while osteoblasts, involved in bone formation, initiate the process of new bone formation. Osteoclasts secrete proteolytic lysosomal enzymes, which efficiently break down the bone matrix akin to acids, converting calcium salts into a soluble form that can be absorbed into the bloodstream [37]. The remodeling process holds particular significance for the long bones responsible for supporting the limbs. These bones feature wider ends and narrower middles, imparting additional strength to the joints. Osteoclasts play a vital role within the bones, operating within the bone marrow cavity and cancellous bone spaces to widen these areas. Additionally, they work on the outer surfaces, diminishing bony processes like the epiphyseal regions of the upper and lower limbs [32]. Osteoclasts are active behind the epiphyseal growth zone. Their function within the bone involves breaking down immature bone, facilitating its transformation into compact, mature bone (known as lamellar bone). This process entails the release of elongated tubular spaces, which act as nuclei for the formation of osteons. Osteons are the bony structures through which blood vessels pass, contributing to the bone’s vascular network and overall integrity [38]. While osteoclasts degrade old bone at the epithelial ends of the bone, osteoblasts within the growth zone initiate the formation of a new epiphysis. Concurrently, within each tubular space liberated by osteoclasts inside the bone, osteoblasts play a crucial role in depositing new layers of bone, thus contributing to bone remodeling and maintenance [39].

A bone that remains unused, such as a lower limb immobilized after trauma, is susceptible to resorption. This is because the rate of bone destruction surpasses the rate of bone formation in such circumstances [40].

Bones experiencing heightened stress undergo permanent remodeling. For instance, the femur undergoes complete replacement approximately every 6 months. Beyond altering bone structure, remodeling also plays a role in regulating the concentration of ionic calcium in the bloodstream. This mineral is essential for various physiological processes such as nerve signaling, cell membrane integrity, and blood clotting [41]. Bone turnover is increased in women after menopause because the rate of bone resorption is greater than that of bone formation [42].

3.2 Fragility fractures

The primary consequence associated with osteoporosis is the heightened susceptibility to fractures. While there exists a recognized correlation between bone density and the risk of osteoporotic fractures, it is not inevitable that every individual diagnosed with osteoporosis will experience fractures [43]. The likelihood of fractures occurring is contingent not only upon bone fragility but also on the level of trauma sustained. Typically, osteoporotic fractures are linked to falls from low heights, a scenario for which the elderly are particularly prone [44, 45]. Various factors contribute to the heightened propensity for falls among the elderly, including diminished visual acuity, vestibular dysfunction, cognitive impairment, musculoskeletal conditions, and the usage of certain medications [46, 47, 48].

Nonetheless, severe hypotension frequently emerges as a predominant characteristic. In cases where osteoporosis is suspected, assessing bone mineral density stands out as the premier diagnostic approach, aiding physicians in gauging fracture susceptibility. Fracture risk assessment can be performed using both clinical methods, which entail identifying risk factors, and paraclinical approaches, which encompass various techniques for evaluating bone properties. Individuals diagnosed with fragility fractures demonstrate an increased relative risk (RR) of suffering subsequent fractures. Patients diagnosed with fragility fractures exhibit an elevated relative risk (RR) of experiencing subsequent fractures. The principal risk factors associated with fractures (RR ≥ 2) include age, bone mineral density, previous history of fragility fractures, family history of fragility fractures, early menopause before the age of 45, glucocorticoid therapy, extended periods of immobilization, susceptibility to falls, malabsorption disorders, chronic renal failure, and undergoing transplantation [49, 50, 51, 52, 53, 54].

Moderate risk factors (where the relative risk, RR, falls between 1 and 2) include conditions such as rheumatoid arthritis, Bechterew’s disease, and the use of anticonvulsant medications. Additionally, low calcium intake, diabetes insipidus, estrogen deficiency, primary hyperparathyroidism, hyperthyroidism, smoking, and excessive alcohol consumption are also considered moderate risk factors for certain health conditions [49, 55].

There is a clear relationship between these risk factors and low bone density or other causes of osteoporotic fractures. For the correct identification of possible risk factors, both the anamnesis and the physical examination of the patients must be completed with biochemical tests (blood count, ESR, calcium, kidney and liver tests, alkaline phosphatase) and markers of bone turnover [56, 57, 58]. Obesity has been considered advantageous for maintaining healthy bones due to the higher bone mineral density seen in overweight individuals [59]. More than half of women and approximately one-third of men will experience osteoporotic fractures at some point in their lives. While there may be no symptoms prior to the fracture, assessing bone mineral density and other risk factors can help identify individuals at high risk. It is crucial to prioritize the measurement of bone mineral density and identification of other risk factors to accurately gauge the likelihood of fracture in patients, whether or not they have a history of previous fractures. This is imperative because fractures, often devoid of symptoms other than the fracture itself, can lead to severe consequences and may signal disease progression. The impact of obesity on fracture risk varies depending on the location, leading to increased risk for certain fractures, such as those involving the humerus or ankle, while decreasing the risk for others, such as fractures of the hip or pelvis [60].

Over the past two decades, more advanced technology has been developed for determining bone mass, and more techniques are available. With these bone densitometry techniques, the clinician can detect low bone mass prior to fracture. This is beneficial for patients because early treatment of osteoporosis can be initiated and thus osteoporotic fractures can be prevented.

In addition to the usual medical history considerations for trauma patients, special attention must be given to specific issues in elderly individuals. Assessing pre-injury mobility, concurrent medical conditions and medications, as well as cognitive and nutritional status is crucial. These factors significantly influence decision-making and the planning of therapy in collaboration with the patient.

4. Involvement of cysteine and homocysteine in osteoporosis

Four mechanisms are proposed in the literature regarding the involvement of homocysteine in bone remodeling: increased osteoclasts activity, decreased osteoblasts activity, decreased bone blood flow, and direct action of the Hcy molecule on the bone matrix [7].

Several studies regarding the link between homocysteine and bones are presented in Table 1.

Study ID	Outcomes	Conclusion
Miyao et al. [61]	Polymorphism of methylenetetrahydrofolate reductase (MTHFR), bone mineral density	The MTHFR gene polymorphism demonstrates a notable correlation with bone mineral density (BMD) in postmenopausal women, affecting both lumbar and total body BMD levels
Van Meurs et al. [62]	Homocysteine levels and the risk of incident osteoporotic fracture	Elevated homocysteine levels emerge as a robust and independent risk factor for osteoporotic fractures in older individuals, encompassing both men and women
Gjesdal et al. [63]	Hcy, folate, and vitamin B12 and the methylenetetrahydrofolate reductase (MTHFR) 677C → T and 1298A → C polymorphisms	High total homocysteine levels and low folate levels were linked to decreased bone mineral density in women, whereas no such association was observed in men
Perier et al. [64]	Homocysteine, serum albumin, and serum creatinine	Plasma homocysteine levels exhibit a significant correlation with aging and serve as a marker of frailty, indicative of overall health and nutritional status, physical activity, and renal function impairment
Rumbak et al. [65]	Homocysteine, serum folate, red blood cell folate, and serum vitamin B12	Levels of homocysteine, folate, or vitamin B12 showed no association with bone mineral density (BMD) in a population of healthy Croatian women aged 45–65
Jianbo et al. [66]	Plasma total homocysteine, glucose, serum creatinine, plasma insulin, serum osteocalcin, serum 25-hydroxyvitamin D, serum folate, and vitamin B12	Diabetic patients with osteoporosis exhibited higher levels of homocysteine compared to those without osteoporosis
Sandeep et al. [67]	Serum homocysteine levels	Individuals with elevated circulating levels of homocysteine demonstrated a decreased bone mineral density (BMD), establishing an association between homocysteine and the risk of developing osteoporosis
Garcia-Alfaro et al. [68]	Hcy, creatinine, calcium, phosphorus, vitamin D, and parathormone levels	Plasma levels of homocysteine do not show a relationship with BMD in the lumbar spine (L1–L4), femoral neck, or total hip
Widjaja et al. [69]	Homocysteine	Elevated homocysteine levels did not increase the incidence of osteoporotic fractures. However, homocysteine levels increased with aging and correlated with bone mineral density
Watanabe et al. [70]	Total procollagen type 1 amino-terminal propeptide (total P1-NP), intact parathyroid hormone (intact PTH), and homocysteine	Preoperative osteoporosis and elevated serum homocysteine levels were identified as risk factors for intraoperative periprosthetic fractures

Table 1.

Research on the relationship between homocysteine and bone.

Numerous studies indicate that elevated plasma levels of homocysteine are associated with osteoporotic fractures [62, 63, 64, 71]. The exact mechanism proposed to explain this association is not elucidated. One proposed hypothesis suggests that hyperhomocysteinemia could impact the bone matrix and reduce bone quality by hindering the formation of cross-links between collagen fibers, thus impeding the development of a robust collagen network structure [72]. It is assumed that Hcy interferes with collagen cross-linking in bone, thereby weakening the bone structure [72]. Another hypothesis suggests that hyperhomocysteinemia triggers the formation and activity of osteoclasts by generating intracellular reactive oxygen species (ROS) within the bone marrow, thereby elevating bone resorption. Moreover, heightened levels of homocysteine have been associated with reduced expression of osteocalcin and increased expression of osteopontin, disrupting normal osteoblast function. This disruption ultimately leads to diminished bone formation and contributes to the development of osteoporosis [72, 73]. Another hypothesis proposed refers to the fact that homocysteine, involved in the methylation process, could disrupt DNA methylation and gene expression, which can lead to changes in bone structure. The hypothesis suggesting that changes in methylation capacity could influence bone metabolism is bolstered by research demonstrating a correlation between a decreased S-adenosylmethionine (SAM) to S-adenosylhomocysteine (SAH) ratio in bone and diminished bone strength observed in hyperhomocysteinemic rats [71]. The results of the study conducted by Garcia-Alfaro et al. [68] evaluated a group of postmenopausal women and reported that homocysteine levels were not related to BMD in the lumbar spine (L1-L4), femoral neck, and total hip [68].

Conflicting data persists regarding the relationship between high homocysteine levels and low bone mineral density. Epidemiological evidence is supported by genetic studies showing an association between the common MTHFR 677C_T polymorphism and osteoporosis risk.

The concentrations of cystine and cysteine were found to be reduced in individuals with osteoporosis, suggesting that cysteine may act as a protective factor for bone mineral density [74, 75]. Cystine and cysteine may accelerate bone regeneration by activating osteoblast (OB) differentiation [76]. Bone regeneration frequently relies on signals from osteogenesis-inducing factors for a successful outcome. N-acetyl cysteine (NAC), a small antioxidant molecule, potentially regulates osteoblastic differentiation [76]. The study by Yamada et al. [76] indicates that N-acetyl cysteine (NAC) can serve as an osteogenesis-enhancing molecule, accelerating bone regeneration by stimulating the differentiation of osteogenic lineages. The systematic review and meta-analysis of different metabolites in osteopenia and osteoporosis by Wang et al. [77] indicated a positive association between cystine and BMD.

Baines et al. [16] examined the relationship between plasma cysteine and related thiols, as well as bone turnover markers, and folate and vitamin B6 levels with calcaneal bone mineral density (BMD) in 328 women, categorized based on their BMD measurements [16]. Their findings revealed a noteworthy association between plasma Cys levels and bone mineral density. A decrease in Cys concentration, potentially influenced by smoking or reduced homocysteine flux, could diminish its availability for collagen formation [16]. This might prompt heightened osteoclast activation, potentially due to relative hyperhomocysteinemia, leading to increased utilization of Cys in cysteine proteases.

5. Conclusions

It is evident that changes in the metabolism of cysteine and homocysteine can contribute to a disorder of bone remodeling, favoring the occurrence of osteoporosis. Increased Hcy levels result in the dysregulation of the transsulfuration pathway as L-cysteine is synthesized from Hcy. Epidemiological cohort studies demonstrate robust associations between serum homocysteine concentrations and a heightened incidence of fractures. The results of clinical trials investigating vitamin treatments for Hcy lowering have not established a consensus on the effectiveness of vitamin supplementation in reducing homocysteine levels, and thereby osteoporosis.

Conflict of interest

The authors declare no conflict of interest.