A Key Metabolic Regulator of Bone and Cartilage Health

Abstract

Taurine, a cysteine-derived zwitterionic sulfonic acid, is a common ingredient in energy drinks and is naturally found in fish and other seafood. In humans, taurine is produced mainly in the liver, and it can also be obtained from food. In target tissues, such as the retina, heart, and skeletal muscle, it functions as an essential antioxidant, osmolyte, and antiapoptotic agent. Taurine is also involved in energy metabolism and calcium homeostasis. Taurine plays a considerable role in bone growth and development, and high-profile reports have demonstrated the importance of its metabolism for bone health. However, these reports have not been collated for more than 10 years. Therefore, this review focuses on taurine-bone interactions and covers recently discovered aspects of taurine's effects on osteoblastogenesis, osteoclastogenesis, bone structure, and bone pathologies (e.g., osteoporosis and fracture healing), with due attention to the taurine-cartilage relationship.

Keywords: Cartilage; Osteoarthritis; Osteoblasts; Osteoclasts; Osteoporosis; Taurine.

Fig. 1.

Biosynthetic pathways of taurine in humans. Orange and blue arrows indicate synthesis pathways (I) and (II) from cysteine, respectively. Synthesis pathway (III) is indicated in green arrows [13] (created with BioRender.com). CBS, cystathionine β-synthase; CDO, cysteine dioxygenase; CSAD, cysteine sulfinate decarboxylase; HDH, hypotaurine dehydrogenase; CAD, candysteic acid decarboxylase; ADO, 2-aminoethanethiol dioxygenase.

Fig. 2.

Human taurine tissue content, distribution, and excretion. The plasma level of taurine is the sum of the dietary intake and the endogenous synthesis by the liver, while the kidney is the primary organ for its excretion. Tissues with remarkably high taurine accumulation include the retina, heart, lung, and skeletal muscle, as well as some hematopoietic cells such as erythrocytes, platelets, and white blood cells. Data represents the amount per day. Data taken from [13] (created with BioRender.com).

Fig. 3.

Cytoprotective roles of taurine against cell damage. Antioxidant activity: taurine facilitates the release of the nuclear factor E2-related factor (Nrf2), a redox-sensitive transcription factor, by the Kelch-like ECH-associated protein 1 (Keap1), and the translocation of Nrf2 to the nucleus and its binding to the antioxidant response element (ARE) activates the gene transcription of antioxidant enzymes. Anti-apoptotic activity: taurine’s contribution to calcium homeostasis blocks the activation of the calpain-dependent apoptotic cascade. Anti-inflammatory activity: taurine represses the degradation of the inhibitor of nuclear factor-κB (NF-κB) alpha (IκBα) and keeps NF-κB inactivated; it also prevents its translocation to the nucleus and reduces the transcription of genes encoding proinflammatory cytokines (created with BioRender.com). APAF1, apoptosis protease-activating factor-1; Bax, Bcl-2-associated X protein; BCl-2, B-cell lymphoma 2; CASP3, caspase-3; Ikβα, nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor, alpha; Keap1, Kelch-like ECH-associated protein 1; MyD88, innate immune signal transduction adaptor; Nrf2, NEMO, NF-kappa-B essential modulator; sMaF, musculoaponeurotic fibrosarcoma; TLR, Toll-like receptor.

Fig. 4.

Potential actions of taurine in bone and cartilage. Taurine promotes osteogenesis by upregulating Runt-related transcription factor 2 (RUNX2), as well as increasing alkaline phosphatase (ALP) activity and calcium deposition by osteoblasts; it also plays an antioxidant role in osteoblasts and osteocytes. In osteoclastogenesis, taurine exerts an inhibitory function. In chondrogenic differentiation, taurine promotes the synthesis of extracellular matrix components such as glycosaminoglycans and collagen, in addition to increasing the expression of SRY-box transcription factor 9 (SOX9). In bone and cartilage health, taurine promotes the formation of bone calluses in the consolidation process and promotes osseointegration. In pathological conditions such as osteoporosis, taurine supplementation increases bone mass, while in osteoarthritis, rheumatoid arthritis, and some bone tumors there are alterations in the metabolism of taurine (created with BioRender.com). TauT, taurine transporter.