Forkhead box O proteins in chondrocyte aging and diseases

Abstract

As people age, the progressive loss of cartilage integrity occurs, accompanied by a decline in the capacity to repair. This results in decreased resilience and increased susceptibility of cartilage to various physiological stressors, which raises the risk of developing osteoarthritis (OA). Therefore, restoring the regenerative capacity of chondrocytes and slowing down the aging process could be promising therapeutic strategies to mitigate or even reverse age-related joint diseases. Forkhead box class O (FoxO) proteins are a family of transcription factors that play a crucial role in various cellular processes linked to aging. Their significant functions in cell cycle regulation, apoptosis, and resistance to oxidative stress highlight their importance in maintaining cellular homeostasis and promoting longevity. In this review, we introduce the structures and functions of FoxO proteins in chondrocytes, focusing on their spatiotemporal regulation of epigenetics during chondrocyte differentiation stages in different layers. The critical roles of FoxO proteins in maintaining chondrocyte homeostasis are summarized, alongside a discussion of how FoxO dysfunction contributes to aging and OA. Furthermore, therapeutic strategies targeting FoxO proteins to mitigate aging-related cartilage degradation and decelerate OA progression are explored. Finally, potential directions for future research are proposed to deepen the current understanding of FoxO proteins.

The translational potential of this article: FoxO transcription factors, especially FoxO1 and FoxO3, are promising therapeutic targets for promoting longevity, stimulating cartilage regeneration, and treating aging-related diseases like OA.

Keywords: Aging; Cartilage regeneration; Chondrocyte; Forkhead box class O; FoxO; Osteoarthritis; Rejuvenation.

Graphical abstract

Fig. 1

The structure, transcriptional regulation, post-transcriptional modification, and biological functions of FOXO3 gene. Transcription of FOXO3 is initiated by RNA polymerase and modulated by upstream signals, including TGF-β/SMAD pathways. The promoter and operator regions precede the gene's coding exons (Exon 1–4), which include functional domains such as the forkhead box (FKH), nuclear localization sequence (NLS), nuclear export sequence (NES), and transactivation domain (TAD). The transcription of the FOXO3 gene is suppressed by various microRNAs, such as miR-29a, miR-30b-5p, miR-29b-3p, and miR-155. Non-coding RNAs, such as circular RNAs (has_circ_0006404, mmu_circ_0002207), POU3F3, and lncRNA H19, act as molecular sponges for miRNAs, binding to them and preventing them from targeting FOXO1 mRNA. Translation of FOXO3 mRNA produces a protein subject to multiple post-translational modifications, including acetylation, phosphorylation, ubiquitination, O-GlcNAcylation, and methylation, modulating its activity and stability. Notably, METTL3-mediated m6A methylation regulates mRNA processing at the exon 4 region, influencing RNA stability. Functionally, FoxO3 plays crucial roles in cellular processes such as inflammation alleviation (via IL-1β/TNF-α), osteogenic differentiation (via RUNX1), autophagy restoration (via AMPK), ferroptosis suppression (via NF-κB), metabolism regulation (via Sirt1), and antioxidative stress responses (via PI3K/Akt).

Fig. 2

Comprehensive overview of FOXO signaling pathways and regulatory mechanisms in cellular homeostasis. Various processes are involved in regulating the cellular location and function of FoxOs. Upstream, FoxO is modulated by various receptor-mediated pathways, including TGF-β/SMAD, EGF/ERK, Insulin/IGF-PI3K-AKT, and IL-6/IL-8-JAK/STAT. For instance, insulin/insulin-like growth factors activate transmembrane receptors, initiating downstream signaling events that culminate in the phosphorylation and activation of AKT. AKT is a central negative regulator, which phosphorylates FoxO proteins, leading to their nuclear export and degradation via the ubiquitin-proteasome system. AMPK, activated by energy stress (ATP/ADP imbalance), enhances FoxO nuclear localization and transcriptional activity, counteracting mTORC1 signaling. SIRT1 and PRKAG1 modulate FoxO activity via deacetylation and NAD⁺-dependent mechanisms, while SET9 and PRMT1 mediate methylation. In the nucleus, activated FOXO factors drive gene expression programs involved in cell cycle arrest, oxidative stress resistance, apoptosis, autophagy, metabolism, immune regulation, and atrophy. MicroRNAs, including miR-24, miR-23a, and miR-145, negatively interfere with the interactions between FoxOs with SMAD3/4, further modulating FOXO-mediated transcription.

Fig. 3

Role of FoxO in cartilage homeostasis and potential therapies targeting FoxOs for chondrocyte rejuvenation and cartilage regeneration. The protein levels of FoxOs decrease from the superficial and mid zones to the deep zone, particularly in weight-bearing areas of the cartilage compared to non-weight-bearing areas. The downregulation of FoxOs leads to several detrimental effects on cartilage, including loss of chondrogenic progenitor cells, increased inflammatory mediators, impaired migration and chondrogenic ability, increased matrix-degrading enzymes, susceptibility to mechanical stress, increased cell apoptosis, compromised autophagy, and abnormal lipid metabolism. These changes are accompanied by decreased chondrogenic ability, cell loss, and genomic instability, ultimately resulting in cartilage degeneration and OA pathology. Various endogenous and exogenous modulators, including miRNA, lncRNA, exosomes, DNA, hormones, metal ions, natural compounds, nanoparticles, drugs, cytokines, biological, and chemical components, can restore FoxO activity, thereby counteracting cartilage degeneration and offering potential therapeutic strategies for OA treatment.