PPARβ/δ: A master regulator of mesenchymal stem cell functions

Pectolinarigenin ameliorates osteoporosis via enhancing Wnt signaling cascade in PPARβ-dependent manner

BACKGROUND

Osteoporosis is a prevalent metabolic bone disease in older adults. Peroxisome proliferator-activated receptor β (PPARβ), the most abundant PPAR isotype expressed in bone tissues, plays a critical role in regulating the energy metabolism of osteoblasts. However, the botanical compounds targeting PPARβ for the treatment of osteoporosis remain largely unexplored.

PURPOSE

To discover a potent PPARβ agonist from botanical compounds, as well as to investigate the anti-osteoporosis effects and to elucidate the underlying mechanisms of the newly identified PPARβ agonist.

METHODS

The PPARβ agonist effects of botanical compounds were screened by an in vitro luciferase reporter gene assay. The PPARβ agonist effects of pectolinarigenin (PEC) in bone marrow mesenchymal stromal cells (BMSCs) were validated by Western blotting. RNA-seq transcriptome analyses were conducted to reveal the underlying osteoporosis mechanisms of PEC in BMSCs. The PPARβ antagonist (GSK0660) and Wnt signaling inhibitor (XAV969) were used to explore the role of the PPARβ and Wnt signaling cascade in the anti-osteoporosis effects of PEC. PEC or the PEG-PLGA nanoparticles of PEC (PEC-NP) were intraperitoneally administrated in both wild-type mice and ovariectomy-induced osteoporosis mice to examine its anti-osteoporotic effects in vivo.

RESULTS

PEC, a newly identified naturally occurring PPARβ agonist, significantly promotes osteogenic differentiation and up-regulates the osteogenic differentiation-related genes (Runx2, Osterix, and Bmp2) in BMSCs. RNA sequencing and functional gene enrichment analysis suggested that PEC could activate osteogenic-related signaling pathways, including Wnt and PPAR signaling pathways. Further investigations suggested that PEC could enhance Wnt/β-catenin signaling in a PPARβ-dependent manner in BMSCs. Animal tests showed that PEC-NP promoted bone mass and density, increased the bone cell matrix protein, and accelerated bone formation in wild-type mice, while PEC-NP also played a preventive role in ovariectomy-induced osteoporosis mice via maintaining the expression level of bone cell matrix protein, balancing the rate of bone formation, and slowing down bone loss. Additionally, PEC-NP did not cause any organ injury and body weight loss after long-term use (11 weeks).

CONCLUSION

PEC significantly promotes bone formation and reduces bone loss in both BMSCs and ovariectomy-induced osteoporosis mice via enhancing the Wnt signaling cascade in a PPARβ-dependent manner, providing a new alternative therapy for preventing estrogen deficiency-induced osteoporotic diseases.

A positive feedback loop: RAD18-YAP-TGF-β between triple-negative breast cancer and macrophages regulates cancer stemness and progression

As a key regulator of the DNA translesion synthesis (TLS) pathway, RAD18 is error-prone and contributes to the accumulation of DNA mutations. Our previous study showed that it plays an essential role in the progression of multiple tumors. However, the mechanism through which RAD18 influences triple-negative breast cancer (TNBC), especially the interaction between tumor cells and the tumor microenvironment, remains elusive. In this study, we showed that RAD18 expression is markedly higher in patients with high T stage TNBC and inversely correlated with prognosis. High expression of RAD18 facilitated a highly stem-cell phenotype through the Hippo/YAP pathway, which supports the proliferation of TNBC. In addition, the cytokine byproduct TGF-β activates macrophages to have an M2-like tumor-associated macrophage (TAM) phenotype. Reciprocally, TGF-β from TAMs activated RAD18 in TNBC to enhance tumor stemness, forming a positive feedback loop. Inhibition of YAP or TGF-β breaks this loop and suppresses cancer stemness and proliferation In nude mice, RAD18 promoted subcutaneous transplanted tumor growth and M2-type TAM recruitment. Collectively, the RAD18-YAP-TGF-β loop is essential for the promotion of the stemness phenotype by TNBC and could be a potential therapeutic target for TNBC.

PPARβ/δ accelerates bone regeneration in diabetic mellitus by enhancing AMPK/mTOR pathway-mediated autophagy

BACKGROUND

Diabetic patients are more vulnerable to skeletal complications. Peroxisome proliferators-activated receptor (PPAR) β/δ has a positive regulatory effect on bone turnover under physiologic glucose concentration; however, the regulatory effect in diabetes mellitus has not been investigated yet. Herein, we explored the effects of PPARβ/δ agonist on the regeneration of diabetic bone defects and the osteogenic differentiation of rat bone marrow mesenchymal stem cells (rBMSCs) under a pathological high-glucose condition.

METHODS

We detected the effect of PPARβ/δ agonist on osteogenic differentiation of rBMSCs in vitro and investigated the bone healing process in diabetic rats after PPARβ/δ agonist treatment in vivo. RNA sequencing was performed to detect the differentially expressed genes and enriched pathways. Western blot was performed to detect the autophagy-related protein level. Laser confocal microscope (LSCM) and transmission electron microscope (TEM) were used to observe the formation of autophagosomes.

RESULTS

Our results demonstrated that the activation of PPARβ/δ can improve the osteogenic differentiation of rBMSCs in high-glucose condition and promote the bone regeneration of calvarial defects in diabetic rats, while the inhibition of PPARβ/δ alleviated the osteogenic differentiation of rBMSCs. Mechanistically, the activation of PPARβ/δ up-regulates AMPK phosphorylation, yielding mTOR suppression and resulting in enhanced autophagy activity, which further promotes the osteogenic differentiation of rBMSCs in high-glucose condition. The addition of AMPK inhibitor Compound C or autophagy inhibitor 3-MA inhibited the osteogenesis of rBMSCs in high-glucose condition, suggesting that PPARβ/δ agonist promotes osteogenic differentiation of rBMSCs through AMPK/mTOR-regulated autophagy.

CONCLUSION

In conclusion, our study demonstrates the potential role of PPARβ/δ as a molecular target for the treatment of impaired bone quality and delayed bone healing in diabetic patients for the first time.

EB virus-induced ATR activation accelerates nasopharyngeal carcinoma growth via M2-type macrophages polarization

Chronic inflammation induced by persistent viruses infection plays an essential role in tumor progression, which influenced on the interaction between the tumor cells and the tumor microenvironment. Our earlier study showed that ATR, a key kinase participant in single-stranded DNA damage response (DDR), was obviously activated by Epstein-Barr virus (EBV) in nasopharyngeal carcinoma (NPC). However, how EBV-induced ATR activation promotes NPC by influencing inflammatory microenvironment, such as tumor-associated macrophages (TAMs), remains elusive. In this study, we showed that EBV could promote the expression of p-ATR and M2-type TAMs transformation in clinical NPC specimens. The expression of p-ATR and M2-type TAMs were closely correlated each other and involved in TNM stage, lymph node metastasis and poor prognosis of the patients. In addition, the expression levels of CD68⁺CD206⁺, Arg1, VEGF, and CCL22 were increased in EB⁺ CNE1 cells, and decreased when ATR was inhibited. In the nude mice, EBV-induced ATR activation promoted subcutaneous transplanted tumor growth, higher expression of Ki67 and lung metastasis via M2-type TAMs recruitment. Experimental data also showed that the polarization of M2, the declined tumor necrosis factor-α (TNF-α) and increased transforming growth factor-β (TGF-β) were associated with ATR. Meanwhile, ATR activation could promote PPAR-δ and inhibited c-Jun and p-JNK expression, then downregulate JNK pathway. Collectively, our current study demonstrated the EBV infection could activate the ATR pathway to accelerate the transition of TAMs to M2, suggesting ATR knockdown could be a potential effective treatment strategy for EBV-positive NPC.

PPAR Beta/Delta and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor family. Three different isoforms, PPAR alpha, PPAR beta/delta and PPAR gamma have been identified. They all form heterodimers with retinoic X receptors to activate or repress downstream target genes dependent on the presence/absence of ligands and coactivators or corepressors. PPARs differ in their tissue expression profile, ligands and specific agonists and antagonists. PPARs attract attention as potential therapeutic targets for a variety of diseases. PPAR alpha and gamma agonists are in clinical use for the treatment of dyslipidemias and diabetes. For both receptors, several clinical trials as potential therapeutic targets for cancer are ongoing. In contrast, PPAR beta/delta has been suggested as a therapeutic target for metabolic syndrome. However, potential risks in the settings of cancer are less clear. A variety of studies have investigated PPAR beta/delta expression or activation/inhibition in different cancer cell models in vitro, but the relevance for cancer growth in vivo is less well documented and controversial. In this review, we summarize critically the knowledge of PPAR beta/delta functions for the different hallmarks of cancer biological capabilities, which interplay to determine cancer growth.

PPARβ/δ: Linking Metabolism to Regeneration

In contrast to the general belief that regeneration is a rare event, mainly occurring in simple organisms, the ability of regeneration is widely distributed in the animal kingdom. Yet, the efficiency and extent of regeneration varies greatly. Humans can recover from blood loss as well as damage to tissues like bone and liver. Yet damage to the heart and brain cannot be reversed, resulting in scaring. Thus, there is a great interest in understanding the molecular mechanisms of naturally occurring regeneration and to apply this knowledge to repair human organs. During regeneration, injury-activated immune cells induce wound healing, extracellular matrix remodeling, migration, dedifferentiation and/or proliferation with subsequent differentiation of somatic or stem cells. An anti-inflammatory response stops the regenerative process, which ends with tissue remodeling to achieve the original functional state. Notably, many of these processes are associated with enhanced glycolysis. Therefore, peroxisome proliferator-activated receptor (PPAR) β/δ—which is known to be involved for example in lipid catabolism, glucose homeostasis, inflammation, survival, proliferation, differentiation, as well as mammalian regeneration of the skin, bone and liver—appears to be a promising target to promote mammalian regeneration. This review summarizes our current knowledge of PPARβ/δ in processes associated with wound healing and regeneration.