Aryl hydrocarbon receptor (AhR)-mediated signaling as a critical regulator of skeletal cell biology

Radix Rehmanniae Praeparata promoted zebrafish fin regeneration through aryl hydrocarbon receptor-dependent autophagy

HEADINGS ETHNOPHARMACOLOGICAL RELEVANCE

Rehmanniae Radix Praeparata (RRP), a staple in traditional Chinese medicine, is derived from Rehmannia glutinosa Libosch and is renowned for its wound-healing properties. Despite its clinical prevalence, the molecular mechanisms underlying RRP's wound-healing effects have not been fully elucidated.

AIM OF THE STUDY

This research endeavored to delineate the molecular and cellular mechanisms underlying the beneficial effects of RRP on wound healing, utilizing a zebrafish model.

MATERIALS AND METHODS

Zebrafish larvae at 3 days post-fertilization were amputated at the fin and subsequently treated with RRP. The pro-wound healing and regenerative effects of RRP were evaluated through morphological analysis, assessment of cell proliferation and apoptosis, Additionally, mechanistic insights were gained through a comprehensive approach encompassing network pharmacology analysis, cell tracing, RNA-sequencing, CRISPR/Cas9 gene editing, and pharmacological inhibition.

RESULTS

Our findings demonstrate that RRP significantly accelerates caudal fin regeneration in zebrafish following injury by suppressing cell apoptosis, promoting cell proliferation, and upregulating the expression of regenerative-related genes. Furthermore, RRP triggers autophagy signals during the regenerative process, which is attenuated by the autophagy inhibitor chloroquine (CQ). Notably, the administration of RRP enhances the expression of ahr1 and ahr2 in the regenerating fin. Genetic knockout of ahr1a, ahr1b, or ahr2 using CRISPR/Cas9, or pharmacological blockade of AHR signals with the antagonist CH-223191, diminishes the regenerative potential of RRP. Remarkably, zebrafish lacking ahr2 completely lose their fin regeneration ability. Additionally, inhibition of AHR signaling suppresses autophagy signaling during fin regeneration.

CONCLUSIONS

This study uncovers that RRP stimulates fin regeneration in zebrafish by inducing AHR signals and, at least partially, activating the autophagy process. These findings provide novel insights into the molecular mechanisms underlying the wound-healing effects of RRP and may pave the way for the development of novel therapeutic strategies.

The Role of Aryl Hydrocarbon Receptor in Bone Biology

Aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, is crucial in maintaining the skeletal system. Our study focuses on encapsulating the role of AhR in bone biology and identifying novel signaling pathways in musculoskeletal pathologies using the GEO dataset. The GEO2R analysis identified 8 genes (CYP1C1, SULT6B1, CYB5A, EDN1, CXCR4B, CTGFA, TIPARP, and CXXC5A) involved in the AhR pathway, which play a pivotal role in bone remodeling. The AhR knockout in hematopoietic stem cells showed alteration in several novel bone-related transcriptomes (eg, Defb14, ZNF 51, and Chrm5). Gene Ontology Enrichment Analysis demonstrated 54 different biological processes associated with bone homeostasis. Mainly, these processes include bone morphogenesis, bone development, bone trabeculae formation, bone resorption, bone maturation, bone mineralization, and bone marrow development. Employing Functional Annotation and Clustering through DAVID, we further uncovered the involvement of the xenobiotic metabolic process, p450 pathway, oxidation-reduction, and nitric oxide biosynthesis process in the AhR signaling pathway. The conflicting evidence of current research of AhR signaling on bone (positive and negative effects) homeostasis may be due to variations in ligand binding affinity, binding sites, half-life, chemical structure, and other unknown factors. In summary, our study provides a comprehensive understanding of the underlying mechanisms of the AhR pathway in bone biology.

Effect of TCDD exposure in adult female and male mice on the expression of miRNA in the ovaries and testes and associated reproductive functions

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant found widely across the world. While animal and human studies have shown that exposure to TCDD may cause significant alterations in the reproductive tract, the effect of TCDD on the expression of miRNA in the reproductive organs has not been previously tested. In the current study, we exposed adult female or male mice to TCDD or vehicle and bred them to study the impact on reproduction. The data showed that while TCDD treatment of females caused no significant change in litter size, it did alter the survival of the pups. Also, TCDD exposure of either the male or female mice led to an increase in the gestational period. While TCDD did not alter the gross morphology of the ovaries and testes, it induced significant alterations in the miRNA expression. The ovaries showed the differential expression of 426 miRNAs, of which 315 miRNAs were upregulated and 111 miRNA that were downregulated after TCDD exposure when compared to the vehicle controls. In the testes, TCDD caused the differential expression of 433 miRNAs, with 247 miRNAs upregulated and 186 miRNAs downregulated. Pathway analysis showed that several of these dysregulated miRNAs targeted reproductive functions. The current study suggests that the reproductive toxicity of TCDD may result from alterations in the miRNA expression in the reproductive organs. Because miRNAs also represent one of the epigenetic pathways of gene expression, our studies suggest that the transgenerational toxicity of TCDD may also result from dysregulation in the miRNAs.

The kynurenine pathway in HIV, frailty and inflammaging

Kynurenine (Kyn) is a circulating tryptophan (Trp) catabolite generated by enzymes including IDO1 that are induced by inflammatory cytokines such as interferon-gamma. Kyn levels in circulation increase with age and Kyn is implicated in several age-related disorders including neurodegeneration, osteoporosis, and sarcopenia. Importantly, Kyn increases with progressive disease in HIV patients, and antiretroviral therapy does not normalize IDO1 activity in these subjects. Kyn is now recognized as an endogenous agonist of the aryl hydrocarbon receptor, and AhR activation itself has been found to induce muscle atrophy, increase the activity of bone-resorbing osteoclasts, decrease matrix formation by osteoblasts, and lead to senescence of bone marrow stem cells. Several IDO1 and AhR inhibitors are now in clinical trials as potential cancer therapies. We propose that some of these drugs may be repurposed to improve musculoskeletal health in older adults living with HIV.

Deciphering the roles of aryl hydrocarbon receptor (AHR) in regulating carcinogenesis

Aryl hydrocarbon receptor (AHR) is a ligand-dependent receptor that belongs to the superfamily of basic helix-loop-helix (bHLH) transcription factors. The activation of the canonical AHR signaling pathway is known to induce the expression of cytochrome P450 enzymes, facilitating the detoxification metabolism in the human body. Additionally, AHR could interact with various signaling pathways such as epidermal growth factor receptor (EGFR), signal transducer and activator of transcription 3 (STAT3), hypoxia-inducible factor-1α (HIF-1α), nuclear factor ekappa B (NF-κβ), estrogen receptor (ER), and androgen receptor (AR) signaling pathways. Over the past 30 years, several studies have reported that various chemical, physical, or biological agents, such as tobacco, hydrocarbon compounds, industrial and agricultural chemical wastes, drugs, UV, viruses, and other toxins, could affect AHR expression or activity, promoting cancer development. Thus, it is valuable to overview how these factors regulate AHR-mediated carcinogenesis. Current findings have reported that many compounds could act as AHR ligands to drive the expressions of AHR-target genes, such as CYP1A1, CYP1B1, MMPs, and AXL, and other targets that exert a pro-proliferation or anti-apoptotic effect, like XIAP. Furthermore, some other physical and chemical agents, such as UV and 3-methylcholanthrene, could promote AHR signaling activities, increasing the signaling activities of a few oncogenic pathways, such as the phosphatidylinositol 3-kinase/protein kinase B (PI3K/AKT) and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathways. Understanding how various factors regulate AHR-mediated carcinogenesis processes helps clinicians and scientists plan personalized therapeutic strategies to improve anti-cancer treatment efficacy. As many studies that have reported the roles of AHR in regulating carcinogenesis are preclinical or observational clinical studies that did not explore the detailed mechanisms of how different chemical, physical, or biological agents promote AHR-mediated carcinogenesis processes, future studies should focus on conducting large-scale and functional studies to unravel the underlying mechanism of how AHR interacts with different factors in regulating carcinogenesis processes.

Orchiectomy sensitizes cortical bone in male mice to the harmful effects of kynurenine

Kynurenine (Kyn) is a tryptophan metabolite that increases with age and promotes musculoskeletal dysfunction. We previously found a sexually dimorphic pattern in how Kyn affects bone, with harmful effects more prevalent in females than males. This raises the possibility that male sex steroids might exert a protective effect that blunts the effects of Kyn in males. To test this, orchiectomy (ORX) or sham surgeries were performed on 6-month-old C57BL/6 mice, after which mice received Kyn (10 mg/kg) or vehicle via intraperitoneal injection, once daily, 5×/week, for four weeks. Bone histomorphometry, DXA, microCT, and serum marker analyses were performed after sacrifice. In vitro studies were performed to specifically test the effect of testosterone on activation of aryl hydrocarbon receptor (AhR)-mediated signaling by Kyn in mesenchymal-lineage cells. Kyn treatment reduced cortical bone mass in ORX- but not sham-operated mice. Trabecular bone was unaffected. Kyn's effects on cortical bone in ORX mice were attributed primarily to enhanced endosteal bone resorption activity. Bone marrow adipose tissue was increased in Kyn-treated ORX animals but was unchanged by Kyn in sham-operated mice. ORX surgery increased mRNA expression of the aryl hydrocarbon receptor (AhR) and its target gene Cyp1a1 in the bone, suggesting a priming and/or amplification of AhR signaling pathways. Mechanistic in vitro studies revealed that testosterone blunted Kyn-stimulated AhR transcriptional activity and Cyp1a1 expression in mesenchymal-linage cells. These data suggest a protective role for male sex steroids in blunting the harmful effects of Kyn in cortical bone. Therefore, testosterone may play an important role in regulating Kyn/AhR signaling in musculoskeletal tissues, suggesting crosstalk between male sex steroids and Kyn signaling may influence age-associated musculoskeletal frailty.

The Potential Influence of Uremic Toxins on the Homeostasis of Bones and Muscles in Chronic Kidney Disease

Patients with chronic kidney disease (CKD) often experience a high accumulation of protein-bound uremic toxins (PBUTs), specifically indoxyl sulfate (IS) and p-cresyl sulfate (pCS). In the early stages of CKD, the buildup of PBUTs inhibits bone and muscle function. As CKD progresses, elevated PBUT levels further hinder bone turnover and exacerbate muscle wasting. In the late stage of CKD, hyperparathyroidism worsens PBUT-induced muscle damage but can improve low bone turnover. PBUTs play a significant role in reducing both the quantity and quality of bone by affecting osteoblast and osteoclast lineage. IS, in particular, interferes with osteoblastogenesis by activating aryl hydrocarbon receptor (AhR) signaling, which reduces the expression of Runx2 and impedes osteoblast differentiation. High PBUT levels can also reduce calcitriol production, increase the expression of Wnt antagonists (SOST, DKK1), and decrease klotho expression, all of which contribute to low bone turnover disorders. Furthermore, PBUT accumulation leads to continuous muscle protein breakdown through the excessive production of reactive oxygen species (ROS) and inflammatory cytokines. Interactions between muscles and bones, mediated by various factors released from individual tissues, play a crucial role in the mutual modulation of bone and muscle in CKD. Exercise and nutritional therapy have the potential to yield favorable outcomes. Understanding the underlying mechanisms of bone and muscle loss in CKD can aid in developing new therapies for musculoskeletal diseases, particularly those related to bone loss and muscle wasting.

Disruption of the tumor suppressor-like activity of aryl hydrocarbon receptor by arsenic in epithelial cells and human lung cancer

As the most classic and extensively studied transcription factor in response to environmental toxic chemicals, the human aryl hydrocarbon receptor (AHR) has been implicated in mediating some oncogenic responses also. Limited information is available, however, on whether arsenic, a widely presented environmental carcinogen, can regulate AHR to exert its carcinogenic activity. Through chromatin immunoprecipitation and sequencing (ChIP-seq), CRISPR-Cas9 gene editing, RNA-seq, and immunohistochemistry (IHC), in this report we provided evidence showing that arsenic enforces TGFβ and other oncogenic signaling pathways in bronchial epithelial cells through disrupting the tumor suppressor-like activity of AHR. AHR is normally enriched on a number of oncogenic genes in addition to the known phase I/II enzymes, such as genes in TGFβ and Nrf2 signaling pathways and several known oncogenes. Arsenic treatment substantially reduced the binding of AHR on these genes followed by an increased expression of these genes. CRISPR-Cas9-based knockout of AHR followed by RNA-seq further demonstrated increased expression of the TGFβ signaling and some oncogenic signaling pathway genes in the AHR knockout cells. IHC studies on human tissue samples revealed that normal human lung tissues expressed high level of AHR. In contrast, the AHR expression was diminished in the lung cancer tissues. Accordingly, the data from this study suggest that AHR has tumor suppressor-like activity for human lung cancer, and one of the carcinogenic mechanisms of arsenic is likely mediated by the inhibition of arsenic on the tumor suppressor-like activity of AHR.

A new insight into the aryl hydrocarbon receptor/cytochrome 450 signaling pathway in MG63, HOS, SAOS2, and U2OS cell lines

Osteosarcoma is the most common malignant tumor of bone, with rapid progressive growth, early distant metastases, and frequent recurrence after surgical treatment. Osteosarcoma is characterized by changes in the ratio and expression of different cytochrome P450 (CYP) isoforms that can affect the effectiveness of anticancer therapies. The inducible expression of CYP1 genes depends on the ligand-dependent functionality of the aryl hydrocarbon receptor (AHR). In this study, we examined the AHR/CYP1 signaling pathway in four osteosarcoma cell lines (MG63, HOS, SAOS2, and U2OS) induced by the known AHR ligands: indirubin, indole-3-carbinol, and beta-naphthoflavone. Using qPCR and Western blot analysis, we explored the effects of these ligands on the expression of the CYP1 genes and studied the correlation between these responses and the changes in the mRNA and protein levels of AHR and the AHR nuclear translocator (ARNT) in these osteosarcoma cell lines. The results show that the AHR/CYP1 signaling pathway retains its function only in MG63 and HOS cells, and is impaired in SAOS2 and U2OS cells. Our data should be taken into account when recommending new strategies for the treatment of osteosarcoma and when evaluating new drugs against osteosarcoma in vitro.