Do hypoxia and L-mimosine modulate sclerostin and dickkopf-1 production in human dental pulp-derived cells? Insights from monolayer, spheroid and tooth slice cultures

Hyperbaric Oxygen Therapy Does Not Have a Negative Impact on Bone Signaling Pathways in Humans

Introduction: Oxygen is emerging as an important factor in the local regulation of bone remodeling. Some preclinical data suggest that hyperoxia may have deleterious effects on bone cells. However, its clinical relevance is unclear. Hence, we studied the effect of hyperbaric oxygen therapy (HBOT) on serum biomarkers reflecting the status of the Wnt and receptor activator of NF-κB ligand (RANKL) pathways, two core pathways for bone homeostasis. Materials and methods: This was a prospective study of 20 patients undergoing HBOT (mean age 58 yrs., range 35–82 yrs.) because of complications of radiotherapy or chronic anal fissure. Patients were subjected to HBOT (100% oxygen; 2.4 atmospheres absolute for 90 min). The average number of HBOT sessions was 20 ± 5 (range 8–31). Serum hypoxia-inducible factor 1-α (HIF1-α), osteoprotegerin (OPG), RANKL, and the Wnt inhibitors sclerostin and dickkopf-1 (DKK1) were measured at baseline and after HBOT by using specific immunoassays. Results: HIF-1α in eight patients with measurable serum levels increased from 0.084 (0.098) ng/mL at baseline to 0.146 (0.130) ng/mL after HBOT (p = 0.028). However, HBOT did not induce any significant changes in the serum levels of OPG, RANKL, sclerostin or DKK1. This was independent of the patients’ diagnosis, either neoplasia or benign. Conclusion: Despite the potential concerns about hyperoxia, we found no evidence that HBOT has any detrimental effect on bone homeostasis.

The Influence of Pro-Inflammatory Factors on Sclerostin and Dickkopf-1 Production in Human Dental Pulp Cells Under Hypoxic Conditions

Sclerostin (Sost) and dickkopf (Dkk)-1 are inhibitors of the Wnt signaling pathway that plays a role in regenerative processes. Hypoxia-based strategies are used for regenerative approaches, but the influence of hypoxia on Sost and Dkk-1 production in a pro-inflammatory environment is unclear. The aim of this study was to assess if pro-inflammatory molecules have an influence on Sost and Dkk-1 production in dental pulp cells (DPC) under normoxia and hypoxia. Human DPC were treated with interleukin (IL)-1β, tumor necrosis factor (TNF)α or transforming growth factor (TGF)β, with L-mimosine (L-MIM) or hypoxia or a combination. Sost and Dkk-1 mRNA and protein levels were measured with qPCR and western blot, respectively. TNFα, TGFβ, L-MIM, or combined treatment did not modulate Sost and Dkk-1. IL-1β downregulated Sost at the mRNA level. Hypoxia alone and together with inflammatory markers downregulated Dkk-1 at the mRNA level. Sost and Dkk-1 protein production was below the detection limit. In conclusion, there is a differential effect of hypoxia and IL-1β on the mRNA production of Sost and Dkk-1. Pro-inflammatory molecules do not further modulate the effects of L-MIM or hypoxia on Sost and Dkk-1 production in DPC.

The role of sclerostin and dickkopf-1 in oral tissues - A review from the perspective of the dental disciplines

Wnt signaling is of high relevance in the development, homeostasis, and regeneration of oral tissues. Therefore, Wnt signaling is considered to be a potential target for therapeutic strategies. The action of Wnt is tightly controlled by the inhibitors sclerostin (SOST) and Dickkopf (DKK)-1. Given the impact of SOST and DKK-1 in hard tissue formation, related diseases and healing, it is of high relevance to understand their role in oral tissues. The clinical relevance of this knowledge is further underlined by systemic and local approaches which are currently in development for treating a variety of diseases such as osteoporosis and inflammatory hard tissue resorption. In this narrative review, we summarize the current knowledge and understanding on the Wnt signaling inhibitors SOST and DKK-1, and their role in physiology, pathology, and regeneration in oral tissues. We present this role from the perspective of the different specialties in dentistry, including endodontics, orthodontics, periodontics, and oral surgery.

Angiogenin production in response to hypoxia and l-mimosine in periodontal fibroblasts

BACKGROUND

A major mediator of angiogenesis is angiogenin, which is expressed in the early phase of healing in oral tissue engineering strategies. It is unclear how angiogenin is regulated in the periodontal tissue. The objective of this study was to reveal the regulation of angiogenin in response to hypoxia and the hypoxia mimetic agent l-mimosine in periodontal fibroblasts.

METHODS

Human fibroblasts of the periodontal ligament (PDLF) and the gingiva (GF) in monolayer and spheroid cultures were exposed to hypoxia or l-mimosine. The production of angiogenin was evaluated at mRNA and protein levels with reverse transcription quantitative polymerase chain reaction and enzyme-linked immunosorbent assays, respectively. Echinomycin, an inhibitor of hypoxia-inducible factor (HIF)-1 activity, was used to test the involvement of HIF-1.

RESULTS

Our data show that hypoxia and l-mimosine can increase angiogenin mRNA and protein levels in PDLF monolayer cultures. In GF monolayer cultures, we found an increase of angiogenin at the mRNA level in response to hypoxia. The increase of angiogenin can be blocked by inhibition of HIF-1 signaling via echinomycin. In PDLF and GF spheroid cultures, the impact of hypoxia and l-mimosine did not reach the level of significance.

CONCLUSION

Hypoxia and the hypoxia mimetic agent l-mimosine can increase the production of angiogenin via HIF-1 signaling in PDLF monolayer cultures but not in spheroid cultures. GF were less sensitive to the impact of hypoxia and l-mimosine. Overall, these results suggest a link between hypoxia, HIF-1 signaling and angiogenin in the periodontium.