Cited2 modulates hypoxia-inducible factor-dependent expression of vascular endothelial growth factor in nucleus pulposus cells of the rat intervertebral disc

Increased HIF-2α activity in the nucleus pulposus causes intervertebral disc degeneration in the aging mouse spine

Hypoxia-inducible factors (HIFs) are essential to the homeostasis of hypoxic tissues. Although HIF-2α, is expressed in nucleus pulposus (NP) cells, consequences of elevated HIF-2 activity on disc health remains unknown. We expressed HIF-2α with proline to alanine substitutions (P405A; P531A) in the Oxygen-dependent degradation domain (HIF-2αdPA) in the NP tissue using an inducible, nucleus pulposus-specific K19CreERT allele to study HIF-2α function in the adult intervertebral disc. Expression of HIF-2α in NP impacted disc morphology, as evident from small but significantly higher scores of degeneration in NP of 24-month-old K19CreERT; HIF-2αdPA (K19-dPA) mice. Noteworthy, comparisons of grades within each genotype between 14 months and 24 months indicated that HIF-2α overexpression contributed to more pronounced changes than aging alone. The annulus fibrosus (AF) compartment in the 14-month-old K19-dPA mice exhibited lower collagen turnover and Fourier transform-infrared (FTIR) spectroscopic imaging analyses showed changes in the biochemical composition of the 14- and 24-month-old K19-dPA mice. Moreover, there were changes in aggrecan, chondroitin sulfate, and COMP abundance without alterations in NP phenotypic marker CA3, suggesting the overexpression of HIF-2α had some impact on matrix composition but not the cell phenotype. Mechanistically, the global transcriptomic analysis showed enrichment of differentially expressed genes in themes closely related to NP cell function such as cilia, SLIT/ROBO pathway, and HIF/Hypoxia signaling at both 14- and 24-month. Together, these findings underscore the role of HIF-2α in the pathogenesis of disc degeneration in the aged spine.

Increased HIF-2α Activity in the Nucleus Pulposus Causes Intervertebral Disc Degeneration in the Aging Mouse Spine

Hypoxia-inducible factors (HIFs) are essential to the homeostasis of hypoxic tissues. Although HIF-2α, is expressed in nucleus pulposus (NP) cells, consequences of elevated HIF-2 activity on disc health remains unknown. We expressed HIF-2α with proline to alanine substitutions (P405A;P531A) in the Oxygen-dependent degradation domain (HIF-2αdPA) in the NP tissue using an inducible, nucleus pulposus-specific K19 CreERT allele to study HIF-2α function in the adult intervertebral disc. Expression of HIF-2α in NP impacted disc morphology, as evident from small but significantly higher scores of degeneration in NP of 24-month-old K19 CreERT ; HIF-2α dPA (K19-dPA) mice. Noteworthy, comparisons of grades within each genotype between 14 months and 24 months indicated that HIF-2α overexpression contributed to more pronounced changes than aging alone. The annulus fibrosus (AF) compartment in the 14-month-old K19-dPA mice exhibited lower collagen turnover and Fourier transform-infrared (FTIR) spectroscopic imaging analyses showed changes in the biochemical composition of the 14-and 24-month-old K19-dPA mice. Moreover, there were changes in aggrecan, chondroitin sulfate, and COMP abundance without alterations in NP phenotypic marker CA3, suggesting the overexpression of HIF-2α had some impact on matrix composition but not the cell phenotype. Mechanistically, the global transcriptomic analysis showed enrichment of differentially expressed genes in themes closely related to NP cell function such as cilia, SLIT/ROBO pathway, and HIF/Hypoxia signaling at both 14- and 24-months. Together, these findings underscore the role of HIF-2α in the pathogenesis of disc degeneration in the aged spine.

Inducing Angiogenesis in the Nucleus Pulposus

Bone morphogenetic protein (BMP) gene delivery to Lewis rat lumbar intervertebral discs (IVDs) drives bone formation anterior and external to the IVD, suggesting the IVD is inhospitable to osteogenesis. This study was designed to determine if IVD destruction with a proteoglycanase, and/or generating an IVD blood supply by gene delivery of an angiogenic growth factor, could render the IVD permissive to intra-discal BMP-driven osteogenesis and fusion. Surgical intra-discal delivery of naïve or gene-programmed cells (BMP2/BMP7 co-expressing or VEGF165 expressing) +/- purified chondroitinase-ABC (chABC) in all permutations was performed between lumbar 4/5 and L5/6 vertebrae, and radiographic, histology, and biomechanics endpoints were collected. Follow-up anti-sFlt Western blotting was performed. BMP and VEGF/BMP treatments had the highest stiffness, bone production and fusion. Bone was induced anterior to the IVD, and was not intra-discal from any treatment. chABC impaired BMP-driven osteogenesis, decreased histological staining for IVD proteoglycans, and made the IVD permissive to angiogenesis. A soluble fragment of VEGF Receptor-1 (sFlt) was liberated from the IVD matrix by incubation with chABC, suggesting dysregulation of the sFlt matrix attachment is a possible mechanism for the chABC-mediated IVD angiogenesis we observed. Based on these results, the IVD can be manipulated to foster vascular invasion, and by extension, possibly osteogenesis.

Conditional Deletion of HIF-2α in Mouse Nucleus Pulposus Reduces Fibrosis and Provides Mild and Transient Protection From Age-Dependent Structural Changes in Intervertebral Disc

Hypoxia-inducible factors (HIFs) are critical to the development and homeostasis of hypoxic tissues. Although HIF-2α, one of the main HIF-α isoforms, is expressed in nucleus pulposus (NP) cells, its functions remain unknown. We deleted HIF-2α in the NP tissue using a notochord-specific FoxA2Cre allele to study HIF-2α function in the adult intervertebral disc. Unlike observations in HIF-1αcKO mice, fate mapping studies using Rosa26-mTmG reporter showed that HIF-2α loss in NP did not negatively impact cell survival or affect compartment development. Rather, loss of HIF-2α resulted in slightly better attributes of NP morphology in 14-month-old HIF-2αcKO mice as evident from lower scores of degeneration. These 14-month-old HIF-2αcKO mice also exhibited significant reduction in NP tissue fibrosis and lower collagen turnover in the annulus fibrosis (AF) compartment. Imaging-Fourier transform-infrared (FTIR) analyses showed decreased collagen and protein content in the NP and maintained chondroitin sulfate levels in 14-month-old HIF-2αcKO . Mechanistically, global transcriptomic analysis showed enrichment of differentially expressed genes with Gene Ontology (GO) terms related to metabolic processes and cell development, molecular functions concerned with histone and protein binding, and associated pathways, including oxidative stress. Noteworthy, these morphological differences were not apparent in 24-month-old HIF-2αcKO , indicating that aging is the dominant factor in governing disc health. Together these data suggest that loss of HIF-2α in the NP compartment is not detrimental to the intervertebral disc development but rather mitigates NP tissue fibrosis and offers mild but transient protection from age-dependent early degenerative changes. © 2022 American Society for Bone and Mineral Research (ASBMR).

Current Knowledge and Future Therapeutic Prospects in Symptomatic Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration is the main source of intractable lower back pain, and symptomatic IVD degeneration could be due to different degeneration mechanisms. In this article, we describe the molecular basis of symptomatic IVD degenerative disc diseases (DDDs), emphasizing the role of degeneration, inflammation, angiogenesis, and extracellular matrix (ECM) regulation during this process. In symptomatic DDD, pro-inflammatory mediators modulate catabolic reactions, resulting in changes in ECM homeostasis and, finally, neural/vascular ingrowth-related chronic intractable discogenic pain. In ECM homeostasis, anabolic protein-regulating genes show reduced expression and changes in ECM production, while matrix metalloproteinase gene expression increases and results in aggressive ECM degradation. The resultant loss of normal IVD viscoelasticity and a concomitant change in ECM composition are key mechanisms in DDDs. During inflammation, a macrophage-related cascade is represented by the secretion of high levels of pro-inflammatory cytokines, which induce inflammation. Aberrant angiogenesis is considered a key initiative pathologic step in symptomatic DDD. In reflection of angiogenesis, vascular endothelial growth factor expression is regulated by hypoxia-inducible factor-1 in the hypoxic conditions of IVDs. Furthermore, IVD cells undergoing degeneration potentially enhance neovascularization by secreting large amounts of angiogenic cytokines, which penetrate the IVD from the outer annulus fibrosus, extending deep into the outer part of the nucleus pulposus. Based on current knowledge, a multi-disciplinary approach is needed in all aspects of spinal research, starting from basic research to clinical applications, as this will provide information regarding treatments for DDDs and discogenic pain.

Intervertebral Disk Degeneration: The Microenvironment and Tissue Engineering Strategies

Intervertebral disk degeneration (IVDD) is a leading cause of disability. The degeneration is inevitable, and the mechanisms are complex. Current therapeutic strategies mainly focus on the relief of symptoms, not the intrinsic regeneration of the intervertebral disk (IVD). Tissue engineering is a promising strategy for IVDD due to its ability to restore a healthy microenvironment and promote IVD regeneration. This review briefly summarizes the IVD anatomy and composition and then sets out elements of the microenvironment and the interactions. We rationalized different scaffolds based on tissue engineering strategies used recently. To fulfill the complete restoration of a healthy IVD microenvironment, we propose that various tissue engineering strategies should be combined and customized to create personalized therapeutic strategies for each individual.

Regeneration in Spinal Disease: Therapeutic Role of Hypoxia-Inducible Factor-1 Alpha in Regeneration of Degenerative Intervertebral Disc

The intervertebral disc (IVD) is a complex joint structure comprising three primary components—namely, nucleus pulposus (NP), annulus fibrosus (AF), and cartilaginous endplate (CEP). The IVD retrieves oxygen from the surrounding vertebral body through CEP by diffusion and likely generates ATP via anaerobic glycolysis. IVD degeneration is characterized by a cascade of cellular, compositional, structural changes. With advanced age, pronounced changes occur in the composition of the disc extracellular matrix (ECM). NP and AF cells in the IVD possess poor regenerative capacity compared with that of other tissues. Hypoxia-inducible factor (HIF) is a master transcription factor that initiates a coordinated cellular cascade in response to a low oxygen tension environment, including the regulation of numerous enzymes in response to hypoxia. HIF-1α is essential for NP development and homeostasis and is involved in various processes of IVD degeneration process, promotes ECM in NP, maintains the metabolic activities of NP, and regulates dystrophic mineralization of NP, as well as angiogenesis, autophagy, and apoptosis during IVD degeneration. HIF-1α may, therefore, represent a diagnostic tool for early IVD degeneration and a therapeutic target for inhibiting IVD degeneration

Cartilage Endplate Stem Cells Transdifferentiate Into Nucleus Pulposus Cells via Autocrine Exosomes

Stem cells derived from cartilage endplate (CEP) cells (CESCs) repair intervertebral disc (IVD) injury; however, the mechanism remains unclear. Here, we evaluated whether CESCs could transdifferentiate into nucleus pulposus cells (NPCs) via autocrine exosomes and subsequently inhibit IVD degeneration. Exosomes derived from CESCs (CESC-Exos) were extracted and identified by ultra-high-speed centrifugation and transmission electron microscopy. The effects of exosomes on the invasion, migration, and differentiation of CESCs were assessed. The exosome-activating hypoxia-inducible factor (HIF)-1α/Wnt pathway was investigated using lenti-HIF-1α and Wnt agonists/inhibitors in cells and gene ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis in normal and degenerated human CEP tissue. The effects of GATA binding protein 4 (GATA4) on transforming growth factor (TGF)-β expression and on the invasion, migration, and transdifferentiation of CESCs were investigated using lenti-GATA4, TGF-β agonists, and inhibitors. Additionally, IVD repair was investigated by injecting CESCs overexpressing GATA4 into rats. The results indicated that CESC-Exos promoted the invasion, migration, and differentiation of CESCs by autocrine exosomes via the HIF-1α/Wnt pathway. Additionally, increased HIF-1α enhanced the activation of Wnt signaling and activated GATA4 expression. GATA4 effectively promoted TGF-β secretion and enhanced the invasion, migration, and transdifferentiation of CESCs into NPCs, resulting in promotion of rat IVD repair. CESCs were also converted into NPCs as endplate degeneration progressed in human samples. Overall, we found that CESC-Exos activated HIF-1α/Wnt signaling via autocrine mechanisms to increase the expression of GATA4 and TGF-β1, thereby promoting the migration of CESCs into the IVD and the transformation of CESCs into NPCs and inhibiting IVDD.

RACK1 mediates the advanced glycation end product-induced degradation of HIF-1α in nucleus pulposus cells via competing with HSP90 for HIF-1α binding

Hyperglycemia can drive advanced glycation end product (AGE) accumulation and associated nucleus pulposus cell (NPC) dysfunction, but the basis for this activity has not been elucidated. Hypoxia-inducible factor-1α (HIF-1α) is subject to cell-type-specific AGE-mediated regulation. In the current study, we assessed the mechanistic relationship between AGE accumulation and HIF-1α degradation in NPCs. Immunohistochemical staining of degenerated nucleus pulposus (NP) samples was used to assess AGE levels. AGE impact on NPC survival and glycolysis-related gene expression was assessed via 3-(4,5)-dimethylthiazol(-z-y1)-3,5-di-phenyltetrazolium bromide assay and quantitative reverse-transcription polymerase chain reaction (qRT-PCR), while HIF-1α expression in NPCs following AGE treatment was monitored via Western blot analysis and qRT-PCR. Additionally, a luciferase reporter assay was used to monitor HIF-1α transcriptional activity. The importance of the receptor for activated C-kinase 1 (RACK1) as a mediator of HIF-1α degradation was evaluated through gain- and loss-of-function experiments. Competitive binding of RACK1 and HSP90 to HIF-1α was evaluated via immunoprecipitation. Increased AGE accumulation was evident in NP samples from diabetic patients, and AGE treatment resulted in reduced HIF-1α protein levels in NPCs that coincided with reduced HIF-1α transcriptional activity. AGE treatment impaired the stability of HIF-1α, leading to its RACK1-mediated proteasomal degradation in a manner independent of the canonical PHD-mediated degradation pathway. Additionally, RACK1 competed with HSP90 for HIF-1α binding following AGE treatment. AGE treatment of NPCs leads to HIF-1α protein degradation. RACK1 competes with HSP90 for HIF-1α binding following AGE treatment, resulting in posttranslational HIF-1α degradation. These results suggest that AGE is an intervertebral disc degeneration risk factor, and highlight potential avenues for the treatment or prevention of this disease.

The potential role and trend of HIF‑1α in intervertebral disc degeneration: Friend or foe? (Review)

Lower back pain (LBP) is one of the most common reasons for seeking medical advice in orthopedic clinics. Increasingly, research has shown that symptomatic intervertebral disc degeneration (IDD) is mostly related to LBP. This review first outlines the research and findings of studies into IDD, from the physiological structure of the intervertebral disc (IVD) to various pathological cascades. The vicious cycles of IDD are re-described in relation to the analysis of the relationship among the pathological mechanisms involved in IDD. Interestingly, a ‘chief molecule’ was found, hypoxia-inducible factor-1α (HIF-1α), that may regulate all other mechanisms involved in IDD. When the vicious cycle is established, the low oxygen tension activates the expression of HIF-1α, which subsequently enters into the hypoxia-induced HIF pathways. The HIF pathways are dichotomized as friend and foe pathways according to the oxygen tension of the IVD microenvironment. Combined with clinical outcomes and previous research, the trend of IDD development has been predicted in this paper. Lastly, an early precautionary diagnosis and treatment method is proposed whereby nucleus pulposus tissue for biopsy can be obtained through IVD puncture guided by B-ultrasound when the patient is showing symptoms but MRI imaging shows negative results. The assessment criteria for biopsy and the feasibility, superiority and challenges of this approach have been discussed. Overall, it is clear that HIF-1α is an indispensable reference indicator for the accurate diagnosis and treatment of IDD.

Low energy extracorporeal shock wave therapy combined with low tension traction can better reshape the microenvironment in degenerated intervertebral disc regeneration and repair

BACKGROUND

Low-tension traction is more effective than high-tension traction in restoring the height and rehydration of a degenerated disc and to some extent the bony endplate. This might better reshape the microenvironment for disc regeneration and repair. However, the repair of the combination of endplate sclerosis, osteophyte formation, and even collapse leading to partial or nearly complete occlusion of the nutrient channel is greatly limited.

PURPOSE

To evaluate the effectiveness of low-intensity extracorporeal shock wave therapy (ESWT) combined with low tension traction for regeneration and repair of moderately and severely degenerated discs; to explore the possible mechanism of action.

STUDY DESIGN

Animal study of a rat model of degenerated discs.

METHODS

A total of thirty-five 6-month old male Sprague-Dawley rats were randomly assigned to one of five groups (n=7, each group). In Group A (model group), caudal vertebrae were immobilized using a custom-made external device to fix four caudal vertebrae (Co7-Co10) whereas Co8-Co9 underwent 4 weeks of compression to induce moderate disc degeneration. In Group B (experimental control group), as in Group A, disc degeneration was successfully induced after which the fixed device was removed for 8 weeks of self-recovery. The remaining three groups of rats represented the intervention Groups (C-E): after successful generation of disc degeneration in Group C (com - 4w/tra - 4w) and Group D (com - 4w/ESWT), as described for group A, low-tension traction (in-situ traction) or low-energy ESWT was administered for 4 weeks (ESWT parameters: intensity: 0.15 Mpa; frequency: 1 Hz; impact: 1,000 each time; once/week, 4 times in total); Group E (com - 4w/tra - 4w/ESWT): disc degeneration as described for group A, low-tension traction combined with low-energy ESWT was conducted (ESWT parameters as Group D). After experimentation, caudal vertebrae were harvested and disc height, T2 signal intensity, disc morphology, total glycosaminoglycan (GAG) content, gene expression, structure of the Co8-Co9 bony endplates and elastic moduli of the discs were measured.

RESULTS

After continuous low-tension traction, low energy ESWT intervention or combined intervention, the degenerated discs effectively recovered their height and became rehydrated. However, the response in Group D was weaker than in the other intervention groups in terms of restoration of intervertebral disc (IVD) height, whereas Group E was superior in disc rehydration. Tissue regeneration was evident in Groups C to E using different interventions. No apparent tissue regeneration was observed in the experimental control group (Group B). The histological scores of the three intervention groups (Groups C-E) were lower than those of Groups A or B (p<.0001), and the scores of Groups C and E were significantly lower than those of Group D (p<.05), but not Group C versus Group E (p>.05). Compared with the intervention groups (Groups C-E), total GAG content of the nucleus pulposus (NP) in Group B did not increase significantly (p>.05). There was also no significant difference in the total GAG content between Groups A and B (p>.05). Of the three intervention groups, the recovery of NP GAG content was greatest in Group E. The expression of collagen I and II, and aggrecan in the annulus fibrosus (AF) was up-regulated (p<.05), whereas the expression of MMP-3, MMP-13, and ADAMTS-4 was down-regulated (p<.05). Of the groups, Group E displayed the greatest degree of regulation. The trend in regulation of gene expression in the NP was essentially consistent with that of the AF, of which Group E was the greatest. In the intervention groups (Groups C-E), compared with Group A, the pore structure of the bony endplate displayed clear changes. The number of pores in the endplate in Groups C to E was significantly higher than in Group A (p<.0001), among which Group C versus Group D (p=.9724), and Group C versus Group E (p=.0116). There was no significant difference between Groups A and B (p=.5261). In addition, the pore diameter also increased, the trend essentially the same as that of pore density. There was no significant difference between the three intervention groups (p=.7213). It is worth noting that, compared with Groups A and B, peripheral pore density and size in Groups D and E of the three intervention groups recovered significantly. The elastic modulus and diameter of collagen fibers in the AF and NP varied with the type of intervention. Low tension traction combined with ESWT resulted in the greatest impact on the diameter and modulus of collagen fibers.

CONCLUSIONS

Low energy ESWT combined with low tension traction provided a more stable intervertebral environment for the regeneration and repair of moderate and severe degenerative discs. Low energy ESWT promoted the regeneration of disc matrix by reducing MMP-3, MMP-13, and ADAMTS-4 resulting in inhibition of collagen degradation. Although axial traction promoted the recovery of height and rehydration of the IVD, combined with low energy ESWT, the micro-nano structure of the bony endplate underwent positive reconstruction, tension in the annulus of the AF and nuclear stress of the NP declined, and the biomechanical microenvironment required for IVD regeneration and repair was reshaped.

Nucleus Pulposus Cell Conditioned Medium Promotes Mesenchymal Stem Cell Differentiation into Nucleus Pulposus-Like Cells under Hypoxic Conditions

Low back pain (LBP) is a major physical and socioeconomic challenge worldwide. Nucleus pulposus (NP) is directly associated with LBP due to intervertebral disc (IVD) degeneration. IVD degeneration is mainly caused by structural and matrix-related changes within the IVD occurring during aging and degeneration. Mesenchymal stem cells (MSCs) can differentiate into multiple mesenchymal lineages under specific stimulatory conditions. This study is aimed at evaluating the effectiveness of the nucleus pulposus cell (NPC) conditioned medium for promoting the expression of MSCs and at confirming the expression of healthy NP phenotypic markers recently recommended by the Spine Research Interest Group. Expression was investigated using quantitative polymerase chain reaction (qPCR) and western blotting under normoxic and hypoxic conditions. qPCR and western blotting demonstrated significant upregulation of NP marker expression in MSCs cultured under hypoxic conditions and treated with the 50% or 100% NPC conditioned medium, compared with those cultured under normoxic conditions. Upregulation was highest in the presence of the 100% NPC conditioned medium compared with the control group (aggrecan, p < 0.01; brachyury, p < 0.05; collagen II, p < 0.001; KRT8, p < 0.01; KRT19, p < 0.001; and Shh, p < 0.01). The expression levels of genes in MSCs treated with the 50% NPC conditioned medium also showed upregulation compared with the control group (collagen II, p < 0.05; KRT8, p < 0.05; and KRT19, p < 0.01). These findings suggested that the NPC conditioned medium stimulated MSC differentiation into an NP-like phenotype with distinct characteristics. The results could inform strategies for IVD regeneration.

CITED2 and the modulation of the hypoxic response in cancer

CITED2 (CBP/p300-interacting transactivator with Glu/Asp-rich C-terminal domain, 2) is a ubiquitously expressed protein exhibiting a high affinity for the CH1 domain of the transcriptional co-activators CBP/p300, for which it competes with hypoxia-inducible factors (HIFs). CITED2 is particularly efficient in the inhibition of HIF-1α-dependent transcription in different contexts, ranging from organ development and metabolic homeostasis to tissue regeneration and immunity, being also potentially involved in various other physiological processes. In addition, CITED2 plays an important role in inhibiting HIF in some diseases, including kidney and heart diseases and type 2-diabetes. In the particular case of cancer, CITED2 either functions by promoting or suppressing cancer development depending on the context and type of tumors. For instance, CITED2 overexpression promotes breast and prostate cancers, as well as acute myeloid leukemia, while its expression is downregulated to sustain colorectal cancer and hepatocellular carcinoma. In addition, the role of CITED2 in the maintenance of cancer stem cells reveals its potential as a target in non-small cell lung carcinoma and acute myeloid leukemia, for example. But besides the wide body of evidence linking both CITED2 and HIF signaling to carcinogenesis, little data is available regarding CITED2 role as a negative regulator of HIF-1α specifically in cancer. Therefore, comprehensive studies exploring further the interactions of these two important mediators in cancer-specific models are sorely needed and this can potentially lead to the development of novel targeted therapies.

Effects of IL-1β, IL-20, and BMP-2 on Intervertebral Disc Inflammation under Hypoxia

Intervertebral disc (IVD) is an avascular tissue under hypoxic condition after adulthood. Our previous data showed that inflammatory cytokines (interleukin (IL)-1β), IL-20, and bone morphogenetic protein-2 (BMP-2) play important roles in the healing process after disc injury. In the current study, we investigated whether IL-1β, IL-20, or BMP-2 modulate the expression of pro-inflammatory cytokines, chemotaxis factor, and angiogenesis factor on IVD cells under hypoxia. IVD cells were isolated from patients with intervertebral disc herniation (HIVD) at the levels of L4–5 and L5–S1. We found that the expression of IL-1β, IL-20, BMP-2, hypoxia-inducible factor (HIF)-1α, IL-6, IL-8, angiogenetic factor (vascular endothelial growth factor (VEGF)), chemotactic factor (monocyte chemoattractant protein 1 (MCP-1)), and matrix metalloproteinase-3 (MMP-3) was upregulated in IVD cells under hypoxia conditions. In addition, IL-1β upregulated the expression of pro-inflammatory cytokines (IL-6 and IL-8), VEGF, MCP-1, and disc degradation factor (MMP-3) in IVD cells under hypoxia conditions. IL-20 upregulated MCP-1 and VEGF expression. BMP-2 also upregulated the expression of MCP-1, VEGF, and IL-8 in IVD cells under hypoxia conditions. Treatment with antibody against IL-1β decreased VEGF and MMP-3 expression, while treatment with IL-20 or BMP-2 antibodies decreased MCP-1, VEGF, and MMP-3 expression. Moreover, IL-1β modulated both the expression of IL-20 and BMP-2, but IL-20 only modulated BMP-2 either under a hypoxic or normoxic condition. Therefore, we concluded that the inflammation, chemotaxis, matrix degradation, and angiogenesis after disc herniation are influenced by the hypoxic condition and controlled by IL-1β, IL-20, and BMP-2.

Elevated expression of hypoxia-inducible factor-2alpha regulated catabolic factors during intervertebral disc degeneration

HEADINGS AIMS

The present study determined whether nucleus pulposus (NP) cells express hypoxia-inducible factor-2alpha (HIF-2α) and assessed its role in regulating the expression of catabolic factors during intervertebral disc degeneration.

MATERIALS AND METHODS

Human degenerated NP tissues were acquired to examine the HIF-2α expression levels using immunohistochemistry, western blotting, and Real-time PCR. Human NP cells were cultivated under normoxic or hypoxic conditions, and the HIF-2α expression was determined. Then, human NP cells were treated with HIF-2α plasmids, HIF-2α siRNA, and tumor necrosis factor-alpha (TNF-α) to evaluate the role of HIF-2α in regulating matrix metalloproteinase (MMP) and aggrecanase expression. An in vivo rabbit disc degeneration model was established to demonstrate that HIF-2α plays a critical role in disc degeneration.

KEY FINDINGS

We found that HIF-2α had a markedly elevated expression in human degenerated discs in the Grade III stage. HIF-2α protein and gene transcript levels in vitro were relatively higher under hypoxic conditions. The expression of MMP-13, ADAMTS-4 was decreased significantly in HIF-2α silencing condition, while the over-expression resulted in significantly increased levels of MMP-13 and ADAMTS-4. When cytokine TNF-α was added, HIF-2α was induced by nuclear factor-κB (NF-κB). The in vivo experiments showed that the HIF-2α controlled the catabolic factors MMP-13 and ADAMTS-4 that regulated the collagen II and aggrecan metabolism in disc degeneration.

SIGNIFICANCE

HIF-2α is a catabolic regulator in disc degeneration and directly controls the catabolic genes. The suppression of HIF-2α expression leads to decelerates extracellular matrix degradation that might represent a therapeutic target for the degenerative disc.

RNA binding protein HuR regulates extracellular matrix gene expression and pH homeostasis independent of controlling HIF-1α signaling in nucleus pulposus cells

Nucleus pulposus (NP) cells reside in the hypoxic niche of the intervertebral disc. Studies have demonstrated that RNA-binding protein HuR modulates hypoxic signaling in several cancers, however, its function in the disc is unknown. HuR did not show cytoplasmic translocation in hypoxia and its silencing did not alter levels of Hif-1α or HIF-targets in NP cells. RNA-Sequencing data revealed that important extracellular matrix-related genes including several collagens, MMPs, aggrecan, Tgf-β3 and Sdc4 were regulated by HuR. Further analysis of HuR-silenced NP cells confirmed that HuR maintained expression of these matrix genes. We confirmed decreased levels of secreted collagen I and Sdc4 and increased pro-MMP13 in HuR-knockdown cells. In addition, messenger ribonucleoprotein immunoprecipitation demonstrated HuR binding to Tgf-β3 and Sdc4 mRNAs. Interestingly, while HuR bound to Hif-1α and Vegf mRNAs, it was clear that compensatory mechanisms sustained their expression when HuR was silenced. Noteworthy, despite the presence of multiple HuR-binding sites and reported interaction in other cell types, HuR showed no binding to Pgk1, Eno1, Pdk1 and Pfkfb3 in NP cells. Metabolic studies showed a significant decrease in the extracellular acidification rate (ECAR) and mitochondrial oxygen consumption rate (OCR) and acidic pH in HuR-silenced NP cells, without appreciable change in total OCR. These changes were likely due to decreased Ca12 expression in HuR silenced cells. Taken together, our study demonstrates for the first time that HuR regulates extracellular matrix (ECM) and pH homeostasis of NP cells and has important implications in the maintenance of intervertebral disc health.

The transcription factor Vezf1 represses the expression of the antiangiogenic factor Cited2 in endothelial cells

Formation of the vasculature by angiogenesis is critical for proper development, but angiogenesis also contributes to the pathogenesis of various disorders, including cancer and cardiovascular diseases. Vascular endothelial zinc finger 1 (Vezf1), is a Krüppel-like zinc finger protein that plays a vital role in vascular development. However, the mechanism by which Vezf1 regulates this process is not fully understood. Here, we show that Vezf1^−/− mouse embryonic stem cells (ESC) have significantly increased expression of a stem cell factor, Cbp/p300-interacting transactivator 2 (Cited2). Compared with WT ESCs, Vezf1^−/− ESCs inefficiently differentiated into endothelial cells (ECs), which exhibited defects in the tube-formation assay. These defects were due to reduced activation of EC-specific genes concomitant with lower enrichment of histone 3 acetylation at Lys²⁷ (H3K27) at their promoters. We hypothesized that overexpression of Cited2 in Vezf1^−/− cells sequesters P300/CBP away from the promoters of proangiogenic genes and thereby contributes to defective angiogenesis in these cells. This idea was supported by the observation that shRNA-mediated depletion of Cited2 significantly reduces the angiogenic defects in the Vezf1^−/− ECs. In contrast to previous studies that have focused on the role of Vezf1 as a transcriptional activator of proangiogenic genes, our findings have revealed a role for Vezf1 in modulating the expression of the antiangiogenic factor Cited2. Vezf1 previously has been characterized as an insulator protein, and our results now provide insights into the mechanism, indicating that Vezf1 can block inappropriate, nonspecific interactions of promoters with cis-located enhancers, preventing aberrant promoter activation.

The Role of Hypoxia in Angiogenesis and Extracellular Matrix Regulation of Intervertebral Disc Cells During Inflammatory Reactions

BACKGROUND

The intervertebral disc (IVD) is an avascular structure, and is therefore stable under hypoxic conditions. Previous studies have demonstrated that hypoxia might be related to symptomatic degenerative disc diseases (DDDs); however, the pathomechanism is still poorly understood.

OBJECTIVE

To identify the effect of hypoxia on the production of inflammatory mediators, angiogenic factors, and extracellular matrix-regulating enzymes of IVD cells during inflammatory reactions.

METHODS

Human nucleus pulposus (NP) and annulus fibrosus (AF) cells harvested during surgery for DDDs were cultured in macrophage conditioned media or interleukin (IL)-1β-stimulated media under hypoxic (2%) and normoxic (21%) conditions. Hypoxia-inducible factor-1α transcription factor activation was analyzed by western blotting. IL-6, IL-8, vascular endothelial growth factor (VEGF), vascular cell adhesion molecule (VCAM), matrix metalloproteinase (MMP)-1, MMP-3, tissue inhibitor of metalloprotease (TIMP)-1, and TIMP-2 in conditioned media were measured by an enzyme-linked immunosorbent assay.

RESULTS

NP cells expressed higher hypoxia-inducible factor-1α in the IL-1β-stimulated group under hypoxic condition. MMP-1 was significantly increased in the AF cells under hypoxic condition; TIMP-1 and TIMP-2 were significantly decreased in both naïve NP and AF cells during hypoxia. Both cells in macrophage conditioned media significantly diminished the production of IL-6 and VCAM, while VEGF significantly increased during hypoxia. After 1 ng/mL IL-1β stimulation, IL-8, VEGF, MMP-1, and MMP-3 were significantly increased in both cell types during hypoxia, while VCAM, TIMP-1, and TIMP-2 were decreased.

CONCLUSION

We found that hypoxia can enhance the angiogenic ability of IVD during inflammatory reactions, and cause progress in development of DDD via extracellular matrix regulation in this in vitro study.

RNA sequencing identifies gene regulatory networks controlling extracellular matrix synthesis in intervertebral disk tissues

Degenerative disk disease of the spine is a major cause of back pain and disability. Optimization of regenerative medical therapies for degenerative disk disease requires a deep mechanistic understanding of the factors controlling the structural integrity of spinal tissues. In this investigation, we sought to identify candidate regulatory genes controlling extracellular matrix synthesis in spinal tissues. To achieve this goal we performed high throughput next generation RNA sequencing on 39 annulus fibrosus and 21 nucleus pulposus human tissue samples. Specimens were collected from patients undergoing surgical discectomy for the treatment of degenerative disk disease. Our studies identified associations between extracellular matrix genes, growth factors, and other important regulatory molecules. The fibrous matrix characteristic of annulus fibrosus was associated with expression of the growth factors platelet derived growth factor beta (PDGFB), vascular endothelial growth factor C (VEGFC), and fibroblast growth factor 9 (FGF9). Additionally we observed high expression of multiple signaling proteins involved in the NOTCH and WNT signaling cascades. Nucleus pulposus extracellular matrix related genes were associated with the expression of numerous diffusible growth factors largely associated with the transforming growth signaling cascade, including transforming factor alpha (TGFA), inhibin alpha (INHA), inhibin beta A (INHBA), bone morphogenetic proteins (BMP2, BMP6), and others.

CLINICAL SIGNIFICANCE

this investigation provides important data on extracellular matrix gene regulatory networks in disk tissues. This information can be used to optimize pharmacologic, stem cell, and tissue engineering strategies for regeneration of the intervertebral disk and the treatment of back pain. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1356-1369, 2018.

Down-regulation of CITED2 attenuates breast tumor growth, vessel formation and TGF-β-induced expression of VEGFA

While we previously demonstrated that CITED2 expression in primary breast tumor tissues is elevated relative to normal mammary epithelium and inversely correlated with patient survival, its functional impact on primary tumor development and progression remained unknown. To address this issue, we examined the effect of CITED2 silencing on the growth of human breast cancer cell lines MDA-MB-231 and MDA-MB-468 following orthotopic administration in vivo. Here, we show that CITED2 silencing significantly attenuated MDA-MB-231 primary tumor growth concordant with reduced tumor vascularization, while MDA-MB-468 primary tumor growth and tumor vascularization remained unaffected. Correspondingly, expression of VEGFA was significantly reduced in shCITED2-expressing MDA-MB-231, but not MDA-MB-468 tumors. Consistent with the observed pattern of vascularization and VEGFA expression, we found that TGF-β stimulation induced expression of VEGFA and enhanced CITED2 recruitment to the VEGFA promoter in MDA-MA-231 cells, while failing to induce VEGFA expression in MDA-MB-468 cells. Further supporting its involvement in TGF-β-induced expression of VEGFA, CITED2 silencing prevented TGF-β induction of VEGFA expression in MDA-MB-231 cells. Collectively, these data indicate that CITED2 regulates primary breast tumor growth, likely by influencing tumor vasculature via TGF-β-dependent regulation of VEGFA.

IL-17 induces reactive astrocytes and up-regulation of vascular endothelial growth factor (VEGF) through JAK/STAT signaling

Spinal cord injury is a grave neurological disability resulting in neuron degeneration and permanent paralysis. The inflammation triggered by the injury would promote the spinal cord lesion in turn. Activated astrocytes during inflammatory response could promote glial scar formation and contribute to the progression of the spinal cord injury. Interleukin 17 (IL-17) was upregulated in inflammatory responses to contusion or compression of the spinal cord. in this study, IL-17 could induce reactive astrocytes which was indicated by a well-known hallmark glial fibrillary acidic protein (GFAP) in vitro and in vivo. Moreover, we demonstrated that the upregulation of VEGF was induced by IL-17 human astrocytoma cells. In our further investigation, IL-17 induced the expression of VEGF in spinal cord injury by activating JAK/STAT signaling pathway both in vitro and in vivo. In addition, we also found that IL-17 significantly changed tissue preservation and residual urine volumes and blood-spinal cord-barrier integrity in vivo. This newly found IL-17-JAK/STAT-VEGF axis improves our understanding of the molecular mechanism of spinal cord injury during inflammatory response and provides another potential target of spinal cord injury.

The influence of oxygen concentration on the extracellular matrix production of human nucleus pulposus cells during isolation-expansion process

Nucleus pulposus (NP) cells locate in the center of avascular intervertebral discs, and thus have presumably adapted to a hypoxic environment. The purpose of this study was to investigate the influences of hypoxic condition, during isolation-expansion of human NP cells, on the cellular proliferation and extracellular matrix (ECM) synthesis in later three-dimensional cultures. Human NP tissues were obtained from patients who underwent lumbar disc surgeries. Immediately after retrieval, NP tissues from each patient were divided into two aliquots for in vitro cultivation either under classical normoxic (21% O₂ ) or hypoxic (3.5% O₂ ) condition. After isolation-expansion processes, microtissues of NP cells were formed and the analysis was performed after one-week culture. Experiments of pretreatment with TGF-β1 or lovastatin were designed to investigate if the isolation-expansion conditions affect the responsiveness to later exogenous treatments. Hypoxic isolation-expansion stimulated NP cell proliferation during monolayer culture. Hypoxia also upregulated mRNA levels of SOX9 and HIF-1α but downregulated type X collagen as well as improved aggrecan and type II collagen synthesis. Although TGF-β1 had no substantial effect, lovastatin pretreatment showed a greater enhancement on type II collagen expression in hypoxic group. Normoxia negatively affected the biochemical composition of regenerated ECM attributable to downregulation of SOX9 and HIF-1α, while hypoxia enhanced cellular proliferation, improved matrix production, and maintained a functional phenotype of NP cells. Hypoxic isolation-expansion of human NP cells is important to achieve better regenerative cells for later cultivation or cell transplantation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 1575-1582, 2017.

Molecular mechanisms of biological aging in intervertebral discs

Advanced age is the greatest risk factor for the majority of human ailments, including spine-related chronic disability and back pain, which stem from age-associated intervertebral disc degeneration (IDD). Given the rapid global rise in the aging population, understanding the biology of intervertebral disc aging in order to develop effective therapeutic interventions to combat the adverse effects of aging on disc health is now imperative. Fortunately, recent advances in aging research have begun to shed light on the basic biological process of aging. Here we review some of these insights and organize the complex process of disc aging into three different phases to guide research efforts to understand the biology of disc aging. The objective of this review is to provide an overview of the current knowledge and the recent progress made to elucidate specific molecular mechanisms underlying disc aging. In particular, studies over the last few years have uncovered cellular senescence and genomic instability as important drivers of disc aging. Supporting evidence comes from DNA repair-deficient animal models that show increased disc cellular senescence and accelerated disc aging. Additionally, stress-induced senescent cells have now been well documented to secrete catabolic factors, which can negatively impact the physiology of neighboring cells and ECM. These along with other molecular drivers of aging are reviewed in depth to shed crucial insights into the underlying mechanisms of age-related disc degeneration. We also highlight molecular targets for novel therapies and emerging candidate therapeutics that may mitigate age-associated IDD. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1289-1306, 2016.

Interleukin-17 upregulates vascular endothelial growth factor by activating the JAK/STAT pathway in nucleus pulposus cells

OBJECTIVES

Intervertebral disc (IVD) related diseases and age-related IVD degeneration are responsible for significant morbidity. Inflammatory mediators and pro-inflammatory cytokines, including interleukin (IL)-17, show elevated expression in degenerated disc tissue. IL-17 is reported to transduce signals across the cell membrane predominantly via the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signal transduction pathway, leading to transcriptional activation of target genes.

METHODS

In this study, we investigated whether the JAK/STAT pathway plays a role in IL-17-mediated signaling in the nucleus pulposus (NP) cells of IVDs. Vascular endothelial growth factor (VEGF) and IL-17 were found to be highly expressed in human degenerated NP tissue. In isolated rat NP cells, IL-17-induced VEGF expression in a time- and dose-dependent manner. Rat NP cells were co-transfected with VEGF promoter plasmid along with constitutively active STAT1, STAT3 or JAK2 plasmid. VEGF promoter activity was found to be increased by STAT1, STAT3 and JAK2 in IL-17-treated cells. Transfection of cultured rat NP cells with STAT1 or STAT3 lentiviral short hairpin RNAs or treatment with the JAK2 inhibitor AG490 significantly reduced IL-17-stimulated VEGF expression.

CONCLUSIONS

IL-17 upregulated VEGF expression in rat NP cells mediated by the JAK/STAT pathway, and elevated levels of IL-17 and VEGF are present in human degenerated NP tissue. These findings provide new insight into the pathology of IVD degeneration.

Hypoxia promotes noncanonical autophagy in nucleus pulposus cells independent of MTOR and HIF1A signaling

Nucleus pulposus (NP) cells reside in the avascular and hypoxic microenvironment of intervertebral discs. Importantly, many activities related to survival and function of NP cells are controlled by the HIF-family of transcription factors. We hypothesize that NP cells adapt to their hypoxic niche through modulation of macroautophagy/autophagy. In various cell types, hypoxia induces autophagy in a HIF1A-dependent fashion; however, little is known about hypoxic regulation of autophagy in NP cells. Hypoxia increases the number of autophagosomes as seen by TEM analysis and LC3-positive puncta in NP cells. Hypoxic induction of autophagy was also demonstrated by a significantly higher number of autophagosomes and smaller change in autolysosomes in NP cells expressing tandem-mCherry-EGFP-LC3B. Increased LC3-II levels were not accompanied by a concomitant increase in BECN1 or the ATG12-ATG5 complex. In addition, ULK1 phosphorylation at Ser757 and Ser777 responsive to MTOR and AMPK, respectively, was not affected in hypoxia. Interestingly, when MTOR activity was inhibited by rapamycin or Torin1, LC3-II levels did not change, suggesting a novel MTOR-independent regulation. Noteworthy, while silencing of HIF1A affected hypoxic induction of BNIP3, it did not affect LC3-II levels, indicating hypoxia-induced autophagy is HIF1-independent. Importantly, there was no change in the number of LC3-positive autophagosomes in NP-specific Hif1a null mice. Finally, inhibition of autophagic flux did not affect the glycolytic metabolism of NP cells, suggesting a possible nonmetabolic role of autophagy. Taken together, our study for the first time shows that NP cells regulate autophagy in a noncanonical fashion independent of MTOR and HIF1A signaling.

Hypoxia regulates sumoylation pathways in intervertebral disc cells: implications for hypoxic adaptations

OBJECTIVE

To explore the hypoxic regulation of sumoylation pathways and cell viability in nucleus pulposus (NP) and annulus fibrosus (AF) cells.

DESIGN

Expression of small ubiquitin-like modifier (SUMO) molecules, SUMO E1 activating enzymes SAE1 and SAE2, SUMO E2 conjugating enzyme UBC9, and de-sumoylation enzyme sentrin/SUMO-specific proteases (SENP)1 was immunolocalized in rat intervertebral disc (IVD) cells. NP and AF cells were cultured in hypoxia and cell viability was evaluated by quantifying cell proliferation, cellular senescence, apoptosis, and cell cycle distribution. Hypoxic regulation of sumoylation pathways was studied by analyzing the transcription and expression of SUMO molecules and sumoylation enzymes. Loss of function study using SENP1 siRNA was performed to investigate the regulatory role of sumoylation on the function of hypoxia inducible factor 1α (HIF-1α) and the hypoxic tolerance of IVD cells.

RESULTS

Sumoylation pathways were expressed in IVD cells and localized predominantly in nuclei. Both NP and AF cells maintained viability under hypoxia and upregulated the expression of SENP1. In NP cells hypoxia transiently increased the expression of SUMO-1, SUMO-2/3, SAE2, and UBC9, whereas SUMO-1 was elevated while SUMO-2/3, SAE1, SAE2, and UBC9 were reduced by low oxygen tensions in AF cells. Although downregulation of SENP1 decreased the transcriptional activity of HIF-1α, the viability of disc cells showed no significant loss under hypoxia.

CONCLUSIONS

NP and AF cells equally tolerate oxygen deficiency, but differently regulate the sumoylation pathways under hypoxia. The distinct sumoylation dynamics may help extend our understanding of the cell-specific regulation of the molecular basis that promotes cell survival in the hypoxic IVD.

Class I and IIa HDACs Mediate HIF-1α Stability Through PHD2-Dependent Mechanism, While HDAC6, a Class IIb Member, Promotes HIF-1α Transcriptional Activity in Nucleus Pulposus Cells of the Intervertebral Disc

The objective of this study was to determine the role of histone deacetylases (HDACs) in regulating HIF-1α protein stability and activity in nucleus pulposus (NP) cells. Treatment of NP cells with pan-HDAC inhibitor TSA resulted in decreased HIF-1α levels under both normoxia and hypoxia in a dose-dependent fashion. TSA-mediated HIF-1α degradation was rescued by concomitant inhibition of not only the 26S proteasome but also PHD2 function. Moreover, TSA treatment of PHD2(-/-) cells had little effect on HIF-1α levels, supporting the notion that inhibition of PHD2 function by HDACs contributed to HIF-1α stabilization. Surprisingly, class-specific HDAC inhibitors did not affect HIF-1α protein stability, indicating that multiple HDACs controlled HIF-1α stability by regulating HIF-1α-PHD2 interaction in NP cells. Interestingly, lower-dose TSA that did not affect HIF-1α stability decreased its activity and target gene expression. Likewise, rescue of TSA-mediated HIF-1α protein degradation by blocking proteasomal or PHD activity did not restore HIF-1 activity, suggesting that HDACs independently regulate HIF-1α stability and activity. Noteworthy, selective inhibition of HDAC6 and not of class I and IIa HDACs decreased HIF-1-mediated transcription under hypoxia to a similar extent as lower-dose TSA, contrasting the reported role of HDAC6 as a transcriptional repressor in other cell types. Moreover, HDAC6 inhibition completely blocked TSA effects on HIF-1 activity. HDAC6 associated with and deacetylated HSP90, an important cofactor for HIF-1 function in NP cells, and HDAC6 inhibition decreased p300 transactivation in NP cells. Taken together, these results suggest that although multiple class I and class IIa HDACs control HIF-1 stability, HDAC6, a class IIb HDAC, is a novel mediator of HIF-1 activity in NP cells possibly through promoting action of critical HIF-1 cofactors. © 2016 American Society for Bone and Mineral Research.

Angiopoietin-1 receptor Tie2 distinguishes multipotent differentiation capability in bovine coccygeal nucleus pulposus cells

BACKGROUND

The intervertebral disc (IVD) has limited self-healing potential and disc repair strategies require an appropriate cell source such as progenitor cells that could regenerate the damaged cells and tissues. The objective of this study was to identify nucleus pulposus-derived progenitor cells (NPPC) and examine their potential in regenerative medicine in vitro.

METHODS

Nucleus pulposus cells (NPC) were obtained from 1-year-old bovine coccygeal discs by enzymatic digestion and were sorted for the angiopoietin-1 receptor Tie2. The obtained Tie2- and Tie2+ fractions of cells were differentiated into osteogenic, adipogenic, and chondrogenic lineages in vitro. Colony-forming units were prepared from both cell populations and the colonies formed were analyzed and quantified after 8 days of culture. In order to improve the preservation of the Tie2+ phenotype of NPPC in monolayer cultures, we tested a selection of growth factors known to have stimulating effects, cocultured NPPC with IVD tissue, and exposed them to hypoxic conditions (2 % O2).

RESULTS

After 3 weeks of differentiation culture, only the NPC that were positive for Tie2 were able to differentiate into osteocytes, adipocytes, and chondrocytes as characterized by calcium deposition (p < 0.0001), fat droplet formation (p < 0.0001), and glycosaminoglycan content (p = 0.0095 vs. Tie2- NPC), respectively. Sorted Tie2- and Tie2+ subpopulations of cells both formed colonies; however, the colonies formed from Tie2+ cells were spheroid in shape, whereas those from Tie2- cells were spread and fibroblastic. In addition, Tie2+ cells formed more colonies in 3D culture (p = 0.011) than Tie2- cells. During expansion, a fast decline in the fraction of Tie2+ cells was observed (p < 0.0001), which was partially reversed by low oxygen concentration (p = 0.0068) and supplementation of the culture with fibroblast growth factor 2 (FGF2) (p < 0.0001).

CONCLUSIONS

Our results showed that the bovine nucleus pulposus contains NPPC that are Tie2+. These cells fulfilled formally progenitor criteria that were maintained in subsequent monolayer culture for up to 7 days by addition of FGF2 or hypoxic conditions. We propose that the nucleus pulposus represents a niche of precursor cells for regeneration of the IVD.

Aquaporin 1 and 5 expression decreases during human intervertebral disc degeneration: Novel HIF-1-mediated regulation of aquaporins in NP cells

Objectives of this study were to investigate whether AQP1 and AQP5 expression is altered during intervertebral disc degeneration and if hypoxia and HIF-1 regulate their expression in NP cells. AQP expression was measured in human tissues from different degenerative grades; regulation by hypoxia and HIF-1 was studied using promoter analysis and gain- and loss-of-function experiments. We show that both AQPs are expressed in the disc and that mRNA and protein levels decline with human disease severity. Bioinformatic analyses of AQP promoters showed multiple evolutionarily conserved HREs. Surprisingly, hypoxia failed to induce promoter activity or expression of either AQP. While genomic chromatin immunoprecipitation showed limited binding of HIF-1α to conserved HREs, their mutation did not suppress promoter activities. Stable HIF-1α suppression significantly decreased mRNA and protein levels of both AQPs, but HIF-1α failed to induce AQP levels following accumulation. Together, our results demonstrate that AQP1 and AQP5 expression is sensitive to human disc degeneration and that HIF-1α uniquely maintains basal expression of both AQPs in NP cells, independent of oxemic tension and HIF-1 binding to promoter HREs. Diminished HIF-1 activity during degeneration may suppress AQP levels in NP cells, compromising their ability to respond to extracellular osmolarity changes.

Soluble factors from the notochordal-rich intervertebral disc inhibit endothelial cell invasion and vessel formation in the presence and absence of pro-inflammatory cytokines

BACKGROUND

Chronic low back pain can be associated with the pathological ingrowth of blood vessels and nerves into intervertebral discs (IVDs). The notochord patterns the IVD during development and is a source of anti-angiogenic soluble factors such as Noggin and Chondroitin sulfate (CS). These factors may form the basis for a new minimally invasive strategy to target angiogenesis in the IVD.

OBJECTIVE

To examine the anti-angiogenic potential of soluble factors from notochordal cells (NCs) and candidates Noggin and CS under healthy culture conditions and in the presence of pro-inflammatory mediators.

DESIGN

NC conditioned media (NCCM) was generated from porcine NC-rich nucleus pulposus tissue. To assess the effects of NCCM, CS and Noggin on angiogenesis, cell invasion and tubular formation assays were performed using human umbilical vein endothelial cells (HUVECs) ± tumor necrosis factor alpha (TNFα [10 ng/ml]). vascular endothelial growth factor (VEGF)-A, MMP-7, interleukin-6 (IL-6) and IL-8 mRNA levels were assessed using qRT-PCR.

RESULTS

NCCM (10 & 100%), CS (10 and 100 μg) and Noggin (10 and 100 ng) significantly decreased cell invasion of HUVECs with and without TNFα. NCCM 10% and Noggin 10 ng inhibited tubular formation with and without TNFα and CS 100 μg inhibited tubules in Basal conditions whereas CS 10 μg inhibited tubules with TNFα. NCCM significantly decreased VEGF-A, MMP-7 and IL-6 mRNA levels in HUVECs with and without TNFα. CS and Noggin had no effects on gene expression.

CONCLUSIONS

We provide the first evidence that soluble factors from NCs can inhibit angiogenesis by suppressing VEGF signaling. Notochordal-derived ligands are a promising minimally invasive strategy targeting neurovascular ingrowth and pain in the degenerated IVD.

Defining the phenotype of young healthy nucleus pulposus cells: recommendations of the Spine Research Interest Group at the 2014 annual ORS meeting

Low back pain is a major physical and socioeconomic problem. Degeneration of the intervertebral disc and especially that of nucleus pulposus (NP) has been linked to low back pain. In spite of much research focusing on the NP, consensus among the research community is lacking in defining the NP cell phenotype. A consensus agreement will allow easier distinguishing of NP cells from annulus fibrosus (AF) cells and endplate chondrocytes, a better gauge of therapeutic success, and a better guidance of tissue-engineering-based regenerative strategies that attempt to replace lost NP tissue. Most importantly, a clear definition will further the understanding of physiology and function of NP cells, ultimately driving development of novel cell-based therapeutic modalities. The Spine Research Interest Group at the 2014 Annual ORS Meeting in New Orleans convened with the task of compiling a working definition of the NP cell phenotype with hope that a consensus statement will propel disc research forward into the future. Based on evaluation of recent studies describing characteristic NP markers and their physiologic relevance, we make the recommendation of the following healthy NP phenotypic markers: stabilized expression of HIF-1α, GLUT-1, aggrecan/collagen II ratio >20, Shh, Brachyury, KRT18/19, CA12, and CD24.

Stem cell therapy for intervertebral disc regeneration: obstacles and solutions

Intervertebral disc (IVD) degeneration is frequently associated with low back and neck pain, which accounts for disability worldwide. Despite the known outcomes of the IVD degeneration cascade, the treatment of IVD degeneration is limited in that available conservative and surgical treatments do not reverse the pathology or restore the IVD tissue. Regenerative medicine for IVD degeneration, by injection of IVD cells, chondrocytes or stem cells, has been extensively studied in the past decade in various animal models of induced IVD degeneration, and has progressed to clinical trials in the treatment of various spinal conditions. Despite preliminary results showing positive effects of cell-injection strategies for IVD regeneration, detailed basic research on IVD cells and their niche indicates that transplanted cells are unable to survive and adapt in the avascular niche of the IVD. For this therapeutic strategy to succeed, the indications for its use and the patients who would benefit need to be better defined. To surmount these obstacles, the solution will be identified only by focused research, both in the laboratory and in the clinic.

Loss of HIF-1α in the notochord results in cell death and complete disappearance of the nucleus pulposus

The intervertebral disc (IVD) is one of the largest avascular organs in vertebrates. The nucleus pulposus (NP), a highly hydrated and proteoglycan-enriched tissue, forms the inner portion of the IVD. The NP is surrounded by a multi-lamellar fibrocartilaginous structure, the annulus fibrosus (AF). This structure is covered superior and inferior side by cartilaginous endplates (CEP). The NP is a unique tissue within the IVD as it results from the differentiation of notochordal cells, whereas, AF and CEP derive from the sclerotome. The hypoxia inducible factor-1α (HIF-1α) is expressed in NP cells but its function in NP development and homeostasis is largely unknown. We thus conditionally deleted HIF-1α in notochordal cells and investigated how loss of this transcription factor impacts NP formation and homeostasis at E15.5, birth, 1 and 4 months of age, respectively. Histological analysis, cell lineage studies, and TUNEL assay were performed. Morphologic changes of the mutant NP cells were identified as early as E15.5, followed, postnatally, by the progressive disappearance and replacement of the NP with a novel tissue that resembles fibrocartilage. Notably, lineage studies and TUNEL assay unequivocally proved that NP cells did not transdifferentiate into chondrocyte-like cells but they rather underwent massive cell death, and were completely replaced by a cell population belonging to a lineage distinct from the notochordal one. Finally, to evaluate the functional consequences of HIF-1α deletion in the NP, biomechanical testing of mutant IVD was performed. Loss of the NP in mutant mice significantly reduced the IVD biomechanical properties by decreasing its ability to absorb mechanical stress. These findings are similar to the changes usually observed during human IVD degeneration. Our study thus demonstrates that HIF-1α is essential for NP development and homeostasis, and it raises the intriguing possibility that this transcription factor could be involved in IVD degeneration in humans.

Regulation of CYP11B1 and CYP11B2 steroidogenic genes by hypoxia-inducible miR-10b in H295R cells

Although numerous studies have shown that hypoxia affects cortisol and aldosterone production in vivo, the underlying molecular mechanisms regulating the steroidogenic genes of these steroid hormones are still poorly known. MicroRNAs are post-transcriptional regulators that control diverse biological processes and this study describes the identification and validation of the hypoxia-inducible microRNA, miR-10b, as a negative regulator of the CYP11B1 and CYP11B2 steroidogenic genes in H295R human adrenocortical cells. Using the human TaqMan Low Density miRNA Arrays, we determined the miRNA expression patterns in H295R cells under normoxic and hypoxic conditions, and in cells overexpressing the human HIF-1α. Computer analysis using three in silico algorithms predicted that the hypoxia-inducible miR-10b molecule targets CYP11B1 and CYP11B2 mRNAs. Gene transfection studies of luciferase constructs containing the 3'-untranslated region of CYP11B1 or CYP11B2, combined with miRNA overexpression and knockdown experiments provide compelling evidence that CYP11B1 and CYP11B2 mRNAs are likely targets of miR-10b.

Responses and adaptations of intervertebral disc cells to microenvironmental stress: a possible central role of autophagy in the adaptive mechanism

Intervertebral discs comprise the largest avascular cartilaginous organ in the body, and its nutrient condition can be impaired by degeneration, aging and even metabolic disease. The unique microenvironment brings special stresses to various disc cell types, including nucleus pulposus cells, notochordal cells, annulus fibrosus cells and endplate chondrocytes. These cells experience nutrient starvation, acidic stress, hypoxic stress, hyperglycemic stress, osmotic stress and mechanical stress. Understanding the detailed responses and complex adaptive mechanisms of disc cells to various stresses might provide some clues to guide therapy for disc degeneration. By reviewing the published literatures describing disc cells under different hostile microenvironments, we conclude that these cells exhibit different responses to microenvironmental stresses with different mechanisms. Moreover, the interaction and combination of these stresses create a complex environment that synergistically increase or decrease influences on disc cells, compared with the effects of a single stress. Interestingly, most of these stresses activate autophagy, a self-protective mechanism by which dysfunctional protein and organelles are degraded. It is becoming clear that autophagy facilitates the cellular adaptation to stresses and might play a central role in regulating the adaptation of disc cells under stress. Therefore, autophagy modulation might be a potential therapeutic method to treat disc degeneration.

Variations of CITED2 are associated with congenital heart disease (CHD) in Chinese population

CITED2 was identified as a cardiac transcription factor which is essential to the heart development. Cited2-deficient mice showed cardiac malformations, adrenal agenesis and neural crest defects. To explore the potential impact of mutations in CITED2 on congenital heart disease (CHD) in humans, we screened the coding region of CITED2 in a total of 700 Chinese people with congenital heart disease and 250 healthy individuals as controls. We found five potential disease-causing mutations, p.P140S, p.S183L, p.S196G, p.Ser161delAGC and p. Ser192_Gly193delAGCGGC. Two mammalian two-hybrid assays showed that the last four mutations significantly affected the interaction between p300CH1 and CITED2 or HIF1A. Further studies showed that four CITED2 mutations recovered the promoter activity of VEGF by decreasing its competitiveness with HIF1A for binding to p300CH1 and three mutations decreased the consociation of TFAP2C and CITED2 in the transactivation of PITX2C. Both VEGF and PITX2C play very important roles in cardiac development. In conclusion, we demonstrated that CITED2 has a potential causative impact on congenital heart disease.

Human dendritic cells exhibit a pronounced type I IFN signature following Leishmania major infection that is required for IL-12 induction

Leishmania major-infected human dendritic cells (DCs) exhibit a marked induction of IL-12, ultimately promoting a robust Th1-mediated response associated with parasite killing and protective immunity. The host cell transcription machinery associated with the specific IL-12 induction observed during L. major infection remains to be thoroughly elucidated. In this study, we used Affymetrix GeneChip (Affymetrix) to globally assess the host cell genes and pathways associated with early L. major infection in human myeloid-derived DCs. Our data revealed 728 genes were significantly differentially expressed and molecular signaling pathway revealed that the type I IFN pathway was significantly enriched. Addition of a neutralizing type I IFN decoy receptor blocked the expression of IRF7 and IL-12p40 during DC infection, indicating the L. major-induced expression of IL-12p40 is dependent upon the type I IFN signaling pathway. In stark contrast, IL-12p40 expression is not elicited by L. donovani, the etiological agent of deadly visceral leishmaniasis. Therefore, we examined the gene expression profile for several IFN response genes in L. major versus L. donovani DC infections. Our data revealed that L. major, but not L. donovani, induces expression of IRF2, IRF7, and IFIT5, implicating the regulation of type I IFN-associated signaling pathways as mediating factors toward the production of IL-12.

SHH-dependent knockout of HIF-1 alpha accelerates the degenerative process in mouse intervertebral disc

Hypoxia-inducible factor-1alpha (HIF-1 alpha) has been reported to have an important role in the metabolism and synthesis of extracellular matrix of the nucleus pulposus cells (NPCs) and was assumed to be involved in the process of intervertebral disc degeneration. The objective of this study was to investigate the role of HIF-1alpha in disc degeneration in vivo using a conditional HIF-1alpha knockout (KO) mouse model. ShhCre transgenic mice were mated with HIF-1 alpha fl/fl mice to generate conditional HIF-1alpha KO mice (HIF-1alpha fl/fl-ShhCre+). Three mice of each genotype (Wide-type and HIF-1alpha KO) at the age of 3 days, 6, and 12 weeks were sacrificed after genotyping. Five lumbar disc samples were harvested from each mouse, with a total of 45 disc samples for each genotype. In situ hybridization and immunohistochemical analysis were used to check the efficacy of HIF-1alpha knockout. Histological grading of the disc degeneration was performed according to the classification system proposed by Boos et al. Picro-sirius red staining, Safranine O/fast green staining and immunohistochemical study were used to evaluate the expression of aggrecan, type-II collagen and vascular endothelial growth factor (VEGF). Histologic analysis revealed more NPC deaths and signs of degeneration in HIF-1alpha KO mice and the degeneration scores of HIF-1alpha KO mice were significantly higher than those of the Wide-type mice at the age of 6 weeks and 12 weeks. There were less expressions of aggrecan, type-II collagen and VEGF in the intervertebral discs of HIF1-alpha KO mice than in those of wild-type mice. Taken together, the results of our study indicated that HIF-1alpha is a pivotal contributor to NPC survival and the homeotasis of extracellular matrix through the HIF-1alpha/VEGF signaling pathway, and plays an important role in the development of disc degeneration.

HIF-1-PHD2 axis controls expression of syndecan 4 in nucleus pulposus cells

Intervertebral disc degeneration is the leading cause of chronic back pain. Recent studies show that raised level of SDC4, a cell-surface heparan sulfate (HS) proteoglycan, plays a role in pathogenesis of disc degeneration. However, in nucleus pulposus (NP) cells of the healthy intervertebral disc, the mechanisms that control expression of SDC4 and its physiological function are unknown. Hypoxia induced SDC4 mRNA and protein expression by ~2.4- and 4.4-fold (P<0.05), respectively, in NP cells. While the activity of the SDC4 promoter containing hypoxia response element (HRE) was induced 2-fold (P<0.05), the HRE mutation decreased the activity by 40% in hypoxia. Transfections with plasmids coding prolyl-4-hydroxylase domain protein 2 (PHD2) and ShPHD2 show that hypoxic expression of SDC4 mRNA and protein is regulated by PHD2 through controlling hypoxia-inducible factor 1α (HIF-1α) levels. Although overexpression of HIF-1α significantly increased SDC4 protein levels, stable suppression of HIF-1α and HIF-1β decreased SDC4 expression by 50% in human NP cells. Finally, suppression of SDC4 expression, as well as HS function, resulted in an ~2-fold increase in sex-determining region Y (SRY)-box 9 (Sox9) mRNA, and protein (P<0.05) and simultaneous increase in Sox9 transcriptional activity and target gene expression. Taken together, our findings suggest that in healthy discs, SDC4, through its HS side chains, contributes to maintenance of the hypoxic tissue niche by controlling baseline expression of Sox9.

CITED2 mutation and methylation in children with congenital heart disease

Background

The occurrence of Congenital Heart Disease (CHD) is resulted from either genetic or environmental factors or the both. The CITED2 gene deletion or mutation is associated with the development of cardiac malformations. In this study, we have investigated the role of CITED2 gene mutation and methylation in the development of Congenital Heart Disease in pediatric patients in China.

Results

We have screened 120 pediatric patients with congenital heart disease. Among these patients, 4 cases were detected to carry various CITED2 gene heterozygous mutations (c.550G > A, c.574A > G, c.573-578del6) leading correspondingly to the alterations of amino acid sequences in Gly184Ser, Ser192Gly, and Ser192fs, respectively. No CITED2 gene mutations were detected in the control group. At the same time, we found that CITED2 mutations could inhibit TFAP2c expression. In addition, we have demonstrated that abnormal CITED2 gene methylation was detected in most of the tested pediatric patients with CHD, which leads to a decrease of CITED2 transcription activities.

Conclusions

Our study suggests that CITED2 gene mutations and methylation may play an important role in the development of pediatric congenital heart disease.

Influence of hypoxia in the intervertebral disc on the biological behaviors of rat adipose- and nucleus pulposus-derived mesenchymal stem cells

Adipose-derived mesenchymal stem cells (ADMSCs) and nucleus pulposus-derived mesenchymal stem cells (NPMSCs) are two cell candidates for cell-based therapies for intervertebral disc (IVD) regeneration. However, little work has been done to determine the influence of hypoxia in the IVD on the biological behaviors of ADMSCs and NPMSCs. This study aimed to investigate the viability, proliferation and differentiation of rat ADMSCs and NPMSCs in the hypoxic environment of IVD in vitro. ADMSCs and NPMSCs isolated from 6 SD rats were cultured under normoxia (20% O2) and hypoxia (2% O2) mimicking the standard condition and hypoxic environment of the IVD for 14 days. Cell viability was determined by the annexin-V-FITC/propidium iodide double-staining assay and cell proliferation was measured by MTT assay. The expression of hypoxia-inducible factor-1α, glucose transporter (GLUT)-1, GLUT-3 and vascular endothelial growth factor-A at the mRNA level was examined by RT-PCR. In cells cultured in three-dimensional micromass and differentiation medium, aggrecan, collagen-II and Sox-9 expression at mRNA and protein levels were examined by RT-PCR and Western blot. Hypoxia inhibited the viability and proliferation of both ADMSCs and NPMSCs, but promoted the chondrocytic differentiation of ADMSCs and NPMSCs. Compared to ADMSCs, NPMSCs showed greater viability, proliferation and chondrocytic differentiation under hypoxia. In conclusion, hypoxia in the IVD had a significant impact on the viability, proliferation and chondrocytic differentiation of ADMSCs and NPMSCs. NPMSCs exhibited more potent biological activity than ADMSCs in the hypoxic environment of the IVD and may represent another candidate for cell-based therapy for IVD regeneration.

Differential phenotypic behaviors of human degenerative nucleus pulposus cells under normoxic and hypoxic conditions: influence of oxygen concentration during isolation, expansion, and cultivation

BACKGROUND CONTEXT

Intervertebral discs (IVDs) are the largest avascular structures in the body; therefore, cells within these discs might be adapted to low-oxygen conditions. Although it has been demonstrated that a low oxygen concentration could promote synthesis of the extracellular matrix by IVD cells in the in vitro culture, isolation, expansion, and cultivation of IVD cells under classical tissue culture O2 saturation could still be detrimental.

PURPOSE

To investigate the phenotypic differences between human degenerative nucleus pulposus (NP) cells during isolation and expansion under normoxic (Nx: 21% O2) or hypoxic (Hx: 3.5% O2) conditions.

STUDY DESIGN

We investigated in vitro isolation, expansion, and cultivation of human NP cells.

METHODS

Human NP tissue samples were obtained from patients who underwent lumbar disc surgeries. Nucleus pulposus cells were then isolated, expanded, and cultivated under normoxic or hypoxic conditions. To determine whether the effects of normoxic expansion are reversible, another group of cells was isolated and expanded in normoxic conditions and then cultivated under hypoxic conditions (Nx→Hx group). Cellular proliferation, RNA expression of selected genes, and immunohistochemical staining were performed to evaluate the phenotypic behaviors of human NP cells under different conditions.

RESULTS

Expressions of Type II collagen and aggrecan in the Nx→Hx group were significantly higher than those in the normoxic group but were significantly lower than those in the hypoxic group. The normoxic group showed higher expression of matrix metalloproteinase (MMP)-2 and MMP-13 than did the other groups. Expression levels of hypoxia-inducible factors (HIFs) were significantly higher in the normoxic groups; however, a greater degree of HIF-1α staining was found in the hypoxic group, whereas a greater degree of HIF-2α staining was found in the normoxic group.

CONCLUSIONS

Human degenerative NP cells isolated, expanded, and cultivated in hypoxic conditions could better preserve the cells' regenerative potential. Compromised properties that were observed during isolation and expansion under normoxic conditions could only be partially rescued by later hypoxic cultivation. The superior phenotypic behaviors of human NP cells under hypoxia may be related to higher HIF-1α production and lower HIF-2α production. Cells that are isolated, expanded, and cultivated under hypoxic conditions may show better regenerative results when transplanted; therefore, the isolation and expansion processes of human degenerative NP cells should be managed in a hypoxic environment.

Regulatory role of hypoxia inducible factor in the biological behavior of nucleus pulposus cells

Intervertebral disc (IVD) degeneration is implicated as a major cause of low back pain. The alternated phenotypes, reduced cell survival, decreased metabolic activity, loss of matrix production and dystrophic mineralization of nucleus pulposus (NP) cells may be key contributors to progressive IVD degeneration. IVD is the largest avascular structure in the body, characterized by low oxygen tension in vivo. Hypoxia-inducible factor (HIF) is a master transcription factor that is induced upon hypoxia and directs coordinated cellular responses to hypoxic environments. This review summarizes relevant studies concerning the involvement of HIF in the regulation of biological behaviors of NP cells. We describe current data on the expression of HIF in NP cells and further discuss the various roles that HIF plays in the regulation of the phenotype, survival, metabolism, matrix production and dystrophic mineralization of NP cells. Here, we conclude that HIF may be a promising target for the prevention and treatment of IVD degeneration.

Hypoxia-inducible factor (HIF)-1α and CCN2 form a regulatory circuit in hypoxic nucleus pulposus cells: CCN2 suppresses HIF-1α level and transcriptional activity

The objective of the study was to investigate if hypoxia-inducible factor (HIF)-1α and connective tissue growth factor (CCN2) form a regulatory network in hypoxic nucleus pulposus (NP) cells. A decrease in CCN2 expression and proximal promoter activity was observed in NP cells after hypoxic culture. Analysis of both human and mouse CCN2 promoters using the JASPAR core database revealed the presence of putative hypoxia response elements. Transfection experiments showed that both promoter activities and CCN2 expression decreases in hypoxia in a HIF-1α-dependent fashion. Interestingly, deletion analysis and mutation of the hypoxia responsive elements individually or in combination resulted in no change in promoter activity in response to hypoxia or in response to HIF-1α, suggesting an indirect mode of regulation. Notably, silencing of endogenous CCN2 increased HIF-1α levels and its target gene expression, suggesting a role for CCN2 in controlling basal HIF-1α levels. On the other hand, treatment of cells with rCCN2 resulted in a decrease in the ability of HIF-1α transactivating domain to recruit co-activators and diminished target gene expression. Last, knockdown of CCN2 in NP cells results in a significant decrease in GAG synthesis and expression of AGGRECAN and COLLAGEN II. Immunohistochemical staining of intervertebral discs of Ccn2 null embryos shows a decrease in aggrecan. These findings reveal a negative feedback loop between CCN2 and HIF-1α in NP cells and demonstrate a role for CCN2 in maintaining matrix homeostasis in this tissue.

Reconstruction of an in vitro niche for the transition from intervertebral disc development to nucleus pulposus regeneration

The nucleus pulposus (NP) plays a prominent role in both the onset and progression of intervertebral disc degeneration. While autologous repair strategies have demonstrated some success, their in vitro culture system is outdated and insufficient for maintaining optimally functioning cells through the required extensive passaging. Consequently, the final population of cells may be unsuitable for the overwhelming task of repairing tissue in vivo and could result in subpar clinical outcomes. Recent work has identified synovium-derived stem cells (SDSCs) as a potentially important new candidate. This population of precursors can promote matrix regeneration and additionally restore the balance of catabolic and anabolic metabolism of surrounding cells. Another promising application is their ability to produce an extracellular matrix in vitro that can be modified via decellularization to produce a tissue-specific substrate for efficient cell expansion, while retaining chondrogenic potential. When combined with hypoxia, soluble factors, and other environmental regulators, the resultant complex microenvironment will more closely resemble the in vivo niche, which further improves the cell capacity, even after extensive passaging. In this review, the adaptive mechanisms NP cells utilize in vivo are considered for insight into what factors are important for constructing a tissue-specific in vitro niche. Evidence for the use of SDSCs for NP regeneration is also discussed. Many aspects of NP behavior are still unknown, which could lead to future work yielding key information on producing sufficient numbers of a high-quality NP-specific population that is able to regenerate deteriorated NP in vivo.

The effects of oxygen tension and antiaging factor Klotho on Wnt signaling in nucleus pulposus cells

Introduction

The goals of this study were to examine the oxemic regulation of Wnt signaling to explore whether Wnt signaling accelerates the age-related degeneration of nucleus pulposus cells, and if so, to define the mechanism underlying this effect. We investigated the expression of Klotho, a newly identified antiaging gene, and whether its regulation is attributable to the suppression of Wnt signaling.

Methods

Rat nucleus pulposus cells were cultured under normoxic (21% O₂) or hypoxic (2% O₂) conditions, and the expression and promoter activity of Wnt signaling and Klotho were evaluated. The effect of Klotho protein was examined with transfection experiments, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, senescence-associated β-galactosidase staining, and cell-cycle analysis. To determine the methylation status of the Klotho promoter region, bisulfite genomic sequencing analysis was performed. Its relation with the activation of Wnt signaling was assessed. We also examined whether the expression of Klotho could block the effects of pathological Wnt expression in nucleus pulposus cells.

Results

Nucleus pulposus cells exhibited increased β-catenin mRNA and protein under the hypoxic condition. Klotho protein was expressed in vivo, and protein and messenger RNA expression decreased under the hypoxic condition. Klotho treatment decreased cell proliferation and induced the quiescence of nucleus pulposus cells. In addition, Klotho treatment inhibited expression of β-catenin gene and protein compared with untreated control cells.

Conclusions

These data indicate that Wnt signaling and Klotho form a negative-feedback loop in nucleus pulposus cells. These results suggest that the expression of Klotho is regulated by the balance between upregulation and downregulation of Wnt signaling.

Tonicity enhancer binding protein (TonEBP) and hypoxia-inducible factor (HIF) coordinate heat shock protein 70 (Hsp70) expression in hypoxic nucleus pulposus cells: role of Hsp70 in HIF-1α degradation

The objective of our study was to examine the regulation of hypoxic expression of heat shock protein 70 (Hsp70) in nucleus pulposus cells and to determine if Hsp70 promoted hypoxia-inducible factor (HIF)-1α degradation. Rat nucleus pulposus cells were maintained in culture in either 21% or 1% oxygen. To determine the regulation of Hsp70 expression by tonicity enhancer binding protein (TonEBP) and HIF-1/2, loss-of-function and gain-of-function experiments and mutational analysis of the Hsp70 promoter were performed. Hypoxia increased Hsp70 expression in nucleus pulposus cells. Noteworthy, hypoxia increased TonEBP transactivation and mutation of TonE motifs blocked hypoxic induction of the Hsp70 promoter. In contrast, mutation of hypoxia response element (HRE) motifs coupled with loss-of-function experiments suggested that HIF-1 and HIF-2 suppressed Hsp70 promoter activity and transcription. Interestingly, HIF-α interferes with TonEBP function and suppresses the inductive effect of TonEBP on the Hsp70 promoter. In terms of Hsp70 function, when treated with Hsp70 transcriptional inhibitor, KNK437, there was an increase in HIF-1α protein stability and transcriptional activity. Likewise, when Hsp70 was overexpressed, the stability of HIF-1α and its transcriptional activity decreased. Hsp70 interacted with HIF-1α under hypoxic conditions and evidenced increased binding when treated with MG132, a proteasomal inhibitor. These results suggest that Hsp70 may promote HIF-1α degradation through the proteasomal pathway in nucleus pulposus cells. In hypoxic and hyperosmolar nucleus pulposus cells, Hsp70, TonEBP, and HIFs form a regulatory loop. We propose that the positive regulation by TonEBP and negative regulation of Hsp70 by HIF-1 and HIF-2 may serve to maintain Hsp70 levels in these cells, whereas Hsp70 may function in controlling HIF-1α homeostasis.

Expression of prolyl hydroxylases (PHDs) is selectively controlled by HIF-1 and HIF-2 proteins in nucleus pulposus cells of the intervertebral disc: distinct roles of PHD2 and PHD3 proteins in controlling HIF-1α activity in hypoxia

Adaptive response to hypoxia in nucleus pulposus cells of the intervertebral disc is regulated by the hypoxia-inducible factors, HIF-1α and HIF-2α. Moreover, oxygen-dependent turnover of HIF-1α in these cells is controlled by the prolyl-4-hydroxylase domain (PHD) family of proteins. Whether HIF homologues control expression of PHDs and whether PHDs control hypoxia-inducible factor (HIF) turnover and/or activity under hypoxia is not known. Here, we show that in nucleus pulposus cells, hypoxia robustly induces PHD3 expression and, to a lesser extent, of PHD2 and PHD1. Reporter analysis shows that the hypoxic induction of the PHD2 promoter is HIF-1α dependent, whereas PHD3 promoter/enhancer activity is dependent on both HIF-1α and HIF-2α. Lentiviral delivery of HIF-1α, ShHIF-1α, and ShHIF-1β confirmed these observations. Noteworthy, HIF-1α maintains basal expression of PHD1 in hypoxia at the posttranscriptional level. Finally, loss of function studies using lentiviral transduction of ShPHDs clearly shows that even at 1% O(2), PHD2 selectively degrades HIF-1α. In contrast, in hypoxia, PHD3 enhances HIF-1α transcriptional activity without affecting protein levels. To correlate these observations with disc disease, a condition characterized by tissue vascularization, we analyzed human tissues. Increased PHD1 mRNA expression but decreased PHD2 and PHD3 expression is observed in degenerate tissues. Interestingly, the hypoxic responsiveness of all the PHDs is maintained in isolated nucleus pulposus cells regardless of the disease state. We propose that PHD2 and PHD3 can be used as a biomarker of tissue oxygenation in the disc and that, as such, it may have important clinical implications.

HIF-1α and HIF-2α degradation is differentially regulated in nucleus pulposus cells of the intervertebral disc

Studies of many cell types show that levels of hypoxia inducible factor (HIF)-1α and HIF-2α are primarily controlled by oxygen-dependent proteasomal degradation, catalyzed by HIF prolyl-hydroxylases (PHDs). However, in the hypoxic niche of the intervertebral disc, the mechanism of HIF-α turnover in nucleus pulposus cells is not yet known. We show that in nucleus pulposus cells HIF-1α and HIF-2α, degradation was mediated through 26S proteasome irrespective of oxygen tension. It is noteworthy that HIF-2α degradation through 26S proteasome was more pronounced in hypoxia. Surprisingly, treatment with DMOG, a PHD inhibitor, shows the accumulation of only HIF-1α and induction in activity of its target genes, but not of HIF-2α. Loss and gain of function analyses using lentiviral knockdown of PHDs and overexpression of individual PHDs show that in nucleus pulposus cells only PHD2 played a limited role in HIF-1α degradation; again HIF-2α degradation was unaffected. We also show that the treatment with inhibitors of lysosomal proteolysis results in a strong accumulation of HIF-1α and to a much smaller extent of HIF-2α levels. It is thus evident that in addition to PHD2 catalyzed degradation, the HIF-1α turnover in nucleus pulposus cells is primarily regulated by oxygen-independent pathways. Importantly, our data clearly suggests that proteasomal degradation of HIF-2α is not mediated by a classical oxygen-dependent PHD pathway. These results for the first time provide a rationale for the normoxic stabilization as well as the maintenance of steady-state levels of HIF-1α and HIF-2α in nucleus pulposus cells.