Temporal transcriptome of mouse ATDC5 chondroprogenitors differentiating under hypoxic conditions

Bioenergetic Phenotyping of DEN-Induced Hepatocellular Carcinoma Reveals a Link Between Adenylate Kinase Isoform Expression and Reduced Complex I-Supported Respiration

Hepatocellular carcinoma (HCC) is the most common form of liver cancer worldwide. Increasing evidence suggests that mitochondria play a central role in malignant metabolic reprogramming in HCC, which may promote disease progression. To comprehensively evaluate the mitochondrial phenotype present in HCC, we applied a recently developed diagnostic workflow that combines high-resolution respirometry, fluorometry, and mitochondrial-targeted nLC-MS/MS proteomics to cell culture (AML12 and Hepa 1-6 cells) and diethylnitrosamine (DEN)-induced mouse models of HCC. Across both model systems, CI-linked respiration was significantly decreased in HCC compared to nontumor, though this did not alter ATP production rates. Interestingly, CI-linked respiration was found to be restored in DEN-induced tumor mitochondria through acute in vitro treatment with P1, P5-di(adenosine-5′) pentaphosphate (Ap5A), a broad inhibitor of adenylate kinases. Mass spectrometry-based proteomics revealed that DEN-induced tumor mitochondria had increased expression of adenylate kinase isoform 4 (AK4), which may account for this response to Ap5A. Tumor mitochondria also displayed a reduced ability to retain calcium and generate membrane potential across a physiological span of ATP demand states compared to DEN-treated nontumor or saline-treated liver mitochondria. We validated these findings in flash-frozen human primary HCC samples, which similarly displayed a decrease in mitochondrial respiratory capacity that disproportionately affected CI. Our findings support the utility of mitochondrial phenotyping in identifying novel regulatory mechanisms governing cancer bioenergetics.

Hypoxia classifier for transcriptome datasets

Molecular gene signatures are useful tools to characterize the physiological state of cell populations, but most have developed under a narrow range of conditions and cell types and are often restricted to a set of gene identities. Focusing on the transcriptional response to hypoxia, we aimed to generate widely applicable classifiers sourced from the results of a meta-analysis of 69 differential expression datasets which included 425 individual RNA-seq experiments from 33 different human cell types exposed to different degrees of hypoxia (0.1–5%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {O}_{2}$$\end{document}O2) for 2–48 h. The resulting decision trees include both gene identities and quantitative boundaries, allowing for easy classification of individual samples without control or normoxic reference. Each tree is composed of 3–5 genes mostly drawn from a small set of just 8 genes (EGLN1, MIR210HG, NDRG1, ANKRD37, TCAF2, PFKFB3, BHLHE40, and MAFF). In spite of their simplicity, these classifiers achieve over 95% accuracy in cross validation and over 80% accuracy when applied to additional challenging datasets. Our results indicate that the classifiers are able to identify hypoxic tumor samples from bulk RNAseq and hypoxic regions within tumor from spatially resolved transcriptomics datasets. Moreover, application of the classifiers to histological sections from normal tissues suggest the presence of a hypoxic gene expression pattern in the kidney cortex not observed in other normoxic organs. Finally, tree classifiers described herein outperform traditional hypoxic gene signatures when compared against a wide range of datasets. This work describes a set of hypoxic gene signatures, structured as simple decision tress, that identify hypoxic samples and regions with high accuracy and can be applied to a broad variety of gene expression datasets and formats.

Adenylate Kinase 4-A Key Regulator of Proliferation and Metabolic Shift in Human Pulmonary Arterial Smooth Muscle Cells via Akt and HIF-1α Signaling Pathways

Increased proliferation of pulmonary arterial smooth muscle cells (PASMCs) in response to chronic hypoxia contributes to pulmonary vascular remodeling in pulmonary hypertension (PH). PH shares numerous similarities with cancer, including a metabolic shift towards glycolysis. In lung cancer, adenylate kinase 4 (AK4) promotes metabolic reprogramming and metastasis. Against this background, we show that AK4 regulates cell proliferation and energy metabolism of primary human PASMCs. We demonstrate that chronic hypoxia upregulates AK4 in PASMCs in a hypoxia-inducible factor-1α (HIF-1α)-dependent manner. RNA interference of AK4 decreases the viability and proliferation of PASMCs under both normoxia and chronic hypoxia. AK4 silencing in PASMCs augments mitochondrial respiration and reduces glycolytic metabolism. The observed effects are associated with reduced levels of phosphorylated protein kinase B (Akt) as well as HIF-1α, indicating the existence of an AK4-HIF-1α feedforward loop in hypoxic PASMCs. Finally, we show that AK4 levels are elevated in pulmonary vessels from patients with idiopathic pulmonary arterial hypertension (IPAH), and AK4 silencing decreases glycolytic metabolism of IPAH-PASMCs. We conclude that AK4 is a new metabolic regulator in PASMCs interacting with HIF-1α and Akt signaling pathways to drive the pro-proliferative and glycolytic phenotype of PH.

ATDC5 cells as a model of cartilage extracellular matrix neosynthesis, maturation and assembly

Fibrillar collagens and proteoglycans (PGs) are quantitatively the major constituents of extracellular matrices (ECM). They carry numerous crucial post-translational modifications (PTMs) that tune the resulting biomechanical properties of the corresponding tissues. The mechanisms determining these PTMs remain largely unknown, notably because available established cell lines do not recapitulate much of the complexity of the machineries involved. ATDC5 cells are a model of chondrogenesis widely used for decades, but it remains described mostly at histological and transcriptional levels. Here, we asked to what extent this model recapitulates the events of ECM synthesis and processing occurring in cartilage. Insulin-stimulated ATDC5 cells exhibit up- or down-regulation of more than one-hundred proteins, including a number of known participants in chondrogenesis and major markers thereof. However, they also lack several ECM components considered of significant, yet more subtle, function in cartilage. Still, they assemble the large PG aggrecan and type II collagen, both carrying most of their in vivo PTMs, into an ECM. Remarkably, collagen crosslinking is fully lysyl oxidase (LOX)-dependent. The ATDC5 model recapitulates critical aspects of the cartilage ECM-processing machinery and should be useful to decipher the mechanisms involved. Proteomics data are available via ProteomeXchange with identifier PXD014121. SIGNIFICANCE: The present work provides the first proteome characterization of the ATDC5 chondrogenesis model, which has been used for decades in the field of cartilage biology. The results demonstrate the up- and down-regulation of more than one hundred proteins. Overall, specific drawbacks of the model are pointed out, that will be important to take into consideration for future studies. However, major cartilage components are massively assembled into an extracellular matrix and carry most of their post-translational modifications occurring in cartilage tissue. Unlike other available established cell lines, the ATDC5 model recapitulates major aspects of cartilage biosynthesis and should be useful in investigating the mechanisms that regulate collagen maturation events.

GSEA-InContext: identifying novel and common patterns in expression experiments

Motivation

Gene Set Enrichment Analysis (GSEA) is routinely used to analyze and interpret coordinate pathway-level changes in transcriptomics experiments. For an experiment where less than seven samples per condition are compared, GSEA employs a competitive null hypothesis to test significance. A gene set enrichment score is tested against a null distribution of enrichment scores generated from permuted gene sets, where genes are randomly selected from the input experiment. Looking across a variety of biological conditions, however, genes are not randomly distributed with many showing consistent patterns of up- or down-regulation. As a result, common patterns of positively and negatively enriched gene sets are observed across experiments. Placing a single experiment into the context of a relevant set of background experiments allows us to identify both the common and experiment-specific patterns of gene set enrichment.

Results

We compiled a compendium of 442 small molecule transcriptomic experiments and used GSEA to characterize common patterns of positively and negatively enriched gene sets. To identify experiment-specific gene set enrichment, we developed the GSEA-InContext method that accounts for gene expression patterns within a background set of experiments to identify statistically significantly enriched gene sets. We evaluated GSEA-InContext on experiments using small molecules with known targets to show that it successfully prioritizes gene sets that are specific to each experiment, thus providing valuable insights that complement standard GSEA analysis.

Availability and implementation

GSEA-InContext implemented in Python, Supplementary results and the background expression compendium are available at: https://github.com/CostelloLab/GSEA-InContext.

Adenylate kinase 4 modulates oxidative stress and stabilizes HIF-1α to drive lung adenocarcinoma metastasis

Background

Adenylate kinase 4 (AK4) has been identified as a biomarker of metastasis in lung cancer. However, the impacts of AK4 on metabolic genes and its translational value for drug repositioning remain unclear.

Methods

Ingenuity upstream analyses were used to identify potential transcription factors that regulate the AK4 metabolic gene signature. The expression of AK4 and its upstream regulators in lung cancer patients was examined via immunohistochemistry. Pharmacological and gene knockdown/overexpression approaches were used to investigate the interplay between AK4 and its upstream regulators during epithelial-to-mesenchymal transition (EMT). Drug candidates that reversed AK4-induced gene expression were identified by querying a connectivity map. Orthotopic xenograft mouse models were established to evaluate the therapeutic efficacy of drug candidates for metastatic lung cancer.

Results

We found that HIF-1α is activated in the AK4 metabolic gene signature. IHC analysis confirmed this positive correlation, and the combination of both predicts worse survival in lung cancer patients. Overexpression of AK4 exaggerates HIF-1α protein expression by increasing intracellular ROS levels and subsequently induces EMT under hypoxia. Attenuation of ROS production with N-acetylcysteine abolishes AK4-induced invasion potential under hypoxia. Pharmacogenomics analysis of the AK4 gene signature revealed that withaferin-A could suppress the AK4-HIF-1α signaling axis and serve as a potent anti-metastatic agent in lung cancer.

Conclusions

Overexpression of AK4 promotes lung cancer metastasis by enhancing HIF-1α stability and EMT under hypoxia. Reversing the AK4 gene signature with withaferin-A may serve as a novel therapeutic strategy to treat metastatic lung cancer.

Electronic supplementary material

The online version of this article (10.1186/s13045-019-0698-5) contains supplementary material, which is available to authorized users.

Cancer Stem Cells: Emergent Nature of Tumor Emergency

A functional analysis of 167 genes overexpressed in Krebs-2 tumor initiating cells was performed. In the first part of the study, the genes were analyzed for their belonging to one or more of the three groups, which represent the three major phenotypic manifestation of malignancy of cancer cells, namely (1) proliferative self-sufficiency, (2) invasive growth and metastasis, and (3) multiple drug resistance. 96 genes out of 167 were identified as possible contributors to at least one of these fundamental properties. It was also found that substantial part of these genes are also known as genes responsible for formation and/or maintenance of the stemness of normal pluri-/multipotent stem cells. These results suggest that the malignancy is simply the ability to maintain the stem cell specific genes expression profile, and, as a consequence, the stemness itself regardless of the controlling effect of stem niches. In the second part of the study, three stress factors combined into the single concept of "generalized cellular stress," which are assumed to activate the expression of these genes, were defined. In addition, possible mechanisms for such activation were identified. The data obtained suggest the existence of a mechanism for the de novo formation of a pluripotent/stem phenotype in the subpopulation of "committed" tumor cells.

Modulation of anti-cancer drug sensitivity through the regulation of mitochondrial activity by adenylate kinase 4

BACKGROUND

Adenylate kinase is a key enzyme in the high-energy phosphoryl transfer reaction in living cells. An isoform of this enzyme, adenylate kinase 4 (AK4), is localized in the mitochondrial matrix and is believed to be involved in stress, drug resistance, malignant transformation in cancer, and ATP regulation. However, the molecular basis for the AK4 functions remained to be determined.

METHODS

HeLa cells were transiently transfected with an AK4 small interfering RNA (siRNA), an AK4 short hairpin RNA (shRNA) plasmid, a control shRNA plasmid, an AK4 expression vector, and a control expression vector to examine the effect of the AK4 expression on cell proliferation, sensitivity to anti-cancer drug, metabolome, gene expression, and mitochondrial activity.

RESULTS

AK4 knockdown cells treated with short hairpin RNA increased ATP production and showed greater sensitivity to hypoxia and anti-cancer drug, cis-diamminedichloro-platinum (II) (CDDP). Subcutaneous grafting AK4 knockdown cells into nude mice revealed that the grafted cells exhibited both slower proliferation and reduced the tumor sizes in response to CDDP. AK4 knockdown cell showed a increased oxygen consumption rate with FCCP treatment, while AK4 overexpression lowered it. Metabolome analysis showed the increased levels of the tricarboxylic acid cycle intermediates, fumarate and malate in AK4 knockdown cells, while AK4 overexpression lowered them. Electron microscopy detected the increased mitochondrial numbers in AK4 knockdown cells. Microarray analysis detected the increased gene expression of two key enzymes in TCA cycle, succinate dehydrogenase A (SDHA) and oxoglutarate dehydrogenease L (OGDHL), which are components of SDH complex and OGDH complex, supporting the metabolomic results.

CONCLUSIONS

We found that AK4 was involved in hypoxia tolerance, resistance to anti-tumor drug, and the regulation of mitochondrial activity. These findings provide a new potential target for efficient anticancer therapies by controlling AK4 expression.

The immediate early gene product EGR1 and polycomb group proteins interact in epigenetic programming during chondrogenesis

Initiation of and progression through chondrogenesis is driven by changes in the cellular microenvironment. At the onset of chondrogenesis, resting mesenchymal stem cells are mobilized in vivo and a complex, step-wise chondrogenic differentiation program is initiated. Differentiation requires coordinated transcriptomic reprogramming and increased progenitor proliferation; both processes require chromatin remodeling. The nature of early molecular responses that relay differentiation signals to chromatin is poorly understood. We here show that immediate early genes are rapidly and transiently induced in response to differentiation stimuli in vitro. Functional ablation of the immediate early factor EGR1 severely deregulates expression of key chondrogenic control genes at the onset of differentiation. In addition, differentiating cells accumulate DNA damage, activate a DNA damage response and undergo a cell cycle arrest and prevent differentiation associated hyper-proliferation. Failed differentiation in the absence of EGR1 affects global acetylation and terminates in overall histone hypermethylation. We report novel molecular connections between EGR1 and Polycomb Group function: Polycomb associated histone H3 lysine27 trimethylation (H3K27me3) blocks chromatin access of EGR1. In addition, EGR1 ablation results in abnormal Ezh2 and Bmi1 expression. Consistent with this functional interaction, we identify a number of co-regulated targets genes in a chondrogenic gene network. We here describe an important role for EGR1 in early chondrogenic epigenetic programming to accommodate early gene-environment interactions in chondrogenesis.

The small MAF transcription factors MAFF, MAFG and MAFK: current knowledge and perspectives

The small MAFs, MAFF, MAFG and MAFK have emerged as crucial regulators of mammalian gene expression. Previous studies have linked small MAF function, by virtue of their heterodimerization with the Cap 'n' Collar (CNC) family of transcription factors, to the stress response and detoxification pathways. Recent analyses have revealed a complex regulatory network involving small MAF transcription factors and other cellular proteins. The expression and activity of small MAFs are tightly regulated at multiple levels. With regard to their clinical importance, small MAFs have been linked to various diseases, such as diabetes, neuronal disorders, thrombocytopenia and carcinogenesis. A better understanding of the molecular mechanisms governing the activity of small MAFs will provide novel insights into the control of mammalian transcription and may lead to the development of novel therapeutic strategies to treat common human disorders.

Defined xenogeneic-free and hypoxic environment provides superior conditions for long-term expansion of human adipose-derived stem cells

Development and implementation of therapeutic protocols based on stem cells or tissue-engineered products relies on methods that enable the production of substantial numbers of cells while complying with stringent quality and safety demands. In the current study, we aimed to assess the benefits of maintaining cultures of adipose-derived stem cells (ASCs) in a defined culture system devoid of xenogeneic components (xeno-free) and hypoxia over a 49-day growth period. Our data provide evidence that conditions involving StemPro mesenchymal stem cells serum-free medium (SFM) Xeno-Free and hypoxia (5% oxygen concentration) in the culture atmosphere provide a superior proliferation rate compared to a standard growth environment comprised of alpha-modified Eagle medium (A-MEM) supplemented with fetal calf serum (FCS) and ambient air (20% oxygen concentration) or that of A-MEM supplemented with FCS and hypoxia. Furthermore, a flow cytometric analysis and in vitro differentiation assays confirmed the immunophenotype stability and maintained multipotency of ASCs when expanded under xeno-free conditions and hypoxia. In conclusion, our data demonstrate that growth conditions utilizing a xeno-free and hypoxic environment not only provide an improved environment for the expansion of ASCs, but also set the stage as a culture system with the potential broad spectrum utility for regenerative medicine and tissue engineering applications.

Hypoxia and adipose-derived stem cell-based tissue regeneration and engineering

INTRODUCTION

Realization that oxygen is one of the key regulators of development and differentiation has a profound significance on how current cell-based and tissue engineering applications using adipose-derived stem cells (ASCs) can be further improved.

AREAS COVERED

The article provides an overview of mechanisms of hypoxic responses during physiological adaptations and development. Furthermore, a synopsis of the hypoxic responses of ASCs is provided, and this information is presented in context of their utility as a major source of stem cells across the regenerative applications explored to date.

EXPERT OPINION

The reader will obtain insight into a highly specific area of stem cell research focusing on ASCs and hypoxia. In order to enhance the level of comprehension, a broader context with other stem cell and experimental systems is provided. It is emphasized that the pericellular oxygen tension is a critical regulatory factor that should be taken into account when devising novel stem cell-based therapeutic applications along with other parameters, such as biochemical soluble factors and the growth substrates.

Artefactual effects of oxygen on cell culture models of cellular senescence and stem cell biology

In life sciences, modelling of the in vivo conditions using in vitro models is an important tool to generate knowledge. Although aerobic organisms including mammals depend on accurate oxygen tension, mimicking physiological conditions in cell culture experiments is not very common. Due to the need for simple technical and experimental design, the requirement for simulating the in vivo oxygen tension parameters has been neglected over long time. Fortunately, due to increasing knowledge in recent years the attention has shifted towards this scientific demand. In this short review, we summarize data substantiating the necessity to adequately mimic physiological oxygen tension using cell culture models in life science research.

Localization of adenylate kinase 4 in mouse tissues

Adenylate kinase (AK) is a key enzyme in the high-energy phosphoryl transfer reaction in living cells. Of its isoforms, AK4 has a similar sequence and subcellular localization to that of AK3 in the mitochondrial matrix. However, unlike AK3, AK4 lacks the guanosine triphosphate: adenosine monophosphate phosphotransferase activity. To elucidate the physiological role of AK4, we explored the protein localization of AK4 in various mouse tissues by immunohistochemical analysis. AK4 protein was detected in the kidney, liver, brain, heart, stomach, intestine, and gonads but not in the lung and spleen. Interestingly, cell-type specific expression was evident in the brain, gastrointestinal tract, and gonads. In the cerebellum, AK4 was detected in granular cells but not in Purkinje cell bodies. In the gastrointestinal tract, AK4 was highly expressed in epithelia. In the ovary, AK4 was detected in oocytes and corpora lutea. In the testis, AK4 was detected in spermatocytes but not in spermatogonia. Our findings demonstrate that AK4 localizes uniquely in a cell-type and tissue-specific manner in mouse tissues.

Enzymatically inactive adenylate kinase 4 interacts with mitochondrial ADP/ATP translocase

Adenylate kinase 4 (AK4) is a unique member with no enzymatic activity in vitro in the adenylate kinase (AK) family although it shares high sequence homology with other AKs. It remains unclear what physiological function AK4 might play or why it is enzymatically inactive. In this study, we showed increased AK4 protein levels in cultured cells exposed to hypoxia and in an animal model of the neurodegenerative disease amyotrophic lateral sclerosis. We also showed that short hairpin RNA (shRNA)-mediated knockdown of AK4 in HEK293 cells with high levels of endogenous AK4 resulted in reduced cell proliferation and increased cell death. Furthermore, we found that AK4 over-expression in the neuronal cell line SH-SY5Y with low endogenous levels of AK4 protected cells from H(2)O(2) induced cell death. Proteomic studies revealed that the mitochondrial ADP/ATP translocases (ANTs) interacted with AK4 and higher amount of ANT was co-precipitated with AK4 when cells were exposed to H(2)O(2) treatment. In addition, structural analysis revealed that, while AK4 retains the capability of binding nucleotides, AK4 has a glutamine residue instead of a key arginine residue in the active site well conserved in other AKs. Mutation of the glutamine residue to arginine (Q159R) restored the adenylate kinase activity with GTP as substrate. Collectively, these results indicate that the enzymatically inactive AK4 is a stress responsive protein critical to cell survival and proliferation. It is likely that the interaction with the mitochondrial inner membrane protein ANT is important for AK4 to exert the protective benefits to cells under stress.

Crystal structure of human adenylate kinase 4 (L171P) suggests the role of hinge region in protein domain motion

It is well known that motion of LID and NMP-binding (NMP(bind)) domains in adenylate kinase (AK) is important in ligand binding and catalysis. However, the nature of such domain motions is poorly characterized. One of the critical hinge regions is hinge IV, which connects the CORE and LID domains. In addition, the hinge IV contains a strictly conserved residue, L171, in the AK family. To investigate the role of hinge IV, crystal structure of human adenylate kinase 4 (AK4) L171P mutant was determined. This mutation dramatically changes the orientation of the LID domain, which could be described as a novel twisted-and-closed conformation in contrast to the open and closed conformations in other AKs. This mutant provides a new example of domain motions in AK family.

Chondrogenic differentiation of growth factor-stimulated precursor cells in cartilage repair tissue is associated with increased HIF-1alpha activity

OBJECTIVE

To investigate the chondrogenic potential of growth factor-stimulated periosteal cells with respect to the activity of Hypoxia-inducible Factor 1alpha (HIF-1alpha).

METHODS

Scaffold-bound autologous periosteal cells, which had been activated by Insulin-like Growth Factor 1 (IGF-1) or Bone Morphogenetic Protein 2 (BMP-2) gene transfer using both adeno-associated virus (AAV) and adenoviral (Ad) vectors, were applied to chondral lesions in the knee joints of miniature pigs. Six weeks after transplantation, the repair tissues were investigated for collagen type I and type II content as well as for HIF-1alpha expression. The functional role of phosphatidylinositol 3-kinase (PI3K)/mammalian target of rapamycin (mTOR) and mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) signaling on BMP-2/IGF-1-induced HIF-1alpha expression was assessed in vitro by employing specific inhibitors.

RESULTS

Unstimulated periosteal cells formed a fibrous extracellular matrix in the superficial zone and a fibrocartilaginous matrix in deep zones of the repair tissue. This zonal difference was reflected by the absence of HIF-1alpha staining in superficial areas, but moderate HIF-1alpha expression in deep zones. In contrast, Ad/AAVBMP-2-stimulated periosteal cells, and to a lesser degree Ad/AAVIGF-1-infected cells, adopted a chondrocyte-like phenotype with strong intracellular HIF-1alpha staining throughout all zones of the repair tissue and formed a hyaline-like matrix. In vitro, BMP-2 and IGF-1 supplementation increased HIF-1alpha protein levels in periosteal cells, which was based on posttranscriptional mechanisms rather than de novo mRNA synthesis, involving predominantly the MEK/ERK pathway.

CONCLUSION

This pilot experimental study on a relatively small number of animals indicated that chondrogenesis by precursor cells is facilitated in deeper hypoxic zones of cartilage repair tissue and is stimulated by growth factors which enhance HIF-1alpha activity.

Hypoxia promotes chondrogenesis in rat mesenchymal stem cells: a role for AKT and hypoxia-inducible factor (HIF)-1alpha

Mesenchymal stem cells (MSCs) are multipotent cells capable of developing along the chondrogenic, osteogenic and adipogenic lineages. As such, they have received interest as a potential cell source for tissue engineering strategies. Cartilage is an avascular tissue and thus resides in a microenvironment with reduced oxygen tension. The aim of this study was to examine the effect of a low oxygen environment on MSC differentiation along the chondrogenic route. In MSCs exposed to chondrogenic growth factors, transforming growth factor-beta and dexamethasone, in a hypoxic environment (2% oxygen), the induction of collagen II expression and proteoglygan deposition was significantly greater than that observed when cells were exposed to the chondrogenic growth factors under normoxic (20% oxygen) conditions. The transcription factor, hypoxia-inducible factor-1alpha (HIF-1alpha), is a crucial mediator of the cellular response to hypoxia. Following exposure of MSCs to hypoxia (2% oxygen), HIF-1alpha translocated from the cytosol to the nucleus and bound to its target DNA consensus sequence. Similarly, hypoxia evoked an increase in phosphorylation of both AKT and p38 mitogen activated protein kinase, upstream of HIF-1alpha activation. Furthermore, the PI3 kinase/AKT inhibitor, LY294002, and p38 inhibitor, SB 203580, prevented the hypoxia-mediated stabilisation of HIF-1alpha. To assess the role of HIF-1alpha in the hypoxia-induced increase in chondrogenesis, we employed an siRNA knockdown approach. In cells exposed to HIF-1alpha siRNA, the hypoxia-induced enhancement of chondrogenesis, as evidenced by upregulation of collagen II, sox-9 and proteoglycan deposition, was absent. This provides evidence for HIF-1alpha being a key mediator of the beneficial effect of a low oxygen environment on chondrogenesis.

Hypoxia Inducible Factor-1α Deficiency Affects Chondrogenesis of Adipose-Derived Adult Stromal Cells

Increased cartilage-related disease, poor regeneration of cartilage tissue, and limited treatment options have led to intense research in tissue engineering of cartilage. Adipose-derived adult stromal cells (ADAS) are a promising cell source for skeletal tissue engineering; understanding ADAS cellular signaling and chondrogenesis will advance cell-based therapies in cartilage repair. Chondrocytes are unique-they are continuously challenged by a hypoxic microenvironment. Hypoxia inducible factor-1-alpha (HIF-1alpha), a critical mediator of a cell's response to hypoxia, plays a significant role in chondrocyte survival, growth arrest, and differentiation. By using an established in vitro 3-dimensional micromass system, we investigated the role of HIF-1alpha in chondrogenesis. Targeted deletion of HIF-1alpha in ADAS substantially inhibited the chondrogenic pathway specifically. In marked contrast, deletion of HIF-1alpha did not affect osteogenic differentiation but enhanced adipogenic differentiation. This study demonstrates the critical and specific interplay between HIF-1alpha and chondrogenesis in vitro.