Elucidating Mechanisms of Osteogenesis in Human Adipose-Derived Stromal Cells via Microarray Analysis

Immunophenotyping and transcriptional profiling of in vitro cultured human adipose tissue derived stem cells

Adipose-derived stem cells (ASCs) have become an important research model in regenerative medicine. However, there are controversies regarding the impact of prolonged cell culture on the ASCs phenotype and their differentiation potential. Hence, we studied 10 clinical ASCs replicates from plastic and oncological surgery patients, in six-passage FBS supplemented cultures. We quantified basic mesenchymal cell surface marker transcripts and the encoded proteins after each passage. In parallel, we investigated the differentiation potential of ASCs into chondrocytes, osteocytes and adipocytes. We further determined the effects of FBS supplementation and subsequent deprivation on the whole transcriptome by comprehensive mRNA and miRNA sequencing. Our results show that ASCs maintain differentiation potential and consistent profile of key mesenchymal markers, with apparent expression of distinct isoforms, in long-term cultures. No significant differences were observed between plastic and oncological surgery cohorts. ASCs in FBS supplemented primary cultures are almost committed to mesenchymal lineages as they express key epithelial-mesenchymal transition genes including early mesenchymal markers. Furthermore, combined mRNA/miRNA expression profiling strongly supports a modulatory role for the miR-30 family in the commitment process to mesenchymal lineages. Finally, we propose improvements to existing qPCR based assays that address alternative isoform expression of mesenchymal markers.

Analysis of the Pro- and Anti-Inflammatory Cytokines Secreted by Adult Stem Cells during Differentiation

Adipose-derived stromal/stem cells (ASCs) are adult stem cells that have the potential to differentiate into mesenchymal lineage cells. The abundance of ASCs in adipose tissue and easy accessibility with relatively little donor site morbidity make them attractive candidate cells for tissue engineering and regenerative medicine. However, the underlying inflammatory process that occurs during ASC differentiation into adipocytes and osteoblast has not been extensively investigated. ASCs cultured in osteogenic and adipogenic differentiation medium were characterized by oil red o staining and alizarin red staining, respectively. ASCs undergoing osteogenic and adipogenic differentiation were isolated on days 7, 14, and 21 and assessed by qRT-PCR for the expression of pro- and anti-inflammatory cytokines. ASCs undergoing osteogenic differentiation expressed a distinct panel of cytokines that differed from the cytokine profile of ASCs undergoing adipogenic differentiation at each of the time points analyzed. Mapping the cytokine expression profile during ASC differentiation will provide insight into the role of inflammation in this process and identify potential targets that may aid in enhancing osteogenic or adipogenic differentiation for the purposes of tissue engineering and regenerative medicine.

Platelet-derived growth factor and spatiotemporal cues induce development of vascularized bone tissue by adipose-derived stem cells

Vasculature is essential to the functional integration of a tissue-engineered bone graft to enable sufficient nutrient delivery and viability after implantation. Native bone and vasculature develop through intimately coupled, tightly regulated spatiotemporal cell-cell signaling. The complexity of these developmental processes has been a challenge for tissue engineers to recapitulate, resulting in poor codevelopment of both bone and vasculature within a unified graft. To address this, we cultured adipose-derived stromal/stem cells (ASCs), a clinically relevant, single cell source that has been previously investigated for its ability to give rise to vascularized bone grafts, and studied the effects of initial spatial organization of cells, the temporal addition of growth factors, and the presence of exogenous platelet-derived growth factor-BB (PDGF-BB) on the codevelopment of bone and vascular tissue structures. Human ASCs were aggregated into multicellular spheroids via the hanging drop method before encapsulation and subsequent outgrowth in fibrin gels. Cellular aggregation substantially increased vascular network density, interconnectivity, and pericyte coverage compared to monodispersed cultures. To form robust vessel networks, it was essential to culture ASCs in a purely vasculogenic medium for at least 8 days before the addition of osteogenic cues. Physiologically relevant concentrations of exogenous PDGF-BB (20 ng/mL) substantially enhanced both vascular network stability and osteogenic differentiation. Comparisons with the bone morphogenetic protein-2, another pro-osteogenic and proangiogenic growth factor, indicated that this potential to couple the formation of both lineages might be unique to PDGF-BB. Furthermore, the resulting tissue structure demonstrated the close association of mineral deposits with pre-existing vascular structures that have been described for developing tissues. This combination of a single cell source with a potent induction factor used at physiological concentrations can provide a clinically relevant approach to engineering highly vascularized bone grafts.

Differences in osteogenic differentiation of adipose-derived stromal cells from murine, canine, and human sources in vitro and in vivo

BACKGROUND

Given the diversity of species from which adipose-derived stromal cells are derived and studied, the authors set out to delineate the differences in the basic cell biology that may exist across species. Briefly, the authors found that significant differences exist with regard to proliferation and osteogenic potentials of adipose-derived stromal cells across species.

METHODS

Adipose-derived stromal cells were derived from human, mouse, and canine sources as previously described. Retinoic acid, insulin-like growth factor-1, and bone morphogenetic protein-2 were added to culture medium; proliferation and osteogenic differentiation were assessed by standardized assays. In vivo methods included seeding 150,000 adipose-derived stromal cells on a biomimetic scaffold and analyzing healing by micro-computed tomography and histology.

RESULTS

Adipose-derived stromal cells from all species had the capability to undergo osteogenic differentiation. Canine adipose-derived stromal cells were the most proliferative, whereas human adipose-derived stromal cells were the most osteogenic (p < 0.05). Human cells, however, had the most significant osteogenic response to osteogenic media. Retinoic acid stimulated osteogenesis in mouse and canine cells but not in human adipose-derived stromal cells. Insulin-like growth factor-1 enhanced osteogenesis across all species, most notably in human- and canine-derived cells.

CONCLUSIONS

Adipose-derived stromal cells derived from human, mouse, and canine all have the capacity to undergo osteogenic differentiation. Canine adipose-derived stromal cells appear to be the most proliferative, whereas human adipose-derived stromal cells appear to be the most osteogenic. Different cytokines and chemicals can be used to modulate this osteogenic response. These results are promising as attempts are made to optimize tissue-engineered bone using adipose-derived stromal cells.

Studies in adipose-derived stromal cells: migration and participation in repair of cranial injury after systemic injection

BACKGROUND

Adipose-derived stromal cells are a multipotent cell type with the ability to undergo osteogenic differentiation. The authors sought to examine whether systemically administered adipose-derived stromal cells would migrate to and heal surgically created defects of the mouse cranial skeleton.

METHODS

Mouse adipose-derived stromal cells were harvested from luciferase-positive transgenic mice; human adipose-derived stromal cells were harvested from human lipoaspirate and labeled with luciferase and green fluorescent protein. A 4-mm calvarial defect (critical sized) was made in the mouse parietal bone; skin incisions alone were used as a control (n = 5 per group). Adipose-derived stromal cells were injected intravenously (200,000 cells per animal) and compared with saline injection only. Methods of analyses included micro-computed tomographic scanning, in vivo imaging system detection of luciferase activity, and standard histology.

RESULTS

Migration of adipose-derived stromal cells to calvarial defect sites was confirmed by accumulation of luciferase activity and green fluorescent protein stain as early as 4 days and persisting up to 4 weeks. Little activity was observed among control groups. Intravenous administration of either mouse or human adipose-derived stromal cells resulted in histologic evidence of bone formation within the defect site, in comparison with an absence of bone among control defects. By micro-computed tomographic analysis, human but not mouse adipose-derived stromal cells stimulated significant osseous healing.

CONCLUSIONS

Intravenously administered adipose-derived stromal cells migrate to sites of calvarial injury. Thereafter, intravenous human adipose-derived stromal cells contribute to bony calvarial repair. Intravenous administration of adipose-derived stromal cells may be an effective delivery method for future efforts in skeletal regeneration.

Deleterious effects of freezing on osteogenic differentiation of human adipose-derived stromal cells in vitro and in vivo

Human adipose-derived stromal cells (hASCs) represent a multipotent stromal cell type with a proven capacity to undergo osteogenic differentiation. Many hurdles exist, however, between current knowledge of hASC osteogenesis and their potential future use in skeletal tissue regeneration. The impact of frozen storage on hASC osteogenic differentiation, for example, has not been studied in detail. To examine the effects of frozen storage, hASCs were harvested from lipoaspirate and either maintained in standard culture conditions or frozen for 2 weeks under standard conditions (90% fetal bovine serum, 10% dimethyl sulfoxide). Next, in vitro parameters of cell morphology (surface electron microscopy [EM]), cell viability and growth (trypan blue; bromodeoxyuridine incorporation), osteogenic differentiation (alkaline phosphatase, alizarin red, and quantitative real-time (RT)-polymerase chain reaction), and adipogenic differentiation (Oil red O staining and quantitative RT-polymerase chain reaction) were performed. Finally, in vivo bone formation was assessed using a critical-sized cranial defect in athymic mice, utilizing a hydroxyapatite (HA)-poly(lactic-co-glycolic acid) scaffold for ASC delivery. Healing was assessed by serial microcomputed tomography scans and histology. Freshly derived ASCs differed significantly from freeze-thaw ASCs in all markers examined. Surface EM showed distinct differences in cellular morphology. Proliferation, and osteogenic and adipogenic differentiation were all significantly hampered by the freeze-thaw process in vitro (*P < 0.01). In vivo, near complete healing was observed among calvarial defects engrafted with fresh hASCs. This was in comparison to groups engrafted with freeze-thaw hASCs that showed little healing (*P < 0.01). Finally, recombinant insulin-like growth factor 1 or recombinant bone morphogenetic protein 4 was observed to increase or rescue in vitro osteogenic differentiation among frozen hASCs (*P < 0.01). The freezing of ASCs for storage significantly impacts their biology, both in vitro and in vivo. The ability of ASCs to successfully undergo osteogenic differentiation after freeze-thaw is substantively muted, both in vitro and in vivo. The use of recombinant proteins, however, may be used to mitigate the deleterious effects of the freeze-thaw process.

Regulation of human adipose-derived stromal cell osteogenic differentiation by insulin-like growth factor-1 and platelet-derived growth factor-alpha

BACKGROUND

Human adipose-derived stromal cells possess a great potential for tissue engineering purposes. The authors' laboratory is interested in harnessing human adipose-derived stromal cells for skeletal tissue regeneration and identifying those factors that enhance human adipose-derived stromal cell osteogenic differentiation. The authors hypothesized that insulin-like growth factor (IGF) and platelet-derived growth factor (PDGF) would stimulate human adipose-derived stromal cell osteogenesis and that IGF would stimulate adipogenesis.

METHODS

Adipose-derived stromal cells were harvested from human lipoaspirate. Previously, a microarray analysis examined gene expression throughout osteogenic differentiation. In a candidate fashion, the authors added recombinant IGF-1 and PDGF-alpha individually and in combination. Osteogenesis and adipogenesis were assessed by alkaline phosphatase, alizarin red, and oil red O staining, and quantitative real-time polymerase chain reaction (RUNX2, ALP, OCN, IGF1, PPARG, LPL, AP2, and GCP1). Finally, intersection between IGF and PDGF signaling pathways was evaluated.

RESULTS

IGF-1 was observed to increase osteogenic differentiation by all markers (p < 0.01). However, PDGF-alpha when added alone primarily did not affect osteogenic markers. PDGF-alpha positively regulated transcription of IGF1. Addition of PDGF-alpha in combination with or before IGF-1 enhanced osteogenesis more than either alone. IGF-1 increased whereas PDGF-alpha diminished human adipose-derived stromal cell adipogenesis.

CONCLUSIONS

IGF signaling significantly increased osteogenesis in human adipose-derived stromal cells and may be used for tissue-engineering purposes. The combination of PDGF and IGF may be more beneficial than either alone in driving adipose-derived stromal cell osteogenesis. Future in vivo applications will focus on the combination of adipose-derived stromal cells, biomimetic scaffolds, and recombinant IGF.