CD146+ Human Umbilical Cord Perivascular Cells Maintain Stemness under Hypoxia and as a Cell Source for Skeletal Regeneration

The human umbilical cord perivascular cells (HUCPVCs) have been considered as an alternative source of mesenchymal progenitors for cell based regenerative medicine. However, the biological properties of these cells remain to be well characterized. In the present study, HUCPVCs were isolated and sorted by CD146(+) pericyte marker. The purified CD146(+) HUCPVCs were induced to differentiate efficiently into osteoblast, chondrocyte and adipocyte lineages in vitro. Six weeks following subcutaneous transplantation of CD146(+) HUCPVCs-Gelfoam-alginate 3D complexes in severe combined immunodeficiency (SCID) mice, newly formed bone matrix with embedded osteocytes of donor origin was observed. The functional engraftment of CD146(+) HUCPVCs in the new bone regenerates was further confirmed in a critical-sized bone defect model in SCID mice. Hypoxic conditions suppressed osteogenic differentiation while increased cell proliferation and colony-forming efficiency of CD146(+) HUCPVCs as compared to that under normoxic conditions. Re-oxygenation restored the multi-differentiation potential of the CD146(+) HUCPVCs. Western blot analysis revealed an upregulation of HIF-1 alpha, HIF-2 alpha, and OCT-4 protein expression in CD146(+) HUCPVCs under hypoxia, while there was no remarkable change in SOX2 and NANOG expression. The gene expression profiles of stem cell transcription factors between cells treated by normoxia and hypoxic conditions were compared by PCR array analysis. Intriguingly, PPAR-gamma was dramatically downregulated (20-fold) in mRNA expression under hypoxia, and was revealed to possess a putative binding site in the Hif-2 alpha gene promoter region. Chromatin immunoprecipitation assays confirmed the binding of PPAR-gamma protein to the Hif-2 alpha promoter and the binding was suppressed by hypoxia treatment. Luciferase reporter assay showed that the Hif-2 alpha promoter activity was suppressed by PPAR expression. Thus, PPAR-gamma may involve in the regulation of HIF-2 alpha for stemness maintenance and promoting the expansion of CD146(+) HUCPVCs in response to hypoxia. CD146(+) HUCPVCs may serve as a potential autologous cell source for bone regeneration.