Altered lineage commitment of bone marrow mesenchymal stem cells in idiopathic osteonecrosis of the femoral head.

Background: Osteonecrosis of the femoral head (ONFH) is an ischaemic disorder often leading to collapse of the femoral head and severe hip dysfunction. Mesenchymal stem cells (MSCs) have a key role in bone repair, through their ability to differentiate into osteoblasts and their paracrine regulation of the bone microenvironment. While altered MSCs behaviour has been reported in some secondary forms of ONFH, the proliferative and differentiation programmes of MSCs in human idiopathic ONFH have not been previously characterized.

Aim: To compare the proliferative capacity, differentiation potential and nuclear factor kappa B (NF-κB) pathway activation of bone marrow MSCs (BM-MSCs) from idiopathic ONFH patients with those from osteoarthritis controls.

Methods: Femoral heads were collected during total hip replacement surgeries. Idiopathic ONFH was defined by imaging and histological criteria. Secondary causes were excluded. BM-MSCs were isolated from trabecular bone cylinders and expanded to passage 2 prior characterizations. Proliferation was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay at various seeding densities. Osteogenic potential was assessed by alkaline phosphatase activity, osteogenic gene expression (RUNX2, ALPL, COL1A1 and BGLAP) and Alizarin Red staining. Adipogenesis was quantified by Oil Red O staining. Expression of NF-κB target genes (IL6, NFKBIA, CCL2) was analyzed by quantitative polymerase chain reaction.

Results: Idiopathic ONFH MSCs exhibited significantly higher proliferation rates than osteoarthritis controls. However, they showed reduced alkaline phosphatase activity and osteogenic gene expression but paradoxically, increased mineralization, suggesting non-canonical mineral deposition mechanisms. These cells also display increased adipogenic differentiation. Importantly, ONFH-MSCs expressed higher, although non-significant levels of certain NF-κB target gene genes, consistent with an activated inflammatory state.

Conclusion: Human BM-MSCs from idiopathic ONFH display a paradoxical phenotype: Hyperproliferative yet osteogenically impaired with greater adipogenesis and activation of NF-κB signalling. This functional compromise and inflammatory bias may underline the failure of bone regeneration in ONFH, highlighting the need for therapies re-directing MSCs fate and modulating the bone marrow niche.