gms | German Medical Science

Background: Osteoporosis (OP) predominantly affects elderly people and is characterized by bone loss, increased fracture risk and reduced regeneration ability. Age-related bone loss correlates with increased bone marrow adiposity due to a shift of osteogenic towards adipogenic differentiation of bone marrow mesenchymal stem cells (MSC). Adipose tissue is metabolically active: It releases proinflammatory (e.g. adipokines) and matrix-degrading proteins (e.g. matrix metalloproteinases, MMPs), which directliy promote bone loss. Therefore, adipocyte-derived factors might influence differentiation of bone marrow-derived MSC. We analyzed the presence of adipokines (visfatin, resistin and leptin) in the bone marrow cavity and their effects on MSC differentiation.

Methods: RNA and MSC were isolated from spongiosa from femoral heads (hip replacement surgery of osteoarthritis patients or after osteoporotic femoral neck fracture). Adipogenic as well as osteogenic MSC differentiation was performed with/without adipokines. For the transfer and differentiation of MSC on cancellous bone, bone fragments were purified and sterilized. Gene expression was evaluated by Realtime PCR. Matrix mineralization was assayed using Alizarin red S staining. Proinflammatory factors were measured by ELISA.

Results: Visfatin and leptin levels were increased in OP bone vs. non-osteoporotic bone. In contrast resistin was reduced (n=14). Visfatin induced the secretion of proinflammatory factors (IL-6, IL-8, MCP-1) during both, osteogenic and adipogenic differentiation. Significantly increased matrix mineralization and downregulated collagen type 1 expression (e.g. d21: -4.6-fold) was detected in osteogenically differentiated cells after visfatin stimulation. In contrast to leptin and resistin, visfatin reduced expression of MMP2, MMP13, TIMP1 and TIMP2 (e.g. d21: -2.4-fold / -3.18-fold / -3.2-fold / -4.3 fold, respectively) during osteogenesis. Visfatin significantly induced MMP13 expression (e.g. d21: 104-fold) during adipogenic differentiation under standard cell culture conditions, however visfatin-induced MMP13 expression as well as IL-6 and IL-8 release was markedly reduced during adipogenic differentiation on purified autologous cancellous bone.

Conclusion: Visfatin and leptin levels were elevated in osteoporotic bone. Therefore, visfatin-mediated increase of matrix mineralization and reduction of collagen type 1 expression might lead to enhanced bone fragility. Visfatin-induced release of proinflammatory cytokines and dysregulated expression of MMPs and TIMPs during MSC-differentiation might therefore influence bone turnover at the adipose tissue/bone interface.