Original ArticleClinicalOsteoblastogenesis and Adipogenesis Are Higher in Osteoarthritic than in Osteoporotic Bone Tissue
Introduction
Osteoporosis (OP) and osteoarthritis (OA) are two common age-related disorders affecting quality of life of the elderly. OP is associated with a reduced bone mass and microarchitectural changes in bone tissue with a consequent increased risk of fracture. OA is a disease associated with articular cartilage degeneration, resulting in joint pain, stiffness and limitation of movement. Bone is also affected in OA as cysts and osteophytes can be present in subchondral bone of the affected joint (1) and changes in trabecular bone tissue distal to the affected joint have been demonstrated (2), suggesting bone may play a role in OA pathogenesis. Although there is some controversy, OA and OP are thought to be inversely related. OA patients have higher bone mass or bone density compared to age- and gender-matched normal or OP subjects. OA and OP are rarely present in the same patient together and if patients with OA experience an osteoporotic fracture, they do so at a later age, suggesting OA may play a protective role in OP development (3).
Bone mass and bone quality depend on the balance between processes of bone resorption and bone formation. Osteoblasts are bone-forming cells originating from the same precursor as fat storing adipocytes—mesenchymal stem cell (MSC). The main transcription factor essential for osteoblast differentiation from MSC is runt-related transcription factor 2 (RUNX2) (4). Another important osteoblast-specific transcription factor is osterix, which acts downstream of RUNX2 5, 6. MSC differentiation to adipocytes is under transcriptional control of peroxisome proliferator-activated receptor γ (PPARγ), a nuclear receptor that activates the expression of adipocyte phenotype-specific genes and adipocyte differentiation 7, 8. MSC differentiation is also determined by the interplay between transcription factors and cofactors. One is a WW domain containing transcription regulator 1 (WWTR1), which enhances the expression of RUNX2 downstream target genes and represses PPARγ-induced gene expression and may function as a transcriptional modifier of MSC differentiation by promoting osteoblastogenesis and impairing adipogenesis (9).
In aging and OP, osteoblastogenesis and adipogenesis are thought to be inversely related. Because MSC is a common precursor of osteoblasts and adipocytes, a shift of MSC differentiation towards the adipocyte lineage could conversely decrease osteoblastogenesis. Using light microscopy on human iliac crest biopsies, Justesen et al. demonstrated that bone marrow adipose tissue volume increases and trabecular bone volume decreases with age and this effect is even more pronounced in patients with OP (10). Similarly, bone marrow adipose tissue quantified with magnetic resonance imaging was inversely related with bone mineral density (BMD) (11). Several studies on rodents also support the inverse relationship between osteoblastogenesis and adipogenesis. Mice administered rosiglitazone (a PPARγ agonist) exhibit bone loss and decreased bone formation with an increased bone marrow adiposity 12, 13 and mice models with reduced PPARγ activity have higher bone mass and increased bone formation 14, 15. There are few papers addressing the relationship between osteoblastogenesis and adipogenesis in OA; however, none were performed on human bone tissue 16, 17.
Studies exploring differences in bone metabolism of OA and OP patients may contribute to understanding underlying pathological mechanisms in both diseases. Therefore, the aim of our study was to determine whether the expression of chosen adipogenic genes is lower in OA compared to OP bone tissue because OA and OP present the opposing bone phenotype. Adipogenesis was evaluated by measuring the expression of predominantly adipocyte-specific isoform PPARγ2 and its downstream genes adiponectin, perilipin 2, angiopoietin-like 4 (ANGPTL4) and fatty-acid binding protein 4 (FABP4). Osteoblastogenesis was evaluated by measuring the expression of RUNX2 and its downstream genes osterix and osteocalcin.
Section snippets
Patients
One hundred three patients undergoing hip arthroplasty due to fragility fracture of the hip or OA were included in our study. Fifty four patients were diagnosed with OP based on a nontraumatic, low-energy hip fracture. Forty nine patients suffering from OA were diagnosed by clinical and radiographic criteria according to Harris hip score. Exclusion criteria for all patients included any history of systemic or metabolic diseases known to impact bone or mineral metabolism or taking any drugs
Results
Osteoblastogenesis and adipogenesis were evaluated by measuring the expression of RUNX2, PPARγ2 and their downstream genes. mRNA levels of the measured genes were normalized to geometric mean of reference genes GAPDH and RPLP0. Normalized mRNA levels for RUNX2, osteocalcin, osterix, PPARγ2, adiponectin, perilipin 2, ANGPTL4, FABP4 and WWTR1 were compared between the OA and OP groups (Figure 1). In the OA group the significantly higher expression of RUNX2 (p <0.001), osterix (p = 0.027),
Discussion
In the present study we attempted to compare osteoblastogenesis and adipogenesis in OA and OP bone tissue. Our results show significantly higher expression of RUNX2, osterix, osteocalcin, PPARγ2 and adiponectin and lower expression of perilipin 2, ANGPTL4 and FABP4 in the OA group. All target genes (except ANGPTL4 in OP) exhibited positive correlations with their transcription factor expression in both groups.
RUNX2 and osterix are transcriptional factors essential for osteoblast differentiation
Acknowledgments
The study was supported by research programs P3-0298 and J3-2330 of the Ministry of Higher Education, Science and Technology of Slovenia. We thank the patients participating in our study for donating bone tissue. We thank Zoran Trošt for valuable advice on qPCR experiments, Igor Locatelli for help with statistical analysis, Professor Roger Pain, Barbara Mlinar and Matjaž Jeras for critical reading of the manuscript and Professor Roger Pain for advice on the English language.
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