The ability of hMSCs to differentiate into several mesenchymal cell lineages including the osteoblast lineage plays a key role in skeletogenesis and bone regeneration. Although the importance of physical factors in the development and maintenance of bone tissue has been recognized for many years and we previously demonstrated that mechanical strain constitutes an inherent stimulus for osteogenic differentiation of undifferentiated hMSCs, there is strong evidence to suggest that obesity is an independent factor in the risk of implant failure due to aseptic loosening or fracture after TJR. While mechanical complications and overload have been widely suggested, we hypothesized that the osteogenic mechanoresponse of hMSCs may be profoundly altered in obese patients.

hMSCs were isolated from bone marrow of 10 donors (BMI ranging from 18.7 to 37.6 kg/m2). The individual response of unidfferentiated hMSCs to cyclic tensile strain (CTS) was determined in a two-armed study design (strained versus unstrained (CTR)) using a 4-point bending device, where strain was restricted to a maximum of 3,000 μstrain. Phenotypic effects were characterized by analyzing cell numbers, cell viability and ALP activity; mRNA levels of marker genes related to early osteogenic differentiation (RUNX2, ALPL, SPARC, SPP1), protein synthesis (COL1A1), and cell cycle (MKI67) were determined by real-time RT-PCR. Possible contributions to anthropomorphometric variables and individual triglycerides, cholesterin, glucose, leptin, adiponectin, resistin, and estradiol levels were evaluated by linear regression analysis.

We found a significant up-regulation of the osteogenic marker genes due to CTS, including RUNX2 (1.9 fold), ALPL (2.4 fold), SPP1 (2.8 fold), and SPARC (4.1 fold), which was accompanied by an increase in cell-based ALP activity from 6.1 ± 1.2 μM/min/106 in CTR to 8.5 ± 1.7 μM/min/106 in CTS (plus 39.6 ± 9.8% SEM, P< 0.05). Cell density was significantly lower following CTS (minus 20.0 ± 4.7%, P< 0.05), which was also found for cell viability (XTT minus 17.8 ± 5.6%, P< 0.05). As a consequence, the phenotypic CTS response (ALP activity w/o normalization) ranged widely between donors (−30.8% to +60.1%) and was highly significant inverse correlated to donor’s BMI (r= −0.91, P< 0.0001). Additionally, leptin and estradiol levels determined within bone marrow plasma were significantly correlated with the phenotypic mechanoresponse (r=−0.71, P=0.028, and r=0.67; P=0.039; respectively).

The findings demonstrate that the osteogenic mechanosensitivity of hMSCs is highly affected by physiological factors related to donor’s BMI. Such an upstream imprinting process within bone marrow may be an important area of further research, since obesity-linked problems constitute increasing concerns in orthopaedic surgery within the western world.