Circulating osteogenic precursor cells in type 2 diabetes mellitus

Abstract

Context: Type 2 diabetes mellitus (T2D) is associated with an increased risk of fractures and low bone formation. However, the mechanism for the low bone formation is not well understood. Recently, circulating osteogenic precursor (COP) cells, which contribute to bone formation, have been characterized in the peripheral circulation.

Objective: Our objective was to characterize the number and maturity of COP cells in T2D.

Patients, design, and setting: Eighteen postmenopausal women with T2D and 27 controls participated in this cross-sectional study at a clinical research center.

Main outcome measures: COP cells were characterized using flow cytometry and antibodies against osteocalcin (OCN) and early stem cell markers. Histomorphometric (n = 9) and molecular (n=14) indices of bone turnover and oxidative stress were also measured.

Results: The percentage of OCN(+) cells in peripheral blood mononuclear cells was lower in T2D (0.8 ± 0.2 vs. 1.6 ± 0.4%; P < 0.0001), whereas the percentage of OCN(+) cells coexpressing the early marker CD146 was increased (OCN(+)/CD146(+): 33.3 ± 7 vs. 12.0 ± 4%; P < 0.0001). Reduced histomorphometric indices of bone formation were observed in T2D subjects, including mineralizing surface (2.65 ± 1.9 vs. 7.58 ± 2.4%, P = 0.02), bone formation rate (0.01 ± 0.1 vs. 0.05 ±0.2 μm(3)/um(2) · d, P = 0.02), and osteoblast surface (1.23 ±0.9 vs. 4.60 ± 2.5%, P = 0.03). T2D subjects also had reduced molecular expression of the osteoblast regulator gene Runx2 but increased expression of the oxidative stress markers p66(Shc) and SOD2.

Conclusions: Circulating OCN(+) cells were decreased in T2D, whereas OCN(+)/CD146(+) cells were increased. Histomorphometric indices of bone formation were decreased in T2D, as was molecular expression of osteoblastic activity. Stimulation of bone formation may have beneficial therapeutic skeletal consequences in T2D.

Fig. 1.

Cell populations in T2D and controls. A, Representative flow cytometry dot plots showing sorting strategy for OCN⁺/CD146⁺ and OCN⁺/CD146⁻ cells. Total PBMC were first gated to exclude CD15⁺ cells. The OCN⁺/CD146⁺ and OCN⁺/CD146⁻ cells were then sorted out of the CD15⁻ populations (FSC, forward scatter; SSC, side scatter); B, the percentage of PBMC that was positive for OCN was lower in the T2D subjects (n = 18) compared with the controls (n = 27); C, the percentage of OCN⁺ cells that was also positive for CD146 was greater in the T2D subjects; D, HbA1c levels in the overall group (T2D plus controls combined) correlated inversely with the population of OCN⁺ cells (r = −0.49; P = 0.001), although within the T2D group alone, the correlation between HbA1c and OCN⁺ cells was not significant; E, HbA1c levels in the overall group (T2D plus controls combined) correlated with the subpopulations of OCN⁺ cells with the early stromal CD146 marker (OCN⁺/CD146⁺: r = 0.59; P < 0.0001), although within the T2D group alone, the correlation between HbA1c and OCN⁺/CD146⁺ cells was not significant. *, P < 0.05.

Fig. 2.

Expression analysis for molecular markers in sorted OCN⁺/CD146⁺ cells. T2D subjects (n = 7) had decreased expression of the osteoblastic master gene Runx2 and the glucose transporter GLUT-1 compared with controls (n = 7) but greater expression of the oxidative stress markers p66^Shc and SOD2. *, P < 0.05.

Fig. 3.

Histomorphometric changes in bone formation. A and B, Tetracycline double-labeled bone biopsies in a 58-yr-old T2D Caucasian woman (A) and a 57-yr-old Caucasian female control (B). Bone formation is decreased in T2D with reduced mineralizing surface. C, Higher glucose levels were associated with lower mineral apposition rate in the T2D subjects.