We read with interest the recent article by Gharibjanian et al.1 regarding the osteogenic potential of bone morphogenetic protein-2 (BMP-2) covalently bound to poly(lactic-co-glycolic acid) and polycaprolactone, on MC3T3-E1 preosteoblast cells in two-dimensional cultures. We write to provide additional comments and perspectives on the study. An up-regulation was noted in alkaline phosphatase and osteocalcin activity at day 4 in cells cultured on poly(lactic-co-glycolic acid) plus BMP-2, but not in cells cultured on polycaprolactone plus BMP-2 or polycaprolactone alone (Fig. 1). Osteocalcin and alkaline phosphatase up-regulation was higher in the control group compared with the polycaprolactone plus BMP-2 group or the polycaprolactone group (Fig. 1) at day 4. This was despite the fact that the peak in BMP-2 release from polymers (Fig. 4) occurring at day 2 was higher in the polycaprolactone plus BMP-2 group (approximately 300 pg) compared with poly(lactic-co-glycolic acid) plus BMP-2 (approximately 150 pg). We wonder whether this occurred because of cell adhesion differences on the polymer surface secondary to differential topographic surface changes in polycaprolactone compared with poly(lactic-co-glycolic acid) or controls. Variation in cell adhesion based on surface topography of biodegradable polymers such as poly(lactic acid), poly(lactic-co-glycolic acid), and polycaprolactone is well recognized.2 Also, extremes of pH such as exposure to chloric acid or sodium hydroxide have previously been shown to nanostructure these polymer surfaces and alter cell-surface mechanical stress interactions.2,3 Cytoskeletal stress is known to influence osteoinduction in osteoprecursor cells such as MC3T3-E1 cells.4 It is plausible therefore that exposure to 6N sulfuric acid during the covalent binding of BMP-2 could have altered the polycaprolactone surface, causing reduced cell adhesion that in turn decreased osteogenesis.
It is interesting that after the initial surge in BMP-2 release from polycaprolactone, the levels were subtherapeutic from days 6 to 10, with an inexplicable late peak at day 12 (Fig. 4). This late peak could be attributable to chemical degradation of the polycaprolactone. We wonder whether the authors conducted any studies to investigate this phenomenon and the structural integrity of the polycaprolactone surface. If the acid hydrolysis essential to covalent bonding process indeed causes chemical degradation of polycaprolactone, this would have obvious implications in future clinical use where structural integrity of scaffolds in bone reconstruction is of utmost importance.
Finally, our experience with the use of mesenchymal stem cells in osseous tissue engineering indicates a differential response to growth factors such as BMP-2 in three-dimensional versus two-dimensional cultures.5 Because three-dimensional cultures mimic in vivo conditions more closely than two-dimensional cultures, it would be interesting to see similar studies on three-dimensional scaffolds. Our previous work also underscored the necessity of osteoinductive agents such as BMP-2 for robust osteogenesis in three dimensions.5 The delivery of osteoinductive agents by means of bioactive scaffolds is an innovative technique. We compliment the authors on a nicely conducted study and thank the Editor for the opportunity to share our thoughts on this article.
Clifford T. Pereira, M.B.B.S.
Weibiao Huang, Ph.D.
Gregory Sayer, B.A., B.S.
Reza Jarrahy, M.D.
George Rudkin, M.D.
Timothy A. Miller, M.D.
Plastic Surgery Molecular Biology Laboratory
Department of Veterans Affairs
Greater Los Angeles Healthcare System
Los Angeles, Calif.
The authors have no financial interest or commercial association with any of the subject matter or products mentioned in this letter.
1. Gharibjanian NA, Chua WC, Dhar S, et al. Release kinetics of polymer-bound bone morphogenetic protein-2 and its effects on the osteogenic expression of MC3T3-E1 osteoprecursor cells. Plast Reconstr Surg
2. Lee SJ, Khang G, Lee YM, Lee HB. Interactions of human chondrocytes and NIH/3T3 fibroblasts on chloric acid-treated biodegradable polymer surfaces. J Biomater Sci Polym Ed
3. Marletta G, Ciapetti G, Satriano C, Perut F, Sarerno M, Baldini N. Improved osteogenic differentiation of human marrow stromal cells cultured on ion-induced chemically structured poly-epsilon-caprolactone. Biomaterials
4. McBeath R, Pirone DM, Nelson CM, Bhadriraju K, Chen CS. Cell shape, cytoskeletal tension, and RhoA regulate stem cell lineage commitment. Dev Cell
5. Huang W, Carlsen B, Wulur I, et al. BMP-2 exerts differential effects on differentiation of rabbit bone marrow stromal cells grown in two dimensional and three-dimensional systems and is required for in vitro bone formation in a PLGA scaffold. Exp Cell Res
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