This study was undertaken to characterize the duration of long-term growth factor delivery by poly (lactic-co-glycolic-acid)-polyethylene glycol (PLGA/PEG) microspheres and to evaluate the potential of long-term delivery of insulin and insulin-like growth factor-1 (IGF-1) for the de novo generation of adipose tissue in vivo. PLGA/PEG microspheres containing insulin and IGF-1, separately, were produced by a double-emulsion solvent- extraction technique. In the first phase of the experiment, the in vitro release kinetics of the microspheres were evaluated for the optical density and polyacrylamide gel electrophoresis of solutions incubated with insulin- containing microspheres for four different periods of time (n = 1). The finding of increased concentrations of soluble insulin with increased incubation time confirmed continual protein release. In the second stage of the experiment, 16 rats were divided equally into four study groups (insulin, IGF-1, insulin + IGF-1, and blank microspheres) (n = 4). Insulin and IGF-1 containing microspheres were administered directly to the deep muscular fascia of the rat abdominal wall to evaluate the potential for de novo adipose tissue generation via adipogenic differentiation from native nonadipocyte cell pools in vivo. Animals treated with blank microspheres served as an external control group. At the 4-week harvest period, multiple ectopic islands of adipose tissue were observed on the abdominal wall of the animals treated with insulin, IGF-1, and insulin + IGF-1 microspheres. Such islands were not seen in the blank microsphere group. Hematoxylin and eosin- stained sections of the growth factor groups demonstrated mature adipocytes interspersed with fibrous tissue superficial to the abdominal wall musculature and continuous with the fascia. Oil-Red-O stained sections demonstrated that these cells contained lipid. Computer-aided image analysis of histologic sections confirmed that there were statistically significant increases in the amount of 'ectopic' adipose neotissue developed on the abdominal wall of animals treated with growth factor microspheres. In conclusion, this study confirms the long-term release of proteins from PLGA/PEG microspheres up to 4 weeks and demonstrates the potential of long- term local insulin and IGF-1 to induce adipogenic differentiation to mature lipid-containing adipocytes from nonadipocyte cell pools in vivo at 4 weeks.
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