De novo adipose tissue generation through long-term, local delivery of insulin and insulin-like growth factor-1 by PLGA/PEG microspheres in an in vivo rat model: A novel concept and capability

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 in sulin and insulin-like growth factor-1 (IGF-1) for the de novo generation o f adipose tissue in vivo. PLGA/PEG microspheres containing insulin and IGF- 1, separately, were produced by a double-emulsion solvent-extraction techni que. 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 microsp heres 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 w ere divided equally into four study groups (insulin, IGF-1, insulin + IGF-1 , and blank microspheres) (n = 4). Insulin and IGF-1 containing microsphere s were administered directly to the deep muscular fascia of the rat abdomin al wall to evaluate the potential for de novo adipose tissue generation via adipogenic differentiation from native nonadipocyte cell pools in vivo. An imals treated with blank microspheres served as an external control group. At the 4-week harvest period, multiple ectopic islands of adipose tissue we re 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 blan k microsphere group. Hematoxylin and eosin-stained sections of the growth f actor groups demonstrated mature adipocytes interspersed with fibrous tissu e superficial to the abdominal wall musculature and continuous with the fas cia. Oil-Red-O stained sections demonstrated that these cells contained lip id. Computer-aided image analysis of histologic sections confirmed that the re were statistically significant increases in the amount of "ectopic" adip ose neotissue developed on the abdominal wall of animals treated with growt h factor microspheres. In conclusion, this study confirms the long-term rel ease 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 dif ferentiation to mature lipid-containing adipocytes from nonadipocyte cell p ools in vivo at 4 weeks.