Obesity morbidity is associated with excess visceral adiposity, whereas sc adipose tissue is much less metabolically hazardous. Human abdominal sc preadipocytes have greater capacity for proliferation, differentiation, and survival than omental preadipocytes. IGF-I is a critical mediator of preadipocyte proliferation, differentiation, and survival through multiple signaling pathways.We investigated IGF-I action in primary cultures of human preadipocytes isolated from sc and omental adipose tissue of obese subjects. IGF-I-stimulated DNA synthesis was significantly lower in omental compared with sc preadipocytes. IGF-I phosphorylation of the IGF-I receptor and the ERK pathway was comparable in sc and omental cells. However, omental preadipocytes had decreased insulin receptor substrate (IRS)-1 protein associated with increased IRS-1-serine636/639 phosphorylation and degradation. IGF-I-stimulated phosphorylation of AKT on serine473 but not threonine308 was decreased in omental cells, and activation of downstream targets, including S6Kinase, glycogen synthase kinase-3, and Forkhead box O1 was also impaired. Cyclin D1 abundance was decreased in omental cells due to increased degradation. Over-expression of IRS-1 by lentivirus in omental preadipocytes increased IGF-I-stimulated AKT-serine473 phosphorylation. The mammalian target of rapamycin (mTOR)-Rictor complex regulates phosphorylation of AKT-serine473 in 3T3-L1 adipocytes, but knockdown of Rictor by lentivirus-delivered short hairpin RNA in sc preadipocytes did not affect AKT-serine473 phosphorylation by IGF-I. These data reveal an intrinsic defect in IGF-I activation of the AKT pathway in omental preadipocytes from obese subjects that involves IRS-1 but probably not mTOR-Rictor complex. We conclude that impaired cell cycle regulation by AKT contributes to the distinct growth phenotype of preadipocytes in visceral fat of obese subjects.
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