Insulin-like growth factor-I is an essential regulator of the differentiation of 3T3-L1 adipocytes.
Murine 3T3-L1 preadipocytes proliferate normally in medium containing fetal calf serum depleted of insulin, growth hormone, and insulin-like growth factor-I (IGF-I). However, the cells do not differentiate into adipocytes in the presence of the hormone-depleted serum. Supplementation of the growth medium with 10-20 nM IGF-I or 2 microM insulin restores the ability of 3T3-L1 cells to develop into adipocytes. The cells acquire an adipocyte morphology, accumulate triglycerides, and express a 450-fold increase in the activity of the lipogenic enzyme glycerol-3-phosphate dehydrogenase. The increase in glycerol-3-phosphate dehydrogenase activity is paralleled by the accumulation of glycerol-3-phosphate dehydrogenase mRNA and mRNA for the myelin P2-like protein aP2, another marker for fat cell development. IGF-I or insulin-stimulated adipogenesis in 3T3-L1 cells is not dependent on growth hormone. Occupancy of preadipocyte IGF-I receptors by IGF-I (or insulin) is implicated as a central step in the differentiation process. The IGF-I receptor binds insulin with a 70-fold lower affinity than IGF-I, and 30-70-fold higher levels of insulin are required to duplicate the effects of an optimal amount of IGF-I. The effects of 10-20 nM IGF-I are likely to be mediated by high affinity (KD = 5 nM) IGF-I receptors that are expressed at a density of 13,000 sites/preadipocyte. In undifferentiated cells the IGF-I receptor concentration is twice that of the insulin receptor. After adipocyte differentiation is triggered, the number and affinity of IGF-I receptors remain constant while insulin receptor number increases approximately 25-fold as developing adipocytes become responsive to insulin at the level of metabolic regulation. Thus, preadipocytes have the potential for a maximal response to IGF-I, whereas the accumulation of more than 95% of adipocyte insulin receptors and the appearance of responsiveness to insulin are consequences of differentiation. IGF-I or insulin is essential for the induction of a variety of abundant and nonabundant mRNAs characteristic of 3T3-L1 adipocytes.
Smith
P J
PJ
Department of Molecular Pharmacology, Atran Laboratories, Albert Einstein College of Medicine, Bronx, New York 10461.
Wise
L S
LS
Berkowitz
R
R
Wan
C
C
Rubin
C S
CS
D000273:Adipose Tissue / Q000166:cytology*; D000818:Animals; D002454:Cell Differentiation / Q000187:drug effects*; D002478:Cells, Cultured; D004274:DNA, Recombinant / Q000378:metabolism; D005993:Glycerolphosphate Dehydrogenase / Q000235:genetics / Q000378:metabolism; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000378:metabolism / Q000494:pharmacology*; D007700:Kinetics; D051379:Mice; D011972:Receptor, Insulin / Q000378:metabolism; D017451:Receptors, Somatomedin; D013002:Somatomedins / Q000494:pharmacology*
D004274:DNA, Recombinant; D007328:Insulin; D017451:Receptors, Somatomedin; D013002:Somatomedins; D007334:Insulin-Like Growth Factor I; D005993:Glycerolphosphate Dehydrogenase; D011972:Receptor, Insulin