Differentiation of 3T3-L1 fibroblasts to adipocytes. Effect of insulin and indomethacin on the levels of insulin receptors.

Differentiating (3T3-L1) and nondifferentiating (3T3-C2) fibroblastic cell lines possess two classes of insulin receptors, high affinity (KD = 1.0 to 3.7 X 10(-9) M) and low affinity (KD = 2.0 to 3.6 X 10(-8) M). Confluent cultures of 3T3-L1 cells induced to differentiate by insulin (1.74 x 10(-6) M) or indomethacin (1.25 x 10(-4) M) exhibit a 3-fold increase in the number of high affinity and low affinity receptors per cell or a 1.5- to 1.8-fold increase in the number of receptors per micron2 of surface area. In contrast, nondifferentiating 3T3-C2 cells treated with insulin or indomethacin lose almost completely the high affinity insulin receptors while retaining the same levels of low affinity receptors. The loss of high affinity receptors of the 3T3-C2 cells is accompanied by the disappearance of the stimulatory effect of insulin on the production of CO2 from glucose and on the uptake of aminoisobutyrate. The levels of high affinity insulin receptors appear to be regulated by different mechanisms in the differentiating (3T3-L1) and nondifferentiating (3T3-C2) cell lines. The mode of this regulation may have a bearing on the ability of a particular cell line to differentiate.

Induction of lipogenesis during differentiation in a "preadipocyte" cell line.

3T3-L1 fibroblasts differentiate in culture into cells having adipocyte character. This transition is accompanied by a 40- to 50-fold rise in the incorporation of [14C]acetate into triglyceride. The increase in lipogenic rate is exactly parallel to a coordinate rise in the activities of the key enzymes of the fatty acid biosynthetic pathway (ATP-citrate lyase, acetyl-CoA carboxylase, and fatty acid synthetase). Immunological studies indicate that the elevated acetyl-CoA carboxylase activity is the product of an increased cellular enzyme level.