Characterization of adrenocorticotropin receptors that appear when 3T3-L1 cells differentiate into adipocytes

The binding of an 125I-labeled analog of ACTH, [125I]Tyr23,Phe2,Nle4-ACTH-(1-38), to differentiated 3T3-L1 fat cells was characterized. Time-dependent binding, which was inhibited by saturating concentrations of unlabeled ACTH (0.44 microM), could be demonstrated in the differentiated cells. Using 0.4 nM [125I]ACTH analog and increasing concentrations of ACTH, the half-maximal concentration for inhibition by ACTH was 4.3 nM. Scatchard analysis demonstrated a single class of ACTH binding. There were approximately 3500 binding sites/cell. The binding of [125I]ACTH analog was specific in that it could be displaced by ACTH, ACTH-(1-19), ACTH-(1-17), and N-acetyl-Ser1-ACTH, but not by high concentrations of insulin, beta-endorphin, or polylysine. There was an excellent correlation between the ability of ACTH and its analogs to inhibit [125I]ACTH analog binding and the ability of ACTH and its analogs to stimulate cAMP production. In contrast, no saturable binding could be demonstrated when undifferentiated 3T3-L1 fibroblasts, which are not responsive to ACTH, were studied. Thus, differentiation of 3T3-L1 cells into the adipocyte form is accompanied by the appearance of receptors for ACTH. These receptors allow the adipocytes to respond to ACTH.