Evaluation of receptor function in ACTH-responsive and ACTH-insensitive adrenal tumor cells

Two variant cell lines (Y6 and OS3), derived from the ACTH-sensitive mouse adrenocortical tumor clone Y1, are defective in the ACTH-sensitive adenylate cyclase system. This study further characterizes the nature of the defects in Y6 and OS3 cells using ACTH1-10, ACTH4-10, and cholera toxin. In Y1 cells, ACTH1-39, ACTH1-10, and ACTH4-10 stimulated steroidogenesis to the same maximum level with Kd' values of 5 x 10(-11) M, 5 x 10(-7) M and 10(-4) m respectively. ACTH1-10 (0.4 mM) and ACTH4-10 (3.2 mM) increased the accumulation of adenosine 3',5'-monophosphate (cAMP) in Y1 cells two- to three-fold. Cholera toxin increased steroidogenesis and cAMP accumulation in Y1 cells with Kd' values of 0.4 ng/mL and 9 ng/mL respectively. Y6 and OS3 cells responded to added cholera toxin with increased cAMP accumulation and increased steroidogenesis but did not respond to ACTH1-39, ACTH1-10, or ACTH4-10 at concentrations effective in Y1 cells. These data are interpreted to suggest that Y6 and OS3 cells are defective in a process or component that links the principal binding regions of the ACTH receptor to the catalytic subunit of the adenylate cyclase system. Attempts to were made to assess the interactions of ACTH with the principal binding regions of the ACTH receptor by analysis of binding of radioactive, iodinated ACTH1-24. ACTH binding, however, showed low affinity, high capacity, and no target-tissue specificity, and was considered not to be useful in evaluating the integrity of the ACTH receptor.