Cyclic AMP-mediated cytoskeletal effects in adrenal cells are modified by serum, insulin, insulin-like growth factor-I, and an antibody against urokinase plasminogen activator

In adrenocortical cells in culture, increased intracellular cyclic AMP resulting from exposure to agents such as ACTH and cholera toxin causes a change in cell morphoology termed 'retraction' or 'rounding'. The breakdown of actin-containing stress fibers in rounding suggested a role for microfilaments in steroidogenesis. Previously, we showed that cultured bovine adrenal cells under standard conditions (medium with 10% fetal bovine serum) do not round in response to intracellular cyclic AMP. Here, we show that these cells do round in defined, serum-free medium. Rounding was maximal within 1 h of addition of 1 nM cholera toxin and after 10 h most cells remained rounded. Cycloheximide at 100 μg/ml did not inhibit the response to cholera toxin. The rounding response was abolished when 10% fetal bovine serum, horse serum, or ether-extracted fetal bovine serum was included in the medium. The inhibitory effect of serum was not mimicked by growth factors with the exception that insulin and insulin-like growth factor-I (IGF-I), while not preventing rounding, accelerated the return of cells to a flattened morphology. A monoclonal antibody against urokinase plasminogen activator completely prevented rounding whereas a monoclonal antibody against tissue plasminogen activator had only a slight effect. Fluorescence visualization of F-actin with N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-phallacidin showed that rounding in cultured bovine adrenocortical cells resembles that defined earlier for human and rat adrenocortical cells and includes depolymerization of actin microfilaments. These cytoskeletal changes in adrenal cells are unlikely to play a role in steroidogenesis; however, they may be involved in tissue remodeling occurring as part of the indirect mitogenic effects of ACTH.