The role of microfilaments in the response of adrenal tumor cells to adrenocorticotropic hormone

Highly purified anti-actin was prepared from sera of patients suffering from hepatitis by affinity chromatography using actin as ligand. The anti-actin was entrapped within liposomes prepared from phosphatidylcholine and phosphatidylserine and presented to Y-1 mouse adrenal tumor cells. Such liposomes were also prepared with entrapped cyclic [³H]AMP; this method of presenting the cyclic nucleotide to the cells lowered the threshold to cyclic AMP by 2 orders of magnitude compared to free nucleotide as judged by production of 20α-dihydroprogesterone. Y-1 cells were incubated with liposomes containing buffer or buffer with anti-actin for 1 h. When the medium was changed and incubation was continued with 0.9% NaCl (saline), adrenocorticotropic hormone (ACTH), or dibutyryl cyclic AMP, it was found that incubation with anti-actin inhibited three steroidogenic responses to ACTH and dibutyryl cyclic AMP: (i) increased production of 20α-dihydroprogesterone; (ii) increased production of pregnenolone in isolated mitochondria prepared from cells previously incubated with either stimulating agent; (iii) increase in the cholesterol content of inner mitochondrial membranes isolated from cells incubated with aminoglutethimide (an inhibitor of side chain cleavage of cholesterol) and ACTH. Liposomes containing buffer were without detectable effect on the response to ACTH. Normal human immunoglobulin G, boiled anti-actin, and anti-actin plus excess actin in liposomes were without effect on the subsequent response to ACTH. These findings, together with previous evidence, suggest that ACTH and cyclic AMP stimulate steroidogenesis by increasing the transport of cholesterol to the mitochondrial side chain cleavage enzyme by a mechanism involving actin.