Association of increased cyclic adenosine 3':5'-monophosphate content in cultured human breast cancer cells and release of hydrolytic enzymes and bone-resorbing activity.

The established human breast cancer cell line MCF-7 resorbs bone directly in vitro independently of viable endogenous bone cells, and this resorption of bone is closely correlated with the release of hydrolytic enzymes by the cultured tumor cells. In this study we have examined the effects of a number of drugs which increase the intracellular cyclic adenosine 3':5'-monophosphate (cyclic AMP) content on the release of hydrolytic enzymes by the tumor cells and their capacity to resorb bone. When MCF-7 cultures were treated with cholera toxin (0.05 to 5 micrograms/ml), a potent adenylate cyclase inducer, mineral-releasing, lysosomal enzyme, and collagenolytic activities increased more than 2-fold in the cell culture medium. Prostaglandin E1 (0.1 microM), 8-bromocyclic AMP (10 mM), and isobutylmethylxanthine (30 microM) caused similar effects. These data suggest that adenylate cyclase activation and increases in cyclic AMP content in the tumor cells caused the release of lysosomal enzymes and collagenolytic activity and caused the resorption of bone. We have previously found that colchicine, a drug which inhibits microtubule assembly, also increased hydrolytic enzyme release and release of bone-resorbing activity. Direct measurements of cyclic AMP were made in MCF-7 cells 3 hr after treatment with colchicine (10 microM) as well as MCF-7 cells treated with prostaglandin E1, cholera toxin, and isobutylmethylxanthine. MCF-7 cells showed a 2-fold increase in cyclic AMP content after treatment with all of these agents, although there was no similar increase in mouse 3T3 cells which do not produce bone-resorbing activity under these conditions. These data suggest that increases in cyclic AMP concentrations in human breast cancer cells lead to release of hydrolytic enzymes and resorption of bone.