Anticarcinogenic effect of N6,O2'-dibutyryl cyclic adenosine 3':5'-monophosphate on 7,12-dimethylbenz(a)anthracene mammary tumor induction in the rat and its relationship to cyclic adenosine 3':5'-monophosphate metabolism and protein kinase.

A single intubation of 7,12-dimethylbenz(a)anthracene (DMBA) (20 mg in 1 ml sesame oil) to female Sprague-Dawley rats at 50 days of age produces primary mammary carcinomas in 80% of rats at 100 to 150 days of age. Administration of N6,O2'-dibutyryl cyclic adenosine 3':5'-monophosphate (DBcAMP) p.o. beginning at 1 day prior to DMBA intubation resulted in marked delay and reduction of tumor production: only 15% as many DBcAMP-treated rats had tumors as in the control group (DMBA only) with 60 days of delay in the first tumor appearance. DMBA-induced tumor production was preceded by changes in the cyclic adenosine 3':5'-monophosphate (cAMP) metabolism and protein kinase of the mammary gland. Within 24 hr post-DMBA intubation, the intracellular cAMP level and adenylate cyclase activity increased with an increase in type I isozyme of cAMP-dependent protein kinase, a form which has been associated with increased proliferative activity and a less differentiated cellular state in other tissues. The increases in cAMP level, adenylate cyclase activity, and the protein kinase activity were transient, and the values decreased to below the control values by Day 10 post-DMBA intubation. In mammary glands of rats that had received DBcAMP, the cAMP level and protein kinase isozyme pattern were similar to those of older rats that are no longer susceptible to the carcinogen. The inhibitory effect on DMBA-induced carcinogenesis may be related to the modifications that DBcAMP induces on cAMP level, adenylate cyclase activity, and cAMP-dependent protein kinase of the mammary gland.

Arrest of hormone-dependent mammary cancer growth in vivo and in vitro by cholera toxin.

Growth of 7,12-dimethylbenz(alpha)anthracene-induced mammary carcinoma in rat was arrested by daily s.c. injections of cholera toxin. At a dose of 2 micrograms/200-g rat, tumors regressed to 50% of their initial size within 2 weeks, and 85% of tumors regressed completely within 4 to 5 weeks. The same response to cholera toxin was observed with another hormone-dependent mammary tumor, MTW9, but not with the hormone-independent tumors, DMBA No. 1 and MT 13762. The latter result was consistent with the lack of response of these hormone-independent tumors to hormone removal (ovariectomy) or N6,O2'-dibutyryl cyclic adenosine 3':5'-monophosphate treatment. The growth-inhibitory effect of cholera toxin was dose dependent, and upon cessation of treatment tumors resumed growth; after complete regression, however, tumors did not reappear until 6 months after termination of the treatment. An amount of cholera toxin as high as 10 micrograms/day/200-g rat s.c. injected over a 6-week period showed no systemic toxicity in the animals. The growth of human breast cancer cells (MCF-7) in culture was also inhibited by a daily supplement of cholera toxin. At a concentration of 100 ng/ml, the cell replication ceased completely within 2 days. The growth inhibitions, both in vivo and in vitro, were accompanied by marked increases in the cellular cyclic adenosine 3':5'-monophosphate content and type II cyclic adenosine 3':5'-monophosphate-dependent protein kinase activity as well as a decrease of estrogen binding activity. Thus, extinction of mammary cancer can be achieved by cholera toxin, an agent that stimulates the intracellular cyclic adenosine 3':5'-monophosphate system.