Retinola Feeding, Hormone Inhibition, and/or Immune Stimulation and the Progression of /V-Methyl-N-nitrosourea-induced Rat Mammary Carcinoma: Suppression by Retinoids of Peptide Hormone-induced Tumor Cell Proliferation in Vivo and in Vitro1

Female Sprague-Dawley rats were treated at 50 and 57 days of age with 2.5 mg per 100 g body weight of A/-methyl-A/nitrosourea (MNU). At 60 days of age, the animals were divided into eight groups (40 rats/group) and treated as follows: (a) controls; (b) immune stimulation (IS); (c) hormone inhibition (HI); (d) HI + IS; (e) retinyl acetate feeding (RA); (f ) RA + IS; (g) RA + HI; and (h) RA + HI + IS. IS treatment was accomplished by three i.p. injections (at 1, 3, and 5 weeks after carcinogen treatment) of a mixture of cell paniculate preparations from pooled MNU-induced rat mammary carcinomas and Freund's (complete) adjuvant. HI treatment consisted of daily s.c. injec tions of tamoxifen (12.5 to 25.0 /^g/IOO g body weight) and CB154 (200 to 400 ¿ig/100g body weight), and RA treatment consisted of daily feeding of retinyl acetate (1.0 ITIM).Both RA alone and HI alone significantly (p < 0.01 to 0.001) reduced mammary carcinoma incidence; HI treatment was significantly (p < 0.01 ) more effective than RA treatment. The combination of RA + HI was significantly (p < 0.01) superior to either treatment alone; RA + HI treatment virtually completely blocked the devel opment of mammary carcinomas at termination of study (a total of only 2 mammary carcinomas was observed in the RA + HI groups of rats at 1 year after carcinogen treatment). IS treat ments did not significantly influence mammary carcinoma inci dence, either alone or in combination with RA and/or HI. Female Sprague-Dawley rats given MNU and subsequently treated daily for 4 weeks with the prolactin secretion-stimulating drug haloperidol (0.05 mg/100 g body weight) responded with a significant (p < 0.001) increase in mammary carcinoma devel opment when compared with control rats. RA treatment of haloperidol-treated rats significantly (p < 0.001) blocked the stimulatory effect of haloperidol on mammary carcinoma devel opment. The addition of insulin (5.0 nQ/rr\\) to the culture media of 2-day organ cultures of MNU-induced rat mammary carcino mas resulted in a significant (p < 0.05) stimulation of [3H] thymidine incorporation into DNA of the cultured cells. Retinoic acid (1 x 10"6 M) significantly (p < 0.05) blocked the stimulatory effect of insulin on [3H]thymidine incorporation into DNA; retinoic acid alone did not significantly affect [3H]thymidine incorporation into DNA. The results of this study demonstrate a striking prevention of the development and progression of MNU-induced rat mammary carcinomas by a combination of two biological response modifier treatments, i.e., RA and HI. Retinoid-induced suppression of peptide hormone-induced stimulation of mammary carcinoma cells in vivo and in vitro has also been demonstrated, suggesting a mechanism by which retinoids may suppress the progression of rat mammary carcinomas.