EFFECT OF MEDROXYPROGESTERONE ACETATE (MPA) AND SERUM FACTORS ON CELL-PROLIFERATION IN PRIMARY CULTURES OF AN MPA-INDUCED MAMMARY ADENOCARCINOMA

The effect of progesterone (Pg), medroxyprogesterone acetate (MPA), es tradiol (E(2)), dihydrotestosterone (DHT) and dexamethasone (DEXA) was studied on the in vitro growth rate of a progestin-dependent (PD), es trogen-sensitive mammary tumor line originated in an MPA-treated BALB/ c mouse (C4-HD), and on its estrogen-resistant variant (C4-HDR). The s pecificity of hormone action was further investigated using the anti-h ormones RU-486 and hydroxyflutamide (FLU). Cell growth was evaluated i n epithelial and fibroblast-enriched cultures using H-3-thymidine and/ or autoradiography and immunocytochemistry. The results indicate that cell growth is directly stimulated by MPA and Pg at concentrations ran ging from 10(-11) to 10(-7) M. RU486 prevented MPA-induced stimulation in concentrations 10 to 100 fold lower than those of MPA. When used a lone, it inhibited cell proliferation only in concentrations higher th an 10(-11)M. At nM concentrations, neither DEXA nor DHT stimulated H-3 -thymidine uptake except DEXA at 100 nM. MPA-induced stimulation was n ot reverted by micromolar concentrations of FLU. As for E(2) (10(-7)-1 0(-9) M) it prevented MPA stimulation only in cultures of estrogen-sen sitive tumors. Progesterone receptors (PR) (475 +/- 115 fmoles/10(5) c ells, n = 5) and estrogen receptors (ER) (ND-115 fmoles/10(5) cells, n = 5) were detected only in epithelial-enriched cultures. Serum from 7 day-MPA-treated mice induced a significant increase of H-3-thymidine uptake; an increase was also obtained with serum from untreated ovarie ctomized animals to which 1 nM-100 nM concentrations of MPA had been a dded. The stimulatory effect of the exogenous MPA was much lower than that of the serum obtained from MPA-treated animals. It is concluded t hat MPA stimulates cell growth of primary cultures of MPA-induced PD t umors via PR. The results provide support for a direct effect of MPA w hich may be mediated or potentiated by serum factors.