Classical membrane progesterone receptors in murine mammary carcinomas: agonistic effects of progestins and RU-486 mediating rapid non-genomic effects.

In this article, we demonstrate the expression of functional progesterone binding sites at the cell membrane in murine mammary carcinomas that are stimulated by progestins and inhibited by antiprogestins. Using confocal immunofluorescence, ligand binding and cell compartment-specific western blots, we were able to identify the presence of the classical progesterone receptors. Medroxyprogesterone acetate (MPA) and RU-486 (1 × 10(-11) and 1 × 10(-8) M) behaved as agonists activating extracellular signal-regulated kinases (ERKs) and progestin-regulated proteins, except for Cyclin D1 and Tissue factor which failed to increase with 1 × 10(-8) M RU-486, an experimental condition that allows PR to bind DNA. These results predicted a full agonist effect at low concentrations of RU-486. Accordingly, at concentrations lower than 1 × 10(-11) M, RU-486 increased cell proliferation in vitro. This effect was abolished by incubation with the ERK kinase inhibitor PD 98059 or by OH-tamoxifen. In vivo, at a daily dose of 1.2 µg/kg body weight RU-486 increased tumor growth, whereas at 12 mg/kg induces tumor regression. Our results indicate that low concentrations of MPA and RU-486 induce similar agonistic non-genomic effects, whereas RU-486 at higher concentrations may inhibit cell proliferation by genomic-induced effects. This suggests that RU-486 should be therapeutically administered at doses high enough to guarantee its genomic inhibitory effect.

Inhibition of mammary tumor growth by estrogens: is there a specific role for estrogen receptors alpha and beta?

To evaluate the extent to which each estrogen receptor (ER) subtype contributes to the stimulation or to the inhibition of mammary tumor growth, we evaluated the effects of specific agonists in MC4-L2 cells, which are stimulated by 17ß-estradiol (E(2)), and in mammary carcinomas of the MPA mouse breast cancer model, which are inhibited by E(2). Both express ERα and ERß. In MC4-L2 cells, 4,4',4"-(4-propyl-(1H)-pyrazole-1,3,5-triyl)trisphenol (PPT; ERα agonist) and (4-hydroxy-phenyl)-propionitrile (DPN; ERß agonist) stimulated cell proliferation, whereas the opposite occurred in C4-HI primary cultures. The inhibitory effect was associated with a decrease in ERα and cyclin D1 expression and an increase in progesterone receptor (PR) expression as well as in the Bax/Bcl-xl ratio. In vivo, mice carrying C4-HI or 32-2-HI tumors were treated with E(2), PPT or DPN (3 mg/kg/day) or with vehicle. PPT and DPN inhibited tumor size, as did E(2), during the first 72 h. After a few days, DPN-treated tumors started to grow again, while PPT-treated tumors remained quiescent for a longer period of time. A pronounced decrease in the mitotic index and an increase in the apoptotic index was associated with tumor regression. All treated tumors showed: (a) an increase in integrin α6 and Bax expression, (b) an increased stromal laminin redistribution, and (c) a decrease in ERα, Bcl-xl and Bcl-2 expression (P < 0.001). Apoptosis-inducing factor (Aif) expression was increased in DPN-treated tumors, while active caspase 9 was up-regulated in PPT-treated mice, demonstrating the involvement of the intrinsic apoptotic pathway in estrogen-induced regression in this model. In conclusion, our data indicate that although there may be some preferences for activation pathways by the different agonists, the stimulatory or inhibitory effects triggered by estrogens are cell-context dependent rather than ER isoform dependent.

The MPA mouse breast cancer model: evidence for a role of progesterone receptors in breast cancer.

More than 60% of all breast neoplasias are ductal carcinomas expressing estrogen (ER) and progesterone receptors (PR). By contrast, most of the spontaneous, chemically or mouse mammary tumor virus induced tumors, as well as tumors arising in genetically modified mice do not express hormone receptors. We developed a model of breast cancer in which the administration of medroxyprogesterone acetate to BALB/c female mice induces mammary ductal carcinomas with a mean latency of 52 weeks and an incidence of about 80%. These tumors are hormone-dependent (HD), metastatic, express both ER and PR, and are maintained by syngeneic transplants. The model has been further refined to include mammary carcinomas that evolve through different stages of hormone dependence, as well as several hormone-responsive cell lines. In this review, we describe the main features of this tumor model, highlighting the role of PR as a trigger of key signaling pathways mediating tumor growth. In addition, we discuss the relevance of this model in comparison with other presently used breast cancer models pointing out its advantages and limitations and how, this model may be suitable to unravel key questions in breast cancer.

Association of estrogen receptor-alpha and progesterone receptor A expression with hormonal mammary carcinogenesis: role of the host microenvironment.

INTRODUCTION: Medroxyprogesterone acetate (MPA) induces estrogen receptor (ER)-positive and progesterone receptor (PR)-positive ductal invasive mammary carcinomas in BALB/c mice. We sought to reproduce this MPA cancer model in C57BL/6 mice because of their widespread use in genetic engineering. Within this experimental setting, we studied the carcinogenic effects of MPA, the morphologic changes in mammary glands that are induced by MPA and progesterone, and the levels of ER and PR expression in MPA-treated and progesterone-treated mammary glands. Finally, we evaluated whether the differences found between BALB/c and C57BL/6 mouse strains were due to intrinsic differences in epithelial cells. METHODS: The carcinogenic effect of MPA was evaluated in C57BL/6 mice using protocols proven to be carcinogenic in BALB/c mice. In addition, BALB/c and C57BL/6 females were treated with progesterone or MPA for 1 or 2 months, and mammary glands were excised for histologic studies and for immunohistochemical and Western blot evaluation of ER and PR. Hormone levels were determined by radioimmunoassay. Isolated mammary epithelial cells were transplanted into cleared fat pads of 21-day-old female Swiss nu/nu mice or control congenic animals. RESULTS: MPA failed to induce mammary carcinomas or significant morphologic changes in the mammary glands of C57BL/6 mice. The expression of ER-alpha and PR isoform A in virgin mice was surprisingly much higher in BALB/c than in C57BL/6 mammary glands, and both receptors were downregulated in progestin-treated BALB/c mice (P < 0.05). PR isoform B levels were low in virgin control mice and increased after progestin treatment in both strains. ER-beta expression followed a similar trend. No differences in hormone levels were found between strains. Surprisingly, the transplantation of the epithelial mammary gland cells of both strains into the cleared fat pads of Swiss (nu/nu) mice abolished the mammary gland morphologic differences and the ER and PR differences between strains. CONCLUSION: C57BL/6 mammary glands are resistant to MPA-induced carcinogenesis and to hormone action. MPA and progesterone have different effects on mammary glands. Low ER-alpha and PR-A levels in untreated mammary glands may be associated with a low-risk breast cancer profile. Although we cannot at this time rule out the participation of other, untested factors, our findings implicate the stroma as playing a crucial role in the strain-specific differential hormone receptor expression and hormone responsiveness.

The in vivo apoptotic effect of interferon alfa-2b on rat preneoplastic liver involves Bax protein.

To determine whether interferon alfa (IFN-alpha) prevents in vivo oncogenesis in very-early-stage cancer cells, we evaluated the action of IFN-alpha2b over preneoplastic foci in rats. Animals were divided into 6 groups: subjected to a 2-phase model (diethylnitrosamine [DEN] plus 2-acetylaminofluorene [2-AAF]) of preneoplasia development (group 1), treated with IFN-alpha2b during the 2 phases (group 2), only during initiation with DEN (group 3), only during administration of 2-AAF (group 4), subjected only to an initiation stage (group 5), and treated with IFN-alpha2b during this period (group 6). The numbers of placental form of rat glutathione S-transferase (rGST-P)-positive foci per liver and the foci as percentage of liver were significantly reduced in groups 2, 3, and 6 but not in group 4. Rats treated with IFN-alpha2b showed a higher apoptotic index (AI) in altered hepatic foci (AHF). Levels of p53 and Bax protein in liver lysates were significantly increased in those animals. Similarly, levels of antiapoptotic proteins Bcl-2 and Bcl-x(L) in mitochondrial fraction were decreased. Finally, increased levels of Bax protein were localized in the mitochondria of rats that received IFN-alpha2b, at least during the DEN phase (groups 2, 3, and 6), whereas mitochondrial Bax expression was not increased in group 4. In conclusion, the preneoplastic hepatocytes in rats that received IFN-alpha2b during the initiation stage undergo programmed cell death as a primary result of a significant increase in the amount and translocation to the mitochondria of Bax protein.

Efecto apoptótico in vivo del interferon alfa-2B (IFN) sobre hígados preneoplásicos de rata / In vivo apoptotic effect of alpha-2B interferon (IFN) on rat preneoplastic liver

Para conocer si el IFN a previene la oncogenesis in vivo, en estadíos tempranos del desarrollo tumoral, evaluamos la acción del IFN a-2b sobre focos preneoplásicos en hígado de rata. Los animales se dividieron en los siguientes grupos: sujetos a un modelo de iniciación-promoción (G1), tratados con IFN a-2b durante: a) iniciación-promoción (G2), b) iniciación (G3), c) promoción (G4); sujetos solo al estadío de iniciación (G5) y tratados con IFNa-2b en este período (G6). El área y el número de los focos preneoplásicos rGST P-positivos se mostraron significativamente disminuidos y el Indice Apoptótico aumentado en los G2, 3 y 6. Los niveles de Bcl-2 y Bcl-xL están disminuidos en los grupos tratados con IFN a-2b y los de Bax mitocondrial aumentados en los G2, 3 y 6. En conclusión, los hepatocitos preneoplásicos de ratas que recibieron IFN a-2b sufren muerte celular programada como resultado de un aumento sustancial de Bax y de su translocación a la mitocondria.

Efecto apoptótico in vivo del interferon alfa-2B (IFN) sobre hígados preneoplásicos de rata / In vivo apoptotic effect of alpha-2B interferon (IFN) on rat preneoplastic liver

Para conocer si el IFN a previene la oncogenesis in vivo, en estadíos tempranos del desarrollo tumoral, evaluamos la acción del IFN a-2b sobre focos preneoplásicos en hígado de rata. Los animales se dividieron en los siguientes grupos: sujetos a un modelo de iniciación-promoción (G1), tratados con IFN a-2b durante: a) iniciación-promoción (G2), b) iniciación (G3), c) promoción (G4); sujetos solo al estadío de iniciación (G5) y tratados con IFNa-2b en este período (G6). El área y el número de los focos preneoplásicos rGST P-positivos se mostraron significativamente disminuidos y el Indice Apoptótico aumentado en los G2, 3 y 6. Los niveles de Bcl-2 y Bcl-xL están disminuidos en los grupos tratados con IFN a-2b y los de Bax mitocondrial aumentados en los G2, 3 y 6. En conclusión, los hepatocitos preneoplásicos de ratas que recibieron IFN a-2b sufren muerte celular programada como resultado de un aumento sustancial de Bax y de su translocación a la mitocondria. (AU)