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


CONCLUSIONS
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.
Montero Girard
Guadalupe
G
Instituto de Investigaciones Oncológicas, Academia Nacional de Medicina, 3092 Las Heras, Buenos Aires 1425, Argentina. guadalupemontero@fibertel.com.ar
Vanzulli
Silvia I
SI
Cerliani
Juan Pablo
JP
Bottino
María Cecilia
MC
Bolado
Julieta
J
Vela
Jorge
J
Becu-Villalobos
Damasia
D
Benavides
Fernando
F
Gutkind
Silvio
S
Patel
Vyomesh
V
Molinolo
Alfredo
A
Lanari
Claudia
C
D000818:Animals; D002273:Carcinogens; D004847:Epithelial Cells / Q000378:metabolism; D004958:Estradiol / Q000378:metabolism; D047628:Estrogen Receptor alpha / Q000096:biosynthesis*; D005260:Female; D006728:Hormones / Q000378:metabolism; D015674:Mammary Neoplasms, Animal / Q000139:chemically induced* / Q000378:metabolism*; D017258:Medroxyprogesterone Acetate; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D011374:Progesterone; D011863:Radioimmunoassay; D011980:Receptors, Progesterone / Q000096:biosynthesis*
D002273:Carcinogens; D047628:Estrogen Receptor alpha; D006728:Hormones; D011980:Receptors, Progesterone; C101118:progesterone receptor A; D011374:Progesterone; D004958:Estradiol; D017258:Medroxyprogesterone Acetate