Regulation of estrogen and progesterone receptor levels in mouse mammary epithelial cells grown in serum-free collagen gel cultures

The effect of collagenase dissociation of virgin mouse mammary glands on the level of mammary epithelial cytosolic estrogen receptors (ER) and progesterone receptors (PR) was assessed. After cell dissociation, ER was present in mammary epithelial cells at concentrations similar to those found in the whole gland. However, PR appeared to be affected by the collagenase treatment. The regulation of ER and PR in mouse mammary epithelial cells isolated by collagenase dissociation and grown within collagen gels was then determined. After 7 days in culture under serum-free conditions inside a collagen gel, PR and, to a lesser extent ER, as characterized by high affinity binding and specificity, were present in the epithelial cells. Although at a low level, the ER were determined to be functional, since estradiol (E2) was able to promote nuclear accumulation of ER and to induce PR. PRL was able to increase cytosolic ER and PR concentrations. The combination of progesterone (P) and PRL was more effective than PRL or P alone in increasing PR. The induction of PR by P and PRL was inhibited when epidermal growth factor was present in the culture medium. Previous studies have shown that P, PRL, and epidermal growth factor, but not E2 (either alone or in combination with these factors) are able to stimulate cell proliferation in vitro. We conclude that the effects of E2 on protein synthesis and proliferation are dissociated in vitro. The difference between the effect of E2 and PRL or P on growth may be related either to the initial concentrations of their respective receptors or estrogen may stimulate growth indirectly.

Correlation between in vitro growth and regulation of estrogen and progesterone receptors in rat mammary epithelial cells

The present studies examine 1) the effect of enzymatic cell dissociation on the level of cytosolic estrogen receptor (ER) and progesterone receptor (PR) for normal rat mammary tissue, 2) the concentrations of ER and PR in rat mammary epithelial (RME) cells cultured within collagen gel, and 3) correlations that may exist between receptor concentration and cultured RME cell proliferation after hormonal stimulation in vitro. After cell dissociation, ER was present in mammary cells at higher concentrations than those found in the whole gland, whereas PR concentrations were similar to those in the whole gland. As characterized by Scatchard analysis, PR and, to a lesser extent, ER can be maintained in cells cultured in serum-free medium within a collagen gel matrix. ER is apparently functional at relatively low levels, since estradiol did induce PR synthesis, and cytosolic ER was reduced by estrogen administration. However, estradiol had no mitogenic effect on RME cells in this system, supporting the hypothesis that there may be a dichotomy between estrogen's effect on growth and progesterone receptor synthesis. PRL plus progesterone act synergistically to induce cell proliferation in our system, and this correlates with increased concentrations of progesterone receptors. Thus, the collagen gel system appears to provide a useful in vitro model for the study of receptor regulation and cell proliferation.

Thioesterase II, a new marker enzyme for human cells of breast epithelial origin

Metabolic, enzymologic, and immunohistochemical techniques have been used to show that a human cell line of breast epithelial origin synthesized medium chain fatty acids via the ubiquitous fatty acid synthetase and a mammary-specific chain-terminating enzyme, thioesterase II. Previous studies in our laboratory with rodents indicated that thioesterase II is expressed exclusively in mammary epithelial cells, an observation consistent with the physiologic role of the enzyme in milk fat synthesis. Results of the present study suggest that the enzyme exhibits a similar cell specificity in its expression in humans and that the specificity is maintained in normal and neoplastic tissues. Thus thioesterase II was detected immunohistochemically in normal human breast epithelia derived from both lactating and nonlactating breast tissue, in cultured cells derived from both primary breast epithelial tumors and from a metastatic tumor of breast origin, and in several human breast epithelial cell lines; the enzyme could not be detected in HeLa cells, in a colon carcinoma, or in a mammary myoepithelial cell line. These findings raise the possibility that thioesterase II may be of use as a diagnostic tool to identify human tumors of breast epithelial origin.

Collagen gel culture system and analysis of estrogen effects on mammary carcinogenesis

The results obtained to date from studies dealing with the role of hormones, including estrogen, on growth of mammary epithelial cells inside the collagen gel are described. The collagen gel matrix culture system appears to be a suitable system to obtain in vivo-like effects of hormones on mammary cell in vitro. The results thus far indicate that prolactin along with progesterone or cortisol can stimulate mammary cell proliferation. Thus far, estrogen has not been found to be mitogenic in our in vitro system.

How culture conditions modulate the morphofunctional differentiation of the human estradiol-sensitive mammary cell line (MCF-7)

The MCF-7 cell line grown on plastic surfaces is widely accepted as a model for hormone sensitivity in molecular biology. However, in vitro results concerning estrogen sensitivity remain controversial. In search of culture conditions most closely simulating the in vivo microenvironment we cultured MCF-7 cells on diverse substrates and in suspension culture. The different factors of the contact environment: (A) influence of diffusive medium, (B) influence of cell to cell contacts, and (C) influence of cell to substrate contacts were considered. Using morphological criteria:phase contrast microscopy, scanning and transmission electron microscopy we observed MCF-7 morphofunctional differentiation under the different culture conditions. Plastic, corneal endothelial cell extracellular matrix, and attached collagen gels imposed a planar medium-aggregate interface. The impermeability of the free surface and the intense basal tension antagonized epithelial polarization. Only at post-confluence did domes and clusters appear above the monolayer. On floating collagen gels and in suspension culture the cells established intimate cell-cell contacts over large surfaces and reconstituted tissular architecture. Three-dimensional growth conditions which approach the in vivo contact environment of epithelial cells should be used instead of the traditional monolayer cultures for assessing hormonal and pharmacological responses of human breast carcinomas.

Growth effect of lithium on mouse mammary epithelial cells in serum-free collagen gel culture

The effect of lithium on the growth of mammary epithelial cells from adult virgin and midpregnant BALB/c or BALB/cfC3H mice was tested in a serum-free collagen gel culture system. The serum-free medium consisted of a 1:1 mixture of Ham's F12 and Dulbecco's Modified Eagle's medium supplemented with insulin, transferrin, cholera toxin, epidermal growth factor (EGF), and bovine serum albumin fraction V (BSA V). A multifold increase in cell number occurred during 10-12 days of culture in this medium. In dose-response studies in which the concentration of each component of this serum-free medium was varied in turn, the addition of LiCL (10 mM) enhanced growth at most concentrations of each factor. However, LiCl could not enhance growth in the absence of insulin or BSA V, but could replace EGF. The optimal concentration of LiCl was 5-10 mM; higher concentrations (20-80 mM) were toxic. KCl (1-10 mM) when added to the serum-free medium slightly stimulated growth; the addition of NaCl to the medium had little effect on growth. LiCl did not enhance the growth of cells from spontaneous mammary tumors of BALB/cfC3H mice.

The use of thioesterase II as a rat mammary epithelial cell-specific marker

The avidin-biotin-peroxidase complex immunoperoxidase technique was employed to determine the intercellular distribution of thioesterase II in rat mammary glands. This enzyme is responsible for shifting the product specificity of the fatty-acid synthetase enzyme complex from long to medium chain fatty acids. Thioesterase II was found exclusively in the cells lining the lumen of the ductal and alveolar structures in glands from mature virgin (150 days old) and pregnant rats. The ductal cell staining intensity was considerably less than that of the alveolar cells in the mature virgin rat glands. No immunoreactive thioesterase II was found in the stromal, adipose, vascular, or myoepithelial components of the gland in the developmental stages examined. In the glands from immature virgin rats (40-45 days old) thioesterase II was again found only in the epithelial cells lining the lumen of the ductal and end-bud structures although this layer was usually more than one cell thick. Quantitative determination of thioesterase II activity in cytosol preparations revealed similar levels in mammary fragments from enzymatically-dissociated glands obtained from mature virgins and in end buds derived from immature virgins, but somewhat higher levels in mammary structures derived from late-pregnant animals. These immunohistological and biochemical results demonstrate thioesterase II's usefulness as a mammary epithelial cell-specific marker.

Growth of mammary epithelial cells from neonatally sex hormone-exposed mice in serum-free collagen gel culture

Neonatal sex hormone treatments are known to cause an increase in mammary tumors in female mice with expressed mammary tumor virus (MTV). The growth of mammary epithelial cells from mice treated neonatally with sex hormones was studied in response to growth-stimulatory factors in a serum-free collagen gel culture system which sustains the growth of normal mammary epithelial cells. Animals were treated with hormones or oil-vehicle for the first 5 days after birth. Cells from control mice at 2 and 3 months of age showed a maximal growth response to insulin at 5-10 micrograms/ml and LiCl at 5-20 mM. Cells responded to epidermal growth factor at all concentrations used (1, 10 and 50 ng/ml). In contrast, mammary epithelial cells from mice treated neonatally with estrogen (estradiol and diethylstilbestrol (DES] showed a reduced growth response to the growth factors tested.

Response of end bud cells from immature rat mammary gland to hormones when cultured in collagen gel

End buds from 4- to 5-week-old rat mammary glands were isolated and cultured within a rat tail tendon collagen gel matrix. Media containing equine serum or porcine serum and cholera toxin promoted growth, but not the production of casein or thioesterase II, nor did they induce a state of differentiation as assessed by cell ultrastructure. Medium supplemented with only 5% porcine serum, insulin and cholera toxin did not support growth or differentiation. However, when prolactin, estradiol, progesterone and hydrocortisone were added to this medium, growth was stimulated greatly and a differentiated state was induced as assessed by the production of casein and thioesterase and by the appearance of a highly secretory ultrastructure.

Effects of phorbol esters on normal and tumorous mouse mammary epithelial cells embedded in collagen gels

The effects of phorbol esters on mammary epithelial cells from BALB/cfC3H/Crgl "midpregnant mice" (i.e., mice at the midterm of pregnancy) and from mammary adenocarcinomas (also from BALB/cfC3H/Crgl mice) grown in a collagen gel matrix were studied. 12-O-Tetradecanoylphorbol 13-acetate (TPA), when added to the media, caused an increased proliferation of both normal and cancerous mammary epithelial cells in a dose-dependent manner. The degree of enhancement of proliferation by TPA ranged from no increase in cell number at 3% swine serum (SW) concentration to two to three times the number of cells in the control cultures when 10 or 25% SW was used. Optimal growth was obtained with a TPA concentration of 0.1 or 1.0 microgram/ml. Increasing the SW concentration (3, 5, 10, or 25%) enhanced the proliferative effect of TPA. Cholera toxin (0.01 microgram/ml) enhanced the proliferative effect of TPA on normal cells but had a variable effect on tumor cells. The addition of TPA also resulted in a morphologic change in the epithelial colonies from midpregnant mice and from mammary tumors and caused them to assume a fibroblastic appearance. The addition of 4 alpha-phorbol or 4 alpha-phorbol 12,13-didecanoate to mammary epithelial cultures had no proliferative or morphologic effect. The results demonstrate that TPA has a growth-promoting effect on normal and cancerous mouse mammary epithelial cells.

Comparison of the growth of normal and neoplastic mouse mammary cells on plastic, on collagen gels and in collagen gels

The growth of normal and neoplastic mouse mammary cells was compared in primary cultures on plastic, on rat tail collagen gels and in rat tail collagen gels. Cells on plastic grew for the first few days, then stopped with only a 1- to 3-fold increase in cell number after 2 weeks in culture. Cells grown on or in collagen gels grew continuously over the 2-week culture period with up to 10-fold increase in cell number for cultures on collagen gels and a 20-fold increase for cells embedded in collagen gels. The difference in growth rates between cells grown in collagen gel and those grown on collagen gels was due, in part, to the three-dimensional growth of the colonies in collagen gel their two-dimensional growth on collagen gel. Cells grown on and in collagen gel can produce an electron-dense basal lamina-like structure which is associated with collagen IV and laminin as judged by immunofluorescence. Cells grown on plastic do not form this structure. Cis-OH-proline blocks the production of collagen and inhibits the growth of the cultured cells indicating collagen production to be involved in growth. Rat tail collagen gels are a superior substratum for the growth of mouse mammary cells and this may be related to the cells' ability to form a collagen IV-containing basal lamina-like structure.

Biosynthesis of medium-chain fatty acids by mammary epithelial cells from virgin rats

Epithelial cells were isolated from the undifferentiated mammary glands of mature virgin female rats, and their lipogenic characteristics were studied. These cells synthesized predominantly medium-chain fatty acids, albeit at a low rate. In contrast, whole tissue from mammary glands of virgin rats synthesized predominantly long-chain fatty acids at a relatively higher rate, indicating that the lipogenic activity is dominated by the adipocyte component of the gland. Enzyme assays revealed that thioesterase II, the enzyme which regulates production of medium-chain fatty acids by the fatty acid synthetase, was present at a high activity in the undifferentiated mammary epithelial cells of virgin rats. Immunohistochemical studies confirmed this observation and showed that the regulatory enzyme was present exclusively in the epithelial cells lining the alveolar and ductal elements of the undifferentiated gland. This study demonstrates that the potential to elaborate tissue-specific medium-chain fatty acids is already expressed in the undifferentiated tissue of virgin rats and is not acquired as a result of the differentiation associated with the lactogenic phase of development. In this species mammary epithelial cells apparently synthesize predominantly medium-chain fatty acids at all stages of development, and only the overall rate of synthesis is increased on induction of the fatty acid synthetase during lactogenesis.

Serum-free growth of normal and tumor mouse mammary epithelial cells in primary culture

Freshly isolated normal and tumor mouse mammary epithelial cells embedded within a collagen gel matrix undergo sustained growth when cultured for as long as 3 wk in a serum-free medium composed of a 1:1 (vol/vol) mixture of Hepesbuffered Ham's F12 and Dulbecco's modified Eagle's medium supplemented with insulin, epidermal growth factor (EGF), transferrin, bovine serum albumin fraction V, and cholera toxin. Of these additives, only insulin, EGF, and albumin are required for the growth of most normal cells. Albumin is not always an absolute requirement for growth but greatly enhances it. Lithium has been found to stimulate the growth of normal cells and can replace EGF. The collagen matrix culture system allows sustained growth of primary cultures of both normal and neoplastic mammary epithelium in serum-free conditions. This serum-free system will be useful in identifying and investigating the role of hormones, growth factors, and nutritional factors in regulating the growth of mammary epithelial cells.

Transplantation of mouse mammary epithelial cells grown in primary collagen gel cultures

A technique for the transplantation of mouse mammary epithelial cells, grown in collagen gels, has been developed that demonstrates that the phenotype of the cells prior to culture was not altered by the culture conditions. Mammary epithelial cells from virgin and midpregnant C57BL/Crgl mice; virgin, midpregnant, and multiparous nonpregnant BALB/cfC3H/Crgl mice; a BALB/c hyperplastic alveolar nodule line, and mammary tumors from BALB/cfC3H/Crgl mice were embedded inside collagen gels and grown for 10 to 14 days in the presence of 25% swine serum plus cholera toxin (0.01 microgram/ml). The epithelial cells increased in number during the culture period. At the end of the culture period, the cells were removed from the collagen gels and transplanted to the gland-free mammary fat pads of 3-week-old syngeneic female mice. Culture in collagen gels increased the number of cells necessary to obtain a high percentage of mammary outgrowths as compared to cells not grown inside collagen gels. In general, mammary cells grown inside collagen gels gave rise to outgrowths, similar in phenotype to those from noncultured cells, and were representative of the tissue of origin. Mammary epithelial cells from C57BL/Crgl virgin donors grown in collagen gels for 10 to 14 days retained their ability to respond to the endogenous hormones of pregnancy and lactation of the host and formed lobuloalveolar structures full of secretion similar to the host's own mammary gland. The data indicate that the growth of mammary epithelial cells in collagen gels and subsequent transplantation into the gland-free fat pads of syngeneic hosts provides a simple system, wherein cells can be grown in vitro and their phenotypes determined in vivo.

Primary culture of human mammary epithelial cells embedded in collagen gels

Human mammary epithelial cells were dissociatd from mastectomy tissues. The contaminating fibroblasts were removed by the use of Percoll density-gradient centrifugation, which utilizes the difference in buoyant densities between epithelial cells and fibroblasts. A preparation highly enriched for mammary epithelial cells ws then embedded in collagen gel and cultured in Ham's F12 medium containing 12.5% horse serum, 2.5% fetal calf serum, 0.1 microgram cholera toxin/ml, an extract prepared from human male urine (L microgram protein/ml), and a hormone combination of 10 microgram insulin/ml, 10 microgram human placental lactogen/ml, 1 microgram aldosterone/ml, and 0.5 microgram hydrocortisone/ml. Sustained growth leading to an increase of tenfold to thirtyfold in cell number over the initial value was accomplished in primary culture, and this growth was maintained even after passage to secondary culture. Deletion of either the urine extract or the hormone combination resulted in less than optimal growth. Subsequent studies showed that hydrocortisone alone could replace the hormone combination. In addition, urine extract could be replaced by extracts prepared from human kidneys or brains. The collagen gel system provies a reproducible and consistent method for sustained three-dimensional growth of mammary epithelial cells from human breast tissue in primary as well as passaged cultures.

Growth factor- and cyclic nucleotide-induced proliferation of normal and malignant mammary epithelial cells in primary culture

Sustained growth of normal mouse mammary epithelial cells in primary culture, leading to an increase in cell number, in response to growth factors [epidermal growth factor (EGF) and fibroblast growth factor (FGF)] or cholera toxin has been achieved by embedding the cells inside collagen cells. Inclusion of agents known to increase the level of cellular cAMP have been found to be favorable for mammary epithelial cell proliferation. Cholera toxin is by far the best of all of the agents tested (prostaglandins E1 and E2, isoproterenol, theophylline, and dibutyryl cAMP). When growth factors (EGF or FGF) are added with cholera toxin, a synergistic effect resulting in a response much greater than with either of them alone is seen. This synergism was best seen in normal mammary epithelial cells from nonpregnant mice. The extent of this synergistic effect was found to be less in normal cells from pregnant mice, suggesting that these cells may be less responsive to EGF during pregnancy. Tumor cells were found to be rather inconsistent in their responses to EGF and cholera toxin, ranging from a minimal response, similar to that of normal cells from pregnant animals, to a maximal response, similar to that of normal cells from nonpregnant animals.

Primary culture of mouse mammary tumor epithelial cells embedded in collagen gels

Mammary tumor epithelial cells from BALB/cfC3H mice were dispersely embedded inside the collagen gels in Ham's F-12 medium containing horse serum. A sustained cell growth leading to a 5- to 10-fold increase in cell number over initial level was observed in less than 2 weeks. The extent of this growth was found to be dependent on serum concentration. However, addition of various protein and steroid hormones, both singly and in combination, to low-serum-containing medium failed to achieve a comparable level of growth to that promoted by higher serum concentration. Mammary tumor cells can now be consistently propagated in primary culture.

Sustained growth in primary culture of normal mammary epithelial cells embedded in collagen gels

Normal mammary epithelial cells from BALB/cfC3H midpregnant mice were freed from stromal cell types by Percoll density gradient centrifugation after collagenase digestion and were then embedded within collagen gels. Sustained growth leading to an increase in cell number was accomplished in response to cholera toxin and high concentrations of horse serum. The extent of growth was found to be dependent on the horse serum concentration, the maximum growth being attained at 50%. A serum concentration of 12.5% horse serum and 2.5% fetal calf serum, along with cholera toxin at 0.01 mug/ml, allowed maintenance but failed to cause any significant increase in cell number during the experimental period of 2 weeks. This same maintenance medium was used to determine the effects of various exogenously added steroids, protein hormones, and organ extracts on the proliferation of mammary epithelial cells in culture. Hormones failed to elicit any proliferative response, but extracts of kidney, brain, uterus, and spleen produced proliferative responses equal to or greater than the response obtained with 50% horse serum and cholera toxin. Kidney extracts prepared from midpregnant mice, virgin mice, and virgin mice given pituitary isografts all showed comparable activities, suggesting that the concentration of stimulatory factor(s) was not influenced by the hormonal status of the donor. Normal mammary epithelial cells that had undergone a 10- to 15-fold increase in cell number over initial values during 2-3 weeks in culture were passaged to secondary gel cultures. Outgrowth similar to those seen in primary culture were seen again in secondary culture. The present system provides a method for sustaining growth in culture of primary mammary epithelial cells from normal tissues.

Isolation and characterization of germ line DNA from mouse sperm

Mouse germ line DNA was isolated from sperm by a physicochemical procedure that preferentially destroys contaminating somatic cell DNA. The use of reducing conditions and chelating agents in combination with phenol permitted extraction of molecular weight DNA from mature sperm nuclei with approximately 80% efficiency. Less than 0.1% somatic cell DNA contamination remained in sperm DNA prepared by this method. Germ line DNA was characterized by determination of its ultraviolet absorbance spectrum, buoyant density in cesium chloride, and melting profile on a hydroxyapatite column. Contamination by mitochondrial DNA was assessed by cesium chloride/ethidium bromide gradient centrifugation. The significance of the mouse germ line DNA isolation procedure is discussed with respect to the possible genetic transmission of mammary tumor virus and leukemia virus, the origin of antibody diversity, and the origin of testicular teratomas.

Sustained growth and three-dimensional organization of primary mammary tumor epithelial cells embedded in collagen gels

We have developed a method for embedding cells within a collagen matrix which allows sustained growth of mouse mammary tumor epithelial cells in primary culture. A characteristic and reproducible pattern of organization and growth occurs: the cells rearrange themselves and produce duct-like structures extending into the matrix, resulting in a three-dimensional outgrowth. Autoradiography showed continuous [3H]thymidine incorporation during 8 weeks in culture. An increase in DNA content of the cultured cells as a function of time was observed. Mouse mammary tumor cells cultured in the conventional monolayer system failed to show any significant increase in cell number during a culture period of 6 weeks. In addition, in such monolayer systems, cells progressively became detached from the dishes in long-term culture. The mammary epithelial cell origin of the collagen gel cell outgrowths was shown by electron microscopic demonstration of polarized cells containing tight junctions and budding mammary tumor virus particles. In addition, in vivo transplantation of collagen gel outgrowths resulted in the development of mammary adenocarcinoma histologically similar to the donor tumor. Cellular outgrowth patterns resembling those from tumor cells were also seen in similar collagen gel cultures of normal mammary cells from mouse and human and of hyperplastic alveolar nodule cells from mouse. The significance and usefulness of this system in comparison to the conventional monolayer system are discussed.

Glucocorticoid regulation of prolactin receptors on mammary cells in culture

Mouse mammary epithelial cultures were examined for the ability to specifically bind [125I]PRL after cultivation on floating collagen gels. Corticosterone, particularly hydrocortisone, were effective in increasing the ability of mouse mammary cells to bind [125I]PRL. The absence of a glucocorticoid in the medium resulted in a loss of PRL binding during the 3 days in culture. 17 beta-Estradiol, progesterone, and aldosterone at equal molar concentration had no or only a small effect in increasing [125I]PRL binding.

Simultaneous production of casein and mammary tumor virus in mouse mammary epithelial cells grown on floating collagen gels

Simultaneous production of casein and mammary tumor virus (MTV) was analyzed in monolayer cultures of mammary epithelial cells from pregnant BALB/cfC3H mice. A comparison of the 2 cell culture substrata, plastic culture dish and floating collagen gel, showed that the latter supported a much higher degree of simultaneous casein and MTV production in the presence of insulin, cortisol and prolactin in serum-free culture medium. Importance of floating collagen gel was further shown by delaying the flotation of gels. When the release of gels was delayed, there were concomitant delays in the increase of casein and MTV production. These results indicate that hormones, nature of substratum and flotation regulate the degree of differentiation of mammary epithelial cells in vitro.

Prolactin receptor on dissociated mammary epithelial cells at different stages of development

Properties of prolactin receptors were measured by monitoring [125I]prolactin binding to specific receptor sites on collagenase-dissociated mammary epithelial cells of virgin, pregnant and lactating mice. On a Scatchard plot the data generated a straight line and the estimated dissociation constant (Kd) and number of receptor sites on lactating cells were 0.9 x 10(-9) and 1540 per cell. The [125I]prolactin binding was inhibited in presence of unlabeled prolactin and other lactogenic polypeptide hormones, but not by nonlactogenic polypeptide hormones. The [125I]prolactin binding was sensitive to pronase and trypsin but not to DNAase, RNAase and hyaluronidase. Scatchard plot analysis further showed that while the number of receptors on mammary cells was variable at different stages of endocrine regulated developmental changes of the gland, Kd of the hormone--receptor complex generally remained similar. The high level of prolactin receptors on mammary cells of virgins was reduced during pregnancy and the lactating mammary cells showed a highly elevated level of prolactin receptors. The results demonstrate that specific prolactin receptors can be measured on collagenase dissociated mammary epithelial cells and this method permits a direct assessment of the number of receptors on a per cell basis rather than indirect estimates, based on average DNA or protein content of the tissue, composed of heterogeneous cell types.

Role of hormones in mammary neoplasia

A testable hypothesis for the role of hormones in mammary carcinogenesis with implications for other endocrine-related carcinogenesis is presented. The hypothesis is based on these observations: (a) hormones are involved, directly or indirectly, in regulating cell division in normal mammary cells: (b) emergence of overt mammary tumors requires hormonal stimulation of cells receiving carcinogenic stimulus; (c) normal mammary cells are of finite divisional capabilities, whereas neoplastic cells appear to have infinite divisional life; and (d) normal cells, when present in large quantities relative to the neoplastic cells, inhibit the growth of the latter cells. According to the hypothesis hormones play at least two roles in mammary carcinogenesis induced by diverse agents, such as viruses, chemicals, and radiation. First, hormones are necessary for DNA synthesis and mitosis of initial transformed cells for their conversion into fixed transformed cells with heritable characteristics. Second, hormones, by increasing the rate of cell division, shorten the reproductive life span of normal cells, eventually causing a reduction in the normal to transformed cell ratio in the population--a condition that allows the emergence of tumor cells by overriding the inhibitory influence of normal cells.

Primary culture of rat mammary epithelial cells. I. Effect of plating density, hormones, and serum on DNA synthesis

Methods for the primary culture of rat mammary epithelial cells and the response of these cells to various hormones and culture parameters are described. In addition, the techniques for subsequent retransplantation of cultured cells into cleared fat pads of isogenic hosts as a test for normalcy of cultured tissue are detailed. A characteristic and reproducible course of the culture was followed for 2 weeks, after which rat mammary epithelial cells ceased to proliferate and were eventually overgrown by fibroblast-like cells. During the initial 2 weeks in culture, mammary cells and contaminating fibroblast-like cells were responsive to polypeptide and steroid hormones and the percentage of proliferating mammary cells could be enhanced with insulin, prolactin, estradiol, progesterone, and hydrocortisone.

Primary culture of rat mammary epithelial cells. II. Cytotoxic effect and metabolism of 7,12-dimethylbenz[a]anthracene and N-nitroso-N-methylurea

To determine the cytotoxicity of 7,12-dimethylbenz[a]anthracene (DMBA) and N-nitroso-N-methylurea (NMU) on primary cultures of rat mammary cells, cultures were exposed to various concentrations of these carcinogens. Cytotoxicity was evident after exposure for 24 hours to as little as 0.1 microgram DMBA/ml. Cytotoxicity was evident after exposure for 2 hours with NMU at concentrations between 80 and 160 microgram/ml. Primary cultures of rat mammary cells were also examined for their ability to metabolize [3H]DMBA into water-soluble products. During a 48-hour period, mammary cells could convert 2.2% [3H]DMBA to water-soluble metabolites. The cells retained (for at least 4 days) small quantities of [3H]DMBA that were insoluble in organic solvents.

Effect of polypeptide hormones on stimulation of casein secretion by mouse mammary epithelial cells grown on floating collagen gels

The in vitro effects of protein hormones on the stimulation of casein secretion by mouse mammary epithelial cells were studied. Mouse mammary glands were enzymatically dissociated and used immediately or were stored frozen and thawed just before use. Cells were cultured on floating collagen gels in the presence of insulin, cortisol and a pituitary or placental polypeptide hormone. Casein, released into the medium, was assayed by a radioimmunoassay against one of the components of mouse casein. Mammary cells released casein into the medium in the presence of as little as 10 ng of ovine prolactin per ml of medium. Human growth hormone stimulated the casein secretion to the same extent as prolactin. Human placental lactogen, ovine and bovine growth hormones were less stimulatory. Luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone had no effect on the stimulation of casein secretion.

Hormonal carcinogenesis: a novel hypothesis for the role of hormones

A testable hypothesis on the role of hormones in carcinogenic processes is presented. The hypothesis has been based on the findings that (1) hormones regulate cell division in normal cells, (2) successful neoplastic transformation requires hormonal stimulation of cells receiving carcinogenic insult, (3) normal cells have finite divisional capabilities, whereas neoplastic cells possess infinite divisional capabilities, and (4) normal cells, when present in high ratio, inhibit the growth of neoplastic cells. According to the hypothesis, hormones are considered to be neither mutagenic nor carcinogenic. (Carcinogen is defined as an agent which initiates normal-to-neoplastic transformation). Instead, they play a dual role in carcinogenesis. First, hormones are considered necessary for the fixation of the cell genome neoplastically transformed by carcinogens. Second, hormones, by enhancing the rate of cell division, shorten the life span of normal cells, thus causing a reduction of the normal- to tumor-cell ratio in the population--a condition which facilitates growth of tumor cells.

Neoplastic transformation of rat mammary cells exposed to 7,12-dimethylbenz[alpha]anthracene or N-nitrosomethylurea in cell culture

Primary cultures of mammary cells from virgin Lewis rats were seeded at 5 X 10(5) cells per cm2 in medium 199 supplemented with 10% fetal calf serum, insulin (5 microgram/ml), prolactin (5 microgram/ml), estradiol (5 ng/ml), progesterone (0.5 microgram/ml), and hydrocortisone (0.5 microgram/ml). On the second or third day of culture, cells were exposed to either 7,12-dimethylbenz[alpha]anthracene (0.1 microgram/ml for 24 hr) or N-nitrosomethylurea (80 microgram/ml for 2 hr). The cells were later assayed for transformation by transplanting 10(6) or 10(5) cells into gland-free mammary fat pads of 3-week-old female hosts. Untreated cells produced only normal mammary outgrowths when transplanted. Cells treated with 7,12-dimethylbenz[alpha]anthracene or N-nitrosomethylurea produced abnormal outgrowths in 11% of the transplants. These abnormal outgrowths ranged from rapidly growing adenocarcinoma to alveolar and ductal hyperplastic lesions. The results indicate that rat mammary epithelial cells can be transformed by exposure to chemical carcinogens in culture and thus represent a potential in vitro model for epithelial cell transformation.

Secretion of casein in cultures of mouse mammary epithelial cells on floating collagen gels

To study the effect of hormones on the secretion of casein, mammary epithelial cells were obtained from mid-pregnant mice and cultured on floating collagen gels in the presence of combinations of insulin, cortisol, and prolactin in serum-free culture medium. After labeling with phosphorus-32 labeled orthophosphate, the cells which had been cultured in the presence of all three hormones secreted hot-trichloroacetic acid precipitable radioactivity into the medium. The secreted materials were identified as three major mouse caseins by sodium dodecyl sulphate-urea polyacrylamide gel electrophoresis followed by autoradiography. The floating collagen gel culture method allows mouse mammary epithelial cells to differentiate and secrete all species of caseins in the presence of hormones. The present culture method and the phosphorus-32 labeling method will be applicable to the study of differentiation of mammary epithelial cells from a variety of mammalian species.

In vitro analysis of proliferating epithelial cell populations from the mouse mammary gland: fibroblast-free growth and serial passage

Normal and neoplastic mouse mammary epithelial cells were cultured in nutrient medium containing D-valine substituted for L-valine. Fibroblast overgrowth was prevented and epithelial cell functions and morphology were retained in cultures maintained in D-valine medium up to 2 months. A nonenzymatic technique was devised to dissociate epithelial cell monolayers. The combined use of this dissociation buffer and D-valine nutrient medium made it possible to passage serially normal and neoplastic mammary epithelial cells. Normal cells were derived from mammary glands of animals stimulated with exogenous hormones for various periods. The period of in vivo hormonal stimulation influenced the ability of normal mammary epithelial cells to attach and proliferate in primary and serially passaged cultures. A greater proportion of cells derived from glands following 2 to 4 weeks of hormonal stimulation were recovered after replating and showed higher labeling indices during serial passage than cells from unstimulated or 5- to 7-week stimulated groups.

A sensitive radioimmunoassay for a component of mouse casein

Mouse casein (m.w. 22,000 daltons) has been purified by employing Sephadex G-100 and DEAE-cellulose column chromatographies. A sensitive radioimmunoassay method has been developed by using [125I]-labelled casein and antiserum elicited in rabbits after injection of glutaraldehyde-treated casein. The assay method is capable of detecting as little as 0.1 ng of casein. The use of the present radioimmunoassay method in detecting casein production in cultured mouse mammary explants has also been demonstrated.

Regulation of mammary tumor virus production by prolactin in BALB/cfC3H mouse normal mammary epithelial cells in vitro

Hormonal regulation of mammary tumor virus (MTV) production, has been analyzed with normal mammary epithelial cells from chronically infected BALB/cfC3H mice. The effect of prolactin in terms of increased MTV production was not reproducibly observed in cells cultured in tissue culture dishes, whereas the cells grown on floating collagen gels consistently responded to prolactin in a dose-dependent manner. Of the three media tested. Dulbecco's modified Eagle's medium was found to be the best in terms of responsiveness to prolactin and in maximal MTV production. Specificity studies with other pituitary and placental hormones in place of prolactin have shown that both growth hormones and human placental lactogen can replace prolactin, whereas follicle-stimulating hormone, luteinizing hormone, and thyrotrophin were ineffective. Contrary to the mammary tumor systems, where it has been shown that insulin and glucocorticoid can maximally stimulate MTV production, these hormones alone elicited only a small response in the absence of prolactin in normal mammary epithelial cells. Although prolactin alone had very little effect by itself, its presence was necessary (permissive effect) in order for the glucocorticoids to be able to maximally stimulate MTV production in normal cells.

Hormonal effects on intracellular and secreted casein in cultures of mouse mammary epithelial cells on floating collagen membranes

Cultured on floating collagen membranes in the presence of lactogenic hormones, dissociated normal mammary epithelial cells from prelactating mice acquire the ultrastructural and biochemical characteristics of differentiated mammary secretory cells in vivo. The cells on floating collagen membranes in medium containing insulin alone have sparse secretory organelles, and a small amount of casein can be detected in these cells with a sensitive radioimmunoassay. These cells resemble counterpart cells in early-pregnant mice. When the cells are exposed to insulin, cortisol, and prolactin, the secretory apparatus is elaborated and significant increases in intracellular and extracellular casein are observed. In this environment, the intracellular casein content is generally four to eight times greater than in freshly dissociated cells or cells cultured in insulin alone. The amount of casein secreted into the medium by floating-collagen-membrane cultures in the three hormones is from 25 to 200 times greater than that secreted by cultures in insulin alone. Cells cultured on plastic substrates in either hormone combination fail to show any increase in intracellular or extracellular casein. On floating collagen membranes, the cells differentiate in response to hormones as they do in vivo and in organ culture. This cell-culture system provides an opportunity to study direct effects of environmental factors on mammary differentiation at the cellular level.

Hormonal control of milk protein synthesis in cultured mouse mammary explants

SDS-urea polyacrylamide gel electrophoresis of mouse milk proteins revealed the presence of three major phosphoproteins (caseins) of m.w. 44,000, 26,000 and 22,000 daltons. By using an antiserum against crude casein fraction, an immunoprecipitation method was developed for the quantitative measurement of the rate of milk protein synthesis in the mouse mammary tissue. Cultivation of mammary explants with insulin, cortisol and prolactin resulted in the induction of milk protein synthesis as evidenced by the incorporation of [3H]amino acid and [32P]orthophosphate into immune precipitable materials. The present immunoprecipitation method coupled with a simplified explant culture technique provides a suitable procedure for the study of mouse mammary gland differentiation.