Prolactin induction of casein mRNA in organ culture. A model system for studying peptide hormone regulation of gene expression

Effects of tripeptides and lactogenic hormones on oligopeptide transporter 2 in bovine mammary gland

This study was conducted to investigate the expression of oligopeptide transporter 2 (PepT2) and its potential function in bovine mammary gland. First, the PepT2 mRNA and protein were determined in cultured mammary epithelial cells. Then the effects of lactogenic hormones (prolactin, hydrocortisone or insulin) and substrate (threonyl-phenylalanyl-phenylalanine) on PepT2 were investigated. The PepT2 mRNA and protein were successfully detected in bovine mammary epithelial cells. PepT2 gene expression was enhanced by the addition of 50, 500 and 5000 ng/ml prolactin, 10 and 100 ng/ml hydrocortisone, and 50, 500, 5000 and 50,000 ng/ml insulin. PepT2 mRNA abundance was increased when 5, 10 and 15% of threonyl-phenylalanyl-phenylalanine was included. Responses of PepT2 to lactogenic hormones and oligopeptide inferred that it may play an important role in bovine mammary gland.

Historical perspectives of prolactin and growth hormone as mammogens, lactogens and galactagogues--agog for the future!

Around 80 years ago researchers first established that the pituitary gland regulates mammary gland function as demonstrated by the ability of its extracts to promote both mammogenesis and lactogenesis in animal models. Little did they realize that in fact two hormones, prolactin (PRL) and growth hormone (GH), were contributing to these effects. By the mid 1930s PRL had been purified as a distinct lactogen, while the galactopoietic effect of GH was confirmed after its purification in the 1940s. Interest in these hormones initially centered about their potential for increasing milk production, while in the latter half of the twentieth century it became obvious that these hormones also had the potential to influence mammary cancer development. During the past 50 years large strides have been made into understanding how these hormones signal to, and within, cells of the mammary gland, paralleling rapid developments in the fields of cellular and molecular biology. In compiling this review we have summarized the progress that has been made to date regarding roles for these hormones in the mammary gland, with a goal of ensuring that some of the seminal literature is not diluted or forgotten. In doing so it is clear that there are lessons to be learned from past experiences, where new methods and technologies will continue to present exciting new opportunities to revisit lingering questions regarding these fascinating hormones and this fascinating organ.

Translational regulation of milk protein synthesis at secretory activation

Studies conducted since the 1970s have revealed that the production of milk proteins in the mammary gland under the influence of lactogenic hormones (insulin, prolactin, and glucocorticoids) is regulated at multiple levels. Whereas earlier studies concentrated on transcriptional regulation and stabilization of milk protein mRNAs, more recent studies have revealed that translation of milk protein mRNAs is also dependent on lactogenic hormones. A general stimulation of translation in mammary epithelial cells is caused by amino acids (as signaling molecules) or by phosphorylation of the translational regulator 4E-BP1 in a synergistic response to signals from insulin and prolactin. However, a selective enhancement of milk protein mRNA translation is caused by cytoplasmic polyadenylation of mRNA, again in a synergistic response to these two hormones. Preliminary evidence indicates that the latter effect depends on the existence of a cytoplasmic polyadenylation element (CPE) in milk protein mRNAs and phosphorylation of its binding protein, CPEB. Experiments in whole animals, organ explants, and cell culture have shown that the poly(A) length of milk protein mRNAs changes as a function of the lactation cycle. Interestingly, cytoplasmic polyadenylation is likely to be responsible for the selective hormone-dependent enhancement of both translation and stability of milk protein mRNAs.

Introduction: secretory activation: from the past to the future

This issue of the Journal of Mammary Gland Biology brings a synthesis of the historical data leading to our understanding of the physiology of lactation up to about 1980 with the new technologies and understanding resulting from the molecular revolution in the late 20th century. We focus specifically on the activation of secretion at parturition, and show that the field is ripe for new investigation.

The role of glucocorticoids in secretory activation and milk secretion, a historical perspective

In this review we present our current understanding of the role of glucocorticoids in secretory activation and milk secretion by looking at the literature from a historical perspective. We begin with the early endocrine ablation experiments and continue from there to show that glucocorticoids are not just necessary for secretory activation and milk secretion--but mandatory. Specifically, we discuss the importance of glucocorticoids to: (1) induce the formation of ultrastructural components necessary to support milk synthesis and secretion, including rough endoplasmic reticulum and tight junction sealing; (2) regulate milk protein gene expression; and (3) prevent the second phase of involution, possibly by preventing the breakdown of the extracellular matrix.

The mineralocorticoid receptor may compensate for the loss of the glucocorticoid receptor at specific stages of mammary gland development

To study the role of glucocorticoid receptor (GR) at different stages of mammary gland development, mammary anlage were rescued from GR-/- mice by transplantation into the cleared fat pad of wild-type mice. In virgin mice, GR-/- outgrowths displayed abnormal ductal morphogenesis characterized by distended lumena, multiple layers of luminal epithelial cells in some regions along the ducts, and increased periductal stroma. In contrast, the loss of GR did not result in overt phenotypic changes in mammary gland development during pregnancy, lactation, and involution. Surprisingly, despite the known synergism between glucocorticoids and prolactin in the regulation of milk protein gene expression, whey acidic protein and beta-casein mRNA levels were unaffected in GR-/- transplants as compared with wild-type transplants. That mineralocorticoid receptor (MR) might compensate for the loss of GR was suggested by the detection of MR in the mammary gland at d 1 of lactation. This hypothesis was tested using explant cultures derived from the GR-/- transplants in which the mineralocorticoid fludrocortisone was able to synergistically induce beta-casein gene expression in the presence of prolactin and insulin. These studies suggest that MR may compensate for the absence of GR at some, but not at all stages of mammary gland development.

Distribution of casein-like proteins in various organs of rat

Casein-like proteins were detected in various organs of rat by use of a specific antiserum raised against rat milk caseins. The antiserum specifically recognized alpha 1-, alpha 2-, beta-, and gamma-caseins in rat milk by Western blot analysis, whereas no immunoreactive band was observed in sera of rat and fetal bovine and in bovine caseins. Immunohistochemical studies of this antiserum on formalin-fixed mammary glands showed that immunoreactive caseins were localized to the apical portion of the cytoplasm in lactating mammary epithelial cells and in the luminal secretion, which indicates a directional secretion of caseins to the lumen by the mammary epithelial cells. With this antiserum, immunoreactive substances were detected in various organs, including the pancreatic ducts and islets of Langerhans, the secretory ducts of salivary glands, zona fasciculata cells and ganglion cells of adrenal gland, distal tubules and convoluted collecting tubules of kidney, epithelial cells of bronchioles and large pneumocytes of the lung, hair follicles, sebaceous glands, and the prickle cell layer of skin, uterine glands and epithelium of the endometrium, hepatic bile ducts, and brain. In Western blot analysis, major immunoreactive substances in the above organ extracts showed a similarity in molecular weight to alpha 2-casein of rat milk. Skin was the only tissue that expressed both alpha 2- and beta-caseins. There were no other immunoreactive bands with similarity to beta- and gamma-caseins in the other organ extracts, but higher molecular weight immunoreactive bands (> 100 kD) were detected in some organ extracts, such as salivary gland, kidney, liver, lung, and uterus. These findings suggest that the alpha 2-casein-like substance is localized not only in the mammary gland but also in a variety of organs and may play an important role as a functional molecule in those organs.

The effect of lactogenic hormones on protein synthesis and amino acid uptake in rat mammary acinar cell culture at various physiological stages

1. The activities of protein synthesis and amino acid uptake at various physiological stages were determined by the incorporation of radioactive materials ([3H]-lysine, [14C]-cycloleucine) in rat mammary epithelial cell cultures. The activity of protein synthesis and amino acid uptake was higher in early lactation than in virgin, pregnant and late lactation stages. 2. Lactogenic hormones (prolactin, hydrocortisone and insulin) treatment related with mammary growth and differentiation increased the activities of protein synthesis and amino acid uptake. But increase of these activities was different at each physiological stage. 3. The effect of prolactin and hydrocortisone on the activities were greater in virgin, pregnant and late lactation than in early lactation. And effect of insulin was greater in pregnant and early lactation than in virgin and weanling.

Posttranscriptional regulation of alpha-casein mRNA accumulation by laminin

We have utilized primary cultures of rat mammary epithelial cells to study mechanisms by which laminin regulates the prolactin-dependent accumulation of alpha-casein mRNA. Mammary cells accumulate approximately fivefold more alpha-casein mRNA when cultured on laminin than when cultured on tissue plastic and the accumulation of alpha-casein mRNA is prolactin dependent. On the basis of transcription assays there is approximately a twofold increase in the alpha-casein mRNA transcription rate in cells cultured on laminin over that of tissue culture plastic. Measurements on the turnover of alpha-casein mRNA show that this mRNA is stabilized fourfold more on laminin than on tissue culture plastic, while there was no significant difference in the turnover of poly(A) RNA on either substratum. These data indicate that laminin regulates the cytoplasmic levels of alpha-casein mRNA accumulation primarily at the post-transcriptional level by increasing the stabilization of this mRNA.

Establishment of bovine mammary epithelial cells (MAC-T): an in vitro model for bovine lactation

The hallmark of differentiated mammary epithelial cells is a copious secretion of milk-specific components regulated by lactogenic hormones. We describe an established clonal cell line produced from primary bovine mammary alveolar cells (MAC-T) by stable transfection with SV-40 large T-antigen. MAC-T cells show a population doubling time of approximately 17 h and have been cultured more than 350 passages without showing any sign of senescence. They show the characteristic "cobblestone" morphology of epithelial cells when grown on plastic substratum. Differentiation was induced by augmenting cell-cell interaction on a floating collagen gel in the presence of prolactin. The differentiated phenotype was characterized to include (1) increased abundance in beta-casein mRNA, (2) increased number and size of indirect immunofluorescent casein secretory vesicles in each cell and (3) alpha s- and beta-casein protein secretion. The clonal nature of the cells, their immortality, and their ability to uniformly differentiate and secrete casein proteins make this cell line unique.

Generation of polyclonal antibodies against purified rat whey acidic proteins and the synthesis of a tracer fusion protein suitable for use in radioimmunoassays

Rodent milk consists mainly of caseins and whey proteins. A major component of the latter group is the whey acidic proteins (WAP) the gene for which has been cloned recently and shown to contain several potential glucocorticoid receptor binding sites. Studies on the regulation of this gene by glucocorticoids would be greatly enhanced by the availability of a radioimmunoassay for WAP. Rat milk was obtained from lactating Sprague-Dawley rats and the WAP purified to homogeneity by ammonium sulphate fractionation, gel filtration and ion exchange chromatography. Rabbit polyclonal antibodies were raised against the purified WAP and used to probe a Western blot of whey proteins. The major band recognized by the antibody corresponded in molecular weight to purified WAP. Problems associated with radiolabelling the tyrosine-free WAP molecule necessitated the fusion of a tyrosine containing protein with the rat milk protein. A rat WAP cDNA clone was ligated to the glutathione transferase gene, the fusion protein expressed in Escherichia coli and purified by a one-step procedure on a glutathione affinity column. Purified WAP readily displaced the radiolabelled recombinant tracer in a radioimmunoassay.

Alpha-lactalbumin production by rat mammary gland: prepartum emergence of resistance to glucocorticoids and progestins

Explants of mammary glands from pregnant or lactating rats were shown to differ markedly in their response to glucocorticoid and progestin in terms of alpha-lactalbumin production. Tissue from day 14 pregnant rats had low initial tissue levels and basal production of alpha-lactalbumin, and showed a dose-biphasic response to glucocorticoids, with significantly higher production at 3 nM than 300 nM RU26988; the progestin ORG 2058 inhibited production. In explants from lactating rats (day 1 and day 5) tissue content and basal production of alpha-lactalbumin were markedly increased compared with day 14 pregnant, but were unaltered by glucocorticoid or progestin. Animals near parturition (day 19, day 20) showed a transitional pattern, with considerable variation between animals. Basal production and tissue content correlated closely; as levels rose, the tissues became progressively less responsive to the effects of both classes of steroid. We interpret these data as evidence for a switch, in terms of glucocorticoid control of alpha-lactalbumin gene expression pre-partum; the mechanism(s) underlying this switch remain to be explored.

Regulation of milk protein synthesis by progesterone in cultured mouse mammary gland

The effect of progesterone on the synthesis of milk proteins, casein and alpha-lactalbumin was investigated by culturing mammary explants from mid-pregnant mice in serum-free medium. The addition of progesterone at concentrations above 10 ng/ml inhibited both the casein and alpha-lactalbumin accumulation that were induced by the synergistic actions of insulin, prolactin and cortisol. The maximal inhibition was attained at a progesterone concentration of 100 ng/ml. The maximal level of inhibition of the alpha-lactalbumin accumulation was about 90% in the presence of insulin and prolactin or insulin, prolactin and 0.01 microgram/ml of cortisol. The inhibition of the casein accumulation by progesterone was about 80% in the presence of insulin and prolactin, and about 40% in the presence of insulin, prolactin and 1 microgram/ml of cortisol, indicating that cortisol partially antagonized the action of progesterone on the casein synthesis. When the inhibitory effect of progesterone on the accumulation of both alpha-lactalbumin and casein was examined in cultured mammary tissues from virgin, early pregnant, mid-pregnant and late pregnant mice, the degree of inhibition was markedly reduced in tissue from late pregnant mice. This indicates that the susceptibility of mammary gland to the inhibitory action of progesterone varies with the developmental stage of the tissue.

Antiprogesterone and antiglucocorticoid actions of RU 486 on rabbit mammary gland explant cultures. Evidence for a persistent inhibitory action of residual progesterone upon the mammary tissue

The antiprogesterone and antiglucocorticoid compound RU 486 added to pregnant rabbit mammary gland explant cultures had no effect alone but significantly stimulated casein production in the presence of ovine prolactin (PRL) in a dose dependent manner. This stimulation was inhibited by progesterone (Pg) and the Pg agonist R5020. When the explants were cultured for 5 days with two changes of medium, to eliminate all steroids, and hormones added afterwards, the effect of PRL was potentiated, Pg was no longer inhibitory and RU 486 had no effect, RU 486 also could inhibit the stimulatory action of glucocorticoids added to the cultures along with PRL. The compound was able to displace [3H]dexamethasone and [3H]R 5020 from mammary gland glucocorticoid and Pg receptors respectively and proved to have a high relative binding affinity (RBA) for both receptors when compared with typical ligands for each receptor. The RBAs of RU 486 and the steroids used in this study to mammary gland glucocorticoid and Pg receptors correlated well with the ability of RU 486 to block their biological activities. These results demonstrate that RU 486 has both antiglucocorticoid and antiprogesterone activities in pregnant rabbit mammary glands as well as the existence of a strong inhibitory residual action of Pg in the gland that persists during the first 48 h of culture and that can be eliminated by RU 486 or after several days of culture with no hormones.

Pituitary-induced lactation in mammary gland explants from the pregnant tammar (Macropus eugenii): a negative role for cyclic AMP

1. alpha-Lactalbumin and casein have been isolated from tammar milk. 2. alpha-Lactalbumin was induced in mammary explants by culture with anterior pituitary. 3. Casein was induced maximally in the presence of a physiological concentration of prolactin alone. 4. Progesterone did not inhibit the prolactin-induced synthesis of casein, alpha-lactalbumin, galactosyltransferase or fatty acids. 5. Both dibutyryl cAMP and a combination of cholera toxin and IBMX did significantly inhibit the induction of casein and alpha-lactalbumin. 6. Progesterone withdrawal is not a component of the lactogenic trigger in this marsupial but cAMP may be a common intracellular signal for negative control of lactogenesis in both marsupials and eutherians.

Hormonally induced elevations of alpha- and beta-casein mRNA levels are blocked by cyclic adenine nucleotide and prostaglandins

Normal mammary gland development during pregnancy follows a coordinated program of morphological development (formation of lobuloalveoli) and biochemical differentiation (casein production). In culture, whole mammary glands of Balb/c mice can be similarly induced by application of a mixture of insulin, prolactin, aldosterone and hydrocortisone (IPAH) for 7 days. Our previous reports have shown that lobuloalveolar development, induced by IPAH, is competitively inhibited by the simultaneous presence of dibutyryl cyclic AMP (Bt2cAMP), prostaglandins (PGs) E1, E2, and B1, and papaverine (pap). However, if this mixture is not added until day 4, lobuloalveolar development is relatively unaffected but casein synthesis is repressed. This report explores the mechanism by which cyclic adenine nucleotides and prostaglandins interfere with the normal developmental pathway. The accumulation of alpha- and beta-casein mRNAs induced by prolactin, hydrocortisone and aldosterone is blocked by the combination of Bt2cAMP, PGs E1, E2, and B1, and pap added to the medium for the final 3 days (days 4-7). Under these conditions the glands retain their lobuloalveoli, and little squamous metaplasia can be discerned. Furthermore, de novo synthesis of both caseins is selectively inhibited, despite the continued presence of casein mRNAs in the glands and normal protein synthesis. In contrast, the synthesis of keratin is stimulated. Incomplete mixtures of Bt2cAMP and pap or the combination of PGs E1, E2, and B1, are only partly effective in preventing the accumulation of casein mRNAs. All three mixtures bring about similar effects on both alpha- and beta-casein mRNAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Progestins specifically suppress alpha-lactalbumin synthesis and secretion

Mammary gland explants from pregnant (day 12-15) rats were cultured with insulin and prolactin, and their content and secretion of alpha-lactalbumin determined after exposure to a wide range of doses (0.01-300 nM) of the specific synthetic progestin (ORG2058), alone or with a maximally stimulatory dose of the highly specific glucocorticoid RU26988. ORG2058 alone suppressed alpha-lactalbumin synthesis below baseline, with a half-maximal effect at a concentration of less than 0.1 nM; RU26988-stimulated secretion was similarly abrogated by ORG2058, similarly with a half maximally effective dose of less than 0.1 nM. We interpret these data as suggesting that (i) given the specificity and doses of the steroids used the effect of progestins on alpha lactalbumin synthesis is directly via progesterone receptor occupancy, and not by competing with glucocorticoids for glucocorticoid receptors and (ii) given the shift to the left in the alpha-lactalbumin response (half maximal less than 0.1 nM ORG2058) compared with receptor binding (Kd (37 degrees C) greater than 1 nM), one possible model for such sensitivity is that of multiple, independent regulatory elements on the chromatin controlling alpha-lactalbumin gene expression, occupancy of any one of which by an activated progesterone receptor is sufficient to abrogate transcription.

Hormonal action controlling mammary activity

Mammary gland differentiation includes multiplication of cells, activation of genes specific to milk synthesis, and activation of "house-keeping" genes. These events are controlled by multiple hormones, the roles of which are not known in detail. Prolactin induction of milk synthesis is accompanied by accumulation of casein messenger ribonucleic acid resulting from acceleration of casein gene transcription as well as stabilization of messenger ribonucleic acid. Prolactin also favors translation of casein messenger ribonucleic acid. Glucocorticoids amplify and progesterone inhibits prolactin action on transcription of casein genes.

Deinduction of transcription of Xenopus 74-kDa albumin genes and destabilization of mRNA by estrogen in vivo and in hepatocyte cultures

The goal of this study is to explain the molecular basis of the marked deinduction of Xenopus albumin synthesis and secretion accompanying the activation of vitellogenin genes by estrogen. We have characterized by restriction analysis, DNA sequencing and hybrid-selected translation of mRNA, a cloned cDNA specifying the two 74-kDa albumins which constitute the predominant circulating form of albumin in Xenopus laevis. Using this recombinant DNA plasmid as a hybridization probe, we have determined the steady-state levels of albumin mRNA, the rate of transcription of the two 74-kDa albumin genes and the stability of the mRNA in male and female Xenopus hepatocytes in vivo and in primary cell cultures following estrogen treatment. In both whole liver and cultured hepatocytes estradiol caused a rapid drop in the steady-state levels of 74-kDa albumin mRNAs, which was reversed spontaneously in the continued presence of the hormone. The concentration of albumin mRNA was substantially higher in male than in female hepatocytes, the hormonal effect being more marked in male than in female hepatocytes. The decrease in steady-state levels of mRNA was anticipated in male hepatocytes by a 70% inhibition of rate of transcription of albumin genes within 2 h of exposure to estradiol, as measured by run-off transcription in liver nuclei isolated from animals treated in vivo or by determining the absolute transcription rate in cell cultures. In the latter the diminished transcription rate returned to normal within 12 h in the continued presence of the hormone. Estradiol caused a threefold destabilization of albumin mRNA in both male and female hepatocyte cultures to t 1/2 = 3 h and 2 h respectively. The combined effects on rate of or transcription and mRNA stability largely explain the changes in the steady-state levels of mRNA caused by hormone administration. Comparison of the kinetics of transcription rates of vitellogenin and albumin genes in vivo and in vitro reveals a striking reciprocity in the selective activation of the inducible genes and deinduction of the constitutively expressed genes at the early stages of response of Xenopus hepatocytes to estrogen.

In vivo lactogenic effects of anti prolactin receptor antibodies in pseudopregnant rabbits

Antibodies generated against partially purified prolactin receptors from rabbit mammary gland membranes were tested for their effects on prolactin binding to receptors and for their in vivo biological potencies. These antibodies are able to inhibit prolactin binding to crude rabbit mammary gland membranes. When administered intravenously or intramuscularly to pseudopregnant rabbits, they induce respectively an accumulation of beta-casein or an enhancement of beta-casein synthesis and mRNA concentration in the mammary gland. Moreover the stimulatory effect of these anti-prolactin receptor antibodies on casein synthesis is totally abolished by a simultaneous treatment with progesterone, which is a potent in vivo inhibitor of prolactin action. These results better establish the prolactin-like activities of these antibodies previously observed in vitro and give strong support to the hypothesis that prolactin molecule is not required beyond the initial binding to its receptor to induce hormonal effects.

Progestin binding in mammary tissue of prepartum, nonlactating and postpartum, lactating cows

Binding of [3H]R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3, 20-=dione) to bovine mammary cytosol indicated the presence of progestin binding sites of high-affinity and low-capacity in tissue from prepartum, nonlactating and from postpartum, lactating cows. To prevent binding of [3H]R5020 to glucocorticoid binding sites, a 200-fold molar excess of nonradioactive cortisol was included during all incubations, thus specific binding was limited to progestin binding sites. Nonradioactive R5020 and progesterone effectively inhibited [3H]R5020 binding to progestin binding sites, while estradiol-17 beta, dihydrotestosterone (17 beta-hydroxy-5 alpha-androstan-3-one), dexamethasone (9-fluoro-11 beta, 17, 21-trihydroxy-16 alpha methyl-1,4-pregnadiene-3,20-dione) or additional cortisol were ineffective. Dissociation constants for especially bound [3H]R5020 in cytosol from mammary tissue of nonlactating and lactating cows were nearly identical, averaging 1.9 (+/- 0.3) and 0.8 (+/- 0.2) x 10(-9)M, respectively. However, binding capacities (fmol/mg cytosolic protein) were greater in cytosol from prepartum, nonlactating (179 +/- 53) than postpartum, lactating (41 +/- 15) cows. Specific binding components in cytosol from lactating cows sedimented iun the 6-7S region on linear sucrose density gradients. When subjected to isoelectric focusing, specific binders with isoelectric points (pI) of approximately 6.1, 7.9 and 8.3 were resolved. The decrease in number of binding sites during lactation was due to the virtual absence of the anionic binding species, suggesting that their presence is necessary for progesterone to inhibit milk secretion.

Hormonal control of casein synthesis in organ culture of the bovine lactating mammary gland

Explants from lactating bovine mammary gland were cultured in vitro in serum-free medium though 1-9 d. casein synthesis was determined by [32P] incorporation into newly synthesized Ca rennin precipitable fraction. High correlation (r = 0.98) was found between incorporation of [32P] and [3H]amino acids in explants cultured under different hormonal regimes, thus indicating that post-translational phosphorylation is not a rate-limiting step in casein synthesis. Hormonal effects on casein synthesis were studied by supplementing the incubating medium with insulin (I), prolactin (PRL), cortisol (F), thyroxine (T4) and triiodothyronine (T3). It was found that both PRL and I were required absolutely for maximal synthesis and almost maximal effect was achieved with 50 ng/ml. The effect of F was less clear, but some increase was achieved at the 200-1000 ng/ml range. T4 and T3 did not affect casein synthesis at a range of 10(-11)-10(-7) M while a significant inhibition was observed at 2 X 10(-5) M. A time-course study of casein synthesis further substantiated the dominant role of PRL in maintenance or even elevation of the initial rate of casein synthesis in the explants, through the first 4 d of incubation.

Characterization and cloning of the mRNAs specific for the lactating mouse mammary gland

We have characterized and cloned the lactation-specific mRNAs of mouse mammary glands. The group of eight milk-protein-specific mRNAs were identified (a) by size and antigenic properties of their translation products in vitro and (b) by characterization of their respective cDNA clones. Two alpha-caseins (43 kDa and 39 kDa) are encoded by mRNAs of 1600 nucleotides and two beta-caseins (26 kDa) are encoded by mRNAs of 1450 nucleotides in length. Three smaller caseins, gamma-casein (23.7 kDa), delta-casein (21 kDa) and epsilon-casein (14.5 kDa) are synthesized by mRNAs of 880, 1150 and 860 nucleotides. Beside these casein mRNAs a mammary specific 620 nucleotide mRNA codes for a novel acidic whey protein (13.7 kDa). cDNA clones corresponding to the mRNAs for the lactation-specific proteins have been isolated from a mammary-specific cDNA library. Cloned alpha-casein cDNA hybridizes to both alpha-casein specific mRNAs and cloned beta-casein cDNAs hybridize with both beta-casein specific mRNAs. By RNA blot analysis we show that the cloned cDNAs for mouse alpha-casein, beta-casein, gamma-casein and epsilon-casein and the acidic whey protein cross-hybridize with mRNAs of the rat, demonstrating partial sequence homology between the corresponding mRNAs of those species.

Cyclic AMP mediated modulation of complement protein production

We have investigated the mechanisms by which cAMP analogues and phosphodiesterase inhibitors, reduced the production of C2 by monocytes in culture. Pulse label studies with 3H-labelled aminoacids showed that dibutyryl cAMP (dbcAMP) impaired the secretion of newly synthesised protein, both total (acid-precipitable) and individual complement proteins (precipitated antibody by antisera to C4, C2, C3, C5, B, P, C3b inactivator and beta 1H). The intracellular degradation of newly synthesised protein was increased in dbcAMP-treated cultures and protein synthesis was reduced. Studies aimed at defining the temporal relationships between these changes showed that protein secretion was impaired on the first day of culture, and increased degradation of newly synthesised protein was obvious by day 2. Protein synthesis was not significantly reduced until day 3 of culture. It is proposed that changes in intracellular cAMP levels may act as a second signal in the control of protein production by monocytes.

Induction of casein synthesis by prolactin and inhibition by progesterone in the pseudopregnant rabbit treated by colchicine without any simultaneous variations of casein mRNA concentration

Prolactin injected into pseudopregnant rabbits induced casein synthesis and it provoked an accumulation of casein mRNA and of ribosomal RNA. Colchicine, which has been shown to block the prolactin signal, totally prevented the accumulation of beta-casein mRNA, when injected with the hormone. However, the drug did not hamper the initiation of casein synthesis and the accumulation of ribosomal RNA. The effect of prolactin injected with colchicine on casein synthesis was totally abrogated by progesterone administered simultaneously and it was essentially unmodified by glucocorticoids. These results suggest that prolactin controls casein gene expression by supporting the accumulation of casein mRNAs and by stimulating the translation of these mRNAs, through independent mechanisms. Progesterone which is known to prevent the accumulation of casein mRNAs is also a potent inhibitor of casein mRNA translation whereas glucocorticoids exert their effect essentially by favouring the accumulation of casein mRNA but not by modifying their translation efficiency.

Role of progesterone and glucocorticoids in the transcription of the beta-casein and 28-S ribosomal genes in the rabbit mammary gland

Isolated mammary nuclei were incubated in the presence of HgCTP and the neosynthesized RNA was isolated with a SH-Sepharose column. The concentration of beta-casein mRNA and 28-S ribosomal RNA in the neosynthesized RNA fractions was measured using [3H]cDNA probes complementary to beta-casein mRNA and 28-S rRNA respectively. Prolactin injected into pseudopregnant animals accelerates the transcription of both genes and increases the stability of the beta-casein mRNA but not of the 28-S rRNA. Progesterone injected simultaneously with prolactin reduced considerably all these effects of prolactin, with a lower efficiency when the highest doses of prolactin were injected. These observations suggest that progesterone attenuates the transfer of prolactin information related to the lactogenesis into the mammary cell. Glucocorticoids injected with prolactin amplify the prolactin action on the expression of the beta-casein gene but not of the 28-S rRNA genes. In the absence of prolactin (a situation obtained by injecting simultaneously CB 154, a drug which inhibits pituitary prolactin secretion) glucocorticoids exhibit no effect. In the lactating rabbit glucocorticoids do not delay significantly the drop of beta-casein gene transcription rate provoked by weaning or by prolactin withdrawal obtained by injecting CB 154. A comparison of the beta-casein mRNA accumulation and the transcription rate of the beta-casein gene indicates that glucocorticoids act essentially by amplifying the activation of beta-casein transcription supported by prolactin, but not by enhancing the stability of the beta-casein mRNA.

Glucocorticoid regulation of alpha-lactalbumin production in long-term cultures of normal rat mammary cells

Cultures of normal rat mammary cells have been developed which produce alpha-lactalbumin (a-LA) for several months in response to physiological levels of corticosterone whereas none is measureable without glucocorticoid. In contrast to recent studies in short-term organ cultures, these long-term cell cultures derived from virginal or mid-pregnant rats show no inhibition of a-LA production at high glucocorticoid concentrations. Comparable a-LA production was observed after a few months whether cells were initially cultured into media with various corticosterone levels or initially into high levels of corticosterone for 3 weeks prior to the shift to various lower doses.

Simultaneous occurrence of pregnancylike lobuloalveolar morphogenesis and casein-gene expression in a culture of the whole mammary gland

Entire second thoracic mammary glands of estrogen- and progesterone-treated immature virgin BALB/c mice were stimulated to pregnancylike lobuloalveolar morphogenesis after 6 days of incubation with insulin (5 micrograms/ml), aldosterone (1 micrograms/ml), growth hormone (5 micrograms/ml), cortisol (5 micrograms/ml), and prolactin (80 ng/ml, present as a contaminant in 5 micrograms/ml growth hormone). The alveolar growth in the glands, as judged by morphological studies, was accompanied by an increase in cell number as a function of incubation time in the hormonal medium. Hybridization of the total RNA from these glands to the casein mRNA specific complementary DNA probe (cDNAcsn) revealed that the level of casein mRNA rises from 0.00012 to 0.005% between 1 and 6 days of incubation. Estimates showed that the concentration of casein mRNA per cell rises 17-fold from 70 molecules on Day 1 to 1200 molecules on Day 6, whereas the number of epithelial cells increases only twofold during the same incubation time. When the growth hormone preparation was totally replaced by 80 ng of prolactin during the 6-day incubation, casein-mRNA levels were found to be 0.0083%. These results demonstrate that a pregnancy-like morphogenesis and concurrent expression of the casein gene in vitro can be achieved in a controlled hormone environment containing high cortisol and low prolactin concentrations. This one-step mammogenesis-lactogenesis culture model should be useful for studying the mechanisms of hormonal regulation of casein-gene expression observed in prepartum mammary gland in vivo.

Characterization of a 7,12-dimethylbenz(alpha)anthracene-induced BALB/c mouse mammary tumor epithelial cell line

Characterization of numerous independently derived cell lines is necessary in order to establish reliable biological criteria applicable to transformed mammary epithelial cells. A mammary tumor cell line (DMBA-2/BALB) and clonal derivatives have been established from an adenocarcinoma induced in vivo in a BALB/c mouse by the administration of the chemical, 7,12-dimethylbenz(alpha)anthracene. The parental and clonal lines in vitro, as well as tumors derived from them in vivo, maintained an epithelial morphology at the light microscope level. Electron microscopic observations demonstrated the presence of desmosomes and numerous microtubules and microfilaments, but low levels of rough endoplasmic reticulum and Golgi apparatus. Growth characteristics were examined, including saturation density levels and colony-forming ability on plastic and in suspension. None of the in vitro growth parameters was found to correlate with the tumorigenicity of the lines in syngeneic mice or with their ability to produce C- or B-type virions by reverse transcriptase assay. Peroxidase/antiperoxidase (PAP) immunological staining failed to detect virus-specified antigens, MuMTV gp52, Rauscher MuLV gp69/71, or BALB virus-2 gp70, in the cells. Although epithelial in appearance, the parental cell line did not appear to be functionally differentiated in culture as evidenced by a very low level of casein messenger RNA. Passage of the parental cell line in vivo in a lactating mouse failed to activate virus expression, but a few cells in scattered focal areas in the tumor did synthesize casein detectable by PAP staining.