Prolactin activation of mammary nitric oxide synthase: molecular mechanisms

Prolactin (PRL) is capable of stimulating both calcium and nitric oxide (NO) accumulation in mammary epithelial cells within 15min. A calcium ionophore was also able to stimulate NO levels to an extent similar to that generated by PRL. Furthermore, maximal concentrations of PRL and the ionophore were not additive, suggesting that they were both using the same pathway, i.e. calcium. Finally, the depletion of intracellular calcium completely abrogated the effect of PRL on NO production. No other pathway known to affect NO synthase (NOS) influenced the action of PRL. Specifically, manipulations of protein phosphatase 2B, protein kinase B (PKB), protein kinase C (PKC), and arginine transport did not alter the activation of NOS by PRL. Therefore, the ability of PRL to stimulate NO production at 15min can be completely explained by its ability to elevate intracellular calcium.

The role of nitric oxide in the biological activity of prolactin in the mouse mammary gland

In mouse mammary epithelial cells, prolactin transiently elevates nitric oxide (NO) to a maximum of 6 nmol/mg protein at 15 min, after which levels fall rapidly. This stimulation can be achieved by as little as 100 ng prolactin/ml and can be mimicked by 100 microg sodium nitroprusside/ml. NO is both necessary and sufficient to mediate the prolactin-induced redistribution of its receptor from internal pools to the cell surface. NO can also enhance DNA synthesis stimulated by submaximal prolactin concentrations (50 ng/ml), but it is not necessary at pharmacological prolactin concentrations (1 microg/ml). In contrast, NO completely inhibits alpha-lactalbumin production. In summary, prolactin transiently elevates NO to enhance DNA synthesis and suppress premature differentiation; thereafter, NO declines, DNA synthesis ceases and differentiation proceeds. This data suggest that NO may mediate some of the effects of prolactin on growth in the mammary gland.

Rapid hormonal regulation of N-acetylglucosamine transferase I

Previous studies have shown that, in unstimulated mammary epithelial cells from virgin mice, prolactin receptors are retained intracellularly because of their incomplete N-glycosylation. Activation of the nitric oxide/cGMP pathway stimulates Nacetylglucosamine (NAG) transferase I activity, completion of terminal glycosylation, and redistribution of the receptors to the cell surface. In this study, it was shown that nitric oxide could stimulate the phosphorylation of NAG transferase I in intact cells and that the cGMP-dependent protein kinase (PKG) could directly phosphorylate the purified enzyme. Furthermore, this modification was associated with enhanced enzymatic activity. Conversely, this stimulation of activity was blocked in intact cells by coincubation with a PKG inhibitor and reversed in the immunoprecipitated enzyme by alkaline phosphatase treatment. Kinetic analysis revealed that this effect on enzyme activity was due to an increase in V(max) without any change in K(m). Therefore, it appears that the nitric oxide/cGMP pathway activates NAG transferase I via direct phosphorylation by PKG.

Regulation of prolactin receptor glycosylation and its role in receptor location

In unstimulated mammary epithelial cells from virgin mice, the prolactin receptor exists as two isoforms: a 78 and a 70 kDa species. Both proteins are reduced to a single 61 kDa molecule after N-glycanase F treatment, indicating that their size difference is solely a result of carbohydrate content. Membrane fractionation experiments reveal that the smaller species is exclusively intracellular, while the larger one is located on the cell surface. Nitric oxide (NO) stimulates the migration of prolactin receptors from an internal pool to the plasmalemma in only 30 min and this redistribution is associated with an increase in molecular weight. Redistribution is blocked by swainsonine, but not by castanospermine or 1-deoxymannojirimycin, suggesting that the glycosylation step involved with translocation is either alpha-mannosidase II or N-acetylglucosamine (NAG) transferase I. The former is unaffected by NO but the activity of the latter is doubled 30 min after exposure to NO. These data suggest that prolactin receptors are retained intracellularly because of their incomplete N-glycosylation and that NO triggers their redistribution by stimulating the completion of this process, in part by increasing NAG transferase I activity.

Transduction pathways involved in rapid hormone receptor regulation in the mammary epithelium

Previous studies have shown that the envelope protein of the mouse mammary tumor virus (MMTV) rapidly upregulates prolactin (PRL) receptors by shifting them from internal pools to the cell surface and downregulates epidermal growth factor (EGF) receptors by inducing their internalization and degradation. This study shows that the effect on PRL receptors is mediated by the nitric oxide (NO)/cGMP pathway, since it can be mimicked by an NO donor or 8-bromo-cGMP and can be blocked by an NO synthase inhibitor. In contrast, the effect on EGF receptors is mediated by tyrosine phosphorylation and phosphatidylinositol 3-kinase (PI3K), since it can be blocked by either a tyrosine kinase inhibitor or by a PI3K inhibitor. Both of these pathways can be activated by a calcium ionophore and inhibited by calcium chelation. Therefore, it appears that the mouse mammary tumor virus envelope protein, like other retroviral envelope proteins, initially elevates cytoplasmic calcium, which can then stimulate both the NO/cGMP and the tyrosine phosphorylation/PI3K pathways, leading to PRL receptor upregulation and EGF receptor downregulation, respectively.

Regulation of mammary hormone receptor metabolism by a retroviral envelope protein

In a previous study, the envelope protein (gp52) of the mouse mammary tumour virus (MMTV) was shown to facilitate mammary gland differentiation by increasing prolactin (PRL) receptors via increased receptor synthesis and via the redistribution of existing receptors from an internal pool. In this study, receptors for other hormones known to affect mammary gland metabolism were investigated. Epidermal growth factor (EGF) stimulates mammary epithelial growth and inhibits differentiation; its receptor is rapidly and dramatically down-regulated by gp52. This is accomplished by its internalization and by decreasing its half-life from 27 h to 2.4 h. Surprisingly, it also increased EGF receptor synthesis, although this effect was not great enough to overcome receptor down-regulation. In contrast, gp52 did not affect the distribution, half-life or synthesis of the insulin receptor. These results demonstrate that MMTV can enhance mammary differentiation by coordinately regulating several hormone receptors: specifically, it can increase the number of receptors for PRL, a differentiative hormone, while decreasing the number of receptors for EGF, a growth/anti-differentiative hormone.

The regulation of mammary prolactin receptor metabolism by a retroviral envelope protein

In a previous study, infection with the mouse mammary tumor virus (MMTV) was shown to increase the sensitivity of the mammary epithelium toward prolactin (PRL); furthermore, this effect could be mimicked by the binding of the MMTV envelope protein (gp52) to its cell receptor. The present work has investigated the possibility that gp52-induced changes in the PRL receptor (PRLR) were responsible for this phenomenon. In vitro, gp52 doubled the PRLR concentration in the plasmalemma of mammary epithelium without affecting the affinity. The origins of these PRLRs were twofold: first, gp52 stimulated PRLR mRNA nearly fivefold, suggesting that some of the receptors were newly synthesized. Second, there was a redistribution of PRLRs within the mammary cell: PRLRs were shifted from an internal pool to the plasma membrane. This relocation was very rapid, occurring within 30 min. There did not appear to be any contribution from alterations in PRLR degradation, since the half-life of PRLR was not affected by gp52. In summary, the MMTV increases the PRL sensitivity of mouse mammary epithelium by elevating PRLRs through both enhanced synthesis and recruitment from microsomes.

Second messengers induced by the envelope protein of a retrovirus

The envelope protein (gp52) of the mouse mammary tumor virus (MMTV) can stimulate RNA synthesis via binding to its cellular receptor on mammary epithelium. This effect was mimicked by either nitric oxide (NO) or 8-bromo-cGMP and was blocked by an NO inhibitor. Furthermore, the effects of gp52 and 8-bromo-cGMP were not additive at maximal concentrations, suggesting that they were using the same signaling route. Finally, gp52 elevated cGMP levels in mammary epithelium. These data suggest that gp52 activates the following transduction pathway in this tissue: gp52-->NO synthase-->NO-->soluble guanylate cyclase cGMP RNA synthesis. In contrast to the mammary gland, gp52 inhibited RNA synthesis in the diaphragm. However, the effect was again mimicked by NO, blocked by an NO inhibitor, and the effects of gp52 and NO were not additive. Therefore, it appears that gp52 is using the NO-cGMP pathway in both tissues, but that muscle tissue may be more susceptible to the toxic effects of NO.

Requirements for mouse mammary tumour virus internalization in mouse mammary epithelial cells

Methylamine, a lysosomotropic alkalinizing agent, blocked mouse mammary tumour virus (MMTV) infection in normal mouse mammary epithelium, suggesting that internalization and acidification are necessary for cell penetration. This mechanism was further supported by the fact that intact MMTV induced the translocation of its cellular binding protein from the plasmalemma to the microsomes; however, isolated gp52, the MMTV envelope protein that binds this receptor, did not redistribute the binding protein. These data suggest that either another viral component, in addition to gp52, is needed for cell entry or that internalization requires receptor aggregation, which only the multivalent viral envelope can induce.

The effect of mouse mammary tumor virus receptor activation on mammary epithelial cell sensitivity toward prolactin

Mammary epithelial cells infected with the mouse mammary tumor virus (MMTV) require less than one-half the concentration of prolactin to elicit alpha-lactalbumin production than uninfected tissue (EC50 = 89 +/- 10 ng/ml vs. 206 ng/ml, respectively). Furthermore, stimulating antibodies to the cellular receptor for MMTV halved the prolactin requirement of MMTV-tissue, while MMTV antibodies that sequestered secreted MMTV increased the prolactin requirement in MMTV+ tissue. These data suggest that the effect of MMTV on mammary epithelial sensitivity toward prolactin is being mediated by its interaction with a cell surface receptor.

Regulation of the mouse mammary tumor virus receptor by phosphorylation and internalization in mammary epithelial cells

The mouse mammary tumor virus enters mammary epithelial cells via a plasma membrane protein that binds to a viral envelope glycoprotein, gp52. In intact cells, this gp52 receptor can be phosphorylated by activators of protein kinase A and protein kinase C (PKC), but this modification does not occur in response to epidermal growth factor, whose receptor is a tyrosine kinase, or to gp52. Phosphorylation of the gp52 receptor rapidly leads to internalization and gradual loss of binding activity. Both the phosphorylation and the internalization induced by PKC are abolished by prior downregulation of this kinase. Although the physiological function of the gp52 receptor is unknown, its binding to gp52 can stimulate several biological activities, including amino acid accumulation. Receptor processing impairs this gp52-induced amino acid uptake, as well as viral infection, by depleting the binding protein at the cell surface. In contrast, PKC augments insulin-induced amino acid transport, and PKC downregulation abolishes the action of insulin, suggesting that insulin and gp52 utilize partially separate pathways leading to amino acid transport. These data further suggest that PKC may be involved in this insulin-stimulated activity.

Regulation of the mouse mammary tumor virus (MMTV) binding site in cultured mammary tissue

The mouse mammary tumor virus (MMTV) initiates infection when the envelope protein, gp52, binds to a cell surface protein and triggers internalization. The hormonal regulation of this protein was studied both in vitro and in vivo. In mammary gland explant culture, levels of the MMTV binding protein were maintained only when prolactin was present in the medium. The further addition of progesterone induced levels to 164% of controls, while estradiol was without effect. These results were reproduced in ovariectomized mice which, in addition, demonstrated a need for estradiol in the maintenance of basal levels. Two growth-promoting agents were also tested in vitro: dibutyryl cAMP decreased levels to 31% of controls, while epidermal growth factor had no effect. Scatchard analyses revealed that all of these changes reflected alterations in binding protein concentration and not affinity. These results suggest that the expression of MMTV from mammary epithelium and the elevation of its binding protein are coordinated.

Tissue distribution of the cellular binding protein for the mouse mammary tumour virus

The envelope protein, gp52, of the mouse mammary tumour virus (MMTV) binds to a cell-surface receptor as a first step in infection. A protein with characteristics of this receptor was measured on freshly isolated cells using, as ligand, 125I-labelled gp52 purified from C3H/HeN mice. The gp52-binding protein was found in all mouse tissues examined, but was present at highest concentrations in the mammary gland and spleen where it reached 4.2 +/- 0.3 (S.E.M.) pmol/mg protein; the dissociation constant was 30 +/- 7 pM. Binding to mammary epithelium could be displaced by either the RIII or 34I-R strains of MMTV, and binding was blocked by antibodies to gp52. Levels in the liver and adrenal glands were only 25% of those in the mammary gland, while the concentrations in the ovary and salivary gland were intermediate. Scatchard analyses of the binding data suggested that there was only a single set of high-affinity binding sites. During late pregnancy and lactation, receptor levels in mammary epithelium rose threefold, while those in the liver and salivary gland were unchanged. This induction would result in the mammary gland having 12 times the gp52-binding protein than other tissues and may result in the preferential reinfection of this tissue during lactation, with subsequent tumour formation.

Thyroid hormone requirement for retinoic acid induction of mouse mammary tumor virus expression

In normal mouse mammary epithelium, insulin, cortisol, and prolactin are absolute requirements for mouse mammary tumor virus expression. Retinoic acid further increased mouse mammary tumor virus expression two- to threefold but only when triiodothyronine was also present; neither retinoic acid nor triiodothyronine alone had any effect.

Cyclic AMP and calcium in mouse mammary tumour virus expression: effects and post-transcriptional site of action

The role of cyclic AMP (cAMP), calcium, calmodulin and protein kinase C (PKC) in the expression of both mouse mammary tumour virus (MMTV) RNA and an MMTV glycoprotein, gp58, was investigated in normal mammary epithelium in culture. None of these second messengers had any effect on MMTV RNA. Dibutyryl cAMP alone had no effect on gp58 levels but, at low concentrations (0.05-0.1 mM), it nearly doubled the induction seen with insulin, cortisol and prolactin; higher concentrations were inhibitory. Although a calcium ionophore (A23187), either alone or with hormones, was ineffective, a calcium channel blocker (verapamil) reduced hormonal induction of gp58 by 80%, and a calmodulin inhibitor (W-13) reduced it by 90%. Two PKC activators, a phorbol ester and a diacylglyceride, were ineffective alone, with hormones or with the calcium ionophore. The following conclusions can be made: (1) cAMP, calcium and calmodulin play an important role in MMTV expression, (2) these second messengers all act post-transcriptionally, since they do not affect MMTV RNA, and (3) PKC does not appear to have a role in MMTV production in normal mammary epithelium.

Differential characteristics of the thoracic and abdominal mammary glands from mice

The anatomical and physiological characteristics of thoracic and abdominal mammary glands were investigated in order to understand why the incidence of mammary tumors is higher in the former. Epithelium in explants from both sets of glands required DNA synthesis, insulin, cortisol, and prolactin for full differentiation as measured by alpha-lactalbumin accumulation. The temporal pattern and magnitude of response were the same with respect to both DNA synthesis and differentiation; however, the epithelium in explants from the thoracic glands required concentrations of hormones for alpha-lactalbumin accumulation only one-half to one-third those from abdominal glands. Tumor distribution did not appear to correlate with mammary gland histology, size, or epithelial content.

Developmental and hormonal regulation of a mouse mammary tumour virus glycoprotein in normal mouse mammary epithelium

A radioimmunoassay was developed and validated for the major glycoprotein (gp58) of the mouse mammary tumour virus (MMTV). Using this assay, the expression of gp58 during pregnancy and lactation was found to parallel that for MMTV RNA. In particular, there was a very rapid induction in late pregnancy and a decline in late lactation, although some residual expression persisted well into involution. In cultures of normal mouse mammary tissue, induction of gp58 occurred after a 24-h lag period and began to reach a plateau after 3 days. Both the insulin and prolactin dose-response curves for gp58 resembled those for MMTV RNA; in contrast, the effects of steroid hormones on gp58 and MMTV RNA were disparate. Although progesterone stimulated the RNA, it only slightly increased gp58 levels; however, the presence of cortisol greatly augmented this stimulation, despite the inability of cortisol to induce RNA at physiological concentrations. These results suggest that insulin, prolactin and progesterone act primarily at the level of RNA accumulation in normal mammary epithelium, while cortisol affects some more distal event.

The role of sex steroids in the expression of MMTV in the normal mouse mammary gland

Infection with the mouse mammary tumor virus (MMTV) is associated with hyperplastic alveolar growth and subsequent tumorigenesis. The role of the sex steroids in the initial phase of this pathological chain of events is investigated in this study. In normal mammary epithelium, progesterone stimulates MMTV RNA expression both in vivo (2.6-fold) and in vitro (2.9-fold); although estradiol is ineffective alone, it does enhance the effect of progesterone (6.8- and 5.7-fold stimulation, respectively). These results suggest that the sex steroids may play an important role in inducing MMTV expression which can then lead to epithelial hyperplasia and, eventually, malignant transformation.

Prolactin regulation of mouse mammary tumor virus (MMTV) expression in normal mouse mammary epithelium

The hormonal regulation of mouse mammary tumor virus (MMTV) RNA in normal mouse mammary epithelium was studied in an explant system. In tissue from parous mice, physiological concentrations of prolactin stimulated MMTV expression, while only pharmacological concentrations of cortisol were effective. Regulation in explants from virgin mice was similar to that in parous animals except that the former were less sensitive to prolactin; this relative unresponsiveness may explain why uninduced tissue from virgin mice does not express MMTV RNA, while that from parous mice does exhibit some basal production. These results suggest that prolactin plays a major role in MMTV expression in normal mammary epithelium and that glucocorticoids may only have a permissive effect or may act through an indirect mechanism requiring high concentrations. These data also suggest that the greater susceptibility of parous mice to MMTV-induced tumorigenesis may reflect the greater prolactin sensitivity in the glands from these animals.

Hormonal regulation of protein kinase C in the mouse mammary gland

The hormonal regulation of protein kinase C (PKC) induction over 3 to 14 days was investigated in the mouse mammary gland in vitro and in vivo. In intact mice, estradiol (1 microgram/mouse injected daily for 2 weeks) stimulated PKC activity 70%, while progesterone (1 mg/mouse injected daily) inhibited it by 30%. Prolactin, whose levels were elevated for 2 weeks by two pituitary isografts, had no effect. When mammary gland explants were cultured in insulin and cortisol, the further addition of estradiol (1 ng/ml), progesterone (1 microgram/ml), or prolactin (1 microgram/ml) did not alter PKC activity after 3 days. These data suggest the following conclusions: although previous studies have implicated prolactin in the transient, calcium-phospholipid activation of PKC, it does not appear to elevate total levels of this kinase over prolonged periods. In contrast, the sex steroids do appear to affect long-term levels of this kinase; furthermore, this latter effect may be indirect.

The interrelationships among poly(ADP-ribosyl)ation, DNA synthesis and mammary gland differentiation

Because mammary epithelium from virgin mice must undergo DNA synthesis prior to differentiation and because poly(ADP-ribosyl)ation has been linked to the cell cycle, it was hypothesized that this requirement for DNA synthesis might be related to the poly(ADP-ribosyl)ation of nuclear proteins. However, 3-methoxybenzamide, an inhibitor of poly(ADP-ribosyl)ation, stimulates alpha-lactalbumin accumulation even when added after DNA replication is completed. Furthermore, in parous mice this compound is still effective when DNA synthesis is blocked by cytosine arabinoside-beta-D-arabinofuranoside. Therefore, poly(ADP-ribosyl)ation appears to be associated, not with DNA synthesis, but with some other event in mammary gland differentiation.

Involvement of protein kinase C in mouse mammary gland development

The relationship between kinase C activity and mammary gland differentiation was investigated by following kinase activity throughout the mouse reproductive cycle and by pharmacologically perturbing the kinase, while monitoring biochemical differentiation. Protein kinase C activity declined during pregnancy and remained low throughout lactation, suggesting an inverse relationship with milk protein expression. This negative association was further supported by the use of quercetin (50-100 mumol/l) and gossypol (50 mumol/l), which are both protein kinase C inhibitors. These compounds doubled alpha-lactalbumin levels in mammary explants cultured with hormones. However, a phorbol ester, known to activate protein kinase C, had no effect on alpha-lactalbumin production, although it did stimulate this milk protein 2.5-fold in the presence of the calcium ionophore, A23187. In the absence of raised calcium levels, protein kinase C activity therefore appeared to be inversely correlated with biochemical differentiation; but, in the presence of increased calcium concentrations, both calcium and the kinase acted synergistically to augment hormone-induced alpha-lactalbumin expression.

Possible roles of calcium and calmodulin in mammary gland differentiation in vitro

In response to insulin, cortisol and prolactin, mammary gland explants from virgin mice will undergo differentiation, as measured by lactose synthetase activity and casein synthesis. W-13 (N-(4-aminobutyl)-5-chloro-2-naphthalenesulphonamide), a calmodulin inhibitor, reduced prolactin-induced differentiation by 80% while inhibiting insulin- and cortisol-induced RNA synthesis only 40%. However, tissue levels of calmodulin did not change during a 3-day culture, with or without prolactin, suggesting that the regulatory component of the calcium-calmodulin system was the intracellular calcium concentration. In support of this hypothesis, verapamil, a calcium channel blocker, also preferentially inhibited prolactin-induced differentiation and prolactin did stimulate calcium accumulation (from a control value of 3.8 +/- 1.3 (S.E.M.) pg calcium/mg wet tissue per 15 min to 61.4 +/- 7.0). On the other hand, A23187, a calcium ionophore which mimicked prolactin actions on calcium accumulation, was not able to induce differentiation in the absence of prolactin; in fact, high concentrations of either A23187 or extracellular calcium actually inhibited lactose synthetase activity, although casein synthesis was relatively unaffected. Therefore, it appears that the calcium-calmodulin system is involved in prolactin-induced differentiation of mammary gland explants but that it cannot be the only mediator of prolactin actions; i.e. it is necessary but not sufficient. There also appears to be another, separate action of calcium on casein synthesis which involves increasing the mammary epithelial sensitivity toward prolactin with respect to casein synthesis but not lactose synthetase activity.

The relationship between adenosine diphosphate-ribosylation and mammary gland differentiation

Poly(adenosine diphosphate [ADP]-ribosyl)ation, although associated with differentiation in many systems, exhibited a reciprocal relationship with mammary gland differentiation, and both the synthetic and degradatory pathways complemented each other in this regard. Poly(ADP-ribosyl)synthetase activity declined during pregnancy and lactation, while poly(ADP-ribose) degradatory activity rose late in pregnancy and peaked during lactation. In explant cultures, similar changes occurred and appeared to be under separate hormonal control; prolactin suppressed the synthetase activity, whereas insulin stimulated the poly(ADP-ribosyl)glycohydrolase activity. This latter effect may be mediated by a decline in cAMP levels for the following reasons: the glycohydrolase is known to be inhibited by cAMp, insulin decreased cAMP concentrations in mammary explants by 70%, and cholera toxin blocked the effects of insulin on poly(ADP-ribose) degradation. This reciprocal relationship between poly(ADP-ribosyl)ation and mammary gland differentiation is further supported by pharmacological studies: in the presence of insulin, cortisol, and prolactin, an inhibitor of the synthetase stimulated alpha-lactalbumin three-fold over hormone stimulation alone. However, this inhibitor was unable to induce differentiation in the absence of prolactin. Therefore, although there is a close association between a decline in enzyme activity and mammary differentiation, the data are insufficient to support a causal relationship.

Enhanced endocrine sensitivity in mouse mammary glands: hormonal requirements for induction and maintenance

This laboratory has previously shown that mammary epithelium from pregnant and parous mice are 5-7 times more sensitive in vitro to insulin, cortisol, and PRL than tissue from virgins, with respect to lactose synthetase activity. In the present studies, virgin mice were treated for 2 weeks with either progesterone or a pituitary allograft to increase PRL levels, and the mammary explants exhibited the same heightened sensitivity to all three hormones that was noted in pregnant animals. Treatment with estradiol or T4 was without effect. However, this effect was transient, since the tissue lost its elevated hormonal responsiveness 4 weeks after progesterone withdrawal, suggesting that the permanent effect seen in nonpregnant nonlactating parous mice involves a more complex series of events than simply elevated progesterone or PRL levels. Even tissue from parous mice reverted to a reduced insulin and PRL sensitivity, if the tissue was initially cultured in the absence of any hormones. Although the increased sensitivity to cortisol did appear to be permanent under the culture conditions used, the maintenance of a heightened insulin responsiveness required the continuous presence of insulin, cortisol, and PRL, while that for PRL needed at least two of these three hormones. In summary, progesterone and/or PRL may be partly responsible for the increased hormonal sensitivity first manifest during pregnancy, but a normal endocrine milieu is required to maintain this responsiveness in vitro.

Enhanced hormonal responsiveness in mammary glands from parous mice: molecular mechanisms

Mammary glands from parous mice required lower concentrations of hormones in vitro than those from virgins to effect differentiation, as measured by lactose synthetase activity. This phenomenon could not be explained by changes in receptor levels, since both mammary gland insulin and prolactin binding, although elevated at midpregnancy, returned to baseline in tissue from parous mice. Ethidium bromide, an intercalating dye, was a potent inhibitor of lactose synthetase induction in explants from virgins but much less potent in tissue from pregnant mice; explants from parous animals displayed intermediate sensitivity, suggesting that DNA structure was permanently altered. However, casein synthesis in glands from parous mice required hormone concentrations as high as in virgins and are just as susceptible to ethidium bromide as in virgins. Similarly, the vulnerability of the casein gene to DNAase I digestion is low in mammary epithelial cells from virgins, high in cells from pregnant mice, and low again in cells from parous animals. These data suggest that during the first pregnancy of mice, there are changes in the chromatin configuration that may facilitate the transcription of milk-related mRNA. Furthermore, after mammary gland involution these changes in the casein gene undergo reversion, while those involved with lactose synthetase activity persist; this may explain the disparate hormonal responsiveness seen in these animals with respect to casein and lactose synthesis.

Loss of differentiative potential of the mammary gland in ovariectomized mice: altered estrogen responsiveness in parous mice

Previously, estrogen had been shown to be essential for the maintenance of differentiative potential in the mammary gland of virgin mice; the purpose of this study was to evaluate the role of these steroids in nonpregnant, nonlactating parous mice. To this end, parous mice were either ovariectomized or adrenalectomized and ovariectomized for 4-6 weeks before hormonally induced differentiation was attempted in culture. Although mammary glands from virgin mice lose their differentiative potential in vitro following ovariectomy, tissue from castrated parous mice exhibit only a partial loss of activity. Since combined adrenalectomy and ovariectomy in parous animals virtually abolished differentiation and since exogenous estradiol preserved it, it would appear that mammary gland differentiation in parous mice, as in virgins, requires estrogens, but that tissue from the former is more sensitive. This difference between virgins and parous mice is not related to differences in either the capacity or affinity of the cytosol estrogen receptor, as these parameters are identical in all experimental groups.

Persistent alterations in hormonal sensitivities of mammary glands from parous mice

Mammary gland explants from virgin mice required supraphysiological concentrations of insulin, cortisol, and PRL for the induction of casein synthesis and lactose synthetase activity; dose-response curves for any one hormone were identical for each parameter of differentiation. Tissue from midpregnant mice was 5-7 times more sensitive to hormonal induction, responding to near-physiological concentrations of insulin, cortisol, and PRL; again, both casein synthesis and lactose synthetase activity required the same amounts of any individual hormone. Induction of lactose synthetase activity in glands from nonlactating nonpregnant parous mice required as little hormone as tissue from pregnant animals; however, induction of casein synthesis required a higher concentration of hormones, as was observed with glands from virgins, leading to a discrepancy between the optimal hormonal concentrations required for these two markers of differentiation in parous mice. Similar data were found in the time courses. Induction in explants from midpregnant animals was 1-2 days faster than that in virgins; in parous mice, induction of lactose synthetase activity was as rapid as in midpregnant mice, but induction of casein synthesis was slower and shifted toward that for the tissue from virgins. These data suggest that those mechanisms responsible for the rapid induction and increased hormonal sensitivity of the mammary gland during pregnancy are still operative after involution of the gland with respect to at least one parameter of differentiation.

Deoxyribonucleic acid synthesis-dependent casein gene expression: species differences

Mammary gland explants from mature virgin mice, rats, and rabbits exhibit an increased rate of both DNA and casein syntheses when cultured in the presence of specific combinations of insulin, hydrocortisone, and PRL. If cytosine arabinoside, a potent inhibitor of DNA synthesis, is added to the culture medium, casein synthesis is inhibited in explants from mice but not in those from rats or rabbits. This inhibition is at the level of accumulation of casein mRNA; an 89% reduction of stimulated levels was observed. The nature of this block was investigated further by examining the general response of mature virgin mouse mammary explants to hydrocortisone and PRL, hormones considered essential for casein gene expression in this species. Cytosine arabinoside did not prevent an increment in either hydrocortisone-induced NADH-cytochrome c-reductase or PRL-induced total RNA synthesis. Previous work has shown that certain insulin-induced responses are also unaffected. Taken collectively, these results suggest that the lesion induced by cytosine arabinoside inhibition of DNA synthesis is distal to the receptor for one or more of these hormones. The necessity for coupling DNA synthesis with overt differentiation in the mature virgin mouse, but not in the rat or rabbit, is one of numerous examples of species variation in regard to the interaction of hormones with the mammary gland.

Insulin is essential for accumulation of casein mRNA in mouse mammary epithelial cells

In the presence of cortisol and prolactin, insulin at concentrations as low as 1 ng/ml significantly stimulates casein synthesis in mammary explants from midpregnant mice; maximal synthesis occurs at 10 ng/ml. However, in the absence of insulin, no detectable immunoprecipitable casein is produced. Insulin also supports enhanced accumulation of casein mRNA in the presence of cortisol and prolactin; neither epidermal growth factor nor somatomedin C has this effect. These inductive actions of insulin are not secondary to a general maintenance effect on the mammary epithelial cell; insulin, epidermal growth factor, and somatomedin C can support the accumulation of RNA in rough endoplasmic reticulum equally well. In addition, these effects do not reflect a specific insulin requirement for prolactin sensitivity; epidermal growth factor can support prolactin-induced total RNA synthesis as well as insulin can. The results demonstrate that, although insulin, epidermal growth factor, and somatomedin C can all function as cell maintenance agents, only insulin, together with cortisol and prolactin, can induce casein mRNA accumulation.

Loss of differentiative potential of the mammary gland in ovariectomized mice: identification of a biochemical lesion

The differentiative functions, lactose synthetase activity and casein synthesis, can be induced in mammary gland explants from intact mice when insulin, cortisol, and PRL are present in the medium. By contrast, the tissue from mice castrated for 1--2 months does not differentiate in vitro. Explants from these ovariectomized animals retain their sensitivity toward insulin, as evidenced by the ability of this hormone to stimulate DNA synthesis, alpha-aminoisobutyric acid accumulation, and glucose-6-phosphate/gluconate-6-phosphate dehydrogenase activities. This tissue also remains sensitive to cortisol, as evidenced by the ability of this steroid to stimulate NADH-cytochrome c reductase activity. However, the tissue from ovariectomized mice has lost biological responsiveness to PRL. Such insensitivity may be due to a deficiency of PRL receptors, which are reduced in the glands from castrated mice to 20--25% of control values. However, a second defect between the receptor and the genome is also likely, since PRL unresponsiveness is still present in the tissue of ovariectomized animals whose mammary PRL-binding has been partially maintained by elevating serum PRL levels with a pituitary transplant. Therefore, this system may be useful for the study of cellular processes related to PRL action beyond the receptor level.

The asynchronous hormonal induction of lactose synthetase components, alpha-lactalbumin and galactosyltransferase, in relation to lactose secretion by mouse mammary explants

Hormonal induction of the lactose synthetase components, alpha-lactalbumin and galactosyltransferase, in relation to the induced levels of lactose synthetase activity and lactose secretion by mammary gland explants from mature virgin mice was examined. During 6 days of culture in the presence of insulin, cortisol, prolactin and triiodothyronine mammary explants accumulated progressively increasing amounts of alpha-lactalbumin. By contrast, galactosyltransferase and lactose synthetase activities were maximal on day 3, and then declined; the time course of lactose secretion was similar to that of galactosyltransferase and lactose synthetase activities. Cortisol concentration studies revealed the following dependencies on the steroid: alpha-lactalbumin content was maximal between 0.01 and 0.1 microgram and, at 10 microgram/ml fell below the baseline level observed without cortisol. On the other hand, galactosyltransferase, lactose synthetase and lactose secretion were only slightly increased at 0.01 microgram/ml, were maximal between 0.1 and 1.0 microgram/ml, and at 10 microgram/ml were still considerably above the baseline levels observed without cortisol. These results indicate that lactose formation and secretion by mammary explants from virgin mice correspond more closely to the hormone-induced activity of galactosyltransferase than to that of alpha-lactalbumin, and that asynchrony can arise between the induction of alpha-lactalbumin and the induction of lactose.

Loss of differentiative potential of the mammary gland in ovariectomized mice: prevention and reversibility of the defect

The differentiative functions, lactose synthetase activity and casein synthesis, are not expressed in vitro in mammary gland explants from female virgin mice castrated for 1-2 months. 17 beta-Estradiol (E2) treatment (3 micrograms, twice weekly) initiated immediately after ovariectomy completely preserves this ability for as long as 2 months; however, elevating serum PRL levels by either perphenazine treatment or pituitary transplantation under the renal capsule is ineffective. Therefore, the loss of differentiative potential in the mammary gland appears to be a result of E3 deprivation and not depressed serum PRL concentrations. E2 therapy for 5 weeks also partially restores this potential in animals whose mammary glands have previously lost it. Complete reversal of this defect occurs when the tissue from mice castrated for 4 weeks is transplanted into the cleared fat pads of intact virgins. These data suggest that E2 has an important role in differentiation of the mouse gland in addition to its known role in mammary epithelial growth.

Stimulation of lactose synthetase activity and casein synthesis in mouse mammary explants by estradiol

17 beta-Estradiol (E2; 1 ng/ml) can significantly (P less than 0.05) augment lactose synthetase activity (85%) and casein synthesis (65%) in mammary gland explants (from midpregnant C3H/HeN mice) cultured in medium 199 containing insulin, cortisol, PRL, and T3. Both T3 (0.65 ng/ml) and a physiological concentration of PRL (50 ng/ml) are required for stimulation by E2; at higher PRL levels, the effects of E2 are obscured. Sex steroid specificity was supported by the observations that the E2 effects are blocked by the antiestrogen nafoxidine (5 micrograms/ml), and that 17 alpha-estradiol (5 ng/ml) was inactive; however, at 5 ng/ml, estrone and diethylstilbestrol were equipotent to E2. Testosterone (5 micrograms/ml) and progesterone (1 microgram/ml) were inhibitory, although progesterone did not suppress casein synthesis. Since E2 did not alter the amount of epithelial DNA, its effects represent an increase in biological activities per cell. E2, therefore, in conjunction with the classical lactogenic hormones and T3 can act directly on mammary tissue by promoting differentiation.

Evidence for a hypothalamic defect in septo-optic dysplasia

A 21-year-old man demonstrated septo-optic dysplasia. Optic and retinal colobomas were present and panhypopituitarism was documented. Releasing hormone studies showed partial luteinizing hormone (LH) response and no follicle-stimulating hormone response to administration of gonadorelin (LH-releasing hormone); thyroid-stimulating hormone (TSH) and prolactin levels were increased normally after administration of protirelin (thyrotropin-releasing hormone). The LH, TSH, and prolactin responses are believed to be evidence of intact pituitary function and suggest that a hypothalamic defect accounts for the hypopituitarism.

The purification and characterization of rabbit placental lactogen

Rabbit placental lactogen, a polypeptide hormone functionally related to the growth hormone/prolactin family, was isolated from placenta by (NH4)2SO4 precipitation, gel filtration and ion-exchange chromatography on DEAE-and CM-cellulose. The hormone was purified to more than 90% homogeneity, as determined by end-group analysis. On disc gel electrophoresis at pH9.0 it migrates as a pair of closely spaced bands with mobilities of 0.489 (minor band) and 0.511 (major band), and its isoelectric point is 6.1. Its mol.wt. is 20600, as determined by sedimentation--equilibrium centrifugation, and 24200, as estimated by gel electrophoresis in sodium dodecyl sulphate. Its amino acid composition resembles that of rabbit growth hormone and rat prolactin, except for a lower glutamic acid and leucine content. Like the prolactins, rabbit placental lactogen has two tryptophan and six cysteine residues, and its N-terminus, valine, is identical with that for human placental lactogen. By radioimmunoassay, it does not cross-react with antisera to either rat growth hormone or rat prolactin; in addition, it does not cross-react with antisera to bovine placental lactogen by double immunodiffusion. The similarity of the biochemical characteristics of rabbit placental lactogen to the other non-primate placental lactogens lends further support to the hypothesis that these molecules occupy a more central position in the growth hormone/prolactin "tree" than do their primate counterparts.

Circulating placental lactogen levels in dairy and beef cattle

Levels of bovine placental lactogen (bPL) have been measured in the serum of dairy and beef cattle and in the milk and amniotic fluid of pregnant animals with a highly specific radioimmunoassay. In both dairy and beef cows, serum bPL levels remain low (less than 50 ng/ml) during the first two trimesters and then rise rapidly between 160 and 200 days of gestation to a plateau. The bPL levels do not decline prior to parturition. During the last trimester, serum levels in dairy cows, 1103+/-342 ng/ml, are significantly higher than those in beef cattle, 650+/-37 ng/ml (P less than 0.01); furthermore, dairy cows having a high milk production also tend to have high bPL levels. Serum levels are almost twice as high in twin pregnancies and are not correlated with fetal sex or birth weight. bPL levels in milk and amniotic fluid from dairy cattle during the last trimester are approximately 86% and 25% of the serum values, respectively, suggesting that bPL enters these fluids by passive diffusion.

Localization and specificity of binding of subprimate placental lactogen in rabbit tissues

A search for specific placental lactogen binding was undertaken in tissues obtained from late pregnant rabbits using the placental lactogens from sheep, cows, and human beings. 125I-labeled ovine lactogen exhibited highest specific binding to the adrenal gland (57.8%), followed by liver (21.5%), ovary (19.9%), mammary gland (15.9%), uterus (12.2%), kidney (8.8%), brain (8.5%), and adipose tissue (7.9%). In liver and mammary gland, the displacement curves for ovine and human lactogen were identical to that for bovine prolactin, indicating that they share the same receptor site. Although the displacement curve for bovine lactogen was parallel to that of the other lactogens the bovine hormone is less active in the radioreceptor assay.

Purification and characterization of bovine placental lactogen

Bovine placental lactogen (bPL), a polypeptide hormone functionally related to bovine growth hormone (bGH) and bovine prolactin (bPL), has been isolated from placentas by pH and ammonium sulfate precipitation, gel filtration, and ion exchange chromatography on DEAE- and CM-cellulose. The hormone has been purified to approximately 99% homogeneity, as determined by end group analysis. On disc gel electrophoresis at pH 9.0 bPL migrates as a pair of closely spaced bands (Rf = 09517 and 0.541) between the positions of bGH and bPR. Its molecular weight, as estimated by gel filtration on Sephadex G-200 in 6 M guanidine hydrochloride and 6.5 mM dithiothreitol, is 22, 150 and its isoelectric point is 5.9. The amino acid composition of bPL closely resembles that of bGH and bPR except for a higher content of serine and glycine and a lower leucine content. Like bPR, it has 2 tryptophans and 6 cysteines, but its COOH-terminal sequence is identical with that of bGH: -Cys-Ala-Phe-OH. By Ouchterlony immunodiffusion, bPL forms lines of partial identity with bGH against bGH antisera and with ovine placental lactogen (oPL) against oPL antisera. In the bPL-antibPL system, oPL forms a line of partial identity while bGH and bPR do not cross-react. However, bPL does not form a precipitin line with bPR antisera. These data would indicate that in terms of structure, and hence molecular evolution, bPL and other subprimate placental lactogens occupy a position more intermediate between growth hormone and prolactin than do the primate placental lactogens.

Growth hormone covalently bound to sepharose or glass. Analysis of ligand release rates and characterization of soluble radiolabeled products

Purified boveine growth hormone labeled enzymatically with iodine-125 was covalently coupled to cyanogen bromide activated Sepharose 4B gel and to diazotized zirconia-clad glass beads. Under the conditions employed, an average of 0.8 and 7.3 mg of hormone were bound per ml of Sepharose and glass, respectively. When the conjugates were incubated in Krebs-Ringer bicarbonate buffer (pH 7.4), three separate radioactive species were detected in the incubation supernatant by chromatography on Sephadex G-75. The elution volumes of two of the species were identical with those of 125-I-labeled growth hormone and Na-125I controls, while the third component eluted as a moleucle of intermediate size. The rate of release of each species from the solid matrix was linear with time over 4 days and increased with temperature from 4 to 37 degrees. Although significantly less growth hormone was released from glass (0.14%/day) than from Sepharose (0.40%/day) at 37 degrees, active hormone in amounts sufficient to be detectable in a biological assay was nevertheless liberated from the former after as little as 4 hr of incubation. By contrast, the rate of release of 125-Iminus- and the intermediate-size compound from glass was significantly greater than from Sepharose, suggesting that protein bound to glass supports is more susceptible to degradation from exposure to ionizing radiation.