Development of insulin-sensitivity by mouse mammary gland in vitro

Regulation of amino acid transporters in the mammary gland from late pregnancy to peak lactation in the sow

Background

Milk protein is crucial for milk quality in sows and health of newborn piglets. Plasma amino acids (AA) in sows are important precursors for milk protein synthesis in the mammary gland. In order to study the regulation of AA transported in sow mammary glands and possible underlying mechanisms, we measured the expression of genes coding for milk proteins, AA transporter expressions, and plasma AA concentrations in sows at three different physiological stages (D-17, D1 and D17 of lactation), and then further investigated the regulation of AA transport across the cell membrane by adaptive mechanisms using pig mammary epithelial cells (PMEC) as an in vitro model. PMEC were cultured in DMEM:F12 with 4 amino acid concentrations (0 × AA complex, 1 × AA complex, 5 × AA complex, and 25 × AA complex). Classes of AA complexes evaluated in this study included neutral AAs (L-Ala + L-Ser + L-Cys), acidic AAs (L-Asp, L-Glu) and neutral + basic AAs (L-Ala + L-Ser + L-Cys + L-Lys).

Results

Our results indicated that mRNA expression of genes coding for milk protein (αs1-casein, αs2-casein, β-casein and κ-casein) increased significantly with the advance of physiological stage (P < 0.05), and plasma concentrations of most AAs including threonine, serine, glutamate, alanine, valine, cysteine, methionine, isoleucine and tyrosine were greater at D1 of lactation compared with D-17 and D17 of lactation (P < 0.05). Additionally, protein and gene expressions of AA transporters including excitatory AA transporter 3 (EAAT3), alanine/serine/cysteine/threonine transporter (ASCT1) and sodium-coupled neutral AA transporter 1 (SNAT2) were greater in lactating sow mammary glands compared with sow mammary glands in late pregnancy (P < 0.05). The mRNA expressions of SLC38A2, SLC1A1, SLC6A14 increased significantly in the cell mediums supplemented with 5 × and 25 × of AA complexes compared with those cells cultured in DMEM/F12 cell medium (P < 0.05). The mRNA expressions of SLC38A, SLC1A4, and SLC6A14 also increased in EBSS cell medium compared to DMEM/F12. However, only mRNA expression of SLC38A decreased when AA complex was added into EBSS (P < 0.05).

Conclusion

AA transportation was positively regulated in sow mammary glands with the advance of physiological stage from late pregnancy to peak of lactation and AA transporters in PMECs were adaptively regulated by changed AA concentrations.

The mechanisms by which nitric oxide affects mammary epithelial growth and differentiation

Nitric oxide (NO) enhances prolactin-stimulated DNA synthesis and inhibits prolactin-induced differentiation in mouse mammary epithelium. The molecular pathways used by NO were determined by employing specific inhibitors of the transducers utilized by NO. Inhibitors of the Jun N-terminal kinase (JNK) blocked the effect of NO on DNA synthesis, although this appeared to involve a protein kinase G (PKG)-independent pathway. In contrast, inhibitors of the extracellular signal-regulated kinase (ERK) prevented NO from suppressing alpha-lactalbumin accumulation and this effect was PKG-dependent. NO can also elevate cAMP through the inhibition of phosphodiesterase 3 and cAMP mimicks the actions of NO on both DNA synthesis and differentiation. However, suppression of cAMP levels did not prevent the effects of NO. Therefore, NO uses two separate pathways to affect mammary epithelium: it stimulates growth via JNK and inhibits differentiation through ERK.

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.

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.

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.

Growth control and differentiation in mammary epithelial cells

Growth and differentiation of the mammary gland are controlled by various hormones and other environmental factors. The role of hormones and growth factors in mammary development is discussed with regard to animal species, physiological stages, and the various experimental systems in vitro. In the female embryo, mammary morphogenesis is induced by the mesenchyme and is hormone independent, whereas androgens cause the partial necrosis of mammary epithelium in the male. Ductal growth during adolescence requires estrogen and prolactin or growth hormone. During pregnancy, progesterone participates in the development of the lobuloalveolar structure of the gland. After parturition, changes in the hormonal environment lead to production and secretion of milk. Proliferation and differentiation of mammary epithelium can be induced in culture systems. Insulin and epidermal growth factor (EGF) stimulate mammary cell proliferation in vitro. EGF is required for the optimal growth of the mammary gland during pregnancy. EGF also appears to play an important role in mammary tumorigenesis in certain mouse strains. Production of milk proteins can be induced in vitro by the synergistic interactions of prolactin, insulin, and glucocorticoids and is inhibited by EGF and progesterone. Complete or partial sequencing of several milk protein genes and comparative analysis have led to identification of a sequence of high homology and conservation in the 5' flanking region that is likely to be involved in the regulation of milk protein gene expression.

Functional differentiation of virgin mouse mammary epithelium in explant culture is dependent upon extracellular proline

Depletion of proline from insulin, hydrocortisone, and prolactin-containing medium prior to incubating virgin mouse mammary explants prevents both DNA synthesis and functional differentiation in the mammary epithelial cells; however, DNA synthesis in the mammary stroma and total incorporation of radioactive amino acids into total protein appears to continue without hindrance. Removal of glycine instead of proline had no deleterious effect on either DNA replication in the hormone-stimulated epithelium or in its functional differentiation. Functional differentiation was determined by the induction of casein and alpha-lactalbumin synthesis in the insulin, hydrocortisone, and prolactin (IFPrl)-treated explant cultures. As a control, the induction of mouse mammary tumor virus (MMTV) gene expression, a corticosteroid-regulated function, was also measured. Neither the absence of proline or glycine prevented the glucocorticoid stimulation of MMTV gene expression. In contrast to mammary tissue from virgin mice, explants from nonpregnant primiparous mice responded fully to IFPrl stimulation with respect to DNA, casein, and alpha-lactalbumin synthesis in medium depleted of proline. These data suggest that the uncommitted epithelium of virgin mouse mammary glands requires the presence of exogenous proline in order to respond to lactogenic hormonal signals. We have demonstrated earlier that DNA synthesis is a prerequisite of functional differentiation in virgin mouse mammary explants (Smith and Vonderhaar, 1981, Dev. Biol., 88:167-179; Vonderhaar and Smith, 1982, J. Cell Sci, 53:97-114), although cytological differentiation proceeded unencumbered in explants prevented from synthesizing DNA. Here, without proline, neither cytological nor functional differentiation can be induced; this suggests that proline provides an essential metabolic interlock in the acquisition of lactogenic hormone responsiveness in uncommitted mouse mammary tissue.

Amino acid transport systems in bovine mammary tissue

Nutrient provision to the lactating mammary gland involves three factors: blood nutrient concentration, blood flow, and cellular uptake. This paper reviews uptake of amino acids by bovine mammary tissue relative to interorgan blood flows, red blood cell contribution, arteriovenous differences, specific mammary amino acid transport systems, and glutathione and gamma-glutamyl transpeptidase. Recent studies with ruminant amino acid blood fluxes and the role of the red blood cell in providing nitrogenous substrates to tissues have brought to light new considerations of nutrient availability to mammary tissue. Previous studies measured arteriovenous differences to quantitate net amino acid uptake. These studies are considered relative to seven specific and separate amino acid transport systems, some of which have been identified in bovine mammary tissue. Uptake of sulfur amino acids by mammary tissue has been of interest because it appeared that insufficient quantities were provided during lactation. Glutathione, a tripeptide, may be a principle source of cysteine to mammary tissue via mechanisms involving gamma-glutamyl transpeptidase, glutathione, and red blood cells. The paper considers these mechanisms in relationship to amino acid transport systems.

Cyclic AMP as a negative regulator of hormonally induced lactogenesis in mouse mammary gland organ culture

In organ cultures of mammary glands from mice in midpregnancy, addition of both insulin and prolactin induces a marked accumulation of alpha-lactalbumin, whereas the augmentation of casein synthesis requires the presence of insulin, prolactin, and cortisol. Addition of 0.5 mM dibutyryl cyclic AMP resulted in complete inhibition of alpha-lactalbumin accumulation and partial inhibition of casein synthesis. Furthermore, either cholera toxin at 0.1-1.0 microgram/ml (a stimulator of adenylate cyclase) or 3-isobutyl-1-methylxanthine (an inhibitor of phosphodiesterase) in combination with 2 mM cyclic AMP, produced a similar pattern of inhibition of alpha-lactalbumin and casein synthesis in cultured tissue. During culture of mammary explants in medium containing no hormone, or insulin alone, or insulin, prolactin, and cortisol, the tissue content of cyclic AMP decreased rapidly, reaching half the initial level in 24-48 hr. These results indicate that cyclic AMP plays "negative" regulatory function in hormonal induction of milk protein synthesis during the development of the mammary gland.

Lactose synthetase activity in mouse mammary glands is controlled by thyroid hormones

Epithelial cells in explants from the mammary glands of euthyroid mature virgin mice are proliferatively dormant. They must undergo DNA synthesis and traverse the cell cycle in vitro before they are able to differentiate fully in response to insulin, hydrocortisone, and prolactin, and synthesize enzymatically active alpha-lactalbumin (measured as lactose synthetase activity). In contrast, glands from hyperthyroid mature virgin mice do not require DNA synthesis in vitro to differentiate. Explants from the euthyroid virgin tissue overcome their dependence on DNA synthesis when 10(-9) M 3,5,3'-triiodo-L-thyronine is added directly to the cultures in addition to the other three hormones. Explants from involuted mammary glands from euthyroid primiparous mice do not require DNA synthesis in vitro to make the milk protein even though they, like explants from mature euthyroid virgin tissue, are proliferatively dormant and do not contain detectable lactose synthetase activity in vivo. Glands from primiparous animals made mildly hypothyroid by ingestion of 0.1% thiouracil in drinking water during 7 wk of involution remain morphologically indistinguishable from glands of their euthyroid counterparts. However, explants from the glands of these hypothyroid animals revert to a state of dependence on DNA synthesis to differentiate functionally. These observations suggest that the dependence on DNA synthesis and cell cycle traversal for hormonal induction of lactose synthetase activity in the mouse mammary gland is controlled by thyroid hormones.

Induction by lithium ion of multiplication of mouse mammary epithelium in culture

Lithium ion at concentrations between 2 and 20 mM simulated the stimulatory effects of insulin on the uptake of alpha-aminoisobutyric acid, synthesis of RNA and DNA, and cell multiplication in mouse mammary gland explants cultured in a chemically defined synthetic medium. Other monovalent cations were virtually ineffective. In most instances the stimulatory effect of lithium ion was somewhat smaller than and additive to that of insulin. However, lithium ion was incapable of substituting for the action of insulin in augmenting milk protein synthesis in mammary explants cultured with other lactogenic hormones, prolactin, and glucocorticoid. The observed similarities of the responses of mammary cells to lithium and insulin suggest that possible importance of cation(s) in the regulation of mammary cell proliferation, which may be a common basis for the action of the two agents. On the other hand, the observed inability of lithium to mimic the lactogenic effect of insulin indicates a specific function of the hormone in the functional differentiation of mammary cells.

Control of amino acid transport in the mammary gland of the pregnant mouse

The regulation of the uptake of the amino acid analog alpha-aminoisobutyric acid was studied in diced mammary glands from pregnant mice. Stimulation of uptake by insulin was not prevented by inhibitors of protein synthesis; protein synthesis inhibitors decreased uptake by 20%; this response occurred more promptly in insulin-treated tissues. Elimination of extracellular amino acids led to a substantial increase in transport which was not abolished by inhibitors of protein synthesis. These results indicate that insulin does not increase amino acid transport in this system by altering synthesis and degradation of transport protein. They are consistent with a model in which the activity of the existing amino acid transport protein in subject to negative feedback regulation from the intracellular amino acid pool.

Insulin regulation of amino acid transport in mesenchymal cells from avian and mammalian tissues

Insulin regulation of amino acid transport across the cell membrane was studied in a variety of mesenchymal cell directly isolated from avian and mammalian tissues or collected from confluent cultures. Transport activity of the principal systems of mediation in the presence and absence of insulin was evaluated by measuring the uptake of representative amino acids under conditions approaching initial entry rates. Insulin enhanced the transport rate of substrate amino acids from the A system(alpha-aminoisobutyric acid, L-proline, glycine, L-alanine and L-serine) in fibroblasts and osteoblasts from chick-embryo tissues, in mesenchymal cells (fibroblasts and smooth muscle cells) from immature rat uterus, in thymic lymphocytes from young rats and in chick-embryo fibroblasts from confluent secondary cultures. In these tissues, the uptake of amino acid substrates of transport systems L and Ly+ (L-leucine, L-phenylalanine, L-lysine) was not affected by the presence of the hormone. No insulin control of amino acid transport was detected in chick-embryo chondroblasts and rat peritoneal macrophages. These observations identify the occurrence of hormonal regulatory patterns of amino acid transport for different mesenchymal cells types and indicate that these properties emerge early during cell differentiation.

Glucose uptake by normal human breast tissue in organ culture. Influence of insulin and other additives

Normal human breast tissue explants were cultured in a synthetic basic medium with and without additives. The mean daily glucose uptake per explant was measured under six basic conditions. Our results show that glucose uptake is strongly related to the glucose concentration of the medium. On the other hand insulin does not affect significantly glucose uptake in vitro, but does enhance mitotic activity. These findings support a role for insulin in promoting D.N.A. synthesis rather than in controlling glucose metabolism of human mammary tissue in vitro.

Effects of hormones on protein and amino acid metabolism in mammary-gland explants of mice

The effects of insulin, cortisol and prolactin on amino acid uptake and protein biosynthesis were determined in mammary-gland explants from mid-pregnant mice. Insulin stimulated [3H]leucine incorporation into protein within 15 min of adding insulin to the incubation medium. Insulin also had a rapid stimulatory effect on the rate of aminoiso[14C]butyric acid uptake, but it had no effect on the intracellular accumulation of [3H]leucine. Cortisol inhibited the rate of [3H]leucine incorporation into protein during the initial 4h of incubation, but it had no effect at subsequent times. [3H]Leucine uptake was unaffected by cortisol, but amino[14C]isobutyric acid uptake was inhibited after a 4h exposure period to this hormone. Prolactin stimulated the rate of [3H]leucine incorporation into protein when tissues were exposed to this hormone for 4h or more; up to 4h, however, no effect of prolactin was detected. At all times tested, prolactin had no effect on the uptake of either amino[14C]isobutyric acid or [3H]leucine. Incubation with actinomycin D abolished the prolactin stimulation of protein biosynthesis, but this antibiotic did not affect the insulin response. A distinct difference in the mechanism of action of these hormones on protein biosynthesis in the mammary gland is thus apparent.

Hormone induction of specific protein synthesis in midpregnant mouse mammary cell culture

Monolayer primary cell cultures of midpregnant mouse mammary cells were subjected to hormone stimulation under strictly defined conditions. Hormonal response was measured in terms of increase in rate of synthesis of mouse casein, using a double antibody precipitation technique. Cells stimulated by insulin plus prolactin plus cortisol plus estradiol plus progesterone showed a marked increase in rate of mouse casein synthesis over the controls. This specific product synthesis, which remains inducible in these cells for at least ten days, was detected either by labelled phosphoric acid or labelled amino acid incorporation. The mouse casein synthesized was identical, as judged by the identification techniques used, to that of mouse milk. Mouse midpregnant mammary explants in organ culture require insulin plus prolactin plus cortisol to express their full lactogenic capabilities. Nevertheless, when the same cells are dispersed and grown in monolayers they require ovarian steroids to elicit a lactogenic response as shown in this study. Ovarian steroids, therefore, are necessary in lactogenesis, although the fundamental nature of their action remains to be established.

Changes in the ribosome content, principal microsomal protein composition, and secretory character of mammary epithelial rough endoplasmic reticulum during differentiation. Evidence that messenger RNAs specific for milk proteins are incorporated into rough endoplasmic reticulum formed de novo after parturition

The equilibrium density distribution, protein composition, and secretory character of mouse mammary epithelial rough microsomes have been determined during differentiation. The density range exhibited by the rough microsomes broadens during mammary development; rough microsomes within the 1.25-1.29 g/ml density range appear soon after conception and then within the 1.30-1.34 range after the onset of lactation. The appearance of these denser microsomes represents the progressive increase of the average ribosome content of the rough endoplasmic reticulum (ER) during gestation and lactation. Fractionation of rough microsomal proteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals that two proteins, having molecular weights of 57,000 and 76,000, occur to a significant extent only during lactation and are then most prominent in the very dense rough microsomes of the 1.30-1.34 range. Nascent polypeptide chains discharged (by incubation with puromycin) from 17-days lactation rough microsomes in either the 1.21-1.29 or 1.30-1.34 density range are distributed equally between the intra- and extravesicular compartments. Whereas 36% of the chains are discharged intravesicularly from 1-day lactation rough microsomes in the 1.30-1.34 range, only 25% are so discharged from those in the 1.21-1.29 range. The results indicate (a) that there is no correlation between the relative levels in lactation rough microsomes of the two microsomal proteins which become prominent during lactation and the extent of secretory activity and (b) that for a short period after parturition the rough ER elements bearing high surface densities of ribosomes have a greater proportion of ribosomes synthesizing milk proteins than the rough ER elements with moderate ribosome densities.

A soluble super-active form of insulin

Insulin-like material can be extracted from insulin-Sepharose. This soluble material elicits super-insulin responses from mammary epithelial cells. It stimulates the accumulation of alpha-aminoisobutyric acid by virgin mouse mammary cells, which are unresponsive to insulin itself. It exerts a greater stimulatory effect on mammary cells from pregnant mice than insulin. It also is more efficacious than insulin as a stimulant of DNA synthesis and the combined activities of glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and phosphogluconate dehydrogenase (EC 1.1.1.43). Its specific biological activity on these cells is more than five times greater than that of insulin.

Is prolactin mitogenic for mammary epithelium?

Prolactin administration to mature virgin mice leads to prolilferation of epithelium of the mammary gland. In organ culture, prolactin is not mitogenic, but insulin and insulin-free serum are. These observations are reconciled by the finding that insensitive epithelium is sensitized to insulin and/or serum by prolactin treatment in vivo. Thus, it appears that prolactin acts indirectly as a mitogen by rendering the cells susceptible to the aforementioned mitogens.

Insulin-sepharose and the dynamics of insulin action

In terms of the rate of accumulation of alpha-aminoisobutyric acid, mammary epithelial cells from mature virgin mice respond rapidly to insulin covalently bound to sepharose particles, but do not respond initially to soluble insulin in vitro. The response to insulin-sepharose is prevented if the cells are first exposed to insulin. These observations may provide new insight into the dynamics of insulin action.

DNA-induced transformation in Drosophila: evidence for transmission without integration

A key feature of the exosome model of transformation in Drosophila melanogaster is that the DNA segments responsible for the phenomenon are not integrated into the linear structure of the chromosomes. This feature is confirmed by the failure to find whole-body transformants in a series of large-scale experiments, and by the absence of evidence for integration under conditions favoring genetic exchange between chromosomes and introduced DNA.