Prolactin and insulin synergize to regulate the translation modulator PHAS-I via mitogen-activated protein kinase-independent but wortmannin- and rapamycin-sensitive pathway

Calorie restriction and rapamycin administration induce stem cell self-renewal and consequent development and production in the mammary gland

Expansion of the mammary epithelial stem cell pool holds promise for consequent mammary gland development and production. Complementary analyses of bovine mammary implants maintained in de-epithelialized mouse mammary fat pad and endogenous mouse mammary gland were performed to elucidate the effect of calorie restriction (CR) on stem cell self-renewal. CR elevated propagation rate and non-adherent mammosphere generation in cultured bovine mammary cells. A corresponding decrease in progenitor-induced colony formation and differentiation marker expression was noted. In the mouse gland, CR enhanced the take rate of transplanted cells and outgrowths' fat pad occupancy. Downregulating mTOR activity by rapamycin administration reproduced CR's effects on stem cell self-renewal within a shorter period. Flow cytometry demonstrated a significant 1.5-fold increase in stem cell number and a corresponding decrease in luminal progenitor and differentiated cells. Consequent effects of rapamycin administration included enhanced ductlet generation in bovine implants and higher milk-protein gene expression in cultured mouse mammary cells. The stimulatory effect of CR on BST-1 expression in both bovine implants and mouse glands resembled that noted in the intestinal Paneth stem cell niche (Yilmaz et al., 2012). A putative niche may also exist in the mammary gland, conveying energy-status information to the insulated stem cells.

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.

Nutrient availability and lactogenic hormones regulate mammary protein synthesis through the mammalian target of rapamycin signaling pathway

The nutritional and endocrine factors affecting protein translation in the bovine mammary gland, and the molecular mechanisms mediating their effects, are not well understood. The objective of this study was to assess the role of the mammalian target of rapamycin (mTOR) signaling pathway in the regulation of mammary protein synthesis by nutrients and hormones. Mammary epithelial acini isolated from lactating dairy cows were treated in medium containing, alone or in combination, a mixture of AA or glucose and acetate (GA) as energy substrates, or a combination of the lactogenic hormones hydrocortisone, insulin, and prolactin (HIP). Changes in the rate of mammary protein synthesis and the phosphorylation state of components of the mTOR signaling pathway were measured. Mammary protein synthesis was 50% higher with increased availability of AA in medium. The presence of GA or treatment of mammary acini with HIP alone did not affect mammary protein synthesis. The stimulation of mammary protein synthesis by AA was enhanced by HIP treatment, but not by the presence of GA in medium. Treatment of mammary acini with HIP induced the phosphorylation of protein kinase B. This effect was augmented in the presence of either AA or GA in medium. The stimulation of mammary protein synthesis by AA and its enhancement by HIP were associated with increased phosphorylation of the mTOR substrates, p70 ribosomal protein S6 kinase-1, and eukaryotic initiation factor 4E (eIF4E)-binding protein-1 (4E-BP1), and dissociation of 4E-BP1 from eIF4E. The results suggest that nutrients and hormones may modulate mammary protein synthesis through the mTOR signaling pathway.

Insulin regulates milk protein synthesis at multiple levels in the bovine mammary gland

The role of insulin in milk protein synthesis is unresolved in the bovine mammary gland. This study examined the potential role of insulin in the presence of two lactogenic hormones, hydrocortisone and prolactin, in milk protein synthesis. Insulin was shown to stimulate milk protein gene expression, casein synthesis and (14)C-lysine uptake in mammary explants from late pregnant cows. A global assessment of changes in gene expression in mammary explants in response to insulin was undertaken using Affymetrix microarray. The resulting data provided insight into the molecular mechanisms stimulated by insulin and showed that the hormone stimulated the expression of 28 genes directly involved in protein synthesis. These genes included the milk protein transcription factor, ELF5, translation factors, the folate metabolism genes, FOLR1 and MTHFR, as well as several genes encoding enzymes involved in catabolism of essential amino acids and biosynthesis of non-essential amino acids. These data show that insulin is not only essential for milk protein gene expression, but stimulates milk protein synthesis at multiple levels within bovine mammary epithelial cells.

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.

Role of amino acids in translational mechanisms governing milk protein synthesis in murine and ruminant mammary epithelial cells

The role of amino acids (AA) on translational regulation in mammary epithelial cells cultured under lactogenic conditions was studied. The rates of total protein synthesis and beta-lactoglobulin (BLG) synthesis in mouse CID-9 cells were 2.1- or 3.1-fold higher, respectively, than in their bovine L-1 counterparts. Total AA deprivation or selective deprivation of Leu had a negative protein-specific effect on BLG synthesis that was more pronounced in bovine cells than in murine cells. Dephosphorylation of eukaryotic initiation factor 4E-binding protein 1 (4E-BP1) and S6 kinase (S6K1) on Thr(389) but not on Ser(411) was also more prominent in bovine cells. Noteably, deprivation of Leu had a less marked effect on BLG synthesis and 4E-BP1 or S6K1 phosphorylation than deprivation of all AA. In AA-deprived CID-9 cells, Leu specifically restored BLG synthesis from pre-existing mRNA whereas AA also restored total protein synthesis. This restoration was associated with a more pronounced effect on 4E-BP1 and S6K1 phosphorylation in bovine versus murine cells. Rapamycin specifically reduced Leu- and AA-stimulated BLG translation initiation in a dose-dependent manner. A further reduction was observed for Leu-treated cells in the presence of LY294002, a PI3K (phosphatidylinositol 3-kinase) inhibitor, which also reduced total protein synthesis. These findings suggest that direct signaling from AA to the translational machinery is involved in determining the rates of milk protein synthesis in mammary epithelial cells.

Inhibition of protein synthesis by Y box-binding protein 1 blocks oncogenic cell transformation

The multifunctional Y box-binding protein 1 (YB-1) is transcriptionally repressed by the oncogenic phosphoinositide 3-kinase (PI3K) pathway (with P3K as an oncogenic homolog of the catalytic subunit) and, when reexpressed with the retroviral vector RCAS, interferes with P3K- and Akt-induced transformation of chicken embryo fibroblasts. Retrovirally expressed YB-1 binds to the cap of mRNAs and inhibits cap-dependent and cap-independent translation. To determine the requirements for the inhibitory role of YB-1 in P3K-induced transformation, we conducted a mutational analysis, measuring YB-1-induced interference with transformation, subcellular localization, cap binding, mRNA binding, homodimerization, and inhibition of translation. The results show that (i) interference with transformation requires RNA binding and a C-terminal domain that is distinct from the cytoplasmic retention domain, (ii) interference with transformation is tightly correlated with inhibition of translation, and (iii) masking of mRNAs by YB-1 is not sufficient to block transformation or to inhibit translation. We identified a noncanonical nuclear localization signal (NLS) in the C-terminal half of YB-1. A mutant lacking the NLS retains its ability to interfere with transformation, indicating that a nuclear function is not required. These results suggest that YB-1 interferes with P3K-induced transformation by a specific inhibition of translation through its RNA-binding domain and a region in the C-terminal domain. Potential functions of the C-terminal region are discussed.

Y box-binding protein 1 induces resistance to oncogenic transformation by the phosphatidylinositol 3-kinase pathway

Y box-binding protein 1 (YB-1) is a multifunctional protein that can act as a regulator of transcription and of translation. In chicken embryo fibroblasts transformed by the oncoproteins P3k (phosphatidylinositol 3-kinase) or Akt, YB-1 is transcriptionally down-regulated. Expression of YB-1 from a retroviral vector induces a strong cellular resistance to transformation by P3k or Akt but does not affect sensitivity to transformation by other oncoproteins, such as Src, Jun, or Qin. The YB-1-expressing cells assume a tightly adherent, flat phenotype, with YB-1 localized in the cytoplasm, and show a greatly reduced saturation density. Both cap-dependent and cap-independent translation is inhibited in these cells, but the activity of Akt remains unaffected, suggesting that YB-1 functions downstream of Akt. A YB-1 protein with a loss-of-function mutation in the RNA-binding motif no longer binds to the mRNA cap structure, is localized in the cell nucleus, does not induce the flat cellular phenotype, and fails to interfere with P3k- or Akt-induced oncogenic transformation. This mutant also does not inhibit cap-dependent or cap-independent translation. These results suggest that YB-1 acts like a rapamycin mimic, inhibiting translational events that are required in phosphatidylinositol 3-kinase-driven oncogenic transformation.

Low-energy laser irradiation enhances de novo protein synthesis via its effects on translation-regulatory proteins in skeletal muscle myoblasts

Low-energy laser irradiation (LELI) drives quiescent skeletal muscle satellite cells into the cell cycle and enhances their proliferation, thereby promoting skeletal muscle regeneration. Ongoing protein synthesis is a prerequisite for these processes. Here, we studied the signaling pathways involved in the LELI regulation of protein synthesis. High levels of labeled [35S]methionine incorporation were detected in LELI cells as early as 20 min after irradiation, suggesting translation of pre-existing mRNAs. Induced levels of protein synthesis were detected up until 8 h after LELI implying a role for LELI in de novo protein synthesis. Elevated levels of cyclin D1, associated with augmented phosphorylation of the eukaryotic initiation factor 4E (eIF4E) and its inhibitory binding protein PHAS-I, suggested the involvement of LELI in the initiation steps of protein translation. In the presence of the MEK inhibitor, PD98059, eIF4E phosphorylation was abolished and levels of cyclin D1 were dramatically reduced. The LELI-induced PHAS-I phosphorylation was abolished after preincubation with the PI3K inhibitor, Wortmannin. Concomitantly, LELI enhanced Akt phosphorylation, which was attenuated in the presence of Wortmannin. Taken together, these results suggest that LELI induces protein translation via the PI3K/Akt and Ras/Raf/ERK pathways.

Characterization of an epithelial cell line from bovine mammary gland

Elucidation of the bovine mammary gland's unique characteristics depends on obtaining an authentic cell line that will reproduce its function in vitro. Representative clones from bovine mammary cell populations, differing in their attachment capabilities, were cultured. L-1 cells showed strong attachment to the plate, whereas H-7 cells detached easily. Cultures established from these clones were nontumorigenic upon transplantation to an immunodeficient host; they exhibited the epithelial cell characteristics of positive cytokeratin but not smooth muscle actin staining. Both cell lines depended on fetal calf serum for proliferation. They exhibited distinct levels of differentiation on Matrigel in serum-free, insulin-supplemented medium on the basis of their organization and beta-lactoglobulin (BLG) secretion. H-7 cells organized into mammospheres, whereas L-1 cells arrested in a duct-like morphology. In both cell lines, prolactin activated phosphorylation of the signal transducer and activator of transcription, Stat5-a regulator of milk protein gene transcription, and of PHAS-I-an inhibitor of translation initiation in its nonphosphorylated form. De novo synthesis and secretion of BLG were detected in differentiated cultures: in L-1 cells, BLG was dependent on lactogenic hormones for maximal induction but was less stringently controlled than was beta-casein in the mouse CID-9 cell line. L-1 cells also encompassed a near-diploid chromosomal karyotype and may serve as a tool for studying functional characteristics of the bovine mammary gland.

A new beta-lactoglobulin-based vector targets luciferase cDNA expression to the mammary gland of transgenic mice

A beta-lactoglobulin (BLG)/luciferase gene vector (p907), composed of a luciferase intronless gene inserted between the second and sixth BLG exons was constructed. Stable transfections of CID-9 cells with this vector, as well as with a series of additional vectors, were performed to define regulatory regions within the BLG sequence, and the contribution of the SV40 polyadenylation (PA) site to luciferase expression. A relatively low level of luciferase activity was supported by vector p907. It was partially rescued by vector p906, in which the BLG 3' region, downstream of the luciferase cDNA, was replaced with the SV40 PA site. Flanking the SV40 region of vector p906, at its 3' end, with BLG sequences of exon 6/intron 6/exon 7 and the 3' region of the gene resulted in vector p904. This vector supported the highest luciferase activity, 10 times or 2.5 times higher than that measured in cells transfected with vectors p907 and p906, respectively. The induced activity supported by vector p904 is attributed to interaction between the SV40 PA site and elements of the distal part of the BLG 3' flanking sequences. The BLG 5' regulatory region of vector p904 encompasses a 3-kb promoter sequences. Deletion of 935 bp of its proximal end resulted in a 60% decrease in luciferase activity. Reduced activity was also seen with vector p915 lacking sequences of exon 1/intron 1/exon 2. This decrease could not be rescued with heterologous sequences of insulin intron 1, inserted upstream of the luciferase cDNA. Two sets of transgenic mice carrying vectors p907 and p904 were generated. Vector p907 supported only marginal luciferase activity in the mammary gland of all transgenic mice tested and luciferase RNA could not be detected by northern analysis. In contrast, 50% of the transgenic mice carrying vector p904 expressed luciferase RNA in the mammary gland and tissue-specific, hormonal-dependent activity was determined. However, the new p904 vector was not able to insulate the transgene from surrounding host DNA sequences, as reflected by its copy number-independent manner of expression. Nevertheless, vector p904 may represent a valuable tool for the expression of cDNAs in the mammary gland of transgenic animals.