Multihormonal regulation of casein gene expression at the transcriptional and posttransciptional levels in the mammary gland

Epigenetic regulation of milk production in dairy cows

It is well established that milk production of the dairy cow is a function of mammary epithelial cell (MEC) number and activity and that these factors can be influenced by diverse environmental influences and management practises (nutrition, milk frequency, photoperiod, udder health, hormonal and local effectors). Thus, understanding how the mammary gland is able to respond to these environmental cues provides a huge potential to enhance milk production of the dairy cow. In recent years our understanding of molecular events within the MEC underlying bovine lactation has been advanced through mammary microarray studies and will be further advanced through the recent availability of the bovine genome sequence. In addition, the potential of epigenetic regulation (non-sequence inheritable chemical changes in chromatin, such as DNA methylation and histone modifications, which affect gene expression) to manipulate mammary function is emerging. We propose that a substantial proportion of unexplained phenotypic variation in the dairy cow is due to epigenetic regulation. Heritability of epigenetic marks also highlights the potential to modify lactation performance of offspring. Understanding the response of the MEC (cell signaling pathways and epigenetic mechanisms) to external stimuli will be an important prerequisite to devising new technologies for maximising their activity and, hence, milk production in the dairy cow.

Genomic organization of the bovine alpha-S1 casein gene

We report the sequence of the complete bovine alpha-s1 casein gene eludicating for the first time the genomic organization of an alpha-s type casein gene. Extending over 17508 bp the gene is split into 19 exons, ranging in size from 24 bp to 385 bp. Except for the translational stop codon not a single coding triplet of the alpha-s1 reading frame is disrupted by any of the splice junctions, which all confirm to known splice consensus sequences. Nine out of 16 coding exons begin with a 'GAX' codon, specific for glutamate. Splicing of this codon from exon 10 to the preceding exon creates a major phosphorylation site. An intron-exon-intron stretch of 154 bp comprising exons 10 and 13 is found precisely duplicated. Associated with the gene, copies of 8 atriodactyla retroposons are found, 6 of which are interspersed into the sequences of the three longest introns. We discuss the possibility that three functional parts of the gene have been recruited and evolutionary conserved at a time before gene diversification gave rise to the separate evolution of alpha- and beta-type casein-genes.

Vectorial secretion of proteoglycans by polarized rat uterine epithelial cells

We have studied proteoglycan secretion using a recently developed system for the preparing of polarized primary cultures of rat uterine epithelial cells. To mimic their native environment better and provide a system for discriminating apical from basolateral compartments, we cultured cells on semipermeable supports impregnated with biomatrix. Keratan sulfate proteoglycans (KSPG) as well as heparan sulfate-containing molecules (HS[PG]) were the major sulfated products synthesized and secreted by these cells. The ability of epithelial cells to secrete KSPG greatly increased in parallel with the development of cell polarity. Furthermore, KSPG secretion occurred preferentially to the apical medium in highly polarized cultures. In contrast, HS(PG) secretion did not increase along with development of polarity, although most HS(PG) (85%) were secreted apically as well. Pulse-chase studies indicated that highly polarized cultures secreted 80-90% of the sulfated macromolecules they synthesized, predominantly to the apical secretory compartment. The half-lives for KSPG and HS(PG) secretion were approximately 3 and 4 h, respectively. Parallel studies of cells cultured on tissue culture plastic-coated with biomatrix indicated that neither the state of confluency nor the biomatrix was primarily responsible for inducing the KSPG secretion observed in polarizing cultures. Experiments with uterine strips indicated that the steroid hormone, 17-beta-estradiol, markedly stimulated synthesis and secretion of sulfated macromolecules, but had no preferential effect on KSPG production. The ratio of KSPG to HS(PG) secretion from uterine strips was similar to that found in the apical medium of highly polarized cell cultures. Thus, the pattern of proteoglycan secretion observed in polarized cell cultures mimicked that observed for uterine cells, although the preferential increase in KSPG production by polarized cells could not be attributed to an estrogen response. Collectively, these studies describe the major sulfated molecules secreted by rat uterine epithelial cells under varying conditions and provide evidence for a novel influence of cell polarity on the cell's ability to secrete sulfated glycoconjugates.

Protein metabolism in the mouse during pregnancy and lactation

Protein synthesis was measured in vivo in the whole body and in a number of individual tissues in mice at various stages of pregnancy and lactation. The absolute rate of protein synthesis in the whole body increased from 640 mg/day in virgin mice to 1590 mg/day by day 18 of pregnancy, and to 2100 mg/day by day 15 of lactation. Large proportions of these increments were contributed by the rapidly growing foetuses and placentae in the pregnant animals and by protein synthesis in the mammary glands during lactation. In addition, a substantial stimulation of growth and protein synthesis was also observed in the liver and the gastrointestinal tract. Gastrocnemius muscle showed no changes in protein metabolism, indicating that in the well-fed mouse this tissue is not required to play a role as a protein reserve during pregnancy and lactation.

Ha-ras oncogene expression directed by a milk protein gene promoter: tissue specificity, hormonal regulation, and tumor induction in transgenic mice

The activated human Ha-ras oncogene was subjected to the control of the promoter region of the murine whey acidic protein (Wap) gene, which is expressed in mammary epithelial cells in response to lactogenic hormones. The Wap-ras gene was stably introduced into the mouse germ line of five transgenic mice (one male and four females). Wap-ras expression was observed in the mammary glands of lactating females in two lines derived from female founders. The tissue-directed and hormone-dependent Wap expression was conferred on the Ha-ras oncogene. The signals governing Wap expression are located within 2.5 kilobases of 5' flanking sequence. The other two lines derived from female founders did not express the chimeric gene. In the line derived from the male founder, the Wap-ras gene is integrated into the Y chromosome. Expression was found in the salivary gland of male animals only. After a long latency, Wap-ras-expressing mice developed tumors. The tumors arose in tissues expressing Wap-ras--i.e., mammary or salivary glands. Compared to the corresponding nonmalignant tissues, Wap-ras expression was enhanced in the tumors.

Influence of a reconstituted basement membrane and its components on casein gene expression and secretion in mouse mammary epithelial cells

When primary mouse mammary epithelial cells are cultured on plastic, they rapidly lose their ability to synthesize and secrete most milk proteins even in the presence of lactogenic hormones, whereas cells cultured on released type I collagen gels show greatly enhanced mRNA levels and secretion rates of beta-casein and of some other milk proteins. We show here that culture on a reconstituted basement membrane from Engelbreth-Holm-Swarm tumor (EHS) allows greater than 90% of cells to produce high levels of beta-casein. By comparison, 30-40% of cells on released type 1 gels and only 2-10% of cells on plastic express beta-casein after 6 days in culture. Because only 40% of cells from late pregnant gland produced beta-casein before culture, the EHS matrix can both induce and maintain an increased level of casein gene expression. Individual basal lamina components were also evaluated. Type IV collagen and fibronectin had little effect on morphology and beta-casein mRNA levels. In contrast, both laminin and heparan sulfate proteoglycan increased beta-casein mRNA levels (1.5- to 4-fold and 2- to 8-fold, respectively). However, for heparan sulfate proteoglycan, increased message was not accompanied by increased secretion of beta-casein. Profound morphological differences were evident between cells cultured on plastic and on EHS matrix, the latter cells forming ducts, ductules, and lumina and resembling secretory alveoli. These results emphasize the vital role of the extracellular matrix in receiving and integrating structural and functional signals that can direct specific gene expression in differentiated tissues.

Cell proliferation and milk protein gene expression in rabbit mammary cell cultures

We analyzed the synthesis of DNA, the rate of cell proliferation, and the expression of milk protein genes in mammary cells grown as primary cultures on or in collagen gels in chemically defined media. We assessed DNA synthesis and cell growth, measured by [(3) H]- thymidine incorporation into acid-insoluble material, DNA content, and cell counts, in a progesterone- and prolactin-containing medium. In some experiments, cultures were pulsed for 1 h with [(3)H]thymidine and dissociated into individual cells which were cytocentrifuged and processed for immunocytochemistry and autoradiography. We analyzed expression of milk protein genes at the transcriptional, translation and posttranslational levels in progesterone-depleted medium in the presence or absence of prolactin. We measured protein secretion by radioimmunoassays with antisera directed against caseins, alpha-lactalbumin and milk transferrin1. We determined protein synthesis by incorporating radio-labeled amino acids into acid-precipitable material and by immunoprecipitating biosynthetically labeled milk proteins. We assessed the accumulation of casein mRNA by hybridizing total cellular RNA extracted from cultured cells with (32)P-labeled casein cDNA probes. On attached collagen gels, the cells synthesized DNA and replicated until they became confluent. The overall protein synthetic activity was low, and no milk proteins were synthesized or secreted even in the presence of prolactin. The block in milk protein gene expression was not restricted to translational or posttranslational events but also included transcription, since no casein mRNA accumulated in these cells. On floating gels, protein synthesis was threefold higher than in cells from attached gels. Overall protein synthesis as well as casein and alpha-lactalbumin synthesis and secretion were prolactin-dependent with maximal stimulation at around 10(-9) M. A marked inhibition occurred at higher hormone concentrations. Casein mRNA accumulated in these cells, provided prolactin was present in the medium. In contrast, these cells did not synthesize DNA, nor did they replicate. In embedding gels, the rate of cell proliferation was exponential over 25 d with a doubling time of approximately 70 h. The overall protein synthesis increase was parallel in time with the increase in cell number. Caseins and alpha-lactalbumin (in contrast to transferrin) were synthesized only in the presence of prolactin. We observed the same hormone dependency as with cells growing on floating gels. The number of casein- and transferring-positive cells was measured after dissociating the cell cultures. At day 12, 60 percent of the total cells stored transferring in small cytoplasmic vesicles, whereas only 25 percent of the cells accumulated casein. Differences in the organization and in the shape of mammary cells depending on cell surface conditions suggest that the geometry of the cells, their interaction with extracellular matrix constituents, and cell-to-cell interactions play a role in the expression of two mammary functions: DNA synthesis and growth, as well as milk protein gene expression.

Characterization of the translation products of the major mRNA species from rabbit lactating mammary glands and construction of bacterial recombinants containing casein and alpha-lactalbumin complementary DNA

Total cytoplasmic polyadenylated RNA from lactating rabbit mammary glands was analysed on methylmercury hydroxide-agarose gels. The size of the most abundant mRNA species ranged between 0.5 and 5.0 kb (kilobases), with major bands at 0.55, 0.84, 0.92, 1.18 and 2.4 kb and discrete minor bands of 1.5, 1.7, 3.0 and 3.9 kb. Translation in vitro of total mRNA with [3H]leucine or [35S]methionine as precursor yielded four major bands with apparent Mr values of 16 000, 25 000, 26 000 and 29 000. The four protein bands were identified by immunoprecipitation by using specific antisera as alpha-lactalbumin and x-, kappa- and alpha-caseins, respectively. Labelling with (35S]cysteine followed by immunoprecipitation with anti-transferrin or anti-alpha-lactalbumin sera allowed the identification of two whey proteins. Translated transferrin was resolved as an 80 000-dalton band and alpha-lactalbumin appeared as a 16 000-dalton protein. A library of recombinant plasmids containing cDNA (complementary DNA) sequences representing cytoplasmic polyadenylated RNA was used to isolate clones for the major rabbit caseins and alpha-lactalbumin. A preliminary characterization of these cDNA clones was achieved by colony hybridization with enriched RNA fractions as probes. Positive clones were identified by use of hybrid-promoted translation in vitro and immunoprecipitation of the translation products. The corresponding mRNA species were further identified by hybridizing RNA blots with radioactively labelled cDNA clones. We present the restriction map of alpha-casein and kappa-casein cDNA clones.