Characterization of lactogenic hormone binding to membranes from ovine and bovine mammary gland and liver

Growth hormone can induce expression of four major milk protein genes in transfected MAC-T cells

Growth hormone (GH) can increase milk production in cattle, and this effect was thought to be mediated by an indirect mechanism because traditional ligand binding assays failed to detect GH binding sites in the mammary gland. However, recent findings that GH receptor (GHR) mRNA and protein are expressed in the epithelial cells of the bovine mammary gland suggest that GH may directly act on these cells to affect milk production. Therefore, the objective of this study was to determine whether GH could affect milk protein gene expression, nutrient uptake, and cell proliferation in bovine mammary epithelial cells using the bovine mammary epithelial cell-derived MAC-T cells as a model. Native MAC-T cells had low expression of GHR. Thus, we transfected them with expression plasmids for GHR and signal transducer and activator of transcription 5 (STAT5), 2 key components of GHR signaling, to maximize their GH response. Growth hormone increased the expression of alphaS1-casein, alphaS2-casein, beta-casein, and alpha-lactalbumin mRNA 16- to 117-fold in the transfected MAC-T cells, whereas it had no effect on the expression of kappa-casein, beta-lactoglobulin, or insulin-like growth factor I mRNA. Cotransfection analyses showed that GH also strongly induced reporter gene expression from alphaS1-casein, alphaS2-casein, beta-casein, and alpha-lactalbumin gene promoters. Growth hormone had no effect on the uptake of 2-deoxyglucose, an unmetabolizable glucose analog, amino acids, or oleic acid; neither did it affect cell proliferation or death. These observations together with the fact that GH receptor mRNA and protein are expressed in the epithelial cells of the bovine mammary gland raise the possibility that GH might act directly on the mammary epithelial cells in cows to stimulate transcription of major milk protein genes, as part of the mechanism by which GH stimulates milk production.

Influence of prepubertal dietary regimen on mammary growth of Holstein heifers

One hundred-sixteen Holstein heifers (mean BW, 175 kg) were randomly assigned to diets of alfalfa silage or corn silage and were fed to gain approximately 725 or 950 g/d in order to study the influence of prepubertal diet and rate of gain on mammary growth and milk production. Blood was collected before puberty for hormone determination, and 8 heifers per group were killed at puberty for evaluation of tissue variables. Serum growth hormone was reduced, and IGF-I was increased, in the group of heifers reared at a high rate of gain on the corn silage diet. Accompanying the decline in growth hormone, total mammary parenchymal DNA and RNA was reduced in heifers reared at a high rate of gain on the corn silage diet. Mammary parenchyma in heifers of the latter group contained a greater volume of adipocytes and a lower volume of epithelial cells than did mammary parenchyma in heifers of other groups. Data are consistent with previous investigations that showed a deleterious effect of prepubertal rapid weight gain on mammogenesis when accompanied by excess body fat deposition. However, this effect did not cause a decline in subsequent milk production.

Solubilization and characterization of lactogenic hormone receptor from kidney of lactating cow

Solubilization of the microsomal fraction from bovine kidney by Triton X-100 or by 3-[(3-cholamidopropyl)-dimethylammonio] 1-propanesulfonate (CHAPS) increased 2-fold the thermodynamic association constant for hGH. While solubilization with CHAPS did not change the 13-fold preferential binding of human growth hormone (hGH) over ovine prolactin (oPRL), solubilization with Triton X-100 increased this preference to 47-fold. The binding was optimal at pH 7-7.5 in the presence of 10 mM of MgCl2 or CaCl2. The association rate with hGH was identical in the microsomal and Triton X-100 solubilized fractions but the dissociation was slower in the latter. Only partial dissociation was observed at neutral pH. Full dissociation was, however, achieved by lowering the pH to 4-5, indicating that the binding was not covalent. Gel filtration studies of the Triton X-100 solubilized fraction after preincubation in the presence of reducing agent revealed two sharp peaks of activity, one having Mr of greater than 700 kDa that represented the aggregated receptor, and the second, with Mr 110-115 kDa. The specificity of the partially purified receptors clearly shows that they are lactogenic and not somatogenic. They resemble lactogenic receptors found in other bovine organs, but differ from other species particularly in their differential affinities of PRL and hGH.

Association of prolactin and insulin receptors with mammogenesis and lobulo-alveolar formation in pregnant ewes

1. Pregnant, multiparous ewes were sacrificed at d 50 (n = 3), 80 (n = 4), 115 (n = 3), or 140 (n = 4) for biochemical, histological and autoradiographical quantitation of mammary growth. 2. Significant increases in concentration of mammary parenchyma DNA (0.25-24 mg/g tissue), total under DNA (57-1304 mg), and total under RNA (36-1504 mg) were observed by d 115. 3. Mammary tissue at d 115 contained the maximal percent of tissue volume occupied by epithelium (41.2%), number of cells per alveolar cross section (36.6) and percent of [methyl-3H]thymidine labeled epithelial cell nuclei (3.5%). 4. Concentration prolactin binding sites were significantly increased at d 115 of gestation, but serum prolactin and growth hormone concentrations remained low until d 140. 5. Mammary insulin binding sites (per unit membrane protein) progressively decreased during gestation. 6. Results suggest that serum growth hormone concentration and quantity of insulin receptors do not limit mammogenesis but greater concentrations of prolactin binding sites coincident with lobulo-alveolar formation, suggest a primary association with mammogenesis in the ewe.

A comparative study of lactogenic hormone binding sites in the adrenal gland, ovary and kidney of the lactating cow

The specific binding of 125I-oPRL to microsomal fractions from the adrenal gland, ovary and kidney of the lactating cow was significantly lower than binding of iodinated hGH. In addition, the ability of oPRL to compete with iodinated hGH as compared to hGH, was 50-fold lower in the adrenal gland 35-fold lower in the ovary and 18-fold lower in the kidney. These results are similar to those obtained in the mammary gland and liver, whereas the ability of oPRL to compete with iodinated hGH was 90-fold lower, as compared to hGH. In the kidney the difference between the binding of iodinated hGH and iodinated oPRL was smaller. Results obtained with a solubilized kidney microsomal fraction also show a slightly higher affinity for oPRL than in other tissues. Thus the phenomena of differential affinities of oPRL and hGH to lactogenic hormone binding sites, characterizes most lactogenic hormone target tissues in the lactating cow. The distribution of these sites in different parts of the tissues was also studied. In the adrenal gland, the binding capacity in the cortex was 8-fold higher than in the medulla. In the ovary most of the binding sites were found in the corpus luteum, while in the kidney the binding capacity was higher in the cortex as compared to the medulla.

Radioimmunoassay for measurement of bovine alpha-lactalbumin in serum, milk and tissue culture media

A sensitive, specific radioimmunoassay for bovine alpha-lactalbumin (alpha-la) suitable for measurement of serum, tissue culture media and milk was developed. alpha-La was not detected in serum from prepubertal male or female cattle, but was detected as early as 60 d of gestation in nulliparous Holstein heifers, the level being greatly increased during the last one third of gestation. Serum from cross bred beef heifers contained less alpha-la and it was not detected until late gestation. Concentrations of alpha-la in serum samples from pregnant multiparous Holstein cows decreased at drying-off and subsequently increased just before parturition. Secretion of alpha-la by mammary tissue explants from steroid-primed prepubertal Holstein heifers was induced by the addition of bovine prolactin, ovine prolactin or human growth hormone to tissue culture media.

Lactogenic hormones: binding sites, mammary growth, secretory cell differentiation, and milk biosynthesis in ruminants

Roles of the lactogenic hormones prolactin and placental lactogen in mammary development in ruminants were reviewed. In contrast with other ruminants, failure to detect lactogenic activity in the serum of pregnant cows (in excess of that attributed to prolactin) suggests that placental lactogen may have little direct effect on mammary growth or lactogenesis. However, replacement and ablation experiments using ergocryptine provide definitive evidence that increased periparturient secretion of prolactin is necessary for maximal milk production in cattle. Quantitative microscopy indicates a relative failure of mammary cells in cows with inhibited secretion of prolactin to differentiate structurally. Prolactin induces synthesis and secretion of alpha-lactalbumin in prepartum bovine mammary tissue. Temporary disruption of mammary microtubules immediately prepartum in pregnant heifers reduced subsequent milk production, biosynthetic capacity, and cellular differentiation. For maximal milk production, mammary secretory cells apparently must respond to lactogenic hormone stimulation during the immediate periparturient period. Colchicine may desensitize the mammary epithelium to prolactin action. Membrane binding of radiolabeled human growth hormone to ruminant mammary gland provides a measure of lactogenic hormone binding sites. Specific binding to 600 micrograms of mammary membrane protein was 296% greater in lactating, compared with nonlactating, pregnant (65 days of gestation) ewes. Binding capacity (fmol/mg membrane protein) averaged 275 +/- 57 in mammary membranes from nonlactating, pregnant ewes (100 days gestation, n = 2) and 2,325 +/- 521 in mammary membranes from lactating ewes (n = 6, 14 to 21 days postpartum). Greater understanding of hormonal regulation of the ruminant mammary gland likely will result in development of techniques to produce milk more efficiently and perhaps capability to evaluate production potential of young animals.