Hormonal factors in lipogenesis in mammary gland

Adipocyte-epithelial interactions regulate the in vitro development of normal mammary epithelial cells

Mammary epithelial organoids (MEO), isolated from pubescent rats, were cultured within a reconstituted basement membrane in transwell inserts, in the presence or absence of mature mammary adipocytes in the lower well. This system allowed for free medium exchange between the two compartments, without direct cell-to-cell contact. When cultured in serum-free medium supplemented with insulin, prolactin, hydrocortisone, progesterone, and various epidermal growth factor (EGF) concentrations, mammary adipocytes did not affect epithelial cell growth, but enhanced epithelial differentiation. Casein and lipid accumulations were monitored as indicators of functional differentiation of MEO. Mammary adipocytes significantly enhanced casein and lipid accumulation within the MEO, independently of EGF concentration. Furthermore, adipocytes induced MEO to preferentially undergo alveolar morphogenesis, inhibited squamous outgrowth, and increased lumen size. These findings demonstrate that morphological and functional differentiation of mammary epithelial cells is profoundly enhanced by the adipose stroma and that these effects are mediated by diffusible paracrine factors. This new model can be exploited in future studies to define the mechanisms whereby hormones and growth factors regulate mammary gland development and carcinogenesis. Moreover, it could complement in vivo reconstitution/transplantation studies, which are currently employed to evaluate the role of specific gene deletions in the regulation of mammary development.

Effect of reproductive states on lipid mobilization and linoleic acid metabolism in mammary glands

Effects of pregnancy and lactation on lipid metabolism in mouse mammary fat pads and nonmammary adipose tissues have been studied. In order to address the question whether the influence of hormonal milieu on lipid metabolism in mammary epithelial cells during pregnancy and lactation is the same as in fat cells, we have studied the mobilization of lipids and metabolism of fatty acids in the intact mammary glands, parenchyma-free mammary fat pads and in the perimetrial fat tissues of virgin, pregnant and lactating mice. Compared to parenchyma-free mammary fat pads, the perimetrial adipose tissues accumulated 5-fold higher levels of triglycerides during pregnancy. Mammary fat cells maintained overall lipid levels during pregnancy and lactation (16-20 micrograms/fat pad). In contrast, lactation depleted total lipid stores from 108 +/- 5 to 24 +/- 4.5 micrograms/fat pad in perimetrial fat pads. Results of comparative analysis of fatty acid composition of mammary fat pads, with and without epithelial tissue, from virgin and lactating mice showed stimulation of 18:2 omega 6 metabolism leading to 130% increase in the ratio 20:4 omega 6 to 18:2 omega 6 in the epithelial compartment. Pregnancy and lactation resulted in the elevation of 20:4 omega 6 levels probably due to a 4-fold increase in delta 5 desaturase activity and a decrease in oxidative degradation of 18:2 omega 6. These results suggest that, unlike other adipose tissues, the metabolic pathways in mammary fat cells are not dedicated to sequestration and accumulation of dietary lipids during pregnancy. Lactation favors mammary epithelial cell-stimulated production of precursors of eicosanoids which are known to have agonist-like effect on mammary epithelial cells.

Mammary development and milk secretion in transgenic mice expressing the sheep beta-lactoglobulin gene

Mammary development and milk secretion were studied in transgenic mice which exhibited mammary tissue-specific expression of the sheep beta-lactoglobulin gene, and secreted significant quantities of the foreign protein in milk. Mammary development was unaffected by transgenesis. Tissue DNA content and the activities of several key enzyme markers of cell differentiation were similar in transgenic mice and non-transgenic controls. Milk yield, whether estimated by pup weight gain or measured by a 3H2O-dilution method, was unchanged by foreign gene expression. Gross milk composition, including milk protein concentration, was also similar in transgenic and non-transgenic animals, even though beta-lactoglobulin accounted for 29% of total milk protein. Therefore the foreign gene product was synthesized at the expense of endogenous milk proteins. However, transgenic mammary tissue in vitro exhibited a significantly higher rate of total protein synthesis than did control tissue. This suggested that a factor limiting milk protein synthesis or secretion in transgenic mice in vivo may have been removed by short-term explant culture of mammary tissue. The results emphasize that the use of transgenesis for manipulating milk composition may depend not only on high-level mammary-specific expression of the foreign gene, but also on the biosynthetic capacity of the mammary gland itself.

Alterations in the activities of rat tissue hexose monophosphate dehydrogenases in response to premature weaning and dietary restriction at mid-lactation

The activities of the hexose monophosphate dehydrogenases increased in adipose tissue, remained unchanged in liver and decreased in mammary gland following the weaning of rats at mid-lactation (day 14). When dietary intake was restricted at mid-lactation, the activities of the hexose monophosphate dehydrogenases increased in adipose tissue, decreased in liver, but were unaltered in mammary gland. Premature weaning on day 14 postpartum resulted in maternal increases in both plasma insulin and glucose, which peaked at day 16. The plasma insulin levels decreased from day 14 to day 18 postpartum in the normal lactating rat, and a similar trend was observed for animals on a restricted dietary intake. Daily food consumption in the lactating rat decreased from 50 g to 20 g after premature weaning. The live weight of pups raised on dams given a restricted food intake from day 14 had decreased by day 17 postpartum, whereas an increase in daily live weight gain was recorded for the litters from the lactating controls. The results demonstrate that the activities of the hexose monophosphate dehydrogenases are regulated differentially between tissues of the lactating rat.

Nutritional and hormonal regulation of malic enzyme synthesis in rat mammary gland

Cytosolic malic enzyme was purified from rat mammary gland by L-malate affinity chromatography. The pure enzyme obtained was used to produce a specific antiserum in a rabbit. Relative synthesis of malic enzyme in the mammary gland of mid-lactating rats was 0.097%, measured by labelling the enzyme in isolated acini. When food was removed, malic enzyme synthesis decreased to 35% and 20% of the control value at 4 and 6 h respectively. Incorporation of [3H]leucine into soluble proteins was constant during the first 6 h of starvation. When lactating rats (maintained with their pups) were starved for 24 h and then re-fed, the relative rate of enzyme synthesis increased 2.5-, 4-, and 4.5-fold at 3 h, 6 h and 18 h respectively after initiation of re-feeding. The relative rate of malic enzyme synthesis was about 50% of normal at 15 h after weaning, whereas the rate of synthesis of soluble proteins did not change. Administration of bromocriptine or adrenalectomy of lactating rats decreased the relative rate of synthesis of malic enzyme by 40% or 30% respectively; these effects were counteracted by hormone supplementation. Hormone therapy also caused an increase in the rate of incorporation of [3H]leucine into soluble proteins and in malic enzyme activity.

Cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase activities of rat mammary tissue

Cyclic nucleotide phosphodiesterase activity in mammary tissue from rats in midlactation was resolved by DEAE-cellulose chromatography into three functionally distinct fractions: a Ca2+/calmodulin-stimulated cyclic GMP phosphodiesterase, a cyclic GMP-stimulated low-affinity cyclic nucleotide phosphodiesterase, and a high-affinity cyclic AMP-specific phosphodiesterase. The absolute activities and relative proportions of high- and low-affinity enzymes resemble those found, for example, in liver, as distinct from those in excitable tissues. Three functional characteristics are described which are peculiar to mammary-tissue phosphodiesterases. Firstly, the concentration of free Ca2+ required to achieve half-maximal activation of the Ca2+/calmodulin-stimulated phosphodiesterase is somewhat higher than for the analogous enzyme in other tissues; secondly, the activity of this enzyme towards cyclic AMP relative to that towards cyclic GMP is unusually low, and thirdly, the low-affinity cyclic nucleotide phosphodiesterase is inhibited by low concentrations of free Ca2+.

Comparison of collagen gels and mammary extracellular matrix as substrata for study of terminal differentiation in rabbit mammary epithelial cells

Mammary epithelial cells were prepared by collagenase digestion of tissue from mid-pregnant rabbits and cultured for up to 6 days on either collagen gels or an extracellular matrix prepared from the same tissue. The behaviour of the cells in serum-supplemented medium containing combinations of insulin, prolactin, hydrocortisone, estradiol and progesterone were monitored by measuring rates of casein synthesis, lactose synthesis, DNA synthesis and protein degradation. After 6 days, epithelial cells on floating collagen gels showed substantial increases in casein synthesis and DNA synthesis over freshly-prepared cells, following a decline during the first 3 days when the collagen gels are contracting. The optimum hormone combination for casein synthesis was insulin + prolactin + hydrocortisone, whereas for optimum DNA synthesis the additional presence of estradiol and progesterone was required. Cells on extracellular matrix showed increased rates of both casein synthesis and DNA synthesis by day 6 in the presence of insulin + prolactin + hydrocortisone, with additional estradiol + progesterone having an inhibitory effect. Whereas on day 2 rates of intracellular protein degradation were generally lower in cells on extracellular matrix, by day 6 rates of protein degradation were lowest in cells cultured on collagen gels with insulin + prolactin + hydrocortisone. In all cases, rates of lactose synthesis fell to low levels as the culture proceeded. Pulse-chase labelling of freshly-prepared cells with [32P]orthophosphate in medium containing serum and insulin + prolactin + hydrocortisone demonstrated that newly-synthesized casein was degraded during its passage through the epithelial cell. The influences of the collagen gels and extracellular matrix and of the hormone combinations on epithelial cell differentiation and secretory activity are discussed.

Control of fatty acid synthesis in lactation

Lactation results not only in an increased rate of fatty acid synthesis in the mammary gland but also in a decreased rate of fatty acid synthesis in adipose tissue and, in the rat at least, an increased rate of hepatic fatty acid synthesis. Progesterone (during pregnancy), prolactin and (in ruminants) GH are implicated in the regulation of the reciprocal changes in fatty acid synthesis in mammary gland and adipose tissue. Progesterone and prolactin, at least, appear to influence the rate of fatty acid synthesis by modulating the insulin-binding capacities of the tissues, but it is clear that steps in the mechanism of action of insulin subsequent to its binding to the receptor are also changed in adipose tissue during lactation.

Protein degradation during terminal cytodifferentiation. Studies on mammary gland in organ culture

1. In mammary gland explants subjected to experimental manipulation, average rates (during 24 h periods) of degradation of fatty acid synthase, casein and cytosol-fraction proteins were measured by a double-isotope method. Rates of degradation of fatty acid synthase were also computed from measurements of changing enzyme amount and rate of synthesis. 2. During the period of most rapid enzyme accumulation there is a transient decrease in the computed rate of degradation of fatty acid synthase. Removal of hormones produces a rapid increase in the computed rate of degradation of the enzyme. 3. The average rate of degradation of fatty acid synthase measured by the double-isotope method is low in the presence of hormones, and increases on hormone removal. This increase in degradation rate is inhibited by adrenaline and further blocked by insulin. NH4Cl (10 mM) also partially inhibits the increase in protein degradation on hormone removal. 4. The pattern of changes in the average rate of degradation of cytosol-fraction proteins is similar to that for fatty acid synthase alone. There is no relationship between subunit molecular weight and rate of degradation under all experimental conditions. 5. Isotope ratios for resolved cytosol protein mixtures are transformed logarithmically to make the standard deviations an estimate of heterogeneity of degradation rates. By this analysis, in some conditions there appears to be significant measureable heterogeneity of degradation rates. 6. Little degradation of casein is measured in the presence of hormones, but a marked increase in the rate of degradation can be measured when hormones are removed. Whereas at 24-48h NH4Cl (10 mM) has little effect on this enhanced rate of degradation, at 48-72h it causes a large decrease in degradation rate. 7. Results are discussed in terms of a two-component degradation system in mammary gland explants.