Regulation of gene expression in the bovine mammary gland by ovarian steroids

It is well established that estrogen is required for mammary epithelial cell proliferation and ductal development in the growing animal, and that lobuloalveolar development during gestation is dependent on progesterone. The effects of these steroid hormones on gene expression in the mammary gland are mediated primarily by their respective nuclear hormone receptors, which function as hormone-bound transcription factors. To gain insight into how estrogen and progesterone regulate mammary gland growth and function in cattle, we and others have characterized the expression patterns of their cognate nuclear hormone receptors in the bovine mammary gland throughout development, pregnancy, and lactation. This work has identified a lack of expression of estrogen receptor beta and a greater abundance of progesterone receptor during lactation in the bovine mammary gland, compared with the rodent gland. We speculate that interactions among the estrogen receptor isoforms that regulate progesterone receptor expression may contribute to these species differences. Further, demonstrated expression of substantial quantities of estrogen receptor within the prepubertal bovine mammary fat pad, along with coordinated insulin-like growth factor-I expression, suggests that this tissue may stimulate parenchymal growth via an estrogen-responsive paracrine mechanism. In addition, the recent availability of bovine genomic sequence information and microarray technologies has permitted the study of global gene expression in the mammary gland in response to the steroid environment. We have identified more than 100 estrogen-responsive genes, of which the majority are novel estrogen gene targets. Estrogen-induced changes in gene expression were consistent with increased mammary epithelial cell proliferation, increased extracellular matrix turnover in parenchyma, and increased extracellular matrix deposition in the fat pad. A comparison of estrogen-responsive genes in the mammary glands of humans, mice, and cattle suggests considerable variation among species, as well as potential differences in regulatory elements in common estrogen receptor gene targets. Continuing studies using advanced molecular techniques should assist in elucidating the complex regulation of mammary function at the transcript level.

Approaches to the manipulation of mammary involution

Mammary involution is a gradual process that occurs following cessation of milking. Regression of mammary secretory tissue accompanies dramatic changes in secretion composition during the transition from lactation to involution. Conversely, rapid differentiation of secretory tissue and copious accumulation of colostrum occur as parturition approaches. The duration of the nonlactating period, mammary gland health, and secretory cell response to hormones influence subsequent lactational performance in most species. Manipulation of the bovine mammary gland in an attempt to hasten involution has been studied. The primary objective of these studies was to determine if hastened involution would decrease new intramammary infections during the early nonlactating period. Results of these studies have also led to a more fundamental understanding of events that occur during physiological transition of the mammary gland. Adequate regression, proliferation, and differentiation of mammary secretory epithelium during the nonlactating period of ruminants appear to be essential for maximal milk production during lactation. Factors that interfere with these mechanisms can adversely affect mammary function during the impending lactation. A greater understanding of these processes may provide new approaches for increasing milk production in dairy cattle.

Milk protein secretion by explants of prepubertal bull mammary tissue: breed differences

Production of milk proteins in response to hormonal stimuli was studied in organ culture of mammary tissue from prepubertal Angus or Holstein bulls. Bulls were injected with estradiol and progesterone for 7 d and slaughtered on d 15. Mammary tissue explants were cultured for 96 h in basal medium containing insulin, hydrocortisone, and triiodothyronine or in stimulatory medium, further supplemented with prolactin. Selected cultures were incubated in the presence of [3H]-amino acids. Content of alpha S1-casein in cultures from Angus bulls was increased 3-fold in medium, and 6-fold in explant homogenates, relative to cultures from Holstein bulls. Concentration of 3H-labeled protein was greater in medium (2.3-fold) of Angus versus Holstein cultures. Overall, alpha-lactalbumin content in medium and homogenates tended to be higher in Angus cultures. Prolactin increased alpha-lactalbumin and casein in medium, and alpha-lactalbumin and [3H]-protein in homogenates. We conclude that mammary tissue of immature bulls can be induced to produce milk proteins and that prolactin enhances production. Genetic (breed) differences in responses were observed, although the greater productivity of explants from Angus compared with explants from Holstein bulls was unexpected. Nevertheless, use of measurements such as these may provide data for sire selection.

Udder health in the periparturient period

The periparturient period is associated with rapid differentiation of secretory parenchyma, intense mammary growth, copious synthesis and secretion, and marked accumulation of colostrum and milk. Udder health during this time is an important factor associated with the production of maximum quantities of high quality milk. Intramammary infections that occur during the dry period can adversely affect udder health, resulting in decreased milk production, altered milk composition, and impaired mammary function. Bovine mammary glands are markedly susceptible to new infections during the periparturient period, especially prior to parturition. Many infections that occur at this time are associated with clinical mastitis during early lactation. Methods of controlling mastitis in the dry period have focused primarily on the use of antibiotics. However, antibiotic therapy at drying off does not appear to prevent new infections in the periparturient period. This is most likely due to lack of persistence of antibiotics. Furthermore, antibiotics used currently are less effective against environmental pathogens, in particular coliform bacteria, which can cause a high proportion of intramammary infections during the periparturient period. Methods of controlling bovine mastitis during the periparturient period is an important area that requires additional research. Procedures need to be developed that are effective against a variety of bacteria, including environmental mastitis pathogens, if additional control is to be achieved.

Caprine mammary differentiation and initiation of lactation following prepartum colchicine infusion

Milk from both colchicine-infused and uninfused udder halves had similar levels of somatic cells, serum albumin, pH, citrate, and lactose throughout the experimental period. Milk citrate and lactose concentrations gradually increased in both colchicine-infused and uninfused udder halves during early lactation while levels of somatic cells and serum albumin decreased as lactation progressed. No differences in parenchymal development or cytological differentiation were observed between colchicine-treated and untreated mammary tissue obtained prepartum, at parturition, or 7 days postpartum. Colchicine-infused udder halves produced about 9% less milk than uninfused controls during the first 30 days of lactation.

Mammary growth pattern in goats during pregnancy and lactation

Nine groups of five dairy goats from four breeds, with each breed represented at least once in each group, were in our experiment to measure mammary gland growth. Groups were virgins, days 90 to 100 pregnancy, 5 days prepartum (145 days pregnancy), 1 or 2 days prepartum, and 1 or 2, 5, 10, 15, and 30 days in lactation. Virgins were 19 mo old while all others had one pregnancy and lactation prior to experiment. Indices of mammary growth included untrimmed and trimmed wet weights of udders, dried fat-free tissue weights, deoxyribonucleic acid and ribonucleic acid contents. Each index was in a regression equation best to describe the pattern of mammary growth. These included linear, quadratic, cubic, and exponential model Y = AebX in which Y was the index of mammary growth and X was the day of pregnancy. Total deoxyribonucleic acid was the best index of mammary growth with a correlation of .95 in the exponential model. The predictive equation for total deoxyribonucleic acid in milligram was Y = 167e.019X on a daily basis, and the rate of growth on a monthly basis was .57. Mammary growth in goats continued into early lactation, peaking at day 5. Ribonucleic acid doubled on the day after parturition, which reflected the rapid increase in protein synthesis at this time.

Induced lactation in dairy ewes by various brief hormone treatments

Hormonal induction of lactation in 40 nonpregnant dairy ewes was attempted by daily injecting subcutaneously 25 mg estradiol-17 beta and 70 mg progesterone for 7 consecutive days. After an interval of 7 days the following treatments were applied: group I (10 ewes), nothing; group II, (10 ewes), 10 mg dexamethasone and 30 mg prednisolone; group III (10 ewes), 10 mg dexamethasone, 30 mg prednisolone, 400 IU insulin; and group IV (10 ewes), 10 mg dexamethasone, 30 mg prednisolone, 400 IU insulin, and 40 mg prostaglandin F2 alpha. Hand milking was initiated at day 16. Thereafter, ewes were milked twice daily for 60 days, and milk yields were recorded and sampled twice weekly for measuring fat, pH, acidity, specific gravity, total solids, lactose, and chloride content. Mean yield of milk of ewes in II was larger than of ewes in I and IV. Four ewes in I, 8 ewes in II, 5 ewes in III, and 3 ewes in IV out of 10 ewes in each group produced more than 200 ml milk daily. The peak milk yield of ewes in I was at day 43, in II at day 28, in III at day 7, and in IV at day 11 of the milking period. Injection of corticoids in the induction scheme of estrogen and progesterone resulted in greater lactogenic response and yields of milk, but injections of insulin or insulin and prostaglandin F2 alpha diminished these responses.

Induction of lactation: lactational, physiological, and hormonal responses in the bovine

Milk yields, physiological responses, and concentrations of plasma hormones were evaluated in 24 attempts to induce lactation in nonlactating dairy cows. Subcutaneous injections of estradiol-17beta and progesterone (.10 and .25 mg/kg body weight per day) for 7 consecutive days were used. Dexamethasone injections (.028 mg/kg body weight per day) on days 18 to 20 were given during 12 attempts at induction. Milking was initiated on day 21. All cows showed proestrus activity within 2 days after the first steroid injection; this subsided, then reappeared in many animals between days 16 to 20. In 14 of 24 attempts mean daily milk production was greater than 5 kg. Actual or projected 305-day lactation milk yields were between 1859 and 5354 kg. However, milk yields of seven induced cows averaged only 73% (32% to 136% range) of their previous natural lactations. Dexamethasone injections increased the number of cows that produced more than 5 kg/day; however, milk yields were not improved. Concentrations of estradiol, estrone, and progesterone in plasma were unaffected by dexamethasone, but concentrations of glucocorticoids in plasma were depressed on days 19 to 22. Concentrations of prolactin (peak and mean) in plasma for six cows each that produced greater or less than 5 kg/day did not differ. However, concentrations of prolactin increased in the week following steroid injections (days 8 to 15) only in those cows that produced greater than 5 kg/day but were elevated in all cows during the 3rd wk.

Induction of lactation by two techniques: success rate, milk composition, estrogen and progesterone in serum and milk, and ovarian effects

Induction of lactation was attempted in 12 heifers and 12 cows with estradiol benzoate (.011 mg/kg body weight per day) subcutaneous for 10 days or that plus progesterone (.1 mg + .25 mg/kg body weight per day) for 7 days. Milking commenced on day 20 for those treated with the mixture and on day 11 for the others. Lactations were induced (minimum of 4.5 kg of milk/day) in five of six heifers and two of six cows by the mixture and in six of six heifers and three of six cows for estradiol benzoate. Milk production was 44% of herdmates in the 16 induced lactations. Cows on the single treatment had lower production than the other three groups. Ovarian status, cycling, cystic, or static, was affected adversely in 5 of 16 animals induced successfully. Two of the 16, both heifers, carried calves to term following induction. The transition to normal composition of milk was slower for single than double treatment. Lactose increased slowly to normal over the 1st wk of milking while protein decreased slowly. Estrogen and progesterone in milk of induced cows were approximately twice as concentrated as in normal postparturient cows, probably because milk production was halved.

Hormonal control of mammogenesis and onset of lactation in cows--a review

Estrogen stimulates development of mammary ducts, and progesterone and estrogen stimulate proliferation of secretory tissues. In vivo, sequential addition of insulin (step 1), glucocorticoid (step 2), and prolcatin (step 3) leads to biosynthesis of casein and lactose. In cows, mammogenesis continues until termination of pregnancy and overlaps onset of lactation. Progesterone probably inhibits differentiation of secretory cells at step 2 or step 3. Sensitivity of individual cells to progestational inhibition may decrease variably which may be interdependent upon relative increases in estrogen, prolactin, corticoids, and growth hormone to cause asynchronies among them at calving. Since prolactin in plasma is not correlated with progesterone or the estrogens, factors other than feed-back effects of ovarian steroids may be responsible for its sustained increase periparturiently. Also, elevated prolactin periparturiently may be unrelated to subsequent rates of lactation because its "basal" concentrations may meet requirements when inhibiting effects of progesterone are removed. This concept is attractive because mammary cells neither are synchronized highly for biosynthesis nor secrete normal milk for several days after calving. At the latter time, concentrations in plasma are low for progesterone and estrogen, similar to 3 days before calving for glucocoiticoids and prolactin, and increasing for insulin. Evidence of lactation under unusual circumstances was discussed.

Hormone induced lactation in the cow. IV. Relationships between lactational performance and hormone concentrations in blood plasma

Concentrations of progesterone, estrogen, and prolactin in plasma were compared among lactations induced in 29 cows by daily subcutaneous injections of .25 mg progesterone and .1 mg estradiol-17beta per kg body weight for 7 days. Superior, median, and inferior lactations were identified by ranking both weightage adjusted maximum milk yields in 7 consecutive days (average 144 +/- 9 kg) and days for milk yield to increase from 5 to 10 kg/day (15 +/- 3 days). Superior and inferior of the former averaged 189 and 101 kg, and their latter averaged 2 and 42 days. Plasma hormones were measured on day 0 before first treatment (day 1), and on days 7, 14, 17, 21, 24, 28, and 35. Superior lactations were associated with below-average progesterone and estrogen in plasma on day 0, rapid decreases in progesterone after day 7 and in estrogen after day 14, and increased prolactin after 7. In comparison, median lactations were associated with elevated progesterone and estrogen after day 17, but prolactin was similar to that of superior lactations. Inferior lactations were associated with decreased prolactin in plasma from days 21 to 35. We hypothesized that (a) first treatment should start 3 to 8 days after estrus, (b) daily doses of estradiol-17beta should be decreased with progesterone unchanged for the 7 days, and (c) estradiol-17beta alone should be continued for 7 days to improve hormonally induced lactations.

Hormone induced lactation in the bovine III. Dynamics of injected and endogenous hormones

Three lactations were induced in two cows in two trials by twice daily subcutaneous injections for seven days. Each injection contained .05 mg/kg body weight of estradiol-17beta and .125 mg/kg body weight of progesterone. Included with the hormones in trial 1 was a combination of 233 muCi of [6,7-hydrogen-3] estradiol-17beta and 12 muCi of [4-carbon-14] progesterone. During trial 2,2050 muCi of [2,4,6,7 hydrogen-3] estradiol-17beta were incorporated with the hormones for one cow and 2344 muCi of [1,2,6,7 hydrogen-3] progesterone for the other. Radioactivity from hydrogen-3 and carbon-14 was detected in urine and feces but not in mammary secretion during trial 1. Isotope recovery from [hydrogen-3] estradiol-17beta was too low to quantify. By day 17, 90% of the carbon-14 from progesterone was accumulated in feces and 8.0% in urine. For trial 2, recoveries of radioactivity by day 28 from urine, feces, and mammary secretions were, respectively, 32.4, 56.2, and less than .002% from [hydrogen-3] estradiol-17beta and 6.0, 87.7, and .25% from [hydrogen-3] progesterone. Estrogen and progesterone in plasma averaged 1.83 and 2.61 ng/ml during days 2 to 7. Estrogen decreased more rapidly than progesterone post-treatment, but both had decreased by days 16 to 19 when average prolactin of plasma increased. Quantitative measures of estradiol 17-beta and progesterone absorption, whether in plasma or excreta, were correlated.

Histology of induced bovine lactogenesis

Nine cycling nonlactating dairy cows were treated hormonally to induce lactation with progesterone, estradiol-17beta, and dexamethasone. Mammary tissue biopsies were taken on days 18, 21, and 23 after initiation of treatment (different three cows for each period). First milking was initiated on day 21 in the latter two groups. Mean differences in tissue development (percent area) between day 18 and 23 were, epithelium + 11.1%, stroma -25.9%, alveolar lumen + 13.9%, adipose + 1.58%, duct - 974%, and + 15 nuclei/alveolar section. On day 18 epithelia containing small lipid droplets predominated but were replaced with mixed lipid droplets by day 23. Epithelia day 18 had elongated dark staining nuclei. Epithelia on day 23 had spherically shaped nuclei often containing several nucleoli and an increased cytoplasmic/nuclear ratio. Marked histological development occurred from 18 to 23 days after initiation of treatment. Mammary tissue development in nongravid hormonally treated animals was equal to or greater than 2-wk-prepartum tissue but less than 2-day-prepartum tissue for natural lactations.