Cabergoline inhibits prolactin secretion and accelerates involution in dairy cows after dry-off.

Dairy cattle require a dry period between successive lactations to ensure optimal milk production. Because prolactin (PRL) is necessary for the initiation and maintenance of milk production, strategies that can inhibit PRL secretion might hasten the involution process. The objective of this study was to determine the effect of the PRL release inhibitor cabergoline on markers of mammary gland involution during the early dry period. To assess the effect of cabergoline treatment on mammary gland involution, 14 Holstein dairy cows in late lactation were treated with either a single i.m. administration of 5.6mg of cabergoline (Velactis, Ceva Santé Animale, Libourne, France, n=7) or placebo (n=7) at the time of dry-off. Blood samples and mammary secretion samples were collected 6d before dry-off and again 1, 2, 3, 4, 8, and 14d following the abrupt cessation of lactation. Blood samples were used to determine plasma PRL concentrations. Mammary secretion samples were used to determine somatic cell count, milk fat, lactose, true protein content, and concentrations of α-lactalbumin, lactoferrin, and citrate. Following the cessation of lactation, changes in mammary secretion composition indicated diminished milk synthesis, including reduced concentrations of α-lactalbumin, citrate, and lactose. In contrast, milk somatic cell count, percent total protein, percent fat content, and lactoferrin concentrations significantly increased as involution progressed. Cabergoline treatment decreased the plasma PRL concentrations during the first week of dry-off, compared with the control treatment. No significant differences in citrate, α-lactalbumin, or protein content were observed between treatment groups. The most dramatic changes in secretion composition as a consequence of cabergoline treatment occurred during the first week of the dry period, when lactose concentrations and the citrate:lactoferrin molar ratio were lower and lactoferrin concentrations higher than in the control cows. Cabergoline treatment also tended to increase fat content and somatic cell count more rapidly following dry-off compared with the control group. These changes in mammary secretion composition following the abrupt cessation of lactation indicate that cabergoline treatment facilitated dry-off and effectively accelerated mammary gland involution.

Ovariectomy improves lactation persistency in dairy cows.

A current trend in the dairy industry is to reduce milk yield at the peak of lactation and improve lactation persistency. Lactation persistency is influenced by livestock management factors, such as feeding level or milking frequency, or by physiological status, including reproductive status or calving period. These factors modulate mammary gland apoptosis and tissue remodeling, which determine the rate of decline of milk yield after the lactation peak. Previous studies on lactating cows suggested that ovarian steroids have a negative effect on milk yield after the peak of lactation. In the present study, 4 Holstein × Normande crossbred multiparous cows were ovariectomized at the time of the lactation peak, and 5 cows underwent sham operations. All of the cows were maintained in lactation for 14 mo and milk yield was recorded daily. At slaughter, mammary epithelial cell apoptosis and mammary tissue remodeling were assessed. Ovariectomized cows had improved lactation persistency and presented an average daily milk gain of 2.5 kg compared with the sham-operated cows between mo 6 and 14 of lactation. The ovariectomy appears to have limited the decline in the milk yield after the peak of lactation by reducing mammary epithelial cell apoptosis [by reducing poly(adenosine diphosphate-ribose) polymerase expression] and mammary-tissue remodeling (by reducing matrix metalloproteinase activity). In conclusion, removal of ovarian secretion via ovariectomy improved the cows' lactation persistency.

Effects of nutrient restriction on mammary cell turnover and mammary gland remodeling in lactating dairy cows.

The aim of this study was to investigate the effects of a severe nutrient restriction on mammary tissue morphology and remodeling, mammary epithelial cell (MEC) turnover and activity, and hormonal status in lactating dairy cows. We used 16 Holstein × Normande crossbred dairy cows, divided into 2 groups submitted to different feeding levels (basal and restricted) from 2 wk before calving to wk 11 postpartum. Restricted-diet cows had lower 11-wk average daily milk yield from calving to slaughter than did basal-diet cows (20.5 vs. 33.5 kg/d). Feed restriction decreased milk fat, protein, and lactose yields. Restriction also led to lower plasma insulin-like growth factor 1 and higher growth hormone concentrations. Restricted-diet cows had lighter mammary glands than did basal-diet cows. The total amount of DNA in the mammary gland and the size of the mammary acini were smaller in the restricted-diet group. Feed restriction had no significant effect on MEC proliferation at the time of slaughter but led to a higher level of apoptosis in the mammary gland. Gelatin zymography highlighted remodeling of the mammary extracellular matrix in restricted-diet cows. Udders from restricted-diet cows showed lower transcript expression of α-lactalbumin and kappa-casein. In conclusion, nutrient restriction resulted in lower milk yield in lactating dairy cows, partly due to modulation of MEC activity and a lower number of mammary cells. An association was found between feed restriction-induced changes in the growth hormone-insulin-like growth factor-1 axis and mammary epithelial cell dynamics.

Review: the cellular mechanisms underlying mammary tissue plasticity during lactation in ruminants.

The mammary tissue is characterized by its capacity to adapt in response to a wide variety of changing conditions. This adaptation capacity is referred to as the plasticity of mammary tissue. In dairy ruminants, lactation is challenged by modifications that can either be induced on purpose, such as by modifying management practices, or occur involuntarily, when adverse environmental constraints arise. These modifications can elicit both immediate changes in milk yield and composition and carryover effects that persist after the end of the challenge. This review focuses on the current knowledge concerning the cellular mechanisms underlying mammary tissue plasticity. The main mechanisms contributing to this phenomenon are changes in the activity and number of mammary epithelial cells (MECs). Changes in the number of these cells result from variations in the rates of cell proliferation and death as well as changes in the rate MEC exfoliation. The number of MECs also depends on the number of resident adult mammary stem cells and their progenitors, which can regenerate the pools of the various mammary cells. Several challenges, including changes in milking frequency, changes in level of feed supply and hormonal manipulations, have been shown to modulate milk yield together with changes in mammary cell activity, turnover and exfoliation. Epigenetic changes may be an additional mechanism of adaptation. Indeed, changes in DNA methylation and reductions in milk yield have been observed during once-daily milking and during mastitis in dairy cows and may affect cell activity persistently. In contrast to what has been assumed for a long time, no carryover effect on milk yield were observed after feed supply challenges in dairy cows and modification of milking frequency in dairy goats, even though the number of mammary cells was affected. In addition, mammary tissue plasticity has been shown to be influenced by the stage of lactation, health status and genetic factors. In conclusion, the cellular mechanisms underlying mammary tissue plasticity are diverse, and the mammary tissue either does or does not show elastic properties (with no permanent deformation), in response to environmental changes.

Hot topic: Prepubertal ovariectomy alters the development of myoepithelial cells in the bovine mammary gland.

Prepubertal ovariectomy can dramatically inhibit mammary development, but the mechanism of inhibition is not well characterized. Holstein heifers were ovariectomized (OVX) or sham operated but left intact (INT) at d 40 and then sacrificed at d 55, 70, 85, 100, 130, or 160 to provide tissues for histologic analysis of cell proliferation. Our histologic analyses unexpectedly revealed a pronounced effect of ovariectomy on myoepithelial cell development. Myoepithelial cells were identified on the basis of location, morphology, and immunohistochemical staining for alpha-smooth muscle actin (SMA). Vascular smooth muscle staining served as an internal positive control for all immunohistochemical analyses. Mammary tissues from d 40 heifers had an abundance of SMA+ cells associated with the ductal parenchyma. In INT heifers, the frequency of SMA+ cells decreased as development progressed. Only a limited number of isolated SMA+ cells were observed in d 70 to d 160 INT heifers. In OVX heifers, SMA+ cells were abundant, had elongated morphology, and frequently stained more intensively than vascular smooth muscle cells. The intense SMA staining and altered morphology was most prominent in older heifers. Limited analysis of gene expression revealed that maspin, a protease inhibitor expressed by myoepithelial cells, was expressed in parenchyma from both INT and OVX heifers. Our hypothesis is that ovarian secretions stimulate epithelial proliferation, and block myoepithelial differentiation. Myoepithelial cells are known to limit parenchymal cell proliferation. Ovariectomy may thus remove an estrogenic growth stimulus and permit the emergence of inhibitory cell populations that further limit parenchymal expansion. Our observation has important implications for control mechanisms that regulate parenchymal development.

Role of ovarian secretions in mammary gland development and function in ruminants.

The mammary gland is a dynamic organ that undergoes cyclic developmental and regressive changes during the lifetime of a female mammal. Mammogenesis begins during embryonic life with the development of the first mammary gland rudiments and ductal system. After birth, during the pre-pubertal period, the ductal growth of the mammary parenchyma occurs through the fat pad. In most of the ruminant species allometric mammary parenchyma development begins with the onset of cyclic ovarian secretions activity. The two main hormones secreted during an ovarian cycle are estradiol and progesterone. These steroid hormones are derived from cholesterol and are synthesized by theca and granulosa cells in ovaries. During puberty, the mammary parenchyma develops in a compact, highly arborescent parenchymal mass surrounded by a dense connective matrix. Ductal elongation and lobulo-alveolar development are accomplished during growth and pregnancy to prepare for future milk production. At the end of lactation, the mammary gland undergoes involution, which corresponds to a regression of the secretory tissue, a reduction in the alveolar size and a loss of mammary epithelial cells (MECs). Ovarian steroids (estradiol and progesterone) appear to be key regulators of the different stages of mammogenesis and mammary function. Through this review, the role and the importance of ovarian steroids on mammary gland and on MECs is described.

Changes in mammary secretory tissue during lactation in ovariectomized dairy cows.

In dairy animals, the milk yield (MY) changes during a lactation and is influenced by several physiological, livestock management and environmental factors. The MY produced by a mammary gland depends on synthetic activity of mammary epithelial cells (MECs) as well as MEC number and mammary secretory tissue organization. It has been suggested that ovarian steroids (estradiol and progesterone) have a negative effect on MY in lactating cows. In a previous study, we showed that the suppression of ovarian secretions by an ovariectomy improved lactation persistency in dairy cows. Here we were interested in the effects of ovariectomy on plasma estradiol and progesterone concentrations and on changes that occur in mammary secretory tissue during lactation. We demonstrated that the ovariectomy of lactating cows at the time of the lactation peak induced a rapid and dramatic drop in plasma progesterone and a smaller reduction in plasma estradiol. Interestingly, the study of the changes in mammary secretory tissue over time revealed that the improvement of MY measured in the ovariectomized cows was associated with a limited increase in estradiol receptivity in MECs, a reduced mammary tissue remodeling and reduced blood protein concentration in milk, in late lactation. These results suggest that ovarian secretions, particularly estradiol and progesterone, act to enhance processes for mammary gland involution in late-lactating dairy cows.

New developments on the galactopoietic role of prolactin in dairy ruminants.

In most mammals, prolactin (PRL) is essential for maintaining lactation and its suppression strongly inhibits lactation. However, the involvement of PRL in the control of ruminant lactation is less clear because inconsistent effects on milk yield have been observed with short-term suppression of PRL by bromocriptine. By contrast, in vitro studies have provided evidence that PRL helps to maintain the differentiation state and act as a survival factor for mammary epithelial cells. Therefore, a series of experiments were conducted to assess the galactopoietic role of PRL. In a first experiment, daily injections of the PRL inhibitor quinagolide reduced milking-induced PRL release and induced a faster decline in milk production. Milk production was correlated with PRL released at milking. Quinagolide reduced mammary cell activity, survival, and proliferation. During the last week of treatments, differential milking (1× vs 2×) was applied. The inhibition of milk production by quinagolide was maintained in the udder half that was milked 2× but not in the udder half milked 1×, suggesting that the response to PRL is modulated at the gland level. In a second experiment, cows were injected with quinagolide, quinagolide + injection of bovine PRL at milking time, or water. As in the first experiment, quinagolide reduced milk, protein, and lactose yields. Although PRL injections at milking time were not sufficient to restore milk yield, they tended to increase milk protein and lactose yields and increased the viability of milk-purified mammary epithelial cells. Recently, we investigated the use of quinagolide at drying off. Treating late-lactation cows with quinagolide decreased milk production within the first day of treatment and induced faster increases in somatic cells and bovine serum albumin content in mammary secretions after drying off, which indicates an acceleration of mammary gland involution. In conclusion, these data, combined with data from other studies, provide a good body of evidence indicating that PRL is galactopoietic in dairy cows. However, the response to PRL appears to be modulated at the mammary gland level.

Effects of ovariectomy in prepubertal goats.

The objectives of this study were to determine the effects of ovariectomy on mammary gland development in prepubertal goats and to validate this model to study mammogenesis in young dairy ruminants. In this experiment, 3 months of aged goats were ovariectomized (ovx) while shammed goats played as surgery controls (sham). Thereafter, sham and ovx goats were slaughtered at 7 months of age to provide tissue for the assays. Results demonstrated that proliferation of mammary of mammary epithelial cells was significantly lower in ovariectomized goats compared to control goats. In ovx animal, epithelium structures were completely overstretched and epithelial ducts were undeveloped with limited branching whereas control animals had classical complex arborescent units with multiple round ductules and limited stroma. Concerning ERalpha (estrogen receptor alpha), PR (progesterone receptor) and P450 (aromatase) expression, results showed number of ERalpha, PR and P450 positive cells was higher in shammed goats compared to ovariectomized goats. All this results suggested that goat mammogenesis and ovarian control are similar to prepubertal heifers and that young goats are a good model to study mammary gland development in ruminants. In conclusion, we demonstrated that ovariectomy of prepubertal goats decreased proliferation of mammary epithelial cells with a profound alteration of cell adhesion molecules.

Constitutively activated CK2 potentially plays a pivotal role in Theileria-induced lymphocyte transformation.

Activation of casein kinase II (CK2) was one of the first observations made on how Theileria parasites manipulate host cell signal transduction pathways and we argue that CK2 induction may in fact contribute to many of the different activation events that have been described since 1993 for Theileria-infected lymphocytes such as sustained activation of transcription factors c-Myc and NF-kappaB. CK2 also contributes to infected lymphocyte survival by inhibiting caspase activation and is probably behind constitutive PI3-K activation by phosphorylating PTEN. Finally, we also discuss how CK2A may act not only as a kinase, but also as a stimulatory subunit for the protein phosphatase PP2A, so dampening down the MEK/ERK and Akt/PKB pathways and for all these reasons we propose CK2 as a central player in Theileria-induced lymphocyte transformation.