Organelles coordinate milk production and secretion during lactation: Insights into mammary pathologies

The mammary gland undergoes a spectacular series of changes during its development and maintains a remarkable capacity to remodel and regenerate during progression through the lactation cycle. This flexibility of the mammary gland requires coordination of multiple processes including cell proliferation, differentiation, regeneration, stress response, immune activity, and metabolic changes under the control of diverse cellular and hormonal signaling pathways. The lactating mammary epithelium orchestrates synthesis and apical secretion of macromolecules including milk lipids, milk proteins, and lactose as well as other minor nutrients that constitute milk. Knowledge about the subcellular compartmentalization of these metabolic and signaling events, as they relate to milk production and secretion during lactation, is expanding. Here we review how major organelles (endoplasmic reticulum, Golgi apparatus, mitochondrion, lysosome, and exosome) within mammary epithelial cells collaborate to initiate, mediate, and maintain lactation, and how study of these organelles provides insight into options to maintain mammary/breast health.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

Influence of bovine mastitis on lipolysis ond proteolysis in milk

Lipolysis and proteolysis in milk were determined before, during, and after experimentally induced mastitis. Streptococcus agalactiae was infused into one quarter of five cows to elicit an infection. Milk protease activity was higher during infection, but milk lipase activity was unchanged. Lipolytic damage to milk fat and proteolytic damage to milk casein occurred in the udder prior to milking during an infection. Lipolysis increased due to increased susceptibility of the milk fat to lipase action during infection. The mechanism of the increased susceptibility of the fat to lipolysis was not determined. After infections were eliminated, SCC, initial and stored FFA concentrations, and initial tryosine values returned to preinfection levels. However, after infections were eliminated, milk protease activity as determined by an increase in tryosine values remained elevated as milk SCC returned to preinfection levels. Protease activity returned to preinfection levels within 10 d after SCC returned to preinfection levels.

Activity of lysosomal enzymes in murine mammary tissue through pregnancy, lactation, and involution

Murine mammary tissue homogenates obtained during pregnancy, lactation, and involution were assayed for activities of two lysosomal marker enzymes, acid phosphatase, and N-acetyl-beta-D-glucosaminidase. Acid phosphatase activity per milligram protein was relatively constant through pregnancy and lactation, although a decrease was detected at parturition. Acid phosphatase activity per milligram deoxyribonucleic acid was also stable through pregnancy and lactation except for a peak of activity during lactogenesis. Acid phosphatase activity per gram wet weight also remained stable during pregnancy and lactation, but activity was significantly higher during lactation than pregnancy. Glucosaminidase activity, whether expressed as milligrams deoxyribonucleic acid of milligrams protein, tended to decrease during pregnancy, decreased further with lactogenesis, and remained significantly lower throughout lactation. Glucosaminidase activity per gram wet weight increased as pregnancy progressed and slowly decreased through lactogenesis to midpregnancy levels, which remained stable throughout the remainder of lactation. Both acid phosphatase and glucosaminidase activities were slightly higher at the end of lactation, and both decreased within 24 h of weaning. Significant increases in activities of both enzymes were observed from d 1 to 4 of involution.

Factors affecting the distribution of lipoprotein lipase activity between serum and casein micelles in bovine milk

The influence of mastitis and early lactation, and the effect of treating milk with heparin, blood serum and trypsin, on the proportion of lipoprotein lipase (LPL) activity in mild serum was investigated. The relative importance of milk serum LPL and LPL bound to micellar casein in promoting lipolysis was also examined. Colostrum contained LPL activity, 45% of which was found in the serum phase in samples obtained from the first milking post partum, but this value fell to 34% in samples taken 24 h later. The proportion of serum LPL was also increased in milks from quarters infected with Staphylococcus aureus, but not after overnight treatment of normal milk at 4 degrees C with 5% (w/v) blood serum or 2 microgram/ml trypsin. The addition of 5 microgram/ml heparin resulted in a consistent increase in serum LPL which varied between 14 and 50% of total milk LPL. Heparin did not release all the enzyme bound to casein micelles even after a second heparin treatment of resuspended micelles. Serum LPL was more effective in promoting lipolysis and was more responsive to blood serum activation than LPL bound to casein micelles. Lipolysis increased after heparin treatment but the increase was not related to serum LPL activity.

Inhibition of lipolysis in bovine milk by proteose peptone

The ability of total milk proteose peptone and individual proteose peptone components to inhibit lipolysis in milk was examined. Proteose peptone (1.5 mg/ml) when added to milk inhibited lipolysis, and component 3 was a more effective inhibitor than component 5 or component 8-fast. Inhibition by proteose peptone also occurred when the milk was treated with between 2 and 20% v/v bovine blood serum or with 5 micrograms/ml heparin. In vitro activity of milk lipoprotein lipase and its stability were not affected by proteose peptone. It was concluded that proteose peptone does not interact with lipase or its activator and that the mechanism of inhibition involves the substrate.

The relationship between the levels of free fatty acids, lipoprotein lipase, carboxylesterase, N-acetyl-beta-D-glucosaminidase, somatic cell count and other mastitis indices in bovine milk

A large-scale survey of milks from healthy and mastitic bovine quarters was undertaken to establish the influence of mastitic infection on milk lipase activity and free fatty acid (FFA) level. Mastitic milks tended to have higher FFA levels, but lower lipoprotein lipase activities compared with milk from healthy quarters. These effects became significant at relatively severe levels of infection. The elevated FFA was attributable to higher FFA levels on secretion and to greater lipolysis during storage. Levels of carboxylesterase activity increased with severity of mastitis and showed high positive correlation with mastitis indices. Marked increases in carboxylesterase, N-acetyl-beta-D-glucosaminidase and phospholipase occurred following the induction of mastitis by intramammary infusion of Escherichia coli endotoxin, in parallel with changes in somatic cell count and other mastitis indices. Relativity little change in lipoprotein lipase activity was observed.

Observations on the influence of high cell count on lipolysis in bovine milk

The effect that changes in composition which occur in milks possessing high cell counts have on milk lipolysis has been investigated. High cell counts were produced either by intramammary infusion ofEscherichia coliendotoxin orStreptococcus agalactiaeor by addition of washed cells which were isolated from milk obtained from quarters infused with endotoxin. Free fatty acid levels in milk were measured in terms of acid degree value (ADV) either as initial ADV measured immediately after milking or ADV developed after a prescribed incubation period.

There was an increase in initial ADV after the infusion either of endotoxin or ofStr. agalactiaerelative to a control quarter. This increase appeared to be associated with changes in cell count, but in absolute terms the influence of cells on ADV became less as cell count increased. Neither type of infusion had any effect on lipoprotein lipase activity. The addition of washed cells to normal milk resulted in an increase in developed ADV, but the increment was not as large as that produced by the addition of 1% blood serum. When cream and skim-milk from endotoxin-treated quarters and control quarters were mixed in different combinations with and without additional cells, developed ADV was higher in those samples containing endotoxin cream and those with added cells. Milk from a quarter treated with endotoxin was diluted with its own skim-milk to produce different cell counts and ADV was determined after various time intervals at 4 and 37 °C. Lipolysis increased with increasing cell count, but a depression in lipolytic rate was noted after incubation for 6 h at 4 °C and 20 min at 37 °C.

The proportion of skim-milk lipoprotein lipase activity in milk serum was larger both in milks possessing high cell counts and in normal milk adjusted to between 5 and 20 mM-NaCl by addition of solid NaCl. These levels of NaCl inhibited lipolysis.

The possible direct and indirect effects of high cell count on milk lipolysis are discussed.