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

Changes in cathepsin D activity of maternal tissues during lactation and weaning in rats

BACKGROUND

A loss of proteins from maternal tissues during lactation has been demonstrated. Protein loss could be explained by intracellular proteolysis.

METHODS

Cathepsin D activity was studied in the liver, muscle and mammary gland of lactating and weaned rat dams. Lactation was studied at maximal milk production (L-14) and at the final stage of lactation (L-21).

RESULTS

Basal activity (virgin rats) was three times higher in liver and mammary gland than in muscle. At both stages, L-14 and L-21, cathepsin D activity increased in liver (50%) as well as in the gland (164%), but no change was observed in muscle, when compared with controls. Twenty-four hours after litter separation, enzyme activity in the liver decreased to basal levels, while in the mammary gland cathepsin D activity showed a significant decrease but remained higher than control levels.

CONCLUSION

Our results show that liver exhibits adaptive changes in the catabolism of proteins in response to the increased demands imposed by lactation on the maternal organism, and when the stimuli disappear activity returns to basal levels. The high activity in mammary gland indicates fast turnover of structures and biomolecules as an answer to the high synthetic activity in this tissue. Activity remained higher in the weaning rats, as a result of the regression process which the mammary gland is undergoing.

Role of tissue remodeling in mammary epithelial cell proliferation and morphogenesis

Lactation is a physiological process characterized by the secretion of large quantities of protein, carbohydrate, and lipid. To achieve the production, the mammary gland must grow and then differentiate; both processes require extensive tissue remodeling. Remodeling begins with a carefully controlled proteolysis of the extracellular matrix and cell-cell adhesion proteins. Plasmin is a serine protease that has been implicated in the tissue remodeling associated with the declining phase of lactation and mammary involution. As lactation progresses, the quantity of plasmin activity increases within the mammary tissue and milk. This has led to the hypothesis that gradual involution results from progressive tissue remodeling. Hormonal attenuation of gradual involution by bST would slow tissue remodeling and would be permissive for lactation. In vitro results indicate that insulin-like growth factor-I impairs the secretion of plasminogen activator by bovine mammary epithelial cells. As such, a mechanism of action for bST exists.

Degradation of individual intracellular proteins analyzed by two-dimensional gel electrophoresis and computerized video densitometry

A technique is described for the analysis of degradation rates of individual intracellular proteins, based on pulse-chase-labeling of cells using radioactive amino acids [35S]methionine, two-dimensional polyacrylamide gel electrophoresis, fluorography and scanning of the fluorograms by a computerized video densitomter. As compared to scintillation counting of individual protein spots resolved by two-dimensional gel electrophoresis, this method allows a rapid and precise determination of the degradation rates of individual intracellular proteins. In the present study, degradation rates of individual intracellular proteins of normal human skin fibroblasts and skin fibroblasts from patients with Duchenne muscular dystrophy were compared. Rates of degradation for proteins PIIa, PIIb and PIIc recently described as cell-type-specific proteins were significantly enhanced (p less than 0.01) in fibroblast cultures of Duchenne muscular dystrophy origin.

Protein synthesis and degradation in the mammary gland of lactating goats

Lactating goats were given a close arterial infusion of [1-14C]leucine and [4,5-3H]4-methyl-2-oxopentanoic acid into one half of the mammary gland at 2-3 weeks and 34-39 weeks after kidding. Rates of protein synthesis, degradation and net output were determined from measurements of arteriovenous difference and blood flow using a model of leucine metabolism previously developed for muscle (Oddy & Lindsay, 1986). Protein leucine output in milk (Y mumol/min) correlated well with the difference between synthesis and degradation (X mumol/min) derived from the model: Y = 1.30 + 1.24X (r2 = 0.9; n = 9, P less than 0.01). There was substantial synthesis and degradation of protein within the mammary gland. Although only an approximate value could be obtained for the partitioning of protein synthesis and degradation between tissue and milk proteins, there was evidence of appreciable turnover of both. There was no significant difference between mammary leucine and protein metabolism in early and late lactation other than that imparted by a greater mass of mammary tissue in early lactation, although there was a tendency for greater oxidation of leucine in late lactation.

Casein secretion by mammary gland epithelia from collagen gel cultures and lactating glands

Amino acid incorporation experiments show that epithelial cells from lactating mouse mammary glands and from collagen gel culture both synthesize and secrete four principal phosphocaseins (p45, p40, p27, and p23 kD). In both cases, however, the casein production is largely dominated by the p27 species. The average percentage distribution of the above casein species in medium from cultured epithelia is approximately 13%, 6%, 68%, and 14%, respectively; for milk the distribution is approximately 23%, 7%, 54%, and 16%. The predominance of the p27 species is not a consequence of extensive extracellular differential degradation of the secreted caseins since no significant casein degradation was observed in culture medium, either in contact or isolated from epithelial cell monolayers. Synthesis and secretion of all the caseins by cultured epithelia is dependent upon insulin, prolactin, and hydrocortisone. Presumably some intracellular events result in the secretion of p27 as the principal casein in mouse milk. Apparently, some selection factor(s) operate to make p27 a major nitrogenous nutritional component for a newborn mouse. In addition, on a quantitative basis, the relative levels of various caseins secreted by epithelia from lactating mammary glands is essentially duplicated by epithelia in collagen gel culture.

Synthesis, accumulation and turnover of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in cultures of embryonic rat hepatocytes

Glucocorticosteroid, thyroid hormones and cyclic AMP can induce the synthesis of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in cultures of hepatocytes as soon as these cells differentiate from the embryonic foregut. The low levels of both enzymes that can accumulate in such still protodifferentiated hepatocytes are due to low levels of enzyme synthesis. In cultures, the rate of synthesis of both enzymes increases continually in the presence of hormones, showing that maturation of the capacity for synthesis towards the postnatal, fully differentiated situation is occurring in these cells. The turnover rate of both enzymes in embryonic hepatocytes is lower in the presence of hormones than in the absence, but does not change during the culture period. In the presence of hormones the turnover rate is comparable to that found in adult rat liver in vivo. The development of the capacity to accumulate organ-specific enzymes in vitro (and hence the rate of enzyme synthesis) is found to be comparable to that in utero.

Protein degradation in lemna with particular reference to ribulose bisphosphate carboxylase: I. The effect of light and dark

Ribulose bisphosphate carboxylase from Lemna minor resembles the structure reported for the enzyme from other plants. When grown in the light, the enzyme appears to undergo little or no degradation, as measured by a double-isotope method. This situation is similar to that reported for wheat and barley, but is unlike that reported for maize, where the enzyme degrades at the same rate as total protein. Prolonged periods of darkness usually induce leaf senescence, characterized by the rapid degradation of chlorophyll and protein, with ribulose bisphosphate carboxylase undergoing preferential degradation. In L. minor there is selective protein degradation in the dark, but chlorophyll and ribulose bisphosphate carboxylase are stable when fronds are kept in the darkness for up to 8 days. It appears that Lemna is not programmed to senesce, or at least that darkness does not induce senescence in Lemna. Although there is no evidence for in vivo degradation or modification of ribulose bisphosphate carboxylase during prolonged periods of darkness, extracts from fronds which have been kept in the dark for periods in excess of 24 hours convert ribulose bisphosphate carboxylase to a more acidic form. The properties of the dark-induced system which acts on ribulose bisphosphate carboxylase, suggest that it may be a mixed function oxidase. The proposition that the selectivity of protein degradation is genetically determined, so that the rate at which a protein is degraded is determined by its charge or size, was tested for fronds grown in the light or maintained in the dark. There was no significant correlation between protein degradation and either charge or size, in light or dark.

Secretion-coupled protein degradation: studies on mammary casein

Mammary explants from midpregnant rabbits were cultured for 18 h at 37 degrees C with insulin, prolactin and cortisol. Subsequently, explants were labelled for 2 h with inorganic [32P]phosphate, L-[5-3H]proline or L-[4,5-3H]leucine, washed and chased for up to 3 h. The radiolabelling profile of [32P]casein or [3H]casein during the chase period, obtained by isoelectric focussing or immunoprecipitation indicates extensive destruction of neosynthesized casein. The extent of casein destruction in mammary explants in culture (measured after radiolabelling with L-[5-3H]proline), is inversely related to casein secretion. Least casein degradation is observed in explants after 48 h in culture when casein secretion is maximal (observed histochemically). Subsequently, when the extracellular alveolar lumen is filled with secretion products (72 h), rapid intracellular casein destruction is again observed. When the chase was carried out in the presence of drugs which inhibit degradation and/or secretion, the results indicate that secretion-coupled casein degradation is dependent on an intact functional microfilamentous-microtubular network, casein is not degraded by an autophagosome requiring process, degradation is inhibited by leupeptin, amino-acid analogue containing casein does not undergo secretion-coupled degradation and inhibition of N-glycosylation of intracellular vesicular membrane proteins prevents secretion-coupled degradation. Secretion-coupled protein destruction is discussed in relation to the post-translational regulation of the net production of secretory proteins in eukaryotic cells.

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.

Multiple molecular forms of rat mammary glucose-6-phosphate dehydrogenase: proposed role in turnover of the enzyme

Polyacrylamide gel electrophoresis shows that more than 80% of the total glucose-6-phosphate dehydrogenase activity of rat mammary homogenates exists as three dimeric forms. During lactation all three forms are present in equivalent amounts, but 24 h after the removal of the pups to stop lactation, there is a shift to the fastest migrating form (band I). During this period the ratio of enzyme activity to tissue glutathione concentration decreased. Using partially purified preparations of the enzyme it was shown that dithiothreitol could convert all the activity to the slowest migrating form (band III) and that diamide could reverse this change giving rise to band I only. Reduced glutathione could partially convert the enzyme to the band III form. Experiments using iodoacetic acid, iodoacetamide, p-chloromercuribenzoate, and mercuric chloride are also described. Based on these observations it is proposed that band I and band III represent fully oxidized and fully reduced forms of the enzyme, respectively, and band II a partially oxidized form. The oxidized and reduced forms are catalytically equivalent. However, the oxidized form is the most rapidly inactivated by chymotrypsin or mammary microsomes. Mammary microsomes can also catalyze the oxidation of the enzyme. The oxidation precedes the inactivation by microsomes and also occurs at lower microsome concentrations. It is proposed that the microsomal oxidation of the enzyme is the initial step in the turnover of the enzyme.

Susceptibility of muscle soluble proteins to degradation by mast cell chymase

We investigated the in vitro susceptibility of muscle soluble proteins to the major alkaline proteinase (chymase) from skeletal muscle tissue, an enzyme originating from intramuscular mast cells, but also present in certain muscle fibers. Cytoplasmic proteins from rat skeletal muscle tissue were fractionated into four groups according to their different isoelectric points: fraction A (pI 9.5-7.0), B (pI 7.0-5.6), C (pI 5.5-4.5) and D (pI 5.3-3.5). Chromatography of these fractions on octyl-Sepharose CL-4B revealed the presence of a higher percentage of hydrophobic proteins in fraction C and D as compared to fraction A and B. In vitro degradation of these protein fractions by chymase, isolated from rat skeletal muscle tissue, was monitored (a) by measuring the ability of these proteins to bind Coomassie G-250, and (b) by analyzing the digestion mixture in isoelectric focusing gels. Both methods revealed fraction B proteins to be degraded very rapidly. While there was also a significant breakdown of fraction A proteins, fraction C and D proteins were degraded only very slowly, if at all. These differences in degradability are not due to the presence of a proteinase inhibitor in fraction C and D. The results suggest that mast cell chymase preferentially degrades those groups of muscle soluble proteins, the constituents of which have neutral to basic isoelectric points and a relatively low surface hydrophobicity.

The effect of starvation and refeeding on lipogenic enzymes in mammary glands and livers of lactating rats

Lactating rats were starved for 48 h and refed a high-carbohydrate diet for a further 48 h. Starvation stops milk secretion, which resumes shortly after refeeding. Three lipogenic enzymes, fatty acid synthase, glucose 6-phosphate dehydrogenase (EC 1.1.1.49) and 'malic' enzyme (EC 1.1.1.40) all decrease in the mammary gland during starvation and are restored to the pre-starvation levels 48 h after refeeding. The same enzymes in liver also decrease during starvation, but increase to values significantly higher than those for the normal fed rats after refeeding the high-carbohydrate diet. For the fatty acid synthase these values were four times the pre-starvation values. Serum insulin and prolactin concentrations also increased upon refeeding the high-carbohydrate diet.

The differential actions of cortisol on the synthesis and turnover of alpha-lactalbumin and casein and on accumulation of their mRNA in mouse mammary gland in organ culture

Cortisol was previously shown to exert different, concentration-dependent, effects on the accumulation of casein and alpha-lactalbumin in mammary glands from mid-pregnant mice cultured in the presence of insulin and prolactin [Ono & Oka (1980) Cell 19, 473-480]. The present study demonstrated that the addition of 30nM-cortisol to the medium containing insulin and prolactin resulted in a marked enhancement of the rate of synthesis of both alpha-lactalbumin and casein in cultured tissue. The addition of 3 microM-cortisol in combination with insulin and prolactin caused a marked decrease in the rate of alpha-lactalbumin synthesis, but increased casein synthesis substantially. Similar changes were also observed in the amount of translatable mRNA for alpha-lactalbumin and casein in mammary explants cultured with insulin, prolactin and the two concentrations of cortisol. The study of the turnover of the milk proteins in cultured explants showed that virtually all of the casein synthesized remained intact in tissue explants cultured with 3 microM cortisol, whereas about 45% of casein disappeared in 40h from explants cultured with 30nM-cortisol. In contrast, the two concentrations of cortisol did not differentially affect the disappearance of alpha-lactalbumin, which was about 55% in 40h. These results indicate that the concentration-dependent differential actions of cortisol on the accumulation of alpha-lactalbumin and casein are exerted through its effects on the rate of synthesis and turnover of the two proteins as well as on the accumulation of their mRNA species.

Extensive destruction of newly synthesized casein in mammary explants in organ culture

1. Explants of mammary glands of mid-pregnant rabbits that had been cultured for 18h in the presence of insulin, prolactin and cortisol were incubated at 37 degrees C for 2h in Medium 199 containing l-[4,5-(3)H]leucine. After a wash procedure at 4 degrees C, explants were re-incubated at 37 degrees C in fresh medium and the radioactivity of casein polypeptides isolated by isoelectric focusing (at pH 4.6) was followed with time. Casein radioactivity rose during the first hour of re-incubation, but fell markedly during the subsequent hour. 2. Loss of radioactivity represented casein degradation, since less than 10% of newly synthesized casein was found in the incubation medium. 3. Such a loss of radioactivity was not due solely to hydrolysis of signal peptides, since similar results were obtained when l-[5-(3)H]proline, which is not part of casein signal peptides, was the radiolabelled precursor. 4. A dual-isotope experiment using l-[U-(14)C]proline and N-[(3)H]acetyl-d-mannosamine gave similar profiles of radioactivity loss from isoelectrically focused casein, indicating that degradation of mature casein was occurring. 5. Analysis of total pellet and particle-free-supernatant fractions prepared by centrifugation of explant homogenates at 115000g(av.) for 1h did not show loss of radioactivity on re-incubation. Total pellet-protein radioactivity remained constant, whereas total soluble-protein radioactivity increased during the 2h re-incubation period. 6. Radioactivity in a specific particle-free-supernatant polypeptide, the subunit of fatty acid synthetase, mimicked that of the total soluble protein. 7. Addition of cycloheximide (20mug/ml) during the re-incubation period completely blocked the incorporation of radioactivity from l-[5-(3)H]proline into casein and the subsequent fall, indicating that observations were being made on newly synthesized casein. 8. Addition of chloroquine (50mum) did not prevent the increase in radioactivity from l-[5-(3)H]proline into casein during the first hour of re-incubation, but did prevent the loss of radioactivity in the second hour. 9. The intracellular degradation of a newly synthesized milk protein is discussed in relation to the known intracellular degradation of other secretory polypeptides.

Protein degradation in rat liver during post-natal development

Protein degradation rates for liver subcellular and submitochondrial fractions from neonatal (8-day), weanling (25-day) and adult rats were estimated by the double-isotope method with NaH14CO3 and [3H] arginine as the radiolabelled precursors [Dice, Walker, Byrne & Cardiel (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 2093-2097]. Decreased protein degradation rates were found during post-natal development for homogenate, nuclear, mitochondrial, lysosomal and microsomal proteins. A decrease in degradation rates for the immunoisolated subunits of monoamine oxidase and pyruvate dehydrogenase was also observed in neonatal and weanling rats respectively. The results suggest coordinate degradation of the subunits of the multi-subunit enzyme pyruvate dehydrogenase. Pyruvate dehydrogenase has a faster rate of degradation in adult rat liver than does cytochrome oxidase. Data analysis suggests heterogeneity of protein degradation rates in the mitochondrial outer membrane and intermembrane space fractions at each developmental stage but not in the mitochondrial inner membrane or matrix fractions. Results obtained for protein degradation rates in adult rat liver by the method of Burgess, Walker & Mayer [(1978) Biochem. J. 176, 919-926] in general confirmed the results obtained for the adult rat liver by the above method. No evidence of a subunit-size relationship for protein degradation was found for proteins in any subcellular or submitochondrial fraction.